GIFT OF MICHAEL REESE ORGANIC CHEMISTRY PART II. BY W. H. PERKIN, JUN., Ph.D., F.R.S. PROFESSOR OF ORGANIC CHEMISTRY IN OWENS COLLEGE, MANCHESTER AND F. STANLEY KIPPING, Ph.D., D.Sc. (LoxD.), F.R.S. PROFESSOR OF CHEMISTRY IN UNIVERSITY COLLEGE, NOTTINOHAM EDINBURGH AND LONDON W. & R. CHAMBERS, LIMITED PHILADELPHIA: J. B. LIPPINCOTT COMPANY PEEFACE. THE present volume (Part II.) consists principally of a description of the aromatic compounds, and, together with Part L, forms an introduction to Organic Chemistry. The opening chapters of Part II. contain an account of coal- tar and its treatment. This leads naturally to a description of the preparation and properties of benzene, and to a discussion of its constitution in the light of facts previously dealt with ; the student is thus made acquainted with the principal characteristics of aromatic, as distinct from fatty, compounds, and is then in a position to understand the classification of organic substances into these two main divisions. The more important classes of aromatic compounds are then described, but in a somewhat different manner from that adopted in Part L, inasmuch as a general account of the properties of each class of substances is given before, instead of after, the more detailed description of typical compounds ; this course is to a great extent free from the disadvantages which are found to attend its adoption at earlier stages, as the student has by this time acquired some experience of the more systematic method from a study of the summaries given in Part I. Special attention has been given, as before, to questions of constitution, one of the objects being to train the student to think out such matters, and to try and deduce a constitutional 96721 IV PREFACE. formula for a given substance, by comparing its properties with those of others of known constitution ; with this end in view, it has often been thought desirable to withhold the most important evidence in favour of the accepted constitutional formula until the subject had been discussed at some length. The concluding chapters on dyes, alkaloids, and stereo- isomerism will doubtless offer the greatest difficulties, but, considering the importance of the matters with which they deal, their omission or curtailment was deemed inadvisable. The account of the alkaloids should be useful, more particu- larly to medical students, whilst the chapter on dyes deals with a variety of substances of even greater practical value, and indicates the methods employed in one of the most im- portant applications of organic chemistry. The chapter on stereo-isomerism was included because, owing to the import- ance to which this theory has now attained, a text-book on organic chemistry would be incomplete without a brief dis- cussion of the subject. The full directions which are given for the use of models will, it is hoped, lead to a clear con- ception of the views set forth. The practical aspect of the science has again been kept well to the front, a detailed description of the preparation of all the more typical compounds being given (usually in smaller type), in order to facilitate the laboratory work, which must be regarded as a necessary accompaniment to the theoretical knowledge. Our thanks are again due to Dr A. Harden for many valuable suggestions, as well as for help in revising the proof-sheets, and in preparing the index. CONTENTS. PAGE CHAPTER XVII. MANUFACTURE, PURIFICATION, PROPERTIES, AND CONSTITUTION OF BENZENE 295 CHAPTER XVIIL ISOMERISM OF BENZENE DERIVATIVES, AND DETERMINATION OF THEIR CONSTITUTION 310 CHAPTER XIX. GENERAL PROPERTIES OF AROMATIC COM- POUNDS 322 Classification of Organic Compounds 322 General Character of Aromatic Compounds 324 CHAPTER XX. HOMOLOGUES OF BENZENE 328 Toluene Xylenes Mesitylene Cumene Gymene 334-339 Diphenyl Diphenylmethane Triphenylmethane 340 CHAPTER XXI. HALOGEN DERIVATIVES OF BENZENE AND ITS HOMOLOGUES 341 Chlorobenzene Bromobenzene Chlorotoluene Benzyl Chloride 347, 348 CHAPTER XXII. NITRO-COMPOUNDS 350 Nitrobenzene Meta-dinitrobenzene Nitrotoluenes 352-355 CHAPTER XXIII. AMIDO-COMPOUNDS AND AMINES 355 Aniline and its Derivatives 361 Homologues of Aniline Alky lanilines 364 Dipheny lamine and Tripheny lamine 367 Aromatic Amines Benzylamine 368 CHAPTER XXIV. DIAZO-COMPOUNDS AND DERIVATIVES 370 Diazoamido- and Amidoazo-compounds 374 Phenylhydrazine 376 Azo-compounds 377 CHAPTER XXV. SULPHONIC ACIDS AND THEIR DERIVATIVES... 379 CHAPTER XX VI. PHENOLS 385 Monohydric Phenols Phenol Picric Acid Cresols ....391-396 Dihydric Phenols Catechol, Resorcinol, Hydroquinone..398, 399 Trihydric Phenols 399 CHAPTER XXVII. AROMATIC ALCOHOLS, ALDEHYDES, KETONES, AND QUINONES 402 Alcohols Benzyl Alcohol 402, 403 Aldehydes Benzaldehyde 405 Hyd roxy-aldehydes Salicylaldehyde 408, 409 -Ketones Acetophenone 411 Quinones Quinone 413 VI CONTENTS. PAGE CHAPTER XXVIII. CARBOXYLIC ACIDS 416 Berizoic Acid Benzoyl Chloride Benzoic Anhydride Benzamide Berizonitrile 418-421 Substitution Products of Benzoic Acid 422 Toluic Acids 423 Dibasic Acids Phthalic Acid, Phthalic Anhydride, Iso- phthalic Acid, Terephthalic Acid 423-427 Phenylacetic Acid, Phenylpropionic Acid, and Derivatives.. 427 Cinnamic Acid 430 CHAPTER XXIX. HYDROXYCARBOXYLIC ACIDS 433 Salicylic Acid Anisic Acid Protocatechuic Acid Gallic Acid Tannin Mandelic Acid 437-440 CHAPTER XXX. NAPHTHALENE AND ITS DERIVATIVES 442 Naphthalene 442 Naphthalene Tetrachloride Nitro-derivatives Amido- derivatives Naphthols Sulphonic Acids a-Naph- thaquinone /3-Naphthaquinone 450-456 CHAPTER XXXI. ANTHRACENE AND PHENANTHRENE, 457 Anthracene 457 Anthraquinone Alizarin Phenanthrene Phenanthra- quinorie Diphenic Acid 462-471 CHAPTER XXXIL PYRIDINE AND QUINOLINE 471 Pyridine and its Derivatives 472 Piperidine 476 Homologues of Pyridine Pyridinecarboxylic Acids 478 Quinoline 480 Secondary and Tertiary Aromatic Bases 483 CHAPTER XXXIII. ALKALOIDS 484 Alkaloids derived from Pyridine, 488 ; from Quinoline 492 Alkaloids contained in Opium Morphine, &c 495 Alkaloids related to Uric Acid Caffeine, &c 497 Antipyrine, Kairine, Thalline 499 Choline, Beta'ine, Neurine, and Taurine 500 CHAPTER XXXIV. DYES AND THEIR APPLICATION 502 Malachite Green, Pararosaniline, Rosaniline, Methylviolet, Aniline Blue 509-517 The Phthaleins Phenolphthalein, Fluorescei'n, Eosin.. 518-521 Azo-dyes Aniline Yellow, Chrysoidine, Bismarck Brown, Helianthin, Ptesorcin Yellow, Rocellin, Congo -red, Benzopurpurins 522-526 Various Colouring Matters Martins' Yellow, Methylene Blue, Indigo 527 CHAPTER XXXV. STEREO-ISOMERISM 528 ORGANIC CHEMISTRY. PAET II. CHAPTER XVII. .MANUFACTURE, PURIFICATION, PROPERTIES, AND CONSTITUTION OF BENZENE. Distillation of Coal-tar. When coal is strongly heated out of contact with air, it undergoes very complex changes, and yields a great variety of gaseous and liquid products, together with a solid, non-volatile residue of coke. This process of dry or destructive distillation is carried out on the large scale in the manufacture of coal-gas, 1'or which purpose the coal is heated in clay or iron retorts, provided with air-tight doors ; the gas and other volatile products escape from the retort through a pipe, and when distillation is at an end, the coke, a porous mass of carbon, containing the ash or mineral matter of the coal, is withdrawn. The hot coal-gas passes first through a series of pipes or conrfwwr*, kept cool by immersion in watej or simply by exposure to the air, and, as its temperature falls, it deposits a considerable quantity of tar and gas-liquor, which are run together into a large tank ; it is then forced through, or washed with, water, in washers and scrubbers, and, after having been further freed from tar, ammonia, carbon dioxide, and sulphuretted hydrogen by suitable processes of purilication, it 296 -MANUFACTURE, PURIFICATION, PROPERTIES, is led into the gas-holder and used for illuminating and heating purposes. The average volume percentage composition of puri- fied coal-gas is H 2 = 47,CH 4 = 36,CO = 8,C0 2 - 1,N 2 = 4, and hydrocarbons, other than marsh-gas (acetylene, ethylene, benzene, &c.) = 4. The coal-tar and the gas-liquor in the tank separate into two layers ; the upper one consists of gas-liquor or ammoniacal- liquor (a yellow, unpleasant-smelling, aqueous solution of ammonium carbonate, ammonium sulphide, and numerous other compounds), from which practically the whole of the ammonia and ammonium salts of commerce are obtained. The lower layer in the tank is a dark, thick, oily liquid of sp. gr. 1-1 to 1-2, known as coal-tar. It is a mixture of a great number of organic compounds, and, although not long ago it was con- sidered to be an obnoxious bye-product, it is now the sole source of very many substances of great industrial importance. In order to partially separate the several constituents, the tar is submitted to fractional distillation ; it is heated in large wrought-iron stills or retorts, and the vapours which pass off are condensed in long iron or lead worms immersed in water, the liquid distillate being collected in fractions. The point at which the receiver is changed is ascertained by means of a thermometer, which dips into the tar, as well as by the character of the distillate. In this way tar may be roughly separated into the following fractions : I. Light oil or crude naphtha Collected up to 170. II. Middle oil or carbolic oil between 170 and 230. III. Heavy oil or creosote oil 230 270. IV. Anthracene oil above 270. V. Pitch Residue in the still. I. The first crude fraction separates into two layers namely, gas-liquor (which the tar always retains mechanically to some extent) and an oil which is lighter than water, its sp. gr. being about 0-975, hence the name, light oil. This oil is first redistilled from a smaller iron retort and the distillate AND CONSTITUTION OF BENZENE. 297 collected in three principal portions, passing over between 82-110, 110-140, and 140-170 respectively. All these fractions consist principally of hydrocarbons, but contain basic substances, such as pyridine, acid substances, such as phenol or carbolic acid, and various other impurities ; they are, therefore, separately agitated, first with concentrated sulphuric acid, which dissolves out the basic substances, and then with caustic soda, which removes the phenols (p. 385), being washed with water after each treatment ; afterwards they are again distilled. The oil obtained in this way from the fraction collected between 82 and 110 consists principally of the hydrocarbons benzene and toluene, and is sold as ' 90 per cent, benzol;' that obtained from the fraction 110-140 consists essentially of the same two hydrocarbons (but in different proportions) together with xylene, and is sold as ' 50 per cent, benzol.'* These two products are not usually further treated by the tar-distiller, but are worked up in the manner described later. The oil from the fraction collected between 140-170 consists of xylene, pseudocumene, mesityl- ene, &c., and is principally employed as ' solvent naphtha,' also as ' burning naphtha.' II. The second crude fraction, or middle oil, collected between 170 and 230, has a sp. gr. of about 1*002, and con- sists principally of naphthalene and carbolic acid. On cooling, the naphthalene separates in crystals, which are drained and pressed to squeeze out adhering carbolic acid and other sub- stances ; the crude crystalline product is further purified by treatment with caustic soda and sulphuric acid successively, and finally sublimed or distilled. The oil from which the crystals have been separated is agitated with warm caustic soda to dissolve the carbolic acid ; the alkaline solution is then drawn off from the insoluble portions of the oil and * Commercial '90 per cent, benzol' contains about 70 per cent., and '50 per cent, benzol ' about 46 per cent, of pure benzene ; the terms refer to the proportion of the mixture which passes over below 100 when the com- mercial product is distilled. Benzene, toluene, and xylene are known com- mercially as benzol, toluol, and xylol respectively. 298 MANUFACTURE, PURIFICATION, PROPERTIES, treated with sulphuric acid, whereupon crude carbolic acid separates as an oil, which is washed with water and again distilled ; it is thus separated into crystalline (pure) carbolic acid and liquid (impure) carbolic acid. III. The third crude fraction, collected between 230 and 270 is a greenish-yellow, fluorescent oil, specifically heavier than water; it contains carbolic acid, cresol, naphthalene, anthracene, and other substances, and is chiefly employed under the name of 'creosote oil' for the preservation of timber. IV. The fourth crude fraction, collected at 270 and up- wards, consists of anthracene, phenanthrene, and other hydrocarbons which are solid at ordinary temperatures ; the crystals which are deposited on cooling, after having been freed from oil by pressure, contain about 30 per cent, of anthracene, and are further purified by digestion with solvent naphtha, which dissolves the other hydrocarbons more readily than the anthracene ; the product is then sold as ' 50 per cent, anthracene,' and is employed in the manufacture of alizarin dyes. The oil drained from the anthracene is. re- distilled, to obtain a further quantity of the crystalline product, the non-crystallisable portions being known as 'anthracene oil' V. The pitch in the still is run out while still hot, and is employed in the preparation of varnishes, for protecting wood and metal work, and in making asphalt. The following table, taken partly from Ost's Lelirlmch der teclmisclien Cheinie, shows in a condensed form the process of tar distillation and the more important commercial products obtained. Benzene, C 6 H 6 . The crude '90 per cent, benzol' of the tar-distiller consists essentially of a mixture of benzene and toluene, but contains small quantities of xylene and other substances ; on further fractional distillation in specially con- structed apparatus (similar to that employed in the rectifica- tion of spirit), it is separated more or less completely into its AND CONSTITUTION OF BENZENE. 299 300 MANUFACTURE, PURIFICATION, PROPERTIES, constituents. The benzene prepared in this way still contains small quantities of toluene, paraffins, carbon bisulphide, and other impurities, and rnay be further treated in the following manner : It is first cooled in a freezing mixture and the crystals of benzene quickly separated by filtration from the mother-liquor, which contains most of the impurities ; after repeating this process, the benzene is carefully distilled, and the portion boiling at 80-81 collected separately. For ordinary purposes this purification is sufficient, but even now the benzene is not quite pure, and, when it is shaken with cold concentrated sulphuric acid, the latter darkens in colour owing to its having charred and dissolved the impurities ; pure benzene, on the other hand, does not char with sulphuric acid, so that if the impure liquid be repeatedly shaken with small quantities of the acid, until the latter ceases to be dis- coloured, most of the foreign substances will be removed. All coal-tar benzene, which has not been purified by repeated treatment with sulphuric acid, contains an interesting sulphur compound, C 4 H 4 S, named thiophene, which was discovered by V. Meyer ; the presence of this substance is readily detected by shak- ing the sample with a little concentrated sulphuric acid and a trace of isatin (an oxidation product of indigo), when the acid assumes a beautiful blue colour (indophenin reaction) ; thiophene resembles benzene very closely in chemical and physical properties, and for this reason cannot be separated from it except by repeated treat- ment with sulphuric acid, which dissolves thiophene more readily than it does the hydrocarbon. Although the whole of the benzene of commerce (' benzol ') is prepared from coal-tar, the hydrocarbon is also present in small quantities in wood-tar and in the tarry distillate of many other substances, such as shale, peat, &c. ; it may, in fact, be produced by passing the vapour of alcohol, ether, petroleum, or of many other volatile organic substances through a red-hot tube, because under these conditions such compounds lose hydrogen (and oxygen), and are converted into benzene and its derivatives. Benzene may be produced synthetically by simply heating AND CONSTITUTION OF BEXZEXE. 301 ncotyleno at a dull-red heat, when 3 mols. (or 6 vols.) of the latter are converted into 1 mol. (or 2 vols.) of benzene, Acetylene, generated from its copper derivative (part i. p. 83), is collected over mercury in a piece of hard glass- tubing, closed at one end and bent at an angle of about 120; when the tube is about half full of gas, the lower end is closed with a cork, and a piece of copper gauze wrapped round a portion of the horizontal limb, as shown (fig. 19). This portion of the tube is then carefully and strongly heated with a bunsen burner, the other end remaining immersed in the mercury; after a short time vapours appear in the tube, and minute drops of benzene condense on the sides, and if, after heating for about fifteen minutes, the tube be allowed to cool and the cork then removed, the mercury will rise, showing that a diminution in volume has taken place. This conversion of acetylene into benzene is a process of polymerisation, and was first accomplished by Berthelot. It is, at the same time, an exceedingly important synthesis of benzene from its elements, because acetylene may be obtained by the direct combination of carbon and hydrogen. Pure benzene may be conveniently prepared in small quantities by heating pure benzoic acid or calcium benzoate with soda-lime, a reaction which recalls the formation of marsh-gas from calcium acetate, (C 6 H 5 .COO) 2 Ca + 2NaOH = 2C 6 H 6 + CaC0 3 + Na COg, or C 6 H 5 -X X- i" When, however, five or six atoms of hydrogen are displaced by identical atoms or groups, only one substance is produced. When more than two atoms of hydrogen are displaced by atoms or groups which are not all identical, the number of isomerides which can be obtained is very considerable ; in the case of any tri-substitution product, C 6 H 3 X 2 Y, for example, six isomerides might be formed, as may be easily seen by assigning a definite position, say 1, to Y ; the isomerides would then be represented by formulas in which the groups occupied the position 1:2:3, 1:2:4, 1:2:5, 1:2:6, 1:3:4, or 1:3:5, all of which would be different. All the cases of isomerism considered up to the present have been those due to the substituting atoms or groups occupying different relative positions in the benzene nucleus ; as, however, many benzene derivatives contain groups of atoms which themselves exist in isomeric forms, such compounds also exhibit isomerism exactly similar to that already met with in the case of the paraffins, alcohols, &c. There are, for example, two isomeric hydrocarbons of the composition C 6 H 5 -C 3 H 7 , namely, propylbenzene, C 6 H 5 'CH 2 -CH 2 -CH 3 , and isopropylbenzene, C 6 H 5 -CH(CH 3 ) 2 , just as there are two isomeric ethereal salts of the composition C 3 H 7 I. As, moreover, the two propylbenzenes, C 6 1I--C 3 H 7 , are isomeric 316 ISOMERISM OP BENZENE DERIVATIVES. with the three (ortho-, meta-, and para-) ethylmethylbenzenes, C 6 H 4 (C 2 H 5 )-CH 3 , arid also with the three (adjacent, sym- metrical, and asymmetrical) trimethylbenzenes, C 6 H 3 (CH 3 ) 3 , there are in all eight hydrocarbons of the molecular formula C 9 H 12 , derived from benzene. In studying the isomerism of benzene derivatives, the clearest impressions will be gained by invariably making use of a simple, unnumbered hexagon to represent C 6 H 6 , and by expressing the constitutions of simple substitution products by formula such as N0 2 CH 3 N0 2 -Cl ^ J ^ J CH 3 -L J-CH 3 Chlorobenzene. Dinitrobenzene. Nitrophenol. Trimethylbenzene. The omission of the symbols C and H is attended by no disadvantage whatsoever, because, in order to convert the above into the ordinary molecular formulae, it is only necessary to write C 6 instead of the hexagon, and then to count the unoccupied corners of the hexagon to find the number of hydrogen atoms in the nucleus, the substituting atoms or groups being added afterwards. In the case of chlorobenzene, for example, there are five unoccupied corners, so that the molecular formula is C 6 H 5 C1 ; whereas in the case of tri- methylbenzene there are three, and the formula, therefore, is As, however, such graphic formulae occupy a great deal of space, their constant use in a text-book is out of the question, and other methods have to be adopted. The most usual course in the case of the di-derivatives is to employ the terms ortho-, meta-, and para-, or simply the letters o, m, and p, as, for example, ortho-dinitrobenzene or o-dinitrobenzene, meta- nitraniline or m-nitraniline, para-nitrophenol or jp-nitrophenol ; the relative positions of the atoms or groups may also be ex- ISOMERISM OF BENZENE DERIVATIVES. 317 pressed by numbers; ortho-chloronitrobenzene, for example, may be described as 1 :2-chloronitrobenzene, as C 6 H 4 <^i S TQ ( 2 )> or as i 2 C 6 H 4 C1-N0 2 , the corresponding para-compound as l:4-chloro- QJ (1) * 4 nitrobenzene, as C 6 H 4 <^-.^ ,^, or as C 6 H 4 C1-N0 2 . In the case of the tri-derivatives the terms symmetrical, asymmetrical, and adjacent (compare p. 314) may be employed when all the atoms or groups are the same, but when they are different the constitution of the compound is usually expressed with the aid of numbers ; the tribromaniline of the constitution Br Br 1356 for example, is described as C 6 H 2 Br 3 -NH 2 [Br:Br:Br:lS T H 2 ], or as C 6 H 2 Br 3 .NH 2 [3Br:NH 2 = 2:4:6:1], and it is of course quite immaterial from which corner of the imaginary hexagon the numbering is commenced. Determination of the Constitution of Benzene Derivatives. It has been pointed out that the di-substitution products of benzene, such as dibromobenzene, C 6 H 4 Br 2 , dihydroxy- benzene, C 6 H 4 (OH) 2 , and nitraniline, C 6 H 4 (X0 2 )-NH 2 , exist in three isomeric forms, and that their isomerism is due to the different relative positions of the substituting atoms or groups in the benzene nucleus ; it is evident, however, that in order to arrive at the constitution of any one of these substances, and to be able to say whether it is an ortho-, meta-, or para- compound, a great deal of additional information is required. Now the methods which are adopted in deciding questions of this kind at the present time are comparatively simple, but they are based on the results of work which has extended over many years. It has been found, in the first place, that 318 ISOMERISM OF BENZENE DERIVATIVES. a given di-substitution product of benzene may be converted by more or less indirect methods into many of the other di-substitution products of the same series ; or^o-dinitroben- zene, C 6 H 4 (N0 2 ) 2 , for example, may be transformed into o-dia- midobenzene, C 6 H 4 (NH 2 ) 2 , o-dihydroxybenzene, C 6 H 4 (OH) 2 , o-dibromobenzene, C 6 H 4 Br 2 , o-dimethylbenzene, C 6 H 4 (CH,). 7 , and so on, similar changes being also possible in the case of meta- and para-compounds. If, therefore, it can be ascer- tained to which series a given di-substitution product belongs, the constitution of other di-substitution products of this series may be easily determined ; suppose, for example, that it could be proved that of the three dinitrobenzenes, the com- pound melting at 90 is a meta-compound, then it would necessarily follow that the diamido-, dihydroxy-, dibromo-, and other di-derivatives of benzene obtained from this particular dinitro-compound by substituting other atoms or groups for the two nitro-groups, must also be meta-compounds ; it would also be known that the di-derivatives of benzene obtained from the other two dinitrobenzenes, melting at 118 and 173 respectively, in a similar manner must be either ortho- or para-compounds. It was necessary, therefore, in the first place, to determine the constitution of one or two di-derivatives of each series; these substances then served as standards, and the constitu- tion of any other di-derivative was established by converting it by suitable reactions into one of these standards. As an illustration of the methods and arguments originally employed in the solution of problems of this nature, the case of the dicarboxy- and dimethyl-derivatives of benzene may be quoted. Of the three dicarboxybenzenes, C 6 H 4 (COOH) 2 , one namely, phthalic acid (p. 425), is very readily converted into its anhydride, but all attempts to prepare the anhydrides of the other two acids (isophthalic acid and terephthalic acid, pp. 426, 427) result in failure ; it is assumed, therefore, that the acid which gives the anhydride is the o-compound, because, from a study of the behaviour of many other dicar- ISOMERISM OF BENZENE DERIVATIVES. 319 boxylic acids, it has been found that anhydride formation takes place most readily when the two carboxyl-groups are severally combined with two carbon atoms which are them- selves directly united, as, for example, in the case of succinic acid. In other words, if the graphic formulae of succinic acid and of the three dicarboxy-derivatives of benzene be compared, it will be evident that in the o-compound the relative position or state of combination of the two carboxyl- groups is practically the same as in succinic acid, but quite otherwise in the case of the m- and ^-compounds. CH 2 COOH :H 2 COOH -COOH COOH For this, and other reasons not stated here, phthalic acid may be provisionally regarded as an or/Ao-dicarboxy benzene. Again, the hydrocarbon mesitylene or trimethylbenzene, C 6 H 3 (CH 3 ) 3 , may be produced synthetically from acetone (p. 337), and its formation in this way can be explained in a simple manner, only by assuming that mesitylene is a symmetrical trimethylbenzene of the constitution (A). CH 3 COOH Mesitylenic Acid. -COOH Dimethylbenzene. (Isoxylene.) Isophthalic Acid. When this hydrocarbon is carefully oxidised, it yields an acid (B) of the composition C 6 H 3 (CH 3 ) 2 -COOH (by the conversion of one of the methyl-groups into carboxyl), from which a dimethylbenzene, C 6 H 4 (CH 3 ) 2 (C), is easily obtained by the 320 ISOMERISM OF BENZENE DERIVATIVES. substitution of hydrogen for the carboxyl-group. This di- methylbenzene, therefore, is a we/a-cornpound, because no matter which of the original three methyl-groups in mesityl- ene has been finally displaced by hydrogen, the remaining two must occupy the ?ft-position. Now when this dimethyl- benzene is oxidised with chromic acid, it is converted into a dicarboxylic acid (D) namely, isophthalic acid, C 6 H 4 (COOH) 2 , which, therefore, must also be regarded as a meta-compound ; the constitution of two of the three isorneric dicarboxy-deriva- tives of benzene having been thus determined, the third namely, terephthalic acid, can only be the jpara-compound. It is now a comparatively simple matter to ascertain to which series any of the three dimethylbenzenes belongs; one of them having been found to bs the meta-compound, all that is necessary is to submit each of the other two to oxidation, and that which gives phthalic acid will be the ortho-compound, whilst that which yields terephthalic acid will be the para-derivative. Moreover, the constitution of any other di-substitution product of benzene may now be determined without difficulty, provided that it is possible to convert it into one of these standards by simple reactions. As the methods which have just been indicated are based entirely on arguments drawn from analogy, or from deductions as to the probable course of certain reactions, the conclusions to which they lead cannot be accepted without reserve ; there are, however, several other ways in which it is possible to distinguish with much greater certainty between ortho-, meta-, and para-compounds, and of these that employed by Kb'rner may be given as an example. Korner's method is based on the fact that, if any di-sub- stitution product of benzene be converted into a tri-derivative by further displacement of hydrogen of the nucleus, the number of isomerides which may be obtained from an ortho-, meta-, and para-compound is different in the three cases, so that by ascertaining the number of these products the constitution of the original di-derivative may be established. Suppose, ISOMERISM OF BENZENE DERIVATIVES. 321 for example, that one of the three isomeric dibromobenzenes be converted into nitrodibromobenzene by treatment with nitric acid ; then, if it be the or^o-dibromo-compound, it is possible to obtain from it two, but only two, nitrodibromo- beiizenes, because, although there are four hydrogen atoms, any one of which may be displaced by a nitro-group, as represented by the following formulas, Br Br -Br -N0 2 -Br NO., -Br N0 2 -Br I. II. III. IV. the compound of the constitution (in.) is identical with (n.), and (iv.) with (i.), the relative positions of all the atoms being the same in the two cases respectively. If, on the other hand, the dibromobenzene be the meta-com- pouiid, it might yield three, and only three, isomeric nitro- derivatives, which would be represented by the first three of the following formulae, the fourth being identical with the second : Br Br Br -Br X0 2 -Br -Br Finally, if the substance in question be j9ra-dibromo- benzene, it could give only one nitro-derivative, the following four formulae being identical : Br Br XOo It is obvious, then, that this method may be applied in u 322 ISOMERISM OF BENZENE DERIVATIVES. ascertaining to which series any di-substitution product belongs ; it may also be employed in determining the con- stitution of the tri-derivatives in a similar manner. At the present time, therefore, the constitution of any new benzene derivative is, as a rule, very easily ascertained ; it is simply converted into some compound of known constitution, or the number of isomerides obtained from it by substitution is determined. CHAPTER XIX. GENERAL PROPERTIES OF AROMATIC COMPOUNDS. Classification of Organic Compounds. The examples given in the foregoing pages will have afforded some indication of the large number of compounds which it is possible to prepare from benzene r by the substitution of various elements or groups for atoms of hydrogen ; as the substances formed in this way, and many other benzene derivatives which occur in nature, or may be prepared synthetically, still retain much of the characteristic chemical behaviour of benzene, and differ in many respects from the paraffins, alcohols, acids, and all other compounds previously considered (part i.), it is con- venient to class benzene and its derivatives in a separate group. Organic compounds are therefore classed in two principal divisions, the fatty and the aromatic. The word 'fatty/ originally applied to some of the. acids of the C 71 H 2R 2 series (part i. p. 142), is now used to denote all compounds which may be considered as derivatives of marsh-gas, and which cannot be regarded as directly derived from benzene ; all the compounds described in part i. belong to the fatty group or division. Benzene and its derivatives, on the other hand, are classed in the ' aromatic ' group, this term having been first applied to certain naturally occurring compounds (which GENERAL PROPERTIES OF AROMATIC COMPOUNDS. 323 have since been proved to be benzene derivatives) on account of their peculiar aromatic odour. The fundamental distinction between fatty and aromatic compounds is one of constitution. The reasons which have led to the conclusion that benzene contains a closed chain of six carbon atoms being equally 'valid in the case of its deri- vatives, it is assumed that this (or a similar) nucleus' is 'present in all aromatic compounds. The constitution of a fatty com- pound, however, is almost invariably expressed by a formula such as CH 3 -CH 2 .CH 2 .CH 3 , CH 2 (OH).CH(OH).CH 2 (OH),and COOH.CH 2 -CH 2 -COOH, in which the carbon atoms do not form a closed-, but an open-chain ; * such compounds, more- over, may be regarded as derived from marsh-gas by a series of simple steps. For these reasons, compounds belonging to the fatty series are often spoken of as open-chain compounds, in contradistinction to the closed-chain compounds of the aromatic group. It must not, however, be supposed that all aromatic are sharply distinguished in any way from all fatty compounds, or that either class can be defined in any exact terms. Many compounds, the constitutions of which must be represented by closed-chain formula?, are nevertheless placed in the fatty group, simply because to class them in the aromatic division would remove them from those substances to which they are most closely related ; succinimide (part i. p. 237), for example, is a closed-chain compound in the strict sense of the word, but is clearly more conveniently considered in the fatty series, because of its relationship to succinic acid. Although, again, the members of the aromatic group may all be regarded as derivatives of benzene, they may also be considered as derived from marsh-gas, since not only benzene itself, but many other aromatic compounds, may be directly obtained from members * The terms 'open-chain' and 'closed-chain' originated in the chain-like appearance of the graphic formulae as usually written, and are not intended to convey the idea that the atoms are joined together by any form of matter, or that they are all arranged in straight lines. 324 GENERAL PROPERTIES OF AROMATIC COMPOUNDS. of the fatty series by simple reactions, and, conversely, many aromatic compounds may be converted into those of the fatty series. Some examples of the production of aromatic from fatty compounds have already been given namely, the formation of benzene by the polymerisation of acetylene, and that of mesitylene by the condensation of acetone ; these two changes may be expressed graphically in the following manner : C-H * III H r r CO-CH 3 CH 3 CH and may be regarded as typical reactions, because many other substances, similar in constitution to acetylene and acetone respectively, may be caused to undergo analogous transforma- tions. Bromacetylene, CBriCH, for example, may be con- verted into (symmetrical) tribromobenzene, simply by leav- ing it exposed to direct sunlight, 3C 2 HBr = C 6 H 8 Br s ; and methylethyl ketone (a homologue of acetone) is trans- formed into symmetrical triethylbenzene (a homologue of mesitylene) by distilling it with sulphuric acid, 3CH 8 .CO.C 2 H 6 = C 6 H 8 (C 2 H 6 ) 8 + 3H 2 0. General Character of Aromatic Compounds. Although, then, it is impossible to draw any sharp line between fatty and GENERAL PROPERTIES OF AROMATIC COMPOUNDS. 325 aromatic compounds, and many substances are known which form a connecting link between the two divisions, the great majority of aromatic substances differ materially from those of the fatty division in constitution, and consequently also in properties. Speaking generally, aromatic compounds contain a larger percentage of carbon than those of the fatty division, and probably for this reason, they are more frequently crystalline at ordinary temperatures. They are, as a rule, less readily resolved into simple substances than are the members of the fatty series, although in most cases they are more easily con- verted into substitution products. Their behaviour with nitric acid and with sulphuric acid is very characteristic, and distinguishes them from nearly all fatty compounds, inas- much as they are, as a rule, readily converted into nitro- and sulphonic-derivatives respectively by the displacement of hydrogen atoms of the nucleus, COOTT C 6 H 5 -COOH + HN0 8 - C fl H 4 < NO + H 2 C 6 H 6 -OH + 3HN0 8 '= C 6 H 2 (OH)(N0 2 ) 3 C.H..NH, + H 2 S0 4 = C Fatty compounds rarely give sulphonic- or nitro-deriva- tives under the same conditions, but are acted on in such a way that they are resolved into two or more simpler substances. When aromatic nitro-compounds are treated with reducing agents, they are converted into amido-compounds, C 6 H 5 .NO, + 6H - C 6 H 5 -NH 9 + 2H 2 C 6 H 4 (N0 2 ) 2 + 12H = C 6 H 4 (NH 2 ) 2 +~4H 2 0. These amido-compounds differ from the fatty amines in at least one very important respect, inasmuch, as they are con- verted into diazo-r.orn pounds (p. 370) on treatment with nitrous acid in the cold; this behaviour is highly clinracteristic, and 326 GENERAL PROPERTIES OP AROMATIC COMPOUNDS. the diazo-compounds form one of the most interesting and important classes of aromatic substances. It has already been pointed out that benzene does not show the ordinary behaviour of unsaturated fatty compounds, although under certain conditions both the hydrocarbon and its derivatives are capable of forming additive compounds by direct combination with two, four, or six (but not with one, three, or five) monovalent atoms. This fact proves that benzene is not really a saturated compound like methane, or ethane, for example, both of which are quite incapable of yielding derivatives except by substitution. Nevertheless, the conversion of benzene and its derivatives into additive products, is, as a rule, much less readily accomplished than in the case of fatty, unsaturated compounds ; the halogen acids, for example, which unite directly with so many unsaturated fatty compounds, have no such action on benzene and its derivatives, and even in the case of the halogens and nascent hydrogen, direct combination occurs only under particular conditions. The compounds, such as dihydrobenzene, C 6 H g , tetrahydrobenzene, C 6 H 10 , benzene hexachloride, C 6 H 6 C1 , and benzene hexahydride, C 6 H 12 (hexamethylene), obtained in this way, have not yet been very fully investigated, but from what is known of their properties, they form a connecting link between the members of the aromatic and fatty divisions (compare p. 309). When the hydrogen atoms in benzene are displaced by groups or radicles which are composed of several atoms, these groups are spoken of as side-chains; ethylbenzene, C 6 H 5 .CH 2 .CH 3 , benzyl alcohol, C 6 H 5 -CH. 2 .OH, and methyl aniline, C 6 H 5 -IS T H-CH 3 , for example, would each be said to contain a side-chain, whereas the term would not, as a rule, be applied in the case of phenol, C 6 H 5 -OH, nitrobenzene, C 6 H 5 -N0 2 , &c., where the substituting groups are com- paratively simple, and do not contain carbon atoms. Now the character of any particular atom or group in the side-chain, although influenced to some extent by the fact GBNEKAL PROPERTIES OB' AROMATIC COMPOUNDS. 327 that the group is united with the benzene nucleus, is on the whole very similar to that which it possesses in fatty com- pounds. The consequence is that aromatic compounds con- taining side-chains of this kind have not only the properties already referred to, as characteristic of the derivatives of benzene, but show also, to a certain extent, the behaviour of fatty compounds. Benzyl chloride, C 6 H 5 -CH 2 C1, for ex- ample, may be directly converted into the nitro-derivative, C fl H 4 (N0 2 ).CH 2 Cl, and the sulphonic acid, C 6 H 4 (S0 3 H>CH 2 C1, reactions characteristic of aromatic compounds ; on the other hand, the -CH 2 C1 group may be transformed into -CH 2 -OH, -CHO, -COOH, and so on, just as may the same group in ethyl chloride, CH 3 -CH 2 C1, and similar fatty com- pounds, and in all cases the products retain, to some extent, the properties of fatty substances as long as the side-chain remains. The groups forming the side-chains, however, are more easily attacked and removed than the closed-chain or nucleus; when ethylbenzene, C 6 H 5 -CH 2 -CH 3 , or propyl- benzene, C 6 H 5 -CH 2 -CH 2 -CH 3 , for example, is boiled with chromic acid, the side-chain undergoes oxidation, carbon dioxide is evolved, and benzoic acid, C 6 H 5 -COOH, is pro- duced in both cases, the six atoms of carbon in the nucleus being unchanged (p. 417). Although the compounds derived from benzene by direct substitution are very numerous, the aromatic group also contains a great many other substances which are more distantly related to benzene, and which can only be re- garded as derived from it indirectly. The hydrocarbon drphenyl t C 6 H 5 -C 6 H 5 , for example, which, theoretically, is formed by the union of two phenyl or C 6 H 5 - groups, just as dimethyl or ethane, CH 3 -CH 3 , is produced by the combination of two methyl-groups, is an important member of the aromatic division, and, like benzene, is capable of yielding a very large number of substitution products. Other hydrocarbons are known in which the presence of two or more closed carbon chains, combined in different ways, 328 GENERAL PROPERTIES Of AROMATIC COMPOUNDS. must be assumed, as, for example, in the cases of naphthalene (p. 442) and anthracene (p. 437), Naphthalene. Anthracene. and there are also substances, such as pyridine (p. 472) and quinoline (p. 480), in which a nitrogen atom occupies the position of one of the CHEE groups in the closed-chain. N N Pyridine. Quinoline. All these, and many other compounds and their derivatives, are classed as aromatic, because they show the general be- haviour already referred to, and resemble benzene more or less closely in constitution. CHAPTER XX. HOMOLOGUES OF BENZENE. Benzene, the simplest hydrocarbon of the aromatic group, is also the first member of a homologous series of the general formula C TO H 9n _ 6 ; the hydrocarbons ,of this series are derived from benzene by the substitution of alkyl-gronps for hydrogen atoms, just as the homologous series of paraffins is derived from marsh-gas. The second member, toluene or methyl- benzene, C 6 H 5 -CH 3 , like benzene itself, exists in only one form, but the next higher homologue, which has the mole- cular composition C 8 H 10 , occurs in four isomeric forms namely, as ethylbenzene, C 6 H 5 -C 2 H 5 , and as ortho-, meta-, and para-dimethylbenzene, C 6 H 4 (CH 3 ) 2 ; on passing up the series, the number of theoretically possible isomerides rapidly increases. HOMOLOGUES OP BENZENE. 329 By substituting a methyl-group for one atom of hydrogen in the hydrocarbon C 8 H 10 , for example, eight isomerides of the com- position C 9 H 12 may theoretically be obtained, and are, in fact, known ; of these isomerides, five namely, propylbenzene and iso- propylbenzene, C 6 H 5 -C 3 H 7 , and 0-, m-, and /?-methylethylbenzene, C 6 H 4 (CH 3 )-C 2 H 5 , are derived from ethylbenzene, the other three namely, symmetrical, adjacent, and asymmetrical trimethyl- benzene, C 6 H 3 (CH 3 ) 3 , being derived from the dimethylbenzenes. Most of the hydrocarbons of this series, and others which will be mentioned later, occur in coal-tar, from which they are extracted in much the same way as benzene ; it is, however, exceedingly difficult to obtain any of them in a pure state directly from this source by fractional distillation, as the boiling-points of some of the compounds He very close together; nevertheless, the process is now carried out on the large scale with such care and with such perfect apparatus that the purified compounds contain, in some cases, only traces of foreign substances. The homologues of benzene may be obtained by the following general methods : (1) By treating benzene or its homologues with alkyl halogen compounds in presence of anhydrous aluminium chloride (Friedel and Craft's reaction) ; under these condi- tions the hydrogen atoms of the nucleus are displaced by alkyl-groups, benzene and methyl chloride, for example, giving toluene, C 6 H 5 -CH 3 , xylene, C 6 H 4 (CH 3 ) 2 , trimethyl- benzene, C 6 H 3 (CH 3 ) 3 , &c 4 ; whereas ethylbenzene, with the same alkyl compound, yields methylethylbenzene, C 6 H 4 (CH 3 )-C 2 H 5 , dimethylethylbenzene, C 6 H 3 (CH 3 ) 2 -C 2 H 5 , and so on. These syntheses may be expressed by equations such as the following, but the exact nature of the interaction is not known : C 6 H + CH 3 C1 = C fi H 5 -CH, + HC1 C 6 H 6 + 2CH 3 C1 - C 6 H 4 (CH 3 ) 2 + 2HC1 C 6 H 5 -C 2 H 5 + CH 3 C1 = C 6 H 4 (CH 8 ).C 2 H 5 + HC1. It is probable that an aluminium compound, such as 330 HOMOLOGUES OF BENZENE. C 6 H 5 -A1 2 C1 5 , is first formed with evolution of hydrogen chloride, this substance then interacting with the alkyl halogen compound to form the hydrocarbon, aluminium, chloride being regenerated, C 6 H 5 -A1 2 C1 5 + CH 3 C1 = C 6 H 5 .CH 3 + A1 2 C1 6 ; an alkyl bromide may be used instead of the chloride, and anhydrous ferric or zinc chloride may be employed in the place of aluminium chloride, but, as a rule, not so success- fully. Anhydrous benzene, or one of its homologues, is placed in a flask connected with a reflux condenser, and about one-third of its weight of anhydrous aluminium chloride added ; the alkyl chloride or bromide is then passed into the liquid if a gas, or poured in, if a liquid, and the mixture heated on a water-bath until the evolution of hydrogen chloride or bromide is at an end ; the apparatus and materials must be dry. In some cases ether, carbon bisulphide, or petroleum is previously mixed with the hydrocarbon in order to dilute it, experience having shown this to be advantageous. When quite cold, water is gradually added to dissolve the aluminium compounds, and after having been separated and dried with calcium chloride, the mixture of hydrocarbons is submitted to fractional distillation ; in some cases a preliminary distillation in steam is advisable.* (2) By treating a mixture, consisting of a halogen deriva- tive of benzene or of one of its homologues, and an alkyl halogen compound, with sodium or potassium (Fittig's re- action) ; this method of formation is similar to that by which the higher paraffins may iJti HynllieticaHy produced from methane, and has the advantage over Friedel and Craft's method that the constitution of the product is known. Bromobenzene and methyl iodide, for example, give toluene, whereas o-, m-, or j>bromotoluene and ethyl iodide yield o-, m-, or ^-ethylmethylbenzene, C 6 H 5 Br + CH 3 I + 2Na = C H 5 .CH 3 + Nal + NaBr C 6 H 4 Br.CH 3 4- C 2 H 5 I + 2K = C 6 H 4 <^ + KBr + KI. * In most cases the detailed description of the preparation of substances is given in small print* HOMOLOGUES OF BENZENE. 331 The bromo-derivatives of the aromatic hydrocarbons are usually employed in such cases because the chloro-derivatives are not so readily acted on, and the iodo-compounds are not so easily prepared ; the alkyl iodides are also used in pre- ference to the chlorides or bromides because they interact more readily. (3) By heating carboxy-derivatives of benzene and its homologues with soda-lime, a method analogous to that employed in converting the fatty acids into paraffins, C H 4 C 9 H 12- Normal. I so. Sp.gr. at 0-899 0-882 0-866 (at 20) 0-881 0-879 B.p. 80-5 110-3 134 157 153. In the case of the cZi-substitution products the gradual variation in physical properties is obscured by the existence of the three (or more) isomeric forms, which themselves show considerable differences, as illustrated by the three isomeric xylenes, C 6 H 4 (CH 3 ) 2 , Orthoxylene. Metaxylene. Paraxylene. Sp. gr. at 0-893 0-881 0-880 B.p. 142-143 139 136-137 (M.p. 15). As a general rule, to which, however, there are some ex- ceptions, para-compounds melt at a higher temperature than the corresponding meta-compounds, and the latter usually at a higher temperature than the corresponding ortho-compounds; the boiling-points also vary, but with less regularity. The homologues of benzene show the characteristic chemical behaviour of the simplest hydrocarbon, inasmuch as they readily yield nitro- and sulphonic-derivatives j toluene, for example, gives nitrotoluene, C 6 H 4 (CH 3 )-N0 2 , and toluene- sulphonic acid, C 6 H 4 (CH 3 )-S0 3 H, xylene yielding nitro- xylene, C 6 H 3 (CH 3 ) 2 -N0 2 , and xylenesulphonic acid, C 6 H 3 (CH 3 ) 2 .S0 3 H. (V In these, and in all similar reactions, the product invariably consists of a mixture of isomerides, the course of the reaction depending both on the nature of the interacting compounds and on the conditions of the experiment (compare p. 351) ; as a rule, the greater the number of alkyl-groups in the hydro- carbon, the more readily it yields nitro- and sulphonic-deri- vatives. The fact that benzene and its homologues gradually dissolve in concentrated sulphuric acid, especially on warming, is some- I10MOLOGUES OF BENZENE. 333 times made use of in separating these aromatic hydrocarbons from the paraffins, as, for example, in the analysis of coal- gas ; their separation from unsaturated fatty hydrocarbons could not of course be accomplished in this way, as the latter are also dissolved by concentrated sulphuric acid. All the homologues of benzene are very stable, and are with difficulty resolved into compounds containing a smaller number of carbon atoms ; powerful oxidising agents, however, such as chromic acid, potassium permanganate, and dilate nitric acid, act on them slowly, the alkyl-groups or side-chains being attacked, and as a rule converted into carboxyl-groups ; toluene and ethylbenzene, for example, give benzoic acid, whereas the xylenes yield dicarboxylic acids (p. 424), C 6 H 5 -CH 3 + 30 = C 6 H 5 .COOH + H 2 C 6 H 5 .CH 2 .CH 3 + 60 = C 6 H 5 .COOH + C0 2 + 2H 2 C 6 H 4 (CH 3 ) 2 + 60 - C 6 H 4 (COOH) 2 + 2H 2 0. Although in most cases oxidation leads to the formation of a carboxy-derivative of benzene, the stable nucleus of six carbon atoms remaining unchanged, some of the homologues are completely oxidised to carbon dioxide (compare p. 337), and benzene itself undergoes a similar change on prolonged and vigorous treatment. Aromatic hydrocarbons, like those of the fatty series, may be regarded as hydrides of hypothetical radicles; in other words, radicles may theoretically be derived from aromatic hydrocarbons by taking away atoms of hydrogen. These radicles have no actual existence, but the assumption is useful in naming aromatic compounds ; the mono- and di-substitution products of benzene, for example, may be regarded as com- pounds of the monovalent radicle phenyl, Cy.H^-, or of the divalent radicle phem/lme^. . C 6 H 4 -xylene depends on the follow- ing facts : (1) When crude xylene is agitated with concentrated sulphuric acid, o- and m-xylene are converted into sulphonic acids, C H,(CH 3 ).,-SO3H ; />xylene remains unchanged, as it is OOO HOMOLOGUES OF BENZENE. only acted on by fuming sulphuric acid. (2) The sodium salt of o-xylenesul phonic acid is less soluble in water than the sodium salt of m-xylenesulphonic acid ; it is purified by recrystallisation, and converted into o-xylene by heating with hydrochloric acid under pressure (p. 381). The three xylenes may all be prepared by one or other of the general methods : when, for example, methyl chloride is passed into benzene in presence of aluminium chloride, o-xylene and a small quantity of the j?-compound are obtained, C 6 H 6 + 2CH 3 C1 = C 6 H 4 (CH 3 ) 2 + 2HC1; toluene, under the same conditions, yields the same two compounds, C 6 H 5 -CH 3 + CH 3 C1 = C 6 H 4 (CH 3 ) 2 + HC1. The non-formation of ??i-xylene in these two cases is accounted for by assuming that the methyl-group first intro- duced into the benzene molecule exerts some directing in- fluence on the position taken up by the second one (p. 351). Orthoxylene is obtained in a state of purity by treating o-bromotoluene with methyl iodide and sodium, C fi H4.tl.3 i "DPI O XJ X^^-"-3 _l_ "POP1 _L WPl . gJtl 4 < -^/~vTq T i v-'ig I^gl 4 < s x ^-ii T JTVJ1^1 3 T n^J , an aromatic alcohol (p. 402), such as benzyl alcohol, also yields the corresponding halogen derivative (benzyl chloride), containing the halogen in the side-chain, Halogen derivatives may also be obtained by distilling halogen acids with lime, C 6 H 4 but they are best prepared from the corresponding toluidines by Sandmeyer's method, Toluidine. Diazotoluene Chloride. Chlorotoluene. Orthoclilorotoluene boils at 156, metaclilorotoluene at 150, and parachlorotoluene at 160; they resemble chlorobenzene in most respects, but, since they contain a methyl-group, they have also some of the properties of fatty compounds; on oxidation, they are converted into the corresponding chloro- benzoic acids, C 6 H 4 CLCOOH, just as toluene is transformed into benzoic acid. Benzyl chloride, C 6 H 5 -CH 2 C1, although isomeric with the three chlorotoluenes, differs from them very widely, and may be taken as an example of the class of halogen-compounds in which the halogen is present in the side-chain. It can be obtained by treating benzyl alcohol (p. 403) with phos- phorus pentachloride, C 6 H 5 .CH 2 .OH + PC1 5 - C 6 H 5 .CH 2 C1 + POC1 3 + HC1, but is always prepared by passing chlorine into boiling toluene, C 6 H 5 .CH S + C1 2 = C 6 H 5 -CH 2 C1 + HC1. The toluene is contained in a tiask which is heated on a sand- HALOGEN DERIVATIVES OF BENZENE, ETC. 349 bath and connected with a reflux condenser ; a stream of dry chlorine is then passed into the boiling liquid until the theoretical gain in weight has taken place and the product is purified by fractional distillation; the action takes place most rapidly in strong sunlight. Benzyl chloride is a colourless, unpleasant-smelling liquid, boiling at 176; it is insoluble in water, but miscible with alcohol, ether, benzene, &c. It behaves like other aromatic compounds towards nitric acid, by which it is converted into a mixture of isomeric nitro-compounds, C 6 H 4 (X0 2 )-CH 2 C1. At the same time, however, it has many properties in com- mon with the alkyl halogen compounds ; like ethyl chloride, it is slowly decomposed by boiling water, yielding the cor- responding hydroxy-compound, benzyl alcohol (p. 403), C 6 H 5 .CH 2 C1 + H 2 = C 6 H 5 .CH 2 .QH + HC1, and just as ethyl chloride interacts with silver acetate, giving ethyl acetate, so benzyl chloride, under the same conditions, yields the ethereal salt, benzyl acetate, C 6 H 5 -CH 2 C1 + CH 3 .COOAg - CH 3 .COOCH 2 -C 6 H 5 + AgCl. Benzyl chloride is a substance of considerable commercial importance, inasmuch as it is used for the preparation of benzaldehyde (p. 406). Benzol chloride, C 6 H 5 -CHC1 2 , may be obtained by treating benzaldehyde with phosphorus pentachloride, C 6 H 5 -CHO + PC1 5 = C 6 H 5 -CHC1 2 + POC1 3 , but it is prepared by chlorinating toluene just as described in the case of benzyl chloride, except that the process is continued until twice as much chlorine has been absorbed. It is a colourless liquid, boiling at 206, and is extensively used for the preparation of benzaldehyde. Benzotricliloride, or phenylchloroform, C 6 H 5 CC1 3 , is also prepared by chlorinating boiling toluene; it boils at 213, and when heated with water it is converted into benzoic acid, C 6 H 5 -CC1 3 + 2H 2 = C 6 H 5 -COOH + 3HCL 350 NITRO-COMPOUNDS. CHAPTEE XXII. NITRO-COMPOUNDS. It has already been stated that one of the most characteristic properties of aromatic compounds is the readiness with which they may be converted into nitro-derivatives by the substitu- tion of nitro-groups for hydrogen of the nucleus; the com- pounds formed in this way are of the greatest importance, more especially because it is from them that the amido- and diazo-compounds are prepared. Preparation. Many aromatic compounds may be l nitrated ' that is to say, converted into their nitro-derivatives, by dis- solving them in concentrated nitric acid (sp. gr. 1-3 to 1-5), in the cold or at ordinary temperatures, and under such conditions a mononitro-compound is usually produced ; ben- zene, for example, yields nitrobenzene, and toluene, a mixture of o- and p-nitrotoluenes, C 6 H 6 + HN0 3 = C 6 H 5 .N0 2 + H 2 C 6 H 6 .CH 8 + HN0 3 = C H 4 <^ + H 2 0. Some aromatic compounds, however, are insoluble in nitric acid, and are then only very slowly acted on ; in such cases, a mixture of concentrated nitric and sulphuric acids is used. This mixture is also used in many cases, even when the substance is soluble in nitric acid, because the sulphuric acid combines with the water which is produced during the interaction, and thus its presence favours nitration, just as the presence of dehydrating agents favours the formation of ethereal salts from a mixture of an acid and an alcohol. When a large excess of nitric and sulphuric acids is employed, and especially when heat is applied, the aromatic compound is usually converted into (a mixture of isomeric) dinitro- or trinitro-derivatives ; benzene, for instance, yields a mixture NITROCOMPOUNDS. 351 of three dinitro-benzenes, the principal product, however, being the meta-compound, C 6 H 6 + 2IDs T 3 = C 6 H 4 (N0 2 ) 2 + 2H 2 0. As soon as nitration is complete (portions of the product may be tested from time to time), the solution or mixture, having been cooled if necessary, is poured on to ice or into a large volume of water, and the product, which is usually pre- cipitated in crystals, separated by filtration, or if an oil, by extraction with ether, or in some other manner. Generally speaking, the number of hydrogen atoms dis- placed by nitro-groups is greater the higher the temperature and the more concentrated the acid, or acid mixture, em- ployed, but depends to an even greater extent on the nature of the substance undergoing nitration, because the introduc- tion of nitro-groups is facilitated when other atoms or groups, especially alkyl radicles, have already been substituted for hydrogen of the nucleus. The nature of these atoms or groups determines, moreover, the position taken up by the entering nitro-group; if the former be strongly negative or acid in character, as, for example, -N0 2 , -COOH, and -S0 3 H, a ra-nitro-derivative is formed, whereas, when the atom or group in question is a halogen, an alkyl, or an amido- or hydroxyl- group, a mixture of the o- and ^-nitro-derivatives is produced. This directing influence of an atom or group already com- bined with the nucleus, on the position which is taken up by a second atom or group, is by no means restricted to the case of nitro-compounds, but is observed in the formation of all benzene substitution derivatives, except, of course, in that of the mono-substitution products ; so regularly, in fact, is this influence exercised, that it is possible to summarise the course of those reactions which take place in the formation of the best-known di-derivatives in the following statements : The relative position taken up by an atom or group, B, depends on its nature, and on that of the atom or group, A, already united with the nucleus. 352 NITRO-COMPOUNDS. When A = Cl, Br, I, NH 2 , OH, CH 3 , and B = Cl, Br, N0 2 , SO S H, a para-compound is the principal product, but it is usually accompanied by smaller and varying quantities of the ortho- compound. When, on the other hand, A = NOjj, COOH, S0 3 H, CHO, CO.CH 3 , and B - Cl, Br, N0 2 , S0 3 H, a raefa-derivative is the principal product, and only very small quantities of the ortho- and para-compounds are formed. These statements also hold good when two identical atoms or groups are introduced in one operation, since the change really takes place in two stages ; when benzene, for example, is treated with nitric acid, meta-dinitrobenzene is the principal product, whereas with bromine it yields para-dibromobenzene. Properties. As a rule, aromatic nitro-compounds are yellowish, well-defined crystalline substances, and are, there- fore, of great service in identifying hydrocarbons and other liquids ; many of them are volatile in steam, but, with the exception of certain mono-nitro-derivatives, cannot be dis- tilled under ordinary pressure, as when heated strongly they undergo decomposition, sometimes with explosive violence ; they are generally insoluble in water, but soluble in benzene, ether, alcohol, &c. As in the case of the mtro-paraffin8 (part i. p. 181), the nitro-group is very firmly combined, and is not, as a rule, displaced by the hydroxyl-group on treat- ment with potash even at high temperatures. The most important reaction of the nitro-compounds namely, their behaviour on reduction, is described later (p. 356). Nitrobenzene, C 6 H 5 -N0 2 , is usually prepared in the labora- tory by slowly adding to benzene (10 parts) a mixture of nitric acid of sp. gr. 1-45 (12 parts), and concentrated sulphuric acid (16 parts), the temperature being kept below about 40 by cooling in water, and the mixture being NITRO-COMPOUNDS. 353 constantly shaken ; the benzene dissolves in the acids, although it does not appear to do so, because it is quickly converted into the nitro-compoimd, which separates again as a yellowish-brown oil. As soon as all the benzene has been added, the mixture is heated at about 80 for half an hour, then cooled, and poured into a large volume of water ; the nitrobenzene, which collects at the bottom of the vessel, is separated with the aid of a funnel, washed with a little water or dilute soda until free from acid, dried with calcium chloride, and fractionated, in order to separate it from un- changed benzene and from small quantities of dinitrobenzene which may have been produced ; this is very easily accom- plished, as the boiling-points of the three compounds are widely different. On the large scale, nitrobenzene is prepared in a similar manner, but the operation is carried out in iron vessels pro- vided with an arrangement for stirring, and the product is distilled from iron retorts, or, better, in a current of steam. Nitrobenzene is a pale-yellow oil of sp. gr. 1-2 at 0, and has a strong smell which is very like that of benzaldehyde (p. 406) ; it boils at 205, is volatile in steam, and is miscible with organic liquids, but practically insoluble in water; in spite of the fact that it is poisonous, it is often employed instead of oil of bitter almonds for flavouring and per- fuming purposes, under the name of 'essence of mirbane;' its principal use, however, is for the manufacture of aniline (p. 361). Meta-dinitrobenzene, C 6 H 4 (N0 2 ) 2 , is obtained, together with small quantities of the o- and jp-dinitro-compounds, when benzene is gradually added to a mixture of nitric acid (sp. gr. 1-5) and concentrated sulphuric acid, and the whole then heated on a sand-bath, until a portion of the oil, which floats on the surface, solidifies completely when dropped into water; after cooling, the mixture is poured into a large volume of water, the solid product separated by filtration, washed with water, and recrystallised from hot alcohol until its melting- W 354 NITRO-COMPOUNDS. point is constant ; the o- and ^-compounds, formed only in very small quantities, remain dissolved in the mother-liquors. Meta-dinitrobenzene crystallises in pale-yellow needles, melts at 90, and is volatile in steam ; it is only sparingly soluble in boiling water, but dissolves freely in most organic liquids. On reduction with alcoholic ammonium sulphide (p. 357) it is first converted into m-nitraniline (p. 363), and then into m-phenylenediamine or meta-diarnidobenzene, C 6 H 4 (NH 2 ) 2 (p. 364). o-Dinitrolenzene and p-dinitrobenzene are colourless, crystal- line compounds, melting at 118 and 173 respectively; they resemble the corresponding ?7z-compound in their behaviour on reduction, and in most other respects. o-Dinitrobenzene, however, differs notably from the other two isomerides, inas- much as it interacts with boiling soda, yielding o-nitrophenol (p. 393), and, with alcoholic ammonia at moderately high temperatures, giving o-nitraniline (p. 363). A similar be- haviour is observed in the case of other o-dinitro-compounds, the presence of the one nitro-group rendering the other more easily displaceable. Symmetrical trinitrobenzene, C 6 H 3 (N0 2 ) 3 , is formed when the ra-dinitro-compound is heated with a mixture of nitric and anhydrosulphuric acids ; it crystallises in colourless plates and needles, melting at 121-122. The halogen derivatives of benzene are readily nitrated, yielding, however, the o- and jo-mononitro-derivatives only, according to the general rule ; the w-nitro-halogen compounds are therefore prepared by chlorinating or brominating nitro- benzene. All these nitro-halogen derivatives are crystalline, and, as will be seen from the following table, their melting- points exhibit the regularity mentioned above (p. 332), except in the case of ?7i-iodonitrobenzene : Ortho. Meta. Para. Chloronitrobenzene, C 6 H 4 C1-N0 2 , 32-5 44-4 83 Bromonitrobenzene, C 6 H 4 Br.N0 2 , 41-5 56 126 lodonitrobenzene, CgH-NO 49 33 171 NITRO-COM POUNDS. 355 They are, on the whole, very similar in chemical properties, except that, as already pointed out (p. 346), the o- and p- compounds differ from the w-compounds in their behaviour with alcoholic potash and ammonia, a difference which recalls that shown by the three dinitrobenzenes. The nitrotoluenes, C 6 H 4 (CH 3 )-X0 2 , are important, because they serve for the preparation of the toluidines (p. 364). The o- and ^-compounds are prepared by nitrating toluene, and may be partially separated by fractional distillation ; o-nitrotoluene is liquid at ordinary temperatures, and boils at 223, whereas p-nitrotolmne is crystalline, and boils at 237, its melting-point being 54. m-Nitrotoluene is not easily prepared ; it melts at 16, and boils at 230. Many other nitro-compounds are mentioned later. CHAPTER XXIII. AMIDOCOMPOUNDS AND AMINES. The hydrogen atoms in ammonia may be displaced by aromatic radicles, bases, such as aniline, C 6 H 5 -NH 2 , benz- ylamine, C 6 H 3 -CH 2 -NH 2 , and diamidobenzene, C 6 H 4 (NH 2 ) 2 , which are analogous to, and have many properties in common with the fatty amines, being produced; as, however, those compounds which contain the amido-group directly united with carbon of the nucleus differ in many important respects from those in which this group is present in the side-chain, the former are usually called amido-compounds, whereas the latter are classed as aromatic amines, because they are the true analogues of the fatty amines. Amido-compounds. The amido-compounds may, therefore, be regarded as derived from benzene and its homologues by the substitution of one or more amido-groups for hydrogen atoms of -the 3t>6 AMIDO-COMPOUNDS AND AMINES. nucleus ; they may be classed as mono-, di-, tri-, &c., amido- coinpounds, according to the number of such groups which they contain. C 6 H 6 .NH 2 CeH 4 (NH 2 ) 2 C 6 H 3 (NH 2 ) 3 . Amidobenzene (Aniline). Diamidobenzene. Triamidobenzene. With the exception of aniline, all amido-compounds exist in three or more isomeric modifications; there are, for example, three isomeric (o.m.p.) diamidobenzenes, and three isomeric (o.m.p.) amidotoluenes, or toluidines, C 6 H 4 (CH 3 )-NH 2 , a fourth isomeride of the toluidines namely, benzylamine, C 6 H 5 .CH 2 .NH 2 (p. 368), being also known. Preparation. The amido-compounds are almost always prepared by the reduction of the nitro-com pounds ; various reducing agents, such as tin, zinc, or iron, and hydrochloric or acetic acid, are employed, but perhaps the most common one is a solution of stannous chloride in hydrochloric acid, C 6 H 5 -N0 2 + 6H - C 6 H 5 -NH 2 + 2H 2 O C 6 H 4 < + 6H = C 6 H 4 < + 2 H 2 C 6 H 5 .N0 2 + 3SnCl 2 + 6HC1 = C 6 H 5 -NH 2 + 3SnCl 4 + 2H 2 0. Reduction is usually effected by simply treating the nitre-com- pound with the reducing mixture without a special solvent, when a vigorous reaction often ensues, heating being seldom necessary except towards the end of the operation. The solution contains the amido-compound, combined as a salt with the acid which has been employed ; when, however, tin or stannous chloride and hydro- chloric acid have been used, a double salt of the hydrochloric! e of the base and stannic chloride is produced ; in the reduction of nitro- benzene, for example, the double salt, aniline stannichloride, has the composition (C 6 H 5 .NH 2 , HC1) 2 , SnCl 4 . In any case, the salt is decomposed by the addition of excess of caustic soda or lime, and the liberated base either distilled with steam or extracted with ether, or isolated in some other manner suitable to the special case. Recent researches show that the re- duction of nitro-compounds may take place in two stages: in the first place, a derivative of hydroxylamine is produced, C 6 H 5 -N0 2 + 4H = C 6 H 5 .NH-OH + H 2 0, Plienylhydroxylainine, AMIDO-COMPOUNDS AND AMINES. 357 and this, by the further action of the reducing agent, is converted into the aniido-compound. Nitro-compounds may also be reduced to amido-compounds by employing sulphuretted hydrogen in alkaline solution, or, more conveniently, an alcoholic solution of ammonium sulphide, C 6 H 6 -N0 2 + 3SH 2 = C 6 H 5 .NH 2 + 2H 2 + 3S. The nitro-compound is dissolved in alcohol, concentrated ammonia added, and a stream of sulphuretted hydrogen passed into the solution, until reduction is complete, heat being applied if necessary. The solution is then filtered from precipitated sulphur, the alcohol distilled off, and the residue acidified with hydro- chloric acid ; the filtered solution of the hydrochloride of the base is now evaporated to a small bulk and treated with soda, when the base separates as an oil or solid, and may then be purified by dis- tillation, recrystallisation, &c. When there are two or more nitro-groups in a compound, partial reduction may be accomplished either by treating its alcoholic solution with the calculated quantity of stannous chloride and hydrochloric acid, or by adding strong ammonia and passing sulphuretted hydrogen ; in the latter, as in the former case, one nitro-group is reduced before a second is attacked, so that by stopping the current of gas at the right time (usually ascertained by weighing the-sulphuretted hydrogen absorbed), only partial reduction takes place. Dinitrobenzene, for example, can be converted into nitraniline by either of these methods, the latter being the mdre convenient, \S CH + 6H = CH + 2H0. The amido-derivatives of toluene, xylene, &c., are com- mercially prepared by heating the hydrochlorides of the isomeric alkylanilines, such as methylaniline and dimethyl- aniline, at 280-300, when the alkyl-group leaves the nitrogen atom and enters the nucleus (compare p. 365), C 6 H 5 -NH-CH 3 , HC1 = C 6 H 4 < Methylaniline Hydrochloride. p-Toluidine Hydrochloride. 358 AMIDO-COMPOUNDS AND AMINES. In the case of dimethylaniline tins change takes place in two stages, C6 H 6 -N(CH 3 ) 2 , HC1 = ^H^NI^QII , HC1 Dimethylaniline Hydrochloride. Methyl-p-toluidine Hydrochloride. PH" / 3 CaM/C Tkr-rr r^TT TjrNH group a feeble acid character, similar to that of imides (part i. p. 238). Triplienylamine, (C 6 H 5 ) 3 N, may be prepared by heating potassium diphenylamine with monobromobenzene at 300, (C 6 H 6 ) 2 NK + C 6 H 5 Br = (C 6 H 5 ) 3 N + KBr. It is a colourless, crystalline substance, melts at 127, and has no basic properties, as it does not combine even with the strongest acids. Aromatic Amines. The true aromatic amines namely, those compounds in which the amido-group is united with carbon of the side-chain, are of far less importance than the amido-compounds, and only a few substances of this class have been thoroughly investigated. Benzylamine, C 6 H 5 -CH 2 -NH 2 , may, however, be described as a typical aromatic primary nmine. It may be obtained by AMIDO-COMPOUNDS AND AMINES. 369 reducing phenyl cyanide (benzonitrile, p. 421) with sodium and alcohol, C 6 H 5 -CN + 4H = C 6 H 5 -CH 2 .NH 2 , by treating the amide of phenylacetic acid (p. 429) with bromine and potash, C 6 H 5 .CH -CO.NH 2 + Br, + 4KOH = C 6 H 5 .CH 2 -NH 2 + 2KBr + K 2 C0 3 + 2H 2 0, and by heating benzyl chloride with alcoholic ammonia, C 6 H 5 -CH 2 C1 + NH 3 - C 6 H 5 .CH 2 -NH 2 , HC1. All these methods are similar to those employed in the preparation of fatty primary amines. Benzylamine is a colourless, pungent -smelling, strongly basic liquid, boiling at 185; it closely resembles the fatty amines in nearly all respects, and differs from the monamido- compoimds (aniline, toluidine, &c.) in being readily soluble in water, and in not yielding diazo-compounds when its salts are treated with nitrous acid. Like the fatty primary amines, it gives the carbylamine reaction, and is converted into the corresponding alcohol (benzyl alcohol, p. 403) on treatment with nitrous acid. Secondary and tertiary aromatic amines are formed when a primary amine is heated with an aromatic halogen compound, containing the halogen in the side-chain ; when, for example, benzylamine is heated with benzyl chloride, both dibenzylamine and tribenzylamine are produced, just as diethylamine and triethyl- amine are obtained when ethylamine is heated with ethyl bromide, C 6 H 5 .CH 2 -NH 2 + C 6 H 5 .CH 2 C1 = (CeH^CH^NH, HC1 C 6 H 5 .CH 2 .NH 2 + 2C 6 H 5 -CH 2 C1 = (C 6 H 5 -CH 2 ) 3 N, HC1 + HC1. When, therefore, benzyl chloride is heated with ammonia, the pro- duct consists of a mixture of the salts of all three amines. 370 DIAZO-COMPOUNDS AND THEIR DERIVATIVES. CHAPTER XXIV. DTAZO-COMPOUNDS AND THEIR DERIVATIVES. It has already been stated that when the amido-compounds or their salts are treated with nitrous acid in aqueous solution, they yield phenols; this decomposition, however, usually takes place only on warming. If, for example, a well-cooled dilute solution of aniline hydrochloride (1 mol.) be mixed with sodium nitrite (1 mol.), and hydrochloric acid (1 mol.) added to set free the nitrous acid, phenol is not pro- duced, and the solution contains a very unstable substance called diazobenzene chloride, the formation of which may be expressed by the equation C 6 H 5 .NH 2 ,HC1 + N0 2 H = C 6 H 5 -N:NC1 + 2H 2 0. In this respect, then, the amido-compounds differ from the fatty amines ; the latter are at once converted into alcohols by nitrous acid in the cold, whereas the former are first trans- formed into diazo-compounds, which, usually only on warming, decompose more or less readily with formation of phenols (p. 386). All amido-compounds behave in this way, yielding diazo- salts similarly constituted to diazobenzene chloride. The diazo-salts were discovered in 1860 by P. Griess; they may be assumed to be salts of diazobenzene, C 6 H 5 -]S[:N-OH, and its homologues. substances which it has not been found possible to isolate in a pure state and analyse on account of their unstable nature. The diazo-salts (usually spoken of as the diazo-compounds) may nevertheless be isolated without much difficulty, although, as a matter of fact, they are seldom separated from their aqueous solutions, partly because of their explosive character, DIAZO-COMPOUNDS AND THEIR DERIVATIVES. 371 partly because for most purposes for which they are prepared this operation is quite unnecessary. Preparation. Anhydrous diazo-salts may be obtained by treating a well-cooled solution of an amido-compound in absolute alcohol with amyl nitrite and a mineral acid, in absence of any considerable quantity of water, C 6 H 5 .NH 2 ,HC1 + C 6 H U .0-NO = C 6 H 5 .N:NC1 + C 5 H n -OH + H 2 0. Diazobenzene sulphate, C 6 H 5 -N:N-S0 4 H, for example, is pre- pared by dissolving aniline (15 parts) in absolute alcohol (10 parts), adding concentrated sulphuric acid (20 parts), and after cooling in a freezing mixture, slowly running in pure amyl nitrite (20 grams) ; after 10-15 minutes diazobenzene sulphate separates in crystals, which are washed with alcohol and ether, and dried in the air at ordinary temperatures. Diazobenzene chloride and diazobenzene nitrate may be obtained in a similar manner, employing alcoholic solutions of hydrogen chloride and of nitric acid in the place of sulphuric acid. Diazobenzene nitrate, C 6 H 5 -N:N-N0 3 , may also be conveniently isolated as follows : Aniline nitrate is suspended in a small quantity of water, and the liquid saturated with nitrous acid (generated from As 2 O 3 and HNO 3 ), when the crystals gradually dissolve with formation of diazobenzene nitrate ; on the addition of alcohol and ether, this salt separates in colourless needles. Special precau- tions are to be observed in preparing this substance, as, when dry, it is highly explosive, although it may be handled with safety if kept moist. Aqueous solutions of the diazo-salts are prepared by dis- solving the amido-compound in an aqueous mineral acid, and adding the theoretical quantity of a solution of sodium nitrite, after first cooling to (see above, also p. 373). Properties. The diazo-salts are colourless, crystalline com- pounds, very readily soluble in water ; in the dry state they are more or less explosive, and should be handled only with the greatest caution. They are of immense value in syntheti- cal chemistry and in the preparation of dyes, as they undergo a number of remarkable reactions, of which the following are some of the more important, 372 DIAZOCOMPOUNDS AND THEIR DERIVATIVES. When warmed in aqueous solution they decompose rapidly, with evolution of nitrogen and formation of phenols (p. 386), C 6 H 5 -N:N.N0 3 + H 2 - C 6 H 5 .QH + N 2 + HN0 3 C 6 H 4 (CH 8 ).N:NC1 + H 2 - C 6 H 4 (CH 3 )-OH + N 2 + HCL 2>-Diazotoluene Chloride. p-Cresol. When boiled with strong alcohol they yield hydrocarbons, part of the alcohol being oxidised to aldehyde, C 6 H 5 .N:NC1 + C 2 H 5 -OH = C 6 H 6 + N 2 + HC1 + CH 3 -CHO. These two reactions afford a means of obtaining phenols and hydrocarbons from amido-compounds. The diazo-compounds behave in a very remarkable way when treated with cuprous salts ; if, for example, a solution of diazobenzene chloride be warmed with cuprous chloride, nitrogen is evolved, and chlorobenzene is produced. In this reaction, the diazo-salt combines with the cuprous chloride to form an intermediate brownish additive compound, which is decomposed at higher temperatures, cuprous chloride being regenerated ; theoretically, therefore, the reaction is continuous, Htlf C 6 H 6 .N:NC1, Cu 2 Cl 2 = C 6 H 5 C1 + N 2 + Cu 2 Cl 2 . If, instead of the chloride, cuprous bromide or cuprous i^krdide be employed, bromobenzene or iodobenzene is produced, C H 6 .N:NBr,Cu 2 Br 2 - C 6 H 5 Br + N 2 + Cu 2 Br 2 , Additive Compound. Bromobenzene. whereas by using cuprous cyanide, a cyanide or nitrile is formed, CflHg.Njj.CN, Cu 2 (ON) 2 = C 6 H 5 .CN + N 2 + Cu 2 (CN) 2 . Additive Compound. Phenyl Cyanide. In this latter reaction a mixture of cupric sulphate and potassium cyanide is generally used instead of the previously prepared cuprous cyanide. By means of this very important reaction, which was discovered by Sandmeyer in 1884, it is possible to displace the DIAZO-COMPOUNDS AND THEIR DERIVATIVES. 373 NH 2 - group in amide-compounds by Cl, Br, I, CN, and indirectly by COOH (by the hydrolysis of the CN- group), and indeed by other atoms or groups ; as, moreover, the yield is generally good, Sandmeyer's reaction is of great practical value. The amido-compounds being readily obtainable from the nitro-compounds, and the latter from the hydrocarbons, this method affords a means of preparing halogen, cyanogen, and other derivatives indirectly from the hydrocarbons. Gattermann has shown that the decomposition of the diazo- compounds is, in many cases, best brought about by treating the cold solution of the diazo-salt with copper powder (prepared by the action of zinc-dust on a solution of copper sulphate). Monochlor- benzene, for example, is readily obtained from aniline by the following process : Aniline (31 grams) is dissolved in hydrochloric acid (300 grams) and water (150 grams), the solution well cooled with ice, and diazotised by adding gradually a concentrated aqueous solution of sodium nitrite (23 grams). The solution of diazobenzene chloride thus obtained is gradually mixed with copper powder (40 grams), when nitrogen is evolved and chlorobenzene produced, the reaction being complete in about half an hour. The chlorobenzene is then purified by distillation in steam and fractionation. In preparing cyanobenzene, C 6 H 5 -CN, from aniline, aniline sul- phate is diazotised, the solution mixed with potassium cyanide, and then copper powder added. The diazo-com pounds also serve for the preparation of an important class of compounds known as the hydrazines, these substances being obtained by reducing the diazo-compounds, usually with stannous chloride and hydrochloric acid, R.N:XC1 + 4H = R.NH.NH 2 ,HC1. Diazochloride. Hydrazine Hydrochloride. Constitution of Diazo-compounds. That diazobenzene salts have the constitution expressed by the formula C 6 H 5 'N:NR 6 a (where R = Cl, Br, I, N0 3 , HS0 4 , &c.) is shown by the following considerations. On reduction they are converted into phenylhydrazine, C 6 H 5 -NH-NH 2 (the constitution of which is known, p. 376), a fact which shows that the two nitrogen atoms are united together, and that one of them (b) 374 DIAZO-COM POUNDS AND THEIR DERIVATIVES. is combined with the benzene nucleus. Diazobenzene chloride interacts readily with dimethylaniline, giving dimethylamido- azobenzene (p. 376), C 6 H 5 .N:NC1 + C 6 H 6 .N(CH 3 ) 2 = C 6 H 5 .N:N.C 6 H 4 .N(CH 3 ) 2 + HC1, b a and this substance, on reduction, yields aniline and dimethyl- ^9-phenylenediamine (p. 376), C 6 H 5 .N:N-C 6 H 4 .N(CH 3 ) 2 + 4H = C 6 H 5 .NH 2 + NH 2 .C 6 H 4 .N(CH 3 ) 2 . b a These changes can only be explained on the assumption that the acid radicle is attached to the a-nitrogen atom, as in the above formula, because if it were united to the other nitrogen atom (6), as in the formula C 6 H 5 -NC1 : N, for example, such b a products could not be obtained. Free diazobenzene is very unstable, and has not been obtained in a pure state, but it probably has the constitution C 6 H 5 -N:N'OH. Diazoamido- and Amidoazo-compounds. Although some of the more characteristic reactions of diazo-compounds have already been mentioned, there are numerous other changes of great interest and of great com- mercial importance which these substances undergo. When, for example, diazobenzene chloride is treated witli aniline, a reaction takes place similar to that which occurs when aniline is treated with benzoyl chloride (p. 420), and diazoamidobenzene is formed, C 6 H 5 .N:NC1 + NH 2 .C fl H 5 = C 6 H 5 .N:N-NH.C 6 H 5 + HC1 Diazoamidobenzene. C 6 H 5 .COC1 + NH 2 .C 6 H 6 = C 6 H 5 .CO-NH.C 6 H 5 + TIC1. Benzoylaniidobenzene or Benzanilide. As, moreover, other diazo-compounds and other amido- compounds interact in a similar manner, numerous diazoamido- compounds may be obtained. Diazoamidobenzene, C 6 H 5 -N:N-NH-C 6 H 5 , may be de- DIAZOCOMPOUNDS AND THEIR DERIVATIVES. 375 scribed as a typical compound of this class ; it is conveniently prepared by passing nitrous fumes into an alcoholic solution of aniline, the diazobenzene nitrite, which is probably first produced, interacting with excess of aniline, C 6 H 5 -N:]S T ^ T 2 + C 6 H 5 .NH 2 - C 6 H 5 .N:X.NILC 6 H 5 + HN0 2 . Diazoamidobenzene crystallises in brilliant yellow needles, and is sparingly soluble in water, but readily in alcohol and ether ; it does not form salts with acids. Amidoazobenzene, C 6 H 5 -X:X-C 6 H 4 -NH 2 , is formed when diazoamidobenzene is warmed with a small quantity of aniline hydrochloride at 40, intramolecular change taking place, C 6 H 5 .X:.\.XH.C 6 H 5 = C 6 H 6 'N:N.C 6 H 4 .NH 2 . The course of this remarkable reaction, which is a general one, and shown by all diazoamido-compounds, may possibly be explained by assuming that the aniline hydrochloride first decom- poses the diazoamidobenzene, yielding diazobenzene chloride and aniline thus : C 6 H 5 .N:N.NH.C 6 H 5 + C 6 H 5 -NH 2 , HC1 - C 6 H 5 .N:NC1 + 2C 6 H B -NH, The diazobenzene chloride then interacts with excess of aniline in such a way that the diazo-group displaces hydrogen of the nucleus from the j97-a-position to the amido-group, C 6 H 5 -N :NC1 + 2C ti H 5 .NH 2 = C 6 H 5 -N :N-C 6 H 4 .NH 2 + C 6 H 5 -NH 2 , HC1. The change is, therefore, theoretically continuous, the regenerated aniline hydrochloride being able to convert a further quantity of the diazoamidobenzene into the amidoazo-compound. Amidoazobenzene may also be prepared by nitrating azobenzene (p. 378), and then reducing the ^-nitroazo- benzene, C 6 H 5 -X:X-C 6 H 4 -X0 2 , which is produced with ammonium sulphide, a series of reactions analogous to those which occur in the formation of aniline from benzene, and which prove the constitution of amidoazobenzene. Amidoazobenzene crystallises from alcohol in brilliant orange-red plates, and melts at 125; its salts are intensely coloured, the hydrochloride, C 6 H 5 -X:X.C 6 H 4 .NH 2 , HC1, for example, forms beautiful steel-blue needles, and used to come 376 DIAZO-COMPOUNDS AND THEIR DERIVATIVES. into the market under the name of ' aniline yellow ' as a silk dye (p. 524). Other amidoazo-compounds may be obtained directly by treating tertiary alkylanilines (p. 364) with diazo-salts : dimethylaniline, for example, interacts with diazobenzene chloride, yielding dimethyl- amidoazobenzene, C 6 H 5 .N:NC1 + C 6 H 5 .N(CH 3 ) 2 = C 6 H 5 .N:N.C 6 H 4 .N(CH 3 ) 2 , HC1, no intermediate diazoamido-compound being formed, because dimethylaniline does not contain an NIK or NH 2 - group. In this case also the diazo-group, C 6 H 5 -N:N-, takes up the ^-position to the N(CH 3 ) 2 - group, as is shown by the fact that, on reduction, dimethylamidoazobenzene is converted into aniline and dimethyl-p-phenylenediamine, the latter being identical with the base which is produced by reducing ^-nitrosodimcthylaniliiie (p. 367). Phenylhydrazine, C 6 H 5 .NH.NH 2 , a compound of great practical importance, is easily prepared by the reduction of diazobenzene chloride, C 6 H 5 .N:NC1 + 4H = C 6 H 6 .NH-NH 2 , HC1. Aniline (10 grams) is dissolved in concentrated hydrochloric acid (200 c.c.), and to the well-cooled solution sodium nitrite (7 -5 grams) dissolved in water (50 c.c.) is added in small quantities at a time; the resulting solution of diazobenzene chloride is then mixed with stannous chloride (45 grams) dissolved in concentrated hydrochloric acid (45 grams). The precipitate of phenylhydrazine hydrochloride, which rapidly forms, is separated by nitration, dissolved in water, decomposed with potash, and the free base extracted with ether and purified by fractionation. Phenylhydrazine crystallises in colourless prisms, melts at 23, and boils with slight decomposition at 241, so that it is best purified by distillation under reduced pressure. It is sparingly soluble in cold water, readily in alcohol and ether ; it is a strong base, and forms well-characterised salts, such as the hydrochloride, C 6 H 5 -NH-NH 2 , HC1, which crystallises in colourless needles, and is readily soluble in hot water ; solutions of the free base and of its salts reduce Fehling's solution in the cold. The constitution of phenylhydrazine is established by the fact that, when heated with zinc-dust DIAZO-COMPOUNDS AND THEIR DERIVATIVES. 377 and hydrochloric acid, it is converted into aniline and ammonia. Phenylhydrazine interacts readily with aldehydes, ketones, and other substances containing a carbonyl-group, with elimi- nation of water and formation of plienylhydrazones (hydra- zones) ; as these compounds are usually sparingly soluble and often crystallise well, they may frequently be employed with advantage in the identification and isolation of aldehydes, ketones, &c. (part i. p. 133), C 6 H 5 .CHO + C 6 H 5 -NH-NH 2 = C 6 H 5 .CH:N.NH-C 6 H 5 + H 2 Benzaldehyde. Benzaldehyde Hydrazone. C 6 H 5 .CO-CH 3 + C 6 H 5 .XH.NH 2 = Acetophenone. + H a O. Acetophenone Hydrazone. Most hydrazones are decomposed by strong mineral acids, with regeneration of the aldehyde or ketone, and formation of a salt of phenylhydrazine, C 6 H 5 -CH:X.^H C 6 H 5 + H 2 + HC1 - C 6 H 5 -CHO + CeHg.NH-NHjp HC1. The value of phenylhydrazine as a means of detecting and isolating the sugars has been explained (part i. p. 267). In preparing hydrazones, the reacting substances may either be heated together without a solvent, or more frequently the substance is dissolved in water (or alcohol), and the solution of the requisite amount of phenylhyd razine in dilute acetic acid added. On warming, the hydrazone generally separates in a crystalline form, and may be readily purified by recrystallisation. Osazones (part i. p. 268) are prepared by warming an aqueous solution of a sugar, with a large excess of phenylhydrazine dissolved in dilute acetic acid ; after some time the osazone begins to be deposited in a crystalline form, the separation increasing as the liquid cools. Azo-compounds. It has already been shown that when nitro-com pounds are treated with tin and hydrochloric acid, and other acid reduc- 378 DIAZO-COMPOUNDS AND THEIR DERIVATIVES. ing agents, they are converted into amido-compounds, a similar change taking place when alcoholic ammonium sulphide is employed; when, however, nitro-compounds are treated with other alkaline reducing agents, such as sodium amalgam, staunous oxide and soda, or zinc-dust and soda, they yield azo-compounds, such as azobenzene, two molecules of the nitro-compound affording one molecule of the azo- compound, 2C 6 H 5 .N0 2 + 4H - C 6 H 5 .N:N.C 6 H 5 + 2H 2 0. Azobenzene, C 6 H 5 -N:NC 6 H 5 , may be described as a typical example of this class of compounds. It is prepared by agitating nitrobenzene with the calculated quantity of stannous chloride, dissolved in soda, until the odour of nitro- benzene is imperceptible. The reddish precipitate is collected, washed with water, dried, and recrystallised from light petroleum. Azobenzene crystallises in brilliant red plates, melts at 68, and distils at 293; it is readily soluble in ether and alcohol, but insoluble in water. Alkaline reducing agents, such as ammonium sulphide, zinc-dust and soda, &c., convert azobenzene into hydrazobenzene, a colourless, crystalline sub- stance, which melts at 131, C 6 H 5 .N:N.C 6 H 5 + 2H = C 6 H 5 .NH.NH.C 6 H 6 , whereas a mixture of zinc-dust and acetic acid decomposes it, with formation of aniline, C 6 H 5 .N:N.C 6 H 6 + 4H = 2C 6 H 6 .NH 2 . Other azo-compounds behave in a similar manner. Hydrazobenzene, C 6 H 5 -NH-NH-C 6 H 5 , is readily converted into azobenzene by mild oxidising agents such as mercuric oxide, and slowly even when air is passed through its alcoholic solution. When treated with strong acids, it undergoes a very remarkable intramolecular change, and is converted into jo-diamidodiphenyl or benzidine, a strongly basic substance largely used in the prepara- tion of azo-dyes (p. 526), C 6 H 5 .NH-NH.C 6 H 5 = NH 2 -C 6 H 4 .C 6 H 4 .NH 2 . Benzidine. DtA20COMPOUNDS AND THEIR DERIVATIVES. 379 Benzidine may be directly produced by reducing azobenzene with tin and strong hydrochloric acid ; other azo-compounds, such as azo-toluene, CH 3 -C 6 H 4 -N:N-C 6 H 4 -CH 3 , behave in a similar manner, and are readily converted into isomeric alkyl-derivatives of benzidine, such as dimethylbenzidine (tolidine), CHAPTER XXV. SULPHONIC ACIDS AND THEIR DERIVATIVES. When benzene is heated with concentrated sulphuric acid, it gradually dissolves, and benzenes ulphonic acid is formed by the substitution of the sulphonic group -S0 3 H or -S0 2 -OH for an atom of hydrogen, C 6 H 6 + H 2 S0 4 = C 6 H 5 -S0 3 H + H 2 0. The homologues of benzene and aromatic compounds in general behave in a similar manner, and this property of readily yielding sulphonic derivatives by the displacement of hydrogen of the nucleus is one of the important characteristics of aromatic, as distinct from fatty, compounds. The sulphonic acids are not analogous to the alkylsulphuric acids (part i. p. 182), which are ethereal salts, but rather to the carboxylic acids, since they may be regarded as derived from sulphuric acid, S0 2 (OH) 2 , just as the carboxylic acids are derived from carbonic acid, CO(OH) 2 , namely, by the substitution of an aromatic radicle for one of the hydroxyl- groups. Sulphuric acid, S0 2 sulphonic acid, or aniline- >-sulphonic acid, C 6 H 4 (NH 2 )-S0 3 H, is easily prepared by heating aniline sulphate at about 200 for some time. Aniline is slowly added to a slight excess of the theoretical quantity of sulphuric acid contained in a porcelain dish, the mixture being constantly stirred as it becomes solid ; the dish is then gently heated on a sand-bath, the contents being stirred, and care being taken to prevent charring. The process is at an end as soon as a small portion of the product, dissolved in water, gives no oily precipitate of aniline on adding excess of soda. After cooling, a little water is added, the sparingly soluble sul phonic acid separated by filtration, and purified by recry stall isation from boiling water, with addition of animal charcoal (see foot-note, p. 393). Sulphanilic acid crystallises with 2 mols. H 0, and is readily soluble in hot, but only sparingly in cold, water. 384 SULPHONIC ACIDS AND THEIR DERIVATIVES. It forms salts with bases, but it does not combine with acids, the basic character of the amido-group being neutral- ised by the acid character of the sulphonic group; in this respect, therefore, it differs from glycine (part i. p. 292), which forms salts both with acids and bases. When sulphanilic acid is dissolved in dilute soda, the solution mixed with a slight excess of sodium nitrite, and poured into well-cooled, dilute sulphuric acid, diazobenzene- sulphonic acid is formed, P TT X NH 2 , TTH ATr> P TT ^NlN-OH 6 H 4-acid, on nitrating benzenesulphonic acid ; they * The existence of this anhydride (and of that of amidobenzene-m- sulphonic acid), is a very interesting fact, because, as a rule, anhydride formation takes place only between groups in the o-position to one another (compare p. 424). t Many other diazo-compounds which, like diazobenzenesulphonic acid, contain some acid group, are decomposed by halogen acids in a similar manner. SULPHONIC ACIDS AND THEIR DERIVATIVES. 385 resemble sulphanilic acid in properties, and are readily converted into the anhydrides of the corresponding diazobenzenesulphonic acids. Many other sulphonic acids are described later. CHAPTER XXVI. PHENOLS. The hydroxy-com pounds of the aromatic series, such as phenol or hydroxy-benzene, C 6 H 5 -OH, the isomeric hydroxy- toluenes, C 6 H 4 (CH 3 )-OH, and benzyl alcohol, C 6 H 5 .CH 2 -OH, are theoretically derived from the aromatic hydrocarbons by the substitution of hydroxyl-groups for atoms of hydrogen, just as the fatty alcohols are derived from the paraffins. It will be seen, however, from the examples just given that whereas, in benzene, hydrogen atoms of the nucleus must necessarily be displaced, in the case of toluene and all the higher homologues this is not so, since the hydroxyl-groups may displace hydrogen either of the nucleus or of the side- chain. Now the hydroxy-derivatives of benzene, and all those aromatic hydroxy-compounds, formed by the substitu- tion of hydroxyl-groups for hydrogen atoms of the nucleus, differ in many respects not only from the fatty alcohols, but also from those aromatic compounds which contain the hydroxyl-group in the side-chain ; it is convenient, therefore, to make some distinction between the two kinds of aromatic hydroxy-compounds, and for this reason they are classed in two groups, (a) the phenols, and (b) the aromatic alcohols (p. 402). The phenols, then, are hydroxy-compounds in which the hydroxyl-groups are united directly with carbon of the nucleus ; they may be subdivided into monohydric, dihydric, trihydric phenols, &c., according to the number of hydroxyl- groups which they contain. Phenol, or carbolic acid, C r H-;OH, for example, is a monohydric phenol, as are also Y 386 PHENOLS. the three isomeric cresols or hydroxy toluenes, C 6 H 4 (CH 3 )-OH; the three isomeric dihydroxybenzenes, C 6 H 4 (OH) 2 , on the other hand, are dihydric phenols, whereas phloroglucinol, C 6 H 3 (OH) 3 , is an example of a trihydric compound. Many of the phenols are easily obtainable, well-known compounds ; carbolic acid, for instance, is prepared from coal-tar in large quantities ; carvacrol and thymol occur in various plants, and catechol, pyrogallol, &c., may be obtained by the dry distillation of certain vegetable products. Preparation. Phenols may be prepared by treating salts of amido-compounds with nitrous acid in aqueous solution, and then heating until nitrogen ceases to be evolved, C 6 H 5 .NH 2 ,HC1 + HO-NO - C 6 H 5 -OH + N 2 + H 2 + HC1 C 6 H 4 <3 1 HC1 + HO.NO = C fl H 4 <^3 + N 2 + H 2 + HC1. It is possible, therefore, to prepare phenols, not only from the amido-compounds themselves, but also indirectly from the corresponding nitro-derivatives and from the hydro- carbons, since these substances may be converted into amido- compounds, Benzene. Nitrobenzene. Amidobenzene. Phenol. C 6 H 6 C 6 H 5 .N0 2 C 6 H 5 .NH 2 C 6 H 5 .QH. The conversion of an amido-com pound into a phenol really takes place in two stages, as already explained (p. 370) ; at ordinary temperatures the salt of the amido-compound is transformed into a salt of a diazo-compound, but on heating its aqueous solution, the latter decomposes, yielding a phenol, C 6 H 5 .NH 2 , HC1 + HC1 + KN0 2 - C 6 H 5 -N:NC1 + KC1 + 211,0 C 6 H 5 -N:NC1 + H 2 = C 6 H 5 -OH + HC1 + N 2 . The amido-compound, aniline, for example, is dissolved in moderately dilute hydrochloric acid (2 mols.), or sulphuric acid (1 mol.), the solution is cooled in ice or water, and an aqueous solution of sodium nitrite (1 mol.) is slowly added, stirring con- stantly. The mixture is then gradually heated to boiling on a reflux condenser, until the evolution of nitrogen (which at first causes brisk effervescence) is at an end, and the diazo-salt is com- PHENOLS. 387 pletely decomposed ; the phenol is afterwards separated from the tarry matter, which is almost invariably produced, either by dis- tillation in steam, by crystallisation from hot water, or by extrac- tion with ether; in the last case the ethereal solution is usually shaken with soda, which dissolves out the phenol, leaving most of the impurities in the ether. Dihydric phenols may sometimes be prepared from the corresponding di-substitution products of the hydrocarbon, as indicated by the following series of changes : Benzene. Dinitrobenzene. Diamidobenzene. Diazo-salt. Dihydric Phenol. CTJ n TI /^^ n TT x'^'-H-o r TI S^f^ n TJ 6 M 6 U 6 i 4\]sr0 2 6 4 >NH 2 6 4 >N 2 C1 6 4 They may also be obtained from the monohydric compounds in the following manner : Phenol. Nitrophenol. Amidophenol. Diazo-salt. Dihydric Phenol. CH.OH CH<2 C These two methods, however, are limited in their application, because o- and ??j-diamido-com pounds cannot always be con- verted into the corresponding diazo-salts, but more often yield products of quite a different nature ; o- and p-amido-hydroxy- compounds also show an abnormal behaviour with nitrous acid, the former not being acted on at all, the latter only with difficulty. For these reasons dihydric phenols are usually most conveniently prepared by the methods given later. Another important general method of preparing phenols consists in fusing sulphonic acids or their salts with potash or soda ; in this case, also, their preparation from the hydro- carbons is often easily accomplished, since the latter are usually converted into sulphonic acids without difficulty, C 6 H 5 -S0 3 K + KOH = C 6 H 5 .OH* + K 2 S0 3 The sulphonic acid or its alkali salt is placed in an iron, or, better, * In all cases the phenols are present in the product as alkali salts. 388 PHENOLS. nickel or silver dish,* together with excess of solid potash (or soda), and a little water, and the dish is heated over a free flame, the mixture being constantly stirred with a nickel or silver spatula, or with a thermometer, the bulb of which is encased in a glass tube, or covered with silver by electro-deposition ; after the potash and the salt have dissolved, the temperature is slowly raised, during which process the mixture usually undergoes a variety of changes in colour, by which an experienced operator can tell when the decomposition of the sulphonic acid is complete ; as a rule, a temperature considerably above 200 is required, so that simply boiling the sulphonic acid with concentrated potash does not bring about the desired change. When the operation is finished, the fused mass is allowed to cool, dissolved in water, the solution acidified with dilute sulphuric acid, and the liberated phenol ex- tracted with ether, or isolated in some other manner. Dihydric phenols may often be obtained in a similar manner from the disulphonic acids, C 6 H 4 (S0 3 K) 2 + 2KOH = C 6 H 4 (OH) 2 + 2K 2 S0 3 . Owing to the high temperature at which these reactions must be carried out, secondary changes very often occur. When the sulphonic acid contains halogen atoms, the latter are usually displaced by hydroxyl-groups, especially if other acid radicles, such as -N0 2 , or -S0 3 H, are also present; when, for example, chlorobenzenesulphonic acid, C 6 H 4 C1-S0 3 H, is fused with potash, a dihydric phenol, C 6 H 4 (OH) 2 , is produced, the halogen as well as the sulphonic group being eliminated. For this reason also, .compounds such as o- and ^-chloro- nitrobenzene may be converted into the corresponding nitro- phenols (p. 392), even by boiling them with concentrated potash, the presence of the nitro-group facilitating the dis- placement of the halogen atom ; m-chloronitrobenzene, on the other hand, is not acted on under these conditions. Some- times also the process is not one of direct substitution only that is to say, the hydroxyl-groups in the product are not united with the same carbon atoms as those with which the displaced atoms or groups were united; the three (o.m.p.) * Caustic alkalies readily attack platinum and porcelain at high tempera- tures, but have little action on nickel and none on silver. PHENOLS. 389 broinobehzenesulphonic acids, for example, all yield one and the same dihydric phenol namely, the w-compound, resor- cinol, C 6 H 4 (OH) 2 , because the o- and ^-dihydric compounds, which are first produced from the corresponding bromo- sulphonic acids, are converted into the more stable ?/i-deriva- tive by intramolecular change. There are several other less important methods by which phenols may he obtained, as, for example, by distilling hydroxy-acids, such as salicylic acid, with lime, = C 6 H 5 -OH + CO,, a reaction which is similar to that which occurs in preparing the hydrocarbons from the acids. Also by heating other phenols with fatty alcohols in presence of zinc chloride, when the alkyl-group displaces hydrogen of the nucleus, just as in the production of toluidine, &c., from aniline (p. 357), C 6 H 5 .OH + C 2 H 5 -OH - C 6 H 4 -acid when boiled with water, and also because it is used as an antiseptic under the name aseptol. The three (o.m.p.)ciesols or hydroxy toluenes, C 6 H 4 (CH 3 ) -OH, the next homologues of phenol, occur in coal-tar, but cannot be conveniently isolated from this source owing to the difficulty of separating them from one another ; they are prepared from the corresponding toluidines or arnidotoluenes, C 6 H 4 (CH 3 )-NH 2 , by means of the diazo-reaction, or by fusing the corresponding toluenesulphonic acids with potash, C 6 H 4 2 CH(CH 3 ) 2 . Thymol. Carvacrol. Thymol occurs in oil of thyme, together with cymene ; it crystallises in large plates, melts at 51-5, and has a charac- teristic smell like that of thyme. It is only very sparingly soluble in water, and does not give a colouration with ferric chloride ; when heated with phosphoric anhydride, it yields propylene and ?n-cresol, C 6 H 3 (OH)<^ = C 6 H 4 (OH).CH 3 + C S H 6 . Carvacrol occurs in the oil of Origanum hirtum, and is easily prepared by heating camphor with iodine, C 10 H 16 + I 2 - C 10 H 14 + 2HI ; it is an oil boiling at 237, and*its alcoholic solution gives a green colouration with ferric chloride. When heated with phosphoric anhydride, it is decomposed into propylene and o-cresol. 398 PHENOLS. Dihydric Phenols. The isomeric dihydric phenols catechol, resorcinol, and hydroquinone are well-known compounds of considerable importance, and are respectively represented by the formulae '" i i -OH )H Catechol, Resorcinol, Hydroquinone, or or or Ortho-dihydroxybenzene. Meta-dihydroxybenzene. Para-dihydroxybenzene. Catechol, or pyrocatechin, C 6 H 4 (OH) 2 , occurs in catechu, a substance obtained in India from Acacia catechu and other trees, and was first obtained by the dry distillation of this vegetable product ; it may be obtained by fusing phenol -o- sulphonic acid, C 6 H 4 (OH)-S0 3 H, with potash, but is most conveniently prepared by heating guaiacol or methylcatechol (a colourless liquid, boiling at 200, obtained from the tar of beechwood), with concentrated hydriodic acid, It is a colourless, crystalline substance, melting at 104, and is readily soluble in water ; its aqueous solution gives, with ferric chloride, a green colouration, which, on the addition of sodium bicarbonate, changes first to violet and then to red, a reaction which is common to all or^o-dihydric phenols (p. 389). Guaiacol shows a similar behaviour with ferric chloride, but when the hydrogen atoms of both the hydroxyl-groups are displaced, as, for example, in dimethylcatechol or veratrol, C 6 H 4 (OCH 3 ) 2 , there is no colouration. Resorcinol, C 6 H 4 (OH) 2 , is prepared on a large scale by fusing benzene-m-disulphonic acid with potash, V + 2KOH - C 6 1 8*- PHENOLS. 399 but it is also obtained when the para-disulphonic acid, and many other ortho- and para-derivatives of benzene are treated in the same way, owing to intramolecular change taking place (compare p. 388). It is a crystalline substance, melting at 110, and dissolves freely in water, alcohol, and ether; its aqueous solution gives a dark-violet colouration, with ferric chloride and a crystalline precipitate of tribromoresorcinol, C 6 HBr 3 (OH) 9 , with bromine water. When resorcinol is strongly heated for a few minutes with phthalic anhydride (p. 426), or with the anhydride of some other dicarboxylic acid (succinic anhydride, for example), and the yellowish-red mass then dissolved in dilute soda, a yellowish-brown solution, which shows a beautiful green fluorescence, is obtained ; this phenomenon is due to the formation of & fluorescein (p. 520). Other ?ft-dihydric phenols give this fluorescein reaction, which, therefore, affords a convenient and very delicate test for such compounds ; the fluorescein reaction may also be employed as a test for anhydrides of dicarboxylic acids. Resorcinol is used in large quantities in preparing fluorescein, eosin, and azo-dyes. Hydroquinone, or quinol, C 6 H 4 (OH) 2 , is formed, together with glucose, when the glucoside, arbutin a substance which occurs in the leaves of the bear-berry is boiled with water, C 12 H 16 7 + H 2 = C 6 H 4 (OH) 2 + C 6 H 12 6 . It is usually prepared by reducing quinone (p. 413) with sulphurous acid in aqueous solution, and then extracting with ether, C 6 H 4 2 + H 2 SO S + H 2 = C 6 H 4 (OH) 2 + H 2 S0 4 . It melts at 169, is readily soluble in water, and when treated with ferric chloride or other mild oxidising agents, it is con- verted into quinone, C 6 H 4 (OH) 2 + = C 6 H 4 2 + H 2 0. Trihydric Phenols. The three trihydric phenols, C 6 H 3 (OH) 3 , which should 400 PHENOLS. exist in accordance with theory, are all known, and are re- spectively represented by the following formulae : OH OH OH Pyrogallol, Phloroglucinol, Hydroxyhydroquinone 1:2: 3-Trihydroxybenzene. 1:3: 5-Trihydroxy benzene. 1:2: 4-Trihydroxybenzene. Pyrogallol, C 6 H 3 (OH) 3 , sometimes called pyrogallic acid, is prepared by heating gallic acid (p. 439) alone or with glycerol, at about 210, until the evolution of carbon dioxide ceases, C 6 H 2 (OH) 3 .COOH = C 6 H 3 (OH) 3 + C0 2 . It is a colourless, crystalline substance, melting at 115, and is readily soluble in water, but more sparingly in alcohol and ether (the effect of hydroxyl-groups) ; its aqueous - solution gives, with ferric chloride, a red, and with ferrous sulphate containing a trace of ferric chloride, a deep, dark- blue colouration. It dissolves freely in alkalies, giving solutions which rapidly absorb oxygen and turn black on exposure to the air, a fact which is made use of in gas analysis for the estimation of oxygen. Pyrogallol has power- ful reducing properties, and precipitates gold, silver, and mercury from solutions of their salts, being itself oxidised to oxalic and acetic acids ; many other phenols, such as catechol, resorcinol, and hydroquinone, show a similar behaviour, especially in alkaline solution, but the monohydric- compounds are much less readily oxidised, and consequently do not exhibit reducing properties. Pyrogallol and hydro- quinone are used in photography as developers. Like glycerol and other trihydric-compounds, pyrogallol forms mono-, di-, and tri-alkyl-derivatives, such as C 6 H 3 (OH) 2 .OC 2 H 5 , C 6 H 3 (OH)(OC 2 H 5 ) 2 , and C 6 H 3 (OC 2 H 5 ) 3 ; the dimethyl-derivative, C 6 H 3 (OCH 3 ) 2 -OH, occurs in beech- wood tar. PHENOLS. 401 Phloroglucinol, or symmetrical trihydroxy benzene, C 6 H 3 (OH) 3 , is produced when phenol, resorcinol, and many resinous substances, such as gamboge, dragon's-blood, &c., are fused with potash. It is best prepared by fusing resorcinol (1 part) with soda (6 parts) for about twenty-five minutes, or until the vigorous evolu- tion of hydrogen has ceased ; the chocolate-coloured melt is dis- solved in water, acidified with sulphuric acid, extracted with ether, the ethereal extract evaporated, and the residue recrystal- lised from water. It crystallises in colourless prisms, melts at about 218, and is very soluble in water ; the solution, which has a sweet taste, gives, with ferric chloride, a bluish-violet colouration, and when mixed with potash, it rapidly turns brown in con- tact with air owing to absorption of oxygen. When digested with acetyl chloride, phloroglucinol yields a triacetate, C 6 H 3 (C 2 H 3 9 ) 3 melting at 106, and in many other reactions it shows properties in harmony with the formula On the other hand, when treated with hydroxylamine, it gives a trioxime, C 6 H 6 (N-OH) 3 , and in this and other respects it behaves as though it were a triketone of the constitution o H 2 Possibly, therefore, phloroglucinol is capable of existing in two forms, which are convertible, the one into the other, by intramolecular change (part i. p. 195). Hydroxykydroquinone, or trihydroxybenzene, (1:2:4), is formed when hydroquinone is fused with potash. It melts at 140, and is very soluble in water, but its aqueous solution is coloured greenish- brown by ferric chloride, but on the addition of sodium carbonate the colour changes to blue and then to red (p. 389). Z AROMATIC ALCOHOLS, ETC. CHAPTER XXVII. AROMATIC ALCOHOLS, ALDEHYDES, KETONES, AND QUINONES. Alcohols. The aromatic alcohols are derived from the hydrocarbons by substituting hydroxy-groups for hydrogen atoms of the side- chain ; benzyl alcohol, C 6 H 5 -CH 2 -OH, for example, is derived from toluene, tolyl alcohol, C 6 H 4 (CH 3 ).CH 2 -OH, from xylene, and so on. The compounds of this kind have not been very fully investigated, but from what is known of their properties, it is clear that they are very closely related to the alcohols of the fatty series, although, of course, they show at the same time the general behaviour of aromatic substances. They may be prepared by methods exactly analogous to those employed in the case of the fatty alcohols namely, by heating the corresponding halogen derivatives with water, weak alkalies, or silver hydroxide, C 6 H 5 .CH 2 C1 + H 2 = C 6 H 5 .CH 2 .OH + HC1, and by reducing the corresponding aldehydes and ketones, C 6 H 5 .CH 2 .CHO + 2H = C H 5 .CH 2 .CH .OH C 6 H 5 .CO-CH 3 + 2H = C 6 H 6 .CH(OH).CH 3 . Those compounds which, like benzyl alcohol, contain the carbinol group, -CH 2 -OH, directly united with the benzene nucleus, may also be prepared by treating the corresponding aldehydes with potash (compare p. 408), 2C 6 H 5 .CHO + H 2 - C 6 H 5 .CH 2 .OH + C 6 H 5 -COOH. The aromatic alcohols are usually colourless liquids or solids, sparingly soluble in water ; their behaviour with alkali metals, phosphorus pentachloride, and acids, is similar to that of the fatty compounds, as will be seen from a consideration of the properties of benzyl alcohol, one of the few well-known aromatic alcohols. AROMATIC ALCOHOLS, ETC. 403 Benzyl alcohol, phenylcarbinol, or hydroxytoluene, C 6 H 5 .CH 2 -OH, an isomeride of the three cresols (p. 396), occurs in storax (a resin obtained from the tree Styrax officinalis), and also in balsam of Peru and balsam of Tolu, either in the free state or as ethereal salts in combination with cinnamic and benzoic acids. It may be obtained by reducing benzaldehyde (p. 405) with sodium amalgam, C 6 H 5 .CHO + 2H = C 6 H 5 .CH 2 .OH, and by boiling benzyl chloride with a solution of sodium carbonate, C 6 H 5 .CH 2 C1 + H 2 = C 6 H 5 .CH 2 .OH + HC1; but it is most conveniently prepared by treating benzaldehyde with cold potash, 2C 6 H 5 .CHO + H 2 = C 6 H 5 .CH 2 -OH + C 6 H 5 -COOH. The aldehyde (10 parts) is shaken with a solution of potash (9 parts) in water (10 parts) until the whole forms an emulsion, which is then allowed to stand for twenty-four hours ; after adding water to dissolve the potassium benzoate, the solution is extracted with ether, the ethereal extract evaporated, and the benzyl alcohol purified by distillation. Benzyl alcohol is a colourless liquid, boiling at 206 ; it is only sparingly soluble in water, but miscible with alcohol, ether, &c., in all proportions. It dissolves sodium and potassium with evolution of hydrogen, yielding metallic derivatives which are decomposed by water, and, when treated with phosphorus pentachloride, it is converted into benzyl chloride, C 6 H 5 -CH 2 .OH + PC1 5 = C 6 H 5 .CH 2 C1 + POC1 3 + HCL When heated with concentrated acids, or treated with anhydrides or acid chlorides, it gives ethereal salts; with hydrobromic acid, for example, it yields benzyl bromide, C 6 H 5 -CH 2 Br (b.p. 199), and with acetyl chloride or acetic anhydride it gives benzyl acetate, C 6 H 5 .CH 2 -()-CO-CH 3 (b.p. 404 AROMATIC ALCOHOLS, ETC. 206). On oxidation with dilute nitric acid, it is first converted into benzaldehyde and then into benzoic acid, C 6 H 5 -CH 2 .OH + = C 6 H 5 .CHO + H 2 C 6 H 5 .CH 2 -OH + 20 = C 6 H 5 -COOH + H 2 0. All these changes are strictly analogous to those undergone by the fatty alcohols. Saligenin, C 6 H 4 (OH)-CH 2> OH, also known as o-hydroxybenzyl alcohol, or salicyl alcohol, is an example of a substance which is both a phenol and an alcohol. It is produced by the action of dilute acids or ferments on salicin (a glucoside existing in the bark of the willow-tree), which breaks up into saligenin and dextrose, Ci 3 H 18 7 + H 2 O = C 6 H 4 which by the further action of the alkali is converted into a hydroxybenzaldehyde, just as benzalchloride, C 6 H 5 -CHC1 2 , is trans- formed into benzaldehyde (compare p. 406), As a rule, the primary product is the o-hydroxyaldehyde, small quantities of the corresponding jo-compound being produced at the same time. Salicylaldehyde, C 6 H 4 (OH)-CHO (o-hydroxybenzaldehydeW- may be obtained by oxidising saligenin with chromic acid (see above), but it is usually prepared from phenol by Reimer's reaction. Phenol (20 grams) is dissolved in soda (60 grams) and water (120 grams), the solution heated to 60 in a flask provided with a reflux condenser, and chloroform (30 grams) added in small quantities at a time from a dropping funnel. After slowly heating to boiling, the unchanged chloroform is distilled off, the alkaline liquid acidi- 410 AROMATIC ALCOHOLS, ETC. fied and distilled in steam, when a mixture of phenol and salicyl- aldehyde passes over. (The residue in the flask contains j>-hydroxy- benzaldehyde, which may be extracted from the filtered liquid with ether, and purified by recrystallisation.) The oily mixture is ex- tracted from the distillate with ether, and the extract shaken with dilute sodium bisulphite, which dissolves the aldehyde in the form of its bisulphite compound. The aqueous liquid is then separated, acidified, and the regenerated salicylaldehyde extracted with ether and purified by distillation. Salicylaldehyde is a colourless oil which boils at 196, and possesses a penetrating, aromatic odour ; it is moderately soluble in water, its solution giving a deep violet colouration on the addition of ferric chloride. When reduced with sodium amalgam, it yields saligenin, C 6 H 4 (OH)-CH 2 -OH (p. 404), whereas oxidising agents convert it into salicylic acid, C 6 H 4 (OH).COOH. p-Hydroxybenzaldehyde is crystalline, and melts at 116; it dissolves readily in hot water, and gives, with ferric chloride, a violet colouration. m-Hydroxybenzaldehyde is obtained from m-nitrobenzaldehyde by conversion into w-amidobenzaldehyde, and subsequent displace- ment of the amido-group by hydroxyl, by means of the diazo- reaction (p. 372). It crystallises from water in colourless needles, and melts at 104. Anisaldehyde, C 6 H 4 (OCH 3 )-CHO (^-methoxybenzaldehyde), is prepared from oil of aniseed. This ethereal oil contains anethole, C 6 H 4 (OCH 3 ).CH:CH-CH 3 , a crystalline substance which melts at 21 and distils at 232, and which on oxida- tion with potassium bichromate and sulphuric acid is con- verted into anisaldehyde, the propenyl group -CH:CH-CH 3 being oxidised to the aldehyde group. Synthetically, it may be prepared by digesting p-hydroxybenzaldehyde with alco- holic potash and methyl iodide, ACO group is transformed into ^C-OH, and not into ^>CH-OH, as might have been expected from analogy ; again, on treat- ment with phosphorus pentachloride, each oxygen atom is displaced by one atom of chlorine, >-dichlorobenzene, C 6 H 4 <^Qj, being formed, and not a tetrachloro-derivative, -amidophenol, C 6 H 4 (OH)-NH 2 , and ^-phenylenedi- amine, C 6 H 4 (NH 2 ) 2 , whereas o-toluidine, ^-toluylenediamine, C 6 H 4 (NH 2 ) 2 .CH 3 , [NH 2 :NH 2 :CH 3 = 1:4:6], &c., yield tolu- quinone. o * Other quinones, of a somewhat different class to benzoquinone, are described later (pp. 456, 470). 416 AROMATIC ALCOHOLS, ETC. When, however, bleaching-powder is used as the oxidising agent, quinone chlorimides and quinone dichlorodiimides are formed in the place of quinone, NH 2 .C 6 H 4 -OH + 4C1 = NC1:C 6 H 4 :0 + 3HC1 Quinone Chlorimide. NH 2 -C 6 H 4 .NH 2 + 6C1 = NC1:C 6 H 4 :NC1 + 4HC1. Quinone Dichlorodiimide. The quinone chlorimides and dichlorodiimides resemble quinone in many respects ; they are crystalline, readily volatile in steam, and are respectively converted into ^-amidophenol and #?-phenylenedi- amine or their derivatives on reduction. Chloranil, or tetrachloroquinone, O:C 6 C1 4 :O, is produced when chlorine acts on quinone, but it is usually prepared by treating phenol with hydrochloric acid and potassium chlorate, oxidation and chlorination taking place simultaneously, C 6 H 5 -OH + 10C1 + O = O:C 6 C1 4 :O + 6HC1. It crystallises in yellow plates, sublimes without melting, and is sparingly soluble in alcohol, and insoluble in water. It is readily reduced to tetrachlorohydroquirione, OH-C 6 C1 4 -OH, and is therefore a powerful oxidising agent, for which reason it is much employed in colour chemistry, when the use of inorganic oxidising agents is undesirable. CHAPTEK XXVIII. CARBOXYLIC ACIDS. The carboxylic acids of the aromatic series are derived from the aromatic hydrocarbons, just as those of the fatty series are derived from the paraffins namely, by the substitution of one or more carboxyl-groups for a corresponding number of hydrogen atoms. In this, as in other cases, however, one of two classes of compounds may he obtained according as substitution takes place in the nucleus or in the side-chain ; benzene yields, of course, only acids of the first class, such as benzoic acid, C 6 H 5 -COOH, the three (o.m.p.) phthalic acids, C 6 H 4 (COOH) 2 , the three tricarboxylic acids, C 6 H 3 (COOH) 3 , &c., but toluene and all the higher homologues may give CARBOXYLIC ACIDS. 417 rise to derivatives of both kinds as, for example, the three toluic acids, C 6 H 4 (CH 3 )-COOH, and phenylacetic acid, Although there are no very important differences in the properties of these two classes of acids, it is more convenient to describe them separately, taking first those compounds in which the carboxyl-groups are directly united with carbon of the nucleus. Preparation. Such acids may be obtained by oxidising the alcohols or aldehydes, C 6 H 5 -CH 2 -OH + 20 = C 6 H 5 .COOH + H 2 C 6 H 5 -CHO + = C 6 H 5 .COOH, and by hydrolysing the nitriles (p. 421) with alkalies or mineral acids, C 6 H 5 -C2s T + 2H = C 6 H 5 -COOH + NH 3 C 6 H 5 .CH 2 -CN + 2H 2 = C 6 H 5 -CH 2 .COOH + NH 3 , reactions which are exactly similar to those employed in the case of the fatty acids (part i. p. 165). Perhaps, however, the most important method, and one which has no counterpart in the fatty series, consists in oxidis- ing the homologues of benzene with dilute nitric acid or chromic acid, C 6 H 5 -CH 3 + 30 = C 6 H 5 -COOH + H 2 C 6 H B .CH 2 .CH 8 + 60 - C 6 H 5 -COOH + C0 2 + H 2 0. In this way only those acids which contain the carloxyl-group united with the nucleus can be obtained, because the side-chain is always oxidised to -COOH, no matter how many -CH 2 - groups it may contain ; in other words, all homologues of benzene which contain only one side-chain yield benzoic acid, whereas those containing two give one of the phthalic acids. In the latter case, however, one of the side-chains is oxidised before the other is attacked, so that by stopping the process at the right time, an alkyl-derivative of benzoic acid is obtained, C 6 H 4 (CH 3 ) 2 + 30 = C 6 H 4 (CH 3 ).COOH + H 2 C 6 H 4 (CH 3 ).COOH + 30 - C 6 H 4 (COOH), + H 2 0. 2 A 418 CARBOXYLIC ACIDS. Oxidation is frequently carried out by boiling the hydrocarbon (1 vol.) with nitric acid (1 vol.) diluted with water (2-4 vols.) until brown fumes are no longer formed. The mixture is then made slightly alkaline with soda, and any unchanged hydrocarbon and traces of nitro-hydrocarbon separated with a funnel or extracted with ether ; the alkaline solution is then acidified and the acid separated by filtration or extracted with ether, and purified by recrystallisation. Most hydrocarbons are only very slowly attacked by dilute nitric or chromic acid ; in such cases it is advantageous to first substitute chlorine or some other group for hydrogen of the side-chain, as in this way oxidation is facilitated. Benzyl chloride, C 6 H 5 -CH 2 C1, for example, is much more readily oxidised than toluene, wlfereas benzyl acetate, C 6 H 5 -CH 2 -OC 2 H 3 (p. 349), and benzyl ethyl ether, C 6 H 5 -CH 2 -O-C 2 H 5 , are even more readily attacked. . Properties. The aromatic acids are crystalline, and distil without decomposition ; they are sparingly soluble in cold water, but much more readily in hot water, alcohol, and ether. As regards all those properties which are determined by the carboxyl-group, the aromatic acids are closely analogous to the fatty compounds, and give corresponding derivatives, as the following examples show : Benzoicacid, C 6 H 5 .COOH Benzoyl chloride, C 6 H 5 -COC1. Sodium benzoate,C 6 H 5 -COONa Benzamide, C 6 H 5 -CO-NH 2 . Ethyl benzoate, C 6 H 5 -COOC 2 H 5 Benzoic anhydride, (C 6 H 5 -CO) 2 O. When distilled with lime, they are decomposed with loss of carbon dioxide and formation of the corresponding hydro- carbons, just as acetic acid under similar circumstances yields marsh-gas, C 6 H 5 -COOH = C 6 H 6 + C0 2 C 6 H 4 (CH 3 ).COOH = C 6 H 6 -CH 3 + C0 2 . Benzoic acid, C 6 H 5 -COOH, occurs in the free state in many resins, especially in gum benzoin and Peru balsam ; also in the urine of cows and horses, as hippuric acid or benzoyl- glycine, C 6 H 5 -CO-NH.CH 2 .COOH, to the extent of about two per cent. It is generally prepared either by the sublimation of gum CARBOXYLIC ACIDS. 419 benzoin in iron pots, the crude sublimate being purified by recry stall! sation from water, or by treating hippuric acid with hydrochloric acid (part i. p. 292), CeH5-CO.NH.CHg.COOH + HC1 + H 2 = C 6 H 5 -COOH + NH 2 .CH 2 .COOH, HC1. Glycine Hydrochloritle. The urine of horses, cows, or other herbivorous animals is evapor- ated to one-third of its volume, filtered, and acidified with hydro- chloric acid ; the crystals of hippuric acid which are deposited on standing, are collected and boiled for a short time with four parts of concentrated hydrochloric acid, the benzole acid which separates on cooling being purified by recrystallisation ; the mother-liquors contain glycine hydrochloride. Benzoic acid is manufactured by oxidising benzyl chloride (p. 348) with 60 per cent, nitric acid, C 6 H 5 .CH 2 C1 + 20 = C 6 H 5 -COOH + HC1. It may also be prepared by oxidising toluene, or by any other of the general methods. Benzoic acid separates from water in glistening crystals, melts at 120, and boils at 250, but it sublimes very readily even at 100, and is volatile in steam ; it dissolves in 400 parts of water at 15, but is readily soluble in hot water, alcohol, and ether. Its vapour has a characteristic odour, and an irritating action on the throat, causing violent coughing. Most of the metallic salts of benzoic acid are soluble in water and crystallise well; calcium benzoate, (C 6 H 5 -COO) 2 Ca + 3H 2 O, for example, prepared by neutralising benzoic acid with milk of lime, crystallises in needles, and is very soluble in water. The ethereal salts are prepared in precisely the same way as those of the fatty acids (part i. p. 187); ethyl benzoate, for example, C^Hg-COOCgHJ, is obtained by saturating an alcoholic solution of benzoic acid with hydrogen chloride, and after some time pouring the solution into water, the pre- cipitated oil being purified by fractional distillation. It boils at 211, has a pleasant aromatic odour, and is readily hydro- lysed by boiling alcoholic potash, 420 CARBOXYLIC ACIDS. Benzoyl chloride, C 6 H 5 -COC1, is obtained by treating benzoic acid with phosphorus pentachloride. It is a colourless oil, possessing a very irritating odour, and boils at 200 ; it is gradually decomposed by water, yielding benzoic acid and hydrochloric acid. Benzoic anhydride, (C 6 H 5 -CO) 2 0, is produced when benzoyl chloride is treated with sodium benzoate, just as acetic anhy- dride is formed by the interaction of acetyl chloride and sodium acetate (part i. p. 1 60) ; it is a crystalline substance, melting at 42, and closely resembles acetic anhydride in ordinary chemical properties. Benzoyl chloride and benzoic anhydride may be used for the detection of hydroxy-compounds, as they interact with all such substances (although not so readily as the corresponding derivatives of acetic acid, part i. p. 159), yielding benzoyl- derivatives, the monovalent benzoyl-grouip, C 6 H 5 -CO-, taking the place of the hydrogen of the hydroxyl-group, C 6 H 5 -OH + C 6 H 5 -COC1 - C 6 H 5 .O.CO.C 6 H 5 + HC1 Phenyl Benzoate. C 2 H 5 -OH + (C 6 H 5 -CO) 2 - C 2 H 5 .O.CO-C 6 H 5 + C 6 H 5 -COOH. Ethyl Benzoate. Benzoyl -derivatives may be prepared by heating the hydroxy- compovmd with benzoyl chloride or with benzoic anhydride. A more convenient method, however, and one which gives a purer product, is that of Baumann and Schotten : it consists in adding benzoyl chloride and 10 per cent, potash alternately, in small quantities at a time, to the hydroxy- com pound, which is either dissolved or suspended in water, the mixture being well shaken and kept cool during the operation. Potash alone is then added until the disagreeable smell of benzoyl chloride is no longer noticed, and the product finally separated by filtration or by extraction with ether. This method is also used in preparing benzoyl-derivatives of amido-compounds ; aniline, for example, yields benzoyl-aniline, C 6 H 5 -NH 2 + C 6 H 5 .COC1 = C 6 H 5 -NH.CO.C 6 H 5 + HC1. In the above method the alkali serves to neutralise the hydrochloric acid as fast as it is formed, the interaction taking place much more readily in the neutral or slightly alkaline solution. CARBOXYLIC ACIDS. 421 Benzamide, C 6 H 5 -CO-NH 2 , may be taken as an example of an aromatic amide ; it may be obtained by reactions similar to those employed in the case of acetamide (part i. p 162), as, for example, by treating ethyl benzoate with ammonia, C 6 H 5 .COOC 2 H 5 + NHg - C 6 H 6 .CO.NH 2 + C 2 H 5 -OH; but it is most conveniently prepared by triturating benzoyl chloride with dry ammonium carbonate in a mortar, and purifying the product by recrystallisation from water, 2C 6 H 5 .COC1 + (NH 4 ) 2 C0 8 = 2C 6 H 5 .CONH 2 + C0 2 + H 2 + 2HC1. It is a colourless, crystalline substance, melts at 130, and is sparingly soluble in cold, but readily soluble in hot, water ; like other amides, it is decomposed by boiling alkalies, yield- ing ammonia and an alkali salt, C 6 H 5 .CO-NH 2 + KOH - C 6 H 6 .COOK + NH 3 . Benzonitrile, or phenyl cyanide, C 6 H 5 -CN, may be obtained by treating benzamide with dehydrating agents, a method similar to that employed in the preparation of fatty nitriles, C 6 H 5 .CO-NH 2 = C 6 H 6 -CN + H 2 0. Although it cannot be prepared by treating chloro- or bromo- benzene with potassium cyanide (the halogen atom being so firmly held that no interaction occurs), it may be obtained by fusing benzenesulphonic acid with potassium cyanide (or with potassium ferrocyanide, which yields the cyanide), just as fatty nitriles may be prepared by heating the alkylsulphuric acids with potassium cyanide, C 6 H 5 .S0 8 K + KCN = C 6 H 5 -CN + K 2 S0 3 C 2 H 5 -S0 4 K + KCN = C 2 H 5 -CN + K 2 S0 4 . It is, however, most conveniently prepared from aniline by Sandmeyer's reaction namely, by treating a solution of diazo- benzene chloride with potassium cyanide and copper sulphate (P- 372), C 6 H 6 -N 2 C1 + KCN = C 6 H 6 -ClSr + KC1 + N 2 . 422 OARBOXYLIC ACIDS. Benzonitrile is a colourless oil, boiling at 191, and smells like nitrobenzene. It undergoes changes exactly similar to tbose which are characteristic of fatty nitriles, being converted into the corresponding acid on hydrolysis with alkalies or mineral acids, C 6 H 5 -CK + 2H 2 = C 6 H 5 -COOH + NH 3 , and into a primary amine on reduction, C 6 H 5 .ON + 4H = C 6 H 5 .CH 2 -NH 2 . Benzylamine. Other aromatic nitriles, such as the three tolunitriles, C 6 H 4 (CH 3 )-CN, are known, also compounds such as phenyl- acetonitrile (benzyl cyanide, p. 429), C 6 H 5 -CH 2 -CN, which contain the cyanogen group in the side-chain. Substitution Products of Benzoic Acid. Benzoic acid is attacked by halogens (although not so readily as the hydro- carbons), the product consisting of the ?ftefa-derivative (p. 351); when, for example, benzoic acid is heated with bromine and water at 125, m-bromobenzoic acid, C 6 H 4 Br-COOH (m.p. 1 55), is formed. The o- and ^>-bromobenzoic acids are obtained by oxidising the corresponding bromotoluenes with nitric acid; the former melts at 148, the latter at 251. Nitric acid, in the presence of sulphuric -acid, acts readily on benzoic acid, ?n-nitrobenzoic acid, C 6 H 4 (N0 2 )-COOH (m.p. 142), being the principal product; o-nitrobenzoic acid (m.p. 147) and ^9-nitrobenzoic acid (m.p. 240) are obtained by the oxida- tion of o- and p-nitrotoluene respectively (p. 355) ; when these acids are reduced with tin and hydrochloric acid, they yield the corresponding amidobenzoic acids, C 6 H 4 (NH 2 )-COOH, which, like glycine (part i. p. 292), form salts both with acids and bases. When heated with sulphuric acid, benzoic acid is converted into w-sulphobenzoic acid, C 6 H 4 (S0 3 H)-COOH, small quantities of the ^-acid also being produced. The o-acid is obtained by oxidising toluene-o-sulphonic acid ; when treated with ammonia it yields an imide (p. 426), + 2H 3 0, CARBOXYLIC ACIDS. 423 which is remarkable for possessing an exceedingly sweet taste, and which conies into the market under the name of saccharin. The sulphobenzoic acids are very soluble in water ; when fused with potash they yield hydroxy-acids (p. 433), just as benzene- sulphonic acid gives phenol, C 6 H 4 (S0 3 K).COOK + 2KOH = C 6 H 4 (OK).COOK + K 2 S0 3 + H 2 O. The three (o.m.p.) toiuic acids, C 6 H 4 (CH 3 )-COOH, may be produced by oxidising the corresponding xylenes with dilute nitric acid, C 6 H 4 (CH 3 ) 2 + 30 = C 6 H 4 (CH 3 ).COOH + H 2 0, but the o- and ^?-acids are best prepared by converting the corresponding toluidines into the nitriles by Sandmeyer's reaction (p. 372), and then hydrolysing with acids or alkalies, As m-toltiidine cannot easily be obtained, and as w-xylene is only very slowly oxidised by dilute nitric acid, in order to pre- pare ??i-toluic acid, ?w-xylyl bromide, C 6 H 4 (CH 3 )-CH 2 Br (b.p. 215), is first prepared by adding bromine (1 mol.) to boiling w-xylene (1 mol.) ; this product is then heated with sodium ethoxide, in alcoholic solution, to convert it into m-xylyl ethyl ether, C 6 H 4 (CH 3 ).CH 2 .0-C 2 H 5 (b.p. 204), a substance which is readily oxidised by potassium bichromate and sulphuric acid (p. 418), yielding m-toluic acid. The three o-, ra-, p-toluic acids melt at 103, 110, and 180 respectively, and resemble benzoic acid very closely, but since they contain a methyl- group, they have also properties which are not shown by benzoic acid ; on oxidation, for example, they are converted into the corresponding phthalic acids, just as toluene is trans- formed into benzoic acid, 30 = CH Dibasic Acids. The most important dicarboxylic acids are the three 424 CABBOXYLIC ACIDS. (o.m.p.) phthalic acids, or benzenedicarboxylic acids, which are represented by the formulae, COOH COOH \ ^iCOOH COOH ^^^ COOH COOH Phthalic Acid. Isophthalic Acid. Terephthalic Acid. These compounds may be prepared by the oxidation of the corresponding dimethylbenzenes with dilute nitric acid, or more conveniently by treating the toluic acids with potassium permanganate in alkaline solution, Cecils + 60 = c ABa, obtained as a white precipitate by adding barium chloride to a neutral solution of the ammonium salt, is very sparingly soluble in water. Ethyl phthalate, C 6 H 4 (COOC 2 H 5 ) 2 , is readily prepared by saturating an alcoholic solution of phthalic acid (or its anhy- dride) with hydrogen chloride. It is a colourless liquid, boiling at 295. Phthalyl chloride, C 6 H 4 (COC1) 2 , is prepared by heating phthalic anhydride (1 mol.) with phosphorus pentachloride (1 mol.). It is a colourless oil, which boils at 275, and is slowly decomposed by water, with regeneration of phthalic acid. In many of its reactions it behaves as if it had the constitution represented by the formula C 6 H 4 <^QQ^>0 (compare succinyl chloride, part i. p. 237). Phthalic anhydride, C 6 H 4 <^^>0, is formed when phthalic acid is distilled. It sublimes readily in long needles, melts at 128, boils at 284, and is only very gradually decomposed by water, but dissolves readily in alkalies, yielding salts of phthalic acid. When heated in a stream of ammonia /"i/-\ it is converted into phthalimide, C 6 H 4 NH, a sub- stance which melts at 229, and yields a potassium derivative, on treatment with alcoholic potash. There is thus a great similarity between phthalimide and succini- mide (part i. p. 237). Isophthalic acid, C 6 H 4 (COOH) 2 (benzene-m-dicarboxylic acid), is produced by oxidising m-xylene or ?w-xylyl diethyl ether, C 6 H 4 (CH 2 -OC 2 H 5 ) 2 (compare p. 418), with nitric acid or chromic acid ; or from ??i-toluic acid (p. 423) by oxidation with potassium permanganate in alkaline solution. CARBOXYLIC ACIDS. 427 Tt crystallises in needles, melts above 300, and when strongly heated, sublimes unchanged; it is very sparingly soluble in water. Methyl isophthalate, C 6 H 4 (COOCH 3 ) 2 , melts at 65 c Terephthalic acid, C 6 H 4 (COOH) 2 (benzene-^-dicarboxylic acid), is formed by the oxidation of jp-xylene, p-toluic acid, and of all di-alkyl substitution-derivatives of benzene, which, like cymene, CH 3 -C 6 H 4 -CH(CH 3 )2, contain the alkyl-groups in the ^-position. It is best prepared by oxidising j>-toluic acid (p. 423) in alkaline solution with potassium permanganate. Terephthalic acid is almost insoluble in water, and, when heated, sublimes without melting ; the methyl salt, C 6 H 4 (COOCH 3 ) 2 , melts at 140. Acids, such as isophthalic acid and terephthalic acid, which have no definite melting-point, or which melt above 300, are best identified by conversion into their methyl salts, which generally crystallise well, and melt at a comparatively low temperature. For this purpose a centigram of the acid is warmed in a test tube with about three times its weight of phosphorus pentachloride, and the clear solution, which now contains the chloride of the acid, poured into excess of methyl alcohol. As soon as the vigorous reaction has subsided, the liquid is diluted with water, the crude methyl salt collected, recrystallised, and its melting-point deter- mined. Phenylacetic Acid, Phenylpropionic Acid, and their Derivatives. Many cases have already been met with in which aromatic compounds have been found to have certain properties similar to those of members of the fatty series, and it has been pointed out that this is due to the presence in the former of groups of atoms (side-chains) which may be considered as fatty radicles ; benzyl chloride, for example, has some properties in common with methyl chloride, benzyl alcohol with methyl alcohol, benzylamine with methylamine, and so on, simply because similar groups or radicles in a similar state of combin- ation confer, as a rule, similar properties on the compounds 428 CARBOXYLIC ACIDS. in which they occur. Inasmuch, however, as nearly all fatty compounds may theoretically be converted into aromatic com- pounds of the same type by the substitution of a phenyl group for hydrogen, it follows that any series of fatty compounds may have its counterpart in the aromatic group. This is well illustrated in the case of the carboxylic acids, because, corresponding with the fatty acids, there is a series of aromatic acids which may be regarded as derived from them in the manner just mentioned : Formic acid, H-COOH, Benzoic acid, C 6 H 5 -COOH (phenylformic acid). Acetic acid, CH 3 .COOH, Phenylacetic acid, C 6 H 5 -CH 2 .COOH. Propionic acid, CH 3 .CH 2 .COOH, Phenylpropionic acid, C 6 H 5 .CH 2 -CH 2 .COOH. Butyric acid, CH 3 -CH 2 .CH 2 .COOH, Phenylbutyric acid, C 6 H 5 .CH 2 .CH 2 -CH 2 .COOH. With the exception of benzoic acid all the above aromatic acids are derived from the aromatic hydrocarbons by the sub- stitution of carboxyl for hydrogen of the side-chain. They have not only the characteristic properties of aromatic com- pounds in general, but also those of fatty acids, and, like the latter, they may be converted into unsaturated compounds by loss of two or more atoms of hydrogen, giving rise to new series, as the following example will show : Propionic acid, CH 3 .CH 2 -COOH, Phenylpropionic acid, C 6 H 5 -CH 2 .CH 2 .COOH Acrylic acid, CH 2 :CH.COOH, Phenylacrylic acid, C 6 H 5 .CH:CH-COOH. Propiolic acid, CHiC-COOH, Phenylpropiolic acid, C 6 H 5 -CiC-COOH. Preparation. Aromatic acids, containing the carboxyl- group in the side-chain, may be prepared by carefully oxidis- ing the corresponding alcohols and aldehydes, and by hydro- lysing the nitriles with alkalies or mineral acids, C 6 H 5 .CH 2 -CN + 2H 2 = C 6 H 5 .CH 2 .COOH + NH 8 , CARBOXYLIC ACIDS. 429 but these methods are Hunted in application, owing to the difficulty of obtaining the requisite substances. The most important general methods are : (a) By the reduc- tion of the corresponding unsaturated acids, compounds which are prepared without much difficulty (p. 430), C 6 H 5 .CH:CH-COOH + 2H = C 6 H 5 -CH 2 .CH 2 .COOH ; and (b) by treating the sodium compound of ethyl malonate or of ethyl acetoacetate with the halogen derivatives of the aromatic hydrocarbons. As, in the latter case, the pro- cedure is exactly similar to that employed in preparing fatty acids (part i. pp. 189, 194, and 198), one example only need be given namely, the synthesis of phenylpropionic acid. The sodium compound of ethyl malonate is heated with benzyl chloride, and the ethyl benzylmalonate which is thus produced, C 6 H 6 .CH,C1 + CH^a(COOC 2 H 5 ) 2 = C 6 H 5 .CH 2 .CH(COOC 2 H 5 ) 2 + NaCl, Ethyl Benzylmalonate. is hydrolysed with alcoholic potash. The benzylmalonic acid is then isolated, and heated at 200, when it is converted into phenylpropionic acid, with loss of carbon dioxide, C 6 H 5 .CH 2 .CH(COOH) 2 - C 6 H 5 .CH 2 -CH 2 .COOH + C0 2 . It should be remembered that only those halogen derivatives in which the halogen is in the side-chain can be employed in sucli syntheses, because when the halogen is united with the nucleus, as in monochlorotoluene, C 6 H 4 C1-CH 3 , for example, no action takes place (compare p. 346). The properties of two of the most typical acids of this class are described below. Phenylacetic acid, or a-toluic acid, C 6 H 5 .CH 2 -COOH, is pre- pared by boiling a solution of benzyl chloride (1 mol.) and potassium cyanide (1 mol.) in dilute alcohol for about three hours ; the benzyl cyanide which is thus formed is purified 430 CARBOXYLIG ACIDS. by fractional distillation, and the fraction 220-235 (benzyl cyanide boils at 232) is hydrolysed by boiling with dilute sulphuric acid, the product being purified by recrystallisation from water, C 6 H 5 .CH 2 C1 > C 6 H 5 .CH 2 .CN > C 6 H 6 .CH 2 .COOH. Phenylacetic acid melts at 76-5, boils at 262, and crystallises from boiling water in glistening plates ; it has an agreeable, characteristic smell, and forms salts and derivatives just as do benzoic and acetic acids. When oxidised with chromic acid it yields benzoic acid, a change very different to that undergone by the isomeric toluic acids (p. 423), C 6 H 5 .CH 2 .COOH + 30 = C 6 H 5 -COOH + C0 2 + H 2 0, Phenylpropionic acid, C 6 H 5 .CH 2 -CH 2 .COOH (hydrocinn- amic acid), is most conveniently prepared by reducing cinnamic acid (see below) with sodium amalgam, C 6 H 6 .CH:CH.COOH + 2H = C 6 H 5 .CH 2 .CH 2 .COOH. Synthetically, it may be obtained from the product of the action of benzyl chloride on the sodium compound of ethyl malonate (p. 429). It crystallises from water in needles, melts at 47, and distils at 280 without decomposi- tion. Cinnamic acid, or phenylacrylic acid, C 6 H 5 -CH;CH.COOH, is closely related to phenylpropionic acid, and is one of the best-known unsaturated acids of the aromatic series. It occurs in large quantities in storax (Styrax officinalis), and may be easily obtained from this resin by warming it with soda ; the filtered aqueous solution of sodium cinnamate is then acidified with hydrochloric acid, and the precipitated cinnamic acid purified by recrystallisation from boiling water. Cinnamic acid is usually prepared synthetically by heating benzaldehyde with acetic anhydride and anhydrous CARBOXYLIC ACIDS. 431 sodium acetate, a process of condensation which is most simply expressed by the equation, C 6 H 5 -CHO + CH 3 -COOH - C 6 H 5 -CH:CH.COOH + H 2 0. A mixture of benzaldehyde (3 parts), acetic anhydride (10 parts), and anhydrous sodium acetate (3 parts) is heated to boiling in a flask placed in an oil-bath. After about eight hours the mixture is poured into water, and distilled in steam to separate the unchanged benzaldehyde ; the residue is then treated with caustic soda, the hot alkaline solution filtered from oily and tarry impurities, and acidified with hydrochloric acid, the precipitated cinnamic acid being purified by recrystallisation from boiling water. This method (Perkin's reaction) is a general one for the prepara- tion of unsaturated aromatic acids, as by employing the anhydrides and sodium salts of other fatty acids, homologues of cinnamic acid are obtained. When, for example, benzaldehyde is treated with sodium propionate and propionic anhydride, phenylmethylacrylic acid (a.-methylcinnamic acid), C 6 H 5 -CH:C(CH 3 )-COOH, is formed ; phenylisocrotonic acid, C 6 H 5 -CH:CH-CH 2 -COOH, is not obtained by this reaction, because condensation always takes place between the aldehyde oxygen atom and the hydrogen atoms of that -CH 2 - group, which is directly united with the carboxyl-radicle. Phenylisocrotonic acid may, however, be prepared by heating benzaldehyde with a mixture of sodium succinate and succinic anhydride, C 6 H 5 .CHO + COOH.CH .CH,.COOH = C 6 H 5 .CH:CH-CH 2 .COOH + CO 2 + H 2 O. It is a colourless, crystalline substance, which melts at 86, and boils at 302 ; at its boiling-point it is gradually converted into a-naphthol and water (p. 453). Cinnamic acid crystallises from water in needles, and melts at 133. Its chemical behaviour is in many respects similar to that of acrylic acid and other unsaturated fatty acids ; it combines directly with bromine, for example, yielding phenyl aft-dibromopropionic acid, C 6 H 5 -CHBr-CHBr-COOH, and with hydrobromic acid, giving plieHyl-fl-bromopropionic acid, C 6 H 5 -CHBr.CH 2 .COOH ; on reduction with sodium amalgam it is converted into phenyl propionic acid (p. 430), just as acrylic acid is transformed into propionic acid. 432 CARBOXYLIC ACIDS. When distilled with lime, cinnamic acid is decomposed into carbon dioxide, and phenylethylene or styrolene* just as benzoic acid yields benzene, C 6 H 5 -CH:CH.COOH = C 6 H 5 -CH:CH 2 + C0 2 . Concentrated nitric acid converts cinnamic acid into a mix- ture of about equal quantities of o- and p-nitrocinnamic acids, C 6 H 4 (N0 2 ).CH:CH-COOH, which may be separated by con- version into their ethyl salts, C 6 H 4 (N0 2 ).CH:CH-COOC 2 H 5 (fry means of alcohol and hydrogen chloride), and recrystallis- ing these from alcohol, the sparingly soluble ethyl salt of the ^;-acid being readily separated from the readily soluble ethyl o-nitrocinnamate. From the pure ethyl salts the acids are then regenerated by hydrolysing with dilute sulphuric acid. They resemble cinnamic acid closely in properties, and combine directly with bromine, yielding the corresponding nitrophenyl- dibromopropionic acids, C 6 H 4 (N0 2 )-CHBr.CHBr-COOH. Phenylpropiolic acid, C 6 H 5 -C :C-COOH, is obtained by treating phenyldibromopropionic acid, or, better, its ethyl salt, with alcoholic potash, C 6 H 5 -CHBi-.CHBr.COOH = C 6 H 5 .C:C-COOH + 2HBr, a method which is exactly similar to that employed in preparing acetylene by the action of alcoholic potash on ethylene dibromide. It melts at 137, and at higher temperatures, or when heated with water at 120, it decomposes into carbon dioxide andphenylacetylene, a colourless liquid, which boils at 140, and is closely related to acetylene in chemical properties, C 6 H 5 -C IC-COOH = C 6 H 5 .C!CH + CO 2 . o-Nitrophenylpropiolic acid, C 6 H 4 (NO. 2 )-C:C-COOH, maybe simi- larly prepared from o-nitroplienylilibromopropiouic acid; it is a substance of great interest, as when treated with reducing agents, * Styrolene, C 6 H 5 -CH:CH 2 , may be taken as a typical example of an aromatic hydrocarbon containing an unsaturated side-chain. It is a colourless liquid which boils at 145, and in chemical properties shows the closest resemblance to ethylene, of which it is the phenyl substitution product. With bromine, for example, it yields a dibromadditive product, C 6 H 5 -CHBr-CH 2 Br (dibromethylbenzene), and when heated with hydriodic acid, it is reduced to ethylbenzene, CARBOXYLIC ACIDS. 433 such as hydrogen sulphide, or grape-sugar and potash, it is con- verted into indigo blue (Baeyer), 2C 6 H 4 < COOH + 4H = C 16 H 10 N 2 2 + 2C0 2 + 2H 2 O. This method of preparation, however, is not of technical value, owing to the high price of phenylpropiolic acid. CHAPTER XXIX. HYDROXYCARBOXYLIC ACIDS. The hydroxy-acids of the aromatic series are derived from benzole acid and its homologues, by the substitution of hydroxyl-groups for hydrogen atoms, just as glycollic acid, for example, is derived from acetic acid (part i. p. 225) ; like the simple hydroxy-derivatives of the hydrocarbons, they may be divided into two classes, according as the hydroxyl-group is united with carbon of the nucleus or of the side-chain. In the first case the hydroxyl-group has the same character as in phenols, and consequently hydroxy-acids, of this class, as, for example, the three (o.m.p.) hydroxyberizoic acids, C 6 H 4 (OH)-COOH, are both phenols and carboxylic acids; in the second case, however, the hydroxyl-group has the same character as in alcohols, so that the compounds of this class, such as mandelic acid, C 6 H 5 -CH(OH)-COOH, have properties closely resembling those of the fatty hydroxy-acids ; in other words, the differences between the two classes of aromatic hydroxy-acids are practically the same as those between phenols and alcohols. As those acids, which contain the hydroxyl-group united with carbon of the nucleus, form by far the more important class, the following statements refer to them only, except where stated to the contrary. Preparation. The hydroxy-acids may be prepared from the simple carboxylic acids, by reactions exactly similar to those employed in the preparation of phenols from hydro- 2B 434 HYDROXYCARBOXYLIC ACIDS. carbons ; that is to say, the acids are converted into nitro- compounds, then into amido-compounds, and the latter are treated with nitrous acid in the usual manner, r TT rorm _ * r TT XCOOH r xCOOH U 6 J 5 ^U< UgH^-^-Q 6 4 /COOH or, the acids are heated with sulphuric acid, and the sulphonic acids obtained in this way are fused with potash, C 6 H 5 .COOH *C 6 1 It must be borne in mind, however, that as the carboxyl- group of the acid determines the position taken up by the nitro- and sulphonic-groups (p. 352), only the mete-hydroxy- compounds are conveniently prepared in this way directly from the carboxylic acids. The or^o-hydroxy-acids, and in some cases the meta- and para-compounds, are most conveniently prepared from the phenols by one of the following methods : The dry sodium compound of the phenol is heated at about 200 in a stream of carbon dioxide, 2C 6 H 6 .ONa + C0 2 = C 6 H 4 + C 6 H 5 -OH. Under these conditions half the phenol distils over and is re- covered ; but if the sodium compound be first saturated with carbon dioxide under pressure, it is converted into an aromatic deriva- tive of carbonic acid, which, when heated at about 130 under pressure, is completely transformed into a salt of the hydroxy-acid by molecular change, C 6 H 5 -ONa + CO, = C 6 H 5 .0-COONa = C 6 H 4 <^ ONa Sodium Phenyl carbonate. Many dihydric and trihydric phenols may be converted into the corresponding hydroxy-acids, simply by heating them with ammonium carbonate or potassium bicarbonate ; when resorcinol, for example, is treated in this way, it yields a mixture of isomeric resorcylic acids, C 6 H 3 (OH) 2 -COOH. HYPROXYCARBOXYLIC ACIDS. 435 The second general method of preparing hydroxy-acids from phenols consists in boiling a strongly alkaline solution of the phenol with carbon tetrachloride ; the principal product is the ort?u>-&cid, but varying proportions of the para-acid are also formed, CC1 4 + 5NaOH - After the substances have been heated together for some hours, the unchanged carbon tetrachloride is distilled off, the residue acidified, and the solution extracted with ether ; the crude acid, obtained on evaporating the ethereal solution, is then separated from unchanged phenol by dissolving it in sodium carbonate, re- precipitated with a mineral acid, and purified by recrystallisation. The above method is clearly analogous to Reimer's reaction (p. 409), and the changes which occur during the process may be assumed to be indicated by the following equations, in which water is represented instead of soda for the sake of simplicity : C 6 H 5 .OH + CC1 4 . C 6 H 4 3 + HC1 3HC1 Properties. The hydroxy-acids are colourless, crystalline substances, more readily soluble in water and less volatile than the acids from which they are derived ; many of them undergo decomposition on distillation, carbon dioxide being evolved ; when heated with lime they are completely decom- posed, with formation of phenols, C 6 H 4Ba, are obtained in a similar manner, employing excess of the metallic hydroxides; with the exception of the salts of the alkali metals, these di-metallic compounds are insoluble ; they are all decomposed by carbon dioxide, with formation of the mono-metallic salts, Methyl salicylate, C 6 H 4 (OH).COOCH 3 , prepared in the manner described (p. 436), or by distilling a mixture of salicylic acid, methyl alcohol, and sulphuric acid (part i. p. 188), is an agreeably-smelling oil, boiling at 224 ; ethyl salicylate, C 6 H 4 (OH)-COOC 2 H 5 , boils at 223. Methyl methylsalicylate, C 6 H 4 (OCH 3 )-COOCH 3 , is formed when methyl salicylate is heated with methyl iodide and alcoholic potash (1 niol.) ; it is an oil boiling at 228. Methylsalicylic acid, C 6 H 4 (OCH 3 )-COOH, is obtained when its methyl salt is hydrolysed with potash ; it is a crystalline substance, melting at 98-5, and when heated with hydriodic acid it is de- composed, giving salicylic acid and methyl iodide; the other halogen acids have a similar action. m-Hydroxybenzoic acid is prepared by fusing m-sulphoberrzoic acid with potash, and also by the action of nitrous acid on w-amidobenzoic acid (p. 422). It melts at 200, gives no coloura- tion with ferric chloride, and when distilled with lime it is decom- posed into phenol and carbon dioxide. p-Hydroxybenzoic acid is formed, together with salicylic acid, by the action of carbon tetrachloride and soda on phenol ; it may also be obtained from ;?-sulphobenzoic acid by fusion with potash, or by the action of nitrous acid on jt?-amidoberizoic acid. It is prepared by heating potassium phenate in a stream of carbon HYDROXYCARBOXYLIC ACIDS. 439 dioxide at 220 as long as phenol distils over; if, however, the temperature be kept below 150, potassium salicylate is formed ; the residue is dissolved in water, the acid precipitated from the filtered solution by adding hydrochloric acid, and purified by re- crystallisation from water. ^-Hydroxybenzoic acid melts at 210, and is completely decomposed on distillation into phenol and carbon dioxide ; its aqueous solution gives no colouration with ferric chloride. Anisic acid, >-methoxybenzoic acid, C 6 H 4 (OCH 3 )-COOH,is obtained by oxidising anethole, C 6 H 4 (OCH 3 ).CH:CH-CH 3 (the principal constituent of oil of aniseed) with chromic acid, when the group -CH:CH-CH 3 is converted into -COOH (p. 410) ; it may also be prepared from j9-hydroxybenzoic acid by a series of reactions analogous to those employed in the formation of methylsalicylic acid from salicylic acid (see above). Anisic acid melts at 185, and when distilled with lime it is decom- posed, with formation of anisole (p. 392) ; when heated with fuming hydriodic acid, it yields >-hydroxybenzoic acid and methyl iodide. There are six dihydroxylenzvic acids, C 6 H 3 (OH) 2 -COOH, two of which are derived from catechol, three from resorcinol, and one from hydroquinone ; the most important of these is protocatechuic acid, [OH:OH:COOH - 1:2:4], one of the two isomeric catecholcarboxylic acids. This compound is formed on fusing many resins, such as catechu and gum benzoin, and also certain alkaloids, with potash, and it may be prepared synthetically by heating catechol with water and ammonium carbonate r.t 140. It crystallises from water, in which it is very soluble, in needles, melts at 199, and when strongly heated it is decom- posed into catechol and carbon dioxide ; its aqueous solution gives with ferric chloride a green solution, which becomes violet and then red on the addition of sodium bicarbonate. Gallic acid, or pyrogallolcarboxylic acid, C 6 H 2 (OH) 3 -COOH [OH:OH:OH:COOH = 1:2:3:5], is a trihydroxybenzoic acid ; it occurs in gall-nuts, tea, and 440 HYDROXYCARBOXYLIC ACIDS. many other vegetable products, and is best prepared by boiling tannin (see below) with dilute acids. It crystallises in needles, and melts at 220, being at the same time resolved into pyrogallol (p. 400) and carbon dioxide ; it is readily soluble in water, and its aqueous solution gives with ferric chloride a bluish-black precipitate. Gallic acid is a strong reducing agent, and precipitates gold, silver, and platinum from solutions of their salts. Tannin, digallic acid, or tannic acid, C 14 H 10 9 , occurs in large quantities in gall-nuts, and in all kinds of bark, from which it may be extracted with boiling water. It is an almost colourless, amorphous substance, and is readily soluble in water ; its solution possesses a very astringent taste, and gives with ferric chloride an intense dark-blue solution, for which reason tannin is largely used in the manufacture of inks. When boiled with dilute sulphuric acid, tannin is completely converted into gallic acid, a fact which shows that it is the anhydride of this acid, C 14 H 10 9 + H 2 = 2C 7 H 6 5 . Tannin is used largely in dyeing as a mordant, owing to its property of forming insoluble coloured compounds with many dyes. It is also extensively employed in ' tanning ;' when animal skin or membrane is placed in a solution of tannin, or in contact with moist bark containing tannin, it absorbs and combines with the tannin, and is converted into a much tougher material; such tanned skins constitute leather. Mandelic acid, C 6 H 5 .CH(OH)-COOH (phenylglycollic acid), is an example of an aromatic hydroxy-acid containing the hydroxyl-group in the side-chain. It may be obtained by boiling amygdalin (which yields benzaldehyde, hydrogen cyanide, and glucose, p. 405) with hydrochloric acid, but it is usually prepared by treating benzaldehyde with hydrocyanic acid and hydrolysing the resulting hydroxycyanide, a method HYDROXYCARBOXYLIC ACIDS. 441 analogous to that employed in the synthesis of lactic acid from aldehyde (part i. p. 139), C 6 H 5 -CHO + HCN = C 6 C 6 H 5 .CH(OH).CN + 2H 2 = C 6 H 5 -CH(OH).COOH + NH 3 . Mandelic acid melts at 133, is moderately soluble in water, and shows in many respects the greatest resemblance to lactic acid (methylglycollic acid) ; when heated with hydriodic acid, for example, it is reduced to phenylacetic acid (p. 429), just as lactic acid is reduced to propionic acid (part i. p. 227), C 6 H 5 .CH(OH).COOH + 2HI - C 6 H 5 .CH 2 -COOH + I 2 + H 2 O. The character of the hydroxyl-group in mandelic acid is, in fact, quite similar to that of the hydroxyl-group in the fatty hydroxy-acids and in the alcohols, so that there are many points of difference between mandelic acid and acids, such as salicylic acid, which contain the hydroxyl-group united with carbon of the nucleus ; when, for example, ethyl mandelate, C 6 H 5 -CH(OH).COOC 2 H 5 , is treated with caustic alkalies, it does not yield an alkali derivative, although the hydrogen of the hydroxyl-group is displaced on treating with sodium or potassium. Mandelic acid, like lactic acid, contains an asymmetric carbon atom (p. 533), and can, therefore, exist in three optically different forms. The synthetical acid is optically inactive that is to say, it is a mixture of the dextro- and levo- rotatory acids, but the acid prepared from amygdalin is levo- rotatory. The dextro-rotatory acid may be obtained by growing the organism Penicttiium glaucum in a solution of the inactive acid under suitable conditions, when the levo- rotatory acid is destroyed, the dextro-rotatory acid remaining (p. 544). 442 NAPHTHALENE AND ITS DERIVATIVES. CHAPTER XXX. NAPHTHALENE AND ITS DERIVATIVES. All the aromatic hydrocarbons hitherto described, with the exception of diphenyl, diphenylmethane, and triphenylmethane (p. 340), contain only one closed-chain of six carbon atoms, and are very closely and directly related to benzene ; most of them may be prepared from benzene by comparatively simple reactions, and reconverted into this hydrocarbon, perhaps even more readily, so that they may all be classed as simple benzene derivatives. The exceptions just mentioned are also, strictly speaking, derivatives of benzene, although at the same time they may be regarded as hydrocarbons of quite another class, since diphenyl and diphenylmethane contain two, and triphenylmethane three, closed-chains of six carbon atoms. There are, in fact, numerous classes or types of aromatic hydrocarbons, and, just as benzene is the first member of a homologous series and the parent substance of a vast number of derivatives, so also these other hydrocarbons form the starting-points of new homologous series and of derivatives of a different type. The hydrocarbons naphthalene and anthracene, which are now to be described, are perhaps second only to benzene in importance ; each forms the starting-point of a great number of compounds, many of which are extensively employed in the manufacture of dyes. Naphthalene, C 10 H 8 , occurs in coal-tar in larger quantities than any other hydrocarbon, and is easily isolated from this source in a pure condition ; the crystals of crude naphthalene, which are deposited on cooling from the fraction of coal-tar passing over between 170 and 230 (p. 297), are first pressed to get rid of liquid impurities, and then warmed with a small quantity of concentrated sulphuric acid, which converts most of the foreign substances into non-volatile sulphonic acids; NAPHTHALENE AND ITS DERIVATIVES. 443 the naphthalene is then distilled in steam, or sublimed, and is thus obtained almost chemically pure. Naphthalene crystallises in large, lustrous plates, melts at 80, and boils at 218. It has a highly characteristic smell, and is extraordinarily volatile, considering its high molecular weight, so much so, in fact, that only part of the naphtha- lene in crude coal-gas is deposited in the condensers (p. 295), the rest being carried forward into the purifiers, and even into the gas-mains, in which it is deposited in crystals in cold weather, principally at the bends of the pipes, frequently causing stoppages. It is insoluble in water, but dissolves freely in hot alcohol and ether, from either of which it may be crystallised. Like many other aromatic hydrocarbons, it combines with picric acid, when the two substances are dissolved together in alcohol, forming naphthalene picrate, a yellow crystalline compound of the composition, C 10 H 8 ,C 6 H 2 (N0 2 ) 3 .OH, which melts at 149. As the vapour of naphthalene burns with a highly luminous flame, the hydrocarbon is used to some extent for carburetting coal-gas that is to say, for increasing its illuminating power ; for this purpose the gas is passed through a vessel which contains coarsely-powdered naphthalene, gently heated by the gas flame, so that the hydrocarbon volatilises and burns with the gas. The principal use of naphthalene, however, is for the manufacture of a number of derivatives which are employed in the colour industry. Constitution. Naphthalene has the characteristic properties of an aromatic compound that is to say, its behaviour under various conditions is similar to that of benzene and its derivatives, and different from that of fatty compounds ; when treated with nitric acid, for example, it yields a nitro-derivative, and with sulphuric acid it gives sulphonic acids. This similarity between benzene and naphthalene at once suggests a resemblance in constitution, a view which is 444 NAPHTHALENE AND ITS DERIVATIVES. confirmed by the fact that naphthalene, like benzene, is a very stable substance, and is resolved into simpler substances only with difficulty. When, however, naphthalene is boiled with dilute nitric or chromic acid, it is slowly oxidised, yielding carbon dioxide and (or^o)-phthalic acid, C 6 H 4 (COOH) 2 . Now the formation of phthalic acid in this way is a fact of very great importance, since it is a proof that naphthalene contains the group, C that is to say, that it contains a benzene nucleus to which two carbon atoms are united in the ortho-position to one another. Nevertheless, further evidence is required in order to arrive at the constitution of the hydrocarbon, since there are still two carbon and four hydrogen atoms to be accounted for, and there are many different ways in which these might be united with the C 6 H 4 XQ group. Clearly, therefore, it is important to ascertain the structure of that part of the naphthalene molecule which has been oxi- dised to carbon dioxide and water to obtain, if possible, some simple decomposition product in which these carbon and hydro- gen atoms are retained in their original state of combination. Now this can be done in the following way : When nitronaphthalene, C 10 H 7 -N0 2 , a simple mono-substitution product of the hydrocarbon, is boiled with dilute nitric acid, it yields nitrophthalic acid, C 6 H 3 (N0 2 )(COOH) 2 ; therefore, again, naphthalene contains a benzene nucleus, and the nitro- group in nitronaphthalene is combined with this nucleus. If, however, the same nitronaphthalene be reduced to amido- naphthalene, C 10 H 7 -NH 2 , and the latter oxidised, phthalic acid (and not amidophthalic acid) is obtained ; this fact can only be explained by assuming either that the benzene nucleus, which is known to be united with the amido-group, has been NAPHTHALENE AND ITS DERIVATIVES. 445 destroyed, or that the amido-group has been displaced by hydrogen during oxidation. Since, however, the latter alternative is contrary to all experience, the former must be accepted, and it is clear that the benzene nucleus which is contained in the oxidation product of amidonaphthalene is not the same as that present in the oxidation product of nitronaphthalene ; in other words, different parts of the naphthalene molecule have been oxidised to carbon dioxide and water in the two cases, and yet in both the group remains. The constitution of naphthalene must therefore be expressed by the formula CH CH JX CH This will be evident if the above changes be expressed with the aid of this formula. When nitronaphthalene is oxidised, the nucleus B (see below), which does not contain the nitro-group, is destroyed, as indicated by the dotted lines, the product being nitrophthalic acid; when, on the other hand, amidonaphthalene is oxidised, the nucleus A, combined with the amido-group, is attacked and destroyed in preference to the other, and phthalic acid is formed, N0 2 N0 2 COOH Naphthalene. NH 2 Nitronaphthalene. COOH COOH Nitrophthalic Acid. COOH" Plithalic Acid. Amidonaphthalene. The constitution of naphthalene was first established in this 446 NAPHTHALENE AND ITS DERIVATIVES. way by Graebe in 1880, although the above formula had been suggested by Erlenmeyer as early as 1866; that the hydrocarbon is composed of two benzene nuclei partially super- posed or condensed together in the o-position, as shown above, has since been confirmed by syntheses of its derivatives, but even more conclusively by the study of the isomerism of its substitution products. The difficulty of determining and of expressing the actual state or disposition of the fourth affinity of each of the carbon atoms in naphthalene is just as great as in the case of benzene. If the carbon atoms be represented as united by alternate double Unkings, as in the formula on the left-hand side (see below), there is the objection that they do not show, as indicated, the behaviour of carbon atoms in fatty unsatu rated compounds, as explained more fully in the case of benzene. For this reason the formula on the right-hand side (see below) has been suggested as perhaps prefer- able, the lines drawn towards the centres of the nuclei having the same significance as in the centric formula for benzene (p. 307). The simple, double-hexagon formula given above is usually em- ployed for the sake of convenience. Naphthalene may be obtained synthetically by passing the vapour of phenylbutylene, C 6 H 5 .CH 2 .CH 2 -CH:CH 2 * (or of pheriylbutylene dibromide, C 6 H 5 -CH 2 .CH 2 .CfeBr.CH 2 Br), over red-hot lime, the change being a process of destructive distillation, accompanied by loss of hydrogen, similar to, but much simpler than that which occurs in the formation of other aromatic from fatty hydrocarbons (p. 300), /CH:CH C 6 H 5 .CH 2 .CH 2 .CH:CH 2 - C 6 H 4 y Fittig, who showed that a-naphthol (a-hydroxy- naphthalene) is formed on boiling phenylsscrotonic acid (p. 431) with water. This change probably takes place in two stages, the first product being a keto-derivative of naphtha- lene, which passes into a-naphthol by intramolecular change (compare part i. p. 195), CH CH C(OH) The a-naphthol thus obtained is converted into naphtha- lene on distillation with zinc-dust, just as phenol is trans- formed into benzene (p. 331). Isomerism of Naphthalene Derivatives. As in the case of benzene, the study of the isomerism of the substitution pro- ducts of naphthalene affords the most convincing evidence that the accepted constitutional formula is correct. In the first place, naphthalene differs from benzene in yielding two different series of mono-substitution products ; there are, for example, two monochloronaphthalenes, two monohydroxy- naphthalenes, two mononitronaphthalenes, &c. This fact is readily accounted for, as, on considering the constitutional formula of naphthalene, which may be conveniently written or numbered or lettered as shown (the symbols C and H being omitted for the sake of simplicity), it will be evident that the eight hydrogen atoms are not all similarly situated relatively 448 NAPHTHALENE AND ITS DERIVATIVES. to the rest of the molecule. If, for example, the hydrogen atom (1) were displaced by chlorine, hydroxyl, &c., the substi- tution product would be isomeric, but not identical with that produced by the displacement of the hydrogen atom (2). In the first case, the substituting atom or group would be united with a carbon atom which is itself directly united with a carbon atom common to both nuclei, whereas in the other case this would not be so. Clearly, then, the fact that the mono- substitution products of naphthalene exist in two isomeric forms is in accordance with the above constitutional formula. Further, it will be seen that not more than two such isomerides could be obtained, because the positions 1.4.1 '.4' (the four a-positions) are identical, and so also are the positions 2.3.2'.3' (the four /^-positions) ; the isomeric mono-substitution products are, therefore, usually distinguished by using the letters a and ft. When two hydrogen atoms in naphthalene are displaced by two identical groups or atoms, ten isomeric di-derivatives may be obtained. Denoting the positions of the substitueiits by the system of numbering shown above, these isomerides would be 1:2, 1:3, 1:4, 1:4', 1:3', 1:2', 1:1', 2:3, 2:3', 2:2', all other possible positions being identical with one of these ; 2:4', for example, is the same as 1:3', 2': 4 and 3:1', and l':4 is identical with 1 : 4'. The constitution of such a di-derivative is usually expressed with the aid of numbers, as it is necessary to show whether the substituents are combined with the same, or with different, nuclei. When the two atoms or groups are present in the same nucleus, their relative position is similar to the 0-, m-, or ^-position in benzene. The positions 1:2, 2:3, and 3:4 corre- spond with the ortho-, 1 : 3, and 2 : 4, with the meta-, and 1 : 4 with the para-position, and similarly in the case of the other nucleus. The position 1:1' or 4:4', however, is different from any of these, and is termed the peri-position ; groups thus situated behave in much the same way as those in the o-position in the benzene and naphthalene nuclei. NAPHTHALENE AND ITS DERIVATIVES. 449 Derivatives of Naphthalene. The homologues of naphthalene that is to say, its alkyl substitution products, are of comparatively little importance, but it may be mentioned that they may be prepared from the parent hydrocarbon by methods similar to those employed in the case of the corresponding benzene derivatives, as, for example, by treating naphthalene with alkyl halogen com- pounds and aluminium chloride, C 10 H 8 + C 2 H 5 I = C 10 H..C 2 H 5 + HI, and by treating the bromonaphthalenes with an alkyl halogen compound and sodium, C 10 H 7 Br + CH 3 Br + 2Na = C 10 H r CH s + 2NaBr. a-Methylnaphthalene, C 10 H r -CH 3 , is a colourless liquid, boiling at 240-242, but /3-methylnaphthalene is a solid, melts at 32, and boils at 242 ; both these hydrocarbons occur in coal-tar. The halogen mono -substitution products of naphthalene are also of little importance. They may be obtained by treating the hydrocarbon, at its boiling-point, with the halogens (chlor- ine and bromine), but only the a-derivatives are formed in this way. Both the a- and the /^-compounds may be obtained by treating the corresponding naphthols (p. 452), or, better, the naphthalenesulphonic acids (p. 455) with pentachloride or pentabromide of phosphorus, C 10 H 7 .S0 2 C1 + PC1 5 - C 10 H 7 C1 + POC1 3 + SOC1 2 , or by converting the naphthylamines (p. 452) into the corre- sponding diazo-compounds, and decomposing the latter with a halogen cuprous salt (p. 372), C 10 H 7 -NH 2 > C 10 H r .N:NCl > C 10 H 7 C1. All these methods correspond with those described in the case of the halogen derivatives of benzene, and are carried out practically in a similar manner. a-Chloronaphthalene, C 10 H-C1, is a liquid, boiling at about '2 C 450 NAPHTHALENE AND ITS DERIVATIVES. 263, but the ^-derivative is a crystalline substance, melting at 56, and boiling at 264. a-Bromonaphthalene, C 10 H^Br, is also a liquid, which boils at 280, but the ^-derivative is crystalline, and melts at 68. The chemical properties of these, and of other halogen derivatives of naphthalene, are similar to those of the halogen derivatives of benzene ; the halogen atoms are very firmly combined, and are not displaced by hydroxyl-groups on boiling with alkalies, &c. Naphthalene tetrachloride, C 10 H 8 C1 4 , is an important halo- gen additive product, which is produced on passing chlorine into a vessel containing coarsely-powdered naphthalene at ordinary temperatures. It forms large colourless crystals, melts at 182, and is converted into dichloronaphtlialene C 10 H 6 C1 2 (a substitution product), when heated with alco- holic potash ; it is readily oxidised by nitric acid, yielding phthalic and oxalic acids, a fact which shows that all the chlorine atoms are present in one and the same nucleus ; the constitution of the compound is therefore expressed by the t i P TI x-CHCl-CHCl>. formula C 6 H 4 < CH cl.CHCi> The formation of this additive product shows tli.it naphthalene, like benzene, is not really a saturated compound, although it usually behaves as such ; other compounds, formed by the addition of four atoms of hydrogen to naphthalene or to a naphthalene derivative, are known, and experience has shown that when one of the nuclei is thus fully reduced, the atoms or groups of which it is composed acquire the character which they have in fatty compounds, whereas the unreduced nucleus retains the character of that in benzene. The amido-group in the tetrahydro-fi-naphthylamine of the consti- /s-., tution C fi H 4 < ", for example, has the same character as X CH 2 -CH 2 that in fatty amines, whereas in the case of the isomeric tetrahydro- /CH 2 .CH 2 fi-naphthylamine, NH 2 -C 6 H 3 \ " | , the amido-group has the CH 2 -CH 2 same properties as that in aniline, because it is combined with the unreduced nucleus. NAPHTHALENE AND ITS DERIVATIVES. 451 Nitro- derivatives. Naphthalene, like benzene, is readily acted on by concentrated nitric acid, yielding nitro-derivatives, one, two, or more atoms of hydrogen being displaced accord- ing to the concentration of the acid employed and the temperature at which the reaction is carried out ; the presence of sulphuric acid facilitates nitration for reasons already mentioned. The chemical properties of the nitro-naphthalenes are in all respects similar to those of the nitre-benzenes. a-Nitronaphthalene, C 10 HK-N0 2 , is best prepared in small quantities by dissolving naphthalene in acetic acid, adding concentrated nitric acid, and then heating on a water-bath for half an hour; the product is poured into water, and the nitronaphthalene purified by recrystallis- ation from alcohol. On the large scale it is prepared by treating naphthalene with nitric and sulphuric acids, the method being similar to that employed in the case of nitro- benzene. It crystallises in yellow prisms, melts at 61, and boils at 304 ; on oxidation with nitric acid, it yields nitro- phthalic acid (p. 445). ^-Nitronaphthalene is not formed on nitrating naphthalene, but it may be prepared by dissolving /3-nitro-a-naphthylarnine (a compound obtained on treating a-naphthylamine with dilute nitric acid) in an alcoholic solution of hydrogen chloride, adding finely-divided sodium nitrite, and then heating the solution of the diazo-compound (compare p. 371), C 10 H 6 (N0 9 ).N:NC1 + C H 5 -OH -= C 10 H r -:N T 2 + N 2 + HC1 + C 2 H 4 0. It crystallises in yellow needles, melting at 79. The amido-derivatives of naphthalene are very similar in properties to the corresponding benzene derivatives, except that even-the monamido-compounds are crystalline solids ; they have a neutral reaction to litmus, and yet are distinctly basic in character, since they neutralise acids, forming salts, which, however, are decomposed by the hydroxides and carbonates of the alkalies. These amido-compounds, moreover, may be 452 NAPHTHALENE AND ITS DERIVATIVES. converted into diazo-com pounds, amidoazo-compounds, &c., by reactions similar to those employed in the case of the amido-benzenes, and many of the substances obtained in this way, as well as the amido-compounds themselves, are exten- sively employed in the manufacture of dyes. a-Naphthylamine, C 10 H 7 -]S T H 2 , may be obtained by heating a-naphthol with ammonio-zinc chloride, or ammonio-calcium chloride,* C 10 H r OH + NH 3 = C 10 H r ira 2 + H 2 0, but it is best prepared by reducing a-nitronaphthalene with iron-filings and acetic acid, It is a colourless, crystalline substance, melting at 50, and boiling at 300 ; it has a disagreeable smell, turns red on exposure to the air, and its salts give a blue precipitate with ferric chloride and other oxidising agents. On oxidation with a boiling solution of chromic acid, it is first converted into a-naphthaquinone (p. 455), and then into phthalic acid. /2-Naphthylamine is not prepared from /3-nitronaphthalene (as this substance is itself only obtained with difficulty), but from /3-naphthol, as described in the case of the a-compound. It crystallises in colourless plates, melts at 112, and boils at 294 ; it differs markedly from a-naphthylamine in having only a faint odour, and its salts give no colouration with ferric chloride. On oxidation with potassium permanganate, it yields phthalic acid. The two naphthols, or monohydroxy-derivatives of naphthalene, correspond with the monohydric phenols, and * Prepared by passing ammonia over anhydrous zinc or calcium chloride. These compounds decompose when heated, evolving ammonia, and are, therefore, conveniently employed in many reactions requiring the pres- ence of ammonia at high temperatures ; the zinc or calcium chloride resulting from their decomposition also favours the reaction in those cases in which water is formed, as both substances are powerful dehydrating agents. Ammonium acetate may be employed for a similar purpose, as it dissociates at comparatively low temperatures, but its action is less energetic. NAPHTHALENE AND ITS DERIVATIVES. 453 are compounds of considerable importance, as they are exten- sively employed in the colour industry. They both occur in coal-tar, but only in small quantities, and are, therefore, prepared either by diazotising the corresponding naphthyl- amines, C 10 H r XH 2 * C lo H r N:NCl - C 10 H r OH, or by fusing the corresponding sulphonic acids with potash (compare p. 387), C 10 H r S0 8 K + KOH . C 10 H 7 -OH + K 2 S0 8 . Their properties are, on the whole, very similar to those of the phenols, and, like the latter, they dissolve in alkalies, yield- ing metallic derivatives, which are decomposed by carbon dioxide ; the hydrogen of the hydroxyl-group in the naph- thols may also be displaced by an acetyl-group or by an alkyl- group, just as in phenols, and on treatment with pentachloride or pentabromide of phosphorus, a halogen atom is substituted for the hydroxyl-group. The naphthols further resemble the phenols in giving a colour reaction with ferric chloride. In a few respects, however, there are certain differences between the chemical properties of the naphthols and phenols, inasmuch as the hydroxyl-groups in the former more readily undergo change ; when, for example, anaphthol is heated with ammonio-zinc chloride at 250, it is converted into the corresponding amido-compound (see above), whereas the conversion of phenol into aniline requires a temperature of 300-350, other conditions remaining the same. Again, when a naphthol is heated with an alcohol and hydrogen chloride, it is converted into an alkyl-derivative, whereas alkyl- derivatives of phenols cannot, as a rule, be obtained in this way ; in this respect, the naphthols form, as it were, a connecting-link between the phenols and the alcohols. a-Naphthol, C 10 H 7 -OH, is formed, as previously stated (p. 447), on boiling phenylisocrotonic acid with water, an important synthesis, which proves that the hydroxyl-group is in the a-position ; it is prepared from a-naphthylamine or from naphthalene-a-sulphonic acid (see above). It is a colourless, crystalline substance, melting at 94, and boiling at 280 ; it has a faint smell, recalling that of phenol, and it dissolves 454 NAPHTHALENE AND ITS DERIVATIVES. freely in alcohol and ether, but is only sparingly soluble in hot water. Its aqueous solution gives with ferric chloride a violet, flocculent precipitate, consisting probably of an iron compound of a-dinaphthol, OH-C 10 H 6 -C 10 H 6 -OH, an oxida- tion product of the naphthol. a-Naphthol, like phenol, is very readily acted on by nitric acid, yielding a r^'mYro-derivative, C 10 H 5 (N0 2 ) 2 -OH, which crystallises in yellow needles, and melts at 138; this nitro- compound, like picric acid, has a much more strongly marked acid character than the hydroxy-compound from which it is derived, and decomposes carbonates, forming deep- yellow salts which dye silk a beautiful golden yellow; its sodium derivative, C 10 H 5 (N0 2 ) 2 -ONa + H 2 0, is known commercially as Martins' yellow, or naplitlialene yellow. Another dye obtained from a-naphthol is naphthol yellow (p. 527), the potassium salt of dinitro-a-naphtholsulphonic acid, C 10 H 4 (N0 2 ) 2 (OK)-S0 3 K; the acid itself is manufactured by nitrating a-naphtholtri- sulphonic acid (prepared by heating a-naphthol with anhydrosulphuric acid), in which process two of the sulphonic groups are displaced by nitro-groups. /^-Naphthol, prepared by fusing naphthalene-/3-sul phonic acid with potash (p. 453), melts at 122, and boils at 285 ; it is a colourless, crystalline compound, readily soluble in hot water, and like the a-derivative, it has a faint phenol-like smell. Its aqueous solution gives, with ferric chloride, a green colouration and a flocculent precipitate of /3-dinaphthol, OH.C 10 H 6 .C 10 H 6 .OH. Sulphonic Acids. Perhaps the most important derivatives of naphthalene, from a commercial point of view, 'are the various mono- and di-sulphonic acids, which are obtained from the hydrocarbon itself, from the naphthyla mines, and from the naphthols, many of these compounds being used in large quantities in the manufacture of dyes. It would be impossible to give here even the names of the very numerous compounds of this class, but some indication of their properties may be afforded by the following statements : NAPHTHALENE AND ITS DERIVATIVES. 455 Naphthalene is readily sulplionated, yielding two mono- sul phonic acids, C 10 H 7 -S0 3 H, namely, the a- and /3-com pounds, both of which are formed when the hydrocarbon is heated with concentrated sulphuric acid at 80 ; if, however, the operation be carried out at 160, only the /3-acid is obtained, because at this temperature the a-acid is converted into the /3-acid by intramolecular change, just as phenol-o-sulphoriic acid is transformed into the p-acid by heating. The two naphthalenesulphonic acids are crystalline hygroscopic sub- stances, and show all the characteristic properties of acids of this class. Di- and tri-sulphonic acids may be obtained by strongly heating naphthalene with sulphuric or anhydrosulphuric acid. Fourteen isomeric naphthyla?ninemonosidpJtonic acids, C 10 H 6 (XH 2 )-S0 3 H, may theoretically be obtained namely, seven from a-naphthylamine, and seven from the /3-base ; as a matter of fact, nearly all these acids are known. One of the most important, perhaps, is l:4-naphthylaminemonosulphonic acid, or naphthionic acid, which is the sole product of the action of sulphuric acid on a-naphthylamine ; it is a crystalline compound, very sparingly soluble in cold water, and is used in the manufacture of Congo- red (p. 526), and other dyes. The naphtholmonosulphonic acids correspond in number with the naphthylaminemonosulphonic acids, and are also extensively used in the colour industry. a-Naphthaquinone, C 10 H 6 2 , is a derivative of naphthalene corresponding with (benzo)quinone, and, like the latter, it is formed on oxidising various mono- and di-substitution products of the hydrocarbon with sodium bichromate and sulphuric acid, but only those in which the substituting groups occupy thea-positions; a-naphthylamine, 1 : 4-amidonaphthol, and 1:4- diamidonaphthalene, for example, may be employed. As a rule, however, naphthalene itself is oxidised with a boiling solution of chromic acid in acetic acid (a method not applicable for the preparation of quinone from benzene), as the product is then easily obtained in a state of purity. 456 NAPHTHALENE AND ITS DERIVATIVES. a-Naphthaquinone crystallises from alcohol in deep- yellow needles, melting at 125; it resembles quinone in colour, in having a curious pungent smell, and in being very volatile, subliming readily even at 100, and distilling rapidly in steam. Like quinone, moreover, it is readily re- duced by sulphurous acid, yielding 1 : 4-dihydroxynaphthalene, C 10 H 6 (OH) 2 , just as quinone yields hydroquinoue (p. 399). This close similarity in properties clearly points to a similarity in constitution, so that a-naphthaquinone may be represented by the formula, for reasons similar to those stated more fully in the case of quinone. /?-Naphthaquinone, C 10 H 6 2 , isomeric with the a-compound, is formed when a-amido-/?-naphthol is oxidised with potassium bichromate and dilute sulphuric acid, or with ferric chloride ; it crystallises in red needles, decomposes at about 115 without melting, and on reduction with sulphur- ous acid, is converted into 1 : 2-dihydroxynaphthalene. It differs from a-naphthaquinone and from quinone in colour, in having no smell, and in being non-volatile, properties which, though apparently insignificant, are really of some importance, as showing the difference between orfho-quiuoues and jpara-quinones ; the latter are generally deep-yellow, volatile compounds, having a pungent odour, whereas the former are red, non-volatile, and odourless. /3-Naphtha- quinone is an example of an ortho-quinone, and its consti- tution may be represented by the formula, The above description of some of the more important NAPHTHALENE AND ITS DERIVATIVES. 457 naphthalene derivatives will be sufficient to show the close relationship which these compounds bear to the corresponding derivatives of benzene ; although the former exist in a larger number of isomeric forms, they are, as a rule, prepared by the same methods as their analogues of the benzene series, and resemble them closely in chemical properties. It may, in fact, be stated as a general rule, that all general reactions and generic properties of benzene derivatives are met with again in studying naphthalene derivatives. CHAPTER XXXI. ANTHRACENE AND PHENANTHRENE. Anthracene, C 14 H 10 , is a hydrocarbon of great commercial importance, as it is the starting-point in the manufacture of alizarin, the colouring matter employed in producing Turkey- red dye ; it is prepared exclusively from coal-tar. The crude mixture of hydrocarbons and other substances known as '50 per cent, anthracene ' (p. 298) is first distilled with one-third of its weight of potash from an iron retort ; the distillate, which consists almost entirely of anthracene and phenanthrene, is then treated with carbon bisulphide, when the phenanthrene dissolves, leaving the anthracene, which is further purified by crystallisation from benzene. Crude anthracene contains considerable quantities of carbazole, /NH, a colourless, crystalline substance, melting at 238, and PR/ L 6 n 4 boiling at 355. On treatment with potash, this substance is C e H 4 \ converted into a potassium derivative, I /NK, which remains P TT / U 6 rL 4 in the retort, or is decomposed on subsequent distillation ; many other impurities, which cannot readily be separated by crystallisa- tion, are also got rid of in this way. Anthracene crystallises from benzene in colourless, lustrous 458 ANTHRACENE AND PHENANTHRENE. plates, which show a beautiful blue fluorescence ; it melts at 213, boils at about 3GO, and dissolves freely in boiling benzene, but is only sparingly soluble in alcohol and ether. On mixing saturated alcoholic solutions of anthracene and picric acid, anthracene picrate, C 14 H 10 ,C 6 H 2 N"0 2 ) 3 -OH, is deposited in ruby-red needles, which melt at 138; this compound is resolved into its components when treated with a large quantity of alcohol (distinction from phenanthrene picrate, p. 468). Constitution. The behaviour of anthracene towards chlorine and bromine is, on the whole, similar to that of benzene and naphthalene that is to say, it yields additive or substitution products according to the conditions employed ; towards concentrated sulphuric acid, also, it behaves like other aromatic compounds, and is converted into sulphonic acids by substitu- tion. "When treated with nitric acid, however, instead of yielding a nitro-derivative, as was to be expected from the molecular formula of the hydrocarbon (which, from the relatively small proportion of hydrogen, clearly indicates the presence of one or more closed chains), it is oxidised to anthra- quinone, C 14 H 8 2 , two atoms of hydrogen being displaced by two atoms of oxygen ; this change always takes place, even when dilute nitric acid, or some other oxidising agent, is employed, and as it is closely analogous to that which occurs in the conversion of naphthalene, C 10 H 8 , into a-naphthaquinone, C 10 H 6 2 (p. 455), it is an indication of the presence of a closed- chain, oxidation processes of this kind (namely, the substitu- tion of oxygen for an equal number of hydrogen atoms) being unknown in the case of fatty (open-chain) substances. Another highly important fact, owing to its bearing on the constitution of anthracene, is this, that, although the hydro- carbon and most of its derivatives are resolved into simpler substances only with very great difficulty, when this does occur, one of the products is always some benzene derivative, usually phthalic acid. Now, if the molecule of anthracene contained only one ANTHRACENE AND PHENANTHRENE. 459 benzene nucleus, or even if, like naphthalene, it contained two condensed nuclei, there would still be certain carbon and hydrogen atoms to be accounted for, and this could only be done by assuming the presence of unsaturated side- chains ; as, however, all experience has shown that such side-chains in benzene and in naphthalene are oxidised to carboxyl (compare p. 327) with the utmost facility, it is impos- sible to accept the assumption of their presence in anthracene, a compound which is always oxidised to the neutral substance anthraquinone, without loss of carbon. Arguments of this kind lead, therefore, to only one conclusion namely, that the molecule of anthracene is composed only of combined or condensed nuclei ; as, moreover, the hydrocarbon may be indirectly converted into phthalic acid, it must be assumed that two of these nuclei are condensed together in the o-position, as in naphthalene. If, now, an attempt be made to deduce a constitutional formula for anthracene on this basis, and it be further assumed that all the closed-chains are composed of six carbon atoms, as in naphthalene, the following formulae suggest them- selves as the most probable, CH CH CH CH 1 - v^ , i , -s^ ^ -^w CH ' II. although, of course, neither could be accepted as final without further evidence. Experience has shown, however, that formula i. must be taken as representing the constitution of anthracene (formula n. expressing that of phenanthrene, p. 468), because it accounts satisfactorily for all known facts, amongst others, for a number of important syntheses of the hydrocarbon (see below), for the 460 ANTHRACENE AND PHENANTHRENE. relation of anthracene to anthraquinone, and for the isomerism of the anthracene derivatives. It is, nevertheless, just as diffi- cult to determine and to express the actual disposition of the fourth affinity of each carbon atom in anthracene, as in the cases of benzene and naphthalene ; as, however, there are reasons for supposing that the state of combination of the two central CH groups (that is, those which form part of the central nucleus only) is different from that of all the others (inasmuch as they are generally attacked first), and that the two carbon atoms of these groups are directly united, the above formula (i.) is usually written / CH \ or C 6 H 4 C 6 H 4 , ' XCH/ the disposition of the fourth affinities of the carbon atoms in the two C 6 H 4 C 6 H 4 + C li H 5 .CH 3 + 3HCI, Oil the hydranthracene (p. 461) which is formed as an interme- diate product, 4 + 2HC1 > being converted into anthracene by loss of hydrogen, which reduces part of the benzyl chloride to toluene, as shown in the first equation. Anthracene is also formed, together with hydranthracene and phenanthrene (p. 469), when 0?*/7j0-bromo- * The letters or numbers serve to denote the constitution of the anthra- cene derivatives (p. 461). ANTHRACENE AND PHENANTHRENE. 461 benzyl bromide (prepared by brominating boiling o-bromo- toluene, C 6 H 4 Br-CH 3 ) is treated with sodium, here, again, hydranthracene is the primary product, and from it anthracene is formed by loss of hydrogen. Another interesting synthesis may be mentioned namely, the formation of anthracene on treating a mixture of tetra- bromethane and benzene with aluminum chloride, H Br( ? HBr 4. H ^r P r H /? H V fl H^ H 4 = C 6 H */ 6 4 BrCHBr All these methods of formation are accounted for in a simple manner with the aid of the above constitutional formula, the last one especially indicating that the two central carbon atoms are directly united; the formula C 6 H 4 <^ i^ ~^>C 6 H 4 CH will, therefore, be employed in describing the anthracene derivatives. Isomerism of Anthracene Derivatives. Further evidence in support of the above constitutional formula is afforded by the study of the isomerism of the substitution products of anthracene, although, in most cases, all the isomerides theo- retically possible have not yet been prepared. When one atom of hydrogen is displaced, three isomerides may be obtained, since there are three hydrogen atoms (a,/3,y), all of which are differently situated relatively to the rest of the molecule; these mono-substitution products are usually distinguished by the letters a, /?, y, according to the position of the substituent (compare formula p. 460). When two atoms of hydrogen are displaced by similar atoms or groups, fifteen isomeric di-substitution products may be obtained. Hydranthracene, C 6 H 4 <^>C 6 H 4 , a substance of little importance, is formed on reducing anthracene with boiling 462 ANTHRACENE AND PHENANTHRENE. concentrated hydriodic acid, or with sodium amalgam. It is a colourless, crystalline compound, melting at 106-108, and when heated with sulphuric acid, it is converted into anthracene, the acid being reduced to sulphur dioxide. Anthracene dicliloride, C 6 H 4 < \TT,^>C 6 H 4 , like hydran- thracene, is an additive product of the hydrocarbon ; it is obtained when chlorine is passed into a cold solution of anthracene in carbon bisulphide, whereas at 100 substitution CC1 takes place, monochloranthracene, C 6 H 4 <^ I ^/CgH^, and CH dichloranthracene, C 6 H 4 <^ I ^CgH^, being formed ; these substitution products crystallise in yellow needles, melting at 103 and 209 respectively, and they are both converted into anthraquinone on oxidation, a fact which shows the positions of the chlorine atoms. Anthraquinone, C 6 H 4 <^ />C 6 H 4 , is formed, as already mentioned, on oxidising anthracene with chromic or nitric acid. It is conveniently prepared by dissolving anthracene (1 part) in boiling glacial acetic acid, and gradually adding a concentrated solution of chromic acid (2 parts) in glacial acetic acid. As soon as oxidation is complete, the product is allowed to cool, and the anthraquinone, which separates in long needles, is collected and purified either by sublimation or by recrystallisation from acetic acid. Anthraquinone is manufactured by oxidising finely-divided '50 per cent, anthracene,' suspended in water, with the calculated quantity of sodium bichromate and sulphuric acid. The crude anthraquinone is collected on a filter, washed, dried, and heated at 100 with 2-3 parts of concentrated sulphuric acid, by which means the impurities are converted into soluble sulphonic acids, whereas the anthraquinone is not acted on. The almost black product is now allowed to stand in a damp place, when the anthra- quinone gradually separates in crystals as the sulphuric acid ANTHRACENE AND PHENANTHRENE. 463 becomes dilute ; water is then added, and the anthraquinone col- lected, washed, and dried and sublimed. Anthraquinone may be produced synthetically by treating a solution of phthalic anhydride (p. 426) in benzene, with a strong dehydrating agent, such as aluminium chloride, the reaction taking place in two stages ; o-benzoylbenzoic acid is first produced, o-Benzoylbenzoic Acid. but by the further action of the aluminium chloride (or when treated with sulphuric acid), this substance is converted into anthraquinone with loss of 1 molecule of water, A BAB Anthraquinone contains, therefore, two C 6 H 4 <^ groups, united by two C0<^ groups. That the two C0<^ groups occupy the o-position in the one benzene ring (A) is known, because they do so in phthalic acid ; that they occupy the o-position in the second benzene ring (B) has been proved, as follows : When bromophthalic anhydride is treated with benzene and aluminium chloride, bromobenzoylbenzoic acid is produced, and this, when treated with sulphuric acid, yields bromanthraquinone, C 6 H 3 BC 6 H 4 + H 2 O. A BAB The formation of this substance from bromophthalic acid proves, as before, that the two CO\ groups are united to the ring A in the o-position. Now, when bromanthraquinone is heated with potash at 160, it is converted into hydroxyanthraquinone, C 6 H 3 (OH)<^Q>C 6 H 4 , A B and this, on oxidation with nitric acid, yields phthalic acid, >C 6 H 4 , the group A being destroyed ; therefore the two CO< groups are attached to B, as well as to A, in the o-position, and therefore anthraquinone has the constitution represented above, 464 ANTHRACENE AND PHENANTHRENE. a conclusion which affords strong support to the above views regarding the constitution of anthracene. Anthraquinone crystallises from glacial acetic acid in pale- yellow needles, melts at 277, and sublimes very readily at higher temperatures in long, sulphur-yellow prisms; it is exceedingly stable, and is only with difficulty attacked by oxidising agents, by sulphuric acid, or by nitric acid. In all those properties which are connected with the presence of the two carbonyl-groups, anthraquinone resembles the aromatic ketones much more closely than the quinones. It has no smell, is by no means readily volatile, and is not reduced when treated with sulphurous acid ; unlike quinone, there- fore, it is not an oxidising agent. When treated with more powerful reducing agents, however, it is converted into PO oxanthranol, C 6 H 4 <^j^Qjj,p>C 6 H 4 , one of the CCX^ groups becoming ^>CH-OH, just as in the reduction of ketones; on further reduction the other C0<^ group undergoes a similar change, but the product, CeH^^ 6114 ' loses one molecule of water, yielding anthranol, C 6 H 4 <^i ^^^C 6 H 4 , which is finally reduced to hydra nth racene ; when anthra- quinone is distilled with zinc-dust, anthracene is produced. Anthraquinone is only slowly acted on by ordinary sulphuric acid even at 250, yielding anthraquinone-/?-monosulphonic acid, C 6 H 4 <^p^^>C 6 H 3 -S0 3 H ; but when heated with a large excess of anhydrosulphuric acid at 160-170, it yields a mixture of disulphonic acids, C 14 H 6 2 (S0 3 H) 2 . Sodium anthraquinone-monosulphonate, which is used in such large quantities in the manufacture of alizarin (see below), is pre- pared by heating anthraquinone with an equal weight of anhydro- sulphuric acid (containing 50 per cent, of SO 3 ) in enamelled iron pots at 160. The product is diluted with water, filtered from un- changed anthraquinone, and neutralised with soda; on cooling, sparingly soluble sodium anthraquinone-monosulphonate separates ANTHRACENE AND PHENANTHRENE. 465 in glistening plates, and is collected in filter-presses. The more soluble sodium salts of the anthraquinone-disulphonic acids, which are always formed at the same time, remain in solution. Test for Anthraquinone. When a trace of finely-divided anthraquinone is mixed with dilute soda, a little zinc-dust added, and the mixture heated to boiling, an intense red colouration is produced, but on shaking in contact with air, the solution is decolourised ; in this reaction oxanthranol is formed, and this substance dissolves in the alkali, forming a deep-red solution ; on shaking with air, however, it is oxidised to anthraquinone, which separates as a white flocculent precipitate. Alizarin, C 6 H 4 <^>C 6 H 2 (OH) 2 , or a/?-dihydroxyanthra- quinone, occurs in madder (the root of Rubia tinctorum\ a sub- stance which has been used from the earliest times for dyeing purposes, and which owes its tinctorial properties to two sub- stances, alizarin and purpurin (see below), both of which are present in the root in the form of glucosides. Ruberythric acid, the glucoside of alizarin, is decomposed when boiled with acids, or when the madder extract is allowed to undergo fermentation, with formation of alizarin and two molecules of dextrose, C 26 H 28 U + 2H 2 = C U H 8 4 + 2C 6 H 12 9 . Ruberythric Acid. Alizarin. A dye of such great importance as alizarin naturally attracted the attention of chemists, and many attempts were made to prepare it synthetically. This was first accomplished in 1868 by Graebe and Liebermann, who found that alizarin could be produced by fusing a/3-dibromanthraquinone* with potash, C 6 H 4 <^>C 6 H 2 Br 2 + 2KOH = C 6 H 4 C 6 H 2 (OH) 2 + 2KBr, but the process was not a commercial success. * Obtained by heating anthraquinone with bromine and a trace of iodine in a sealed tube at 160. 466 ANTHRACENE AND PHENANTHRENE. At the present day, however, the madder root is no longer used, and the whole of the alizarin of commerce is made from (coal-tar) anthracene in the following manner : Anthracene is first oxidised to anthraquinone, and the latter is converted into anthraquinone-/3-sulphonic acid by the method already described (p. 464) ; the sodium salt of this acid is then fused with soda and a little potassium chlorate, and is thus converted into the sodium derivative of alizarin, 3 .S0 3 Na + 3NaOH + = ro + 2H 2 + Na 2 S0 8 ; from this sodium salt the colouring matter itself is obtained by adding acid. When anthraquinonesulphonic acid is fused with soda, the -S0 3 H group is displaced by -OH in the usual manner, but the hydroxy anthraquinone thus produced is very readily converted into alizarin by the further action of the soda, part of it being reduced to anthraquinone, 2C 6 H 4 C 6 H 3 (OH) = Hydroxyanthraqninone. C 6 H 4 C 6 H 2 (OH) 2 + C 6 H 4 C 6 H 4 . This regeneration of anthraquinone, and consequent diminished yield of alizarin, is prevented by the addition of the oxidising agent (KC10 3 ) ; the operation is usually conducted as follows : Sodium anthraquinonesulphonate (100 parts) is heated in a closed iron cylinder, fitted with a stirrer, with soda (300 parts) and potassium chlorate (14 parts), for two days at 180. The dark- violet product, which consists of the sodium salt of alizarin, is dissolved in water, the solution filtered if necessary, and the alizarin precipitated by the addition of hydrochloric acid. The yellowish crystalline precipitate is collected in filter-presses, washed well with water, and sent into the market in the form of a 10 or 20 per cent. paste. From this product alizarin is obtained in a pure state by vecrystallisation from toluene, or by sublimation. Alizarin crystallises and sublimes in dark-red prisms, which melt at 282, and are almost insoluble in water, but ANTHRACENE AND PHENANTHRENE. 467 moderately soluble in alcohol. It is a dihydroxy-derivative of anthraquinone, and has therefore the properties of a dihydric phenol; it dissolves in potash and soda, forming metallic derivatives of the type C 6 H 4 <^p^^ > C 6 H 2 (OM) 2 , which are soluble in water, yielding intensely reddish-violet solutions. With acetic anhydride it gives a diacetate, C 14 H 6 2 (G 2 H 3 2 ) 2 , melting at 180, and when distilled with zinc-dust, it is reduced to anthracene. The value of alizarin as a dye lies in the fact that it yields magnificently coloured insoluble compounds (called 'lakes') with certain metallic oxides ; the ferric compound, for example, is violet black, the lime compound blue, and the tin and aluminium compounds different shades of red (Turkey- red). A short account of the methods used in dyeing with alizarin is given later (p. 504). Constitution of Alizarin. Alizarin may be synthetically prepared by heating a mixture of phthalic anhydride and catechol with sulphuric acid at 150, = As catechol is o-dihydroxy benzene, it follows that the two hydroxyl-groups in alizarin must be in the o-position to one another, and this substance must, therefore, be represented by one of the following formulae : CO OH CO ^-^^^^^\ ^^^^ OH p 7|^7 -I- ]OH CO CO I. II. Xow alizarin yields two (a 1 and /3 1 ) isomeric mono-nitro- derivatives, C 6 H 4 ^Q>C 6 H(OH) 2 -N0 2 , both of which 468 ANTHRACENE AND PHENANTHRENE. contain the nitro-group in the same nucleus as the two hy d roxy 1-groups. The constitution of alizarin must, therefore, be represented by formula i., as a substance having the constitution n. could only yield one such iiitro-derivative, and this formula has been shown to be correct in many other ways which cannot be discussed here. Besides alizarin, several other dihydroxy- and also trihydroxy- anthraquinones have been obtained, but only those are of value as dyes which contain two hydroxyl-groups in the same positions as in alizarin ; two such derivatives, which possess very valuable dyeing properties, may be mentioned. Purpurm, C 6 H 4 <^ ^>C 6 H(OH) 3 , or ajScMrihydroxyanthraqui- none, is contained in madder root, in the form of a glucoside, and may be artificially prepared by oxidising alizarin with manganese dioxide and sulphuric acid. It crystallises in deep-red needles, melts at 252, and gives, with alumina mordants, a much yellower shade of red than alizarin, and is now used on the large scale for the production of brilliant reds. Anthrapurpurin, C 6 H 3 (OH)<^Q>C6H 2 <^ ( ^, is isomeric with purpurin, and is manufactured by fusing anthraquinone-disul- phouic acid, C 6 H 3 (S0 3 H)<^>C 6 H 3 .SO 3 H, with soda and potass- ium chlorate (see alizarin, p. 466). It crystallises in yellowish- red needles, melts at 330, and is very largely employed in dyeing yellow shades of Turkey-red. Phenanthrene, C 14 H 10 , an isomeride of anthracene, is a hydrocarbon of considerable theoretical interest, although it has no commercial value. It occurs in large quantities in * 50 per cent, anthracene,' from which it may be extracted as already described (p. 457). The resulting crude phenanthrene is converted into the picrate (see below), which is h'rst re- crystallised from alcohol, to free it from anthracene picrate, and then decomposed by ammonia, the hydrocarbon being finally purified by recrystallisation. Phenanthrene crystallises in glistening needles, melts at ANTHRACENE AND PHENANTHRENE. 469 99, and distils at about 340 ; it is readily soluble in alcohol, ether, and benzene. When oxidised with chromic acid, it is first converted into phenanihraquinone, C 14 H 8 2 , isomeric with anthraquinone, and then into diphenic acid, C 14 H 10 4 . This acid is decomposed on distillation with lime, yielding carbon dioxide and diplienyl (p. 340) ; it is therefore diplienyl- dicarboxylic acid, COOH-C 6 H 4 -C 6 H 4 -COOH, and its formation from phenanthrene shows that the latter is also a derivative of diphenyl. Further evidence as to the constitution of phenanthrene is obtained by studying its methods of formation. It is formed, for example, on passing o-ditolyl (prepared from o-bromo- toluene and sodium) or stilbene * through a red-hot tube, and the simplest manner of expressing these two reactions is the following : 2H 2 C 6 H 4 CH 3 C 6 H 4 -CH 3 o-Ditolyl. CH 4 CH 1 II C 6 H 4 CH Phenanthrene. CA-CH C 6 H 6 -CH Stilbene. C 6 H. CH 1 II C 6 H 4 -CH Phenanthrene. Again, phenanthrene is formed, together with anthracene, by the action of sodium on o-bromobenzyl bromide (p. 461), -rj ., T 6 4 = /CH:CH\ C 6 H 4 + H 2 + 4NaBr. * Stilbene, or diphenylethylene, C 6 H 5 -CH : CH-C 6 H 5 , may be prepared by acting on benzal chloride (p. 349) with sodium, 2C 6 H 5 -CHC1 2 + 4Na = C 6 H 5 -CH:CH-C 6 H 5 + 4NaCl. It crystallises in colourless needles, melts at 120, and, like ethylene, com- bines with two atoms of bromine, forming stilbene dibromide, CH 5 -CHBr-CHBr-C 6 H 5 (m.p. 237). 470 ANTHRACENE AND PHENANTHRENE. For these and many other reasons, the constitution of phenan- threne is expressed by the formula, CH = CH When the hydrocarbon is oxidised to phenanthraquinone, the group -CH = CH- becomes -CO CO-, and, on further oxidation to diphenic acid, this group is converted into two carboxyl-groups, CO CO COOH COOH Phenanthraquinone. Diphenic Acid. CH 4 CO Phenanthraquinone, i i , like anthraquinone, is C 6 H 4 CO formed by oxidising the hydrocarbon with chromic acid. It crystallises from alcohol in orange needles, and melts at 198. In chemical properties it shows little resemblance to anthra- quinone, but is closely related to /2-naphthaquinone (p. 456), and is, like the latter, an ortho-diketone (ortho-quinone) ; it is readily reduced by sulphurous acid to diliydroxyphenantlirene, C 14 H 8 (OH) 2 , and it combines with sodium bisulphite, forming a soluble bisulphite compound, C 14 H 8 2 , NaHS0 3 + 2H 2 ; with hydroxylamine it yields a diaxime, C 19 H 8 (C:NOH) 2 . The hydroxy-derivatives of phenanthraquinone, unlike those of anthraquinone, possess no tinctorial properties. Phenanthraquinone may be readily detected by dissolving a small quantity (0-1 gram) in glacial acetic acid (20 c.c.), adding a few drops of commercial toluene, and then mixing the well-cooled solu- tion with sulphuric acid (1 c.c.). After standing for a few minutes, the bluish-green liquid is poured into water and shaken with ether, when the ether acquires an intense reddish- violet colouration ANTHRACENE AND PHENANTHRENE. 471 (Laubenheimer's reaction). Like the indophenin reaction, this test depends on the formation of a colouring matter containing sulphur, produced by the condensation of the phenanthraquinone with the thiotolene, C 4 H 3 S(CH 3 ), which is contained in the crude toluene (p. 334). C 6 H 4 COOH Diphenic acid, i , obtained by the oxidation of C 6 H 4 COOH phenantbrene or of pbenanthraquinone with chromic acid, crystallises from water in needles, and melts at 229. When heated with acetic anhydride it is converted into diphenie anhydride, C 12 H 8 <>0 (m.p. 217). This fact is remarkable, because it shows that in the case of derivatives of hydrocarbons which are composed of condensed benzene nuclei, the ortho-position is not the only one which allows of the formation of an anhydride. Naphthalic acid, C 10 H 6 (COOH) 2 , a derivative of naphthalene in which the carboxyl-groups are in the 1:1'- or peri-position, also forms an anhydride. CHAPTER XXXII. PYRIDINE AND QUINOLTNE. Pyridine and quinoline are two very interesting aromatic bases, and many of their derivatives, more especially those which occur in nature, are well-known and important com- pounds. Coal-tar, though consisting principally of hydrocarbons and phenols, contains also small quantities of pyridine, quino- line, and numerous other basic substances, such as aniline and isoquinoline ; all these bases are dissolved, in the form of sulphates, in the purification of the hydrocarbons, &c., by treatment with sulphuric acid (compare p. 297), and, on afterwards adding excess of soda to the dark acid liquor, they separate again at the surface of the liquid in the form of a dark-brown oil. By repeated fractional distillation a partial 472 PYRIDINE AND QU1NOLINK separation of the various constituents of this oil may be etfected, and crude pyridine, quinoline, &c., may be obtained ; on further purification by crystallisation of their salts, or in other ways, some of these bases may be prepared in a state of purity. Another important source of these compounds is bone-tar or bone-oil, a dark-brown, unpleasant-smelling liquid formed during the dry distillation of bones in the preparation of bone-black (animal charcoal) ; this oil contains considerable quantities of pyridine and quinoline, and their homologues, as well as other bases, and these compounds may be extracted from it with the aid of sulphuric acid, and then separated in the manner mentioned above. Bone-oil, purified by distilla- tion, was formerly used in medicine under the name of Dippel's oil. Pyridine and its Derivatives. Pyridine, C 5 H 5 N, is formed during the destructive distilla- tion of a great variety of nitrogenous organic substances, hence its presence in coal-tar and in bone-oil; it is also formed when various alkaloids are distilled with potash. It may be obtained synthetically by passing a mixture of acetylene and hydrogen cyanide through a red-hot tube, a reaction which is very similar to that which occurs in the formation of benzene from acetylene alone (p. 301), 2C 2 H 2 + HCN - C 5 H 3 N. Pyridine is conveniently prepared in small quantities by distilling nicotinic acid (p. 479), or other pyridinecarboxylic acids, with lime, just as benzene may be prepared from benzoic and phthalic acids in a similar manner, C 5 H 4 N-COOH = C 5 H 5 N + C0 2 C 5 H 3 N(COOH) 2 = C 5 H 5 N + 2C0 2 . For commercial purposes it is usually prepared by the frac- tional distillation of the basic mixture, which is separated from bone-oil or coal-tar as already described; the product PYRIDINE AND QUINOLINE. 473 consists of pyridine, together with small quantities of its homologues. Pyridine is a colourless, mobile liquid of sp. gr. 1-0033 at ; it boils at 1 1 5, is miscible with water in all proportions, and possesses a pungent and very characteristic odour. It is an exceedingly stable substance, as it is not attacked by boiling nitric or chromic acid, and only with difficulty by halo- gens ; in the latter case, substitution products such as mono- bromopyridine, C 5 H 4 BrN, and dibromopyridine, C 5 H 3 Br 2 N, are formed. If, however, a solution of pyridine in hydro- chloric acid be treated with bromine, a crystalline, unstable, additive product, C 5 H 5 NBr 2 , is precipitated, even from very dilute solutions, and the formation of this substance is sometimes used as a test for pyridine. When treated with sodium and alcohol, pyridine is readily reduced, piperidine or hexahydropyridim (p. 476) being formed, C 5 H 5 N + 6H = C 6 H U N. Pyridine is a strong base ; like the amines, it turns red lit- mus blue, and combines with acids to form crystalline salts, such as the hydrocliloride, C 5 H 5 N,HC1, and the sulphate, (C 5 H 5 X) 2 ,H 2 S0 4 . The platinochloride, (C^I^HgPtCle, crystallises in orange-yellow needles, and is readily soluble in water ; when, however, its solution is boiled, a very sparingly soluble yellow salt, (C 5 H 5 N) 2 PtCl 4 , separates, a fact which may be made use of for the detection of pyridine even when only small quantities of the base are available. Another test for pyridine (and its homologues) consists in heating a few drops of the base in a test tube with methyl iodide, when a vigorous reaction takes place, and a yellowish additive product, pyridine methiodide, C 5 H 5 N,CH 3 I, is produced ; if a piece of solid potash be now added, and the contents of the tube again heated, a most pungent and exceedingly disagreeable smell is at once noticed. Constitution. Although pyridine is a powerful base, having 474 PYRIDINE AND QUINOLINE, a pungent odour, and turning red litmus blue, properties which suggest some relation to the fatty amines, a careful consideration of its molecular formula and chemical behaviour shows at once that it is not analogous to the fatty amines in constitution. It is not a primary, nor a secondary amine, because it does not give the carbylamine reaction, and is not acted on by nitrous acid, and it cannot possibly be a tertiary fatty amine, because no reasonable constitutional formula based on this view could be constructed. If, moreover, it be borne in mind that pyridine is extremely stable, the probability of its being a fatty (open-chain) compound at all seems very remote, because if it were, it would be highly unsaturated, and should be readily oxidised and resolved into simpler substances. The grounds for doubting its relation to any fatty compound are, in fact, much the same as those which led to the conclusion that the constitution of benzene is totally different from that of dipropargyl (p. 304). Comparing now the properties of pyridine with those of aromatic compounds, a general analogy is at once apparent ; in spite of its great stability, pyridine is really an unsaturated compound, and, like benzene, naphthalene, and other closed- chain compounds, it yields additive products under certain conditions, although as a rule it gives substitution products. Considerations such as these led to the conclusion, suggested by Korner in 1869, that pyridine, like benzene, contains a closed-chain or nucleus, as represented by the following formula, c and this view has since been confirmed in a great many ways, notably in the following manner : Piperidine, or hexahydro- pyridine, the compound which is formed by the reduction of PYRID1NE AND QUlNOLtNE. 475 pyridine, and which is reconverted into the latter on oxidation with sulphuric acid (p. 477), has been prepared synthetically by a method (p. 478) which shows it to have the constitution (i.); pyridine, therefore, has the constitution (IL), the relation between the two compounds being the same as that between benzene and hexahydrobenzene. CH CH NH Piperidine (I.). p ^iC \^ ^Jc N Pyridine (II.X That the constitution of pyridine is represented by this formula (u.) is also established by a study of the isomerism of pyridine derivatives, and by its relation to quinoline (p. 482) ; it must, therefore, be regarded as derived from benzene by the substitution of trivalent nitrogen N<^ for one of the CH<: groups. The exact nature of the union of the nitrogen and carbon atoms is not known, and as in the case of benzene, several methods of representation (some of which are shown below) have been sug- gested ; of these, the centric formula is perhaps the best, for reasons similar to those already mentioned in discussing the con- stitution of benzene (pp. 306, 307). CH Korner. CH CH Centric Formula. Isomerism of Pyridine Derivatives. The ?nora0-substitution products of pyridine, as, for example, the methylpyridines or picolines, exist in three isomeric forms ; this fact is clearly in accordance with the accepted constitutional formula for pyridine, in which, for the sake of reference, the carbon 476 PYRIDINE AND QUINOLINE. atoms may be numbered or lettered in the following manner, the symbols C and H being omitted as usual : These substitution products, being formed by the displace- ment of any one of the five hydrogen atoms, it is evident that the following three (but not more than three), isomerides may be obtained : : ' I' The positions aa 1 (or 1, 5) are identical, and so also are the positions /3fi l (or 2, 4), but the position y (or 3) is different from any of the others. The ^^'-substitution products exist theoretically in six isomeric forms, the positions of the substituents in the several isomerides being as follows : 1:2, 1:3, 1:4, 1:5, 2:3, 2:4. All other positions are identical with one of these ; 4 : 5, for example, is the same as 1:2, and 3:4 is identical with 2:3. As regards the isomerism of its derivatives, pyridine may be conveniently compared with a mono-substitution product of benzene aniline, for example the effect of substituting a nitrogen atom for one of the CHNH-group, show in some respects the behaviour of secondary amines. When treated with nitrous acid they yield nitroso-derivatives (which give Liebermann's reaction), >XH + HO-NO = >N-NO + H 2 0, and when warmed with an alkyl halogen compound, such as methyl iodide, they are converted into alkyl-derivatives by the substitution of an alkyl-group for the hydrogen atom of the >NH-group, CH 3 I = >N-CH 3 ,HI, 484 PTRIDINE AND QUINOLINE. just as diethylamine, for example, interacts with ethyl iodide, giving triethylamine, (C 2 H 5 ) 2 NH + C 2 H 5 I = (C 2 H 5 ) 2 N.C 2 H 5 ,HI. These alkyl-derivatives of the secondary bases are them- selves tertiary bases, and have the property of forming additive products with alkyl halogen compounds, giving salts corresponding with the quaternary ammonium salts (part i. pp. 204, 205), >N.CH 3 + CH 3 T = :>N.CH 3 ,CH 3 I, or >N(CH S ) 2 I. The hydrogen atom of the>NH-group in secondary bases is also displaceable by the acetyl-group and by other acid radicles. The tertiary bases, such as pyridine and quinoline, in which the nitrogen atom is not directly united with hydrogen, behave in many respects like the tertiary amines ; they do not yield nitroso- nor acetyl-derivatives, but when treated with an alkyl halogen compound they yield additive compounds, cor- responding with the quaternary ammonium salts, without the formation of any intermediate product, CH 3 I = >N,CH 3 I, or These differences in behaviour make it an easy matter to distinguish between secondary and tertiary aromatic bases of this class. CHAPTER XXXIII. ALKALOIDS. The alkaloids, like the carbohydrates (part i. p. 259), do not form a well-defined group, this term being applied to nearly all basic nitrogenous substances which occur in plants, irrespective of any similarity in properties or constitution. Most alkaloids are composed of carbon, hydrogen, oxygen, and nitrogen, and are crystalline and non-volatile, but a few, ALKALOIDS. 485 notably coniine and nicotine, are composed of carbon, hydro- gen, and nitrogen only, and are volatile liquids ; with the exception of these liquid compounds, which are readily soluble, the alkaloids are usually sparingly soluble in water, but dissolve much more readily in alcohol, chloroform, ether, and other organic solvents ; they are all soluble in acids, with which they usually form well-defined, crystalline salts. Many alkaloids have a very bitter taste, and are excessively poison- ous ; many, moreover, are extensively used in medicine, and their value in this respect can hardly be overrated. Generally speaking, the alkaloids are tertiary aromatic bases, but, with few exceptions, their constitutions have not been established, owing partly to their complexity, partly to the difficulties which are experienced in resolving them into simpler compounds which throw any light on the structure of their molecules. Nevertheless, work has been done in this direction, and it is known that many alkaloids are derivatives of pyridine, or of quinoline, because they yield these bases, or their derivatives, when strongly heated with potash, and, on oxidation, usually with potassium permanganate, they give carboxylic acids of pyridine and quinoline. It is a remarkable fact that by far the greater number of alkaloids contain one or two, sometimes three or more, methoxy-groups (-0-CH 3 ), united with a benzene nucleus (as in anisole, C 6 H 5 -0-CH 3 , p. 392), and the determination of the number of such groups in the molecule is of the greatest importance in establishing the constitution of an alkaloid, because in this way some of the carbon and hydro- gen atoms are at once disposed of. The method employed for this purpose depends on the fact that all substances con- taining inethoxy-groups are decomposed by hydriodic acid, yielding methyl iodide and a hydroxy-compound (compare anisole) in accordance with the general equation, n(-0-CH 8 ) + nRl = w(-OH) + wCH 3 I; by estimating the amount of methyl iodide obtained from a ( ALKALOIDS. known weight of a given compound, it is easy, therefore, to determine the number of methoxy-groups in the molecule. This method was first applied by Zeisel, and is of general application, as it affords a means of accurately determin- ing the number of methoxy-groups, not only in alkaloids, but in any other substances in which they occur ; it is carried out as follows : A distilling flask of about 35 c.c. capacity (A, fig. 20), with the side-tube bent as shown, and suspended in a beaker of glycerol, is fixed to the condenser (B) by means of a cork, and connected with an apparatus for generating carbon dioxide. The condenser, through which water at 50 circulates from the bottle (C), is attached to the 'potash bulbs,' which contain water and about 0-5 gram of amorphous phosphorus ; the bulbs are sus- pended in a beaker of water kept at 60, and connected, as shown, with two flasks (D, E), containing respectively 50 c.c. and 25 c.c. of an alcoholic solution of silver nitrate (prepared by adding 100 c.c. of absolute alcohol to a solution of 5 grams of silver nitrate in 12 c.c. of water); In carrying out the estimation, about 0-3 gram of the substance under examination is placed in the flask A, together with 10 c.c. of fuming hydriodic acid, and the temperature of the glycerol bath is gradually raised, until the acid just boils, carbon dioxide, at the rate of about 3 bubbles in 2 seconds, being passed all the time. The methyl iodide thus formed is carried forward through the Condenser into the 'potash bulbs,' where it is freed from hydriodic acid and from small quantities of iodine, which it always contains ; it then passes into the alcoholic silver nitrate solution, and is de- composed with separation of silver iodide. The operation, which occupies about two hours, is at an end when the precipitate in the flask settles, and leaves a clear, supernatant liquid. The contents of flask E are poured into 5 vols. of water and gently warmed ; if, as is usually the case, no precipitation takes place after five minutes, the solution is neglected ; if, however, a precipitate forms, it must be collected and added to that contained in flask D. The alcoholic liquid in flask D is decanted from the precipitate, mixed with water (300 c.c.) and a few drops of nitric acid, and heated to boiling until free from .alcohol ; any pre- cipitate is then added to the main quantity, the whole digested for a few minutes with dilute nitric acid, collected on a filter, dried, and weighed. 488 ALKALOIDS. The extraction of alkaloids from plants, and their subsequent purification, are frequently matters of considerable difficulty, partly because in many cases a number of alkaloids occur together, partly because of the neutral and acid substances, such as the glucosides,* sugars, tannic acid, malic acid, &c., which are often present in large quantities. Generally speak- ing, they may be extracted by treating the macerated plant or vegetable product with dilute acids, which dissolve out the alkaloids in the form of salts ; the filtered solution may then be treated with soda to liberate the alkaloids, which, being sparingly soluble, are usually precipitated, and may be separ- ated by filtration ; if not, the alkaline solution is extracted with ether, chloroform, &c. The products are finally purified by recrystallisation, or in some other manner. Most alkaloids give insoluble precipitates with a solution of tannic, picric, phosphomolybdic, or phosphotungstic acid, and with a solution of mercuric iodide in potassium iodide,t &c. ; these reagents, therefore, are often used for their detec- tion and isolation. Only the more important alkaloids are described in the following pages. Alkaloids derived from Pyridine. Coniine, CgHjyN, one of the simplest known alkaloids, is contained in the seeds of the spotted hemlock (Conium macula- tum), from which it may be prepared by distillation with soda. It is a colourless oil, boiling at 167, and is readily soluble in water ; it has a most penetrating odour, and turns brown * The term glucos\de is applied to all those vegetable products which, on treatment with acids or alkalies, yield a sugar, or some closely allied carbohydrate and one or more other substances (which are frequently phenols or aromatic aldehydes) as decomposition products (compare amyg- dalin, p. 405 ; salicin, p. 404 ; ruberythric acid, p. 465, &c.). f" For the preparation of these solutions larger works must be consulted. In cases of alkaloid poisoning it is usual, after using the stomach-pump, to wash out the stomach with dilute tannic acid, or to administer strong tea (which contains tannin), in order to render the alkaloids insoluble, and, therefore, harmless. ALKALOIDS, 489 on exposure to air. Con line is a strong base, and com- bines with acids to form salts, such as the hydrochloride, C 8 H 17 N,HC1, which are readily soluble in water ; both the base arid its salts are exceedingly poisonous, a few drops of the pure substance causing death in a short time by paralysing the muscles of respiration. Ladenburg has shown that coniine is dextrorotatory a-propyl- piperidine, CH 2 NH and has succeeded in preparing it synthetically, the first instance of the synthesis of an optically active alkaloid. a-Propylpiperidine contains an asymmetric carbon atom (shown in heavy type compare p. 533), and, therefore, like lactic acid, it exists in three modifications, all of which have been synthetically prepared ; the inactive modification may be separated into the two optically active compounds by crystallisation of its tartrate (compare p. 544). x^Nicotine, C 10 H 14 N 2 , is present in the leaves of the tobacco y plant (Nicotiana tabacum), combined with malic or citric acid. Tobacco leaves are extracted with boiling water, the extract concentrated, mixed with milk of lime, and distilled ; the distillate is acidified with oxalic acid, evaporated to a small bulk, decomposed with potash, and the free nicotine extracted with ether. The ethereal solution, on evaporation, deposits the crude alkaloid, which is purified by distillation in a stream of hydrogen. Nicotine is a colourless oil, which boils at 24 1, possesses a very pungent odour, and rapidly turns brown on exposure to air; it is readily soluble in water and alcohol. It is a strong di-acid base, and forms crystalline salts, such as the hydrochloride, C 10 H U N 2 ,2HC1; it combines directly with two molecules of methyl iodide, yielding nicotine dimethiodide, C 10 H 14 N 2 ,2CH 3 I, a fact which shows that it is a di-tertiary base (p. 484). When oxidised with chromic acid, it yields 490 ALKALOIDS. nicotinic acid (pyridine-/?-carboxylic acid, p. 479) ; it is, therefore, a pyridine-derivative, but its constitution has not yet been determined. Nicotine is exceedingly poisonous, two or three drops taken into the stomach being sufficient to cause death in a few minutes. It shows no very characteristic reactions, but its presence may be detected by its extremely pungent odour (which recalls that of a foul tobacco pipe). Piperine, C 17 H 19 N0 3 , occurs to the extent of about 8-9 per cent, in pepper, especially in black pepper (Piper nigrum), from which it is easily extracted. The pepper is powdered and warmed with milk of lime for 15 minutes ; the mixture is then evaporated to dry ness on a water- bath, extracted with ether, the ethereal solution evaporated, and the residual crude pipeline purified by recrystallisation from alcohol. It crystallises in prisms, melts at 128, and is almost insoluble in water ; it is only a very weak base, and when heated with alcoholic potash, it is decomposed into piperidine (p. 476) and piperic acid, C^pNO, + H 2 = C 5 H U N + C 12 H 10 4 . Piperidine. Piperic Acid. Atropine, or dafcurine, C 17 H 2S N0 3 , does not occur in nature, although it is prepared from the deadly nightshade (Atropa belladonna). This plant contains two isomeric and closely related alkaloids hyoscyamine and liyoscine, and the former readily undergoes intramolecular change into atropine on treatment with bases. The plant is pressed, the juice mixed with potash, and extracted with chloroform (1 litre of juice requires 4 grams of potash and 30 grams of chloroform); the chloroform is then evaporated, the atropine extracted from the residue with dilute sulphuric acid, the solution treated with potassium carbonate, and the precipitated alkaloid recrystallised from alcohol. It crystallises from dilute alcohol in glistening prisms, and melts at 115; it is readily soluble in alcohol, ether, and chloroform, but almost insoluble in water. When boiled ALKALOIDS. 491 with baryta water it is readily hydrolysed, yielding tropic acid and a base called tropine, which is a derivative of pyridine, C 17 H 23 N0 3 + H 2 = CA'CtK + C 8 H 15 NO. Tropic Acid. Tropine. Atropine is a strong base, and forms well-characterised salts, of which the sulphate, (C l7 H 23 N0 3 ) 2 ,H 2 S0 4 , is readily soluble, and, therefore, most commonly used in medicine ; both the base and its salts are excessively poisonous, 0-05 0-2 gram causing death. Atropine sulphate is largely used in ophthalmic surgery, owing to the remarkable property which it possesses of dilating the pupil when its solution is placed on the eye. Test for Atropine. If a trace of atropine be moistened with fuming nitric acid, and evaporated to dryness on a water- bath, it yields a yellow residue, which, on the addition of alcoholic potash, gives an intense violet solution, the colour gradually changing to red. Cocaine, C 17 H 21 N0 4 , and several other alkaloids of less importance, are contained in coca leaves (Enjthroxylon coca). The coca leaves are extracted with hot water (80), the solution mixed with lead acetate (in order to precipitate tannin, &c.), filtered, and the lead in the filtrate precipitated with sodium sul- phate; the solution is then rendered alkaline with soda, the cocaine extracted with ether, and purified by recrystallisation from alcohol. Cocaine crystallises in colourless prisms, melts at 98, and is sparingly soluble in water; it forms welUcharacterised salts, of which the hydrochloride, C 17 H 21 N0 4 ,HC1, is most largely used in medicine. Cocaine is a very valuable local anaesthetic, and is used in minor surgical operations, as its local application takes away all sensation of pain ; it is, however, poisonous, one grain injected subcutaneously having been attended with fatal results. When heated with acids or alkalies, cocaine is readily hydrolysed with formation of benzoic acid, methyl alcohol, and ecgonine (a derivative of tetrahydropyridine), C 17 H 21 N0 4 + 2H 2 = C 6 H 5 .COOH + CH 3 -OH + C 9 H 15 N0 3 . 492 ALKALOIDS. Alkaloids derived from Quinoline. Quinine, C 00 H 24 N 2 2 , cinchonine (see below), and several other allied alkaloids, occur in all varieties of cinchona-bark, some of which contain as much as 3 per cent, of quinine. The alkaloids are contained in the bark, combined with tamiic and quinic acids.* The powdered bark is extracted with dilute sulphuric acid, and the solution of the sulphates precipitated with soda. The crude mixture of alkaloids thus obtained is dissolved in alcohol, the solution neutralised with sulphuric acid, and the sulphates, which are deposited, repeatedly recrystallised from water. Quinine sulphate is the least soluble, and separates out first, the sulphates of cinchonine and the other alkaloids remaining in solution ; from the pure sulphate, quinine may be obtained as an amorphous powder by adding ammonia. Quinine crystallises in silky needles, melts at 177, and is only very sparingly soluble in water ; it is only a feeble di- acid base, and generally forms acid salts, such as the sulphate, (C 20 H 24 ]S" 2 2 ) 2 ,H 2 S0 4 + 8H 2 ; many of its salts are soluble in water, and much used in medicine as tonics, and for lower- ing the temperature in cases of fever, &c. Quinine is a di-tertiary base, because it combines with methyl iodide to form quinine dimethiodide, C 20 H 24 N 2 2 ,(CH 3 I) 2 ; it is a derivative of quinoline, because, on oxidation with chromic acid, it yields qtiininic acid (methoxyquinoline-y-car- boxylic acid). COOH /^\ ^ CH 3 Quinine appears to be methoxy-cinchonine, and that it contains one methoxy-group, has been demonstrated by Zeisel's method (p. 486) ; this view accords with the fact * Quinic acid, C 6 H 7 (OH) 4 -COOH, crystallises in colourless prisms, and melts at 162. It is a derivative of benzoic acid, being, in fact, hexahydro- tetrahydroxybenzoic acid. ALKALOIDS. 493 that, whereas cinchonine, on oxidation, yields quinoline-y- carboxylie acid, quinine yields the rnethoxy-derivative of this acid : in spite, however, of a great amount of laborious investigation, the constitution of quinine is still an unsolved problem. Tests for Quinine. If a solution of a salt of quinine be mixed with chlorine- or bromine- water, and then ammonia added, a highly characteristic emerald green colouration is produced ; quinine is also characterised by the fact that dilute solutions of its salts show a beautiful light- blue fluorescence. Cinchonine, C 19 H 22 N 2 0, accompanies quinine in almost all the cinchona-barks, and is present in some kinds (in the bark, China Huanaco) to the extent of 2-5 per cent. In order to prepare cinchonine, the mother-liquors from the crystals of quinine sulphate (see above) are treated with soda, and the precipitate dissolved in the smallest possible quantity of boiling alcohol ; the crude cinchonine, which separates on cooling, is further purified by converting into the sulphate, and crystallising this salt from water. Cinchonine crystallises in colourless prisms, melts at 250, and resembles quinine in ordinary properties; its salts, for example, are antipyretics, but are much less active than those of quinine. Oxidising agents, such as nitric acid and potassium per- manganate, readily attack cinchonine, converting it into a variety of substances, one of the most important of which is cinchoninic acid, or quinoline-y-carboxylic acid, COOH The formation of this acid not only proves that cinchonine is a quinoline-derivative, but also shows the close relationship existing between quinine and cinchouine (see above). 494 ALKALOIDS. Strychnine, C 21 H 22 N 2 2 , and brucine, two highly poisonous alkaloids, are contained in the seeds of Strychnos nux vomica and of Strychnos Ignatii (Ignatius' beans), but they are usually extracted from the former. Powdered nux vomica is boiled with dilute alcohol, the filtered solution evaporated to expel the alcohol, and treated with lead acetate to precipitate tannin, &c. The filtrate is then treated with hydrogen sulphide to precipitate the lead, and the filtered solution mixed with magnesia and allowed to stand. The precipitated alkaloids are separated, and warmed with a little alcohol, which dissolves out the brucine ; the residual strychnine is further purified by recrystallisation from alcohol. The alcoholic solution of the brucine which still contains strychnine is evaporated, and the residue dissolved in dilute acetic acid ; this solution is now evaporated to dryriess on a water-bath, during which process the strychnine acetate decomposes, with loss of acetic acid and separation of the free base. The stable brucine acetate is dissolved again by adding water, the filtered solution treated with soda, and the precipitated brucine purified by re- crystallisation from dilute alcohol. Strychnine crystallises in beautiful rhombic prisms, and melts at 284 ; although it is very sparingly soluble in water (1 part in 4000 at 15), its solution possesses an intensely bitter taste, and is very poisonous. Strychnine is, in fact, one of the most poisonous alkaloids, half a grain of the sulphate having caused death in twenty minutes. Although strychnine contains two atoms of nitrogen, it is, like brucine, only a mon-acid base, forming salts, such as the hydrochloride, C 21 H 22 N 2 2 ,HC1, with one equivalent of an acid ; many of the salts are soluble in water. It is, further- more, a tertiary base, because it combines with methyl iodide to form strychnine methiodide, C 21 H 22 N 2 2 ,CH 3 I. When distilled with potash, strychnine yields, among other products, quinoline ; probably, therefore, it is a derivative of this base. Test for Strychnine. Strychnine is very readily detected, as it shows many characteristic reactions, of which the follow- ing is the most important : When a small quantity of powdered ALKALOIDS. 495 strychnine is placed in a large porcelain basin, a little con- centrated sulphuric acid added, and then a little powdered potassium bichromate dusted over the liquid, an intense violet solution, which gradually becomes bright-red, and then yellow, is produced. Brucine, C 93 H 96 N 2 4 , crystallises in colourless prisms, with 4 mols.. H 2 0, and melts at 178. It is more readily soluble in water and in alcohol than strychnine, and, although very poisonous, it is not nearly so deadly as the latter (its physio- logical effect being only about ^jth of that of strychnine). Although it contains two atoms of nitrogen, brucine, like strychnine, is a mon-acid base. The hydrochloride, for ex- ample, has the composition C 23 H 26 N 2 4 , HClj it is also a tertiary base, because it combines with methyl iodide, to form brucine metliiodide, C 23 H 26 N 2 4 , CH 3 I. Test for Brucine. When a solution of a brucine salt is treated with nitric acid, a deep brownish-red colouration is obtained, and, on warming, the solution becomes yellow; if now stannous chloride be added, an intense violet colouration is produced. This colour reaction serves as a delicate test, both for brucine and for nitric acid, as it may be carried out with very small quantities. Alkaloids contained in Opium. The juice of certain kinds of poppy-heads (Papaver somni- ferum) contains a great variety of alkaloids, of which morphine is the most important, but codeine, narcotine, theba'ine, and papaverine may also be mentioned. All these compounds are present in the juice in combination with meconic acid* and partly also with sulphuric acid. When incisions are made in * Meconic acid, C 5 HO 2 (OH)(COOH) 2 , is a hydroxydiearboxylic acid be- longing to the fatty series. It crystallises with three molecules of water, and gives, with ferric chloride, an intense dark-red colouration. In cases of suspected opium-poisoning this acid is always tested for, owing to the ease with which it can be detected by this colour reaction. 496 ALKALOIDS. the poppy-heads, and the juice which exudes is collected and left to dry, it assumes a pasty consistency, and is called opium. An alcoholic tincture of opium, containing about 1 grain of opium in 15 minims, is known as laudanum. Preparation of Morphine. Opium is extracted with hot water, the extract boiled with milk of lime, and filtered from the precipi- tate, which contains the meconic acid, and all the alkaloids, except morphine. The filtrate is then concentrated, digested with ammonium chloride until ammonia ceases to be evolved (to convert any lime present into soluble calcium chloride), and allowed to stand for some days ; the morphine, which separates, is collected and purified by recrystallisation from fusel oil (part i. p. 99). Morphine, C 17 H 19 N0 3 , crystallises in colourless prisms, with 1 mol. H 2 0, and is only slightly soluble in water and cold alcohol, but dissolves readily in potash and soda, from which it is reprecipitated on the addition of acids ; it has, in fact, the properties of a phenol. At the same time, it is a mon- acid base, and forms well-characterised salts with acids. The hydrochloride, C l7 H 19 N0 3 ,HCl + 3H 2 0, crystallises from water in colourless needles, and is the salt most commonly employed in medicine. Morphine has a bitter taste, and is excessively poisonous, one grain of the hydrochloride having been found sufficient to cause death ; on the other hand, the system may become so accustomed to the habitual use of opium that, after a time, very large quantities may be taken daily without fatal effects. Morphine hydrochloride is extensively used in medicine as a soporific, especially in cases of intense pain, which it relieves in a remarkable manner. Tests for Morphine. Morphine has the property of liberat- ing iodine from a solution of iodic acid. If a little iodic acid be dissolved in water, and a few drops of a solution of morphine hydrochloride added, a brownish colouration is at once produced, owing to the liberation of iodine, and, on adding some of the solution to starch-paste, the well-known deep-blue colouration is obtained. A solution of morphine, or of a morphine salt, gives a deep- ALKALOIDS. 497 blue colouration with ferric chloride, but, perhaps, the most delicate test for the alkaloid is the following : If a trace of morphine be dissolved in concentrated sulphuric acid, the solution kept for 15 hours, and then treated with nitric acid, it gives a bluish-violet colour, which changes to blood-red. This reaction is very delicate, and is well shown by 0-01 milligramme of morphine. The constitution of morphine is still undetermined, but that it is a tertiary base is proved by the fact that, when treated with methyl iodide, it yields morphine methiodide, C 1Y H 19 N0 3 ,CH 3 I. Morphine contains two hydroxyl-groups, one of which is phenolic, the other alcoholic. The third atom of oxygen present in the mole- cule is not ketonic (that is, present as >>CO) ; it must, therefore, be combined with two carbon atoms -C O C- (as in ordinary ether). It is to the presence of the phenolic hydroxyl-group that morphine owes its property of dissolving in alkalies, and giving a blue colour with ferric chloride. If the base be heated with potash and methyl iodide, methyl- morphine, C 17 H 17 NO(OCH 3 )-OH, is produced, a substance which is identical with codeine, an alkaloid which accompanies morphine in opium. Codeine is insoluble in alkalies, and is, therefore, not a phenol ; it behaves, however, like an alcohol, and gives, with acetic anhydride, acetylcodeine, C 17 H 17 NO(OCH 3 )-C 2 H 3 2 . It is very remarkable that morphine is a derivative of phenan- threne, as derivatives of this hydrocarbon are very seldom met with in nature. If morphine be distilled with zinc-dust, a considerable quantity of this hydrocarbon is obtained, together with pyridine, quinoline, and other substances. Alkaloids related to Uric Acid. Caffeine, theine, or methyltheobromine, C 8 H 10 N 4 2 , occurs in coffee-beans (-| per cent.), in tea (2 to 4 per cent.), in kola-nuts (2-5 per cent.), and in other vegetable products. Tea (1 part) is macerated with hot water (4 parts), milk of lime (1 part) added, and the whole evaporated to dry ness on a water- bath ; the caffeine is then extracted from the residue by means of chloroform, the extract evaporated, and the crude base purified by recrystallisation from water. 498 ALKALOIDS. Caffeine crystallises in long, colourless needles, with 1 moL H 2 0, melts at 225, and at higher temperatures sublimes un- decomposed ; it has a bitter taste, and is sparingly soluble in cold water and alcohol. Caffeine is a feeble base, and forms salts only with strong acids; the hydrochloride, C 8 H 10 N" 4 2 ,HC1, is at once decomposed on treatment with water, with separation of the base. The constitution of caffeine has been determined by E. Fischer, who has shown that this substance and uric acid are very closely allied ; caffeine is, therefore, an example of an alkaloid which is not a derivative of pyridine or quinoline. Tests for Caffeine. If a trace of caffeine be evaporated with concentrated nitric acid, it gives a yellow residue (amalinic acid), which, on the addition of ammonia, becomes intensely violet (murexide reaction) ; this reaction is also shown by uric acid (part i. p. 292). A solution of caffeine in chlorine water yields, on evaporation, a yellowish-brown residue, which dis- solves in dilute ammonia, with a beautiful violet-red colouration. Theobromine, C 7 H 8 N 4 2 , occurs in cocoa-beans, from which it may be obtained by treatment with lime, and extraction with alcohol. It crystallises from water, and shows the greatest resemblance to caffeine in properties ; the latter is, in fact, methyltheobromine, and may be obtained directly from theobromine in the following way : Theobromine contains an >NH group, the hydrogen of which is readily displaced by metals (as in succinimide, part i. p. 238), and when treated with an ammoniacal silver nitrate solution, it yields silver theobromine. This substance interacts readily with methyl iodide with formation of caffeine, C 7 H 7 N 4 2 Ag + CH S I = C 7 H 7 N 4 2 -CH 3 + Agl. Silver Theobromine. Caffeine. The relationship between uric acid, theobromine, and caffeine is expressed by the following graphic formulae : /NH.CO-C.NHv /NH.CO.C.N(CH 3 k C0< || >CO C0< >CH N NH C-NH/ X N(CH 3 )-C W Theobromine. xN(CH 3 )-CO.C.N(CH 3 ) x C0 < !!. V / CH ^"~ " i\ N N(CH 3 ) Caffeine. ALKALOIDS. 499 Antipyrine, Katrine, and Tkalline. These three nitrogenous compounds, which do not occur in nature, may be briefly described here as examples of what may be termed ' artificial alkaloids ; ' they are employed in medicine, as substitutes for quinine, for lowering the body temperature in cases of fever. Antipyrine, C 11 H 12 N 2 0, was first obtained by Knorr by treating ethyl acetoacetate (part i. p. 189) with phenyl- hydrazine (p. 376), and then heating the product (phenyl- methylpyrazolone) with methyl iodide, CH 8 .CO.CH 9 .COOC 2 H 5 + C 6 H 5 -NH.NH 9 = C 10 H 10 N 2 + C 2 H 5 -OH + H 2 C 10 H 10 N 2 + CH 3 I = C n H 12 N 2 0,HI. It is a colourless, crystalline compound, melts at 113, and is readily soluble in water and alcohol ; it is a strong mon-acid base, and its salts dissolve freely in water. Its aqueous solu- tion gives a deep-red colouration with ferric chloride, and a bluish-green colouration with nitrous acid. Kairine, or hydroxymethyltetrahydroquinoline, may be obtained indirectly from o-amidophenol, which is first converted into hydroxyquinoline by Skraup's reaction (p. 482); this product is then reduced with tin and hydrochloric acid, and the tetrahydrohydroxyquinoline thus obtained is converted into its methyl-derivative by treating it with methyl iodide. Kairine is a crystalline compound, melting at 114. It is a strong base, and forms crystalline salts, of which the hydro- chloride, C 10 H 13 IS T 0,HC1 + H 2 0, is used in medicine. Thalline, or methoxytetrahydroquinoline, ;H .CH fl is isomeric with kairine, and is obtained by reducing the methoxyquinoline which is prepared from p-methoxy aniline, C 6 H 4 (OCH 3 )-NH 2 , by Skraup's reaction ; it is a crystalline 500 ALKALOIDS. compound, melting at 42, and is used in the form of its sulphate or tartrate. With ferric chloride and other oxidising agents it gives a green precipitate. Antifebrin, or acetanilide, another important febrifuge, has already been described (p. 362). Choline, Beta'ine, Neurine, and Taurine. Certain nitrogenous substances which occur in the animal kingdom may also be referred to in this chapter, because they are basic compounds of great physiological importance ; they really belong, however, to different classes of the fatty series. Choline, or hydroxyethyltrimethylammonium hydroxide, OH.CH 2 .CH 2 .N(CH 3 ) 3 .OH, occurs in the blood, bile, brain- substance, yolk of egg, and in other parts of animal organisms, usually -in the form of lecithin (a compound of choline, glycerol, phosphoric acid, and various fatty acids) ; it also occurs in mustard and in hops. It may be prepared syntheti- cally by warming trimethylamine with ethylene oxide (part i. p. 223) in aqueous solution, N(CH 3 ) 3 + C 2 H 4 + H 2 = C 5 H 15 N02, It is a crystalline, very hygroscopic, strongly basic substance, its aqueous solution having an alkaline reaction, and absorb- ing carbon dioxide from the air ; when treated with hydro- chloric acid it yields the corresponding chloride, OH.CH 9 .CH 2 -N(CH 3 ) 3 .OH + HC1 = OH.CH 2 .CH 2 .N(CH 3 ) 3 C1 + H 2 O, but when boiled with water the base is decomposed into glycol and trimethylamine. Betaine, C 5 H n N0 2 , is formed when choline undergoes mild oxidation ; the acid, which is first produced by the conversion of the -CH 2 'OH group into carboxyl, 33 + 20 = COOH.CH 2 .N-phenylene- diamine, C 6 H 5 .N:N-C 6 H 4 .NH 2 + 4H = C 6 In very many cases, however, the colourless reduction DYES AND THEIR APPLICATION. 507 product differs from the dye in composition, simply in containing two or more additional atoms of hydrogen, and may be readily reconverted into the dye by oxidising agents ; such reduction products are called leucocompounds. Amidoazobenzene, for example, the hydrochloride or oxalate of which is the dye aniline yellow (p. 524), on treat- ment with mild reducing agents, such as zinc-dust and acetic acid, yields amidohydrazobenzene, which is only slightly coloured, C 6 H 5 .N:tf.C 6 H 4 .NH 2 + 2H = C 6 H 5 -NH.NH.C 6 H 4 .NH 2 . The last-named substance is readily oxidised on shaking its alcoholic solution with precipitated (yellow) mercuric oxide, with regeneration of amidoazobenzene, and is, therefore, leuco- amidoazobenzene ; many examples of leuco-compounds will be met with in the following pages. When an insoluble dye yields a soluble leuco-compound, which is very readily reconverted into the dye on oxidation, it may be applied to fabrics in a special manner, as, for example, in the case of dyeing with indigo blue. Indigo blue, C 16 H 10 N 2 2 (p. 527), is insoluble in water, but on reduction it is converted into a readily soluble leuco-base, C 16 H 12 N" 2 2 , known as indigo ivliite : in dyeing with indigo, a solution of indigo white is prepared by reducing indigo, suspended in water, with grape-sugar and soda, or ferrous sulphate and soda, and the fabric is then passed through this solution, whereupon the indigo white diffuses through the walls into the fibres ; on subsequent exposure to the air the indigo white is reconverted into indigo blue by oxidation, and the insoluble dye is thus fixed in the fabric. Some of the more important dyes will now be described : as, however, it would be impossible to discuss fully the constitutions of these compounds, it must be understood that the formulae employed in the following pages are those com- monly accepted, and that most of them have been satis- factorily established. 508 DYES AND THEIR APPLICATION. Derivatives of Triphenylmethane. Triphenylmethane, C 6 H 5 -CH(C 6 H 5 ) 2 (p. 340), or, more strictly speaking, triphenyl carbinol, C 6 H 5 -C(C 6 H 5 ) 2 -OH, is the parent substance of a number of dyes, which are of very great technical importance, on account of their brilliancy : as examples, malachite green, pararosaniline, and rosaniline may be described. Three distinct classes of substances are constantly met with in studying the tripheny 1m ethane group of colouring matters namely, the leuco-base, the colour-base, and the dye itself. The leuco-base (p. 507) is an amido-derivative of triphenyl- methane ; in the case of malachite green, for example, the leuco-base is tetramethyldiamidotriphenylmethane, The colour-base is a derivative of triphenyl carbinol, and is produced from the leuco-base by oxidation, just as triphenyl carbinol results from the oxidation of triphenylmethane (p. 341) ; tetramethyldiainidotriphenyl carbinol, for example, is the colour-base of malachite green, Both the leuco-base and the colour-base are usually colour- less, and the latter also yields colourless, or only slightly coloured, salts on treatment with cold acids; when warmed with acids, however, the colour-base is at once converted into highly coloured salts, which constitute the dye, water being eliminated, C 23 H 26 N 2 + HC1 = C 23 H 25 N 2 C1 + H 2 0. Malachite Green Base. Chloride of Malachite Green. This loss of water must be assumed to be due to com- bination taking place between the hydroxyl-group and the hydrogen atom of the acid employed, and the conversion of DYES AND THEIR APPLICATION. 509 the colourless, into the coloured, salt may be expressed in the following way : TT p/nTT^^6 la 4*^^ /ra 3/2 P W rV^e^-M^J-lg^ i_ IT O tlg-L-^UJlJ^^ TT .ivr/piT trpi U 6 H 5 -l^ n TT _xr/pir v ni + l 2 U. This change resembles the conversion of colourless hydro- quinone into highly coloured quinone (and also that of p- amidophenol into quinone-chlorimide, p. 416), as will be more readily understood if it be represented thus : C 6 H 5 .C(OH).C 6 H 4 .X(CH 3 ) 2 C 6 H 5 .C.C 6 H 4 .N(CH 3 ) 2 (CH 3 ) 2 N, HC1 Hydrochloride of Colour-base. Chloride of Malachite Green. Exactly similar changes may be assumed to take place in the formation of the pararosaniline and rosaniline dyes, and, in fact, in the case of many other colouring matters, some of which are described later. Malachite green (of commerce) is a double salt, formed by the combination of the chloride of tetramethyldiamidotriphenyl carbinol with zinc chloride, and the first step in its manu- facture is the preparation of leuco-malachite green or Leuco-malachite green is obtained by the action of dehydrating agents, generally zinc chloride, on a mixture of benzaldehyde (1 mol.) and dimethylaniline (2 mols.), C 6 H,CHO It is a colourless, crystalline substance, which, when treated with oxidising agents, such as manganese' dioxide and 510 DYES AND THEIR APPLICATION. sulphuric acid, or lead dioxide and hydrochloric acid, yields tetrametliyldiamidotriplienyl carbinol, just as triphenyl- methane, under similar circumstances, yields triphenyl carbinol, This oxidation product is a colourless base, and dissolves in cold acids, yielding colourless solutions of its salts ; when, however, such solutions are warmed, the colourless salts decompose, and lose one molecule of water, intensely green solutions of the dye being obtained ; the formation of the chloride, for example, is expressed by the equation C 23 H 26 N 2 + HC1 = C 23 H 25 N 2 C1 + H 2 0, and its double salt, with zinc chloride (or the oxalate of the base), constitutes the malachite green (Victoria green, benzal- dehyde green) of commerce. Preparation of Malachite Green. Dimethylaniline(10 parts) and benzaldehyde (4 parts) are heated with zinc chloride (4 parts) in a porcelain basin, or enamelled iron pot, for two days at 100, with constant stirring ; the product is then submitted to distillation in steam, to get rid of the unchanged dimethylaniline, and allowed to cool. The leuco-compound is now separated from the aqueous solution of zinc chloride, washed with water, dissolved in as little hydrochloric acid as possible, the solution diluted consider- ably with water, and the calculated quantity of freshly pre- cipitated lead peroxide, (Pb0 2 ), added. The filtered dark-green solution is then mixed with sodium sulphate, to precipitate any lead, again filtered, and the colouring matter precipitated in the form of its zinc double salt, SC^HogNaCl^ZnCls + 2H 2 O, by the addition of zinc chloride and common salt ; this salt is finally purified by recrystallisation. Malachite green, and other salts of the base, such as the oxalate, 2C 23 H 24 N 2 ,3C 2 H 2 4 , form deep-green crystals, and are readily soluble in water ; they are decomposed by alkalies, with separation of the colour-base, tetramethyldiamidotri- phenyl carbinol. DYES AND THEIR APPLICATION. 511 Malachite green dyes silk and wool directly an intense dark-bluish green, but cotton must first be mordanted with tannin and tartar emetic (p. 506), and then dyed in a bath gradually raised to 60. Many other dyes, closely allied to malachite green, are prepared by condensing benzaldehyde with tertiary alkylanilines (p. 366). Brilliant green, for example, is finally obtained when diethyl- aniline is employed instead of dimethylaniline in the above- described process, whereas acid green is obtained from benzal- dehyde and ethylbenzylaniline,* C 6 H 5 -N(C 2 H 5 )-C 7 H 7 , in a similar manner. The salts of these two colouring matters are very sparingly soluble in water, and, therefore, of little use as dyes ; for this reason, the bases are treated with anhydrosulphuric acid, and thus converted into a mixture of readily soluble sulphonic acids, the sodium salts of which constitute the commercial dyes. Silk and wool are dyed in a bath acidified with sulphuric acid (hence the name acid green), and very bright greens are obtained, but these dyes are not suitable for cotton. Pararosaniline and rosaniline are exceedingly important dyes, which, like malachite green, are derived from triphenyl- methane. Whereas, however, malachite green is a derivative of cfo'a??izV?0-triphenylmethane, the rosanilines are all triamido- triphenylmethane derivatives, as will be seen from the follow- ing table : Triphenylmethane. Tolyldiphenylmethane (MethyltriphenylmethaneX Leuco-pararosaniline Leuco-rosaniline (Paraleucaniline). (Leucaniline). Triamidotriphenylmethane. Triamidotolyldiphenylmethane. 4 -^"2 Pararosaniline Base. Rosaniline Base. Triainidotriphenyl Carbinol. Triamidotolyldiphenyl Carbinol. L^" XI J.i Pararosaniline Chloride. Rosaniline Chloride. " Produced by treating aniline with benzylchloride and ethyl bromide 512 DYES AND THEIR APPLICATION. In all these compounds, the amido-groups have been proved to be in the jpara-position to the methane carbon atom. Pararosaniline (of commerce) is the chloride of triamido- triphenyl carbinol, a base which is most conveniently pre- pared by oxidising a mixture of jp-toluidine (1 mol.) and aniline (2 mols.) with arsenic acid, or nitrobenzene (compare rosaniline, p. 513). NH 2 .C 6 H 4 .CH 3 + 2 H0. Probably the j9-toluidine is first oxidised to ^-amidobenzaldehyde, NH 2 -C 6 H 4 -CHO, -which then condenses with the aniline (as in the case of the formation of leuco-malachite green), to form leuco-para- rosaniline ; this compound is then converted into the pararosaniline base by further oxidation. The salts of pararosaniline have a deep magenta colour, and are soluble in warm water; they dye silk, wool, and cotton, under the same conditions as described in the case of malachite green ; pararosaniline is, however, not so largely used as rosaniline. Triamidotriphenyl carbinol, the pararosaniline colour-base, is obtained, as a colourless precipitate, on adding alkalies to a solution of the chloride, or of some other salt ; it crystallises from alcohol in colourless needles, and, when treated with acids, gives the intensely coloured pararosaniline salts. Leuco-pararosaniline, paraleucaniline or triamidotriphenyl- methane, NH 2 -C 6 H 4 -CH(C 6 H 4 -NH 2 ) 2 , is prepared by reducing triamidotriphenyl carbinol with zinc-dust and hydrochloric acid, NH 2 .C 6 H 4 .C(OH)(C 6 H 4 .NH 2 ) 2 + 2H = NH 2 .C 6 H 4 .CH(C 6 H 4 .NH 2 ) 2 + H 2 0. It crystallises in colourless plates, melts at 148, and forms salts, such as the hydrochloride, C l9 H. ig N B ,3~H.C\, with three equivalents of an acid. When the hydrochloride is treated with nitrous acid, it is converted into a tri-diazo-compound, CH(C 6 H 4 'N:NC1) 3 , which, when boiled with water, yields DYES AND THEIR APPLICATION. 513 aurin, C 19 H U 3 (p. 518), and when heated with alcohol, is converted into triphenylmethane, just as diazobenzene chloride, under similar conditions, yields phenol or benzene. Constitution of Pararosaniline. Since triphenylmethane can be obtained from pararosaniline in this way, the latter is a derivative of this hydrocarbon (an important fact, first estab- lished by E. and 0. Fischer in 1878) ; moreover, pararosani- line may be prepared from triphenylmethane, as follows : Triphenylmethane is converted into trinitrotriphenylmethane, ;N"0 2 -C 6 H 4 -CH(C 6 H 4 .j^0 2 ) 2 a compound in which, it has. been shown, that all the nitro-groups are in the ^-position to the methane carbon atom* with the aid of fuming nitric acid > this rlifro-compound, on reduction, yields a substance which is identical with leuco-pararosaniline, and which, on oxidation, is readily converted into the colour-base, triamidotriphenyl carbinol ; this base, when treated with acids, yields salts of pararosaniline, with elimination of water (compare p. 511) : Hydrochloride of Pararosaniline Base Chloride of Pararosaniline. Rosaniline (of commerce), fuchsine, or magenta, is the chloride (or acetate) of triamidotolyldiphenyl carbinol, a base which is produced by the oxidation of equal molecular pro- portions of aniline, o-toluidine, and ^-toluidine (with arsenic acid, mercuric nitrate, nitrobenzene, &c.), the reaction being similar in all respects to the formation of the pararosaniline base from aniline (2 mols.) and ^>-toluidine (1 mol.), o-Toluidine. NH 2 .C 6 H 4 .CH 3 4- 6 XH - 2 + 30 = p-Toluidine. Aniline. Rosaniline Base. * The proofs of this statement are too complex to be given here. 2G 514 DYES AND THEIR APPLICATION. Rosaniline is usually manufactured at the present time by what is termed the ' nitrobenzene process? the ' arsenic acid process ' in which the oxidising agent is arsenic acid heing now little used. To the requisite mixture of aniline, o-toluidine, and jt>-toluidine* (38 parts), hydrochloric acid (20 parts) and nitrobenzene (20 parts) are added, and the whole is gradually heated to 190, small quantities of iron-filings (3-5 parts) being added from time to time (see below). At the end of five hours the reaction is complete, and steam is then led through the mass to drive off any unchanged aniline, toluidine, or nitrobenzene, after which the residue is powdered and extracted with boiling water, under pressure ; lastly, the extract is mixed with salt, and the crude rosaniline chloride which separates purified by recrystallisation. In this reaction the nitrobenzene acts only indirectly as the oxidising agent ; the ferrous chloride, produced by the action of the hydrochloric acid on the iron, is oxidised by the nitrobenzene to ferric chloride, which in its turn oxidises the mixture of aniline and toluidiues to rosaniline, and is itself again reduced to ferrous chloride ; the action is, therefore, continuous, and only a small quantity of iron is necessary. The salts of the rosaniline base with one equivalent of acid, as, for example, the chloride, C 20 H 20 N 3 C1, form magnificent crystals, which show an intense green metallic lustre; they dissolve in warm water, forming deep red solutions, and dye silk, wool, and cotton a brilliant magenta colour, the con- ditions of dyeing being the same as in the case of malachite green. The addition of alkalies to the saturated solution of the chloride of rosaniline destroys the colour, and causes the precipitation of the colour-base, triamidotolyldiplienyl carbinol, C 20 H 20 N 3 -OH (p. 511), which crystallises in colour- less needles, and, on warming with acids, is at once reconverted into the intensely coloured salts. When reduced with tin and hydrochloric acid, the rosaniline salts yield leuco-ros- aniline, C 20 H 21 N 3 (p. 511), a colourless, crystalline substance, * Crude 'aniline-oil,' a mixture of these three bases, is sometimes used instead of the pure compounds. DYES AND THEIR APPLICATION. 515 which, when treated with oxidising agents, is again converted into rosaniline. The constitution of rosaniline has been deduced in the same way as that of pararosaniline (p. 513), since, by means of the diazo-reaction, leuco-rosaniline has been converted into diphenyl-??i-tolylmethane, CH 3 -C 6 H 4 -CH(C 6 H 5 ) 2 ; leuco-rosani- line has, therefore, the constitution (4) (4)' and the rosaniline salts are derived from this base, just as those of pararosaniline and of malachite green are derived from leuco -pararosaniline and leuco-malachite green respectively. Derivatives of Pararosaniline and Rosaniline. The hydrogen atoms of the three amido-groups in pararos- aniline and rosaniline may be displaced by methyl- or ethyl- groups, by heating the dye with methyl or ethyl iodide (chloride or bromide) ; under these conditions, tri-alkyl sub- stitution products are obtained as primary products, one of the hydrogen atoms of each of the amido-groups being dis- placed. When, for example, rosaniline chloride is heated with methyl iodide or chloride, it yields, in the first place, the chloride of trimethyl-Tosamlme, This compound is a reddish-violet dye ; the corresponding trietJnjl-i-osamlme chloride is the principal constituent of Hof- mann's violet, dahlia, primula, &c. dyes, which have now been superseded by more brilliant violets. By the long-continued action of the methyl halogen com- pounds on rosaniline salts, the chloride of hexamethyl-Tosani- line, 516 DYES AND THEIR APPLICATION. is obtained. This substance is a magnificent, bluish-violet dye, but is now little used; it is a tertiary base, and, like dimethylaniline, it combines directly with methyl chloride, forming an additive compound of the constitution which, curiously enough, is green, and was formerly used under the name ' iodine green ' (so called because it was first produced with methyl iodide). Starting, then, from rosaniline, which is a brilliant red dye, and substituting methyl-groups for hydrogen, the colour first becomes reddish-violet, and then bluish-violet, as the number of alkyl-groups increases. This change is more marked when ethyl-groups are introduced, and, still more so, when phenyl- or benzyl-groups are substituted for hydrogen, as, in the latter case, pure blue dyes are produced (see below) ; in fact, by varying the number and character of the substituting groups, almost any shade from red to blue can be obtained. Lastly, it is interesting to note that, when a violet dye, like hexamethylrosaniline, combines with an alkyl halogen com- pound, it is converted into a bright green dye, which, how- ever, is somewhat unstable, and, on warming, readily decom- poses into the alkyl halogen compound and the original violet dye. A piece of paper, for example, which has been dyed with * iodine green ' becomes violet when warmed over a bunsen burner, and methyl chloride is evolved. The alkyl-derivatives of pararosaniline and of rosaniline are no longer prepared by heating the dyes with alkyl halogen compounds, but are obtained by more economical methods. The dyes of this class now actually manufactured, examples of which are described below, are, with few exceptions, deriv- atives of pararosaniline. Methyl violet appears to consist principally of the chloride of j9erctarae%/-pararosaniline ; it is usually manufactured by heating a mixture of dimethylaniline, potassium chlorate, DYES AND THEIR APPLICATION. 517 and copper chloride (or sulphate), at 50-60, for about 8 hours;* the product is treated with hot water, the copper removed by passing sulphuretted hydrogen, the solution concentrated, and the dye precipitated by the addition of salt. Methylviolet comes into the market in the form of hard lumps, which have a green metallic lustre ; it is readily soluble in alcohol and hot water, forming beautiful violet solutions, which dye silk, wool, and cotton, under the same conditions as employed in the case of malachite green (p. 511). When rosaniline is treated with aniline at 100, in the presence of some weak acid, such as acetic, benzoic, or stearic acid (which combines with the ammonia), phenyl-groups dis- place the hydrogen atoms of the amido-groups, just as in the formation of diphenylamine from aniline and aniline hydro- chloride (p. 368), C 6 H 6 .NH 2 + C 6 H 5 .NH 2 ,HC1 = (C 6 H 5 ) 2 NH + NH 3 ,HC1. Here, as in the case of the alkyl-derivatives of rosaniline, the colour of the product depends on the number of phenyl-groups which have been introduced; the mono- and di-phenyl- derivatives are reddish-violet and bluish-violet respectively, whereas triphenylrosaniline is a pure blue dye, known as aniline blue. Aniline blue, C (triphenylrosaniline chloride), is prepared by heating rosaniline with benzoic acid and an excess of aniline at 180 for about 4 hours, and until the mass dissolves in dilute acids, forming a pure blue solution. The product, which contains the aniline blue in the form of the colour-base, is then treated with hydrochloric acid, whereupon the chloride crystallises out in an almost pure condition. * The changes which take place during this remarkable process are doubtless very complex, and cannot be discussed here* 518 DYES AND THEIR APPLICATION. Aniline blue is very sparingly soluble in water, and, in dyeing with it, the operation has to be conducted in alcoholic solution. In order to get over this difficulty, the insoluble dye is treated with anhydrosulphuric acid, and thus converted into a mixture of sulphonic acids, the sodium salts of which are readily soluble, and come into the market under the names ' alkali blue,' t water UueJ &c. In dyeing silk and wool with these colouring matters, the material is first dipped into alkaline solutions of the salts, when a light-blue tint is obtained, and it is not until it has been immersed in dilute acid (to liberate the sulphonic acid), that the true blue colour is developed. Cotton is dyed in the same way, but must first be mordanted with tannin. The tri-hydroxy-derivatives of triphenyl carbinol and of tolyldi- phenyl carbinol, which correspond with the tri-amido-com pounds described above, are respectively represented by the following formulae : Trihydroxytriphenyl Carbinol. Trihydroxytolyldiphenyl Carbinol. These compounds may be obtained from the corresponding tri- amido-derivatives (the colour-bases of pararosaniline and of rosani- line) with the aid of the diazo-reaction ; in other words, the amido- compounds are treated with nitrous acid, and the solutions of the diazo-salts are then heated. The hydroxy-compounds thus produced are, however, unstable, and readily lose one molecule of water, yielding coloured compounds aurin and rosolic acid which corre- spond with the pararosaniline and rosaniline dyes in constitution, Aurin. Rosolic Acid. These substances are of little use as dyes owing to the difficulty of fixing them. The Plitlialeins. The phthaleins, like malachite green and the rosanilines, are derivatives of triphenylmethane, inasmuch as they are substitution products of phthalophenone, a compound formed DYRS AND THEIR APPLICATION. 519 from triphenylcarbinol-o-carboxylic acid, by loss of one mole- cule of water,* CO ( C 6 H 5> 2 = CO<^^>C(C 6 H 5 ) 2 + H 2 0. Phthalophenone is readily prepared by acting on a mixture of phthalyl chloride (p. 426) and benzene, with aluminium crhloride, 2CH = CC(C 6 H 4 -N0 2 ) 2 CO<^t>C(C 6 Dinitrophthalophenone. Diamidophthalophenone. Phenolphthalem. Fluorescein, C 20 H 12 5 , is a very important dye-stuff, produced by heating together phthalic anhydride and resor- cinol, Fluorescein. In this change, two hydrogen atoms of the two benzene rings unite with the oxygen atom of one of the >CO groups of the phthalic anhydride (as in the formation of phenolphthalei'n), a second molecule of water being eliminated from the hydroxyl- groups of the two resorcinol molecules. Phthalic anhydride (5 parts) and resorcinol (7 parts) are heated together at 200 until the mass has become quite solid ; the dark product is then washed with hot water, dissolved in soda, the filtered alkaline solution acidified with sulphuric acid, and the fluorescein extracted with ether. Fluorescein crystallises from alcohol in dark-red crusts ; it is almost insoluble in water, but dissolves readily in alkalies, DYES AND THEIR APPLICATION. 521 forming dark reddish-brown solutions, which, when diluted, show a most magnificent yellowish-green fluorescence (hence the name fluorescein). In the form of its sodium salt, C 20 H 10 5 Xa 2 , fluorescein comes into the market as the dye ' uranin.' Woor and silk are dyed yellow, and at the same time show a beautiful fluorescence, but the colours are faint, and soon fade, hence fluorescein has a very limited application alone, and is generally mixed with other dyes, in order to impart fluorescence. The great value of fluorescein lies in the fact that its derivatives are very important dyes. Eosin, CKcroE ( tetrabromonubr - escein), is formed when fluorescein is treated with bromine, four atoms of hydrogen in the resorcinol nuclei being displaced. Flnorescei'n is treated with the calculated quantity of bromine in acetic acid solution, and the eosin which separates is collected, washed with a little acetic acid, and dissolved in dilute potash. The filtered solution is then acidified, and the eosin extracted with ether. Eosin separates from alcohol in red crystals, and is almost insoluble in water, but dissolves readily in alkalies, forming deep-red solutions, which, on dilution, exhibit a beautiful green fluorescence, but not nearly to the same extent as solutions of fluorescein. Eosin comes into the market in the form of its potassium salt, C 20 H 6 Br 4 5 K 2 (a brownish powder), and is much used for dyeing silk, wool, cotton, and especially paper, which fixes the dye without the aid of a mordant. Silk and wool are dyed with eosin directly in a bath acidified with a little acetic acid ; but cotton must first be mordanted with zinc, lead, or aluminium salts. The shades produced are a beautiful pink, and the materials also show a very beautiful fluorescence. Tetriodofluoresce'in, C 20 H 8 I 4 5 , is also a valuable dye. Its sodium salt, C 20 H 6 I 4 5 Na2, comes into the market under the name 'erythroMii' Many other phthaleins have been prepared by condensing 522 DYES AND THEIR APPLICATION. phthalic acid and its derivatives with other phenols, and then treating the products with bromine or iodine. Azo-dyes. The azo-dyes contain the azo-group, -N:N-, to each of the nitrogen atoms of which a benzene or naphthalene nucleus is directly united. Azobenzene, C 6 H 5 -N:N-C 6 H 5 , the simplest of all azo-compounds, is not a dye, although it is intensely coloured (compare p. 502), and this is true also of other neutral azo-cornpounds ; if, however, one or more hydrogen atoms in such compounds be displaced by amido-, hydroxyl-, or sulphonic-groups, the products, as, for example, Amidoazobenzene, C 6 H 5 -N:N.C 6 H 4 .:N T H 2 , Hydroxy azobenzene, C 6 H 5 - -^ : -^' ^6^4 * OH, Azobenzenesulphonic acid, C 6 H 5 -N:N-C G H 4 -S0 3 H, are yellow or brown dyes. Azo-dyes are usually prepared by one of two general methods namely, by treating a diazo-chloride with an amido- compound* C 6 H 5 'N* T C1 + C 6 H 6 .N(CH 3 ) 2 = C 6 H 5 .N : N.C 6 H 4 .N(CH 3 ) 2 ,HC1, Dimethylamidoazobenzene Hydrochloride. CH 3 .C 6 H 4 .N:NC1 + CH 3 .C 6 H 4 -NH 2 = p-Diazotoluene Chloride. o-Toluidine. Amidoazotoluen e Hydrochloride. or by treating a diazo-chloride with a, phenol, + C 6 H 5 .OH = C 6 H 5 .N:N.C 6 H 4 .OH + HC1, Hydroxyazobeiizene. C 6 H 4 (OH) 2 = C 6 H 5 .N:N-C 6 H 3 (OH) 2 + HC1. Dihydroxyazobenzeue. In the first case the products amidoazo-compounds are basic dyes, whereas in the second case they are acid dyes. * In cases where a diazoamido-compound is first produced (p. 374), an excess of the amido-compound is employed and the mixture warmed until the intramolecular change into the amidoazo-compound is complete. DYES AND THEIR APPLICATION. 523 Another method of some general application for the direct preparation of azo-dyes containing a sulphonic-group, consists in treating diazobenzenesulphonic acid, or its anhydride (p. 384), with an amido-compound or with a phenol : S0 3 H-C 6 H 4 .N:N-OH + C 6 H 5 -NH 2 = S0 3 H.C 6 H 4 .N:N-C 6 H 4 .]S T H 2 + H 2 Amidoazobenzenesulphonic Acid. S0 3 H.C 6 H 4 .N:N.OH + C 6 H 5 -OH = S0 3 H-C 6 H 4 .N:N.C 6 H 4 .OH + H 2 0. Hydroxyazobenzenesulphouic Acid. As, however, the yield is generally a poor one, such dyes are usually prepared by sulphonating the amidoazo- or hydroxy- azo-com pounds. In all these reactions the diazo-group, C 6 H 5 'N:T-, displaces hydrogen of the benzene nucleus from the ^-position to one of the amido- or hydroxyl-groups ; substances such as jo-toluidiue, in which the ^-position is occupied, either do not interact with diazo-chlorides or only do so with great difficulty. The technical operations incurred in the production of azo-colours are, as a rule, very simple. In combining diazo-compounds with phenols, for example, the amido-compound (1 mol.) is dissolved in water and hydrochloric acid (2 mols. ), the solution well cooled with ice, and gradually mixed with the calculated quantity of sodium nitrite (1 mol.); this solution of the diazo-salt is then slowly run into the alkaline solution of the phenol, or its sulphonic acid, care being taken to keep the solution slightly alkaline, otherwise the liberated hydrochloric acid prevents combination taking place. After a short time the solution is mixed with salt, which causes the colouring matter to separate in flocculent masses ; the product is then collected in filter-presses and dried, or sent into the market in the form of a paste. The combination of diazo-compounds with amido-compounds is generally brought about by simply mixing the aqueous solution of the diazo-compound with that of the salt of the amido-compound (compare foot-note, p. 522), and then precipitating the colouring matter by the addition of common salt ; in some cases, however, the reaction takes place only in alcoholic solution. Acid azo-colours (that is, hydroxy- and sulphonic-derivatives) 524 DYES AND THEIR APPLICATION. are taken up by animal fibres directly from an acid bath, and are principally employed in dyeing wool ; they can be fixed on cotton with the aid of mordants (tin and aluminium salts being generally employed), but, as a rule, only with difficulty ; nevertheless some acid dyes, notably those of the Congo-group (p. 526), dye cotton directly without a mordant. Basic azo-dyes are readily fixed on cotton which has been mordanted with tannin, and are very largely used in dyeing calico and other cotton goods. At the present time a great many azo-colours are manu- factured, but only a few of the more typical can be mentioned here. Aniline yellow, a salt of amidoazobenzene (p. 375), is now no longer used in dyeing, because the colour is not fast, and is in many ways inferior to other readily obtainable yellow dyes. Chrysoidine (diamidoazobenzene), C 6 H 5 -N:N.C 6 H 3 (NH 2 ) 2 , is produced by mixing molecular proportions of diazobenzene chloride and m-phenylenediamine (p. 364) in aqueous solution. The hydrochloride crystallises in reddish needles, is moderately soluble in water, and dyes silk and wool directly, and cotton mordanted with tannin, an orange-yellow colour. Bismarck brown, NH 2 .C 6 H 4 -N:Is T .C 6 H 3 (NH 2 ) 2 (triamidoazo- benzene), is prepared by treating m-phenylenediamine hydro- chloride with nitrous acid, one half of the base being con- verted into the diazo-compound, which then interacts with the other half, producing the dye, NH 2 .C 6 H 4 .N:NC1 + C 6 H 4 (NH 2 ) 2 - NH 2 .C 6 H 4 .N:N.C 6 H 3 (NH 2 ) 2 ,HC1. The hydrochloride is a dark-brown powder, and is largely used in dyeing cotton (mordanted) and leather a dark brown. Helianthin (dimethylaraidoozohenzenesulphonic acid) is very easily prepared by mixing aqueous solutions of DYES AND THEIR APPLICATION. 525 diazobenzenesulphonic acid and dimethylaniline hydro- chloride, S0 3 ILC 6 H 4 .N:N.OH + C 6 H 5 -N(CH 3 ) 2 = S0 3 H.C 6 H 4 .IS T :N.C 6 H 4 .N(CH 3 ) 2 + H 2 0. The sodium salt (methylorange) is a brilliant orange-yellow powder, and dissolves freely in hot water, forming a yellow solution, which is coloured red on the addition of acids, hence its use as an indicator. It is seldom employed as a dye, on account of its sensibility to traces of acid. Resorcin yellow (tropseolin 0) is prepared by combining diazobenzenesulphonic acid and resorcinol, and has the con- stitution S0 3 H.C 6 H 4 -]Sr:]S~.C 6 H 3 (OH) 2 . Its sodium salt is a moderately brilliant orange-yellow dye, and is not readily acted on by acids ; it is chiefly employed, mixed with other dyes of similar constitution, in the production of olive- greens, maroons, &c. By using various benzene derivatives, and combining them as in the above examples, yellow and brown dyes of almost any desired shade can be obtained ; in order, however, to produce a red azo-dye, a compound, containing at least one naphthalene nucleus, must be prepared. This can be readily done by combining a benzenediazo-compound with a naphthyl- amine, naphthol, naphthalenesulphonic acid, &c., just as described above. The dyes thus obtained give various shades of reddish-brown or scarlet, and are known collectively as ' Ponceaux' or * Bordeaux.' When, for example, diazoxylene chloride is combined with /?-naphthol, a scarlet dye (scarlet K) of the composition C 6 H 3 (CH 3 ) 2 .N:N.C 10 H 5 (OH).S0 3 Na is formed; another scarlet dye (Ponceau 3R) is produced by the combination of diazo- cumene chloride with /3-naphtholdisulphonic acid, and has the composition C 6 H 2 (CH 3 ) 8 .N:N.C 10 H 4 (S0 8 Na) 2 .()H. Rocellin, S0 3 Na.C 10 H 6 .X:N.C 10 H 6 -OH, a compound pro- duced by combining ^-naphthol with the diazo-compound of naphthionic acid (p. 455), may be mentioned as an example DYES AND THEIR APPLICATION. of an azo-dye containing two naphthalene nuclei. It gives beautiful red shades, very similar to those obtained with the natural dye, cochineal, which rocellin and other allied azo- colours have, in fact, almost superseded. Within the last few years a great number of exceedingly valuable colouring matters have been prepared from benzidine, NH 2 .C 6 H 4 .C 6 H 4 .NH 2 (p. 379), and its derivatives. Benzidine may be compared with two molecules of aniline, and when diazotised it yields the salt of a di-diazo- or tetrazo- dipkenyl, C1N:^C 6 H 4 .C 6 H 4 .N:NC1. This substance inter- acts with amido-com pounds, phenols, and their sulphonic acids, just as does diazobenzene chloride (but with double the quantity), producing a variety of most important colouring matters, known as the dyes of the conga-group. Congo-red, a dye produced by the combination of tetrazo- diphenyl chloride with naphthionic acid, is one of the most valuable compounds of this class. Its sodium salt, is a scarlet powder, which, on the addition of acids, turns blue, owing to the liberation of the free sulphonic acid. The congo-dyes possess the unusual property of com- bining with unmordanted cotton, producing brownish-red shades which are fast to soap. They are much used for dye- ing cotton, but they become dull in time in any atmosphere which contains traces of acid fumes, as, for example, in the air of manufacturing towns, owing to the liberation of the blue sulphonic acids. The Benzopurpurins are also exceedingly valuable dyes of the congo-group ; they are produced by combining tetrazo- ditolyl salts* with the sulphonic acids of a- and /3-naphthyl- amine, and are, therefore, very similar to congo-red in con- *Tolidine, NH 2 -(CH 3 )C 6 H8-C 6 H 3 (CH 3 )-NH2, is produced from nitro- toluene by reactions similar to those by which benzidine is produced from nitrobenzene ; when its salts are treated with nitrous acid they yield salts of tetrazoditolyl, just as benzidine gives salts of tetrazodiphenyl, DYES AND THEIR APPLICATION. 527 stitution. They dye unmordanted cotton splendid scarlet shades, and are used in very large quantities. Various Colouring Matters. Martins' yellow (dhiitro-a-naphthol), C 10 H 5 (N0 2 ) 2 -OH, is obtained by the action of nitric acid on a-naplitliolniono-, or di-sulphonic acid, the sulphonic group or groups being elimin- ated during nitration. The commercial dye is the sodium salt, C 10 H 5 (N0 2 ) 2 -ONa ; it is readily soluble in water, and dyes silk and wool directly an intense golden yellow. When a-naphthol-trisulphonic acid is nitrated, only two of the sulphonic groups are eliminated, and the resulting sub- stance has the formula C 10 H 4 (N0 2 ) 2 (OH).S0 3 H; it is, in fact, the sulphonic acid of Martins' yellow. This valuable dye-stuff is called naphthol yellow, and comes into the market in the form of its potassium salt, C 10 H 4 (M) 2 ) 2 (OH)-S0 3 K; it is very largely used, as the yellow shades are faster to light than those of Martins' yellow. Methylene blue, C 16 H 18 N 3 SC1, was first prepared by Caro, in 1876, by the oxidation of dimethyl-^-phenylenediamine (p. 367) with ferric chloride in presence of sulphuretted hydrogen. Nitrosodimethylaniline (p. 367) is reduced in strongly acid solution with zinc-dust, or with sulphuretted hydrogen, and the solution of dimethyl -p-phenylenediamine thus obtained is treated with ferric chloride in presence of excess of sulphuretted hydrogen. The intensely blue solution thus obtained is mixed with salt and zinc chloride, which precipitate the colouring matter as a zinc double salt, in which form it comes into the market. Methylene blue is readily soluble in water, and is a valuable cotton-blue, as it dyes cotton, mordanted with tannin, a beautiful blue, which is very fast to light and soap; it is not much used in dyeing silk or wool. Indigo, C ]6 H 10 N 2 2 , is a natural dye, which has been used from the earliest times. It is contained in the leaves of the indigo plant (Indigo/era tinctoria) and in woad (Isatis tindoria) 528 DYES AND THEIR APPLICATION. in the form of the glucoside * indican ; ' when the leaves are macerated with water, this glucoside undergoes fermentation, and indigo separates as a blue scum. Indigo comes into the market in an impure condition in the form of dark-blue lumps, and, especially when rubbed, shows a remarkable copper-like lustre \ it is insoluble in water and most other solvents, but dissolves readily in hot aniline, from which it crystallises on cooling ; it sublimes, when heated, in the form of a purple vapour, and condenses as a dark-blue crystalline powder, which consists of pure 'indigotin,' the principal and most valuable constituent of commercial indigo. Reducing agents convert indigo into its leuco-compound, indigo white^ which, in contact with air, is rapidly recon- verted into indigo, a property made use of in dyeing with this substance (p. 507) ; concentrated sulphuric acid dis- solves indigo with formation of indigodisulplionic acid, C 16 H 8 N 2 2 (S0 3 H) 2 , the sodium salt of which is used in dyeing under the name * indigo carmine.' Indigo has been synthetically produced by Baeyer by various reactions, two of the more important of which are mentioned on pp. 408 and 433. CHAPTER XXX Y. STEREO-ISOMERISM. The constant use of graphic formulae in studying carbon compounds was strongly recommended in an early chapter (part i. p. 53), because, as was then pointed out, such formulae afford a fairly sure and complete summary of the chemical properties of the substances which they represent, whereas the ordinary molecular formulae express little, and are besides more difficult to remember. The true significance of graphic formulae was also explained; the lines which are drawn between any two atoms simply express the conclusion that, STEREO-ISOMERISM. 529 as far as can be ascertained experimentally, these particular atoms are directly united, without attempting to give the slightest indication of the nature of this union, or of the direction in which the force of affinity is exerted. AVhen, therefore, formulae such as the following H H H I I I H C H H C Cl H C OH I I I H Cl H are employed, it must not be supposed that they give any idea whatever of the actual form of the molecule, or intend to indicate that all the atoms in the molecule lie in one plane (that is, the plane of the paper) ; such an assumption is unsup- ported by facts, and is, moreover, shown to be incorrect by many considerations, of which the following may be men- tioned. (a) Experience has shown that methylene chloride, CH 2 C1 2 , exists in only one form, and all attempts to obtain an isomeride have failed; yet, if a compound of this com- position were actually represented by the above plane formula, it should be capable of existing in two isomeric forms namely, H H H C Cl and Cl C Cl Cl H because in one case the chlorine atoms would be adjacent, in the other they would be separated by hydrogen atoms, and the relative positions of all the atoms not being identical, the substances themselves could not be so. (b) Again, only two isomeric dichlorethanes namely, CH 2 C1-CH 2 C1 and CH 3 -CHC1 2 , are known, whereas, if ethane and its derivatives were actually composed of atoms, 2 H 530 STERRO-ISOMERISM. all of which lie in one plane, the following five isomeric dichlorethanes should be capable of existence : H H H H H Cl H C C H H C C Cl H C C H Cl Cl Cl H Cl H H Cl H Cl II II H C C Cl H C C H u u These, and a great many other similar cases, show con- clusively that the atoms in the molecule of a carbon com- pound cannot lie in one plane; were this so, it would be impossible to explain the fact that a large number of isomerides which, theoretically, would be capable of existence, have never yet been prepared. If, then, an attempt be made to account satisfactorily for the known isomerism of carbon compounds, it is found that this can be done by assuming that each of the several atoms or groups with which a carbon atom is united is situated at some point on one of four different lines, which are symmetrically arranged in the space around the carbon atom. In other words, it may be supposed that the carbon atom is situated in the centre of an imaginary regular tetrahedron, and that its four affinities (those forces by virtue of which it unites with four atoms or groups) act in the directions of straight lines drawn from the centre of the tetrahedron to the four corners, as represented by the dark lines in the following figure : STEREO-ISOMERISM. 531 Xow this highly important theory, which was advanced by Le Bel and van't Hoff, independently, in 1874, is not based solely on the fact that it explains the non-existence of a larger number of isomerides of a given substance than is actually known ; it is also supported by positive evidence of a very weighty character, and } indeed, may be shown to accord well with all known facts. If, then, this theory be applied in the case of some of the simplest organic compounds, it leads to the following con- clusions : (1) Assuming that one of the hydrogen atoms in marsh- gas, CH 4 , is displaced by an atom X, there can only be one, substitution product of the type CH 3 X, because all the hydrogen atoms are identically situated. (2) Only one di-substitution product of the type CH 2 XY, such as CH 2 C1 2 or CH 2 ClBr (in which X and Y are either identical or dissimilar), is also possible, formulas such as being absolutely identical, although they may appear to be different on paper. Points such as these can only be clearly understood by actually handling models made to represent arrangements of this kind ; * it will then be seen at once that, in whatever manner the positions of the different atoms H H X Y are * In order to facilitate the study of stereochemistry, sets of models similar to those recommended by Friedlander have been specially prepared at the authors' request by Messrs Baird and Tatlock (14 Cross Street, Hatton Garden, London, E.C.), from whom they may be obtained at a cost of eighteen pence. Such sets contain sufficient models for the study of the isomerism of the tartaric acids, but larger sets adapted for the study of the sugars may also be obtained. 532 STEREO-ISOMERISM. varied, only one arrangement is possible, the apparent differ- ence which exists on paper vanishing at once on rotating the models. (3) In the case of the tri-substitution products of methane, also, one form only is possible, where any two of the sub- stituting atoms, or groups of atoms, are the same, as, for example, in the compounds CHC1 3 (CH 3 ) 2 CH. OH (C 2 H 5 ) 2 CH . CH 2 - OH. In all these cases there is perfect agreement between fact and theory, compounds of the given types being known in one form only. (4) If, however, three atoms in marsh-gas be substituted by three different groups, compounds of the type C, H, X, Y, Z* in which the carbon atom is united with four different atoms or groups being obtained, then it is possible to construct two, but only two, different arrangements, which cannot be made to coincide by rotation, or in any other way ; these two forms may be represented by the following figures : Z Z Y In working with the models this is very clearly seen, by first inserting the red, white, blue, and yellow balls into the two india- rubber carbon models, in such a way as to produce identical arrangements ; by then interchanging any two of the balls in one of the models, a form will be obtained which is different from, and which, therefore, cannot be made to coincide with, the other form by rotating. These two arrangements are related to one another, in the same way as an object to its mirror-image that is to say, if one be held before a mirror, the position of X, Y, and Z in relation to H in the mirror-image will be found to be * Or C, r, 6, ?0, y; compare foot-note, p. 536. STEREO-ISOMERISM. 533 identical with those in the other viewed directly, an interesting point, which again is much more clearly seen by using models ; for the sake of convenience, one of these arrangements may be denoted by + , the other by - , the actual choice being im- material. When, therefore, a carbon atom is united to four different atoms or groups, H, X, Y, and Z, the compound which is pro- duced may, theoretically, exist in two distinct modifications, related to one another in the same way as an object to its mirror-image. Any carbon atom united in this way is called an ' asymmetric carbon atom,' on account of its unsymmetrical or asymmetrical nature. Now certain substances, such as active amyl alcohol, sarco- lactic acid, malic acid,* and mandelic acid (p. 440), which have already been described, have the property of rotating the plane of polarised light, and experience has shown that all substances which have this property, when in a liquid state, or in solution, exist in (at least) two forms, one of which rotates the plane of polarisation to the right, the other doing so to precisely the same extent to the left. On considering the constitutional formulae of such optically active organic substances, one remarkable fact is brought to light namely, that the molecule always contains at least one asymmetric carbon atom, as is indicated in the follow- ing formula?, in which the symbol of this particular carbon atom is printed in heavy type : CH,H 3X , H\;H- C 2 H;H 2 -OH H/ Active Amyl Alcohol. Lactic Acid. CH 2 .COOH C 6 H 5 \ OH \/ OH COOH Malic Acid. Mandelic Acid. * These three compounds are described in part i. pp. 105, 227, 239. 534 STEREO-ISOMERTSM. That this property of rotating the plane of polarised light is due to the presence in the molecule of an asymmetric carbon atom is practically proved by the fact that all optically active compounds of known constitution contain a carbon atom united in this way, arid also by the fact that if by any means the asymmetric character of the carbon atom be destroyed, the power of rotating the plane of polarised light also disappears. Sarcoladic acid, for example, is optically active, but when reduced with hydriodic acid, it yields propionic acid, which is inactive, because it does not contain a carbon atom united with four different atoms or groups, OH CH 3 v/H H/ \COOH H/ \COOH Active, Inactive. Malic acid, again, is optically active, but, on reduction, in- active succinic acid is formed, H\ /CH 2 .COOH H CH 2 -COOH ^5? OETXJOOH ^-^ ^ Active. Inactive. A still more instructive case is afforded by active amyl alcohol, and the following derivatives : 3 x/ -OH C 2 H/ \CH 2 T Amyl Alcohol. Amyl Iodide. 3 \^ / C C 2 H CH 2 .CN C 2 H/ X CH 2 .COOH Amyl Cyanide. Methylethylpropionic Acid. These substances, prepared from active amyl alcohol by the STEREO-ISOMERISM. 535 usual series of reactions, are themselves optically active, be- cause they still contain an asymmetric carbon atom ; if, how- ever, the iodide be reduced to the hydrocarbon CH 3 \/H C 2 H / \CH 3 Dimethylethylmethane. the asymmetric character of the carbon atom is destroyed, and a substance is formed which is optically inactive. This relation between the presence of an asymmetric car- bon atom and the property of rotating the plane of polarised light, was first pointed out by Le Bel and van't Hoff, and is now supported by such a mass of evidence that it may be regarded as established. Considering now some of the simplest optically active substances namely, those containing only one asymmetric carbon atom, it may be repeated that they invariably exist in two optically active forms, one of which is dextrorotatory (d or + ), the other levorotatory (/or - ) to exactly the same extent. These two forms are called optical, physical, or stereo- chemical i8omerid.es ; they have the same chemical properties and chemical constitution, because their molecules differ only as regards the arrangement in space. They have also the same melting-point and boiling-point, and are identical in other physical properties, except that they almost invariably differ to a greater or less extent in crystalline form, inasmuch as the crystals of the one are to those of the other as an object to its mirror-image (p. 540). When any substance containing one asymmetric carbon atom is prepared synthetically, the product is found to be optically inactive. When, for example, lactic acid is produced from a-bromopropionic acid, or malic acid from bromosuccinic acid (part i. pp. 226 and 240), the product in each case has no action on polarised light. This is due to the fact that the product contains equal quantities of the d and / forms, and the action on polarised 536 STEREO-ISOMERISM. light of the one is exactly counterbalanced by that of the other. This can be proved by simply dissolving together equal quantities of the d and I forms, and then evaporating the solution, when an inactive product, identical with that produced synthetically, is obtained. When, moreover, this inactive product is a solid, it is found, as a rule, to differ very considerably from the active forms in physical properties ; it has a different melting-point (usually a higher one), different solubility, and a different crystalline form, and is spoken of as the racemic (inactive or i.r.) modification of the compound. Liquid racemic modifica- tions are not known, and it is doubtful whether they are capable of existing. The above statements refer simply to compounds containing only one asymmetric carbon atom. No matter how many carbon atoms the molecule may contain, or what the nature of the other atoms may be, as long as only one of the carbon atoms is combined with four different atoms or groups, the com- pound exists only in the above three optically different forms namely, d, I, and i.r. ; a substance of the constitution H CH 3 .CH 2 .CH 2 -CH 2 C COOH, OH for example, would not form a larger number of optical isomerides than a simple substance such as lactic acid. When, however, a compound contains two asymmetric carbon atoms, a larger number of modifications may exist in accordance with the above theory, as will be seen at once by constructing models in the following manner : I. Make two identical asymmetric carbon atoms, C, r, b, to, y t * each of which, for convenience, may be designated +; now remove y from both models, join the two open ends by means * The letters r, 6, w and y refer to the red, blue, white, and yellow balls in the sets of models. STEREO-ISOMERISM. 537 of the rod, and lay the model on the table, so that the two red balls point upwards. This is one possible modification, a plane figure of which may be obtained by pressing the red balls outwards on the table, when it will appear like this : r ;& + or -10 MODIFICATION L The removal of one of the balls, representing one of the atoms or groups, and the substitution for it of the more complex group (C, ?', b, w), still leaves each carbon atom asymmetrical ; in other words, each is now combined with the four different groups (b), (w), (r), and (C, r, b, iv\ instead of with (r), (b), (w), and (y). II. Repeat the above operations, starting, however, with two identical asymmetric carbon atoms, C, r, b, y, w, which are the mirror-images of those taken in (I.), and which may, therefore, be called ; the plane representation of this model will be r i * w _'_ or MODIFICATION II. This form is quite different from L, because the one can- not possibly be converted into the other by rotation ; if, for example, II. be turned over, the positions of b and w will correspond with those in I., but although the flat images would be the same, the two are not identical, because r, r will 538 STEREO-ISOMEfclSM. now point downwards in II., whereas they pointed upwards in I. ; if, in fact, this model (II.) be held before a mirror, it will be seen that it is not identical with its mirror-image, but that its mirror-image is identical with I. viewed directly. III. If now two different asymmetric carbon atoms, C, r, b, w, y, and C, r, b, y, w, or + and , be joined in the same manner as before, another modification will be obtained which is quite different from I. and II., and which may be represented thus : r I w b + I or w b MODIFICATION III. No other forms different from these three can be con- structed. It is evident, then, that a compound containing two asymmetric carbon atoms may form three distinct modifications. One of these (I.) will be dextrorotatory, because it contains two identical ( + ) asymmetric carbon atoms ; the other (II.) will be levorotatory to exactly the same extent, because it contains two identical (-) asym- metric carbon atoms. The third form, on the other hand, will be optically inactive j the molecule which it represents contains two different asymmetric carbon atoms, one + and the other , and consequently the dextrorotatory action of the one is exactly counterbalanced by the levorotatory action of the other ; in other words, the rotatory power of one part of this molecule is compensated or neutralised by that of the other part ; such a compound is said to be inactive by internal compensation. There is, however, a fourth modification which has not yet been considered in the present case ; by dissolving equal quantities of the two active (d and I) forms, and then evap- STEREO-ISOMERISM. 539 orating, ail inactive or racemic modification may be obtained, just as in the case of the lactic acids, &c., and this form is said to be inactive by external compensation, the action of two separate molecules counterbalancing one another. In order to decide which two of the above three forms represent the active (d and I) modifications of the substance, it is only necessary to determine which two models behave to each other as object to mirror-image. This will be found to be the case with the forms I. and II., which are therefore the active forms; on the other hand, the form III. coincides with its own mirror-image, and is, therefore, inactive. The same conclusions are arrived at by disconnecting and then comparing the asymmetric carbon atoms, when it is easy to see that one of the models is composed of two different arrangements ; this, therefore, is the form which is inactive by internal compensation. Stereo-isomensm of the Tartaric Acids. One of the best examples of the stereo-isomerisni of sub- stances containing two asymmetric carbon atoms is that of the tartaric acids, COOH-CH(OH)-CH(OH).COOH. As will be seen from the constitutional formula, there are two carbon atoms, each of which is united with four different atoms or groups namely, {COOH}, {H}, {OH}, and {CH(OH)-COOH}, and consequently, theoretically, there should be four physically isomeric forms of this acid. As a matter of fact, four modifications are known namely, dextrotartaric, levotartaric, mesotartaric and racemic acid, (part i. p. 245). Dextrotartaric acid and levotartaric acid are the two optically active modifications, and may be respectively repre- sented by the formulae, COOH COOH I I H C OH + OH C H - I and [ C- OH C H + H C-OH I I COOH COOH 540 STEREO-ISOMERISM. The one rotates the plane of polarisation to the right to exactly the same extent as the other to the left ; but in all other respects they are identical, except for slight differences in crystalline form. They possess the same melting-point, and the same solubility in various solvents ; their metallic salts have the same composition, and crystallise with the same number of molecules of water. Their ethereal salts melt and boil at the same temperature ; all their salts, like the acids themselves, are optically active to the same extent, but in opposite directions. In addition to this difference in their action on polarised light, these two active tartaric acids and the corresponding salts show a slight difference in crystalline form, which is exhibited very clearly in the case of the well-defined crystals of their sodium ammonium salts, C 4 H 4 6 Na(NH 4 ) + 4H 2 0. Fig. 21. If these crystals be examined, it will be found that certain faces (those which are darkened in the figures) which are on the right-hand side of the crystals of the dextrorotatory acid, are on the left-hand side of those of the levorotatory acid. The two kinds of crystals are, in fact, related as an object to its mirror-image, as will be seen by holding i. before a mirror, when the darkened faces will appear as in u. viewed directly, and vice versd. A similar difference in the crystalline form is observed in the case of other optically active substances, and such crystals are said to be enantiomorphous. Mesotartaric acid, C 4 H 6 6 , is the simple optically inactive STEREO-1SOMERISM. 541 form of tartaric acid ; that is to say, it is inactive by internal compensation (see above), and may be represented by the formula, COOH I H C OH + H C OH - It differs from the two optically active forms in many re- spects, as, for example, in melting-point, solubility, and crystalline form. It might, in fact, be regarded as quite a different substance from an examination of its physical pro- perties, and of those of its salts, although, in chemical pro- perties, it is identical with the active forms. On the other hand, mesotartaric acid resembles racemic acid very closely in physical properties, but, unlike the latter, it cannot be resolved into two optically active modifications, because it is a simple substance. Racemic acid, C 4 H 6 6 ,C 4 H 6 6 , is the double inactive form of tartaric acid, and is simply composed of equal quanti- ties of dextro- and levo-tartaric acids ; that is to say, it is inactive by external compensation (see above), and may be r\ TT Q 4-4- represented by the formula \ r^^fr\ ^ a ^ so ^e- I L/ 4 1 6 U 6 ~ haves as if it were a distinct substance, as far as physical properties are concerned, which is all the more remarkable when it is borne in mind that racemic acid is obtained on evaporating a solution of equal quantities of the two active modifications, and that it can be again separated into these two forms by the methods given below. It will be seen from the above examples that the existence of physical isomerides, and the number of such modifications, is in complete accordance with the theory of Le Bel and van't Hoff, and a great many other cases might be mentioned in which the agreement is quite as perfect. 542 STEREOISOMERISM. As the number of asymmetric carbon atoms increases, the number of isomerides naturally becomes larger, so that a substance such as saccharic acid (part i. pp. 264, 270), COOH.CH(OH)-CH(OH).CH(OH).CH(OH).COOH, which contains four asymmetric carbon atoms, is capable of existing in ten optically isomeric forms (which may be con- structed with the aid of models). As in the case of chemical isomerism, however, all the theoretically possible isomerides of a given substance have not always been actually obtained owing to experimental diffi- culties ; dimethylsuccinic acid, COOH.CH(CH 3 ).CH(CH 3 ).COOH, for example, like tartaric acid, should exist in four forms, but only two are known, both of which are optically inactive, the two active forms not having yet been isolated. An examination of the models of substances containing two asymmetric carbon atoms that is, of substances derived from the symbol, might lead to the supposition tliat they should exist in more than four modifications. In the first place, the model could be so arranged that the directions of the affinities of the two carbon atoms would be as shown in the figure. If, then, one of the carbon atoms were slowly STEREO-ISOMERISM. 543 rotated about an axis, an infinite number of forms would be pro- duced, all of which would be different, because they would represent different relative positions in space of the atoms constituting the molecule. It would be just the same even if the substance did not contain an asymmetrical carbon atom ; ethane, CH 3 -CH 3 , or ethylene chloride, CH 2 C1-CH S C1, for example, could in this way be represented as existing in an infinite number of modifications. This objection, however, at once disappears on considering the matter a little more carefully. In a compound represented by the above symbol (by attaching atoms or groups to the corners of the imaginary tetrahedra), the atoms or groups united with one of the carbon atoms must exert a certain attraction or repulsion on those united with the other, those which have the greatest affinity for each other striving to approach as nearly as possible, until a certain position of equilibrium, which is the resultant of all the mutual attractions, is reached. This position may be disturbed by the application of heat or of some other force, but on removing the disturbing element, the original form will be restored, so that, under given conditions, the compound only exists in one form, unless, of course, it contains asymmetric carbon atoms. Resolution of Racemic Modifications. The racemic modification of tartaric acid and the corre- sponding forms of other optically active substances namely, of those which are inactive because they are composed of equal quantities of the two opposed active forms may sometimes be resolved into their components by one or other of the follow- ing methods : (1) By crystallisation of the salt formed by the combina- tion of a racemic acid or base with an optically inactive sub- stance. This method was first employed by Pasteur in the case of racemic (tartaric) acid, and depends on the fact that if a solution of sodium ammonium racemate be allowed to crystallise at a particular temperature (below 28), enantio- morphous crystals (right- and left-handed, as shown in the fig., p. 540) are deposited. If now these crystals are sorted mechanically, the right-handed ones being placed in one vessel, the left-handed ones in another, a separation of the 544 STEREO-ISOMERISM. racemic acid into its constituents is accomplished, one kind of crystals being those of the salt of the dextro-acid, the other those of the salt of the levo-acid. If, however, crystallisation take place at temperatures above 28, only one kind of crystal is deposited namely, crystals of sodium ammonium racemate, which do not exist in enantiomorphous forms, and which, indeed, belong to quite a different crystalline system. This method of separation is not applicable in all cases, because, as a rule, the crystals of the salts of the two active components are not sufficiently well defined to allow of their mechanical separation, even if they are deposited separately. (2) A second method, also discovered by Pasteur, consists in fractionally crystallising the salt formed from a racemic acid or base with an optically active substance. This method depends on the fact, that the two constituents of the racemic modification, form, with one and the same optically active substance, salts which differ in solubility, and which, there- fore, can be separated by fractional crystallisation in the ordinary way. If, for example, racemic acid be combined with the optically active base cinchonine (p. 493) or strychnine (p. 494), the product may be resolved into the salts of the dextro- and levo-acids ; in a similar manner the inactive modification of coniine (p. 489) may be resolved into its constituents by fractional crystallisation of the salt which it forms with dextrorotatory tartaric acid. (3) Another method of separation, quite different in principle from the foregoing, depends on the fact that if certain organisms, such as' penicillium glaucum, be placed in a solution of a'racemic modification, they feed on and, therefore, destroy one usually the dextro modification, the result being that, after a time, the solution contains only the levo- isomeride. INDEX. [Where more than one reference is given, and one of them is in heavy type, the latter refers to the systematic description of the substance.] PAGE Acetanilide 360, 362 Acetophenone 411 Acetophenonehydrazone 412 Acetophenoneoxime 412 Acetotoluidide 360 Acetylbenzene 411 Acetylcodeine 497 Acid dyes, 506 ; Acid green 511 AcroleTn 482 Acrylaniline 482 Active amyl alcohol 533 Alizarin, 464, 465 ; constitution of, 467; diacetate, 467 ; dyeing with. ..504 Alkali blue 518 Alkaloids, 484 ; extraction of 488 Alkaloids, contained in opium, 495 ; derived from pyridine, 488 ; derived from quinoline, 492 ; related to uric acid 498 Alkylanilines 364 Amalinic acid. 498 Amidoazobenzene, 375, 522, 524 ; hydrochloride, 375 ; sulphonicacid 523 Amidoazo-compounds 374 Amidoa2Otoluene hydrochloride 522 Amidobenzaldehydes 408, 410 Amidobenzene 361 Amidobenzenesulphonic acid, tn, o. . .384 Amidobenzenesulphonic acid, / 383 Amidobenzoic acid, m, o, p 422 Amido-compounds 325, 355 Amidoethylsulphonic acid 501 Amidonaphthalene 444, 451 -Amido-,3-naphthol 455 i :4-Amidonaphthol 455 Amidophenol, / 415 Amidotoluene 364 Amygdalin 405 Amyl alcohol, 534 ; cyanide, 534 ; iodide 534 ir. 2i Auethole 410, 439 Aniline, 361 ; homologues of, 364 ; hydrochloride, 362 ; platinochlo- ride, 362; stannichloride, 356, 361 ; substitution products of, 363 ; sul- phate, 362; sulphonicacid,/ 383 Aniline blue 517 Aniline yellow 524 Animal charcoal, use of. 393 Anisalcohol 404 Anisaldehyde 404, 410 Anisic acid 404, 41 r, 439 Anisole 392 Anisyl alcohol 410 Anthracene.. 298. 328, 457 Anthracene, constitution of. 458 Anthracene derivatives, isomerism of. 461 Anthracene dichloride 462 Anthracene disulphonic acids 464 Anthracene oil 296, 298 Anthracene picrate 458 Anthranilic acid 422, 437 Anthranol 464 Amhrapurpurin 468 Anthraquinone, 458, 462 ; test for 465 Anthraquinone-/3-monosulphonic acid 464, 466 Anthraquinonedisulphonic acid 468 Anthraquinonesulphonic acid, sodium salt of. 464 Antifebrin 362, 500 Antipyrine 499 Arbutin 399 Aromatic, alcohols, 385, 402 ; alde- hydes, 405 ; amines, 355, 368 ; com- pounds, general properties of, 322 ; halogen derivatives 341 Aseptol 396 Asymmetric carbon atom 533 Atropine, 490; sulphate 491 ; test for.49i INDEX. PAGE Aurin 513, 518 Azobenzene 378, 522 Azobenzenesulphonic acid 522 Azo-compounds 377 Azo-dyes, 506, 522; preparation of. ..523 Basic dyes 506 Baumann and Schotten's method . . . .420 Benzal chloride 341, 342, 349, 407 Benzaldehyde, 405 ; bisulphite comp.4o6 Benzaldehyde green 510 Benzaldoxime 407 Benzamide 421 Benzene, 297, 298; constitution of. ..303 Benzene derivatives, constitution 0^.317 Benzene derivatives, isomerism of.... 310 Benzene hexabromide 303 Benzene hexachloride 303, 326 Benzene hexahydride 326 Benzene homologues, 328 ; properties of, 331 ; oxidation of. 333 Benzene, synthesis of 301, 324 Benzene-;-dicarboxylic acid 426 Benzene-0-dicarboxylic acid 425 Benzene-/-dicarboxylic acid 427 Benzenedisulphonic acid, m, o, p. . . .383 Benzenesulphonamide 383 Benzen'esulphonic acid 382 Benzenesulphonic chloride 383 Benzidine 379, 526 Benzoic acid, 418 ; salts of, 419 ; sub- stitution products of 422 Benzoic anhydride 420 Benzonitrile 421 Benzophenone 340, 412 Benzopurpurin 526 Benzoquinone 413 Benzotrichloride 342, 349 Benzoylaniline 420 Benzoylbenzene 412 Benzoylbenzoic acid, o 463 Benzoyl chloride, Benzoyl-group .... 420 Benzyl, acetate, 349, 403 ; alcohol, 403 ; bromide, 403 ; chloride, 340, 342, 348, 460 ; cyanide, 422, 429 ; ethyl ether, 418 ; radicle 333 Benzylamine 368 Benzylidene radicle 407 Benzylideneacetone 407 Benzylidenehydrazone 407 Benzylidenehydroxycyanide 407 Benzylmalonic acid 429 ii PAGE BetaTne, 500 ; chloride 501 Bismarck brown 524 Bone-oil, Bone-tar 472 Bordeaux 525 Brilliant green 511 Bromacetylene 324 Bromanthraquinone 463 Bromobenzene 303, 347 Bromobenzoic acid, m, o, p 422 Bromobenzoylbenzoic acid 463 Bromobenzyl bromide, o 460, 469 a-Bromonaphthalene 450 /3-Bromonaphthalene 450 Bromonitrobenzene, m, o, p 354 Bromophthalic acid ; anhydride 463 Bromotoluene, o 461 Brucine, 494, 495 ; test for 495 Brucine, ethiodide ; hydrochloride ..495 Butyrolactone 519 Butyrophenone 412 Caffeine, 497 ; hydrochloride 498 Calico-printing 505 Carbazole 457 Carbolic, acid, 297, 298, 391 ; oil. ...296 Carboxylic acids 416 Carvacrol 339, 397 Catechol 398, 467 Catecholcarboxylic acid 439 Catechu 398 Chloracetanilide 363 Chloranil 416 Chloraniline, tn, o, p 363 Chlorobenzene 303, 347 Chlorobenzoic acid 348 Chlorobenzyl chloride, / 343 Chloronaphthalene, -, 449 ; /3- 450 Chloronitrobenzene, m, o, p, 354 ; ^..363 Chjorotoluene, m,o,p 348 Choline, 500 ; chloride 500 Chrysoidine 524 Cinchomeronic acid 483 Cinchona-bark, alkaloids of. 492 Cinchonine 492, 493 Cinchoninic acid 493 Cinnamic, acid, 430 ; aldehyde 405 Closed-chain compounds 323 Coal-tar, distillation of. 295, 299 Coca, alkaloids of 49 1 Cocaine, 491 ; hydrochloride 491 Codeine 495. 497 Coke... 295 INDEX. PAGE Collidines 478 Colour-base 508 Congo group of dyes. 5-4, 526 Congo-red 455, 526 Coniine, 488 ; hydrochloride 469 Creosote oil 296, 298 Cresol, 298 ; Cresol, m, o, p 396 Cumene 338 Cumic acid 338 Cymene 339 Dahlia 515 Daturine 490 Dextrorotatory compounds 535 Dextrotartaric acid 539 Diallyl, Diallyl tetrabromide 303 Diamidoazobenzene ; hydrochloride.. 524 Diamidobenzene 364 Diamidobenzene, m 354 Diamido-compounds 360 Diamidodiphenyl, p 378 i:4-Diamidonaphthalene 455 Diamidophthalophenone 520 Diazoamidobenzene 374 Diazoamido-compounds 374 Diazobenzene, chloride, 371 ; cyanide, 372 ; nitrate, 371 ; sulphate 371 Diazobenzenesulphonic acid . . . .384, 523 Diazo-compounds 325, 344, 370 Diazo-compounds, constitution of 373 Diazocumene chloride 525 Diazotoluene chloride 372 Diazoxylene chloride 525 Dibasic acids 423 Dibenzylamine 369 a/3-Dibromanthraquinone 465 Dibromethy Ibenzene 432 Dibromopyridine 473 Dichloranthracene 462 Dichlorobenzene 303 Dichloronaphthalene 450 Diethylaniline 365 Digallic acid 440 Dihydric phenols 387, 388, 398 Dihydrobenzene 3<>9> 3 2 6 Dihydroxyanthraquinones 468 /3-Dihydroxyanthraquinone 465 Dihydroxyazobenzene' 522 Dihydroxybenzene, m, o, 398 ; p 399 Dihydroxybenzoic acids 439 i:2-Dihydroxynaphthalene 456 1 14-Dihydroxy naphthalene 456 PAGE Dihydroxyphenanthrene 470 Dihydroxyphthalophenone 519 Dimethylainidoazobenzene 376 Dimethylamidoazobenzene hydro- chloride 522 Dimethylamidoazobenzenesulphonic acid 524 Dimethylaniline 358, 366 Dimethylbenzidine 379 Dimethylcatechol 398 Dimethylethylmethane 535 Dimethyl-/-phenylenediamine. .367, 527 Dirnethylpyridines 478 a-Dmaphthol, /3-Dinaphthol 454 Dinitro--disulphon:c acid, potassium salt of 454 Dinitro-a-naphthol 454, 527 Dinitrobenzene, m, 353; o,/....353, 354 Dinitrobenzenes, constitution of 318 Dinitrophthajophenone 520 Diphenic acid 469, 470, 471 Diphenic anhydride 471 Diphenyl, 327, 340, 469 ; ketone, 340, 412 Diphenylamine 359, 367 Diphenyldicarboxylic acid 469 Diphenylethylene 469 Diphenylmethane 340, 413 Diphenyl-w-tolylmethane 511, 515 Dippel's oil 472 Dipropargyl.. 303 Ditolyl, o 469 Dyes and their application 502 Ecgonine 491 Enantiomorphous crystals 540 Eosin, 521; potassium salt of 521 Erythrosin 521 Ethyl, benzenesulphonate, 381 ; ben- zoate, 419 ; benzylmalonate, 429 ; mandelate, 441 ; phthalate, 426 ; salicy late 438 Ethylaniline 3^5 Ethylbenzene 335, 337 Ethylbenzylaniline 511 Fatty compounds 322 Fittig's reaction 330 Fluorescei'n, 425, 520; reaction, 399; sodium salt of. 521 Formanilide 363 Friedel and Craft's reaction 329, 411 Fuchsine 513 iii INDEX. PAGE Gallic acid 439 Gas liquor 295 Glucosides 488 Guaiacol 398 Gum benzoin 418 Heavy oil 296 Helianthin 524 Hemimellitene 338 Hemlock, alkaloids of 488 Hexahydropyridine 473, 476 Hexahydrotetrahydroxybenzoic acid. 492 Hexamethylene 326 Hexamethylrosaniline chloride 515 Hippuric acid 418 Hofmann's violet 515 Hydranthracene 460, 461 Hydrazines 373 Hydrazobenzene 378 Hydrobenzamide 408 Hydrocarbons, aromatic, oxidation of 417 Hydrocinnamic acid 430 Hydroquinone 399, 414 Hydroxyaldehydes, aromatic 408 Hydroxyantbraquinone 463, 466 Hydroxyazobenzene 522 Hydroxyazobenzenesulphonic acid.. .523 Hydroxybenzaldehyde, ;;/, p> 410 ; o 409 Hydroxybenzene 391 Hydroxybenzoic acid, 455 PAGE /3-Naphthaquinone 456 Naphthionic acid 455, 525, 526 -Naphthol, 447, 453 ; /3-Naphthol. . .454 Naphthol yellow 454, 527 Naphthol yellow, potassium salt 527 a-Naphtholdisulphonic acid 527 /3-Naphtholdisulphonic acid 525 -Naphtholmonosulphonic acid 527 Naphtholmonosulphonic acids 455 Naphthols 452 a-Naphtholtrisulphonic acid 454, 527 -Naphthylamine 452, 453 /3-NaphthyIamine 452 Naphthylaminemonosulphonic acids. 455 Naphthylamines 449 i :4-Naphthylaminesulphonic acid 455 Narcotine 495 Neurine, 501 ; chloride 501 Nicotine, 489 ; dimethiodide, 489 ; hydrochloride 489 Nicotinic acid 472, 479, 490 Nightshade, alkaloids of 490 Nitracetanilide, o, p 363 Nitraniline, t/t, 354 ; m, a, p 363 a^-Nitroalizarin, /31-Nitroalizarin 467 Nitrobenzaldehyde, m, o, p 408 Nitrobenzene, 352 ; oxidising action of 480, 514 Nitrobenzoic acid, m, o, p 422 Nitrocinnamic acid, o, p 432 N itro-compounds 325, 350 Nitronaphthalene 444 -Nitronaphthalene 451 /3-Nitronaphthalene 451 jS-Nitro-oe-naphthylamine 451 Nitrophenol, m, o, p 392 Nitrophenyldibromopropionic acid, o, p 432 Nitrophenylpropiolic acid, o 432 Nitrophthalic acid 444 Nitrosodimethylaniline 366, 367, 527 Nitrosomethylaniline 366 Nitrosophenol, p 367 Nitrosopiperidine 477 Nitrotoluene, in, o, p 355 Nux vomica, alkaloids of 494 Oil of aniseed, 410, 439 ; bitter almonds, 405 ; wintergreen 437 Open-chain compounds 323 Opium, 496 ; alkaloids of 495 Optical isomerides 535 V INDEX. PAGE Optically active substances 533 Organic compounds, classification of.322 Ortho-compounds 313 Orthodiketones 470 Orthoquinones 456 Osazones 377 Oxanilide 363 Oxanthrol 464 Papaverine 495 Para-compounds 313 Paraleucaniline 511, 512 Paraquinones 456 Pararosaniline, 511, 512 ; base of, 511 ; chloride, 511, 513 ; constitution of.. 513 Pentamethylene diamine 478 Pentamethylpararosaniline chloride. .516 Pepper, alkaloids of 490 Peri-position 448, 471 Perkin's reaction 431 Peru balsam 418 Phenanthraquinone 469, 470 Phenanthraquinone, bisulphite com- pound of. 470 Phenanthraquinone dioxime 470 Phenanthrene 298, 457, 468 Phenanthrene, constitution of 470 Phenetole 392 Phenol, 297, 391 ; Phenols 385 Phenolphthalei'n 519 Phenolsulphonic acid, o, m, p, 395 ; /.-384 Phenyl benzoate, 420 ; bromide, 347 ; chloride, 347 ; cyanide, 421 ; ethyl ether, 392 ; group, 327 ; iodide, 348 ; methyl ether, 392 ; radicle. . . .333, 390 Phenylacetaldehyde 405 Phenylacetic acid 428, 429 Phenylacetonitrile 422 Phenylacetylene 432 Phenylacrylic acid 428, 430 Phenylarnine 361 Phenyl-/3-bromopropionic acid 431 Phenylbutylene, 446 ; dibromide 446 Phenylbutyric acid 428 Phenylcarbinol 403 Phenylcarbylamine 360, 362 Phenylchloroform 349 Phenyl-/3-dibromopropionic acid. ... 431 Phenylene radicle 333, 390 Phenylenediamine, ;, 354, 524; /. ..414 Phenylenediamine, m, o,p 360, 364 Phenylethane 337 vi PAGB Phenylethyl alcohol 405 Phenylethylene 432 Phenylformic acid 428 Phenylglycollic acid 440 Phenylhydrazine ; hydrochloride 376 Phenylhydrazones 377 _Phenylhydroxylamine 356 Phenylisocrotonic acid 431, 447 Phenylmethane 334 Phenylmethyl carbinol, 412; ketone..4ii Phenylmethylacrylic acid 431 Phenylmethylpyrazolone 499 Phenylpropiolic acid 428, 432 Phenylpropionic acid 428, 430 Phenyltrimethylammonium iodide. . .360 Phloroglucinol 400, 401 Phloroglucinol triacetate 401 Phloroglucinol trioxime 401 Phosphomolybdic acid 488 Phosphotungstic acid 488 Phthalei'ns, the 518 Phthalic acid 425, 444 Phthalic acids, m, o, p 423, 424 Phthalic acids, constitution of 318 Phthalic anhydride 426, 467 Phthalimide 426 Phthalophenone 518 Phthalyl chloride 426 Physical isomerides 535 Picolines 478 Picolinic acid 479 Picric acid '. 394, 488, 502 Piperic acid 477, 490 Piperidine, 473, 476; constitution of .477 Piperine 476, 490 Pitch 296, 298 Ponceau 3R 525 Ponceaux 525 Potassium cresate, 390; diphenyl- amine, 368 ; phenate, 392 ; phthali- mide, 426 ; picrate 394 Primula 515 Propiophenone 412 -Propylpiperidine, d 489 Protocatechuic acid 439 Pseudocumene 338 Purpurin 4^5, 468 Pyridine, 297, 328, 471, 472 ; alkaloids derived from, 488 ; constitution of, 473 ; derivatives, isomerism of, 475 ; homologues of, 478 ; hydrochloride, 473 ; methiodide, 473 ; platino- INDEX. PAGE chloride, 473; sulphate, 473; tests for 473 Pyridine-*,3-dicarboxylic acid 479 Pyridine-^-carboxylic acid 49 Pyridine-^y-dicarboxylic acid 483 Pyridinecarboxylic acid, as., /3, j/ 479 Pyridinecarboxylic acids 478 Pyridinemonocarboxylic acids 479 Pyrocatechin 398 Pyrogallic acid, Pyrogallol 400 Pyrogallolcarboxylic acid 439 Pyrogalloldimethyl ether 400 Quinic acid 49 2 Quinine; dimethiodide ; sulphate 492 Quinine, tests for 493 Quininic acid 49 2 Quinol 399 Quinoline, 328, 471, 480; alkaloids derived from, 492 ; bichromate, 481 ; y-carboxylic acid, 493 ; constitu- tion of, 481 ; hydrochloride, 481 ; methiodide, 481 ; platinochloride, 481 ; sulphate 4 Sl Quinolinic acid, 479, 482 ; anhydride48o Quinone, 413 ; constitution of 414 Quinone chlorimides 416 Quinone dichlorodiimides 416 Quinonedioxime 414 Quinonemonoxime 414 Quinones 413 Racemic acid 539, 541 Racemic modification 536 Racemic .modifications, resolution of. 54i, 543 Reimer's reaction 409, 435 Resorcin yellow 525 Resorcinol 398 Resorcylic acids, the 434 Rocellin 525 Rosaniline, 511, 513; base of, 511; chloride, 511 ; constitution of 511 Rosolic acid 518 Rubery thric acid 465 Saccharin 423 Salicin 404 Salicyl alcohol 404 Salicylaldehyde 409 Salicylic acid, 437 ; salts of. 438 Saligenin, 409, 404; methyl ether. ...404 PAGE Sandmeyer's reaction 347, 348, 372, 421, 423 Sarcolactic acid 533. 534 Scarlet R 525 Secondary aromatic bases 483 Side-chains 3 26 Silver theobromine 498 Skraup's reaction 480, 500 Sodium ammonium racemate 540 Sodium dinitro--naphthol 527 Sodium phenate 390 Sodium phenylcarbonate 434, 437 Sodium picrate 394 Stereo-chemical isomerides 535 Stereo-isomerism Stilbene ; dibromide 7^7x^469 Storax 403. Strychnine ; test for, 494 ; hydro- chloride, 494 ; methiodide 494 Styrolene 432 Substitution, rule of 352 Sulphanilic acid 3 8 3 Sulphobenzoic acid, /;/, o, p 422 Sulphonamides 38 1 Sulphonation 380 Sulphonic acids, 325, 379; chlorides. 381 Tannic acid 44 Tannin 440, 488, 506 Tartaric acids, stereo-isomerism of. . .539 Taurine 5* Terephthalic acid 339, 427 Tertiary aromatic bases 483 Tetrabromethane 461 TetrabromofluoresceTn 521 Tetrachlorohydroquinone 416 Tetrachloroquinone 416 Tetrahydrobenzene 309, 326 Tetrahydro-/3-naphthylamine 450 Tetrahydrohydroxyquinoline 499 Tetramethyldiamidotriphenyl car- binol 508, 509, 510 Tetramethyl-/-diamidotriphenyl- methane 509 Tetrazodiphenyl chloride 526 Tetrazoditolyl salts 526 Tetriodofluorescei'n 521 Thalline 499 Thebai'ne 495 TheTne 497 Theobromine 498 Thiophen 300 INDEX. PAGE Thiotolene 334, 471 Thymol 339, 397 Tobacco, alkaloid of 489 Tolidine 379, 526 Toluene, 297, 334; chlorination of ...342 Toluenesulphonimide, o 422 Toluenesulphonic acid, o 422 Toluenesulphonic acids 383 Toluic acid, 429 ; ;,