THE CHEMISTRY 
 
 OF 
 
 THE DIAZOCOMPOUNDS 
 
 BY 
 
 JOHN CANNELL CAIN 
 
 D.Sc. (MANCHESTER AND TUBINGEN) 
 
 EDITOR Or THE PUBLICATIONS OF THE CHEMICAL SOCIETY 
 
 JOINT AUTHOR OF * THE SYNTHETIC DYESTUFFS AND THE INTERMEDIATE 
 
 PRODUCTS FROM WHICH THEY ARE DERIVED* 
 
 " A OF*V- 
 
 UNIVERSITY 
 
 LONDON 
 
 EDWARD ARNOLD 
 1908 
 
 {Att rights reserved] 
 
DEDICATED 
 TO THE MEMORY OF 
 
 JOHANN PETEK GKIESS 
 
 IN THE SOTH ANNIVERSARY OF THE DISCOVERY OF 
 THE DIAZO-COMPOUNDS (1858-1908) 
 
 174535 
 
PREFACE 
 
 IN this book I have endeavoured to describe our 
 present knowledge of the Diazo-compounds and to 
 give an account of the enormous progress made in 
 this important branch of Organic Chemistry since 
 Griess's epoch-making discovery just fifty years ago. 
 
 I have adopted the plan of giving full details of the 
 simpler preparations and reactions which are being 
 continually carried out in the laboratory, but I have 
 made only short reference to the more involved 
 operations such as would be undertaken by the 
 research chemist, not only to avoid a mass of detail, 
 but also because in such cases the original paper is 
 invariably consulted, and with this in view full 
 references to the literature are given. 
 
 Owing to the very large space necessarily given to 
 a discussion of the theories of the constitution of 
 the diazo-compounds, the practical and descriptive 
 portions have been kept together and the theoretical 
 part reserved until later. / 
 
 Partly on account of this the word t diazonium ' is 
 not used until the theoretical part is reached, where 
 its meaning is explained. 
 
 I have striven to give an exact account of the long 
 controversy between Hantzsch and Bamberger on 
 the constitution of the diazo-compounds. The most 
 
vi PREFACE 
 
 important contributions to the literature have hitherto 
 been made by Hantzsch (Die Diazoverbindungen, Ahrens' 
 Sammlung, 1902) and his former pupil, Eibner (Zur 
 Geschichte der aromatischen Diazoverbindungen, Olden- 
 bourg, 1903), so that the opposite view has, perhaps, 
 not been set forth quite so fully, although the 
 admirable Keport to the British Association by Morgan 
 (Our Present Knowledge of Aromatic Diazo-compounds, 
 1902) leaves nothing to be desired. 
 
 Another difficulty in the way of presenting a clear 
 view of the subject is found in the many cases where 
 work, even supported by numerous analyses, has been 
 shown to be incorrect. This, taken in conjunction 
 with the somewhat authoritative tone of Hantzsch's 
 papers and the unfortunate illness of Bamberger 
 which necessitated cessation of work for some years, 
 may well be responsible for the incomplete accounts 
 of Diazo-chemistry which are occasionally encountered. 
 Finally, a new theory of the constitution of Diazo- 
 compounds is presented in the Appendix to this 
 work. 
 
 J. C. C. 
 
 LONDON, 
 December -, 1907. 
 
CONTENTS 
 
 CHAPTER I 
 
 PAGE 
 
 INTRODUCTION . . . ... . . 1 
 
 CHAPTER II 
 PREPARATION OF THE DIAZO-COMPOUNDS . . . $ 
 
 1. Preparation of dry diazo-salts. 2. Other methods of 
 preparation. 3. Diazotization of amino-phenols and -thio- 
 phenols. Quinonediazides (diazophenols). 4. Thiodiazoles 
 (Diazosulphides). 5. Preparation of diazo-salts in aqueous 
 solution. 6. Other methods of preparing solutions of diazo- 
 salts. 7. The action of nitrous acid on aromatic substances 
 containing more than one amino-group. 8. ' Solid ' diazo- 
 compounds. 
 
 CHAPTER III 
 
 THE MECHANISM OF THE DIAZOTIZING PROCESS . .^. 27 
 
 1. Thermochemistry. 2. Explosibility of dry diazo- 
 compounds. 3. Velocity of diazotization. 
 
 CHAPTER IV 
 
 THE REACTIONS OF THE DIAZO-COMPOUNDS . . . 29 
 1. Action of water. 2. Stability of diazo-solutions. 
 
 CHAPTER V 
 
 THE REACTIONS OF THE DIAZO-COMPOUNDS (continued) 38 
 
 1. Action of alcohols. 2. Influence of substituents. 
 3. Influence of the alcohol used. 4. Influence of temperature 
 and pressure. 5. Influence of other substances. 6. Other 
 methods of reduction. 
 
viii CONTENTS 
 
 CHAPTER VI 
 
 PAGE 
 
 THE REACTIONS OF THE DIAZO-COMPOUNDS (continued) . 43 
 
 1. Replacement of the diazo-group by the halogens Chlo- 
 rine Bromine Iodine Fluorine. 
 
 CHAPTER VII 
 
 THE REACTIONS OP THE DIAZO-COMPOUNDS (continued) . 48 
 
 1. Replacement of the diazo-group by cyanogen. 2. Re- 
 placement of the diazo-group by the cyano-group. 3. Re- 
 placement of the diazo-group by the thiocyano-group. 
 4. Replacement of the diazo-group by the group SH. 5. Re- 
 placement of the diazo-group by sulphur. 6. Replacement of 
 the diazo-group by the sulphonic acid group. 7. Replace- 
 ment of the diazo-group by the nitro-group. 8. Replace- 
 ment of the diazo-group by the nitroso-group. 9. Replacement 
 of the diazo-group by the amino-group. 10. Replacement of 
 the diazo-gr oup by the acetoxy-group. 
 
 CHAPTER VIII 
 ACTION OF VAKIOUS REAGENTS ON DIAZO-COMPOUNDS . 54 
 
 1. Sulpihur dioxide. 2. Replacement of the diazo-group 
 by the sulphinic acid group. 3. Hydrogen sulphide. 4. Re- 
 placement of the diazo-group by the azoimino-group. 5. Benzoyl 
 chloride. 
 
 CHAPTER IX 
 
 FORMATION OF DIPHENYL DERIVATIVES IN THE DIAZO- 
 
 BEACTION 60 
 
 CHAPTER X 
 INTERCHANGE OF GROUPS IN DIAZO-COMPOUNDS . . 63 
 
 CHAPTER XI 
 
 ACTION OF LIGHT ON DIAZO-COMPOUNDS , , .70 
 
 CHAPTER XII 
 
 DlAZOAMINO-COMPOUNDS . 73 
 
CONTENTS ix 
 
 CHAPTER XIH PAOE 
 
 AZO-COMPOUNDS , Y . , . V 80 
 
 1. Azoxy-compounds. 2. Azo-compounds. 3. Aminoazo- 
 compounds. 4. Hydroxyazo-compounds. 5. Rate of 
 formation of amino- and hydroxy-azo-compounds. 6. Constitu- 
 tion of the hydroxyazo-compounds. 7. Mixed azo-compounds. 
 
 CHAPTER XIV 
 METALLIC DIAZO-DERIVATIVES. DIAZO-HYDROXIDES . 96 
 
 CHAPTER XV 
 DIAZO-COMPOUNDS OF THE ALIPHATIC SERIES . . 103 
 
 1. Preparation. 2. Properties of diazoacetic esters. 
 3. Reactions of the aliphatic diazo-compounds. 4. Metallic 
 diazo-compounds of the aliphatic series. 5. Diazoamino-com- 
 pounds of the aliphatic series. 
 
 CHAPTER XVI 
 
 CONSTITUTION OF THE DIAZO-COMPOUNDS . / / . 112 
 
 1. Constitution of the diazo-salts according to Griess. 
 2. Constitution of diazo-compounds according to Kekule. 
 3. Constitution of the diazo-salts according to Blomstrand. 
 4. Constitution of diazobenzene hydroxide to 1834. 
 
 CHAPTER XVH 
 CONSTITUTION OF THE DIAZO-COMPOUNDS (continued) . 123 
 
 1. Constitution of the diazo-compounds according to Hantzsch. 
 
 CHAPTER XVin 
 CONSTITUTION OF THE DEAZO-SALTS AFTER 1894 . . 132 
 
 1. Constitution of the diazo-compounds according to 
 Bamberger. 2. Relation between diazonium compounds 
 and normal or syw-diazo-compounds. 3. Double salts of 
 diazonium halides and metallic salts. 4. Diazonium halides 
 andst/n-diazo-halides. 5. Diazonium perhalides. 6. Rela- 
 tion between syn- and aw^'-compounds. 7. The isomeric 
 diazo-sulphonates and diazo-cyanides. 8. Constitution of the 
 metallic diazo-oxides. 9. Diazo-ethers. 10. Diazo- 
 anhydrides. 11. Diazo-hydroxides. 12. Condition of the 
 non-ionized diazonium hydroxide. 13. Constitution of iso 
 (anti) diazo-hydroxides. 
 
x CONTENTS 
 
 CHAPTER XIX 
 
 PAGE 
 
 OTHER VIEWS OP THE CONSTITUTION OP DIAZO-COM- 
 
 POUNDS FROM 1895 . , > . . . 153 
 1. Constitution of the coloured diazo-salts of Jacobson. 
 2. Constitution of the diazo-salts according to Walther. 
 3. Constitution of diazo-compounds according to Bruhl. 
 4. Constitution of the diazo-compounds according to Dobbie 
 and Tinkler. 5. Constitution of the diazo-compounds 
 according to Armstrong and Robertson. 
 
 CHAPTER XX 
 
 A REVIEW OP THE VARIOUS THEORIES OP THE DIAZO- 
 COMPOUNDS TO 1907 . . . . . . 159 
 
 1. Constitution of the diazo-salts (diazonium salts). 
 2. The labile and stable isomeric diazo-compounds. 
 
 APPENDIX 
 
 A NEW THEORY OP THE CONSTITUTION OP THE DIAZO- 
 COMPOUNDS . . ,. Y . . . . .163 
 
 SUBJECT INDEX . . . . . . .169 
 
 NAME INDEX . 171 
 
 
 
 \ 
 
ABBEEVIATIONS 
 
 ABBREVIATED TITLE. JOURNAL. 
 
 Amer. Chem. J. American Chemical Journal. 
 
 Annalen Justus Liebig's Annalen der Chemie. 
 
 Arch. Pharm Archiv der Pharmazie. 
 
 Atti R. Accad. Lincei .... Atti della Reale Accademia dei Lincei. 
 Ber. Berichte der Deutschen chemischen 
 
 GeseUschaffc. 
 Bull. Acad. Sci. Cracow . . . Bulletin international de 1'Academie 
 
 des Sciences de Cracovie. 
 Bull. Soc. chim Bulletin de la Societe chimique de 
 
 France. 
 Bull. Soc. ind. Nulhouse . . . Bulletin de la Societe industrielle de 
 
 Mulhouse. 
 
 Chem. Weekblad Chemisch Weekblad. 
 
 Chem. Zeit Chemiker Zeitung. 
 
 Compt. rend Comptesrendushebdpmadaires des Sean- 
 ces de 1'Academie des Sciences. 
 
 Gazzetta Gazzetta chimica italiana. 
 
 Jahresber Jahresbericht iiber die Fortschritte der 
 
 Chemie. 
 
 J. pr. Chem Journal fur praktische Chemie. 
 
 J. Buss. Phys. Chem. Soc. . . Journal of the Physical and Chemical 
 
 Society of Russia. 
 J. Soc. Chem. Ind Journal of the Society of Chemical/ 
 
 Industry. 
 J. Soc. Dyers Journal of the Society of Dyers 
 
 Colourists. 
 
 Journ. Chem. Soc Journal of the Chemical Society. 
 
 Phil. Mag Philosophical Magazine (The /iondon, 
 
 Edinburgh and Dublin). / 
 Phil. Trans Philosophical Transactions / the Royal 
 
 Society of London. / 
 
 Proc Proceedings of the Cheixical Society. 
 
 Proc. Boy. Soc Proceedings of the Ro/al Society. 
 
 Trans. . Transactions of the Chemical Society. 
 
 Zeitsch. angew. Chem Zeitschrift fur angswandte Chemie. 
 
 Zeitsch. EleJctrochem Zeitschrift fiir EJektrochemie. 
 
 Zeitsch. Farb.-Ind Zeitschrift fur Farben-Industrie. 
 
 Zeitsch. physical. Chem. . . . Zeitschrift fur physikalische Chemie. 
 Zeitsch. f. Chem Zeitschrift fiir Chemie. 
 
 D.R-P Deutsches Reichs-Patent. 
 
 E.P English Patent. 
 
 F.P. . . French Patent. 
 
THE CHEMISTRY OF THE DIAZO- 
 COMPOUNDS 
 
 CHAPTER I 
 
 INTRODUCTION 
 
 THE diazocompounds were discovered in 1858 by Johann 
 Peter Griess,* who obtained them by treating aromatic ammo- 
 compounds with nitrous acid. Piria had already found, in 
 1849, that asparagine or aminosuccinamic acid is converted 
 into malic acid by the action of nitrous acid, the aniino-group, 
 NH 2 , being substituted by the hydroxyl group, OH, thus 
 
 C 4 H 4 3 (NH 2 ) 2 + 2HN0 2 = C 4 H 4 3 (OH) 2 + 2N 2 + 2H 2 O. 
 In the aromatic series, also. Hunt in the same vear 
 
 ERRATUM 
 Page 25, note ^,for diazo-dyestuffs read disazo-dyestuffs 
 
 Cain. Diazo-compounds 
 
 Ull'UHJ lilt! IWU atUllLH ^Of 
 
 the molecule) of nitrogen, N 2 , they contain, must be considered as 
 equivalent to two atoms of hydrogen, and it is in accordance with this 
 view that the names of the new compounds have been framed.' (Phil. 
 2Vaw., 1864, 154, 668.) 
 
 II Roscoe and Schorlemmer, Treatise on Chemistry, vol. iii, part 3, 311. 
 Griess, private communication to Watson Smith in 1887 ; compare also 
 Watson Smith, J. Soc. Chem. 2nd., 1907, 26, 134. 
 

 THE CHEMISTRY OF THE DIAZO- 
 COMPOTJNDS 
 
 CHAPTER I 
 
 INTRODUCTION 
 
 THE diazo-compounds were discovered in 1858 by Johann 
 Peter Griess,* who obtained them by treating aromatic amino- 
 compounds with nitrous acid. Piria had already found, in 
 1849, that asparagine or aminosuccinamic acid is converted 
 into malic acid by the action of nitrous acid, the amino-group, 
 NH 2 , being substituted by the hydroxyl group, OH, thus 
 
 C 4 H 4 3 (NH 2 ) 2 + 2HN0 2 = C 4 H 4 3 (OH) 2 + 2N 2 + 2H 2 O. 
 In the aromatic series, also, Hunt in the same year showed 
 that aniline, by the same method, was converted into phenol. 
 Then Gerland in 1853 f prepared hydroxybenzoic acid from 
 aminobenzoic acid and also observed the formation of a red 
 intermediate product, the quantity of which was found to 
 increase by working with cold dilute solutions. Gerland was 
 unable to decide as to the constitution of this substance owing 
 to wide variations in the analytical figures. The further 
 investigation of this was suggested to Griess by Kolbe,J with 
 the result that diazoaminobenzoic acid was isolated. Griess 
 then extended his work so successfully that he discovered the 
 existence of an entirely new class of substances, to which the 
 name ' diazo ' was given. An account of this discovery, 
 given by Griess himself, will be of interest. || 
 
 * Annalen, 106, 123. t Ibid., 1854, 91, 185. 
 
 t Obituary notices of Griess, Ber., 1891, 24, 1007. 
 
 Griess says : ' I have come to the conclusion that the two atoms (or 
 the molecule) of nitrogen, N 2 , they contain, must be considered as 
 equivalent to two atoms of hydrogen, and it is in accordance with this 
 view that the names of the new compounds have been framed.' (Phil. 
 Trans., 1864, 154, 668.) 
 
 || Roscoe and Schorlemmer, Treatise on Chemistry, vol. iii, part 3, 311. 
 Griess, private communication to Watson Smith in 1887 ; compare also 
 Watson Smith, J. Soc. Chem. 2nd., 1907, 26, 134. 
 
 B 
 
2 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 'Dr. Gerland, when working in the laboratory of Prof. 
 Kolbe, in Marburg, investigated the action of nitrous acid on 
 amidobenzoic acid at the request of Kolbe. Thus oxybenzoic 
 acid was prepared, indicating a chemical change then con- 
 sidered of much importance. In like manner I investigated 
 a means of converting picramic acid (amidodinitrophenylic 
 acid) into the oxydinitrophenylic acid, C 6 H 2 (N0 2 ) 2 (OH) 2 , but 
 I obtained instead of the latter a compound possessed of such 
 striking and peculiar properties that I at once concluded it 
 must belong to a completely new class of compounds. Analysis 
 soon showed me that this peculiar compound had the com- 
 position C 6 H 2 (lSr0 2 ) 2 N 2 O. Naturally I soon submitted many 
 other amido-compounds in like manner to the action of nitrous 
 acid, and obtained thus, in almost every case, the correspond- 
 ing diazo-compound. But the circumstance to which I was 
 indebted for my success in obtaining the diazo-compounds 
 was that of the treatment of the amido-compounds with 
 nitrous acid in the cold, whereas in the earlier experiments 
 of Hunt and Gerland a higher temperature was always 
 attained, and consequently no diazo-compounds could exist. 
 Having obtained these diazo-compounds, I then tried their 
 action on all possible substances, among which of course are 
 the numerous class of amido-compounds. I found that the 
 diazo-compounds combine directly with these, forming fre- 
 quently brilliantly coloured substances which dye animal 
 fibres directly. The first colouring matter thus prepared by 
 me, which I obtained in the years 1861-2, was the benzeneazo- 
 a-naphthylamine.'* It was first prepared on the large scale, 
 to the best of my recollection, in the years 1865-6 by Caro, 
 who was then chemist in the works of Messrs. Roberts, Dale 
 & Co., of Manchester. I first recommended the oxyazobenzene 
 obtained by me for use as a colouring matter in 1866.'f 
 
 Griess continued his researches on diazo- and azo-com- 
 poundsj during his three years' residence in London as 
 
 * Phil. Trans., 1864, 154, 679. 
 
 t Annalen, 137, 88. 
 
 t Griess's first short preliminary announcement was published in 
 Annalen, 1858, 106, 123 ; see also Proc. Roy. Soc., 1859, 9, 594 ; Phil. 
 Mag. 1859 [iv], 17, 370 ; Compt. rend., 1859, 49, 77. The full paper 
 appeared in Annalen, 1860, 113, 201. See also Annalen, 1860, 113, 337 ; 
 
INTRODUCTION 3 
 
 Hofmann's assistant, and also afterwards while with Messrs. 
 Allsopp, Burton-on-Trent. Here, although busily occupied in 
 the vast brewery, Griess found time in which to prepare a 
 large number of new diazo-compounds, and these were then 
 handed over to his friend, Dr. R. Schmitt, at Dresden for 
 analysis. Hempel's account of this is interesting : ' Regel- 
 massig kamen von Burton an den Ufern des Trent die von 
 Griess dargestellten neuen Korper in kleinen Packeten, urn in 
 Dresden an der Elbe analysirt zu werden. Per Fracht kamen 
 dann wohl gleichzeitig als willkommene Beilage Fasser von 
 Allsopp's beruhmtem Pale Ale in ausgesuchtester Qualitat.' 
 (The new compounds prepared by Griess were regularly sent 
 in small packets from Burton-on-Trent to Dresden to be 
 analysed. At the same time a welcome accompaniment took 
 the form of barrels of Allsopp's finest Pale Ale). Griess's 
 brilliant investigations extended to the preparation of a very 
 large number of diazo-compounds; further, he discovered 
 most of their reactions with other reagents and laid the 
 foundation of the immense edifice of azo-dyestuffs which has 
 since been, and is still being, erected. 
 
 After having described the diazoamino-derivatives of amino- 
 benzoic acid,* aminotoluic acid, and aminoanisic acid, and their 
 reactions, Griess then obtained diazoaminobenzene f and the 
 diazobenzene salts. { Of these the nitrate, the easiest to pre- 
 pare, was used as the starting-point in the preparation of the 
 crystallized sulphate, the platinichloride, the aurichloride, and, 
 some years later, the ferricyanide, the nitroprussiate, and the 
 tin chloride double salt. With the object of preparing the 
 bromide, Griess treated diazoaminobenzene with bromine in 
 ethereal solution and obtained the perbromide C 6 H 5 N 2 Br 3 . 
 
 1861, 117, 1 : 120, 125: 1861, Suppl. I, 100; 1862, 121, 257; 1866, 137, 
 39; Proc. Boy. Soc., 1860, 10, 309, 591; 1862, 11,263; 1863,12,418; 
 1864, 13, 375. A long paper, including most of the earlier work, is in 
 Phil. Trans., 1864, 154, 667 ; and accounts were also published in Journ. 
 Chem. Soc., 1865, 3, 268, 298 ; 1866, 4, 57 ; 1867, 5, 36. For the later 
 work see Ber., 1869, 2, 369 ; 1874, 7, 1618 ; 1876, 9, 132, 627, 1653 ; 1878, 
 11,624; 1879,12,2119; 1881,14,2032; 1882,15,2183; 1883,16,2028; 
 1884, 17, 338. 
 
 * Annalen, 1861, 117, 1. t Ibid.. 1862, 121, 257. 
 
 t Ibid., 1866, 137, 39. 
 
 Ber., 1879, 12, 2119; 1885, 18, 965. 
 
4 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 On the addition of ammonia to this compound, the whole of 
 the bromine was removed and a substance containing three 
 atoms of nitrogen was isolated. This was called diazobenzene- 
 imide and possessed the formula C 6 H 5 N 3 . Griess also studied 
 the formation of metallic diazo-derivatives ; thus, by the action 
 of a concentrated potassium hydroxide solution on a strong 
 solution of diazobenzene nitrate, a substance containing 
 potassium was obtained, and by mixing a solution of this 
 with a silver solution, a substance containing silver was pre- 
 cipitated. The formulae of these metallic derivatives were 
 considered by Griess to be C 6 H 5 N 2 . OK and C 6 H 5 N 2 . OAg 
 respectively (see, however, p. 96). By treating a solution of 
 the potassium compound with acetic acid, a viscous yellow oil 
 was obtained which Griess looked upon as free diazobenzene. 
 With mineral acids it yielded the corresponding salts. 
 
 We now pass on to consider briefly the various recictions 
 which the diazo-salts, in Griess's hands, were found to undergo. 
 By boiling with water, phenols were obtained ; in the case of 
 the nitrate, nitrophenol was formed by the interaction of 
 phenol and the liberated nitric acid. When alcohol was 
 substituted for water benzene was formed, whilst the alcohol 
 was reduced to aldehyde. By the action of hydriodic acid 
 in the cold, iodobenzene was obtained, chlorobenzene by 
 distilling the dry platinichloride with soda, and bromobenzene 
 in the same way from the platinibromide, and also by boiling 
 the perbromide with alcohol. 
 
 By the action of phenols and amines on the diazo-compounds 
 Griess discovered that highly-coloured condensation products 
 were formed. These were the azo-dyestuffs, some of which 
 were immediately prepared on the large scale ; the reaction 
 itself giving the key to an industry which has since attained 
 an enormous importance. 
 
 The next important discovery to be noted is that of the 
 first diazo-compound belonging to the aliphatic series in 1883. 
 Curtius prepared the ethyl ester of diazoacetic acid, pro- 
 ceeding from this to a series of brilliant researches on fatty 
 diazo-compounds, culminating in his discovery of azoimide. 
 In 1894 von Pechmann isolated the simplest member of 
 
INTRODUCTION 5 
 
 the series, namely, diazomethane, since which time a large 
 number of derivatives have been obtained. 
 
 The remarkable influence of small amounts of copper salts 
 on the reactions which the diazo-compounds undergo was 
 discovered in 1884 by Sandmeyer, whose name is associated 
 with this decomposition, and the substitution of finely divided 
 copper for its salts was introduced by Gattermann in 1890. 
 
 In the latter year Meldola discovered that the presence of 
 the diazo-group has, in certain cases, a remarkable effect on 
 the stability of a nitro-group present in the same benzene 
 ring, whereby this group is very readily eliminated. 
 
 This transformation of acidic groups under the influence of 
 the diazo-group has been made the subject of comprehensive 
 researches by Meldola and his colleagues, as well as by 
 Bamberger, Orton, and Hantzsch, and the latter chemist 
 showed in 1896 that in some cases the acidic group attached 
 to the diazo-nitrogen could change places with a halogen atom 
 in the benzene ring. 
 
 In 1894 an important investigation carried out by Schraube 
 and Schmidt, whereby the existence of two isomeric metallic 
 salts of diazobenzene was indicated, led to a thorough 
 examination of the metallic diazo-derivatives, and gave rise 
 to a prolonged discussion of the constitution of the whole 
 class of diazo-compounds. Among the many discoveries 
 which were made in 1895 is specially to be recorded Bam- 
 berger's isolation of the diazoic acids by the oxidation of the 
 diazo-salts. 
 
 The definite proof by Andresen, in that year, that light 
 acted on diazo-compounds with the production of the corre- 
 sponding phenols was followed by the remarkable observation 
 of Orton in 1906, that quantitative yields of phenols were 
 obtained from certain diazo-salts which gave practically no 
 hydroxy-derivative when heated with waiter or acids. 
 
CHAPTEE II 
 PREPARATION OF THE DIAZO-COMPOUNDS.* 
 
 1. Preparation of dry diazo-salts. In preparing the 
 diazo-compounds Griess used, as a source of nitrous acid, the 
 gases evolved by warming a mixture of nitric acid and 
 arsenic trioxide. These gases were passed into either an 
 alcoholic solution of the amine or an aqueous paste of an 
 amino-salt, the experiment being carried out in the cold, 
 when the resulting diazo-compound separated or was precipi- 
 tated by the addition of alcohol and ether. 
 
 In the case of diazobenzene nitrate, used by Griess as the 
 starting-point for the preparation of other diazo-compounds, 
 the nitrous gases were passed into a well-cooled paste of 
 aniline nitrate and water until aniline ceased to be liberated 
 on adding potassium hydroxide to a small test portion, The 
 solution was then filtered and alcohol and ether added to 
 precipitate the diazobenzene nitrate, which separated in white 
 needles. 
 
 A considerable improvement on this method consists in the 
 use of a solution of sodium nitrite as a source of nitrous acid.f 
 By this process the calculated quantity of nitrous acid may 
 be used, and this is so convenient that sodium nitrite is almost 
 entirely employed at the present day in the preparation of 
 diazo-compounds. 
 
 The use of an aqueous solution of sodium nitrite is, how- 
 ever, not very suitable for the preparation of those dry diazo- 
 compounds which are very soluble in water, and in order to 
 avoid the presence of the latter Knoevenagel { used amyl 
 nitrite in alcoholic solution, a method which had been em- 
 
 * The description of the metallic diazo-compounds is reserved until a 
 later chapter (see p. 96). 
 
 t Martius, J. pr. Chem., 1866, 98, 94. 
 | Ber., 1899, 23, 2995. 
 
PREPARATION OF THE DIAZO-COMPOUNDS 7 
 
 ployed by V. Meyer and Ambuhl in the preparation^of diazo- 
 ammobenzene.* This was a great improvement on existing 
 methods, and a large number of dry diazo-salts have been 
 prepared in this way. Pure products are, however, only 
 obtained in the absence of free mineral acid.f The reason 
 of this is that in presence of excess of mineral acid additive 
 compounds of the diazo-chloride with hydrochloric acid are 
 formed. 
 
 The hydrochlorides are therefore prepared by passing 
 dry hydrogen chloride into a solution of the amine in absolute 
 alcohol or ether and heating the product at 40-50 until the 
 last traces of acid have been removed. The dry salt is then 
 dissolved in alcohol and the theoretical quantity of amyl 
 nitrite added at the ordinary temperature. On precipitating 
 with ether the diazo-chloride is obtained in the pure state. 
 This is a somewhat tedious process, and it has been found J 
 that the reaction proceeds even more satisfactorily in the 
 presence of glacial acetic acid, thus avoiding the necessity of 
 preparing the dry aminic hydrochloride. 
 
 In order to prepare diazobenzene chloride the experiment 
 is carried out as follows: Fifty grams of aniline hydro- 
 chloride are dissolved or suspended in about three times the 
 quantity of glacial acetic acid and the mixture stirred by 
 a turbine. A little more than the theoretical quantity of 
 amyl nitrite is now added, care being taken that the tempera- 
 ture does not exceed 10. Any undissolved aniline salt 
 disappears quickly, and the diazotization is complete as soon 
 as a small portion withdrawn and treated with sodium acetate 
 no longer gives a yellow coloration. On adding ether a thick 
 crystalline precipitate of diazobenzene chloride is obtained, 
 which is filtered, washed with ether, and dried in a desiccator. 
 The yield is 53 grams. The sulphate is obtained in a similar 
 way ; in this case aniline sulphate is diazotized in presence of 
 the calculated quantity of sulphuric acid. The separation of 
 the diazo-sulphate is effected by first adding a little alcohol to 
 the mixture with acetic acid and then precipitating with 
 ether. The halogenated diazo-chlorides are prepared in the 
 
 * Annalen, 1889, 251, 56. t Hirsch, Ber., 1897, 30, 1148. 
 
 J Hantzsch and Jochem, Ber., 1901, 34, 3337. 
 
8 CHEMISTEY OF THE DIAZO-COMPOUNDS 
 
 same way, but care must be taken to use the hydrochlorides 
 obtained according to Hirsch's method, avoiding the presence 
 of mineral acid. 
 
 The preparation of diazo-salts which are sparingly soluble 
 in water can, of course, be carried out in aqueous solution. In 
 some cases the diazo-compound is precipitated on addition of 
 sodium nitrite to the acid solution of the amine, whilst in 
 others the insoluble diazo-compound is precipitated on adding 
 the salt of a different acid. 
 
 Many instances of the former case occur among the amino- 
 sulphonic acids. Thus ^9-diazobenzenesulphonic acid is easily 
 obtained* by dissolving sulphanilic acid in dilute aqueous 
 sodium hydroxide, acidifying with hydrochloric acid, 
 and adding the calculated quantity of sodium nitrite, pre- 
 viously dissolved in a small quantity of water. The tempera- 
 ture should be about 5. As soon as the whole of the 
 nitrite has been added the diazo-compound, in the form of 
 the anhydride C 6 H 4 O a N 2 S, separates in fine white needles, 
 which may be filtered, but should not be dried, as they are 
 extremely explosive (see p. 28). The diazo-compound derived 
 from a-naphthylamine-4-sulphonic acid (naphthionic acid) is 
 obtained in a similar manner. The naphthionic acid is dis- 
 solved in alkali and reprecipitated by the addition of mineral 
 acid. On adding the nitrite solution, the white insoluble 
 naphthionic acid is gradually converted into the yellow 
 insoluble diazo-compound. 
 
 Instances of the second method are numerous; thus on 
 adding a solution of sodium picrate to a solution of diazo- 
 benzene nitrate a precipitate of the insoluble diazobenzene 
 picrate is obtained, f A very stable diazo-picrate has also 
 been prepared from >-aminobenzanilide in the same way, J 
 and the picrate of diazophenylindole, C 20 H ]2 7 N 6 , as also the 
 picrate of diazomethylindole, C 15 H 10 O 7 N 6 , can be crystallized 
 from alcohol. The chromate was prepared by Griess and 
 Caro, || who diazotized aniline nitrate by means of a solution 
 
 * Schmitt, Annalen, 1859, 112, 118 ; 1861, 120, 144. 
 
 t Baeyer and Jaeger, Ber., 1875, 8, 894. 
 
 t Morgan and Wootton, Proc., 1906, 22, 23. 
 
 Castellana and d'&ngelo, AttiE. Accad. Lincei, 1905 [v], 14, ii. 145. 
 
 || Jahresber., 1867, 915. 
 
PREPARATION OF THE DIAZO-COMPOUNDS 9 
 
 of calcium nitrite, and then added an equivalent of potassium 
 dichromate and hydrochloric acid, when a precipitate was 
 obtained. They suggested the use of this chromate as an 
 explosive.* 
 
 Diazo-chromates are now usually prepared f by precipita- 
 ting a diazo-solution with sodium dichromate. 
 
 Insoluble diazo-thiosulphates, hydrof erricyanides, and tung- 
 states have been also prepared in a similar manner. J 
 
 An interesting diazo-carbonate is obtained by pouring 
 the diazo-chloride derived from benzoyl-p-phenylenediamine 
 (p-aminobenzanilide) into cold aqueous sodium carbonate, 
 when a yellow precipitate results which has the formula 
 C 6 H 5 . CO.NH.C 6 H 4 . N 2 . HC0 3 . 
 
 By adding sodium acetate to the diazo-chloride, and then 
 treating with excess of sodium nitrite, a crystalline yellow 
 diazo-nitrite is obtained. 
 
 Certain diazo-salts of hydrazoic acid have also been 
 prepared by treating an ethereal solution of the diazo- 
 hydroxide with ethyl azoiminocarboxylate, N 3 . CO.,Et. These 
 salts have the composition Ar . N 2 . N 3 , and are extremely 
 unstable. || 
 
 Diazo-fluorides containing one molecule of hydrofluoric acid 
 are obtained by diazotizing the amine with amyl nitrite in 
 presence of hydrofluoric acid,^[ and diazo-perchl orates are pro- 
 duced by diazotizing amines in presence of perchloric acid. 
 These perchlorates are extremely explosive.** 
 
 2. Other methods of preparation. A modification of 
 Griess's method of diazotizing was used by his co-worker 
 Schmitt,ft wn saturated absolute alcohol with nitrous fumes 
 and poured this over aminophenol hydrochloride. On adding 
 ether the diazophenol was precipitated. 
 
 .* Butt. Soc. chim., 1867 [ii], 7, 270 ; F. P. 73286. 
 t Meldola and Eynon, Trans., 1905, 87, 1 ; Castellana and d'Angelo, 
 loc. cit. 
 
 t Hepburn, J. Soc. Dyers, 1901, 17, 279. 
 Morgan and Micklethwait, Trans., 1905, 87, 921. 
 H Hantzsch, Ber., 1903, 36, 2056. 
 IF Hantzsch and Vock, Ber., 1903, 36, 2059. 
 ** Ber., 1906, 39, 2713, 3146. 
 tt Ber., 1868, 1, 67. 
 
10 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 The direct interaction of fuming nitric acid and amines for 
 the preparation of diazo-nitrates is hardly to be classed as 
 a separate method, as fuming or brown nitric acid always 
 contains nitrous fumes which diazotize the nitrate of the 
 amine. It is interesting, however, in this connexion to note 
 that Stenhouse * obtained the diazodinitrophenol of Griess by 
 pouring boiling nitric acid on picramic acid, and several 
 diazonitrotoluenesulphonic acids have been obtained in the 
 dry state by diluting a solution of the corresponding toluidine- 
 sulphonic acid in fuming nitric acid; nitration and diazo- 
 tization both having been effected, f A more recent example 
 of this is the preparation of 2-nitro-4-diazo-m-xylene-6-sul- 
 phonic acid from m-xylidinesulphonic acid 
 
 (CH 3 ) 2 :NH 2 :S0 3 H = 1:3:4:6.1 
 
 A surprising reaction is the formation of the diazo-deriva- 
 tive of aminophenolsulphonic acid by treating it with nitric 
 acid and carbamide, for one would expect the nitrous acid 
 present to be destroyed by the carbamide and thus prevent 
 diazotization, but recent experiments || have shown that this 
 destruction is very incomplete in the case of concentrated 
 nitric acid. Various other nitrous derivatives have been used 
 occasionally in the preparation of diazo-compounds ; it is even 
 stated that nitric oxide can be substituted for nitrous fumes 
 in the preparation of diazobenzene nitrate.^ 
 
 Other substances which have been used are nitrosyl 
 bromide** and chloride, ff and nitrosulphonic acid.Jt 
 
 The use of barium nitrite instead of sodium nitrite has 
 been suggested for preparing dry diazo-sajts. By using the 
 calculated quantity of sulphuric acid the whole of the mineral 
 
 * Journ. Chem. Soc., 1868, 6, 150. 
 
 t Limpricht, Ber., 1874, 7, 452. 
 
 J Zincke, Annalen, 1905, 339, 202. 
 
 Bennewitz, J. pr. Chem., 1874 [ii], 8, 50, 
 
 || Silberrad and Smart, J. Soc. Chem. Ind., 1906, 25, 156. 
 
 IT Ladenburg, Ber., 1875, 8, 1212. 
 
 ** Koninck, Ber., 1869, 2, 122. 
 
 tt Pabst and Girard, D. R-P. 6034 of 1878, and Ber., 1879, 12, 365 ; 
 compare also Kastle and Keiser, A mer. Chem. J., 1895, 17, 91, who ob- 
 tained a double salt, diazobenzene aniline chloride, C 6 H 5 N 2 C1, C 6 H 5 NH 3 C1, 
 by treating aniline hydrochloride with nitrosyl chloride. 
 
 H Pabst and Girard, loc. cit. Witt, Ber., 1903, 36, 4388. 
 
PREPAKATION OF THE DIAZO-COMPOUNDS 11 
 
 matter is precipitated, and, on filtering, the diazo-salt may be 
 precipitated (with alcohol and ether) free from admixture with 
 inorganic salts. 
 
 It has also been found possible to obtain diazo-compounds 
 without using an amine as the starting-point; thus dry 
 diazobenzene nitrate has been prepared* by the action of 
 nitrous fumes on mercury diphenyl, 
 
 Hg(C 6 H 6 ) 2 + 2N 2 3 = HgC! H 6 . N0 3 + C 6 H 6 . N 3 . NO^_ 
 and when mercury p-ditolyl is substituted for the diphenyl 
 compound, p-diazotoluene nitrate is formed, t By treating 
 a solution of nitrosobenzene in chloroform with nitric oxide, 
 diazobenzene nitrate was obtained by Bamberger, 
 
 C 6 H 6 .NO + 2NO = C 6 H 5 .N 2 . 
 
 and the same diazo-salt has been isolated by passing nitrous 
 fumes into an ethereal solution of nitrosophenylhydrazine.J 
 Finally, by the action of alcoholic hydrochloric acid on nitroso- 
 anilidoacetic acid, the chloride of p-diazophenylhydroxylamine, 
 OH.NH.C 6 H 4 . N 2 C1, is produced. 
 
 3. Diazotizatiou of amino-phenols and -thiophenols. 
 
 Quinouediazides (Diazophenols). The diazo-chlorides of 
 o- and ^-aminophenol are obtained by diazotizing the corre- 
 sponding bases in alcoholic solution with amyl nitrite and 
 hydrochloric acid at 0, and precipitating with ether. || The 
 diazo-salts thus obtained are white. m-Diazophenol chloride 
 is extremely unstable, and loses nitrogen even at 0. 
 
 When the two former salts are dissolved in water and 
 treated with potassium hydroxide or moist silver oxide, 
 hydrochloric acid is split off, and the free diazophenols, or 
 quinonediazides,^r are formed 
 
 HO.C 6 H 4 . N 2 C1 = HC1 + O : C 6 H 4 
 
 These quinonediazides are yellow and soluble in water. 
 
 * Ber., 1897, 30, 509. t Kunz, Ber., 1898, 31, 1528. 
 
 t Riigheimer, Ber., 1900, 33, 1718. 
 0. Fischer, Ber., 1899, 32, 247. 
 
 || Schmitt, Ber., 1868, 1, 67. Hantzsch and Davidson, Ber., 1896, 29, 
 1522. See also Cameron, Amer. Chem. J. t 1898, 20, 229. 
 H For constitution see p. 126. 
 
12 CHEMISTKY OF THE DIAZO-COMPOUNDS 
 
 The substituted aminophenols are converted directly into 
 the quinonediazides on diazotization. The compound derived 
 from aminodinitrophenol (picramic acid) was the first diazo- 
 compound obtained by Griess. 
 
 Quinonediazides are also formed by allowing neutral solu- 
 tions (or solutions containing no free mineral acid) of certain 
 substituted diazo-salts to stand for some time ; thus 2:4: 6-tri- 
 chlorodiazobenzene hydrogen sulphate or nitrate loses one 
 atom of chlorine and becomes converted into 3:5-dichloro- 
 o-quinonediazide, 
 
 / 
 0:C 6 H 2 C1 2 /|| 
 
 and many other halogen substituted anilines behave in the 
 same way.* 
 
 4. Thiodiazoles (Diazosulphides). When o-aminophenyl- 
 mercaptan 
 
 NI 
 
 is treated with nitrous acid an anhydride is obtained, as in 
 the case of the aminophenols, but, unlike the quinonediazides, 
 the o-diazosulphides are colourless, resembling in this respect 
 the azimides obtained by the action of nitrous acid on the 
 o-diamines. They generally crystallize well, have a charac- 
 teristic sweetish odour, and are very feebly basic. Their 
 constitution is probably represented as 
 
 5. Preparation of diaso-salts in aqueous solution. From 
 the foregoing it will have been seen that the preparation 
 of solutions of diazo-salts is a comparatively simple matter, 
 nevertheless there are many amines, mostly substituted, which 
 
 * Orton, Proc. Roy. Soc. t 1903, 71, 153 ; Trans., 1903, 83, 83, 796. 
 t JfcCObon,^naZe, 1893, 277, 209, 218, 232, 237. 
 
PREPAEATION OF THE DIAZO-COMPOUNDS 13 
 
 either resist the action of nitrous acid or, owing to the forma- 
 tion of secondary products, are incapable of yielding diazo- 
 salts. These will be dealt with later. 
 
 In all cases the method of preparation on the large and the 
 small scale is the same, so that a technical recipe may be 
 exactly imitated in the laboratory and vice versa. 
 
 The amine to be diazotized is usually dissolved in about 10 
 parts of water with addition of one equivalent of hydrochloric 
 acid, if necessary by the aid of heat. The solution is then 
 cooled to 0-5 by the direct addition of ice and one and a half 
 to two equivalents of hydrochloric acid added. (When the 
 hydrochloride of the base is easily soluble in hydrochloric 
 acid the whole of the acid may be used in dissolving the 
 amine.) A solution of the calculated quantity of sodium 
 nitrite is now added ; in most cases slowly until only a weak 
 reaction is obtained with starch-iodide paper (this is best 
 prepared from cadmium iodide and starch) after the solution 
 has stood for 3-4 minutes. But in certain cases, especially 
 where there is a great tendency towards the formation of 
 diazoamino-derivatives, as in the case of a-naphthylamine and 
 p-nitroaniline, the nitrite solution is added all at once, the 
 precaution being taken of adding sufficient ice to prevent the 
 temperature rising unduly. Occasionally the secondary reac- 
 tion may be avoided by using a nitrite solution which has been 
 previously acidified with hydrochloric acid or by using a 
 larger excess of acid. The tendency towards the formation of v 
 diazoamino-compounds increases if organic acids are used, \ 
 thus, for example, if two and a half equivalents of acetic acid / 
 are substituted for the same equivalent quantity of hydro- 
 chloric acid, in the case of aniline only about 20 per cent, is 
 converted into the diazo-salt; the diazotization is complete 
 only by the use of eleven equivalents of acetic acid. The use 
 of less hydrochloric acid has a similar effect ; aniline hydro- 
 chloride is only partly diazotized by sodium nitrite, but the 
 quantity converted increases with the concentration of the 
 solution, thus in solutions containing respectively 10,1, and 
 0-1 per cent, of aniline, about 30, 20, and 10 per cent, of the 
 aniline is diazotized.* 
 
 * Altschul, J. pr. Chem., 1896 [ii], 54, 508. 
 
14 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 One of the most frequently prepared diazo-compounds is 
 that derived from p-nitroaniline ; indeed it is stated * that 
 more than 1,000 tons of >-nitroaniline are yearly converted 
 into the diazo-compound for the purpose of producing ' para- 
 nitraniline red ' by combination with /S-naphthol on the cotton 
 fibre. 
 
 A large number of methods of preparing this important 
 diazo-compound have been published, f one of which (Cassella 
 & Co.) is here quoted. 
 
 >-Nitroaniline (21 grams) is dissolved in water with addition 
 of 42 c.c. of hydrochloric acid of 22 Be, and the solution cooled 
 to 5-10. The water and ice used weigh 307 grams. A solu- 
 tion of 11-5 grams of technical sodium nitrite (95 per cent.) in 
 103-5 grams of water is now added all at once and the mixture 
 well stirred until a clear solution is obtained. If the resulting 
 diazo-solution is to be used for combination with /3-naphthol 
 it is first treated with a solution of 25 grams of sodium acetate 
 dissolved in 50 grams of water. J 
 
 Generally speaking, amines such as aniline, the toluidines, the 
 xylidines, >-aminoacetanilide, are diazotized at 0-2. Others, 
 as for example, a- and /3-naphthylamines, the nitroanilines, 
 and diamines, such as benzidine, tolidine, dianisidine, are 
 converted into the diazo-compounds more suitably at about 
 10. Hydrochloric acid is most commonly employed, but 
 sulphuric and acetic acids are also used. 
 
 There are many cases where the diazotization of an ammo- 
 compound is not effected quite so easily as is described above, 
 and special methods have to be employed. Thus many 
 aminoazo-compounds are insoluble in water or acids and are 
 attacked by nitrous acid only with difficulty. At the same 
 time the diazo-compound is often insoluble in water. Such 
 compounds are, for example, p-sulphobenzeneazo-a-naphthyl- 
 arnine and ^-acetylaminobenzeneazo-a-naphthylamine, and 
 these are diazotized by using an excess of sodium nitrite and 
 
 * Schwalbe, Zeitsch. Farb. Ind., 1905, 4, 433. 
 
 t Schwalbe, loc. cit. 
 
 | Compare also Schwalbe, Zeitsch. Farb. Ind., 1905, 4, 433 ; Erban 
 and Mebus, Chem. Zeit., 1907, 31, 663, 678, 687, 1011. 
 
 For a detailed description of the preparation of a number of these 
 see Cain and Thorpe, The Synthetic Dyestuffs, 1905, 226 et seq. 
 
y 
 
 PREPARATION OF THE DIAZO-COMPOUNDS 15 
 
 stirring for several hours, keeping the mixture ice-cold in order 
 to avoid escape of nitrous acid. In order to prevent this 
 escape it has been proposed to diazotize under increased pres- 
 sure.* The amine is introduced into a closed vessel together 
 with the corresponding quantity of mineral acid, and the 
 pressure is then raised by admitting compressed air or other 
 indifferent gas, after which the nitrite solution is added. 
 
 Difficulties have been met with in attempting the diazotiza- 
 tion of substituted amines containing a number of acidic 
 groups ; thus V. Meyer and Stiiber f found it impossible to 
 decompose trinitroaniline 
 
 by treatment with ethyl nitrite in alcoholic solution, and 
 pentabromoaniline also resists diazotization J unless a large 
 excess of sulphuric acid is employ ed. 
 
 This method || is found to be advantageous in diazotizing 
 derivatives of aniline containing several negative groups. 
 The base is dissolved in sulphuric acid (monohydrate), the 
 solution cooled to 10 to 15, and a very concentrated solu- 
 tion of sodium nitrite added in excess during 1-lf hours, the 
 liquid being well stirred, On diluting the solution any un- 
 altered amine is often precipitated and can be removed by 
 filtration. Fuming, 40 per cent., hydrochloric acid may some- 
 times be used instead of sulphuric acid. 
 
 This method has been successfully applied in diazotizing \ 
 dinitro-p-toluidine 
 
 NH 2 
 
 NO, 
 
 cH 3 ir 
 
 * Seidler, D. R-P. 143450. t Annalen, 1873, 165, 187. 
 
 % Noelting, Bull. Soc. ind. Mulhouse, 1887, 57, 30. 
 
 Hantzsch, Ber., 1900, 33, 520. 
 
 || Clans and Wallbaum, J. pr. Chem., 1897 [ii], 56, 48. 
 
 II Claus and Beysen, Annalen, 1891, 266, 224. 
 
16 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 Although, as Griess found, the ortho-aminophenols can be easily 
 diazotized (see p. 11), when the corresponding compounds of the 
 naphthalene series are similarly treated, difficulties often arise 
 owing to the oxidizing action of the nitrous acid. This is 
 especially applicable to the 1 : 2- and 2 : 1-amino-naphthols. 
 In order, therefore, to obtain diazo-salts derived from these 
 substances a number of methods have been employed with the 
 object of avoiding this action. Thus the addition of copper or 
 zinc salts to the solution of the amine or the use of the nitrites 
 of zinc, nickel, mercury, &c., has been found efficacious.* For 
 example, 12 kilos of l-amino-2-naphthol-4-sulphonic acid are 
 mixed with 50 litres of water and ice and a solution of 1 kilo 
 of copper sulphate added. A solution of 3-5 kilos of sodium 
 nitrite is now slowly run in, and after diazotization is complete 
 the solution is filtered and the diazo-compound precipitated 
 with hydrochloric acid. This diazo-compound may be dried 
 and powdered. f A second example is the following : J 48 
 kilos of the above acid are mixed and well stirred with 
 a solution of 33 kilos of zinc sulphate in 33 litres of water 
 containing a little zinc hydroxide. The latter is formed by 
 the addition of about 3 kilos of ammonia to the solution. 
 A concentrated aqueous solution of 14 kilos of sodium nitrite 
 is then added. The reaction is completed by warming for two 
 hours at about 40, and the mass is then acidified with acetic 
 acid. By filtration and crystallization, brilliant bronze needles 
 of the diazo-compound are obtained. 
 
 Another method consists in carrying out the diazotization 
 of these amino-compounds in presence of excess of acetic or 
 oxalic acid. The diazo-compounds derived from such amino- 
 hydroxynaphthalenesulphonic acids are so stable that they 
 can be sulphonated || and nitrated. If 
 
 It is singular that the 2 : 3-aminonaphthols can be smoothly 
 diazotized in the usual manner.** 
 
 6. Other methods of preparing solutions of diazo-salts. 
 In addition to the methods given in 1 and 2 which were 
 used to obtain dry diazo-salts, many other ways of producing 
 
 * E. P. 10235 of 1904. See also D.R-P. 171024, 172446. 
 
 t E. P. 15025 of 1904. 1 F. P. 353786 of 1905. 
 
 D. R-P. 155083, 175593. || D. R-P. 176618, 176620. 
 
 D. R-P. 164665, 176619. ** E. P. 28107 of 1897. 
 
PREPARATION OF THE DIAZO-COMPOUNDS 17 
 
 these compounds in solution have been used. For example, 
 diazobenzene chloride can be obtained by the action of zinc 
 dust and hydrochloric acid on a solution of aniline nitrate,* thus 
 C 6 H 5 . NH 2 , HN0 3 + Zn + 3HC1 = C 6 H 5 . N 2 C1 + ZnCl 2 + 3H 2 0. 
 It is obvious that the action is a reducing one, the nitric acid 
 being converted into nitrous acid by the nascent hydrogen. 
 
 A reaction similar to this is the production of diazo-salts 
 from the nitrites of aromatic amines by treatment with 
 a mineral acid.f 
 
 Bamberger found J that when nitrosoacetanilide, 
 
 C 6 H 5 .NAc.NO, 
 
 was triturated with excess of 50 per cent, potassium hydroxide, 
 the resulting solution showed the presence of a diazo-compound. 
 (The nature of this, existing in an alkaline solution, will be 
 explained later.) 
 
 Certain nitroso-compounds, which contain the nitroso-group 
 in the benzene nucleus may be directly converted into diazo- 
 compounds by the action of three molecular proportions of 
 nitrous acid, thus 
 
 R.NO + 3HN0 2 = R.N 2 . N0 3 + HNO 3 + H 2 0. 
 
 The method has been successfully applied to the preparation 
 of the diazo-derivative of diphenylamine from the _p-nitroso- 
 compound. || 
 
 Quinoneoxime also, when treated with nitrogen trioxide in 
 ethereal solution, yields the corresponding diazo-salt.H 
 
 The formation of diazo-compounds by the interaction of 
 nitrogen peroxide and quinonedioximes ** is of much interest 
 from a theoretical point of view (see p. 163). When, for 
 example, thymoquinonedioxime is treated with nitrogen 
 peroxide a nitrosodiazo-derivative is obtained. 
 
 :NOH 
 
 N0 3 .N 
 
 * Mohlau, D. R-P. 25146, Per., 1883, 16, 3080. 
 t Wallach, Annalen, 1907, 353, 322. 
 \ Ber., 1894, 27, 915 ; compare E. P. 13577 of 1894. 
 0. Fischer and Hepp, Annalen, 1888, 243, 282. 
 || Hantzsch, Ber., 1902, 35, 894. If Jaeger, Ber., 1875, 8, 894. 
 
 ** Oliveri-Tortorici, Gazzetta, 1900, 30, i. 526. 
 
 C 
 
18 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 Diazo-compounds are also obtained by the oxidation of 
 phenylhydrazines with mercuric oxide * or acetate, f with 
 nitrous acid in presence of a strong mineral acid,} and with 
 bromine, and also by the action of acidic chlorides on 
 thionylphenylhydrazone. || 
 
 An electrolytic process for the preparation of diazo-salts has 
 been patented by Boehringer & Sons.^j" As the method is 
 carried out at temperatures of from 40 to 90, under which 
 conditions the diazo-salt would be very quickly decomposed, 
 the latter is immediately combined with a hydroxyl compound, 
 such as jS-naphthol-3 : 6-disulphonic acid ('R salt'). A solution 
 containing a mixture of aniline, sodium nitrite, and R salt is 
 charged into a suitable cell at the platinum electrode whilst 
 dilute sodium hydroxide surrounds the nickel cathode. On 
 electrolyzing, the diazo-salt is formed and at once condenses 
 with the R salt, with production of the azo-dyestuff. 
 
 A somewhat indirect method of obtaining diazo-salts was 
 observed by Lauth,** who found that certain azo-dyestuffs 
 were split up into quinones and diazo-compounds by treatment 
 with an oxidizing agent, such as lead peroxide and sulphuric 
 acid. It has been found alsoff that by the action of red 
 fuming nitric acid on azo-dyestuffs, a reaction first studied by 
 Meldola.Jt oxidation and nitration takes place, and diazo- 
 compounds, together with nitro- derivatives of the second 
 constituent of the dyestuff, are formed. 
 
 7- The action of nitrous acid on aromatic substances 
 containing more than one amino-gronp. In investigating 
 the action of nitrous acid on diamines or triamines of the 
 aromatic series, one would expect each amino-group to become 
 converted into the corresponding diazo-group. Although in 
 most cases, perhaps, this is the primary action, yet very often 
 some secondary reaction ensues with such rapidity that no 
 
 * E. Fischer, Annalen, 1879, 199, 320. 
 
 t Bamberger, Ber., 1899, 32, 1809. 
 
 t Altschul, J. pr. Chem., 1896 [ii], 54, 496. 
 
 Michaelis, Ber., 1893, 26, 2190. 
 
 || Annalen, 1892, 270, 116. 
 
 [ D. R-P., 152926 of 1902 ; E.P. 2608 of 1904. 
 
 ** Bull. Soc. chim., 1891 [iii], 6, 94. 
 
 tt 0. Schmidt, Ber., 1905, 38, 3201. 
 
 $| Proc. y 1894, 10, 118 ; Trans., 1889, 55, 608; 1894, 65, 841. 
 
PREPARATION OF THE DIAZO-COMPOUNDS 19 
 
 diazo-salt can be isolated by the usual means, and special 
 methods have to be employed. 
 
 Very striking differences in behaviour are exhibited by the 
 three phenylenediamines. When a dilute solution of sodium 
 nitrite is added to a dilute solution of the sulphate of 
 o-phenylenediamine, aziminobenzene is formed * according to 
 the equation 
 
 C.H/555 1 + HN 2 = C 6 H/ 1 )>NH + 2H 2 0. 
 
 VU Jig N 
 
 Griess obtained this substance by acting on o-phenylene- 
 diamine hydrochloride with >-diazobenzenesulphonic acid.f 
 It has not yet been found possible to prepare the tetrazo- 
 compound. In the case of o-tolylenediamine a similar reaction 
 takes place, and it has been shown J that the amino-group 
 which is in the meta-position relative to the methyl group is 
 converted into the diazo-group before internal condensation, 
 resulting in the formation of the azimino-compound, takes 
 place. 
 
 m-Phenylenediamine behaves very differently. When a 
 solution of the hydrochloride is treated with sodium nitrite, 
 the well-known dyestuff ' Bismarck Brown ' is obtained. This 
 is the hydrochloride of bisbenzeneazophenylenediamine. If 
 the nitrite is added suddenly, a certain amount of nitroso-m- 
 phenylenediamine is formed. 
 
 When the reaction, however, is carried out in a different 
 manner, both ammo-groups may be diazotized, forming a 
 tetrazo- or bisdiazo-compound. Thus Griess || showed that 
 the reaction could be successfully brought about by taking 
 care that both the nitrite solution and hydrochloric acid are 
 always in excess of the diamine. A two per cent, solution of 
 m-phenylenediamine hydrochloride is prepared, and, on the 
 other hand, a dilute solution of sodium nitrite of specific 
 gravity 1.1. To the latter is added an equal volume of 
 hydrochloric acid of specific gravity 1.15, and then the 
 diamine solution added slowly, keeping the mixture well 
 
 * Ladenburg, Per., 1876, 9, 221. t Ber., 1882, 15, 2195. 
 
 1 Noelting and Abt., Ber., 1887, 20, 2999. 
 
 Tauber and Walder, Ber., 1900, 33, 2116. || Ber., 1886, 19, 317. 
 
20 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 stirred until the dark yellow solution of the tetrazo-compound 
 is obtained. A later modification of this method is the 
 following:* 80 c.c. of fuming hydrochloric acid are diluted 
 with about 400 grams of ice and cooled with a freezing mix- 
 ture. To this is added a solution of 15 grams of sodium 
 nitrite in cold water, so that a strong solution of nitrous acid 
 is obtained. To this solution is added quickly a cold solution 
 of 9 grams of m-phenylenediamine hydrochloride to which 
 10 c.c. of strong hydrochloric acid had been added. The 
 mixture is well stirred during the operation, and a clear 
 yellow solution of the tetrazo-compound is obtained. Other 
 methods consist in adding the nitrite solution to a mixture of 
 the diamine with a large excess of hydrochloric acid,f or in 
 pouring a mixture of the diamine and nitrite into ice-cold 
 dilute hydrochloric acid.J The dry tetrazo-chloride has also 
 been prepared. 
 
 The tetrazo-compound derived from m-tolylenediamine is 
 prepared similarly. || 
 
 Substituted m-phenylenediamines, containing the sub- 
 stituent attached to that carbon atom which is in the ortho- 
 position to both amino-groups are, as a rule, easily tetrazo- 
 tized. Thus the m-tolylenediaminesulphonic acid of formula 
 
 CH 3 
 
 NH/\NH 2 
 
 S0 3 H I 
 
 is tetrazotized without difficulty ,lf as are also such diamino- 
 hydroxy- compounds as those of the formulae 
 
 OH OH OH OH 
 
 NH 
 
 * Tauber and Walder, Ber., 1897, 30, 2901. 
 
 t E. P. 1593 of 1888. J Epstein, D. R-P. 103660 of 1899. 
 
 Hantzsch and Borghaus, Ber.. 1897, 30, 93. 
 
 || D. R-P. 103685 of 1899. 
 
 IT E. P. 17546 of 1892. ** E. P. 18624 of 1900, D. R-P., 168299. 
 
PREPARATION OF THE DIAZO-COMPOUNDS 21 
 
 In order to diazotize only one ammo-group in sulphonated 
 m-diamines, the solution of the base is mixed with the calcu- 
 lated quantity of alkali nitrite and then mineral acid added ; 
 by this means the diamine is always in contact with the 
 requisite quantity of free nitrous acid, and the diazotization 
 proceeds smoothly.* 
 
 Griess also studied f the action of nitrous acid on p-pheny- 
 lenediamine, and stated that the principal product of the 
 reaction when carried out in the usual way consisted of 
 aminodiazobenzene chloride, one only of the amino-groups 
 having been diazotized. It was found, however, that by this 
 method a mixture of the diazo- and the tetrazo-compounds 
 was obtained. { 
 
 Later, Griess was successful in preparing the tetrazo-com- 
 pound by using the same method as he had employed in the 
 preparation of the m-tetrazobenzene chloride, and the dry 
 tetrazo-sulphate has been obtained in small amount. || 
 
 The use of a diazo- and tetrazo-compound derived from 
 >-phenylenediamine has become of very great practical im- 
 portance in the manufacture of azo-dyes, but as it is essential 
 that a single compound and not a mixture of diazo- and 
 tetrazo- should be prepared, and, further, that no large excess 
 of nitrous acid should be present, these compounds are now 
 prepared indirectly. 
 
 For this purpose either ^-nitroaniline or ^-aminoacetanilide 
 is used as the starting-point. If a compound involving the 
 use of the diazo-chloride is required, the above substances 
 are diazotized in the usual way,^J and, after coupling with the 
 desired component, the nitro-group is reduced by sodium 
 sulphide solution, or the acetyl group is removed by heating 
 with sodium hydroxide. In each case, if X denotes the 
 component, we obtain the compound 
 
 * D. R-P. 152879. t Ber., 1884, 17, 697. 
 
 | Nietzki, Ber., 1884, 17, 1350. Ber., 1886, 19, 317. 
 
 || Hantzsch and Borghaus, loc. cit. 
 
 IT It is singular that, although ^-aminoacetanilide, NH 2 . C 6 H 4 . NHAc, 
 is very easily diazotized, methyl-p-phenylenediamine, NH 2 . C 6 H 4 . NHMe, 
 cannot be thus transformed, nitrogen being evolved even below by 
 the action of nitrous acid. (Hantzsch, Ber., 1902, 35, 896.) 
 
22 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 If the tetrazo-compound had been desired, this product is 
 now diazotized in the usual way, and the diazo-compound 
 coupled with a molecule of the same component X, or a diffe- 
 rent one Y, giving us a dyestuff derived from the tetrazo- 
 compound of p-phenylenediamine, of formula 
 
 In the event of the component X containing an amino- 
 group, and at the same time (as is usual) belonging to the 
 naphthalene series, care is taken to use the calculated quantity 
 of sodium nitrite (one molecule), when the NH 2 group united 
 to the benzene ring is completely diazotized, leaving the other 
 NH 2 group intact. This can generally be also diazotized by 
 using a second molecule of nitrite. 
 
 We have seen already (p. 15) that certain substituted 
 amines present difficulties to the diazotizing process, some, in 
 fact, being incapable of diazotization by the usual method. 
 Similar examples occur amongst the substituted diamines: 
 thus o-nitro-p-phenylenediamine cannot be directly converted 
 into the tetrazo-compound, but only the diazo-compound is 
 formed. Even an excess of nitrite fails to convert more than 
 one amino-group into the diazo-group,* the constitution of 
 the product being in all probability 
 
 NO^ 
 
 V 'N 2 C1 
 
 The nitro-p-phenylenediamine is best diazotized by dis- 
 solving the hydrochloride in water, adding an excess of acetic 
 acid, and then excess of sodium nitrite at 5-10. It is very 
 remarkable that if the diazo-compound is coupled with a 
 component such as K salt and an azo-dye formed, the remain- 
 ing amino-group may now be easily converted into the 
 diazo-group. f Other instances of this are known in the 
 naphthalene series (see p. 24). Differences in the behaviour 
 of two amino-groups in the substituted benzene molecule had 
 indeed been detected by Griess, who found that p-diamino- 
 
 * Billow, Ber. } 1896, 29, 2285. t E. P. 6630 of 1892. 
 
 
PREPARATION OF THE DIAZO-COMPOUNDS 23 
 
 benzole acid yielded >-aminodiazobenzoic acid and not the 
 tetrazo-derivative.* 
 
 The diamines of the diphenyl series have attained very 
 great importance owing to their use in the production of 
 dyestuffs which dye cotton without the aid of a mordant. 
 The simplest of these is benzidine, 
 
 which presents no difficulty in undergoing diazotization (con- 
 trary to the statement of Kaufler f), the most suitable tem- 
 perature being 8-10, and both amino-groups being easily 
 diazotized. 
 
 It is also possible to obtain the monodiazo-compound by 
 mixing solutions of benzidine hydrochloride and tetrazo- 
 diphenyl chloride and allowing the mixture to remain for two 
 or three days at 10-204 After filtering off the dark -coloured 
 insoluble by-products, the solution contains principally 
 aminodiazodiphenyl chloride 
 
 Other diamines, such as tolidine, dianisidine, ethoxybenzi- 
 dine, dichlorobenzidine, nitro- and dinitro-benzidine, diamino- 
 stilbenedisulphonic acid, are tetrazotized in exactly the same 
 manner as benzidine. 
 
 Turning now to the naphthalene series the phenomena 
 observed in the diamines of the benzene series are again 
 present. 
 
 Those diamines containing the amino-groups in the ortho 
 or peri positions yield with nitrous acid azimino-compounds 
 (2 : 3-naphthylenediamine) . Their sulphonic acids behave 
 similarly. |[ 
 
 The meta-diamines behave like the m-diamines of the 
 benzene series, giving brown colouring matters. 
 
 I : 4-Naphthylenediamine is diazotized with still greater 
 difficulty than >-phenylenediamine. Nitrous acid acts in this 
 case also as an oxidizing agent, and 1 : 4-naphthaquinone is 
 
 * Ber., 1884, 17, 603. t Annalen, 1907, 351, 151. 
 
 t Tauber, Ber., 1894, 27, 2627. Ber., 1894, 27, 765. 
 
 II E. P. 8645 of 1895. 
 
24 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 formed. In order to obtain the diazo- or tetrazo-derivative 
 the same method is adopted as in the case of p-phenylene- 
 diamine, namely, to convert one amino-group into the acetyl- 
 amino-group and then to diazotize the remaining amino- 
 group. * If this is then coupled with a suitable component, 
 forming an azo-dyestuff, the acetyl group may be split off 
 and the free amino-group now diazotized. 
 
 The same procedure is used in preparing the diazo- or 
 tetrazo-derivatives of the l:4-naphthylenediaminesulphonic 
 acids, except in the case of the acid containing the S0 3 H group 
 in the position 2 
 
 NH, 
 
 (XH 
 
 This acid exhibits a great tendency towards the formation 
 of oxidation products when treated with sodium nitrite in 
 the presence of mineral acids, but the diazotization proceeds 
 smoothly when acetic or oxalic acids are used.f It is re- 
 markable that only one amino-group is attacked, and it has 
 been found impossible to prepare a tetrazo-derivative. This 
 behaviour is to be attributed, perhaps, to the protective action 
 of the sulphonic acid group, and consequently the diazo- 
 compound probably possesses the constitution 
 
 (XH 
 
 N 2 X 
 
 X being the organic acid radical. 
 
 If now this diazo-compound is coupled with a phenol or 
 naphthol, the resulting azo-dyestuff is easily diazotized. This 
 behaviour is analogous to that exhibited by o-nitro-p-pheny- 
 lenediamine (p. 22). 
 
 It has been found also that if the monoacetyl derivative of 
 
 * E.P. 18783 of 1891. 
 t E. P. 2946 of 1896. 
 
PREPARATION OF THE DIAZO-COMPOUNDS 25 
 
 this sulphonic acid is prepared, the formula of which is 
 probably 
 
 NH 
 
 NH.CO.CH, 
 
 the free amino-group readily undergoes diazotization.* 
 
 The remaining naphthylenediamines and their sulphonic 
 acids are easily converted into the tetrazo-compounds.f 
 
 The transference of triamines into the corresponding diazo- 
 com pounds cannot be illustrated by many examples, as cases of 
 this are rare. The best known are probably those of rosaniline 
 and para-rosaniline. These bases, containing, of course, three 
 free ammo-groups, were diazotized by Caro and Wanklyn J and 
 E. and O. Fischer, who thus prepared compounds containing 
 three diazo-groups. 
 
 The formation of diazoamino-compounds in this reaction 
 has also been observed. || 
 
 8. ' Solid diazo-componnds '. Mention has already been 
 made of the great technical importance of diazotized p-nitro- 
 aniline, owing to its use in the production of the ' para-red ' 
 by combination with /3-naphthol on the cotton fibre. 
 
 In order to enable the dyer to avoid the preparation of this 
 and other diazo-compounds in the dyehouse, several processes 
 have been adopted for the purpose of supplying the users 
 with the diazo-compound ready made. 
 
 Such preparations mostly consist of a paste of the diazo- 
 compound in a very concentrated form, or of a sparingly 
 soluble diazo-salt. A remarkable compound, produced by the 
 action of alkalis on the diazo-chloride, which is very stable 
 
 * E.P. 17064 of 1896. 
 
 t E.P. 26020 of 1896; see also Lange, Chem. Zeit., 1888, 12, 856. 
 Kaufler and Karres (Ber., 1907, 4O, 3263) could only diazotize one 
 ammo-group of 2 : 7-naphthylenediamine, using amyl nitrite in alcoholic 
 solution, but patents have been taken out formerly for diazo-dyestuffs 
 from the tetrazo-compound. 
 
 % Zeitscli. f. Chem., 1866, 511. Annalen, 1878, 194, 269. 
 
 |i Pelet and Eedard, Bull. Soc. chim., 1904 [iii], 81, 644. 
 
26 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 and which yields the diazo-chloride on acidifying, has also 
 been put on the market. The nature of this substance is 
 fully discussed on p. 96. 
 
 It is prepared by treating the >-nitrodiazobenzene chloride 
 or other diazo-salts containing nitro- or halogen-groups with 
 caustic alkali at 60-70.* The diazo-salts prepared from 
 aniline and its homologues are treated at 120.f 
 
 The substances formed may be dried or used as a paste ; by 
 the action of a mineral acid the free diazo-chloride is regenerated. 
 
 The diazo-compound of ^?-nitroaniline, after having been 
 treated in this manner with alkali, is known as * Nitrosamine 
 red in paste '. Another way in which to obtain the diazo-salt 
 in a more stable condition is to mix it with a solution of 
 sodium a-naphthalenesulphonate, J sodium nitrobenzenesul- 
 phonate, or sodium naphthalenedisulphonates, || and the 
 tetrazo-salts of benzidine, &c., can be condensed with sodium 
 2-naphthol-3 : 6 : 8-trisulphonate, or sodium 2-naphthol-l-sul- 
 phonate, when additive compounds, and not azo-dyestuffs, are 
 obtained.^ Further, the zinc chloride double salts of diazo- 
 tized aminoazo-compounds are also prepared.** All these 
 stable compounds may be dried. 
 
 A simpler method is to diazotize the nitroaniline in a very 
 concentrated solution by passing nitrous acid gas through 
 a solution of p-nitroaniline in sulphuric acid, or even to 
 evaporate the diazo-solution (prepared from sulphuric acid) 
 in a vacuum at a temperature not exceeding 45. Anhydrous 
 sodium sulphate is now added, which, with the excess of sul- 
 phuric acid, is converted into the bisulphate, and the paste, 
 which soon solidifies, may be powdered, ft The substance 
 obtained from diazotized p-nitroaniline in this way is called 
 'Azophor red P.N. ', 'Nitrazol C', Azogen red, and Benzo- 
 nitrol; and that from diazotized dianisidine 'Azophor blue 
 D '. (For instances of the elimination of groups during diazo- 
 tization see p. 63.) 
 
 * E. P. 20605 of 1893. f E. P. 3397 of 1894, 13460 of 1895. 
 
 I E. P. 18429 of 1894. D. R-P. 88949 of 1894. 
 
 || D. R-P. 94280 of 1894. IT E. P. 8989 of 1895, 11757 of 1895. 
 
 ** E. P. 1645 of 1896 ; D. R-P. 89437 of 1896. 
 tt E. P. 21227 of 1894 ; D. R-P. 85387 of 1894; E. P. 15353 of 1897. 
 
CHAPTEK III 
 
 THE MECHANISM OF THE DIAZOTIZING PROCESS 
 
 1. Thermochemistry. The formation of diazo-compounds 
 proceeds with absorption of heat; the reaction is thus an 
 endothermic one. 
 
 The development of heat which is observed in the usual 
 process of preparing these compounds is due to the formation 
 of water and sodium chloride. 
 
 The values which have been recorded for the heat of forma- 
 tion of diazo-compounds are as follows : 
 
 Diazobenzene nitrate . . . 47-4 calories.* 
 Diazobenzene chloride . 44-0 
 
 o-Diazotoluene chloride . . 41-8 
 
 p-Diazo toluene chloride . . . 42-3 J 
 
 2. Explosibility of dry diazo-compounds. From the fact 
 that the formation of diazo-compounds is accompanied by 
 absorption of heat, it was to be expected that these substances 
 would be unstable, and it is found that nearly all diazo-salts 
 are very liable to explode when in the dry state; the most 
 unstable in this respect being those containing nitro-groups. 
 Thus diazobenzene nitrate J is more explosive than the sulphate, 
 and a case is on record where jp-nitrodiazobenzene nitrate 
 exploded violently when lightly touched with a platinum 
 spatula. 
 
 Great care must be taken, therefore, in handling these sub- 
 stances, as they are extremely unreliable, and may never be 
 regarded as safe. Diazobenzene chloride, usually looked on 
 as comparatively stable, exploded on one occasion, apparently 
 
 * Berthelot and Vielle, Compt. rend., 1881, 92, 1076. 
 t Vignon, Bull. Soc. chim., 1888 [ii], 49, 906. 
 t Knoevenagel, Ber., 1890, 23, 2994. 
 Bamberger, Ber., 1895, 28, 538. 
 
28 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 spontaneously, with very great violence ; * and a violent ex- 
 plosion of dry diazobenzenesulphonic acid, which had been 
 prepared some years previously, occurred in 1901. f An 
 exactly similar accident befell the author of this book 
 in 1896. 
 
 In spite of the danger of working with such substances, 
 determinations of the temperature at which diazo-compounds 
 explode have been made. Thus dry m- and >-nitrodiazo- 
 benzene chlorides explode at 118 and 85 respectively ,J and 
 diazobenzene nitrate explodes above 90. 
 
 3. Velocity of diazotization. The rate at which amines 
 are diazotized has been determined by Hantzsch and Schu- 
 mann. || Diazotization, of course, proceeds with extreme 
 rapidity under ordinary conditions, and the experiments were 
 therefore conducted with -ZV/1000 solutions. Using a colori- 
 metric method for estimating the nitrous acid, it was found 
 that, in presence of an excess of acid, the rate of diazotization 
 of aniline, >-toluidine, m-xylidine, p-bromoaniline, and^?-nitro- 
 aniline is the same in each case. Further, if the temperature 
 is raised, the rate is increased. 
 
 The reaction which takes place is of the second order, and 
 the values obtained for the velocity-constant 
 
 t(a-x) 
 
 were 0-036 for aniline, 0-038 for ^-toluidine, 0-041 for m-xyli- 
 dine, and 0-045 for p-bromoaniline in .ZV/1000 solution with one 
 molecule of free acid at 0. 
 
 Schumann then measured the velocity by observing the 
 fall of electrical conductivity which takes place during 
 diazotization. ^f He was able to confirm the previous experi- 
 ments, and concluded that all aromatic amines are diazotized 
 at approximately the same speed. 
 
 * Hantzsch, Ber., 1897, 30, 2342, footnote. 
 
 t Wichelhaus, Ber., 1901, 34, 11. 
 
 t Oddo, Gazzetta, 1895, 25, i. 327. 
 
 Berthelot and Vielle, Compt. rend., 1881, 92, 1074. 
 
 U Ber., 1899,32, 1691. 
 
 IT Ber., 1900, 33, 527. 
 
CHAPTEE IV 
 THE REACTIONS OF THE DIAZO-COMPOUNDS 
 
 1. Action of water. When a diazo-salt is heated with 
 water a phenol is formed * according to the equation 
 X.N 2 .HS0 4 + H 2 O = X.OH + H 2 S0 4 + N 2 , 
 X denoting the aromatic nucleus. 
 
 The reaction is best carried out in the presence of sulphuric 
 acid. If the diazo-nitrate is used the nitric acid liberated 
 attacks the phenol, forming nitrophenols. 
 
 The ordinary method of carrying out the operation is to 
 diazotize in presence of sulphuric acid and then to add, if neces- 
 sary, a further quantity of sulphuric acid. The solution is 
 then either directly boiled until no further evolution of nitrogen 
 takes place, or steam may be passed into the solution, or the 
 solution may be added slowly to boiling dilute sulphuric acid. 
 
 In one or other of these ways most diazo-compounds yield 
 the corresponding phenol, which is isolated by the usual 
 means. For example, 4 : 4'-dihydroxydiphenyl is obtained 
 in the following manner. 25 grams of benzidine are dis- 
 solved by the aid of heat in 500 c.c. of water and 30 c.c. of 
 concentrated hydrochloric acid. The solution is cooled to 5 
 by adding ice, and then 18 grams of sodium nitrite dissolved 
 in a small quantity of water are poured in slowly, the tempera- 
 ture not being allowed to rise above 10 ; this is effected by 
 adding more ice if necessary. 
 
 100 grams of concentrated sulphuric acid are now added, 
 and steam passed into the mixture until crystals of dihydroxy- 
 diphenyl begin to separate out and the solution gives no 
 further colour with an alkaline solution of R salt or /3-naphthol. 
 On cooling, the precipitate is filtered, dissolved in dilute caustic 
 soda, the solution filtered from any insoluble matter, and 
 * Griess, Annalen, 1866, 137, 67. 
 
30 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 reprecipitated by hydrochloric acid. The dihydroxydiphenyl 
 is recrystallized from dilute alcohol when it is obtained pure.* 
 
 The process is carried out on the large scale in the manufacture 
 of several naphthol- and dihydroxynaphthalene-sulphonic 
 acids, and a classical example of this decomposition, as applied 
 to diazo-compounds derived from triamines, is the production 
 of aurin from para-rosaniline. f 
 
 Owing to the very great reactivity of the diazo-salts and their 
 well-known capacity of coupling or combining with phenols, 
 it is obvious that there is a great tendency for secondary 
 reactions to take place, interfering, to a certain extent, with 
 the quantitative production of the hydroxy-compounds ; 
 further, owing to the extreme differences in the relative 
 stability of diazo-salts, other more obscure side reactions are 
 liable to intervene, particularly in those cases where the 
 decomposition can be completed only by long heating. 
 
 Secondary reactions occur indeed even in the simplest case ; 
 thus in the decomposition of diazobenzene sulphate a small 
 quantity of hydroxydiphenyl is formed in consequence of the 
 action of some of the undecomposed diazo-compound on 
 phenol. J 
 C 6 H 5 . N 2 . HS0 4 + C 6 H 5 .OH = C 6 H 6 . C 6 H 4 . OH + H 2 S0 4 + N 2 . 
 
 In very many cases also, particularly in the naphthalene 
 series, the solution becomes deeply coloured owing to the 
 coupling of the diazo-compound with the naphthol formed, with 
 production of the azo-dyestuff. Thus in the decomposition 
 of diazo-a-naphthalene-4-sulphonie acid this reaction in variably 
 occurs, even in the presence of sulphuric acid ; consequently 
 a large excess of acid is usually taken in order to limit this 
 formation of colouring matter as far as possible. 
 
 There are many cases recorded in the literature where it 
 has been found impossible to obtain even a trace of an 
 hydroxy-compound by carrying out the decomposition in the 
 manner described above. Most of these occur among extremely 
 stable diazo-compounds, such as those derived from the halogen 
 or nitro-substituted amines. || 
 
 * Compare also Hirsch, Ber., 1889, 22, 335. 
 
 t Annalen, 1878, 194, 301. J Hirsch, Ber., 1890, 23, 3705. 
 
 Trans., 1903, 83, 221. || Amer. Chem. J., 1889, 11, 319. 
 
THE REACTIONS OF THE DIAZO- COMPOUNDS 31 
 
 Recognizing that the cause of this might be due to an 
 insufficiently high temperature, Heinichen* adopted the 
 method of heating the strong diazo-solution with concentrated 
 sulphuric acid, whereby the boiling-point becomes raised to 
 150. In this way he obtained 2 : 6-dibromophenol from the 
 corresponding diazo-salt after the usual method had failed. 
 
 As, however, the stability of diazo-compounds has been 
 shown to increase with addition of sulphuric acid,f this 
 method is not always successful. 
 
 A novel way of attacking the problem is that described by 
 Kalle & Co. J The non-production of phenols in certain cases 
 being evidently due, as already indicated, to condensation 
 between the diazo-compound and the phenol formed, any 
 process depending on the removal of the latter when set free 
 would be expected to stand more chance of success. The 
 method adopted by this firm is therefore to carry out the 
 decomposition by dropping the diazo-solution into a mixture 
 of dilute sulphuric acid and sodium sulphate heated to 135- 
 145, and allowing any volatile products to distil over. In 
 this way the temperature is kept high without using con- 
 centrated sulphuric acid. 
 
 By this means a good yield of guaiacol is obtained from the 
 diazo-salt of o-anisidine,|| and the diazo-salts of s-tribromo- 
 and s-trichloro-aniline, which under no other conditions could 
 be made to yield phenols, gave a small yield of s-tribromo- 
 phenol and s-trichlorophenol respectively.^ Another method 
 of procedure is to decompose the diazo-compound at the 
 moment of its formation by adding a solution of sodium 
 nitrite to a boiling solution of the base in hydrochloric acid. 
 In this way a good yield of p-nitro-o-cresol can be obtained 
 from p-nitro-o-toluidine, but if the decomposition is carried 
 out in the usual way internal condensation occurs, with 
 
 * Anndlen, 1889, 253, 281. t Ber., 1905, 38, 2511. 
 
 t E. P. 7233 of 1897. 
 
 An alternative method consists in adding the diazo-solution to 
 a boiling 50 per cent, aqueous solution of copper sulphate. (D. R-P. 
 167211, Soc. Chim. des Usines du Rhdne.) 
 
 || The diazo-sulphate of jp-anisidine yields quinol on heating with 
 water to 140. (Salkowski, Ber., 1874, 7, 1008.) 
 
 ff Cain, Trans., 1906, 87, 19. 
 
32 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 formation of nitroindazole.* Certain derivatives of o-anisi- 
 dine, the diazo-compounds of which have failed to yield the 
 corresponding substituted guaiacol, would, in all probability, 
 behave in the normal manner if one of the above methods 
 were applied. f 
 
 Various other apparent exceptions to the general rule have 
 been described by Wroblewski.J who obtained the substituted 
 hydrocarbons only, and not the phenols, from the diazo-salts 
 derived from dibromoaniline, dibromo->-toluidine, and bromo- 
 and chloro->-toluidine. These diazo-salts have been recently 
 examined, with the result that, in each case, the corresponding 
 phenol was obtained. Wroblewski's results were probably 
 due, as indeed he himself suggests, || to the presence of alcohol 
 used in the preparation. The production of dibromophenetole 
 by heating the diazo-compound of dibromo-o-phenetidine 1f is 
 probably explained in the same way. 
 
 A case which does not seem to accord with this explanation 
 is that of ethyl diazogallate, which can be crystallized from 
 water, and when heated with water in a sealed tube for four 
 hours to 220 yields ethyl gallate, the nitrogen having been 
 completely eliminated.*'* 
 
 By treating aminoindazole with nitrous acid and warming 
 the resulting diazo-compound with water, Bamberger ff pre- 
 pared a new class of diazo-compounds, to which the name 
 ' triazolens ' is given. He formulated the compound according 
 to the equation 
 
 Aminoindazole. Indazoletriazolen. 
 
 but Hantzsch JJ regarded it as a diazide 
 
 * Witt, Noelting, and Grandmougin, Ber., 1890, 23, 3635. 
 
 t Meldola, Woolcott, and Wray (Trans., 1896, 69, 1327) obtained 
 resins only by boiling the diazo-salts of _p-bromo- and ^-nitro-o-anisidine 
 with water or dilute sulphuric acid. 
 
 t Ber., 1874, 7, 1061. 
 
 Cain, Trans., 1906, 89, 19. [| Ber., 1884, 17, 2704. 
 
 % Mohlau and Oehmichen, J. pr. Chem., 1881 [ii], 24, 476. 
 
 ** Power and Sheddon, Trans., 1902, 81, 77. 
 
 ft Ber., 1899, 32, 1773. JJ Ber., 1902, 35, 89. 
 
THE REACTIONS OF THE DIAZO-COMPOUNDS 33 
 
 C 
 
 N:N 
 
 In the diphenyl series certain exceptions to the general rule 
 have been observed. On heating the tetrazo-salts prepared 
 from dianisidine and 3 : 3'-dichlorobenzidine 
 
 NIL 
 
 NH 
 
 .ca 
 
 .CH 3 
 
 with dilute sulphuric acid, no phenol whatever was obtained,* 
 the products being apparently of a quinonoid character. By 
 using Heinichen's method a small amount of the dihydroxy- 
 compound was obtained only in the latter case. An attempt 
 to replace both the diazo-groups in ethoxytetrazodiphenyl 
 sulphate 
 
 N 2 .HS0 4 
 
 .HS0 4 
 
 led to an interesting result, f It was found that the diazo- 
 groups varied greatly in stability, and an intermediate product 
 was isolated having the formula 
 
 * Cain, Trans., 1903, 83, 688. 
 t Cain, Trans., 1905, 87, 5. 
 
34 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 N 2 .HS0 4 
 
 >H 
 
 2. Stability of diazo-solutions. - Very great differences 
 occur between the various diazo-salts with regard to their 
 power of resisting decomposition by water. Many decompose 
 rapidly at the ordinary temperature, whilst others remain 
 apparently unchanged after prolonged boiling. 
 
 Several cases of great stability are described by Griess; 
 most of them occur among the halogen or nitro-substituted 
 diazo-salts. Experiments of a somewhat qualitative character 
 were performed by Oddo,* who diazotized a number of amines 
 at various temperatures, and determined the quantity of the 
 diazo-compound formed. He found that at 100-105 much 
 diazo-compound is produced from m- and ^?-nitroaniline, 
 >-chloroaniline, l:3:4-dinitroaniline, and 1:2:5- and 1:3:6- 
 nitrotoluidine, whilst little is obtained with m-chloro- and 
 bromo-aniline, o-nitroaniline, p-aminobenzoic acid, and 1:5:2- 
 nitrotoluidine. At 80-85 much diazo-compound is obtained 
 with the substances named above as giving little at 100-105, 
 whilst small yields are furnished by aniline, p-toluidine, and 
 a- and /3-naphthylamine ; at 60-65 the four bases last named 
 give good yields of diazo-compounds, whilst o-toluidine and 
 _p-xylidine give very poor ones. At 40-45 $>-xylidine in 
 turn gives a good yield of diazo-compound. 
 
 A considerably more exact method of determining the 
 stability consists in titrating from time to time a portion 
 of a diazo-solution with a fixed amount of sodium /3-naph- 
 tholsulphonate (Schaffer's salt) solution. f The increasing 
 amount of diazo-solution required to combine with the whole 
 
 * Gazzetta, 1895, 25, i. 327 ; 1896, 26, ii. 541. 
 t Hirsch, Ber., 1891, 24, 324. 
 
THE REACTIONS OF THE DIAZO-COMPOUNDS 35 
 
 of the naphthol solution is a measure of the advance of the 
 decomposition. 
 
 The main results obtained in this way are given in the 
 table on p. 37. 
 
 This titration method has been used by some later investi- 
 gators, * but it is not suitable for exact measurements, owing 
 to the possibility of secondary reactions taking place between 
 the diazo-compound and the alkali or sodium acetate, which 
 must be added to the naphtholsulphonic acid in order to effect 
 complete combination. (For an account of this secondary 
 reaction see p. 96.) 
 
 Hausser and Muller f introduced an entirely different 
 method from the foregoing. They heated solutions of various 
 diazo-compounds at fixed temperatures and measured the 
 volume of nitrogen evolved. 
 
 The decomposition belongs to the class of unimolecular 
 processes, and is represented by the well-known expression 
 
 1, A 
 
 C = T log -T 
 
 t 3 A x 
 
 By comparing the values of the constant obtained the relative 
 stability of the diazo-compounds may be determined. 
 
 A constant value was obtained only in the case of the 
 diazo-salts from sulphanilic acid and ^-toluidinesulphonic 
 acid. In the case of the other amines examined the values 
 for C were not constant, and from these results somewhat 
 erroneous conclusions were drawn. Hantzsch, J using the 
 same method, measured the rate of decomposition of the 
 diazo-chlorides prepared from aniline, >-bromoaniline, p-tolu^ 
 idine, >-anisidine, and i/r-cumidine, and showed that at 25 all 
 these substances gave a constant value for 
 
 1 . A 
 -log 
 
 t b A-x 
 
 It must be noted that the experiments of Hausser and 
 Muller and of Hantzsch were carried out with solutions pre- 
 
 * Buntrock, Leipziger Monatsschrift fur Textil-Industrie, 1898, 608 ; 
 Schwalbe, Zeitsch. Farb.-Ind., 1905, 4, 433. 
 
 t Bull. Soc. chim., 1892 [iii], 7, 721 ; 1893, 9, 353. Compt. rend., 1892, 
 114, 549, 669, 760, 1438. 
 
 t Ber., 1900, 33, 2517. 
 
 D 2, 
 
36 CHEMISTRY OF THE DIAZO -COMPOUNDS 
 
 pared by dissolving the dry diazo-salt in water. Recent 
 investigations have shown* that solutions of diazo-salts 
 prepared directly from the following amines aniline, the 
 toluidines, sulphanilic acid, the nitroanilines, >-aminoacet- 
 anilide, 3r3'-dichlorobenzidine, a- and /?-naphthylamine, and 
 a number of sulphonic acids derived from the two last, decom- 
 pose in accordance with the above formula. This holds good 
 at temperatures ranging from 20 to 100. The diazo-salt 
 prepared from m-toluidine is the most unstable of those 
 examined, the value of C at 20 being 0-00208. o-Toluidine 
 comes next with a value of C of 0-00187; aniline gives a 
 value for C of 0-00072. 
 
 The diazo-salts of the nitroanilines are extremely stable, 
 the ortho-compound being the most and the para- the least 
 stable. The value of C for o-nitrodiazobenzene chloride is 
 0-00555 at 100. Exceptions to the rule are shown by certain 
 tetrazo-salts and those diazo-salts which are insoluble in 
 water. 
 
 The rate of decomposition of diazo-salts increases rapidly 
 with the temperature,! the values of C obtained being in 
 accordance with Arrhenius's formula for the temperature 
 coefficient, namely 
 
 Ct 1 = Ct e A ( T i~ T o) :T i T o- 
 
 The rate of decomposition (in the case of diazobenzene 
 salts) is independent of the quantity of mineral acid present 
 (except sulphuric acid, which tends to withdraw water from 
 the sphere of action), and is independent of the nature of the 
 acid. Equivalent solutions of diazobenzene chloride, bromide, 
 sulphate, nitrate, and oxalate decompose at the same rate. J 
 
 The presence of colloidal platinum or silver increases the 
 rate of the decomposition, owing to catalytic action. Finally, 
 it may be useful to append a table showing the relative 
 
 * Cain and Nicoll, Trans., 1902, 81, 1412 ; 1903, 83, 206. 
 
 t Cain and Nicoll, Trans., 1903, 83, 470; Euler, Annalen, 1902, 325, 
 
 OQO 
 
 {'Cain, Ber., 1905, 38, 2511 ; Euler, loc. cit. 
 
 Euler, Ofversigt af Kongl. Vetenskaps. Akad. Forhandl. Stockholm, 
 1902, No. 2, 227. Compare also Schwafbe, Ber., 1905, 38, 2196, 3071 ; 
 Cain, Ber., 1905, 38, 2511. 
 
THE REACTIONS OF THE DIAZO-COMPOUNDS 37 
 
 stability of various diazo-salts as determined by various 
 observers. The amine giving the most stable diazo-salt is 
 at the top. 
 
 
 
 
 Il 
 
 
 
 o g 
 
 I 
 
 l I 
 
 '3 g 
 
 .3 
 
 ! 
 
 -a 
 
 Aniline 
 \^-Cumidin 
 o-Toluidin 
 tw-Toluidi 
 
 
 O) d> O> 
 
 .S S .S 
 9' D *o 
 
 g - 
 
 1 1 S iS g 
 
 ep 
 
 ' 
 
 "3 .2 
 
 > 
 
 rO - 
 
 s 
 
 
 
 
 II 
 
 ips 
 -si?,-!-! 
 
 MsffsiS 1 
 
 <J ^ 6 CQL 6 &, 
 
CHAPTEK Y 
 
 THE KEACTIONS OF THE DIAZO-COMPOUNDS 
 
 (continued) 
 
 1. Action of alcohols. The action of alcohol on diazo- 
 compounds was, of course, studied by Griess, who obtained 
 benzene from diazobenzene salts, and dinitrophenol from 
 diazodinitrophenol. 
 
 The production of the hydrocarbon or complete elimination 
 of the diazo-group by the action of boiling alcohol was, for 
 many years, regarded as a general reaction, in spite of the 
 observation of Wroblewski, * who found that the diazo-salt 
 of chlorotoluidine gave, not the chloro-hydrocarbon, but 
 the corresponding chlorophenetole. Four years later also 
 Hayduckf showed that when o-toluidinesulphonic acid was 
 diazotized, and the resulting diazo-salt boiled with alcohol, 
 phenetolesulphonic acid was obtained. 
 
 A striking application of the reaction was made by E. and 
 O. Fischer in 1878, { who showed that when the diazo- 
 compound of paraleucaniline was boiled with alcohol the three 
 diazo-groups were eliminated, with formation of triphenyl- 
 methane ; diazo-leucaniline, in the same way, gave tolyl- 
 diphenylmethane. 
 
 When the bisdiazo-derivative of benzidine is warmed with 
 
 ethyl alcohol to 40-45 only one diazo-group is eliminated, 
 
 the second requiring a higher temperature for its removal 
 
 C1.N 2 .C 6 H 4 .C 6 H 4 .N 2 C1 -> C 6 H 6 .C 6 H 4 .N 2 C1 
 
 -> C 6 H 5 . C 6 H 5 . 
 
 Examples of the formation of ethers in this reaction were, 
 however, rapidly multiplying, amongst which may be men- 
 tioned the cases of m-aminobenzenedisulphonic acid, || cumi- 
 
 * Ber., 1870, 3, 98. t Annalen, 1874, 172, 215. 
 
 | Annalen, 1878, 194, 242. Ber., 1898, 31, 479. 
 
 || Zander, Annalen, 1879, 198, 1. 
 
THE REACTIONS OF THE DIAZO-COMPOUNDS 39 
 
 dinesulphonic acid, * aminotetramethylbenzene, cumidine, f 
 and o-toluidinedisulphonic acid, J all of which yielded, when 
 diazotized and then treated with alcohol, the corresponding 
 ethyl ethers. 
 
 The reaction may thus proceed in two ways, according to 
 the following equations : 
 
 i. R.N 2 X + C 2 H 5 . OH = RH + C 2 H 4 + N 2 + HX 
 ii. R.N 2 X + 2 H 5 . OH = R.O.C 2 H 5 + N 2 + HX ; 
 where R denotes a hydrocarbon radical and X an acid 
 radical. A systematic investigation into the whole question 
 was next undertaken by Remsen and his pupils, and it was 
 very quickly demonstrated that the normal reaction is the 
 formation of ethers in accordance with the second of the 
 foregoing equations. || The course of the reaction is, how- 
 ever, somewhat complicated, and depends on many factors, 
 such as the position and nature of the substituents, the 
 pressure at which the operation is carried on, &c. 
 
 2. Influence of substituents. The presence of the acid 
 radicals, C0 2 H, Cl, Br, N0 2 , &c., tends to induce the complete 
 elimination of the diazo-group, and this influence is greatest 
 when these radicals are in the ortho-position with respect to 
 the diazo-group; their influence is less in the meta-position 
 and least in the para-position. 
 
 Thus of the chlorodiazobenzene nitrates, the ortho- and 
 meta-compounds yield only chlorobenzene with ethyl alcohol, 
 but the para-compound gives rise to the formation of a little 
 >-chlorophenetole.^y In the case of the diazobenzoic acids the 
 ortho-compound gives benzoic acid only, whilst the meta- and 
 para- yield the alkyloxy-derivatives. ** 
 
 Another interesting example is that of the nitrodiazo- 
 benzene salts. When heated with methyl alcohol the ortho- 
 
 * Haller, Ber., 1884, 17, 1887. 
 t Hofmann, Ber., 1884, 17, 1917. 
 | Hasse, Annalen, 1885, 23O, 286. 
 Ber., 1885, 18, 65. 
 
 || Remsen and Palmer, Amer. Chem. J., 1886, 8, 243. 
 T Cameron, Amer. Chem. J., 1898, 20, 229. 
 
 ** Remsen and Orndorff,^mer. Chem. J., 1887, 9, 387. Compare also 
 Griess, Ber., 1888, 21, 978. 
 
40 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 compound gives 87 per cent, of the theory of nitrobenzene ; 
 from the meta-compound 51 per cent, is obtained, together 
 with a little m-nitroanisole, whilst the para-derivative gives 
 about 40 per cent, of nitrobenzene and 8 to 17 per cent, of 
 p-nitroanisole. 
 
 In the naphthaline series the 1 : 2-, 2:1-, and 1 : 4-nitrodiazo- 
 naphthalene sulphates all yield nitronaphthalene with ethyl 
 alcohol,* whereas the ethoxy-derivative is obtained from both 
 a- and )3-diazonaphthalene sulphates.f 
 
 3. Influence of the alcohol used. The tendency towards 
 the formation of hydrocarbons is increased as the molecular 
 weight of the alcohol increases. Diazobenzene chloride and 
 sulphate with methyl alcohol yield anisole as the sole product, 
 no benzene being formed. J With ethyl alcohol the chief pro- 
 duct is phenetole, but a little benzene is also obtained. o-Diazo- 
 toluene sulphate with methyl alcohol yields tolyl methyl ether 
 and only a trace of toluene. 
 
 The diazo-salts of m-chloro- and m-bromo-aniline also yield 
 only the corresponding halogen derivatives of benzene when 
 heated with ethyl alcohol, but when methyl alcohol is used 
 the chief product in each case is the halogenated anisole, only 
 small quantities of chloro- and bromo-benzene being produced. 
 The diazo-sulphates of >-chloro- and >-bromo-aniline illustrate 
 this point very clearly ; with ethyl alcohol no ethers are ob- 
 tained, whilst with methyl alcohol the ethers are the sole 
 products. || The higher alcohols behave in a similar manner 
 to methyl and ethyl alcohols. With diazobenzene chloride 
 7i- and ^so-propyl alcohol yield phenyl propyl ethers but no 
 trace of propaldehyde or acetone; amyl alcohol gives both 
 phenyl amyl ether and valeraldehyde or its condensation pro- 
 ducts ; and benzyl alcohol gives benzaldehyde with only a 
 little phenyl benzyl ether. Glycerol behaves like propyl 
 alcohol, giving the monophenyl ether, whilst mannitol and 
 benzoin are not attacked.^ 
 
 * Orndorff and Cauffman, Amer. Chem. J., 1892, 14, 45. 
 t Orndorff and Kortright, Amer. Chem. J., 1891. 13, 153. 
 J Hantzsch and Jochem, Ber., 1901, 34, 3337. 
 
 Bromwell, Amer. Chem. J., 1897, 19, 561. || Cameron, loc. cit. 
 
 IT Hantzsch and Vock, Ber., 1903, 36, 2061. Compare also Orndorff 
 and Hopkins, Amer. Chem. J., 1893, 15, 518. 
 
THE REACTIONS OF THE DIAZO-COMPOUNDS 41 
 
 Phenol acts similarly, thus, when a solution of diazobenzene 
 sulphate is warmed with phenol, diphenyl ether is obtained.* 
 In alkaline solution, however, an azo-compound is produced 
 (see p. 86). 
 
 4. Influence of temperature and pressure. The influence 
 of these factors in the decomposition is somewhat difficult to 
 separate, as when the pressure is varied, the boiling-point of 
 the solvent changes. 
 
 In the case of the diazo-compound prepared from p-tolu- 
 idine-o-sulphonic acid,f the decomposition proceeds slowly at 
 the ordinary pressure. When this pressure is raised by 500 
 m.m. an almost quantitative yield of the ethoxy-compound is 
 obtained, but below this pressure the yield is diminished, as 
 shown by the following numbers : 
 
 Pressure in mm. 800 700 600 500 400 300 210 120 
 
 69 ' 8 63 ' 2 57 ' 7 52 ' 8 48 ' 7 434 40 ' 6 37 ' 2 
 
 With methyl alcohol the methoxy-compound is obtained, and 
 alteration of pressure has no influence on the course of the 
 reaction.! 
 
 From a large number of cases, however, which have been 
 examined, involving the use of both methyl and ethyl alcohol, 
 it is found that the yield of alkyloxy-derivative increases with 
 the pressure. 
 
 5. Influence of other substances. If the decomposition 
 is carried out with the addition of sodium ethoxide, sodium 
 hydroxide, potassium carbonate, or zinc dust, a remarkable 
 effect is produced. The alkyloxy-formation is almost entirely 
 inhibited, and the reaction proceeds mainly with elimination 
 of the diazo-group. Thus in the case of ^-diazotoluene nitrate 
 and sulphate the ordinary treatment with methyl alcohol 
 results in the production of a good yield of the methoxy- 
 derivative. When, however, sodium methoxide or any of the 
 
 * Hofmeister, Annalen, 1871, 159, 191. 
 
 t Remsen and Palmer, Amer. Chem. J., 1886, 8, 243 ; Remsen and 
 Dashiell, ibid., 1893, 15, 105. 
 
 t Parks, Amer. Chem. J., 1893, 15, 320. 
 
 Shober, Amer. Chem. J., 1893. 15, 379 ; Metcalf, ibid., 301 ; Beeson, 
 ibid., 1894, 16, 235 ; Shober and Kiefer, ibid., 1895, 17, 454 ; Chamber- 
 lain, ibid., 1897, 19, 531. 
 
42 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 above substances are present no alkyloxy-compound is obtained, 
 but the product consists mainly of toluene.* 
 
 The rule holds good also for tetrazo-compounds of the 
 diphenyl series ; thus the tetrazo-chloride of o-ditolyl gives, 
 with methyl alcohol, dimethoxy-o-ditolyl, and with ethyl 
 alcohol a mixture of diethoxy-m-ditolyl and m-ditolyl ; but in 
 the presence of sodium methoxide, hydroxide, or zinc dust, no 
 alkylated compound is formed, f 
 
 6. Other methods of reduction. The reduction of diazo- 
 salts to the corresponding hydrocarbon may, of course, be 
 effected by reducing agents instead of alcohol : thus Baeyer 
 and Pfitzinger J introduced the method of reducing the diazo- 
 salt to the hydrazine with stannous chloride, and removing the 
 group NH.NH 2 , by oxidation with boiling cupric sulphate 
 solution, and by treating diazobenzene formate with stannous 
 formate solution, benzene, together with a little diphenyl, &c., 
 is produced. 
 
 C 6 H 6 . N 2 C1 + SnCl 2 + H 2 = C 6 H 6 + N 2 + SnOCl 2 + HC1. 
 The reduction is also effected by adding sodium stannite to 
 a solution of a diazo-compound in sodium hydroxide, || 
 C 6 H 5 . N 2 C1 + NaOH + Na 2 Sn0 2 
 
 = C 6 H 6 + N a + Na 2 Sn0 3 + NaCl, 
 
 by the use of hypophosphorous acid,! an alkaline solution of 
 sodium hyposulphite,** and also when diazides of sulphonic 
 acids are boiled with copper powder and formic acid.ff 
 
 * Beeson, loc. cit. ; Chamberlain, loc. cit. ; Griffin, Amer. Chem. /., 
 1897, 19, 163 ; Moale, ibid., 1898, 20, 298. 
 
 t Winston, ibid., 1904, 31, 119. + Ber., 1885, 18, 90, 786. 
 
 Gasiorowski and Waijss, Ber., 1885, 18, 337; Culrnann and Gasio- 
 rowski, J. pr. Chem., 1889 [ii], 40, 97. 
 
 || Friedlander, Ber., 1889, 22, 587. Compare also Eibner, Ber., 1903, 
 36, 813. 
 
 IF Mai, Ber., 1902, 35, 162. ** Grandmougin, Ber., 1907, 40, 858. 
 
 tt Ber., 1890, 23, 1632. 
 
CHAPTEE VI 
 
 THE REACTIONS OF THE DIAZO-COMPOUNDS 
 
 (continued) 
 
 1. Replacement of the diazo-gronp by the halogens. 
 
 1. Chlorine. Although chloro- derivatives are obtained 
 when a diazo-salt is heated with concentrated hydrochloric 
 acid,* the yield is usually very poor,f and Griess observed that 
 the replacement was more successful when the platinichloride 
 of the diazo-compound was heated with sodium hydroxide, 
 thus 
 
 (C 6 H 6 N 2 ) 2 PtCl 6 = 2C 6 H 5 C1 + Pt + 2C1 2 + N 2 . 
 
 A much more convenient method, however, was introduced 
 by Sandmeyer in 1884.J 
 
 In investigating the action of cuprous acetylide on diazo- 
 benzene chloride, he noticed that chlorobenzene was produced, 
 and showed that this was due to the cuprous chloride formed 
 during the reaction. 
 
 The replacement is carried out by adding the diazo-solution 
 to a boiling 10 per cent, solution of cuprous chloride in 
 hydrochloric acid. Nitrogen is evolved and the mass distilled 
 with steam when chlorobenzene passes over. 
 
 The cuprous chloride may be prepared by heating to boiling 
 a mixture of copper sulphate (250 parts), sodium chloride 
 (120 parts), and water (500 parts). Concentrated hydrochloric 
 acid (1,000 parts) and copper turnings (130 parts) are now added, 
 and the temperature maintained until the mixture loses its 
 colour. The solution is decanted from any undissolved copper 
 and the weight made up to 2,036 parts by the addition of 
 
 * Griess, Per., 1885, 18, 960. 
 
 t When, however, a-diazoanthraquinone is treated with hydrochloric 
 acid, the chloro-derivative is readily obtained, and the presence of 
 cuprous salts is not essential (D. R-P. 131538). 
 
 t Ber., 17, 1633, 2650; 1885, 18, 1492, 1496; 1890, 23, 1880; see also 
 Ber., 1886, 19, 810; 1890, 23, 1628; Annalen, 1893, 272, 141. 
 
44 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 concentrated hydrochloric acid. A 10 per cent, solution of 
 cuprous chloride is obtained, which is preserved in an atmo- 
 sphere of carbon dioxide.* 
 
 Gattermann then demonstrated f that the addition of very 
 finely-divided copper to a solution of the diazo-chloride in 
 hydrochloric acid effected the replacement at the ordinary tem- 
 perature. Gattermann used copper precipitated from copper 
 sulphate solution with zinc, but Ullmann has shown that the 
 f copper bronze ' of commerce may be used equally effec- 
 tively.:): 
 
 A modification of this method consists in using copper 
 sulphate solution to which is added hydrochloric acid and 
 sodium hypophosphite. 
 
 The ' Sandmeyer ' reaction, as it is usually called, is con- 
 sidered to be accompanied by the intermediate formation of 
 a compound of the diazo-chloride with the cuprous chloride, 
 and it is important in carrying out this operation that the 
 possibility of the formation of phenols and azo-compounds 
 should be avoided as far as possible. The production of 
 a phenol is due to the decomposition of the diazo-compound 
 before it has been converted into the cuprous chloride com- 
 pound, or if it is added too slowly to the latter. 
 
 According to Erdmann,|| the normal decomposition of the 
 diazo-cuprous chloride compound takes place rapidly and 
 smoothly only above a certain temperature, which is different 
 for each compound ; these temperatures are about 0, 27, and 
 30-40 in the case of the cuprous chloride derivatives of 
 diazobenzene, o-diazotoluene, and ^-diazotoluene respectively. 
 Below these points, the evolution of nitrogen takes place too 
 slowly and is incomplete, part of the diazo-cuprous chloride 
 compound being reduced to an azo-compound by the liberated 
 cuprous chloride. It has been found that the quantity of cuprous 
 chloride required may be reduced to 1/21 and 1/28 molecule 
 per molecule of amine without appreciably reducing the yield 
 of chlorobenzene and m-chloronitrobenzene respectively. If This 
 
 * Feitler, Zeitsch. physikal Chem., 1889, 4, 68. 
 
 t Ber., 1890, 23, 1218; 1892, 25, 1091. 
 
 t Ber., 1896, 29, 1878. Angeli, Ber., 1891, 24, 952. 
 
 || Annalen, 1893, 272, 141. 
 
 H Votocek, Chem. Zeit. Rep., 1896, 20, 70. 
 
THE REACTIONS OF THE DIAZO-COMPOUNDS 45 
 
 is considered to be due to the diazobenzene chloride becoming 
 first reduced to phenylhydrazine by the cuprous chloride, which 
 then becomes cupric chloride ; the phenylhydrazine is then 
 oxidized in presence of hydrochloric acid to chlorobenzene by 
 the cupric chloride, and the cuprous chloride would then be 
 re-formed to play the same part again. In confirmation of 
 this explanation, it is found that phenylhydrazine is oxidized 
 under the conditions named to chlorobenzene by both cupric 
 and ferric chlorides, but no phenylhydrazine can be detected 
 in the Sandmeyer reaction, owing possibly to its momentary 
 existence. It is also worthy of note that a copper salt is not 
 necessary in the preparation of iodobenzene by this method, 
 and this may be due to the fact that hydriodic acid is itself 
 a reducing agent.* 
 
 Still another variation of Sandmeyer's method consists in 
 electrolysing a solution of a diazo-compound to which cupric 
 chloride has been added. A thick copper wire is used as 
 the anode, and a cylinder of sheet copper as the cathode ; 
 with a current density of 2-1 amperes per sq. dcm., and an 
 E. M. F. of 10 volts, nitrogen was evolved, and a yield of 64 
 per cent, of the theory of chlorobenzene was obtained, f 
 
 In certain cases the reaction takes a different course from 
 that already described ; thus Gattermann J found that two 
 benzene nuclei could condense to form diphenyl derivatives, 
 and the reaction has been extended by Ullmann, who has 
 prepared a large number of diphenyl compounds by acting on 
 nitrodiazo-compounds with cuprous chloride. 
 
 2. Bromine. The diazo-group is replaced by bromine in the 
 same manner as by chlorine. || In Griess's method a perbromide 
 is obtained by adding hydrobromic acid and bromine water to 
 the diazo-compound,1f and this on being boiled with alcohol 
 yields the bromo-derivative thus 
 
 * Walter, J.pr. Chem., 1896 [ii], 53, 427. 
 t VotoSek and Zenisek, Zeitsch. Elektrochem., 1899, 5, 485. 
 Ber., 1890, 23, 1226. 
 
 Ber., 1901, 34, 3802 ; D. R-P. 126961. See also p. 61. 
 Phil. Trans., 1864, 154, 673; Annalen, 1866, 137, 49. 
 iff If a diazophenol is used, a bromodiazophenol is formed ; cp. J. pr. 
 Chem., 1881 [ii], 24, 449; Annalen, 1886, 234, 1; and it is remarkable 
 that diazosulphanilic acid is indifferent to bromine (Armstrong, Proc., 
 1899, 15, 176). 
 
46 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 C 6 H 5 . N 2 . N0 3 + HBr + Br 2 = C 6 H 5 . N 2 Br.Br 2 + HNO 3 
 C 6 H 5 . N 2 Br.Br 2 + C 2 H 5 . OH 
 
 = C 6 H 5 Br + N 2 + 2HBr + CH 3 . CHO. 
 
 In addition to bromobenzene, >-bromophenetole is formed 
 (cp. p. 38) ; when ether or glacial acetic acid is used instead 
 of alcohol, bromobenzene alone is produced.* The platini- 
 bromide of the diazo-bromide may also be treated in the same 
 way as the platinichloride. 
 
 In Sandmeyer's reaction cuprous bromide is substituted for 
 cuprous chloride. 
 
 In order to prepare -bromonaphthalene, Oddo f modified 
 Gattermann's process as follows : 14-3 grams of /3-naphthyl- 
 amine are diazotized and added to a mixture of 36 grams 
 of potassium bromide with 100 grams of water and 30 
 grams of moist copper powder previously heated to 50-70. 
 The whole is heated in a reflux apparatus for 15 minutes, and 
 then distilled in steam. A yield of 46-48 per cent, of the 
 theory is obtained. 
 
 3. Iodine. lodo-derivatives are easily prepared from the 
 diazo-compounds by treating the latter with hydriodic acid. 
 A solution of a little more than the theoretical quantity of 
 sodium or potassium iodide is added to the solution of the 
 diazo-chloride or sulphate. After standing and warming 
 until the evolution of nitrogen has ceased, the liquid is 
 usually made alkaline and the iodo-compound, if it is volatile, 
 distilled with steam. In other cases it may be filtered off. 
 
 4. Fluorine. The diazo-group may be replaced by fluorine 
 by treating the diazo-salt with a solution of hydrogen fluoride 
 in water. J 
 
 The substitution has also been effected from diazoamino- 
 compounds. Thus, if diazoaminobenzene is added to fuming 
 hydrofluoric acid, fluorobenzene is produced, and on mixing 
 diazobenzene piperidide (from diazobenzene nitrate and piper- 
 
 * Saunders, Amer. Chem. J. y 1891, 13, 486. 
 
 t Gazzetta, 1890, 2O, 631. 
 
 1 Ber., 1879, 12, 581 ; 1889, 22, 1846. 
 
 Schmitt and Gehren, J. pr. Chem., 1870 [ii], 1, 395. 
 
THE REACTIONS OF THE DIAZO-COMPOUNDS 47 
 
 idine *) with concentrated hydrofluoric acid, fluorobenzene is 
 formed, thus 
 
 C 6 H 5 . N 2 . NC 6 H 10 + 2HF = C 6 H 6 F + N 2 + NHC 6 H 10 , HF. 
 
 The Sandmeyer reaction is considered by Hantzsch and 
 Blagden f to be a somewhat complicated one, the final result 
 being due to the simultaneous effect of three concurrent 
 actions, namely, (1) the formation of a labile (diazonium, 
 see p. 133) cuprous double salt, which then decomposes in 
 such a way that the radical originally attached to the copper 
 migrates to the aromatic nucleus ; (2) a catalytic action, which 
 is the main action when copper powder is used, whereby 
 nitrogen is eliminated from the diazo-salt, and the acid 
 radical becomes united with the aromatic nucleus ; (3) the 
 formation of azo-compounds, the cuprous being oxidized to 
 a cupric salt. 
 
 The first two reactions proceed when ^-bromodiazobenzene 
 bromide is subjected to the action of cuprous chloride dis- 
 solved in methyl sulphide. The product consists chiefly of 
 chloro->-bromobenzene mixed with a little p-dibromobenzene. 
 
 i. 2C 6 H 4 Br.N 2 Br + Cu 2 Cl 2 = Cu 2 Br 2 + 2N 2 + 2C 6 H 4 ClBr. 
 
 ii. C 6 H 4 Br.N 2 Br = N 2 + C 6 H 4 Br 2 . 
 
 When cuprous bromide is allowed to react with p-bromo- 
 diazobenzene chloride, p-dibromobenzene and a little chloro- 
 ^-bromobenzene are produced. In both examples the first 
 reaction is the chief one, and, under certain conditions, is the 
 only one. Thus cuprous iodide furnishes iodo-derivatives 
 only, with various diazo- chlorides and bromides, and cuprous 
 chloride and diazobenzene iodide yield chlorobenzene, no iodo- 
 benzene being produced. 
 
 The third reaction, namely, the formation of azo-compounds, 
 occurs when cuprous chloride, dissolved in hydrochloric acid, 
 is added to the cold solution of the diazo-salt. Under these 
 conditions, aniline, o-chloroaniline, and the o- and p-toluidines 
 yield considerable quantities of azo-compound, but the nitro- 
 amines give diphenyl derivatives (see p. 61). 
 
 * Baeyer and Jaeger, Ber., 1875, 8. 893. 
 t Ber., 1900, 33, 2544. 
 
CHAPTEE VII 
 
 THE REACTIONS OF THE DIAZO-COMPOUNDS 
 
 (continued) 
 
 1. Replacement of the diazo-group by cyanogen. This 
 is one of the most important of the diazo-decompositions, as it 
 serves to introduce an additional carbon atom into the mole- 
 cule ; moreover, the nitriles formed in this way mostly yield 
 the corresponding carboxylic acids.* 
 
 The preparation of p-toluonitrile is carried out as follows : 
 50 grams of copper sulphate are dissolved in 200 c.c. of water 
 by heating on the water-bath, and a solution of 55 grams of 
 potassium cyanide in 100 c.c. of water added gradually with 
 continuous heating. Care must be taken to perform the 
 operation under a hood as cyanogen is evolved. To this hot 
 solution is now added during about ten minutes a diazo-solution 
 prepared from 20 grams of ^9-toluidine, 50 grams of concen- 
 trated hydrochloric acid and 16 grams of sodium nitrite. The 
 whole is now heated on the water-bath for a quarter of an 
 hour and the toluonitrile distilled over with the steam. Here 
 again care must be taken to get rid of the vapours as hydrogen 
 cyanide is evolved. The nitrile distils as a yellow oil, which is 
 purified by distillation, f 
 
 By treating a solution of diazobenzene chloride with potas- 
 sium cyanide in the cold, a double compound of the diazo- 
 cyanide and hydrogen cyanide, C 6 H 5 . N 2 . CN, HCN, is formed. J 
 
 The replacement is also effected by adding copper powder to 
 a mixture of the diazo-salt and potassium cyanide, exactly as 
 in the case of the preparation of the chloride. 
 
 * Sometimes, however, these are formed with difficulty, owing probably 
 to steric hindrance. Hofmann, Ber., 1884, 17, 1914 ; Ktister and Stall- 
 berg, Annalen, 1894, 278, 207 ; Cain, Ber., 1895, 28, 967. 
 
 t Gattermann, Practical Methods of Organic Chemistry. 
 
 1 Gabriel, Ber., 1879, 12, 1637. 
 
 Gattermann, Hausknecht, Cantzler, and Ehrhardt, Ber., 890, 23, 
 1218. 
 
THE REACTIONS OF THE DIAZO-COMPOUNDS 49 
 
 2. Replacement of the diazo-gronp by the cyano-gronp. 
 
 This is effected by adding potassium cyanate to a diazo- 
 sulphate and treating the mixture with copper powder, when 
 the corresponding carbimide is obtained, thus 
 
 C 6 H 5 . N 2 . HS0 4 + KCNO = C 6 H 5 . N : CO + N 2 + KHS0 4 . 
 
 The potassium cyanate is prepared in the following manner : 
 100 grams of finely-powdered and sieved potassium ferro- 
 cyanide are mixed with 75 grams of powdered potassium 
 dichromate, each ingredient being first thoroughly dried. 
 This mixture is added, in portions of 3-5 grams at a time, to 
 an iron dish heated over a three-flame burner. The mass 
 becomes black and is well stirred but should not be heated 
 to the melting-point. On cooling it is extracted with five times 
 its volume of 80 per cent, alcohol, and the cold solution stirred, 
 when a white crystal powder of potassium cyanate separates, 
 which is filtered and washed with small quantities of ether. 
 
 For the decomposition, 10 grams of aniline are dissolved in 
 100 grams of water and 20 grams of concentrated sulphuric 
 acid, the solution cooled with ice, and diazotized with 7-5 
 grams of sodium nitrite. To the diazo-solution is added a 
 concentrated aqueous solution of 9 grams of potassium cyanate 
 and then 5 grams of copper powder, when evolution of nitrogen 
 begins. A second 5 grams of copper powder is added and an 
 oily layer of phenylcarbimide separates on the top of the 
 liquid. This is skimmed off with a glass spoon, extracted 
 with chloroform, and the chloroform solution filtered by the 
 aid of the pump. More copper powder is added to the original 
 solution until no more nitrogen is evolved and any further 
 quantity of phenylcarbimide collected in the same way. The 
 chloroform solutions are now separated from water, dried and 
 freed from chloroform by evaporation. The residual oil, on 
 distillation, yields pure phenylcarbimide.* 
 
 3. Replacement of the diazo-gronp by the thiocyano- 
 group. This reaction is carried out by the aid of copper 
 thiocyanate. For example : 31 grams of aniline are dissolved 
 in 100 grams of concentrated sulphuric acid and 200 grams of 
 water, and diazotized with 23 grams of sodium nitrite. To this 
 
 * Ber., 1890, 23, 1220; ibid., 1892, 25, 1086. 
 
 
 
50 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 solution is added a concentrated solution of 35 grams of potas- 
 sium thiocyanate, and a paste of copper thiocyanate, obtained 
 by dissolving 80 grams of copper sulphate and 150 grams of 
 ferrous sulphate in water, precipitating with 35 grams of 
 potassium thiocyanate and filtering. Nitrogen is evolved 
 when this paste is added to the diazo-solution, and the reaction 
 is complete after the whole has stood for three hours; the 
 phenylthiocarbimide is then distilled with steam and rectified,* 
 
 4. Replacement of the diazo-group by the group SH. 
 
 When a diazo-sulphonate is warmed with an alcoholic solution 
 of potassium sulphide nitrogen is evolved, and a thiophenol- 
 sulphonic acid is formed, f Thus if the diazo-derivative of 
 sulphanilic acid is used potassium ^-thiophenolsulphonic acid 
 
 results 
 
 /SK 
 
 C 6 H/ + N 2 
 S0 3 X S0 3 K 
 
 These thiophenols or mercaptans are also obtained by 
 hydrolysing the xanthates produced by treating a diazo-salt 
 with potassium xanthate.J 
 
 By hydrolysing the xanthate from diazotized sulphanilic acid, 
 in addition to the mercaptan, there is formed the ethosulphide 
 
 S0 3 K>C H 4 . S.CS.OEt -* S0 3 K.C 6 H 4 .SH 
 and S0 3 K.C 6 H 4 .SEt. 
 
 5. Replacement of the diazo-group by sulphur. When 
 hydrogen sulphide or ammonium sulphide acts on a solution 
 of diazobenzene chloride or sulphate, the diazo-group is re- 
 placed by sulphur, and phenyl sulphide (C 6 H 6 ) 2 S is produced ;|| 
 and if a solution of o-diazobenzoic acid sulphate is added to a 
 cold saturated solution of sulphur dioxide in which copper 
 powder is suspended, nitrogen is evolved, much copper passes 
 into solution, part of the sulphur dioxide being oxidized to 
 sulphuric acid, and the chief product is dithiosalicylic acid, 
 
 * Ber., 1890, 23, 738; compare Ibid., 770. 
 
 t Klason, Ber., 1887, 20, 349. 
 
 1 Leuchart, J. pr. Chem., 1890 [ii], 41, 179. 
 
 Walter, Proc., 1895, 11, 141. 
 
 || Graebe and Mann, Ber., 1882, 15, 1683. 
 
THE REACTIONS OF THE DIAZO-COMPOUNDS 51 
 
 (C 6 H 4 . CO 2 H) 2 S, which is obtained in a yield of about 50 per 
 cent, of the theoretical.* 
 
 Another way in which sulphides are formed is by treating 
 diazobenzene chloride with a colourless solution of copper 
 sulphate (1 mol.) in sodium thiosulphate (6 mols.), that is, 
 cuprous sodium thiosulphate. Phenyl sulphide is formed 
 along with benzeneazodiphenyl. 
 
 Sulphanilic acid and o- and _p-toluidine yield also the corre- 
 sponding sulphides, but no diphenyl derivatives are produced, 
 When a-naphthylamine is similarly treated there is no forma- 
 tion of sulphide, but a-azonaphthalene is obtained, f 
 
 6. Replacement of the diazo-gronp by the sulphonic acid 
 group. Both the thiophenols and the disulphides yield the 
 corresponding sulphonic acids on treatment with alkaline 
 permanganate solution.J 
 
 7- Replacement of the diazo-gronp by the nitre-group. 
 This is brought about by treating a diazo-salt with nitrous 
 acid and cuprous oxide. The amine is dissolved in two mole- 
 cules of dilute nitric acid (hydrochloric acid is to be avoided), 
 and, after being diazotized, a second molecule of sodium nitrite 
 is added. The solution is then poured on finely-divided 
 cuprous oxide and the reaction usually proceeds in the cold. 
 
 For example, the cuprous oxide is prepared by dissolving 
 together 50 grams of copper sulphate and 15 grams of grape 
 sugar in 100 grams of water. The solution is boiled and 20 
 grams of caustic soda, dissolved in 60 grams of water, added 
 all at once. The mixture is neutralized with acetic acid. 
 
 On the other hand, 9 grams of aniline are dissolved in 50 
 grams of water and 20 grams of concentrated nitric acid 
 (sp. gr. 1.4) ; 15 grams of sodium nitrite, dissolved in 50 grams 
 of water, are added and then the diazo-solution is poured on 
 the cuprous oxide gradually. When the reaction is finished 
 the nitrobenzene is extracted by distillation with steam. 
 
 The replacement also proceeds to a small extent in the 
 
 * Henderson, Amer. Chem. J., 1899, 21, 206. 
 
 t Bornstein, Ber., 1901, 34, 3968. 
 
 J F. Bayer & Co., D. R-P. 70286 of 1892 ; E. P. 11865 of 1892. 
 
 Ber., 1887, 20, 1495. 
 
 E 2 
 
52 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 absence of cuprous oxide; thus when 2 : 4 : 6-tribromodiazo- 
 benzene sulphate is treated with 20 molecular proportions 
 of potassium nitrite, the corresponding 2:4: 6-tribromo-l- 
 nitrobenzene is formed, together with the quinonediazide 
 
 Nitrobenzene is also formed when diazobenzene perbromide 
 is shaken with aqueous sodium hydroxide in the cold.f 
 
 The diazo-group of diazobenzene nitrate may also be re- 
 placed by the nitro-group by making use of the crystalline 
 double salt of formula Hg(N0 2 ) 2 , 2C 6 H 5 .N 2 .N0 3 , which is 
 obtained by mixing solutions of diazobenzene nitrate and 
 potassium mercuric nitrate. When this salt is boiled with 
 water, it yields phenol and nitrophenol, but when treated 
 with copper powder, a quantitative yield of nitrobenzene is 
 produced. J 
 
 Another method consists in mixing diazo-sulphates with 
 a freshly prepared suspension of cupro-cupric sulphite and 
 treating the mixture with excess of an alkali nitrite. By 
 this means 2:4: 6-tribromodiazobenzene sulphate gives a 65 
 per cent, yield of 2 : 4 : 6-tribromo-l -nitrobenzene, and /?-diazo- 
 naphthalene sulphate furnishes a 25 per cent, yield of j3-nitro- 
 naphthalene. 
 
 8. Replacement of the diazo-group by the nitroso-gronp. 
 
 This is effected by treating a diazobenzene chloride solution 
 with an alkaline solution of potassium ferrocyanide.|| 
 
 9. Replacement of the diazo-group by the amino-gronp. 
 
 This replacement is effected by adding hydroxylamine to 
 
 * Orton, Trans., 1903, 83, 806. 
 
 t Bamberger, Ber., 1894, 27, 1273. 
 
 t Hantzsch and Blagden, Ber., 1900, 33, 2544. 
 
 Hantzsch and Blagden, loc. cit. 
 
 || Bamberger and Storch, Ber., 1893, 26, 471. 
 
THE REACTIONS OF THE DIAZO-COMPOUNDS 53 
 
 a solution of a diazo-salt;* thus, aniline may be obtained 
 from diazobenzene chloride, and>-toluidine from its diazo-salt. 
 An interesting example occurs in the anthracene series. When 
 the anhydride of l-diazoanthraquinone-2-sulphonic acid 
 
 3 
 
 is suspended in water and treated with ammonia or ammonium 
 carbonate, nitrogen is evolved, and the original aminosulphonic 
 acid is obtained. Further, when this diazo-compound is 
 treated with hydroxylamine, a diazohydroxyamide 
 
 OH . NH . N 2 . C 10 H 6 2 . S0 3 Na 
 
 is formed, which is transformed by concentrated sulphuric acid 
 into l-amino-4-hydroxyanthraquinone-2-sulphonic acid. 
 
 A similar reaction takes place when hydrazine is substituted 
 for hydroxylamine ; in this way, both the amino- and hydroxyl- 
 groups are introduced into the molecule when the diazo-group 
 is eliminated.! 
 
 10. Replacement of the diazo-group by the acetoxy- 
 group. Meldola and EastJ found that when certain azo- 
 derivatives of /3-naphthylamine, containing an amino-group, 
 are diazotized in warm glacial acetic acid, the diazo-group is 
 replaced by the acetoxy-group, and Orndorff has shown that 
 this reaction may be applied generally for the preparation of 
 aromatic acetates. 
 
 The acetoxy-group may be readily converted into the 
 hydroxy-group by hydrolysis, so that this method is useful 
 in effecting the replacement of the diazo- by the hydroxy- 
 group in such cases where the normal decomposition with 
 water does not take place. 
 
 * Mai, Ber., 1892, 25, 372. t Wacker, Ber., 1902, 35, 2593, 3920. 
 
 J Trans., 1888, 53, 460. Amer. Chem. J., 1888, 10, 368. 
 
CHAPTER VIII 
 
 ACTION OF VARIOUS REAGENTS ON DIAZO- 
 COMPOUNDS 
 
 1. Snlphur dioxide. When diazobenzene chloride is 
 treated with sulphur dioxide in aqueous solution in the cold, 
 reduction takes place with formation of a hydrazine, and at 
 the same time a second reaction proceeds by which the nitro- 
 gen is eliminated and the sulphonic acid group takes its place. 
 These two products condense together in the nascent state 
 and a sulphazide is formed* 
 
 2C 6 H 6 . N 2 C1 + 3S0 2 + 4H 2 
 
 = C 6 H 5 . NH.NH.S0 2 . C 6 H 6 + N 2 + 2H 2 S0 4 + 2HC1. 
 
 These compounds are also formed by dissolving the amine 
 in 95 per cent, alcohol, saturating the solution with sulphur 
 dioxide, and adding a concentrated aqueous solution of potas- 
 sium nitrite.f 
 
 A differently constituted product results when the neutral 
 diazo-compound, prepared from >-nitroaniline (p-nitrodiazo- 
 benzene hydroxide, N0 2 . C 6 H 4 . N 2 . OH), is dissolved in abso- 
 lute alcohol and subjected to the action of dry sulphur dioxide 
 at 0-5. )-Nitrobenzenediazo-p-nitrobenzenesulphone 
 
 N0 2 .C 6 H 4 .N 2 .S0 2 .C 6 H 4 .N0 2 
 is formed.]: 
 
 These sulphones are also obtained by treating a diazo-salt 
 with benzenesulphinic acid. 
 
 Condensation products in which two benzene nuclei exist 
 are also obtained when a diazo-salt is subjected to the action 
 of sulphur dioxide in presence of a not too large excess of 
 
 * Koenigs, Ber., 1877, 10, 1531. t Ulatowski, Ber., 1887, 20, 1238. 
 
 t Ekbom, Ber., 1902, 35, 656. 
 
 Hantzsch and Singer, Ber., 1897, 30, 312. 
 
ACTIONS OF REAGENTS ON DIAZO-COMPOUNDS 55 
 
 sulphuric acid.* Diazobenzene chloride, under these conditions, 
 yields a compound of formula 
 
 C H 5 . N : N.C 6 H 4 . NH.NH.S0 3 H, 
 
 and m-diazotoluene chloride a compound of analogous con- 
 stitution. The reaction takes a different course when sulphites 
 are employed. With neutral alkali sulphites the correspond- 
 ing diazo-salts are obtained 
 
 C 6 H 5 . N 2 C1 + K 2 S0 3 = C 6 H 5 . N 2 . S0 3 K + KCl.f 
 Acid sulphites furnish hydrazinesulphonic acids of formula 
 C 6 H 5 . NH.NH.SOgX.J Also when a solution of sodium hypo- 
 sulphite is allowed to react with diazobenzene sulphate or 
 chloride, the chief product is sodium phenylhydrazine-j3- 
 sulphonate. There are also formed small quantities of 
 diazobenzeneimide and benzenesulphonphenylhydrazine. In 
 alkaline solution, the diazo-group is replaced by hydrogen 
 (see p. 42). 
 
 2. Replacement of the diazo-group by the sulphinic 
 acid group. The formation of sulphinic acids by the direct 
 action of sulphurous acid on diazo-salts was first observed by 
 Muller and Wiesinger,|| but the replacement is best carried 
 out by Gattermann's method, using copper powder. If A solu- 
 tion of the diazo-sulphate, containing an excess of sulphuric 
 acid, is saturated with sulphur dioxide, the solution being 
 kept cold. Each 100 c.c. of the solution should absorb about 
 15 grams of the gas. Copper powder is now added gradually 
 to the solution (which should be clear), ice being added to 
 keep the solution cold during the operation. The addition of 
 copper is continued with vigorous stirring until no more 
 nitrogen is evolved. As some sulphur dioxide is carried off 
 
 * 15 grams of aniline and 50 grams of concentrated sulphuric acid. 
 Troger, Hille,and Vesterling, J. pr. Chem., 1905 [ii], 72, 511; Troger 
 and Schaub, Arch. Pharm., 1906, 244, 302 ; Troger and Franke, ibid., 
 307; Troger, Warnecke, and Schaub, ibid., 312; Troger, Berlin, and 
 Franke, ibid., 326. 
 
 t Griess, Ber., 1876, 9, 1653. 
 
 t Schmitt and Glutz, Ber., 1869, 2, 51 ; Strecker and Romer, Ber., 
 1871, 4, 784; E. Fischer, Ber., 1875, 8, 589. 
 
 Grandmougin, Ber., 1907, 40, 422. || Ber., 1879, 12, 1348. 
 
 IT Ber., 1899, 32, 1136. 
 
56 CHEMISTRY OF THE DIAZO -COMPOUNDS 
 
 with the nitrogen, a further quantity is passed through the 
 mixture during the reaction. The sulphinic acid is extracted 
 from the product by means of ether. In the case of the 
 diazotized naphthylamines it is better to add the diazo- 
 solution to a mixture of copper powder and a saturated 
 solution of sulphurous acid. 
 
 3. Hydrogen sulphide. When hydrogen sulphide is 
 passed through an aqueous, nearly neutral, solution of jp-nitro- 
 diazobenzene chloride at 0, the diazo-sulphide 
 
 (N0 2 .C 6 H 4 .N 2 ) 2 S 
 is produced. 
 
 In hydrochloric acid solution, the mercaptan hydrosulphide, 
 NO 2 , C 6 H 4 . N 2 SH.H 2 S, is first formed, and on prolonging 
 the passage of the gas, the disulphide, (NO 2 . C 6 H 4 .N 2 ) 2 S 2 , 
 results.* 
 
 On warming diazotized sulphanilic acid with alcoholic 
 potassium sulphide, the diazo-nitrogen is expelled and the 
 dipotassium salt of jo-thiophenolsulphonic acid is produced 
 
 and mercaptan combines with diazo-salts to form an inter- 
 mediate compound which loses nitrogen on warming 
 
 N 2 \ ^ p TT /N 2 S.C 2 H 5 p TT /S.C 2 H 5 
 
 Similarly phenyl mercaptan forms corresponding thiophenol 
 ethers. J 
 
 4. Replacement of the diazo-gronp by the azoimino-gronp. 
 
 1. Action of ammonia. Griess examined the action of 
 concentrated aqueous ammonia on diazobenzene nitrate, and 
 obtained an extremely unstable compound which decomposed 
 into phenol, aniline, and nitrogen. 
 
 This substance was shown by von Pechmann ]| to consist of 
 bisdiazobenzeneamide, the reaction proceeding as follows 
 
 2C 6 H 5 . N 2 C1 + 3NH 3 = C 6 H 5 . N 2 . NH.N 2 . C 6 H 5 + 2NH 4 C1. 
 
 * Bamberger and Kraus, Ber., 1896, 29, 272. 
 
 t Ber., 1887, 20, 350. J Hantzsch and Freese, Ber., 1895, 28, 3237. 
 
 Annalen, 1866, 137, 81. 
 
 || Ber., 1894, 27, 898; ibid., 1895, 28, 171. 
 
ACTIONS OF REAGENTS ON DIAZO-COMPOUNDS 57 
 
 A similar substance is obtained from^-diazotoluene chloride, 
 but jp-nitrodiazobenzene chloride yields only jp-dinitrodiazo- 
 aminobenzene under the same conditions. 
 
 On extending this reaction to diazobenzene perbromide, 
 Griess obtained the first of a very important series of new 
 compounds, namely, the diazoimides, containing three atoms of 
 nitrogen united together. The empirical formula is C 6 H 5 N 3 , 
 and Kekule* proposed for it the constitutional formula 
 
 / N 
 
 C 6 H 5 .N<|| 
 X N 
 C 6 H 5 . NBr.NBr 2 + NH 3 = C 6 H 5 . N 3 + 3HBr. 
 
 Diazobenzeneimide or phenylazoimide is a yellow oil 
 possessing a stupefying odour. It boils at 59 under a 
 pressure of 12 mm. and explodes when heated at the ordinary 
 pressure. When it is heated with hydrochloric acid nitrogen 
 is evolved and chlorobenzene is obtained*; with sulphuric 
 acid two-thirds of the nitrogen is eliminated and aminophenol 
 is producedf. Diazobenzeneimide is also obtained when 
 hydroxylamine acts on diazobenzene sulphate,! 
 
 C 6 H 5 . N 2 . HS0 4 + NH 2 . OH = C 6 H 6 . N 3 + H 2 + H 2 SO 4 
 and by the elimination of water from nitrosophenylhydrazine 
 
 , 
 = C 6 H 6 .N<Jj+H 2 
 
 NO N N 
 
 2. Action of hydrazine. The action of diazo-salts on 
 phenylhydrazine was first studied by Griess, who obtained 
 diazobenzeneimide by treating phenylhydrazine with m-diazo- 
 benzoic acid 
 
 +2C 6 H S .NH.NH 2 
 = C 6 H 6 . N 3 + C 6 H 5 . NH 2 + C0 2 H.C 6 H 4 . N 3 + NH 2 .C 6 H 4 . C0 2 H 
 
 * Ber., 1886, 19, 313. t Ber., 1894, 27, 192. 
 
 I Ber., 1892, 25, 372 ; 1893, 26, 1271 ; compare also Forster and Fierz, 
 Trans., 1907, 91, 855, 1350. 
 Ber., 1876, 9, 1659. 
 
58 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 and also by the interaction of diazobenzene and m-hydrazino- 
 benzoic acid 
 
 = C 6 H 5 . N 3 + C 6 H 5 . NH 2 + C0 2 H.C 6 H 4 . N 3 
 
 + NH 2 . C 6 H 4 . C0 2 H + 2H 2 0. 
 
 The same compound was obtained by E. Fischer by acting on 
 phenylhydrazine with diazobenzene sulphate 
 C 6 H 6 . N 2 . S0 4 H + C 6 H 6 . NH.NH 2 
 
 = C 6 H 5 . N 3 + C 6 H 6 .NH 2 . H 2 S0 4 * 
 
 It has been shown, however, that in addition to the above 
 compound a substance is formed of formula 
 C 6 H 5 .N 2 .N(C 6 H 6 ).NH 2 , 
 
 to which the name diazobenzenephenylhydrazide is given, f 
 When this compound is oxidized with dilute permanganate 
 solution, bisdiazobenzenediphenyltetrazone is obtained. This 
 compound, of formula 
 
 C 6 H 6 . N 2 . N(C 6 H 6 ).N : N.N(C e H 6 ).N : N.C 6 H 6 , 
 contains a chain of no less than eight nitrogen atoms. 
 
 When hydrazine itself is substituted for the phenyl deriva- 
 tive two reactions proceed : on the one hand, we have the 
 formation of diazobenzeneimide and ammonia, and on the 
 other, aniline and azoimide are produced, thus 
 
 C 6 H 5 . N 2 . NH.NH 2 = C 6 H 6 . N 3 + NH 3 
 and C 6 H 5 . N 2 . NH.NH 2 = C 6 H 5 . NH 2 + N 3 H ; J 
 
 the latter reaction, however, proceeds to only a slight extent. 
 
 3. Action of azoimide. The azoimides are also obtained 
 by adding a solution of azoimide or its sodium salt to a diazo- 
 solution containing excess of sulphuric acid. The resulting 
 azoimide is extracted with ether. (For the preparation of 
 azoimide by reactions which do not involve the use of diazo- 
 
 * Ber., 1877, 10, 1334 ; Annalen, 1878, 190, 94 ; compare also Griess, 
 Ber., 1887, 20, 1528 ; ibid., 1888, 21, 3415. 
 t Wohl and Schiff, Ber., 1900, 33, 2741. 
 t Ber., 1893, 26, 88, 1263. 
 Noelting and Michael, Ber., 1893, 26, 86. 
 
ACTIONS OF REAGENTS ON DIAZO-COMPOUNDS 59 
 
 compounds, textbooks on Organic Chemistry should be con- 
 sulted.) 
 
 5. Benzoyl chloride. When diazo-salts are treated 
 with an aqueous suspension of benzoyl chloride and copper 
 powder, dibenzoylhydrazines, RN(CO.C 6 H 5 )N(CO.C 6 H 5 )R, are 
 obtained.* 
 
 * Biehringer and Busch, Ser., 1902, 36, 1964. 
 
CHAPTEE IX 
 
 FORMATION OF DIPHENYL DERIVATIVES IN THE 
 DIAZO-REACTION 
 
 BY acting on diazobenzene nitrate with potassium ferro- 
 cyanide, Griess * obtained azobenzene, a substance having the 
 formula C 18 H 14 N 2 , and a brownish oil. The second of these 
 was shown later to be benzeneazodiphenyl 
 
 C 6 H 5 .N 2 .C 6 H 4 .C 6 H 6 .t 
 
 Griess also observed the formation of p-diphenol by the 
 decomposition of the double salt, (C 6 H 5 . N 2 C1) 2 , SnCl 4 .J 
 
 As will be seen later (p. 73) diazo-salts combine with amines 
 to form diazoamino-compounds, and these pass, by molecular 
 change, into aminoazo-compounds. In the simplest case, that 
 of diazobenzene chloride and aniline, in addition to aminoazo- 
 benzene, o- and p-aminodiphenyl are formed. 
 
 A similar case has not been observed, however, when a diazo- 
 salt acts on a phenol, but when nitrosophenol is thus treated, 
 diphenyl derivatives are largely produced. || 
 
 Also in the preparation of phenol from diazobenzene sul- 
 phate o- and >-hydroxydiphenyl are formed. If 
 
 It is evident, therefore, that at the moment when the diazo- 
 nitrogen separates from the benzene nucleus, two of the latter 
 unite at this point. 
 
 Diphenyl may be prepared in good yield by Gattermann's 
 method of adding copper powder to a solution of diazobenzene 
 sulphate in alcohol.** 31 grams of aniline are dissolved in 
 
 * Annalen, 1866, 137, 39 ; Ber., 1876, 9, 132. 
 
 t Locher, Ber., 1888, 21, 911 ; compare also p. 62. 
 
 1 Ber., 1885, 18, 960. 
 
 $ Hirsch, Ber., 1892, 25, 1973 ; see also Heusler, Annalen, 1890, 260, 
 227. 
 
 || Borsche, Ber., 1899, 32, 2935; Annalen, 1900, 312, 211. 
 
 IT Hirsch. Ber., 1890, 23, 3705; J. pr. Chem., 1885 [ii], 32, 117; com- 
 pare also Norris, Macintyre, and Corse, Amer. Chem. J., 1903, 29, 120. 
 
 ** Ber., 1890, 23, 1226. 
 
FORMATION OF DIPHENYL DERIVATIVES 61 
 
 40 grams of concentrated sulphuric acid and 150 grams of 
 water, and the solution diazotized in the usual manner with 
 23 grams of sodium nitrite. 100 grams of alcohol (90 per 
 cent.) are now added, and then 50 grams of copper powder. 
 Nitrogen is evolved and the temperature rises to about 30-40. 
 After about one hour, when the reaction is finished, the whole 
 is distilled with steam; alcohol passes over, and when the 
 distillate gives, on addition of water, a solid substance, the 
 receiver is changed, and crystals of diphenyl are collected. 
 Instead of copper powder, 100 grams of zinc dust or iron 
 powder may be used. 
 
 Diphenyl is also obtained by the action of stannous chloride 
 on diazobenzene chloride or formate.* 
 
 A large number of similar condensation products are 
 obtained by subjecting mixtures of diazo-salts and hydro- 
 carbons or similar ring-compounds to the action of aluminium 
 chloride; thus diphenyl is obtained from diazobenzene chlo- 
 ride and benzene, and the corresponding phenyl derivative 
 results from the condensation of this diazo-salt with thiophen, 
 pyridine, and quinoline.f 
 
 In applying the cuprous chloride and copper powder methods 
 for the production of chloro-derivatives to the case of many 
 nitrodiazo- and chloronitrodiazo-salts, a remarkable tendency 
 towards the formation of diphenyl derivatives has been 
 observed. 
 
 Thus, when o-nitrodiazobenzene chloride is acted on by 
 copper powder, a yield of 60 per cent, of 2 : 2'-dinitrodiphenyl 
 is obtained ; J by using cuprous chloride a yield of 68 per cent. 
 was observed. 
 
 The diphenyl reaction is also brought about by treating 
 diazo-salts with cuprous oxide dissolved in ammonia,|| and 
 corresponding derivatives are formed by treating diazo-salts 
 with zinc 
 
 * Culmann and Gasiorowski, J. pr. Chem., 1889 [ii], 40, 97. 
 
 t Mdhlau and Berger, Ber., 1893, 26, 1994; see also Kiihling, Ber., 
 1895, 28, 41 ; 1896, 29, 165, and Bamberger, Ber., 1895, 28, 403. 
 
 + Niementowski, Ber., 1901, 34, 3325. 
 
 Ullmann and Forgan, Ber., 1901, 34, 3802; D. R-P. 126961. 
 
 || Vorlander and F. Meyer, Annalen, 1902, 320, 122. 
 
 IT Bamberger and Tichwinsky, Ber., 1902, 35, 4179. Tichwinsky, 
 J. Russ. Phys. Chem. Soc., 1903, 35, 155, 675 ; 1904, 36, 1052. 
 
62 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 A similar condensation takes place in the naphthalene series ; 
 thus when jS-diazonaphthalene sulphate is dissolved in alcohol 
 and treated with zinc dust, to which has been added a very 
 little powdered copper sulphate, /3-dinaphthyl is formed,* and 
 when a cold neutral solution of diazobenzene chloride is mixed 
 with a solution containing 1 molecular proportion of copper 
 sulphate and 6 molecular proportions of sodium thiosulphate 
 (a solution of the salt Cu 2 S 2 O 3 , 3 Na 2 S 2 3 , 6 H 2 0), benzene- 
 azodiphenyl, C 6 H 5 . N 2 . C 6 H 4 . C 6 H 5 , is produced, together with 
 phenyl sulphide, (C 6 H 6 ) 2 S.t 
 
 * Chattaway, Trans., 1895, 67, 653. 
 t BOrnstein, Ber. t 1901, 34, 3968. 
 
CHAPTER X 
 
 INTERCHANGE OF GROUPS IN DIAZO-COMPOUNDS 
 
 A CURIOUS reaction was noticed by Meldola,* who found 
 that when 3 : 4-dinitro-o-anisidine 
 
 NH, 
 
 is diazotized in acetic acid solution, the resulting diazo-com- 
 pound only contains one nitro-group ; the other having been 
 eliminated during diazotization, a substance of formula 
 
 CH, 
 
 being obtained.f The nitro-group has thus been replaced by 
 hydroxyl in the process, being itself liberated in the form of 
 nitrous acid. 
 
 In a similar manner dinitro-p-anisidine 
 
 NIL 
 
 O.CH 3 
 
 on being diazotized in presence of acetic acid J loses a nitro- 
 group, the diazo-compound formed giving with -naphthol 
 a substance of formula 
 
 * Trans., 1900, 77, 1172. t Proc., 1901, 17, 135. 
 
 I Meldola and Eyre, Trans., 1902, 81, 988. 
 
64 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 N 2 .C 10 H 6 .OH 
 IH 
 
 the diazo-compound itself not having been isolated. 
 
 In nitric or sulphuric acid solution the nitro-group remains 
 unaffected, but in presence of hydrochloric acid the nitro- 
 group adjacent to the diazo-group is replaced by chlorine. 
 
 Meldola and his pupils have found that when a nitro-group 
 is in the ortho- or para-position with'respect to an amino-group, 
 no displacement of the nitro-group takes place on diazotization 
 unless there is a second nitro-group adjacent to the first 
 (mobile) group. 
 
 It has further been observed that when a methoxy-group 
 is in the para-position with respect to the amino-group, and at 
 the same time has a nitro-group in an adjacent position, de- 
 methylation takes place on diazotization.* 
 
 Thus the compounds 
 
 OMe OMe 
 
 and 
 
 yield the corresponding quinonediazides of dinitrobenzene 
 O O 
 
 and 
 
 Also when m-phenylenediaminedisulphonic acid is tetrazo- 
 tized, a sulphonic acid group is replaced by hydroxyl with 
 formation of tetrazophenolsulphonic acid.f 
 
 In some other cases which have been observed, it has been 
 possible to obtain a nitrodiazo-compound which, even on dilu- 
 
 * Meldola and Stephens, Trans., 1905, 87, 1205. 
 t E. P. 18283 of 1903. 
 
INTERCHANGE OF GROUPS 
 
 65 
 
 tion with water, soon loses a nitro-group. Thus if the dinitro- 
 -naphthylamine of formula 
 
 N0 2 
 
 (XT 
 
 *\A/ 
 
 NO, 
 
 is diazotized in concentrated sulphuric acid solution and 
 poured into ice-cold water, a precipitate is formed after a short 
 time consisting of a diazo-oxide, to which is assigned the 
 formula 
 
 
 
 NO 
 
 The nitro-group in the a-position is thus replaced by 
 hydroxyl. Similarly from the dinitronaphthylamine 
 
 N0 
 
 the corresponding mononitrodiazo-oxide 
 
 is obtained. The formation of the nitrodiazo-oxide 
 
 N, 
 
 * Gaess and Ammelburg, Per., 1894, 27, 2211. 
 
66 CHEMISTRY OF THE DIAZO- COMPOUNDS 
 
 from the corresponding dinitro-a-naphthylamine 
 
 NH 
 
 takes place in exactly the same way.* 
 
 In all the above cases it will have been noted that the 
 nitro-group which is eliminated reappears as free nitrous acid. 
 This has led Meldola and Eyre f to make the experiment of 
 starting the diazotization of the above-mentioned dinitro-o- 
 anisidine with a small quantity of nitrous acid (one quarter of 
 the theoretical amount was used) ; and they observed that the 
 diazotization was continued by the nitrous acid thus eliminated. 
 
 This transformation is not confined to those diazo-compounds 
 containing only nitro-groups. Many other cases are known ; 
 thus Meldola and Streatf eild J found that when the sulphate 
 of dibromo-/3-naphthylamine 
 
 Br 
 
 was diazotized in presence of acetic acid and the resulting 
 mixture raised to the boiling-point, the normal reaction, 
 namely, replacement of the diazo-group by hydroxyl (see 
 p. 29) did not take place, but bromine was displaced and a 
 diazo-oxide was formed 
 
 5N 
 
 Jr 
 
 In a similar manner chlorobromo--naphthylamine yielded 
 a bromodiazo-oxide 
 
 * Friedlander, Ber., 1895, 28, 1951. 
 
 t Trans., 1901, 79, 1076. \ Trans., 1895, 67, 908, 
 
INTERCHANGE OF GROUPS 67 
 
 Cl 
 
 H 
 
 Similar substitutions of a halogen-group by hydroxyl have 
 been observed to take place by merely treating the diazo-salt 
 with alkalis. Thus 2:4: 6-tribromodiazobenzene chloride 
 yields the dibromodiazo-oxide 
 
 :0* 
 
 The same reaction takes place in the case of 2-chloro-3- 
 nitroaniline-5-sulphonic acid, 
 
 NH N 
 
 =0 t 
 
 and the tetrazo-derivative of 2-chloro-m-phenylenediamine 
 5-sulphonic acid, under the same conditions, loses chlorine, 
 a hydroxyl-group taking its place 
 
 Cl OH O 
 
 !1 ClN/^-N^Cl ClN/\: N 2 J 
 
 "* u * u 
 
 J0 3 H S0 3 H S0 3 H 
 
 A sulphonic acid group also undergoes this change ; thus the 
 compound 
 
 * Bamberger and Kraus, Vierteljahrssch. Ges. Zurich, 1899, 24, 257 ; 
 Ber., 1906, 39, 4248 ; Bamberger, Annalen, 1899, 305, 289. Compare 
 also Silberstein, J. pr. Client. 1883 [ii], 27, 98. 
 
 t Badische Anilin- und Soda-Fabrik, D. R-P. 141750. 
 
 J E. P. 16811 of 1901. 
 
68 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 S0 3 H P 
 
 r m 
 
 yields 
 
 S0 3 H/V\N S 
 
 when diazotized and rendered alkaline.* 
 
 The replacement proceeds even when the diazo-salt of a 
 weak acid such as the acetate, carbonate, bicarbonate, oxalate, 
 &c., is allowed to stand ; this takes place in the case of 2 : 5 : 6- 
 trichloroaniline-m-sulphonic acid, onitroaniline-p-sulphonic 
 acid, and 2:4-dinitroaniline.t 
 
 The transformation of 2:4: 6-tribromo- and trichloro-diazo- 
 benzene takes the same course as shown above, J and a similar 
 phenomenon occurs in the case of a considerable number of 
 halogen-derivatives of the benzene and naphthalene series. 
 
 Some striking molecular transformations have been observed 
 by Hantzsch.|| If >-chlorodiazobenzene thiocyanate (prepared 
 by adding potassium thiocyanate to the diazo-chloride) is 
 dissolved in alcohol containing a trace of hydrochloric acid, 
 the thiocyano-group changes place with the chlorine atom, 
 and on adding ether to the solution, p-thiocyanodiazobenzene 
 chloride is precipitated, thus 
 
 No . SON N C1 
 
 .31 SON 
 
 Similarly, many brominated diazo-chlorides pass into chlori- ; 
 nated diazo-bromides ; H for example, 2:4: 6-tribromodiazo- 
 benzene chloride is converted into chlorodibromodiazobenzene 
 bromide.** 
 
 * E. P. 23998 of 1902. t E. P. 20551 of 1901. 
 
 1 Orton, Proc. Boy. Soc., 1902, 71, 153. 
 
 Orton, Proc., 1902, 18, 252; Trans., 1903, 83, 796; 1907, 91, 1554; 
 Badische Anilin- und Soda-Fabrik, E. Ps. 1561, 6615 of 1902 ; 16995, 
 27372 of 1903 ; 4997 of 1904; Noeltingand Battegay, Ber., 1906, 39, 79. 
 
 || Ber., 1896, 29, 947. 
 
 1" Hantzsch, Schleissing, and Jager, Ber., 1897, 30, 2334 ; see also Ber., 
 1898, 31, 1253. 
 
 ** Hantzsch and Smythe, Ber., 1900, 33, 505. 
 
INTERCHANGE OF GROUPS 69 
 
 This transformation has been studied quantitatively, and 
 has been found to proceed according to the following laws : 
 
 (1) The bromine atoms are replaced only when present in 
 the para- or ortho-position with respect to the diazo-group, 
 those in the ortho-position being most readily removed. A 
 bromine atom in the meta-position is not affected. 
 
 (2) The ease of transformation increases with the number 
 of bromine atoms present. 
 
 (3) The transformation constant, calculated from the equa- 
 tion for a unimolecular reaction, 
 
 i *i A 
 
 k= t^A^' 
 
 increases with the temperature and is also influenced by the 
 solvent, having its minimum value in water, and becoming 
 greater as the series of alcohols is ascended. 
 
 (4) The diazo-salts containing two bromine atoms are 
 stable when dry, but are rapidly transformed in ethyl alcohol ; 
 2:4: 6-tribromodiazobenzene chloride becomes transformed 
 even in the dry state. 
 
 A corresponding isomeric change does not take place in the 
 case of tri-iododiazobenzene chloride or tribromodiazobenzene 
 fluoride.* 
 
 Lastly, a remarkable change is undergone by 1-nitrodiazo- 
 /S-naphthalene chloride which is transformed in presence of 
 glacial acetic acid into l-chlorodiazo-/?-naphthalene nitrite, f 
 
 * Hantzsch, Per., 1903, 36, 2069. 
 t Morgan, Trans., 1902, 81, 1376. 
 
CHAPTEE XI 
 ACTION OF LIGHT ON DIAZO-COMPOUNDS 
 
 MOST investigators who have worked with diazocompounds 
 have noticed that they are very easily changed by the action 
 of light. Thus Berthelot and Vielle in 1881 * recorded the 
 observation that when diazobenzene nitrate was exposed to 
 light it became rose-coloured. 
 
 This decomposition has been made the basis of photographic 
 processes; thus Feer in 1889f exposed a film coated with a 
 mixture of a diazo-sulphite and a phenol or amine to light. 
 
 A decomposition of the former occurred which was followed 
 by the formation of an azo-compound, and hence the produc- 
 tion of a coloured negative. 
 
 Green, Cross, and Bevan J coated films with diazotized pri- 
 muline, the decomposition of which was proportional to the 
 intensity of the light ; this formed the ' negative ', and a ' posi- 
 tive ' was developed by treatment with an amine or a phenol. 
 Those parts of the negative which had been exposed to bright 
 light gave no colour with the component, owing to the destruc- 
 tion of the diazo-compound with evolution of nitrogen and 
 formation of a phenol. They concluded, however, that 
 union of the diazo-compound with the medium (cellulose) was 
 necessary, for the free diazo-primuline when exposed to light 
 in a thin film was either not decomposed at all or only after 
 long exposure. 
 
 Andresen examined the behaviour of the diazo-salts of the 
 two naphthylamines, and showed that the reaction was similar 
 to that effected by heat, namely, that phenols were formed 
 thus 
 
 K.N 2 C1 + H 2 = R.OH + N 2 + HC1. 
 
 Kuff and Stein || arrived at the following conclusions with 
 
 * Compt. rend., 1881, 92, 1074. t D. R-P. 53455. 
 
 t D.R-P. 56606, Ber., 1890, 23, 3131 ; J. Soc. Chem. Ind., 1890, 9, 1001. 
 Photographische Correspondenz, 1895. 
 || Ber., 1901, 34, 1668. 
 
ACTION OF LIGHT ON DIAZO-COMPOUNDS 71 
 
 regard to the action of light on substituted diazobenzene 
 chlorides. 
 
 Those which contain a negative group (OH, NO 2 , CO 2 H) in 
 the para-position are more sensitive than those containing 
 a similarly situated positive group (Cl, CH 3 ) ; the influence of 
 the nitro-group is greatest. Ortho- and para-substituted 
 groups have about the same effect, either in increasing or 
 decreasing sensitiveness ; this effect is always less than that of 
 a meta-group. In the case of diazo-salts derived from different 
 nuclei, the sensitiveness to light increases with the number of 
 atoms in the nucleus ; thus the diazo-salt from 3-aminocarba- 
 zole is nearly five times as sensitive as that from >-toluidine. 
 
 As regards the decomposition of diazo- and tetrazo-com- 
 pounds an equal number of diazo-groups are destroyed by light 
 in the same time ; thus the same number of minutes is neces- 
 sary to decompose completely the diazo-salt from an JV/10 
 solution of >-aminodiphenyl as from an N/2Q solution of 
 benzidine. 
 
 Orton, Coates, and Burdett* were the first to investigate 
 extensively this reaction. Solutions of the diazo-salts of 
 various aromatic amines were found to decompose under the 
 action of light in the manner indicated by Andresen, but the 
 mechanism whereby the phenolic decomposition is effected 
 must be very different from that induced by the action of 
 heat, for the remarkable fact was discovered that many of the 
 diazo-compounds which are decomposed by water or acids 
 only with very great difficulty, and then only to a very slight 
 extent, for example, the diazo-salt of 2:4: 6-tribromoaniline, 
 undergo rapid transformation under the action of light with 
 quantitative formation of the corresponding phenol. 
 
 A similar instance of this difference in stability towards 
 heat and light had been noticed by Meldola, Woolcott, and 
 Wray,f wno found that the compound 
 
 -O 
 
 * Trans., 1907, 91, 35. t Trans., 1896, 69, 1327. 
 
72 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 was stable towards boiling water, but that it decomposed 
 gradually and became brown on exposure to light. 
 
 The S2/7&-diazo-cyanide of 2 : 4 : 6-tribromoaniline in benzene 
 solution changes under the action of light into the corre- 
 sponding a7ii-compound.* 
 
 * Ciusa, Atti. E. Accad. Lincei, 1906 [v], 15, ii, 136 ; for an explanation 
 of the terms syn and anti see p. 123. 
 
CHAPTEK XII 
 
 DIAZOAMINO-COMPOUNDS 
 
 THE diazoamino-compounds are formed by the condensation 
 of a diazo-salt with primary or secondary amines in presence 
 of sodium acetate, thus 
 
 (1) C 6 H 5 . N 2 .C1 + NH 2 . C 6 H 5 = C 6 H 5 . N 2 . NH.C 6 H 6 + HC1. 
 
 (2) C 6 H 6 . N 2 C1+ NH(C 2 H 6 ).C 6 H 6 = 
 
 C,H, . N 2 . N(C 2 H 6 ).C 6 H 6 + HC1. 
 
 The preparation of diazoaminobenzene is carried out as 
 follows : 10 grams of aniline are dissolved in 100 c.c. of 
 water and concentrated hydrochloric acid corresponding to 
 12 grams HC1. The solution is diazotized by adding a solu- 
 tion of 8 grams of sodium nitrite with the usual precautions. 
 On the other hand, 10 grams of aniline are dissolved in 
 50 grams of water and exactly the theoretical quantity of 
 hydrochloric acid. After cooling this solution with ice it is 
 added to the diazo-solution, and then, immediately, a cold 
 concentrated solution of 50 grams of sodium acetate. After 
 standing for half an hour the diazoaminobenzene is filtered 
 off, washed with water, dried on a porous plate, and crystal- 
 lized from light petroleum. 
 
 When aromatic diazo-compounds are allowed to act on 
 aliphatic amines, similar diazoamino-compounds are obtained. 
 Thus methylamine and ethylamine yield with diazobenzene, 
 diazobenzenemethylamide (phenylmethyltriazen), 
 
 and diazobenzene-ethylamide (phenylethyltriazen), 
 
 C 6 H 6 .N 2 .NH.C 2 H 6> 
 
 respectively.* 
 
 These compounds possess the formulae assigned to them, 
 and are not tautomeric. f 
 
 * Dimroth, Ber., 1903, 36, 909 ; 1905, 38, 670, 2328 ; compare also 
 Goldschmidt and Holm., Ber., 1888, 21, 1016. 
 t Dimroth, Eble, and Gruhl, Ber., 1907, 40, 2390. 
 
74 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 The formation takes place also when an alkali nitrite is 
 added to a solution of an amine containing no free mineral 
 acid 
 
 2C 6 H 5 .NH 2 .HCl + NaN0 2 
 
 = C 6 H 6 . N 2 . NH.C 6 H 5 + NaCl + HC1 + 2H 2 0. 
 
 If two molecules of a diazo-salt condense with one of a 
 primary amine, a bisdiazoamino-compound is formed 
 
 2C 6 H 5 . N 2 C1 + C 6 H 6 . NH 2 = (C 6 H 5 . N 2 ) 2 N.C 6 H 5 4- 2HC1. 
 
 A modification of this method is to allow a molecule of 
 a diazo-salt to act on a molecule of a diazoamino-compound 
 C 6 H 5 . N 2 C1 + C 6 H 5 . N 2 . NH.C 6 H 6 = (C 6 H 5 .N 2 ) 2 N.C 6 H 5 + HC1 .* 
 
 The primary monoamines of the benzene series all yield 
 diazoamines, those containing the groups Cl, NO 2 , ON, &c., 
 most readily, but the monoalkylated monoamines of this 
 series show a tendency to form azo-compounds ; for example, 
 methylaniline, when treated with diazobenzenesulphonic acid, 
 yields a mixture of the diazoamino-compound, 
 S0 3 H.C 6 H 4 .N 2 .N(CH 3 ).C 6 H 6 , 
 and the isomeric aminoazo-compound, 
 
 S0 3 H.C 6 H 4 . N 2 . C 6 H 4 . NH.CH 3 . f 
 
 A number of bases, as for example diphenylamine, the 
 naphthylamines and their monoalkyl-derivatives, m-phenyl- 
 enediamine and certain of its homologues and substitution 
 products, form aminoazo-compounds direct. Dimethylaniline 
 and some other tertiary amines also yield aminoazo-compounds ; 
 here, of course, no diazoamine can be formed. Griess discovered 
 the remarkable fact that the same compound is obtained from, 
 for example, diazobenzene chloride and ^-toluidine on the 
 one hand, and p-diazotoluene chloride and aniline on the 
 other. 
 
 According to the above equations one would expect two 
 different diazoamino-compounds to be formed thus 
 
 (1) C 6 H 5 . N 2 C1 + NH 2 . C 6 H 4 . CH 3 
 
 = C 6 H 5 . N 2 . NH.C 6 H 4 . CH 3 + HC1. 
 
 * Ber., 1894, 27, 703. 
 
 t Bernthsen and Goske, Ber., 1887, 20, 925 ; Bamberger and Wulz, 
 Ber., 1891, 24, 2082. 
 
DIAZOAMINO-COMPOUNDS 75 
 
 (2) CH 3 . C 6 H 4 . N 2 C1 + NH 2 . C 6 H 6 
 
 = CH 3 . C 6 H 4 . N 2 . NH.C 6 H 5 + HC1. 
 
 If one supposes, however, that an intermediate product is 
 formed of the formula 
 
 C 6 H 5 . NH.NC1.NH.C 6 H, . CH 3 , 
 (a) (b) 
 
 then, by the elimination of hydrogen chloride, either of the 
 above formulae is obtained according as to which hydrogen 
 atom (a) or (b) is removed.* 
 
 In order to decide which of the above formulae is correct, 
 use is made of the compound with phenylcarbimide. This 
 combines with the diazoamino-compound to form a substance 
 of formula 
 
 (1) C 6 H & . 
 
 ~~Ig.Vfr.ij 
 
 or (2) C& 
 
 When this is decomposed with dilute sulphuric acid, phenyl- 
 >-tolylcarbamide, phenol, and nitrogen are formed, so that its 
 constitution must be represented by (1), for (2) would give 
 diphenylcarbamide. This conclusion is also confirmed by the 
 fact that whichever way the compound is prepared it yields 
 only one acetyl derivative, namely, diazobenzene #>-aceto- 
 toluidide, which, when decomposed by acids, yields aceto- 
 toluidide.f The constitution of the diazoamino-compound is 
 therefore C 6 H 6 .N 2 .NH.C 6 H 4 .CH 3 , t and it is found that in 
 these reactions the imino-group is always attached to the 
 electronegative, and the diazo-group to the electropositive 
 nucleus. 
 
 When the alkyl derivatives of the mixed diazoamino-com- 
 pounds (that is, compounds in which the two radicals combined 
 with the group N 3 H are different) are examined, it is found 
 that three isomeric substances exist. These are formed : 
 I. By the action of X.N 2 C1 on Y.NH.R. 
 
 II. By the action of Y.N 2 C1 on X.NHR. 
 
 * V. Meyer, Ber., 1881, 14, 2447 ; 1888, 21, 1016, 3004. 
 t von Pechmann, Ber., 1895, 28, 869. 
 I Goldschmidt, Ber., 1888, 21, 2578. 
 
76 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 III. By the alkylation of X.N 3 HY with RI and caustic 
 potash. X and Y represent the two radicals united with the 
 group N 3 H, and R represents a univalent alkyl-group. 
 
 The isomerides obtained by direct alkylation are also 
 formed when the compounds obtained according to (I) and 
 (II) are heated together in equimolecular proportions.* 
 
 Migration of the diazo-group. An interesting variation in 
 this reaction is that in which the migration of the diazo-group 
 occurs. Thus when diazotized sulphanilic acid and ^9-toluidine 
 hydrochloride are mixed together at the diazo- and amino- 
 groups change places, and there results a mixture of >-diazo- 
 toluene chloride and sulphanilic acid.f In neutral solution, 
 however, the normal diazoamino-compound, 
 
 CH 3 . C 6 H 4 . N 2 . NH.C 6 H 4 . S0 3 H, 
 is formed. 
 
 A corresponding interchange takes place between m- or 
 >-nitrodiazobenzene chloride and >-toluidine. When, how- 
 ever, >-diazotoluene chloride is mixed with m- or >-nitro- 
 aniline or sulphanilic acid no migration of the diazo-group 
 takes place. J If diazobenzene chloride and p-bromoaniline 
 are allowed to interact, aniline and p-bromodiazobenzene 
 chloride are formed. 
 
 This migration of the diazo-nucleus is probably associated 
 with the changes which occur when this group passes from 
 the ' diazonium ' (see chap, xviii) to the diazo condition. 
 
 Bamberger || found that when an alkali i'so-diazo-oxide is 
 dissolved in cold mineral acid, nitrous acid is formed 
 
 R.N:N.OH + H 2 = R.NH 2 + HNO 2 
 R.NH 2 + HN0 2 + HC1 = R.N 2 C1 + 2H 2 O. 
 
 It is interesting to note that diazoamino-compounds may 
 be obtained without the use of diazo-compounds ; thus they 
 
 * Meldola and Streatfeild, Trans., 1886, 49, 624 ; 1887, 51, 102, 434 ; 
 1888, 53, 664; 1889, 55, 412 ; 1890, 57, 785. 
 t Griess, Ber., 1882, 15, 2190. 
 1 Schraube and Fritsch, Ber., 1896, 29, 287. 
 Hantzsch and F. M. Perkin, Ber., 1897, 30, 1412. 
 || Ber., 1895, 28, 826. 
 
DIAZOAMINO-COMPOUNDS 77 
 
 are formed by the interaction of nitrosoamines and primary 
 aromatic amines ; for example 
 
 C.H 6 (CH 3 )N.NO + H 2 N.C 6 H 6 = C 6 H 6 (CH 3 )N.N 2 . C 6 H 5 . 
 Nitrosoacetanilide also reacts in a similar way 
 
 = C 6 H 6 . NH.N a . C 6 H 8 + CH 3 . CO 2 H, 
 
 and two molecules react with one molecule of aniline in alka- 
 line solution to form a bisdiazoamino-compound, 
 
 -CH 6 + 2CH 3 . C0 2 H. 
 
 An isomeride of diazoaminobenzene is said to result when 
 aniline is diazotized in presence of acetic acid instead of 
 a mineral acid.* Its constitution is supposed to be 
 
 N.CH 6 
 
 but the existence of such a compound must be accepted with 
 reserve, f 
 
 Reactions of the diazoamino-componnds. The diazoamino- 
 compounds usually have a yellow colour, and do not dissolve 
 in acids. They may generally be crystallized without decom- 
 position, and are much more stable than the diazo-compounds. 
 
 When boiled with hydrochloric acid, nitrogen is evolved 
 
 C 6 H 5 . N 2 . NH.C e H 5 + H 2 = C 6 H 6 . OH + C e H 6 . NH 2 + N 2 . 
 On heating with cuprous chloride and hydrochloric acid, 
 chlorobenzene and aniline are formed 
 
 C 6 H 6 . N 2 . NH.C e H 6 + HC1 = C 6 H 6 C1 + C 6 H 5 . NH 2 + N 2 . 
 
 Hydrazines are obtained by reduction with zinc dust and 
 acetic acid 
 
 C 6 H 6 .N 2 . NH.C 6 H 6 + 2H 2 = C 6 H 6 .NH.NH 
 
 * Orloff, J. Buss. Phys. Chem. Soc., 1906, 38, 587. 
 t Compare also Vaubel, Zeitsch. angew. Chem. t 1900, 13, 762 ; 1902, 15, 
 1209. 
 
78 CHEMISTRY OF THE DIAZO -COMPOUNDS 
 
 and with nitrous acid two molecules of a diazo-compound are 
 produced 
 
 C 6 H 6 . N 2 . NH.C 6 H 5 + HN0 2 + 2HC1 = 2C 6 H 5 . N 2 C1 + 2H 2 0. 
 
 By boiling a diazoamino-compound with sulphurous acid in 
 alcoholic solution, the diazo-group is replaced by the sulphonic 
 acid group 
 
 C 6 H 6 . N 2 . NH.C 6 H 5 + 2S0 2 + 2H 2 O 
 
 = C 6 H 5 . S0 3 H + N 2 + C 6 H 5 . NH 2 . H 2 S0 3 . 
 
 A very important reaction is that which takes place when 
 a diazoamino-compound is warmed with a mixture of an 
 amine and its hydrochloride ; a molecular change occurs with 
 formation of aminoazo-compounds 
 
 C 6 H 6 .N 2 .NH.C 6 H 6 -* C 6 H 5 .N 2 .C 6 H 4 .NH 2 . 
 
 The velocity of the transformation of diazoamino- into 
 aminoazo-compounds under the influence of aniline hydro- 
 chloride has been shown by Goldschmidt and his pupils to be 
 in accordance with the law of unimolecular reactions, in 
 which 
 
 1 a * 
 
 k=llog 
 
 t a x 
 
 In the case of diazoaminobenzene dissolved in aniline con- 
 taining aniline hydrochloride, the rate of reaction is pro- 
 portional to the concentration of the aniline hydrochloride, 
 and increases with the temperature ; as is usual in the case of 
 a unimolecular reaction, the velocity is independent of the 
 concentration of the diazoaminobenzene. 
 
 The transformation is also effected by other aniline salts, 
 such as the dichloroacetate or trichloroacetate, but the 
 rate in this case is slower than when the hydrochloride is 
 used.f 
 
 Benzenediazoamino-p-toluene becomes converted into diazo- 
 aminobenzene and ^-toluidine, the former of which then 
 undergoes transformation in accordance with the above rules. 
 
 * Ber., 1896, 29, 1369, 1899 ; Zeitsch. physikal. Chem., 1899, 29, 89. 
 t Compare also Jungius, Chem. Weekblad, 1905, 2, 246. 
 
DIAZOAMINO-COMPOUNDS 79 
 
 The conversion takes place more slowly when the diazo- 
 group is in the ortho-position with respect to the amino- 
 group. Thus the value of the constant in the case of diazo- 
 aminobenzene at 45 is 0-081, whilst the corresponding value 
 for diazoamino-p-toluene is only 0-0095, the solution in each 
 case being semi-normal. 
 
CHAPTER XIII 
 
 AZO-COMPOUNDS 
 
 THE azo-compounds, like the diazo-, contain the group .N 2 ., 
 but with the important difference that, whereas in the latter 
 only one organic radical is united to the .N 2 . group, the other 
 free linking being combined with an acid radical, thus R.N 2 . Cl J 
 in the former two organic radicals are united to the N 2 group, 
 thus R.N 2 . R. 
 
 The groups attached to the nitrogen atoms may be either 
 (1) aromatic, (2) aliphatic, or (3) one aromatic and one ali- 
 phatic group, giving the mixed azo-compounds. 
 
 The first representative of this class of compounds was 
 obtained by Mitscherlich * by the distillation of nitrobenzene 
 with alcoholic potash. Mitscherlich called the substance ' azo- 
 benzide', the modern name being of course azobenzene, 
 C 6 H 6 . N 2 . C 6 H 5 . He considered that the substance was formed 
 by the replacement of one atom of nitrogen for one atom of 
 hydrogen in benzene, but Zinin showed that ' azoxybenzide ' 
 was always formed in this reaction, and this on distillation 
 yielded ' azobenzide '.f Zinin also, by the reduction of this 
 substance with hydrogen sulphide and treatment of the pro- 
 duct with sulphuric acid, obtained the sulphate of benzidine, 
 which he considered was formed by the direct reduction of 
 ' azobenzide '. Hofmann, however,J showed that in this reduc- 
 tion, hydrazobenzene, C 6 H 6 .NH.NH.C 6 H 5 , was first formed, 
 which, in presence of sulphuric acid, underwent molecular 
 change into the isomeric benzidine, NH 2 .C 6 H 4 .C 6 H 4 .NH 2 , 
 and also proved by a vapour density determination that ' azo- 
 benzide' must have a formula double that which had been 
 previously given to it. 
 
 * Anndlen, 1834, 12, 311. t J.pr. Chem., 1845, 38, 93. 
 
 t Jahresber., 1863, 424. 
 
AZO-COMPOUNDS 81 
 
 The azo-compounds occupy an intermediate position between 
 the nitre-compounds and the corresponding amines; thus in 
 the case of nitrobenzene we have 
 
 Nitrobenzene. Azoxybenzene. 
 
 C 6 H 6 .N C 6 H 6 -NH 
 
 -* II -* I - C e H 6 .NH 2 . 
 
 C 6 H 5 .N C 6 H 5 .NH 
 
 Azobenzene. Hydrazobenzene. Aniline. 
 
 1. Azoxy-componnds. These are obtained by the reduction 
 of nitro- or nitroso-compounds with methyl- or ethyl-alcoholic 
 potash 
 
 4C 6 H 5 .N0 2 + 3CH 3 .ONa = 2(C 6 H 6 .N) 2 + 3H.CO 2 Na + 3H 2 O. 
 Other reducing agents which may be employed are sodium 
 amalgam and alcohol, zinc dust and alcoholic ammonia, and 
 arsenious acid in alkaline solution. 
 
 Further, azoxy-compounds are obtained by oxidizing amino- 
 and azo-compounds with alkaline potassium permanganate 
 or ferricyanide and by the oxidation of /S-phenylhydroxyl- 
 amine, C 6 H 5 .NH.OH, in the air. 
 
 By treatment with energetic reducing agents, azoxy-com- 
 pounds yield products of various degrees of reduction ; thus 
 with iron filings azo-compounds are formed, with ammonium 
 sulphide, hydrazo-compounds, and, finally, acid reducing agents 
 furnish amino-compounds ; the acid used in the latter case 
 obviously serves to bring about a molecular change in the 
 hydrazo-compound first formed. 
 
 The simplest member of the series, azoxybenzene, is pre- 
 pared as follows. 10 parts of sodium are dissolved in 100 
 parts of methyl alcohol, 15 parts of nitrobenzene added, and 
 the whole heated for 3 hours on a boiling-water bath in an 
 apparatus connected with an inverted condenser. The alcohol 
 is then distilled off, the residue of sodium formate a,nd azoxy- 
 benzene extracted with water, and the azoxybenzene allowed 
 to crystallize out ; the yield is about 90 to 92 per cent, of the 
 theoretical.* Azoxybenzene is insoluble in water, but crystal- 
 lizes from alcohol in long, yellow, rhombic needles melting at 
 
 * Ber., 1882, 15, 865; 1883, 16, 81. 
 G 
 
82 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 36. On heating with concentrated sulphuric acid it undergoes 
 isomeric change and is converted into hydroxyazobenzene 
 
 /N.C 6 H 6 N.C 6 H 6 
 
 o< i -> j 
 
 X N.C 6 H 5 N.C 6 H 4 .OH* 
 
 2. Azo-componnds. As already indicated these are obtained 
 by the reduction of nitro- or azoxy-compounds. The reducing 
 agents used are, in the case of nitro-compounds, (1) zinc dust 
 and alcoholic potash; (2) sodium amalgam and alcohol; (3) 
 stannous chloride dissolved in caustic soda, and, in the case of 
 azoxy-compounds, iron filings (p. 81). 
 
 Other methods of formation are : (1) by the oxidation of 
 hydrazo- or amino-compounds by potassium permanganate in 
 alkaline solution f or by potassium f erricyanide ; (2) by the 
 interaction of nitrosobenzene and aniline, 
 
 C 6 H 5 . NO + NH 2 . C 6 H 6 = C 6 H 6 . N 2 . C 6 H 
 
 of nitrosobenzene and phenylhydra^zine, or of nitrosobenzene 
 and s-diphenylhydrazine.|| Azobenzene is also formed when 
 phenylhydrazine is treated with bleaching powder solution,^ or 
 when diazo-compounds are treated with the same reagent ; thus 
 diazotized sulphanilic acid yields 2 : 2'-dinitroazobenzene-4 : 4'- 
 disulphonic acid, S0 3 H.C 6 H 3 (NO 2 ).N : N.C 6 H 3 (N0 2 ).S0 3 H ; 
 some 4 : 6-dichloro-2-nitroaniline is formed at the same time, 
 and this substance is also produced when diazobenzene chloride 
 is treated with bleaching powder. 
 
 When the sodium ^so-diazo-oxide derived from diazosul- 
 phanilic acid is used, the calcium salt of ^9-nitroaminobenzene- 
 sulphonic acid, N0 2 . NH.C 6 H 4 . S0 3 H, is formed.** 
 
 An interesting synthetic method for the production of these 
 substances is that due to Meldola ;ff trinitroacetylaminophenol 
 
 * Ber., 1880, 13, 525 ; Annalen, 1882, 215, 218. 
 t Annalen, 1867, 142, 364. 
 
 j Baeyer, Ber., 1874, 7, 1638 ; Bamberger, Ber., 1893, 26, 473, 483. 
 Mills, Trans., 1895, 67, 925. 
 |1 Bamberger, Ber., 1900, 33, 3508. 
 IT Ber., 1897, 30, 284. 
 
 ** Zincke, Ber., 1895, 28, 2948 ; Zincke and Kuchenbecker, Annalen 
 1903, 33O, 1 ; see also Lenz, Annalen, 1903, 330, 370. 
 tt Trans., 1906, 89, 1943. 
 
AZO-COMPOUNDS 83 
 
 is condensed with phenylhydrazine, forming a hydrazo-com- 
 pound which passes by oxidation into the azo-compound thus 
 
 OH 
 
 
 N0 2 
 
 NH.CO.CH 3 
 
 The azo-compounds are usually strongly coloured owing to 
 the presence of the chromophoric group ,N':N.* They are 
 readily acted on by sulphuric or nitric acids, chlorine, &c. 
 
 Azobenzene is prepared from azoxybenzene by distilling a 
 mixture of one part of the latter with three parts of iron filings 
 from a small retort. Care must be taken that the materials 
 are quite dry. The reddish distillate is crystallized from 
 light petroleum and forms red plates melting at 68 and boil- 
 ing at 293. 
 
 Complex azo-compounds are also obtained by treating diazo- 
 salts with potassium ferrocyanide.f 
 
 3. Aminoazo-compounds. These are formed by the follow- 
 ing reactions J : 
 
 (1) Intramolecular rearrangement of diazoamines 
 
 C 6 H 6 . N 2 . NH.C 6 H 5 = C 6 H 5 . N 2 . C 6 H 4 . NH 2 . 
 
 (2) Reduction of >-nitroazo-derivatives by an alkaline 
 sulphide. 
 
 (3) Combination of a diazotized monoacetyldiamine with 
 an amine or phenol and subsequent hydrolysis of the acetyl- 
 derivative.|| 
 
 (4) The action of nitrosobenzene on monoacetyldiamines 
 and hydrolysis of the acetyl-derivative.^f 
 
 (5) The alkaline reduction of nitroamines.** 
 
 * See, however, Baly and Tuck, Trans., 1906, 89, 982. 
 
 t Griess, Annaien, 1866, 137, 39 ; Ber., 1876, 9, 132 ; Ehrenpreis, Bull. 
 Acad. Sci. Cracow, 1906, 265. 
 
 I Meldola and Eynon, Trans., 1905, 87, 1. 
 
 Meldola, Trans., 1883, 43, 425. 
 
 || Nietzki, Ber., 1884, 17, 343. IT Mills, Trans., 1895, 67, 928. 
 
 * Haarhaus, Annaien, 1865, 135, 164 ; Mixter, Amer. Chem.J., 1883, 5, 
 283 ; Nietzki, Ber., 1884, 17, 345 ; Graff, Annaien, 1885, 229, 341 ; 
 Noelting and Binder, Ber., 1887, 2O, 3016 ; Meldola and Andrews, Trans., 
 1896, 69, 10 ; Noelting and Fourneaux, Ber. f 1897, 30, 2938. 
 
84 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 (6) Combination of a monodiazo-chromate, prepared from 
 a diamine by diazotizing only one amino-group and precipi- 
 tating with sodium dichromate, with an amine or phenol, f 
 
 (7) Interaction of certain amines (the naphthylamines and 
 2 : 4-tolylenediamine, for example) and the para-diazoimides.J 
 
 The usual method of preparation, however, is to allow a 
 diazo-salt to react with amine in presence of sodium acetate ; 
 thus diazobenzene chloride unites with a solution of a-naphthyl- 
 amine hydrochloride to form benzeneazo-a-naphthylamine 
 
 C 6 H 5 . N 2 C1 + C 10 H 7 . NH 2 . HC1 + 2C 2 H 3 2 Na 
 
 = C 6 H 5 . N 2 . C 10 H 6 . NH 2 + 2NaCl + 2C 2 H 4 O 2 . 
 
 It is a fairly general rule that the diazo-group enters the 
 aminic nucleus in the para-position with respect to the amino- 
 group when this is unoccupied, but otherwise it enters the 
 ortho-position. 
 
 In the following formulae the position of the entering diazo- 
 group is shown by the asterisk. 
 
 NH, 
 
 NH 2 
 
 ,H 
 
 CCr 
 
 In the case of diamines combination only takes place with 
 the meta- derivative; thusm-phenylenediamine andm-tolylene- 
 diamine 
 
 NH, 
 
 and 
 
 ! NH f 
 
 N 
 
 form azo-dyes, the diazo-group entering the para-position 
 
 t Meldola and Eynon, loc. cit. 
 
 J Morgan and Micklethwait, Trans., 1907, 91, 1512. 
 
AZO-COMPOUNDS 85 
 
 with respect to an aminogroup. It is even possible for a 
 second diazo-complex to combine at the carbon atom between 
 the amino-groups. The question of the influence of substitu- 
 tion in such diamines on the formation of aminoazo-compounds 
 has been made the subject of comprehensive researches by 
 Morgan, who has arrived at the following conclusions * : 
 
 (1) The mono-substituted meta-diamines and the di-substi- 
 tuted meta-diamines, containing one free para-position with 
 respect to an amino-group, react readily with diazo-salts 
 to furnish para-ammoazo-dyestuffs 5 f and this reaction takes 
 place with equal readiness both with the primary meta- 
 diamines of this type and with their completely alkylated 
 derivatives.! 
 
 (2) The di-para-substituted primary meta-diamines 
 
 NB 
 
 react with diazo-salts to form ortho-aminoazo-derivatives, but 
 the reaction takes place much less readily than with those 
 diamines having one free para-position, and the yield of azo~ 
 product is frequently very smalL 
 
 (3) The nature of the substituents X and Y exerts some 
 influence on the course of the azo-condensation, for when they 
 are methyl-groups the base (4 : 6-diamino-m-xylene) reacts 
 with diazotized aniline and its homologues, but when both 
 substituents are halogen atoms (chlorine, bromine, or iodine) 
 the condensation does not occur with these simple diazo-salts, 
 but only with those derived from the nitroanilines. When only 
 one methyl-group is replaced by chlorine or bromine, reaction 
 with diazotized aniline and ^-toluidine still occurs, but the 
 yield of o-aminoazo-derivative is extremely small. || 
 
 (4) The presence of a nitro-group in one of the two substi- 
 tuted para-positions facilitates the azo-condensation, particu- 
 larly when the diazo-salt also contains a substituent nitro- 
 group.l 
 
 * Trans., 1907, 91, 370. t Trans., 1900, 77, 1205 ; 1902, 81, 89. 
 
 J Trans., 1902, 81, 656. Trans., 1902, 81, 1379; 1905, 87, 935. 
 
 || Trans., 1902, 81, 1379 ; 1905, 87, 937. IT Trans., 1905, 87, 940. 
 
86 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 (5) The progressive alkylation of the di-para-substituted 
 meta-diamines rapidly reduces their capacity for forming 
 azo-derivatives. The symmetrically and unsymmetrically 
 dimethylated diamines give mixtures of diazoamines and 
 aminoazo-compounds,* whilst the trimethylated diamines 
 readily furnish diazoamines and show scarcely any tendency 
 to form azo-derivatives. Finally, the interaction of the 
 di-para-substituted m-diamines and diazo-salts is entirely 
 prevented by the complete alkylation of these bases, f 
 
 The aminoazo-compounds are usually crystalline and 
 soluble in alcohol. They are yellow to red or brown in colour, 
 and many of them are used as dyestuffs. 
 
 The simplest member, aminoazobenzene, was introduced 
 into commerce in 1863 by Simpson, Maule, and Nicholson 
 under the name of ' aniline yellow ' ; at the present day it is 
 only used as an intermediate product for the manufacture of 
 induline, &c., but its sulphonic acids find extensive application 
 as { fast yellow '. 
 
 4- Hydroxyazo-compounds. These are obtained by the 
 action of a diazo-salt on phenols and their derivatives in 
 alkaline solution,! thus 
 
 C 6 H 5 . N 2 C1 + C 6 H 6 . OH = C ft H 5 . N 2 . C 6 H 4 . OH + HC1. 
 
 The same rules apply here with regard to the position 
 taken by the diazo-complex as in the case of the aminoazo- 
 compounds, namely, that the para-position to the hydroxyl- 
 group, if free, is occupied. Otherwise the ortho-position is 
 taken. In the case of /3-naphthol, the diazo-group enters 
 the a-position. 
 
 It has been found, however, that by the action of diazo- 
 benzene or ^>-diazotoluene on phenol, small amounts of the 
 corresponding o-hydroxyazo-compounds are formed. 
 
 * Trans., 1905, 87, 946 ; 1906, 89, 1057 ; 1907, 91, 368. 
 
 t Trans., 1902, 81, 656. 
 
 % In acid solution, diphenyl ether is produced, 
 
 C 6 H 5 . N 2 . S0 4 H + C 6 H 5 . OH = (C 6 H 5 ) 2 + H 2 S0 4 + N 2 
 (Hofmeister, Annalen, 1871, 159, 191), to the extent of a 4 per cent, 
 yield. 
 
 Bamberger, Ber., 1900, 33, 3188. 
 
AZO-COMPOUNDS 87 
 
 With two molecules of a diazo-compound, the bisazo- 
 derivative is formed 
 
 OH 
 
 and with three molecules the trisazo-compound 
 
 OH 
 RN/\N 2 R* 
 
 a-Naphthol yields with one molecule of a diazo-salt, first, 
 the monoazo-compound 
 
 and with two molecules the bisazo-compound 
 
 OH 
 >R 
 
 fl 
 
 Resorcinol, like phenol and m-phenylenediamine, can also 
 combine with two molecules of a diazo-compound ; thus with 
 one molecule it yields 
 
 OH OH OH 
 
 * Grandmougin and Freimann, Ber., 1907,40, 2662; Heller and Nfitzel, 
 J. pr. Chem., 1907 [ii], 76, 58. 
 
 t Orndoff and Ray, Ber., 1907, 40, 3211. 
 
88 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 For a more detailed description of the amino- and hydroxy- 
 azo-dyestuffs a larger work must be consulted.* 
 
 5. Bate of formation of amino- and kydroxyazo-com- 
 ponnds. This has been measured by Goldschmidt and his 
 pupils. The case of the formation of methyl-orange from 
 jp-diazobenzenesulphonic acid and dimethylaniline hydro- 
 chloride was studied first, the method of procedure being to 
 withdraw samples of the mixture at given periods of time, 
 and after acidifying, to determine the quantity of diazo- 
 compound present by measuring the nitrogen evolved on 
 boiling. 
 
 The conclusions arrived at were : (1) In the combination of 
 the hydrochloride of a tertiary amine with diazobenzene- 
 sulphonic acid, it is the base liberated from the hydrochloride 
 by hydrolysis that acts with the diazo-compound. (2) Excess 
 of hydrochloric acid retards the combination. (3) The con- 
 centration of the hydrochloride or of the diazo-compound 
 has no influence on the reaction. (4) The combination pro- 
 ceeds at the same rate when other acids of the same strength 
 are used (hydrobromic, nitric, &c.), but much more quickly 
 when weak acids such as acetic, the chloroacetic acids, formic, 
 propionic, levulinic, or lactic acids are employed. Investiga- 
 tions were also made as to the influence of the base used; 
 comparisons of the velocity of formation of the azo-compound 
 between dimethyl-, diethyl-, and dipropyl-aniline show that 
 the replacement of methyl by ethyl lowers, and that of ethyl 
 by propyl increases, the velocity. Dimethyl- and diethyl-m- 
 toluidine combine more rapidly, and dimethyl- and diethyl- 
 m-chloroanilines more slowly, than the corresponding un- 
 substituted compounds. In the formation of hydroxyazo- 
 compounds from phenols and diazo-salts in alkaline solution 
 the active agents are the free phenol liberated by the hydro- 
 lytic action of the water on the alkali salt and the syn- 
 diazo-compound (for an explanation of this see p. 123). An 
 excess of alkali retards the combination, and the more con- 
 
 * See Billow, Chemische Technologic der Azo-Farbstoffe, 1897; Pauli, 
 Synthese der Azo-Farbstoffe, 1904 ; Cain and Thorpe, Synthetic Dyestutfs, 
 1905, p. 47. 
 
AZO-COMPOUNDS 89 
 
 centrated the solution of the phenol and diazo-salt the more 
 time is required for the combination. Similar results were 
 obtained when sodium diazo-oxide (see p. 99) was used instead 
 of the diazo-salt. 
 
 6. Constitution of the hydroxyazo-componnds. The earlier 
 assumptions that the hydroxyazo-compounds possessed the 
 formula R.N:N.K.OH were somewhat shaken by the 
 discovery that, by the action of phenylhydrazine on a- and 
 /S-naphthaquinones, compounds resulted which were identical 
 with those obtained by treating a- and /3-naphthol with diazo- 
 salts,* and it was found that phenylhydrazine and benzo- 
 quinoneoxime yielded a condensation product which was 
 identical with p-hydroxyazobenzene, thus 
 
 C 6 H 6 . N 2 . C 6 H 4 . OH C,H 4 . NH.N : C 6 H 4 : 
 
 ^-Hydroxyazobenzene. Benzoquinonephenylhydrazone. 
 
 The question was attacked for many years chiefly from the 
 purely chemical side. Goldschmidt and his pupils, f McPher- 
 son,J and Hewitt, expressed the opinion that para-hydroxy- 
 azo-compounds were true phenols, and ortho-compounds 
 quinones, whilst Jacobson.|| Meldola,T and Nietzki and 
 Kostanecki ** adhered to the view that both series of com- 
 pounds possessed the phenolic constitution. 
 
 On the other hand, Farmer and Hantzsch,f f from determin- 
 ations of electrical conductivity, and Mohlau and KegelJi from 
 the reactions with carbinols, expressed the opinion that both 
 ortho- and para-hydroxyazo-compounds in the free state were 
 really quinones, the metallic salts, however, being phenolic in 
 character. 
 
 The views generally held about this time, therefore (1900- 
 
 * Zincke, Ber., 1884, 17, 3026; 1887, 20, 3171 ; compare also Lieber- 
 mann, Ber., 1883, 10, 2858. 
 
 t Ber., 1890, 23, 487 ; 1891, 24, 2300; 1892, 25, 1324. 
 
 | Ber., 1895, 28, 2414 ; Amer. Chem. J., 1899, 22, 364 ; Auvers, Ber., 
 1896, 29, 2361 ; 1900, 33, 1302. 
 
 Trans., 1900, 77, 99, 712. || Ber., 1888, 21, 414. 
 
 IT Trans., 1888, 53, 460 ; 1889, 55, 114, 603 ; 1891, 59, 710 ; 1893, 63, 
 923 ; 1894, 65, 834. 
 
 ** Ber., 1890, 23, 3263 ; 1891, 24, 1592, 3977. 
 
 tt Ber., 1899, 32, 3089. JJ Ber., 1900, 33, 2858. 
 
90 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 1903), were that the metallic salts and alkyl ethers of all 
 hydroxyazo-compounds, as well as the acyl-derivatives of 
 >-hydroxyazo-compounds, were true azo- (phenolic) com- 
 pounds. 
 
 It was considered, further, that the free ortho-compounds 
 were quinonoid in constitution, as were also the acid additive 
 products of both series of compounds, for example 
 
 R.NH.N:CH:0 
 
 Opinions as to the constitution of the free para-hydroxy- 
 compounds were, however, divided, as has been shown. 
 
 From this period onwards a number of researches rather 
 tend to show that the phenolic constitution for both the free 
 ortho- and para-compounds is to be adopted. 
 
 Thus the properties of m-hydroxyazophenol were found by 
 Jacobson and Honigsberger to agree closely with those of the 
 para-derivative,* and these chemists concluded that the free 
 para-compounds, as well as their additive compounds with 
 acids, were to be regarded as azo (phenolic) in constitution. 
 
 This view was confirmed by the researches of Borsche,f 
 and Goldschmidt and Low-Beer showed that the earlier 
 opinion of Goldschmidt as to the quinonoid character of the 
 ortho-compounds had been based on incorrect data, and con- 
 cluded that all hydroxyazo-compounds possessed the azo 
 (phenolic) structure. 
 
 Most of the later work has confirmed this conclusion,! 
 although Tuck, from an examination of the absorption spectra, 
 inclines to the view that the free ortho-compounds are quino- 
 noid, whilst the para-compounds, their hydrochlorides, and the 
 hydrochlorides of the ortho-compounds, are phenolic in 
 structure. 
 
 * Ber., 1903, 36, 4093. 
 
 t Annalen, 1904, 334, 143; 1905, 340, 85; 1907, 357, 171. 
 
 | Hewitt and Mitchell, Proc., 1905, 21, 298; Mitchell, Trans., 1905, 
 87, 1229 ; Willstatter and Veraguth, Ber., 1907, 40, 1432 ; Auvers, Ber., 
 1907, 40, 2154. 
 
 Trans., 1907, 91, 449; compare also Hewitt and Mitchell, Trans., 
 1907, 91, 1251. 
 
AZO-COMPOUNDS 91 
 
 7. Mixed azo-componnds. These are represented by the 
 general formula A.N : N.B where A is an aliphatic and B an 
 aromatic group. 
 
 The simplest member of this class, namely, benzeneazo- 
 methane, is best prepared by treating phenylhydrazine with 
 formaldehyde.* It is a yellow, volatile oil, boiling at 150 
 with decomposition. 
 
 The first representative of this class, however, prepared 
 directly from a diazo-compound was obtained by V. Meyer 
 and Ambuhl ; f by the action of diazobenzene nitrate on sodium 
 nitroethane they prepared benzeneazonitroethane 
 
 C 6 H 5 .N 2 .C 2 H 4 .N0 2 . 
 
 This is probably not a true azo-compound, but a hydrazone- 
 derivative having the constitution 
 
 C 6 H 6 .N 2 H:C 2 H 3 .N0 2 . 
 
 Shortly afterwards Friese J described benzeneazonitro- 
 methane as being obtained by treating sodium nitromethane 
 with diazobenzene nitrate, but Bamberger has shown that 
 Friese's compound was nitroformazyl, produced according to 
 the equation 
 
 CH 3 . NO, + 2C 6 H 6 . N 2 . OH = NO, . 
 
 This behaviour of nitromethane is exceptional, as formazyl- 
 derivatives are not produced with the higher homologues of 
 nitromethane ; under certain conditions, however, the simple 
 compound nitroformaldehydrazone, NO 2 . CH : N 2 H.C 6 H 6 , is 
 obtained. 
 
 By the action of diazo-salts on the sodium-derivative of 
 acetoacetic ester V. Meyer obtained a compound which was 
 first considered to be a true mixed azo-compound, thus 
 
 CH 3 . CO.CH(C0 2 Et) . N 2 . C 6 H 5 ,U 
 
 but later he considered that, owing to its supposed insolubility 
 in alkali, the substance possessed a hydrazone structure 
 CH 3 . CO.C(C0 2 Et) : N.NH.C 6 H 5 .1[ 
 
 * Baly and Tuck,Trans., 1906, 89, 986 ; see also Tafel, Ber., 1885, 18, 1742. 
 t Ber., 1875, 8, 751, 1053. | Ber., 1875, 8, 1078. 
 
 Ber., 1894, 27, 155 ; 1900, 33, 2043 ; compare also Oddo and Ampola, 
 Gazzetta, 1893, 23, i. 257, naphthylazonitroehane. 
 || Ber., 1876, 9, 384 ; 1878, 11, 1418 ; 1884, 17, 1928. 
 1 Ber., 1888, 21, 12. 
 
92 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 He adhered to this opinion, although he found that the com- 
 pound was really soluble in alkali.* Kjellin also adopted 
 this view, and noticed that apparently two isomeric condensa- 
 tion products were formed ; these he considered to be stereo- 
 isomeric hydrazones, thus 
 
 CH 3 . CO.C.CO 2 Et CH 3 . CO.C.C0 2 Et 
 
 C 6 H 6 .NH.N N.NH.C 6 H 5 t 
 
 BUlow, however, showed that not only was the compound 
 very readily soluble in alkali, but that it could not be acetyl- 
 ated and was not acted on by benzoyl chloride or methyl iodide, 
 so that the original theory of V. Meyer was correct, and the 
 compound must be regarded as the true azo-derivative 
 
 CH 3 . CO.CH(C0 2 Et).N : N.C 6 H 6 .t 
 
 Billow obtained the same compound from diazobenzene 
 chloride and sodium benzene-i'so-diazo-oxide. 
 
 By the action of a diazo-salt on the sodium-derivative of 
 ethyl methylacetoacetate, the acetyl-group is eliminated and 
 the compound was written CH 3 . CH(C0 2 Et).N:N.C 6 H 5 ,|| 
 the corresponding acid being CH 3 . CH(C0 2 H).N 2 . C 6 H 5 . 
 This, however, was found to be identical with the condensa- 
 tion product formed by the action of phenylhydrazine on 
 pyroracemic acid, CH 3 . C(C0 2 H) : N.NH.C 6 H 5 ,f 
 
 In order to decide which of these formulae was correct 
 Japp and Klingemann ** treated the so-called benzeneazo- 
 acetone f f with sodium ethoxide and ethyl chloroacetate, and 
 reduced the corresponding acid. The substance obtained was 
 anilinoacetic acid, proving that the . CH 2 . C0 2 H group had 
 combined with the nitrogen atom attached to the phenyl- 
 group. The second formula for benzeneazoacetone is there- 
 
 * Per., 1888, 21, 2121. 
 
 t Ber., 1897, 30, 1965; compare also Favrel. Compt. rend.. 1901. 132 ; 
 983 ; 1898, 127, 116. 
 t Ber., 1899, 32, 197. 
 Ber., 1898, 31, 3122. 
 
 j| Japp and Klingemann, Ber., 1887, 2O, 2942, 3284, 3398. 
 1F E. Fischer and Jourdan, Ber., 1883, 16. 2241. 
 ** Trans., 1888, 63, 521, 538. 
 tt V. Meyer and Miinzer, Ber., 1878, 11, 1417, 
 
 CH 8 .CO.CH 2 .N 2 .C 6 H 6 or CH S . CO.CH : N.NH.C 6 H 6 . 
 
AZO-COMPOUNDS 93 
 
 fore the correct one, and the compound is really a mono- 
 hydrazone of pyruvic aldehyde. This was also confirmed by 
 the fact that it yields an osazone with phenylhydrazine. 
 
 This conclusion is, however, not to be applied universally, 
 for when diazobenzene chloride acts on the sodium derivative 
 of acetoacetaldehyde, benzeneazoacetaldehyde, 
 CH 3 . CO.CH(CHO).N 2 . C 6 H 5 , 
 
 is formed, and this, with phenylhydrazine, yields the corre- 
 sponding hydrazone 
 
 CH 3 . CO.CH(CH:N.NH.C 6 H 6 ).N 2 . C 6 H 5 .t 
 
 When diazobenzene chloride acts on acetonedicarboxylic 
 acid in presence of sodium acetate, the bishydrazone of 
 niesoxalaldehyde, CO(CH:N.NH.C 6 H 5 ) 2 , is obtained.:): If, 
 however, the ethyl ester of acetonedicarboxylic acid is sub- 
 jected to the action of >-nitrodiazobenzene, a compound is 
 obtained which reacts partly as a hydrazone and partly as an 
 azo-derivative. This is therefore to be regarded as contain- 
 ing a labile hydrogen atom, indicated by a star in the 
 formula 
 
 H* 
 N0 2 .C 6 H 4 .N.N.C.C0 2 Et 
 
 CO.CH 2 .C0 2 Et. 
 
 When a hydrazone such as is described above, in which the 
 group C 6 H 5 .NH.N:C: is combined with H, CO 2 H, or COR, 
 each of the latter groups can be replaced by the action of 
 diazobenzene in alkaline or acetic acid solution. Thus, by 
 the action of diazobenzene on malonic ester, V. Meyer and 
 Munzer|| also obtained a condensation product which they 
 regarded as 
 
 t Claisen and Beyer, Ber., 1888, 21, 1697 ; compare also Ber., 1892, 
 25, 3190. 
 
 | v. Pechinann and Jenisch, Ber., 1891, 24, 3255 ; v. Pechmann and 
 Vanino, Ber., 1892, 25, 3190. 
 
 Billow and HCpfner, Ber., 1901, 34, 71 ; compare also Billow and 
 Hailer, Ber., 1902, 35, 915. 
 
 I! Loc. cit. 
 
94 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 but which is now considered to be the phenylhydrazone of 
 malonic ester 
 
 If the acid itself is used, f ormazylcarboxylic acid and f ormazyl 
 result f 
 
 C,H 5 . NH.N : Ct' C 6 H 5 . NH.N : 
 
 Formazylcarboxylic acid. Formazyl. 
 
 The latter is also formed by the action of diazobenzene on 
 the ethyl hydrogen salt of phenylhydrazonemalonic acid. 
 
 By the further action of diazobenzene on formazyl, or its 
 carboxylic acid, formazylazobenzene 
 
 / 2 ^6^5 
 
 is obtained, as well as by allowing diazobenzene and acet- 
 aldehyde to interact in alkaline solution. J 
 
 These compounds are usually dark red, crystalline sub- 
 stances, and on reduction they give colourless hydrazones. 
 
 Formazylcarboxylic acid is also obtained by the action of 
 diazobenzene on ethyl acetoacetate under certain conditions ; 
 the first product of the reaction being the hydrazone 
 
 By the action of diazobenzene on ethyl oxalacetate the 
 hydrazone, C 6 H 5 .NH.N:C (C0 2 Et).CO.CO 2 Et, is first formed, 
 and with a further molecule of diazobenzene, ethyl di- 
 phenylformazylformate, C 6 H 6 .N:N.C(N.NH.C 6 H 5 ).C0 2 Et, is 
 produced.! 
 
 Further confirmation of the hydrazone constitution of such 
 condensation products is afforded by the action of diazo- 
 
 * Compare also Favrel, Compt. rend., 1899, 128, 829 ; 1901, 132, 1336. 
 t Compare also Busch and Wolbring, J. pr. Chem., 1905 [ii], 71, 366. 
 t Bamberger and Miiller, Ber., 1894, 27, 147. 
 v. Pechmann, Ber., 1892, 25, 3175. 
 || Bamberger, Ber., 1892, 25, 3201, 3539, 3547. 
 
 1" Rabischong, Butt. Soc. chim., 1904 [iii], 31, 76, 83 ; compare also 
 Favrel, Compt. rend., 1899, 128, 318. 
 
AZO-COMPOUNDS 95 
 
 benzene on ethyl cyanoacetate whereby the phenylhydra- 
 zone of the latter is produced.* 
 
 The question of the constitution of the mixed azo- compounds 
 cannot, however, be said to be finally settled by fixed rules. 
 Some appear to be tautomeric substances, others true hydra- 
 zones, and many undoubtedly true azo-compounds. In all 
 probability the course of the reaction depends on the constitu- 
 tion of the diazo-compound used, the discussion of which is 
 postponed until later (see p. 
 
 * Kriickeberg, J. pr. Chem., 1892 [ii], 46, 579, 47, 591, 49, 321; 
 compare also Uhlmann, ibid. 51, 217 ; Marquardt, ibid. 52, 160; Favrel, 
 Compt. rend., 1900, 131, 190 ; 1907, 145, 194. 
 
 t For mixed bisazo-compounds, see Duval, Compt. rend., 1907,144, 1222. 
 
CHAPTEE XIV 
 
 METALLIC DIAZO-DERIVATIVES. 
 DIAZO-HYDROXIDES 
 
 BY adding a cold saturated solution of diazobenzene nitrate 
 to a large excess of concentrated aqueous potassium hydroxide, 
 evaporating the resulting yellow liquid, and extracting with 
 alcohol, Griess obtained a substance containing potassium, to 
 which he gave the formula C 6 H 5 N 2 . OK. 
 
 When this compound was treated with acetic acid a yellow 
 oil was obtained which Griess regarded as free diazobenzene, 
 C 6 H 4 N 2 . 
 
 The potassium compound was examined by Curtius,* who 
 found, however, that it contained only two-thirds of the 
 nitrogen required by the above formula. Also when diazo- 
 benzene sulphate was neutralized with barium hydroxide and 
 the mixture extracted with ether, a yellow substance was 
 obtained which melted at 3. This contained only two atoms 
 of nitrogen to three benzene nuclei, although no nitrogen had 
 been evolved as gas. 
 
 Griess's product was shown later by Bamberger to consist 
 chiefly of the ^so-salt. 
 
 A very important addition to the chemistry of the metallic 
 diazo-derivatives was made by Schraube and Schmidt in 
 1894.f These chemists found that when a 10 per cent, 
 solution of ^9-nitrodiazobenzene chloride was quickly added to 
 an 18 per cent, solution of sodium hydroxide at 50-60, golden- 
 yellow plates separated which no longer combined with 
 /3-naphthol, and which they considered to be sodium p-mtro- 
 phenylnitrosoamine, N0 2 . C 6 H 4 . NNa.NO. 
 
 When an ice-cold, aqueous solution of this was treated with 
 
 * Ber., 1890, 23, 3035. t JBer., 1894, 27, 514. 
 
METALLIC DIAZO-DERIVATIVES 97 
 
 acetic acid, a pale yellow precipitate was obtained which was 
 regarded as the free ^-nitrophenylnitrosoamine, 
 
 N0 2 .C 6 H 4 .NH.NO; 
 
 this, like the sodium salt, did not couple with alkaline 
 /9-naphthol. When hydrochloric acid was substituted for acetic 
 acid, >-nitrodiazobenzene chloride was slowly regenerated. 
 
 On treatment with methyl iodide, the sodium salt gave the 
 nitrosoamine of p-nitromonomethylaniline 
 N0 2 .C 6 H 4 .NMe.NO. 
 
 Schraube and Schmidt also investigated the properties of 
 Griess's potassium salt, and showed that it differed from the 
 compound described by them in that it coupled with alkaline 
 /3-naphthol. On being heated with concentrated aqueous 
 potassium hydroxide, however, it was converted into the 
 potassium salt of phenylnitrosoamine, C 6 H 5 .NK.NO, which 
 no longer combined with /3-naphthol, and with methyl iodide 
 gave the nitrosoamine of methylaniline. This potassium salt, 
 on neutralization with acetic acid, gave a colourless oil which 
 combined with /3-naphthol solution, and on adding an excess of 
 acetic acid to the oil, a solution of diazobenzene acetate was 
 obtained. 
 
 From this work Schraube and Schmidt drew the following 
 conclusions : 
 
 (1) The alkali salts of diazobenzene can exist in two forms, 
 namely, the diazo-metallic salts, C 6 H 5 . N : N.OR, and the 
 nitrosoamines, C 6 H 5 . NR.NO (R denoting the metal)^ 
 
 (2) Free phenylnitrosoamine, C 6 H 5 . NH.NO, does not exist, 
 as a solution of its non-combining sodium salt, when acidified 
 with acetic acid, immediately gives an azo-dyestuff with 
 /J-naphthol. 
 
 (3) ^9-Nitrodiazobenzene appears to exist only in the 
 nitrosoamine form, and its alkali salt does not exist in the 
 ' oxime ' condition. 
 
 These conclusions, as will be shown, do not truly represent 
 the course of the reactions (see p. 98). 
 
 In a paper published shortly after the appearance of 
 Schraube and Schmidt's work, Bamberger stated that he 
 had earlier discovered a derivative of /3-naphthylamme cor- 
 
 H 
 
98 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 responding to the formula C 10 H 7 .NH.NO, which did not 
 couple with /3-naphthol, but did so after treatment with a 
 mineral acid.* 
 
 He considered that the influence of the latter was to effect 
 a transformation into the isomeric diazo-compound, thus 
 C 10 H 7 .NH.NO -> C 10 H 7 .N:N.OH. 
 
 Confirmation of this view was afforded by von Pechmann 
 and Frobenius, who stated uhat the silver salt of ^-nitro- 
 phenylnitrosoamine, when treated with methyl iodide, yielded 
 an oxygen-ether of ^9-nitrodiazobenzene 
 
 N0 2 .C 6 H 4 .N:N.O.CH 3 . 
 
 The conclusion is emphasized, therefore, that the hydroxide 
 corresponding with these compounds exhibits the phenomenon 
 of tautomerism.f That is to say, that the hydroxide can 
 act either as 
 
 N0 2 . C0H 4 . N; N.OH or N0 2 . C 6 H 4 . NH.NO. J 
 
 It was thus established that two isomeric forms of the 
 metallic diazo-compounds exist ; the modification described by 
 Schraube and Schmidt may be called the stable or iso- 
 modification, and the labile or normal form is that which 
 couples with phenols mucji more readily than its isonieride. 
 
 Most of the metallic diazo-compounds exist in these two 
 modifications, but the presence of negative groups in the 
 aromatic nucleus greatly diminishes the stability of the 
 normal modification. Although, for the purpose of defining 
 these isomeric compounds, it has been necessary to mention : 
 the constitutions which were assigned to them at the time of i; 
 their discovery, the subsequent developments of the views on 
 this subject have been so extensive, and the discussion so 
 prolonged, that an account of this must be postponed. We 
 
 * Ber>, 1894, 27, 679. 
 
 t Per., 1894, 27, 672 ; compare also Bamberger, ibid., 679. 
 
 | Although Hantzsch would not accept the views of Bamberger and 
 v,on Pechmann, yet he arrived at this conclusion from his own work 
 some years later (p. 151). 
 
 The presence of alkali has a great effect on the combining power 
 of the tw.o isomerides : Schraube and Schmidt had a large excess of alkali 
 present when they noticed that the stable form did not combine, but 
 when less is used it does combine, although much more slowly than the 
 labile form. 
 
METALLIC DIAZO-DERIVATIVES 99 
 
 shall therefore proceed to a description of some of the more 
 important compounds of this class. 
 
 Potassium benzenediazo-oxide (normal, labile, or syn-sali). 
 10 c.c. of a 15 per cent, solution of diazobenzene chloride 
 are dropped slowly into a mixture of 140 grams of potassium 
 hydroxide and 60 grams of water cooled to 5. The tempera- 
 ture is allowed to rise to 15-20, whereby the potash becomes 
 completely dissolved, and the precipitated potassium benzene- 
 diazo-oxide is collected. This is pressed on porous porcelain. 
 One gram of the crude product is now dissolved in 3 c.c. of 
 absolute alcohol at 5, the solution quickly filtered, and 8-10 
 times its volume of dry ether added. The salt is obtained in 
 this way in snow-white, silky needles, which are very hygro- 
 scopic and soon become pink.* 
 
 sT/^-Diazo-oxides are also obtained by treating nitroso- 
 acylanilides with potassium hydroxide,f 
 
 Ar.N(NO)Ac + 2KOH = Ar.N : N.OK + KOAc + H 2 0, 
 and by the reduction of salts of diazoic acid, 
 
 Ar.N 2 O.OK + 2H = H 2 Q + Ar.N : N.OK. 
 
 When the normal salts are treated with acids the correspond- 
 ing hydroxides are not formed, but the yellow, explosive 
 diazo-anhydrides are produced (see p. 148). 
 
 Potassium benzenediazo-oxide (iso-, stable, or anti-salt). 
 This is obtained by heating the strongly alkaline solution of 
 the diazo-chloride to 130-140 until the product no longer 
 combines with /3-naphthol (compare p. 98). 
 
 Another method of preparing the ^so-metallic compounds 
 consists in treating o- or ^9-hydroxybenzylated nitrosoaryl- 
 derivatives of the type NO.NAr.CH 2 . C 6 H 4 . OH with very 
 dilute aqueous potassium hydroxide 
 
 OH.C 6 H 4 . CH 2 . N(C 6 H 5 ).NO + KOH 
 
 = OH.C 6 H 4 . CH 2 .OH + C 6 H 5 . NK.NO. 
 
 The potassium -iso-diazo-oxide is formed together with 
 hydroxy benzyl alcohol. J 
 
 * Bamberger, Per., 1896, 29, 461. 
 t Bamberger, Ber., 1894, 27, 915. 
 I Bamberger and Miiller, Annalen, 1900, 313, 97. 
 H 2, 
 
100 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 iso-Diazo-oxides are also formed by the reduction of iso- 
 diazoic acids, and by heating secondary nitrosoamines with 
 potassium hydroxide.* 
 
 When the potassium ^so-diazo-oxides are treated with acetic 
 or mineral acids, the corresponding hydroxides are obtained, 
 which, with alkalis, regenerate the metallic iso-salt. 
 
 The hydroxides of m>-diazobenzene, ^so-diazo-p-toluene, 
 p-chloro- and p-bromo-^so-diazobenzene, a- and /3-^so-diazo- 
 naphthalene, and potassium ^so-diazobenzenesulphonate are 
 all colourless, whilst the hydroxides of o- and p-mtico-iso- 
 diazobenzene are yellow, f 
 
 ^so-Diazobenzene hydroxide is a colourless oil which is 
 readily soluble in ether. It is very unstable. 
 
 The hydroxides of the remaining substances mentioned 
 above are white crystalline solids.t 
 
 The action of alkalis on diazo-salts sometimes, however, 
 proceeds differently . Thus 2:4:6-tribromodiazobenzene gives 
 rise to 3:5-dibromo-o-benzoquinonediazide 
 
 Br 
 
 6 
 
 (see p. 67), and o-diazotoluene furnishes indazol 
 
 C H <!L 
 
 (see p. 31). 
 
 Diazobenzene hydroxide (diazonium hydroxide). This 
 hydroxide does not correspond with either of the two fore- 
 going potassium salts according to Hantzsch, although 
 Bamberger regards it as the hydroxide derived from the ; 
 labile salt (see p. 144). 
 
 For the preparation, 0-7 gram of pure diazobenzene chloride 
 is dissolved in about 50c.c. of ice-cold water, and about 
 
 * Bamberger, Ber., 1894, 27, 1179; Ber. t 1900, 33, 1957. 
 
 t Bamberger, Ber., 1896, 29, 446. 
 
 j Bamberger, Ber., 1896, 29, 1383. i 
 
 Bamberger, Annalen, 1899, 305, 289. 
 
METALLIC DIAZO-DEEIVATIVES 101 
 
 0-8 gram (the theoretical amount is 0-62) of freshly-precipi- 
 tated moist silver oxide mixed with ice, added, and the whole 
 shaken for five minutes. The filtrate consists of a practically 
 pure solution of the hydroxide. 
 
 This solution has a strongly alkaline reaction, and combines 
 instantly with /S-naphthol. The pure solution is colourless. 
 The hydroxide is also obtained by treating a solution of the 
 diazo-sulphate with the calculated amount of baryta.* The 
 solution is very unstable, even at it decomposes and be- 
 comes dark coloured. 
 
 Other diazo-hydroxides are prepared in a similar manner. 
 
 Reduction of the metallic diazo-oxides. Both series of 
 diazo-oxides, when treated with sodium amalgam in presence 
 of excess of sodium hydroxide, yield the corresponding 
 hydrazine.f 
 
 Oxidation of the metallic diazo-compounds. Aromatic 
 diazoic-acids. When an alkaline solution of diazobenzene is 
 oxidized by potassium ferricyanide, a substance is obtained 
 to which the name benzenediazoic acid is given 
 
 C 6 H 5 .N 2 2 H. 
 
 It is formed in white leaflets, melting at 46-46-5, sparingly 
 soluble in water, but readily so in organic solvents or alkalis. 
 It forms well-defined salts.J 
 
 Potassium permanganate may also be used as the oxidizing 
 agent. 
 
 A better yield is obtained by oxidizing potassium benzene- 
 ^so-diazo-oxide with potassium ferricyanide. || The compound 
 may also be prepared by treating diazobenzene perbromide 
 with sodium hydroxide,^ or by the action of nitrogen pen- 
 toxide on aniline in ethereal solution at 20.*"* 
 
 When benzenediazoic acid is slowly heated, or when it is 
 
 * Hantzsch, Ber., 1898, 31, 340. t Hantzsch, Ber., 1898, 31, 340. 
 I Bamberger and Storch, Ber.., 1893, 26, 471 ; Bamberger, ibid., 1894, 
 27, 359. 
 
 Bamberger and Landsteiner, Ber., 1893, 26, 482. 
 || Bamberger, Ber., 1894, 27, 914. 
 I Bamberger, Ber., 1894, 27, 1273. 
 ** Bamberger, Ber., 1894, 27, 584. 
 
102 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 treated with mineral acids, it undergoes intramolecular change 
 with formation of a mixture of o- and ^-nitroaniline. 
 
 When heated with potassium hydroxide to about 290 it is 
 decomposed into aniline and nitric or nitrous acid. By gentle 
 reduction with zinc and acetic acid, diazobenzene is formed, 
 and with sodium amalgam, phenylhydrazine is produced.* 
 
 On account of the conversion into nitroaniline, Bamberger 
 regarded benzenediazoic acid as the simplest aromatic nitra- 
 mine, or phenylnitramine, C 6 H 5 .NH.NO 2 , and represented 
 the change into nitroaniline as follows 
 NH.N0 2 NH 
 
 The proof of this constitution was found in the study of the 
 action of hypochlorite on the diazoic acid, for a chloro- 
 derivative was obtained which gave the characteristic re- 
 actions of a chloroimide, and underwent molecular change 
 even more readily than the parent compound, forming p-chloro- 
 0-nitroaniline. 
 
 The constitution of the chloro-compound is therefore 
 C 6 H 5 .NCl.N0 2 .t 
 
 Benzenediazoic acid forms two methyl esters ; with methyl 
 iodide the sodium salt gives the a-ester, C 6 H 5 . NMe.N0 2 , and 
 the silver salt yields the /?-ester, C 6 H 5 .N:NO 2 Me. 
 
 Benzenediazoic acid is therefore, as Bamberger had shown 
 to be the case with diazobenzene hydroxide, a tautomeric 
 substance, thus 
 
 C 6 H 6 .NH.N0 2 ^ C 6 H 5 .N:N0 2 H. 
 
 This conclusion, after a considerable amount of discussion,! 
 was confirmed by Hantzsch, who showed that the compound 
 reacted as a pseudo-acid. 
 
 * Ber., 1894, 27, 365. t Ber., 1894, 27, 361. 
 
 I Bamberger, Ber., 1894, 27, 2601 ; 1897, 30, 1248 ; Annalen, 1900, 311, 
 99; Hantzsch, Ber., 1894, 27, 1729; 1898, 31, 177 ; 1899, 32, 1722. 
 Ber., 1902, 35, 258. 
 
OHAPTEE xv 
 
 DIAZO-COMPOUNDS OF THE ALIPHATIC SERIES* 
 
 1. Preparation. The amines of the aliphatic series do 
 not react with nitrous acid as do those of the aromatic series. 
 Only in certain cases is there a departure from the usual 
 reaction of substitution of the amino- by the hydroxyl-group, 
 and then the product has not, as might be expected, a compo- 
 sition similar to that of an aromatic diazo-salt, but the nitrogen 
 atoms are each linked to the aliphatic nucleus, thus 
 
 The reason of this difference in behaviour will be explained 
 in the discussion of the constitution of the aromatic diazo-salts 
 (see p. 167). 
 
 The first number of the series was obtained by Schiff and 
 Meissen in 1881, who prepared diazocamphor from camphor- 
 imide.f This diazo-compound was also obtained by Angeli by 
 the action of nitrous acid on aminocamphor. J 
 
 The principal worker in this field of research is, however, 
 Curtius, who, a little later, succeeded in diazotizing the ethyl 
 ester of glycocoll, or ethyl aminoacetate, a reaction which 
 proceeds in two stages ; the first stage is the formation of the 
 nitrite of the aliphatic amine, 
 
 HC1.NH 2 . CH 2 . CO 2 Et + NaNO 2 
 
 = NaCl + HN0 2 . NH 2 . CH 2 . C0 2 Et, 
 
 and this then loses water with production of the diazo- 
 compound 
 
 HN0 2 . NH 2 . CH 2 . C0 2 Et = 2H 2 O + N 2 : CH.C0 2 Et. 
 
 * Compare also Curiius,Diazoverbindungen der Fettreihe. 1886; Demetre 
 Vladesco, Sur les composts diazoiques de la serie grasse, 1891. 
 t Gazzetta, 1881, 11, 171. 
 
 1 See also Angeli, Ber., 1904, 37, 2080, footnote. 
 Ber., 1883, 16, 2230; J. pr. Chem., 1888 [ii], 38, 401. 
 
104 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 The free fatty acids do not yield diazo- derivatives, as these are 
 immediately decomposed, so that the esters, amides, &c., must 
 be used. The constitution of diazoacetic ester is proved by (1 ) 
 the ready substitution of the two atoms of nitrogen by two 
 atoms of iodine, yielding di-iodoacetic ester, 
 
 ,N 
 
 ^^ 
 
 i 
 
 CH< || CHI 2 
 
 N+I 2 =| 
 2 Et CO 2 Et 
 
 and (2) the reduction to ammonia and glycocoll 
 
 CH 
 
 C0 
 
 | +NH 3 
 
 Et C0Et 
 
 Similarly diazosuccinic acid yields ammonia and aspartic acid 
 
 / N 
 C0 2 Et . (\ || C0 2 H . CH.NH 2 
 
 | X N + 3H 2 = | +NH, 
 
 CH 2 .C0 2 H CH 2 .C0 2 H 
 
 The preparation of diazoacetic ester is carried out as follows* : 
 Five grams of sodium acetate are dissolved in two litres of 
 water in a ten-litre separating funnel ; to this solution one 
 kilo of the finely powdered hydrochloride of ethyl aminoacetatet 
 is added, and then 750 grams of sodium nitrite. The mixture 
 is shaken until the temperature has fallen to about 0. Five 
 c.c. of ten per cent, sulphuric acid and half a litre of ether are 
 then added and the whole again well shaken. During this 
 period the gradual solution of the still undissolved salts cools 
 the mixture and prevents the reaction from becoming too 
 violent. As soon as the action slackens, the ethereal solution 
 of ethyl diazoacetate is run off, fresh ether added, and ten 
 per cent, sulphuric acid run in from time to time in small 
 quantities until red fumes are evolved. The ethereal solution 
 is then run off, added to that already obtained, washed with 
 small quantities of dilute sodium carbonate solution until the 
 washings assume a deep yellow colour and have an alkaline 
 
 * Silberrad, Trans., 1902, 81, 600. 
 
 t For the preparation of this compound see Hantzsch and Silberrad, 
 Ber., 1900, 33, 70. 
 
DIAZO-COMPOUNDS OF ALIPHATIC SERIES 105 
 
 reaction. The ethereal solution is dried by shaking with fused 
 calcium chloride, and freed from ether on the water-bath. 
 The yield amounts to 770 grams, or 947 per cent, of the 
 theoretical quantity. 
 
 2. Properties of diazoacetic esters. The esters of diazo- 
 acetic acid are liquids which solidify at very low temperatures. 
 They are citron -yellow, and possess a characteristic odour. On 
 being warmed to 100 the colour changes to a deep orange ? 
 but, on cooling, the original colour reappears. The esters boil 
 without decomposition under very low pressures, and even at 
 the ordinary pressure by quick distillation over the free flame 
 the greater part of the liquid passes over unchanged ; the rest 
 decomposes with slight detonation, and forms a thick, white 
 cloud. 
 
 The ethyl ester is extraordinarily volatile, and rapidly 
 vaporizes in a vacuum over sulphuric acid. 
 
 The esters distil mostly unchanged with steam, the volatility 
 increasing with the weight of the ester radical, whilst the 
 solubility in water at the same time decreases. 
 
 The diazo -compounds of the fatty esters are miscible in all 
 proportions with alcohol, ether, benzene, light petroleum, &c. 
 
 Methyl diazoacetate, N 2 : CH.C0 2 . CH 3 , boils at 129 
 under a pressure of 721 mm. Its sp.gr. is 1'139 at 21. It is 
 moderately soluble in water, and has a neutral reaction. 
 
 Ethyl diazoacetate, N 2 :CH.CO 2 .C 2 H 5 , crystallizes in a 
 mixture of ether and solid carbon dioxide to a crystalline 
 mass, which melts at -24. It boils at 143-144 under 721 mm. 
 pressure, and its sp. gr. is 1-073 at 22. On gentle warming it 
 takes fire and burns quickly with a luminous flame. It does 
 not explode by concussion, but on adding concentrated sul- 
 phuric acid a violent explosion occurs ; this also takes place on 
 heating it with certain organic nitro-compounds such as 
 nitroaldehydes. 
 
 The ester is sparingly soluble in water, and has a neutral 
 reaction. By heating diazoacetic ester near its boiling-point 
 nitrogen is evolved, and finally fuinaric ester remains 
 
 2 N 2 : CH.C0 2 Et = 2N 2 + C 2 H 2 (C0 2 Et) 2 . 
 
106 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 As an intermediate product in the reaction there is formed 
 azinsuccinic ester * 
 
 H.C0 2 Et 
 
 CH.CO 2 Et 
 
 Amyl diazoacetate, N 2 : CH.C0 2 . C^H lt , boils at 160 under 
 a pressure of 721 mm. It is insoluble in water, and has a 
 neutral reaction. 
 
 When diazoacetic esters are mixed with aqueous potassium 
 hydroxide or baryta water the corresponding metallic salts 
 are formed. These are stable, however, only in cold, aqueous 
 solution, and Curtius was unable to isolate them or to prepare 
 the free acid by treatment with mineral or organic acids. 
 Thiele, however, by another method, succeeded in preparing 
 the pure sodium salt (see p. 109). 
 
 With concentrated aqueous ammonia, diazoacetic esters 
 yield diazoacetamide, N 2 ; CH.CO.NH 2 , which crystallizes from 
 warm alcohol or water in large, gold-yellow, transparent 
 prisms. These crystals melt at 114 with copious evolution 
 of gas, 
 
 3. Reactions of the aliphatic diazo-compounds. The 
 
 reactions of the fatty diazo-compounds are very similar to those 
 of the aromatic ; thus, with water, nitrogen is evolved, and the 
 corresponding hydroxyester produced. 
 
 The reaction in the case of ethyl diazoacetate has been 
 quantitatively studied by Fraenkel.f As in the case of the 
 diazo-compounds of the aromatic series the reaction is 
 unimolecular, and the rate is proportional to the concentration 
 of the hydrogen ions, these exerting a catalytic influence. 
 
 The presence of neutral salts destroys the regularity of the 
 decomposition and introduces secondary reactions. 
 
 The other decompositions of diazoacetic ester are as follows 
 
 With alcohol it yields ethylglycollic ester. 
 
 With picric acid it yields picrylglycollic ester. 
 
 With benzaldehyde it yields benzoylglycollic ester. 
 
 * Curtius, Ber., 1885, 18, 1302. 
 
 t Zeitsch. physical. Chem., 1907, 60, 202. 
 
DIAZO-COMPOUNDS OF ALIPHATIC SERIES 107 
 
 With hydrochloric acid it yields monochloroacetic ester. 
 With iodine it yields di-iodoacetic ester. 
 A concentrated solution of hydrofluoric acid yields with 
 diazoacetic ester, diglycollic ester 
 
 2CH : N 2 . C0 2 R + H 2 O = O/ >* + 2N 
 
 On reduction, diazoacetic esters yield the original amino- 
 compound, a hydrazine being formed as intermediate product, 
 thus 
 
 N 2 : CH.C0 2 R + 2H 2 = NH 2 . NH 2 . CH 2 . C0 2 R. 
 
 A very singular reaction takes place with concentrated 
 aqueous alkalis. An acid is produced having the same com- 
 position as diazoacetic acid, but possessing a greater molecular 
 weight. This was considered by its discoverer * to be triazo- 
 acetic acid, composed of three molecules of diazoacetic acid, 
 but it was later shown that the substance was really bis- 
 diazoacetic acid 
 
 With potassium sulphite, diazoacetic ester gives the potas- 
 sium salt of ethyl sulphohydrazimethylenecarboxylic acid 
 
 ,NH 
 
 C0 2 Et.CH< | 
 N 
 
 When ethyl diazoacetate is treated with potassium or 
 sodium ethoxide, the corresponding salt 
 
 NK 
 
 b N 
 of ethyl 2\so-diazoacetate 
 
 C0 2 Et. 
 is obtained. This ester is an oil which does not dissolve in 
 
 * Curtius, J.pr. Chem., 1889 [ii], 39, 116. 
 
 t Hantzsch and Silberrad, Ber., 1900, 33, 58; compare also Curtius, 
 Darapsky, and Mflller, Ber., 1907, 40, 1176, 1194. 
 I Von Pechmann, Ber., 1895, 28, 1847. 
 
108 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 water, and, unlike the diazoacetate, does not form an additive 
 compound with sulphites.* 
 
 The simplest diazo-compound of the aliphatic series, namely, 
 diazomethane, 
 
 N 
 
 was prepared by von Pechmann in 1894.f This substance, 
 which is a yellow gas, is obtained by warming nitrosoacyl- 
 derivatives of methylamine, CH 3 .NAc.NO, with aqueous or 
 alcoholic alkalis. One part by volume of nitrosourethane 
 (from 1 to 5 c.c.), together with 30-50 c.c. of pure ether, and 
 1-2 parts by volume of 25 per cent, methyl-alcoholic potassium 
 hydroxide, are warmed in a small flask fitted with a reflux 
 condenser on the water-bath. J The mixture becomes yellow, 
 and a yellow gas is evolved. The heating is continued until 
 the solution becomes colourless. 
 
 The distillate consists of an ethereal solution of diazo- 
 methane, the yield of which is about 50 per cent, of the 
 theory. 
 
 Diazomethane is also obtained by the interaction of 
 hydroxylamine and dichloromethylamine 
 
 CH 3 . NC1 2 + H 2 N.OH = 2HC1 + H 2 + CH 2 N 2 . 
 
 The disulphonic acid of diazomethane 
 
 (S0 3 H) 2 C^| 
 
 is obtained in a remarkable manner. When potassium cyanide 
 is treated with an aqueous solution of potassium bisulphite in 
 presence of potassium hydroxide, and the product acidified, 
 ammomethanedisulphonic acid results, which, on treatment 
 with nitrous acid, furnishes the corresponding diazo- 
 derivative.il 
 
 * Hantzsch and Lehmann, Ber., 1901, 34, 2506. 
 
 t Ber., 1894, 27, 1888. J Ber., 1895, 28, 855. 
 
 Bamberger and Renauld, Ber., 1895, 28, 1682 ; compare also Thiele 
 and. Meyer, Ber., 1896, 29, 961. 
 
 || Von Pechmann and Manck, Ber., 1895, 28, 2374 ; von Pechmann, 
 Ber., 1896, 29, 2161. 
 
DIAZO-COMPOUNDS OF ALIPHATIC SERIES 109 
 
 Phenyldiazomethane is obtained by decomposing potassium 
 benzyldiazo-oxide (p. 110) with water * 
 
 C 6 H 6 . CH 2 . N : N.OK = C 6 H 5 . CH< || + H 2 O 
 
 N 
 
 It is a dark red-brown oil which has only a faint odour, and 
 is slightly volatile. It decomposes when rapidly heated. 
 When distilled under the ordinary pressure it is mostly 
 decomposed with formation of stilbene 
 
 2C 6 H 5 . CH : N 2 = C 6 H 5 . CH : CH.C 6 H 6 + 2N 2 . 
 
 When warmed with water it yields benzyl alcohol, and in its 
 other reactions it resembles diazomethane. 
 
 Another interesting method of obtaining aliphatic diazo- 
 compounds is that due to Traube, which consists in treating 
 the sodium or lead salt of iso-nitraminoacetic acid, 
 
 H0 2 N 2 .CH 2 .C0 2 H, 
 
 with sodium amalgam at 0. Reduction takes place, and the 
 sodium salt of diazoacetic acid is produced.f A metallic salt 
 of diazoacetic acid was thus isolated in the pure state for the 
 first time. 
 
 By treating aminoguanidine nitrate with nitrous acid it 
 was thought that the corresponding diazoguanidine nitrate 
 was formed, J but this was later shown to be a derivative of 
 carbaminoiminoazoimide 
 
 / 
 
 NH 2 .C(NH).N< 
 
 4. Metallic diazo-componuds of the aliphatic series. 
 
 The compounds of this class were obtained by Hantzsch and 
 Lehmann|| by treating nitrosoalkylurethanes with concen- 
 trated potassium hydroxide solution or ethereal potassium 
 ethoxide, thus 
 
 * Hantzsch and Lehmann, Ber., 1902, 35, 897. 
 t Ber., 1896, 29, 667. 
 
 I Thiele, Annalen, 1892, 270, 1 : E. P. 2194 of 1892 ; Thiele and 
 Osborne, Ber., 1897, 30, 2867 ; Annalen, 1899, 305, 64. 
 Hantzsch and Vagt, Annalen, 1901, 314, 339. 
 H Ber., 1902, 35, 897. 
 
110 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 N 5 N - OK + H 
 
 . N : N.OK + EtOH + KEtCO 3 
 
 These salts are highly unstable ; with water they decompose 
 with explosive violence. The metallic salt obtained from 
 nitrosomethylurethane forms diazomethane, and that derived 
 from nitrosobenzylmethane gives phenyldiazomethane. 
 
 Potassium methyldiazo-oxide, CH 3 . N; N.OK + H 2 O, separ- 
 ates in white crystals when nitrosomethylurethane is gradually 
 added to a very concentrated aqueous solution of potassium 
 hydroxide at 0. As excess of alkali is used, the reaction 
 proceeds according to the equation 
 
 = K 2 CO a + C 2 H 5 . OH + CH 3 . N : N.OK, H 2 0. 
 
 Potassium benzyldiazo-oxide, C 6 H 5 . CH 2 . N : N.OK + H 2 3 
 is obtained in a similar manner from nitrosobenzylurethane. 
 
 On decomposition with water it yields, as chief products, 
 phenyldiazomethane and potassium hydroxide, and as second- 
 ary products, benzyl alcohol and nitrogen, thus 
 
 - 6 
 C 6 H 5 .CH 2 .N:N.OH( 
 
 \C 6 H 5 .CH 2 .OH + N 2 
 
 It is found that only esters of a-amino-acids yield diazo- 
 esters ; /?- and y-amino-esters, on the other hand, do not form 
 diazo-compounds, and an a/3-diamino-ester therefore yields 
 an a-diazo-/3-hydroxy-ester. 
 
 Further, only fatty compounds in which the amino-group, 
 carbonyl, and at least one hydrogen atom are attached to the 
 same carbon atom, yield diazo-compounds with nitrous acid;"* 
 thus, for example, diazoacetophenone is obtained by adding 
 sodium nitrite solution to an aqueous solution of the hydro- 
 chloride of aminoacetophenone, and then dropping acetic acid 
 into the cold solution. A solid substance separates, which is 
 
 * Curtius and Muller, Ber.. 1904, 37, 1261 ; compare also Angeli, Ber., 
 1904, 37, 2080. 
 
DIAZO-COMPOUNDS OF ALIPHATIC SERIES 111 
 
 washed with sodium carbonate solution and crystallized from 
 light petroleum. Yellow needles are obtained, possessing the 
 constitution 
 
 X N * 
 C 6 H 5 .CO.CH<J 
 
 X N 
 
 5. Diazoamino-compounds of the aliphatic series. The 
 
 simplest representative of the aliphatic diazoamino series, 
 namely, diazoaminomethane, is obtained by the action of 
 magnesium methyl iodide on methylazoimide f and decom- 
 position of the resulting compound with water. J 
 Its formation is represented by the equations 
 
 CH 3 . Mgl + CH 3 . N 3 = CH 3 . N : N.N(CH 3 ).MgI 
 
 CH 8 . N : N.N(CH 3 ).MgI + ELO 
 
 = CH 3 .N : N.NH.CH 3 + MgI(OH). 
 
 Diazoaminomethane is a colourless liquid, boiling at 92, 
 which solidifies when immersed in a mixture of solid carbon 
 dioxide and ether; the solid melts at 12. When heated 
 quickly it explodes, and it decomposes with acids according to 
 the equation 
 
 N 3 (CH 3 ) 2 H + 2HC1 = CH 3 C1 + N 2 + NH 2 . CH 3 , HC1. 
 
 * Angeli, Ber., 1893, 26, 1715 ; Gfizzetta, 1895, 25, ii, 494. 
 t Prepared by treating sodium azoimide with methyl sulphate (Dimroth 
 and Wislicenus, Ber., 1905, 38, 1573). 
 J Dimroth, Ber., 1906, 39, 3905. 
 
CHAPTER XVI 
 
 CONSTITUTION OF THE DIAZO-COMPOUNDS 
 
 ON account of the extraordinary controversy which has 
 raged round the subject of the constitution of the diazo- 
 compounds, it has appeared more advisable to deal with this 
 question separately and more fully than would otherwise 
 have been possible. 
 
 As will have been gathered from the account already given, 
 the first question calling for attention is that of the constitution 
 of the diazo-salts, and then naturally follows that of the two 
 classes of isomeric metallic diazo-derivatives. 
 
 1. Constitution of the diazo-salts according to Griess. 
 The first attempt to assign a constitutional formula to a 
 diazo-salt was made by Griess, who gave to diazobenzene 
 nitrate the formula C 6 H 4 N 2 , HNO 3 .* 
 
 Griess considered that in a diazo-compound two atoms of 
 hydrogen of the benzene nucleus were substituted by two 
 atoms of (monoatomic) nitrogen. 
 
 In 1859 Wurtz f suggested that each atom of nitrogen was 
 tervalent, and that a bivalent group N 2 " was present. 
 
 Erlenmeyer { and Butleroff developed this idea and gave 
 to diazobenzene the formula 
 
 C,H/ 
 
 which Griess adopted. || 
 
 The idea that two hydrogen atoms of the benzene nucleus 
 were substituted by nitrogen was still present. 
 
 Griess also, in the same year, considered that a possible 
 formula for diazobenzene nitrate was C 6 H 4 :NjN, HN0 3 . 
 
 * Phil. Trans., 1864, 154, 667. 
 
 + Repert. de Chimie pure, 1858-9, 1, 348. 
 
 t Zeitsch.f. Chem., 1861, 176; 1863, 678. 
 
 Zeitsch.f. Chem., 1863, 511. || Ber., 1874, 7, 1618. 
 
CONSTITUTION OF THE DIAZO-COMPOUNDS 113 
 
 About this time Griess discovered that the diazoamino- 
 compound obtained from aniline and bromodiazobenzene 
 nitrate was identical with that prepared from bromoaniline 
 and diazobenzene nitrate, and therefore put forward for 
 diazoaminobenzene the symmetrical formula 
 
 C 6 H 4 : NH.NH.NH : C 6 H 4 . 
 
 It is interesting to notice in passing that in the aliphatic 
 series the diazo-group is actually united with two valencies 
 of carbon, thus 
 
 /N 
 diazomethane CH 2 \ 
 
 X N 
 
 2. Constitution of diazo-compounds according to Kekule. 
 In 1886 Kekule' advanced the view that diazo-compounds 
 contained the group .N : N., and considered that the behaviour 
 of these compounds was not in accord with Griess's idea that 
 two atoms of hydrogen in the benzene nucleus were displaced;* 
 thus the formation of diazobenzene nitrate proceeded, according 
 to Kekule', as follows 
 
 C 6 H 5 .NH 2) HN0 3 - C 6 H 6 .N:N.N0 3 . 
 
 Kekule"s opinion that, in diazobenzene nitrate, there were 
 five, and not four, atoms of hydrogen attached to the benzene 
 nucleus was proved by the fact that penta-substituted deriva- 
 tives of aniline could be converted into the corresponding diazo- 
 salt without suffering any loss of their substituents.f 
 The formulae which Kekule' introduced were thus 
 Free diazobenzene . . . C 6 H 5 . N : N.OH 
 Diazobenzene sulphate . . C 6 H 5 . N : N.HSO 4 
 Diazobenzene platinichloride . (C 6 H 5 . N : N.Cl) 2 PtCl 4 
 Potassium salt . . . C H 5 .N:N.OK 
 Diazoaminobenzene . . C 6 H 5 . N : N.NH.C 6 H 5 
 Diazobenzene perbromide . C 6 H 5 N : NBr, Br 2 
 or . C 6 H 5 .NBr.NBr 2 
 
 Whereas Griess regarded the diazobenzene salts as additive 
 
 * Lehrbuch der organischen Chemie, II. 717. 
 
 t Langfurth and Spiegelberg, Annalen, 1878, 191, 205 ; 1879, 197, 
 305 ; compare also ibid., 1874, 174, 355 ; 1880, 215, 103. 
 
114 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 compounds of diazobenzene and acids, Kekule* looked upon 
 diazobenzene as playing the part of a base analogous to 
 ammonium. 
 
 Further, he explained the difference in stability between 
 a diazo-salt such as diazobenzene chloride and an azo-com- 
 pound 
 
 C 6 H 5 .N:NC1 C 6 H 6 .N:N.C 6 H 5 
 
 as being due to each nitrogen atom being united with a phenyl 
 group in the latter, whilst in the former the union of chlorine 
 and nitrogen rendered the compound similar to chloride of 
 nitrogen. Diazoaminobenzene was formed, according to 
 Kekule', by the union of the acidic part of the diazo-salt with 
 a hydrogen atom of aniline, and the residues of both uniting 
 thus 
 
 The compound thus was an anilide, and hence was called 
 diazobenzene anilide. 
 
 This conception of the constitution of diazoaminobenzene is 
 supported by the fact that, like many hydrazones, it forms 
 metallic compounds, the hydrogen of the NH group being 
 replaceable. 
 
 Diazobenzene perbromide was considered by Kekule' to be 
 either an additive compound of diazobenzene bromide with 
 one molecule of bromine (1) or a compound of formula (2) 
 C 6 H 6 . N : NBr, Br 2 C 6 H 6 . NBr.NBr 2 
 
 (1) P) 
 
 the former of which was regarded as the correct one. 
 
 The constitution of diazobenzene imide was correctly written 
 by Kekule' as 
 
 / 
 
 C 6 H 5 .N/J 
 
 and he pointed out that although free diazobenzene (or diazo- 
 benzene hydroxide), C 6 H 6 . N : N.OH, was very unstable, yet 
 certain other diazo-compounds could exist in the free state, 
 such as, for example, the diazophenols. The reason of this was, 
 that whilst one nitrogen of the diazo-group was attached to 
 
CONSTITUTION OF THE DIAZO-COMPOUNDS 115 
 
 the benzene ring, the other was united to the oxygen atom of 
 the phenol group, thus 
 
 O 
 
 This view was confirmed by the fact that the diazo- 
 derivatives of phenol ethers for example, anisole behave as 
 derivatives of diazobenzene and not of diazophenol. Thus 
 salts of nitrodiazoanisole with mineral acids are easily obtain- 
 able, whilst those of diazophenol are not ; further, free diazo- 
 anisole does not exist. 
 
 The diazosulphonic acids possessed a similar constitution to 
 the diazophenols ; thus diazotized sulphanilic acid was 
 
 S0 8 
 C 6 H/> 
 
 Kekule* also pointed out that the diamines could be divided 
 into three classes according to their behaviour on diazotiza- 
 tion, namely, (1) those in which only one amino-group was diazo- 
 tized, (2) those in which both ammo-groups were diazotized, or 
 (3) those in which one amino-group was diazotized and the 
 other took part in the reaction.* 
 
 Many examples of these three classes have already been 
 described in the foregoing pages. The difference in stability 
 between the aromatic diazo- and azo-compounds was explained 
 by Kekule* to lie in the fact that whilst both contained the 
 group C 6 H 5 .N:, in the latter series it was attached to a 
 benzene radical, whilst in the former a halogen or acid radical 
 was united with it. 
 
 Kekules theory of the constitution of diazo-compounds was 
 universally adopted, and held its own for thirty years, until, 
 in fact, the discovery of the isomeric metallic salts, giving, as 
 it did, an immense impetus to the study of their constitution, 
 led to the abandonment of Kekul^'s theory in favour of that 
 of Blomstrand. 
 
 
 
 * Compare Holleman, Zeitsch. f. Chem., 1865, 557 ; Hofmann, Annalen, 
 I860, 115, 251. 
 
 I 2, 
 
116 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 3. Constitution of diazo-salts according to Blomstrand. 
 
 An entirely novel view of the constitution of the diazo-salts 
 was published by Blomstrand in 1869.* This chemist ex- 
 plained the formation of diazoaminobenzene and of diazo- 
 benzene nitrate in the following way. 
 
 In order to obtain a diazo-compound from an amine and one 
 molecule of nitrous acid (HONO), three atoms of hydrogen 
 must be present in order to become replaced by an atom of 
 nitrogen. In the preparation of diazoaminobenzene from an 
 alcoholic solution of aniline and nitrous acid, two molecules of 
 aniline are required to furnish these three hydrogen atoms; 
 consequently a simple diazo-compound is not the final pro- 
 duct 
 
 C 6 H 5 .NH;Hi C 6 H 5 .NHf 
 
 C 6 H 5 .NjHHJ C 6 H 6 .N^N 
 
 If, however, the starting-point is a salt of aniline, which, 
 according to the ammonium theory of Berzelius, is a salt of 
 a substituted ammonium (C 6 H 5 .NH 3 ), the three necessary 
 hydrogen atoms are now present. Further, in the latter is 
 present a quinquevalent nitrogen atom, whilst in diazoamino- 
 benzene the two atoms of nitrogen are in the tervalent 
 condition. 
 
 The formation of diazobenzene nitrate is therefore to be 
 regarded as follows 
 
 V ill v 
 
 C 6 H 5 . N.O.N0 2 + HO.N : O = C 6 H 5 . N.O.N0 2 + 2H 2 O. 
 
 i 
 
 The three atoms of hydrogen attached to the quinquevalent 
 nitrogen atom in aniline nitrate have thus been replaced by 
 a tervalent nitrogen atom. 
 
 This theory of the constitution of the diazo-salts was also 
 put forward independently by Strecker in 1871 f and by 
 Erlenmeyer in 1874 J without the knowledge of Blomstrand's 
 paper or of each other. 
 
 * Chemie der Jetztzeit, 1869, No. 4, 272. 
 
 + Ber., 1871, 4, 786. { Ber., 1874, 7, 1110. 
 
 $ The Chemie der Jetztzeit seems to be a very obscure publication. No i 
 copy exists in the Patent Office library or that of the Chemical Society. 
 
CONSTITUTION OF THE DIAZO-COMPOUNDS 117 
 
 That these two chemists had been anticipated was shown 
 in a paper by Blomstrand in 1875,* who thus established his 
 claim to priority. 
 
 Blomstrand pointed out the superiority of his formula to 
 that of Kekule' in that no change in the valency of the aniline- 
 nitrogen was postulated, a change for which there is no justi- 
 fication 
 
 V III 
 
 Kekule- C 6 H 5 . NH 3 C1 -> C 6 H 5 .N:NC1 
 
 v v 
 
 Blomstrand C 6 H 5 .NH 3 C1 - C 6 H 5 .NC1 
 
 ,N 
 
 He also explained the instability of the diazo-salts by 
 referring to the unusual replacement of three atoms of 
 hydrogen by one of nitrogen in an ammonium salt. 
 
 Blomstrand agreed with Erlenmeyer in adopting the formula 
 C 6 H 5 . NBr : NBr 2 for diazobenzene perbromide. 
 
 In later papers Blomstrand developed his theory more fully 
 in the light of recent work and suggested the names: 
 ' Azoammonium ' compounds for the diazo-salts; 'azo '-com- 
 pounds for not only the stable compounds such as 
 
 C 6 H 6 .N:N.C 6 H 5 , 
 
 but even for potassium diazobenzene sulphonate, 
 C 6 H 6 .N:N.S0 3 K; 
 
 ' diazo '-compounds for the aliphatic diazo-compounds of 
 Curtius, and ' iso-azo ' compounds for the labile isomerides of 
 Hantzsch (see later) and the labile forms of metallic salts and 
 hydroxides.f 
 
 Blomstrand regarded the unstable azoammonium compounds 
 as readily undergoing change into the azo-compounds under 
 the influence of reagents such as phenols, &c. ; for examplt 
 
 * Ber., 1875, 8, 51. 
 
 + Ada Reg. Soc. Physiogr. Lund., 6, 1 ; J. pr. Chern., 1896 [ii], 53, 
 169. 
 
118 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 R.N.C1 
 
 + C 6 H 5 . OK = KC1 + R.N : N.C 6 H 4 . OH 
 
 R.N.C1 
 and ]|| + K 2 S0 3 = KC1 + R.N : N.S0 3 K. 
 
 He pointed out that a sharp distinction must be drawn 
 between the quinquevalent salt-forming nitrogen atom in the 
 azoammonium compounds, and the tervalent non-salt-forming 
 nitrogen atom in the azo-compounds. 
 
 The chief reason why Blomstrand's theory of the constitu- 
 tion of diazo-salts (azoammonium salts) was not accepted was 
 due to the objection of E. Fischer, who showed that this 
 constitution did not explain the formation of phenylhydra- 
 zine, discovered by him in 1875, by simple reduction of a 
 diazo-salt.* 
 
 Fischer pointed out that when diazobenzene nitrate was 
 treated with an equimolecular quantity of potassium sulphite, 
 a yellow crystalline salt, C 6 H 5 . N 2 . S0 3 K, was formed, but when 
 an excess of potassium bisulphite was used, a colourless salt, 
 C 6 H 5 . N 2 H 2 . S0 3 K, was obtained. The latter, potassium 
 phenylhydrazine sulphonate, had already been prepared by 
 Strecker and Romer in 1871.f 
 
 The former salt had all the properties of a diazo-compound, 
 and on treatment with zinc dust and acetic acid passed into 
 the latter, which was, therefore, a product of reduction. It 
 had no longer the properties of the diazo-compound but was 
 converted into this by gentle oxidation. 
 
 On treatment with hydrochloric acid, the sulphonic acid 
 group was eliminated, and there resulted the hydrochloride of 
 phenylhydrazine. Strecker and Romer formulated these com- 
 pounds as follows 
 
 C 6 H 6 . NH.S0 3 K C 6 H 5 . N.S0 3 K 
 
 N 
 
 H 
 
 Potassium phenylhydrazine Potassium benzenediazo- 
 sulphonate. sulphonate. 
 
 (The latter formula, it will be noticed, differs from that 
 suggested by Blomstrand.) 
 
 * Ber., 1877, 10, 1331. t Ber., 1871, 4, 784. 
 
CONSTITUTION OF THE DIAZO-COMPOUNDS 119 
 
 Fischer pointed out that in order to explain the formation 
 of phenylhydrazine from a diazobenzene salt according to the 
 Blomstrand theory, it would be necessary to assume the 
 change of quinquevalent nitrogen into tervalent by the addi- 
 tion of hydrogen, and also the change of tervalent into quin- 
 quevalent nitrogen by the withdrawal of hydrogen, 
 
 v 
 C 6 H 5 .N.C1 in v 
 
 + 4H ^ C 6 H 5 .NH.NH 3 C1 
 
 i 
 
 in 
 
 a procedure which was extremely improbable. 
 
 On the other hand, if we assume that no change of valency 
 occurs when phenylhydrazine is formed, we have 
 C 6 H 5 .N.C1 C 6 H 5 .NH 3 C1 
 
 i 
 
 NH 
 
 giving us for free phenylhydrazine the formula 
 
 C 6 H 6 .NH 2 
 
 Fischer proved, however, that this formula could not repre- 
 sent the constitution of phenylhydrazine* in the following 
 way: phenylhydrazine and ethyl bromide unite to form the 
 compound 
 
 C 6 H 6 .N 2 H 2 (C 2 H 6 )<Cf* 
 
 which is also produced by the addition of ethyl bromide to 
 phenylethylhydrazine. As the latter substance, however, is 
 derived from ethylaniline by the substitution of the remaining 
 ^T-hydrogen atom by the group NH, and thus possesses the 
 constitution 
 
 therefore the compound of phenylethylhydrazine and ethyl 
 bromide must contain the complex > N.NH 2 , and consequently 
 phenylhydrazine itself must be C 6 H 5 . NH.NH 2 . Fischer thus 
 
 * Annalen, 1877, 190, 67. 
 
120 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 concluded that its formation from diazobenzene chloride could 
 only be explained by the aid of Kekule's formula 
 
 C 6 H 5 . N : NCI -^ C 6 H 5 . NH.NH 3 C1. 
 When the whole question of the constitution of the diazo- 
 compounds was undergoing renewed investigation many years 
 later, Blomstrand explained that the formation of phenyl- 
 hydrazine might be expressed as follows 
 
 R.N i N p ^ ^TT 
 
 + 4H = K , , * = R.NH.NH 3 C1 
 
 and pointed out that it was impossible to postulate a double 
 linking between the two nitrogen atoms as shown in the 
 
 formula 
 
 C 6 H 6 .NH 2 
 
 NH 
 
 when complete reduction had taken place.* 
 
 The objections of Fischer, however, as has been said, were 
 taken as final, and it was only in 1895 that the formula of 
 Blomstrand was again adopted. 
 
 It seems suitable, at this stage, to postpone further inquiry 
 as to the constitution of the diazo-salts until we have con- 
 sidered a little more fully that of the free diazobenzene, for 
 the two are very closely connected. 
 
 4. Constitution of diazobenzene hydroxide to 1894. In 
 
 chapter xiii we have seen that when diazo-compounds are con- 
 densed with various substances with the formation of mixed 
 azo-compounds, the resulting substances were regarded in 
 some cases as true azo-compounds and in others as hydrazones. 
 It was therefore argued by the workers in this field that the 
 original diazo-compound (or rather the hydroxide which might 
 be supposed to be formed) might also be represented by a 
 tautomeric formula 
 
 R.N : N.OH R.NH.NO 
 
 the former of which would give rise to true azo-compounds, 
 and the latter to the hydrazones.f 
 
 * J.pr. Chem., 1896 [ii], 53, 176. 
 
 t V. Meyer, Ber., 1888, 21, 15 ; Japp and Klingemann, Ber., 1891, 24, 
 2264 ; von Pechmann, Ber., 1891, 24, 3255 ; Bamberger, Ber.. 1891, 24, 
 3264; 1893,26,495. 
 
CONSTITUTION OF THE DIAZO-COMPOUNDS 121 
 
 It has also been shown that Bamberger's discovery of the 
 diazoic acids lent support to this view * (p. 102). 
 
 Further confirmation was adduced by the experiments of 
 von Pechmann f on the action of acetic anhydride on an 
 alkaline solution of diazobenzene. He found that an acety- 
 lated nitrosoamine was formed which was identical with the 
 nitrosoamine prepared by O. Fischer { by the action of nitrous 
 acid on acetanilide. , 
 
 Von Pechmann therefore concluded that primary amines 
 by successive treatment with (1) nitrous acid, (2) acetic 
 anhydride, or (1) acetic anhydride, (2) nitrous acid, yielded 
 the same product, and that free diazobenzene was to be regarded 
 as the anilide of nitrous acid 
 
 OH.NO C 6 H 5 .NH.NO 
 
 Nitrous acid. Anilide of nitrous acid. 
 
 In confirmation of this view von Pechmann showed that 
 nitrosoanilides (prepared from an anilide and nitrous acid) 
 actually coupled with primary amines and phenols, yielding 
 diazoamino- and hydroxyazo-compounds respectively, the 
 acetyl group being at the same time split off. 
 
 The discovery by von Pechmann and Frobenius that the 
 methyl ether prepared from the silver salt of >-nitrodiazo- 
 benzene, to which reference has already been made (p. 98), was 
 isomeric with that obtained from the sodium salt of Schraube 
 and Schmidt, gave emphasis to the view of the tautomeric 
 nature of diazobenzene 
 
 C 6 H 6 .N:N.OH or C 6 H 5 .NH.NO 
 N0 2 . C 6 H 4 . N : N.OAg NO 2 . C 6 H 4 . NNa.NO 
 
 Silver salt of P. and F. Sodium salt of S. and S. 
 
 (See, however, p. 147.) 
 
 Further work bearing on this point was immediately pub- 
 lished by Bamberger.|| By treating a /3-diazonaphthalene 
 solution with concentrated aqueous sodium hydroxide, an iso- 
 
 * Hantzsch regarded benzenediazoic acid as 
 C 6 H 5 .N-N.OH 
 
 O 
 
 (Ber., 1894, 27, 1730). 
 
 t Ber., 1894, 27, 651. J Ber., 1877, 10, 959. 
 
 Ber., 1894, 27, 672. || Ber., 1894, 27, 679. 
 
122 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 meric substance was obtained which was called /3-^so-diazo- 
 naphthalene, C 10 H 7 .NH.NO, and which did not form an 
 azo-compound with alkaline phenols. When, however, it was 
 subjected to the action of a mineral acid, molecular change 
 took place rapidly, and /8-diazonaphthalene was re-generated 
 C 10 H 7 . NH.NO -> C 10 H 7 . N : N.OH. 
 
 Bamberger drew the following conclusions as to the mecha- 
 nism of diazotization. The properties of benzenediazoic acid, 
 especially its transformation to o-nitroaniline, led him to 
 suggest that the first stage in the process of nitration of a 
 primary amine was the formation of a diazoic acid, for he had 
 succeeded in obtaining benzenediazoic acid by the action of 
 nitrogen pentoxide on aniline 
 
 C 8 H 6 . NH 2 + N 2 6 -* C 6 H 5 . NH.N0 2 . 
 
 Similarly, by the action of nitrous acid on a primary amine, 
 the first product was the nitrosoamine (iso-diazo-compound), 
 C 8 H B . NH 2 + N 2 3 - C 6 H 6 . NH.NO ; 
 
 the ordinary form of diazobenzene, C 6 H 6 . N : N.OH, would 
 then result by molecular change from this. 
 
 Bamberger found, in confirmation of this view, that under 
 certain conditions, many primary amines yielded the iso- 
 diazo-compound as first product.* 
 
 Further, these isomeric forms of diazobenzene give (accord- 
 ing to Bamberger) metallic salts, 
 
 C 6 H 5 .NK.NO C 6 H 5 . N : N.OK 
 
 Potassium salt of Potassium salt of 
 
 tso-diazobenzene. diazobenzene. 
 
 of which the iso-salt (like fcso-diazobenzenej does not couple 
 with phenols, and is transformed into the normal diazo-salt 
 by mineral acids (see, however, p. 144). 
 
 * Ber., 1894, 27, 1948. 
 
CHAPTER XVH 
 
 CONSTITUTION OF THE DIAZO-COMPOUNDS 
 
 (continued) 
 
 1. Constitution of the diazo-componnds according to 
 Hantzsch. An important contribution to the current ideas 
 was next made by Hantzsch.* 
 
 He introduced the theory that the constitution of the 
 isomeric diazo-compounds was exactly analogous to that of 
 the isomeric oximes, according to which the former existed as 
 stereoisomeric substances of the general formulae 
 C 6 H 5 .N C 6 H 6 .N 
 
 . xl L 
 
 syn. anti. 
 
 Hantzsch pointed out that the development of the chemistry 
 of the isomeric diazo-compounds had undergone a precisely 
 similar course to that of the isomeric oximes ; to the isomeric 
 diazo-compounds had been assigned the formulae 
 
 C 6 H 5 .N:N.OH and C 6 H 5 .NH.NO, 
 
 just as, after the discovery of * 2so-benzaldoxime ', its constitu- 
 tidn was very generally regarded as being structurally different 
 from that of the normal oxime, thus 
 
 C 6 H 5 .CH.NH 
 C 6 H 5 .CH:N.OH 
 
 Y 
 
 Stable oxime. Labile oxime. 
 
 He showed that the formulae advocated by Schraube and 
 Schmidt, and confirmed by Bamberger, for the metallic diazo- 
 compounds for example, 
 
 C 6 H 6 . NK.NO C 6 H 5 . N : N.OK 
 
 iso-salt. Normal salt. 
 
 required that, in the change from normal to iso-salt, the 
 
 * Ber. t 1894, 27, 1702. 
 
124 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 potassium should wander from the oxygen, for which it has 
 an enormous affinity, to the nitrogen which has little attrac- 
 tion for it. As this transformation takes place, in the case 
 of >-nitrodiazobenzene, at the ordinary temperature and in 
 aqueous solution, the current theory of its mechanism could 
 not be accepted. Further, all substances which exhibit 
 tautomerism in their salts, such as nitrous, cyanic, hydro- 
 cyanic, and sulphurous acids, would also have to be considered 
 as displaying structural isomerism in these, but no structural 
 isomeric salts had been found the origin of which isomerism 
 lay in a dissociable group (H or Me) which could alter its 
 position in the molecule. Hence Hantzsch concluded that 
 such isomerism must be steric and not structural. The fact 
 that structurally isomeric alkyl derivatives had been obtained 
 could not be used as a proof of the structural difference of the 
 original substances; Schraube and Schmidt had concluded 
 that because an ^V-ether (I) was formed from the iso-diazo- 
 benzene salts and alkyl iodide the original salt had the 
 composition (II) 
 
 C 6 H 5 . N(Alk).NO C 6 H 5 . NH.NO 
 
 but this could not be maintained, for if the -iso-diazobenzene 
 salts were nitrosoamines, their alkyl derivatives ought, by 
 alkaline hydrolysis, to yield the corresponding nitrosoamines ; 
 thus C 6 H 5 .NAlk.NO should give C 6 H 5 . NH.NO, but Bam- 
 berger had shown that the normal metallic diazo-salt and not 
 the ^so-salt was formed in each ease.* 
 
 The production of an JV-ether from the potassium salt, and 
 an -ether from the silver salt (p. 121) of ^>-nitrodiazobenzene, 
 found an analogy in the case of the oximes. 
 
 Hantzsch, therefore, was of the opinion that there was no 
 proof of the structural isomerism of the free phenylnitroso- 
 amines with the true diazo-compounds, and stated that if 
 a derivative of diazobenzene in which the dissociable hydrogen 
 or metallic atom was replaced by a non-dissociable group 
 should exist in two isomeric forms, of which the one showed 
 
 * Hantzsch himself showed later, however (see p. 147), that in this 
 reaction the iso-salts are actually produced. 
 
CONSTITUTION OF THE DIAZO-COMPOUNDS 125 
 
 the reactions of a true diazo-compound (for example, coupling 
 with /3-naphthol) and the other those of the iso-diazo-com- 
 pounds, such isomerides would be structurally identical, and 
 their difference would be due to stereoisomerism. 
 
 As an example of such isomerism, Hantzsch * described a 
 series of new diazoamino-compounds which he regarded as 
 stereoisomeric with those already known; these were, how- 
 ever, soon shown by Bamberger f not to have any existence, 
 as the substances described by Hantzsch were identical with 
 the bisdiazobenzeneanilides of von Pechmann.J 
 
 A second example was given, namely, a new (syri) form of 
 potassium benzenediazosulphonate which, as will be shown, 
 gave rise to a long controversy as to its nature. 
 
 For the purpose of determining which diazo-com pounds 
 belong to the syn- and which to the anti-series, Hantzsch 
 took for example those compounds which were considered to 
 form anhydrides, such as diazosulphanilic acid, diazophenol, &c. 
 At that time these were supposed to have the constitution 
 
 C 6 H 4 .N NO 2 .C 6 H 4 .N 
 
 I II I II 
 S0 3 . N O N 
 
 Anhydride of diazobenzene- Nitrodiazophenol. 
 
 sulphonic acid. 
 
 If we now imagine the ring to be broken by addition of 
 
 * Ber., 1894, 27, 1857. t Ber. y 1894, 27, 2596. 
 
 J Ber., 1894, 27, 703. This is an exceedingly good example of the 
 importance of carrying out complete and exhaustive analyses in organic 
 research. For ' benzene-svw-diazoanilide ' 
 
 C 6 H 6 .N 
 
 C 6 H 5 .NH.N 
 Hantzsch gave the following numbers : 
 
 Found. : C = 72-7, H = 6.0, N = 20-8, 
 C ]2 H n N 3 requires C = 73-1, H = 5-6, N = 21-3. 
 The compound was really bisdiazobenzeneanilide, 
 
 C 6 H 5 . N : N.N. C 6 H 5 ) N : N C 6 H 5 or C 1S H 16 N B , 
 
 which requires C = 71-8, H = 5-0, N = 23 2, for which von Pechmann 
 found C = 71-8, 72-3 ; H = 5-0, 5-8 ; N = 23-5 ; and Bamberger, 
 N = 23-2, 2347 per cent. 
 
 For ' jp-toluene-sww-diazotoluide ' Hantzsch gave 
 
 Found. : N = 19-5, 
 C 14 H 5 N 3 requires N = 18-7, 
 
 whereas the substance really possessed the formula C 21 H 21 N 5 , which 
 requires N = 204 per cent. 
 
126 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 water, salt-formation, &c., it is reasonable to suppose that the 
 group attached to the nitrogen atom should retain its position, 
 thus 
 
 C 6 H 4 . N C 6 H 4 N 
 
 S0 3 . N S0 8 H HO.N 
 
 but as all these cyclic compounds readily couple with 
 /3-naphthol, it is to be concluded that ordinary diazo-com- 
 pounds have a similar configuration, or, in other words, those 
 diazo-compounds which combine readily with /3-naphthol 
 belong to the syn-seriea, whilst those combining with diffi- 
 culty or not at all are a7ii-compounds.* 
 
 Hantzsch explained the fact that many diazo-compounds, 
 when left for some time in alkaline solution, lose their power 
 of coupling with ^-naphthol by saying that the sy^-compound 
 (alkali-labile) was transformed into the anti-compound (alkali- 
 stable) as follows 
 
 C 6 H 4 .N C 6 H 4 N C 6 H 4 .N 
 
 | || +H 2 -* | || + KOH -+ | || 
 
 S0 3 . N S0 3 H HO.N S0 3 K N.OK 
 
 syn. anti. 
 
 Another method of determining the configuration of the 
 isomeric diazo-compounds was drawn from the analogy to the 
 oximes. In this class of compounds intramolecular change 
 proceeds only in the case of the s^-compounds, so that only 
 the diazo-compounds belonging to the same series could 
 decompose according to the equation 
 
 The diazo-compounds which correspond to this condition 
 * Hantzsch at a later date accepted the formulae 
 
 C H <U 
 
 and : C 6 H 4 (N0 2 ) : N 2 for these compounds, so that this particular 
 argument cannot be maintained. This constitution of the quinonediazides 
 (diazophenols) was adduced by L. Wolff (Annalen, 1900, 312, 119 etseq.) 
 from the fact that compounds containing undoubtedly the grouping 
 
 O.C. 
 
 had properties quite different from the former. 
 
CONSTITUTION OF THE DIAZO-COMPOUNDS 127 
 
 are the normal compounds, so these were to be regarded, 
 according to Hantzsch, as syn-compounds. 
 The decomposition thus takes place as follows : 
 
 C.HJ.N C 6 H 5 
 
 II - I +N 2 
 Xj.N X 
 
 On the other hand, the ow^compounds have a tendency to 
 decompose into two residues, each containing a nitrogen atom, 
 thus 
 
 C 6 H 5 .N 
 
 -|H> - C 6 H 6 .N: + :NX 
 N.X 
 
 In this way Hantzsch explained the formation of nitroso- 
 benzene by the oxidation of the ^so-diazo-compounds 
 C 6 H 6 .N 
 
 !r.OH + C ' H6 ' ] 
 
 A third method of distinguishing between the syn~ and 
 anti-compounds was to be found in their difference in 
 explosibility, the normal or si/Ti-diazo-compounds being much 
 more explosive than the em^i-compounds. 
 
 It is to be mentioned here that Hantzsch regarded diazo- 
 benzene chloride as a s^w-compound, 
 
 C 6 H S .N 
 
 0,1 
 
 but soon adopted another view (see p. 133). 
 
 Hantzsch's main conclusions were, therefore, that the 
 ordinary normal diazo-compounds were sy^-diazo-compounds, 
 and the so-called i'so-diazo-compounds (nitrosoamine formula) 
 were cmi-diazo-compounds. Bamberger,* in criticizing these 
 views of Hantzsch, denied that it was possible to draw a 
 parallel between the stereoisomeric oximes and the isomeric 
 diazo-compounds. He based his objections on the very great 
 difference between the latter, a difference which was greatly 
 in contrast to the very small one existing between the oximes, 
 and which could not be due to stereoisomerism. Moreover, 
 
 * Ber., 1894, 27, 2582. 
 
128 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 the isomeric oximes gave corresponding isomeric ethers, but 
 from the diazo- and ^so-diazo-silver salts only the one normal 
 ether was obtained with methyl iodide. 
 
 He maintained that the difference between the normal and 
 the -iso-diazo-compounds was best explained by the presence 
 of the labile hydrogen atom (*) 
 
 C 6 H 5 . N : N.OH* C 6 H 6 . NH* NO 
 
 Normal. iso. 
 
 Further, the objection of Hantzsch that in the normal and 
 m>-potassium benzenediazo-oxides 
 
 C 6 H 6 . N : N.OK and C 6 H 6 . NK.NO 
 
 Normal. iso. 
 
 it was unlikely that the potassium atom should leave the 
 oxygen and become attached to the nitrogen atom was met 
 by Bamberger by the reminder that many compounds con- 
 taining the imino-group for example, azoimide and benzimin- 
 azole readily dissolve in alkali, and he pointed out, also, that 
 the positive group C 6 H 5 . N 2 in the salt C 6 H 5 . N 2 . OK greatly 
 lessened the affinity of potassium for the oxygen atom, just as 
 in potassium oxide, K.O.K., one potassium atom is easily 
 removed. 
 
 As was briefly indicated above (p. 124), it was obviously of 
 great importance in connexion with Hantzsch' s stereochemical 
 theory to adduce evidence of the existence of isomerides other 
 than those capable of being explained by the presence of 
 a labile hydrogen atom which could give rise to isomerism as 
 shown by the formulae 
 
 C 6 H 6 . N : N.OH and C 6 H 5 . NH.NO. 
 
 Hantzsch's first example of stereoisomeric diazoamino- 
 benzenes was, as already mentioned, shown to be based on an 
 error, and considerable interest, therefore, was attached to the 
 other example of stereoisomeric benzenediazosulphonates. 
 
 Isomeric benzenediazosulphonates. | It has been already 
 explained (p. 55) that by treating diazobenzene chloride with 
 neutral potassium sulphite, potassium benzenediazosulphonate 
 is formed (' Strecker's salt '). In 1894 Hantzsch J described a 
 
 t These must not be confused with the salts of diazobenzenesulphonic 
 acid prepared by diazotizing sulphanilic acid. 
 | Ber., 1894, 27, 1726. 
 
CONSTITUTION OF THE DIAZO-COMPOUNDS 129 
 
 new isomeride which he obtained by pouring diazobenzene 
 nitrate solution into an ice-cold solution of neutral potassium 
 sulphite containing an excess of potassium carbonate. Orange 
 plates separated which were readily soluble in water and con- 
 tained one molecule of water of crystallization. The substance 
 was very unstable, and coupled readily with j3-naphthol, and 
 Hantzsch assigned to it the sy^-configuration 
 
 C 6 H 5 .N 
 
 S0 3 K.N 
 
 The clear dark-yellow aqueous solution of this new sub- 
 stance gradually became paler on standing, and crystals of 
 the Strecker salt separated out. This was much more stable 
 than the s^/ft-compound, and gave no colour reaction at all 
 with alkaline phenol solution. It was, therefore, the anti- 
 compound. 
 
 Bamberger considered* that the isomeric benzenediazo- 
 sulphonates of Hantzsch 
 
 C 6 H 5 .N C 6 H 5 .N 
 
 L 
 
 syn. anti. 
 
 might just as well be represented as 
 
 C 6 H 5 .N:N.O.S0 2 K and C 6 H 5 . N : N.S0 3 K 
 
 Potassium diazobenzene Potassium benzenediazo- 
 
 sulphite. sulphonate. 
 
 in view of the fact that Hantzsch's new salt gave all the re- 
 actions of a sulphite, and he maintained that Hantzsch had 
 not proved that the two were stereoisomerides. 
 
 He showed also that Strecker's salt, on acidification, did 
 not pass into the diazo-salt as did ^so-diazo-compounds. 
 
 The same view of the constitution of these salts was 
 expressed by Claus.f 
 
 In reply to this criticism, Hantzsch % showed that both 
 modifications gave the same ions, (C 6 H 5 . N 2 S0 3 ) and K, in 
 solution, so that they must both have the structure 
 
 * Ber., 1894, 27, 2586, 2930. t J. pr. Chem., 1894 [ii], 50, 239. 
 I Ber., 1894, 27, 2099, 3527. 
 
 S0 3 K.N N.S0 3 K 
 
130 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 C 6 H 5 . N 2 . S0 3 K ; he pointed out, further, that the sulphite 
 reaction showed only that the new salt decomposed readily 
 with separation of sulphurous acid, just as does the compound 
 Hg(S0 3 K) 2 , which in aqueous solution gives the three ions 
 
 Hg(S0 3 ) 21 K, K. 
 
 He maintained that a diazo-sulphite, C 6 H 5 . N 2 . 0.SO 2 K, must 
 give three ions in solution and not two, although Ostwald 
 had informed Bamberger* that only two ions, namely, 
 
 - + 
 
 (C 6 H 5 N 2 S0 3 ) and K, 
 
 were to be expected according to analogy. 
 
 Up to this point we may summarize the evidence as proving 
 the existence of two isomeric compounds, C 6 H 5 . N 2 . (S0 3 K). 
 As we have seen, Hantzsch supposed that only one consti- 
 tutional formula was possible for these, and that the 
 substances were therefore stereoisomeric. Other formulae 
 have, however, been advocated, but the discussion of these 
 must be postponed until the constitution of the diazo-salts 
 (chlorides, &c.) have been more fully dealt with (p. 140). 
 
 Isomeric diazo-cyanides. In 1895 Hantzsch and Schultze 
 succeeded in preparing isomeric diazo-cyanides by the action 
 of potassium cyanide on ^-chloro- and >-nitro-diazobenzene 
 chloride solution, f 
 
 A little more than the theoretical quantity of potassium 
 cyanide is added to the hydrochloric acid solution of the 
 diazo-salt, and care must be taken that sufficient hydrochloric 
 acid is present to ensure an acid reaction at the end of the 
 operation. 
 
 At a low temperature (below 5) the primary, syn, or labile 
 compound is obtained, whilst at higher temperatures the 
 secondary, anti, or stable compound is produced. 
 
 Both compounds are coloured, crystalline, and almost in- 
 soluble in water, and the labile variety passes into the stable 
 form slowly in the solid state but quickly in alcoholic 
 solution. The labile isomerides couple with /3-naphthol, and 
 are explosive, whilst the stable do not possess these properties. 
 
 The two compounds behave very differently when treated 
 
 * Ber., 1894, 27, 2934. t Ber., 1895, 28, 666. 
 
CONSTITUTION OF THE DIAZO-COMPOUNDS 131 
 
 with copper powder; the si/Ti-compounds derived from both 
 >-chloro- and >-nitroaniline yield >-chlorobenzonitrile and 
 ^-nitrobenzonitrile respectively, whilst the cwi-compounds 
 are entirely without action. Further, the s^/Ti-compounds 
 yield azo-dyestuffs with R salt, but the cmfa'-compounds 
 do not. These are substantial chemical differences in the 
 behaviour of these substances which are not usually met 
 with in stereoisomeric compounds (see p. 127). 
 
 Hantzsch regarded these differences, however, as merely 
 showing that the one compound was more stable than the 
 other, and considered the existence of these substances to be 
 a proof of his stereochemical theory, formulating them as 
 C1.C 6 H 4 .N C1.C 6 H 4 .N 
 
 CN.N N.CN 
 
 Labile (syri). Stable (anti). 
 
 As in the case of the isomeric diazosulphonates, we have 
 first to discuss the later development of the theory of the 
 constitution of the diazo-salts with mineral acids before 
 describing the criticism to which these two series of 
 isomerides have been subjected. 
 
 E 2 
 

 CHAPTEK XVIII 
 
 CONSTITUTION OF THE DIAZO-SALTS AFTER 1894 
 
 1. Constitution of the diazo-componnds according to 
 Bamberger. In 1894 Bamberger * stated that, in the diazo- 
 salts, the radical (C 6 H 5 . N 2 ) is strongly positive, and that even 
 negatively substituted diazo-salts, such as bromo- and nitro- 
 diazobenzene nitrates, showed a neutral action towards litmus 
 or Congo-red, and were not hydrolytically dissociated in 
 solution. He suggested, therefore, that the formula 
 
 C 6 H 5 .N!N.X 
 
 was worth consideration. Shortly afterwards,! he pointed out 
 that there were no compounds known in which tervalent 
 nitrogen was combined with a negative complex, such as 
 N0 3 , 01, &c., to form a salt. Hence he concluded that the 
 nitrogen atom united with such groups in the diazo-salts must 
 be quinquevalent, and therefore that Kekule"s formula, 
 
 C 6 H 5 .N:NC1, 
 which was commonly accepted, could not be correct. 
 
 He adopted instead of this, or the one just referred to, the 
 old formula suggested by Blomstrand (p. 116), C 6 H 5 . NCI : N, 
 for the diazo-salts with mineral acids.J 
 
 When the negative group was withdrawn from this (by 
 formation of the hydroxide, for example) the nitrogen atom 
 to which it was attached became tervalent, thus 
 
 C 6 H 5 .N(OH)iN -> C 6 H 5 .N:N.OH. 
 Diazobenzene. tso-diazobenzene. 
 
 It will be noticed that Bamberger here proposed a formula for 
 the i'so-compounds differing from the nitrosoamme formula, and 
 a convincing proof of the correctness of this formula for the 
 iso-diazo-compounds was apparently given by the discovery 
 
 * Ber., 1894, 27, 3417. t Ber., 1895, 28, 242. 
 
 I Ber., 1895, 28, 444. 
 
CONSTITUTION OF DIAZO-SALTS AFTER 1894 133 
 
 that these were formed by the action of hydroxylamine on 
 the nitroso-compounds, thus 
 
 C 6 H 6 . NO + H 2 N.OH = C 6 H 5 . N : N.OH + H 2 O * 
 but Hantzsch f showed that, in reality, the normal compound 
 was formed as follows 
 
 C 6 H 6 . NO + H.NH.OH = C 6 H 5 . N 
 C 6 H 5 .N.OH C 6 H 6 .N 
 
 HO.N.H HO.N 
 
 Bamberger's view of the constitution of the diazobenzene 
 salts was not at first accepted by Hantzsch, who maintained 
 that diazobenzene chloride in the dry state possessed the 
 constitution 
 
 C 6 H 6 .N 
 
 C1.N 
 
 and when dissolved in water was to be regarded as the 
 hydrochloride of s2/7i-diazobenzene hydroxide 
 
 C 6 H 5 .N 
 
 HO.N, HC1 
 
 Very soon, however, Hantzsch gave up the latter idea and 
 adopted Blomstrand's formula for the diazo-salts. He pre- 
 ferred also to call these ' diazonium ' salts, from their analogy 
 to the ammonium salts. || 
 
 Bamberger had shown that the diazo-salts had, like the 
 alkali salts, a neutral reaction in solution. That the diazo-salts 
 are electrolytically dissociated in solution had indeed been 
 demonstrated by Goldschmidt in 1890, who also found that 
 they form two ions;! and Hantzsch now made a careful 
 comparison of the electrical conductivities of various diazo- 
 salts and salts of the alkali metals, and was able to show that 
 the degree of ionization is about the same in the two cases ; 
 
 * Bamberger, Ber., 1895, 28, 1218. t Ber., 1905, 38, 2056. 
 
 I Compare also Angeli, Ber., 1904, 37, 2390. 
 Ber., 1895, 28, 676. || Ber., 1895, 28, 1734. 
 
 1 Ber., 1890, 23, 3220. 
 
134 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 that is to say, diazonium salts are dissociated almost to the 
 same extent as the corresponding potassium or ammonium 
 salts. 
 
 Further analogy was illustrated by Hantzsch's discovery of 
 various double salts of the diazo-salts with cobalt nitrite, 
 mercuric chloride, and mercuric cyanide. 
 
 Having thus developed the idea of * diazonium ' as a com- 
 plex alkali metal, Hantzsch agreed with Bamberger in regard- 
 ing the salts of ' diazonium' with acids as possessing the 
 constitution which had been attributed to them by Blom- 
 strand, Erlenmeyer, and Strecker, namely, C 6 H 5 . NCI : N. 
 
 The metallic salts, cyanides, sulphonates, &c., belonged to 
 the ordinary c diazo ' form ; for example, C 6 H 5 . N :N.OK could, 
 as we have seen, exist in two stereoisomeric modifications. 
 
 The cyclic diazo-compounds were divided into two groups. 
 On the one hand, Hantzsch assigned to the diazo-compound 
 'prepared from sulphanilic acid the formula which had been 
 already given to it by Strecker, namely, 
 
 C 6 H 4 
 
 and the diazo-phenols and naphthols, which are anhydrides, he 
 regarded as possessing the formulae 
 
 N N 
 
 C 6 H/\N and C 10 H 
 
 respectively. 
 
 To this pronouncement of Hantzsch, Bamberger replied 
 that the diazonium radical is not directly comparable with an 
 alkali metal, as its behaviour varies with the substituting 
 group in the aromatic nucleus. Moreover, the electrical con- 
 ductivity experiments of Goldschmidt and Hantzsch were not 
 a trustworthy basis upon which to speculate as to the nature 
 of diazonium.* 
 
 * Ber., 1896, 29, 446, 564, 608. 
 
CONSTITUTION OF DIAZO-SALTS AFTER 1894 135 
 
 Hantzsch now elaborated his arguments in favour of re- 
 garding the diazonium radical as a ' compound alkali metal ' 
 of the same strength as ammonium or potassium. He pointed 
 out that the alkali salts of all strong monobasic acids, such as 
 hydrochloric, sulphuric, &c., all undergo very extensive electro- 
 lytic dissociation in solutions of moderate dilution, but are 
 not hydrolytically dissociated. The dissociation and mole- 
 cular conductivity are only very slightly increased by further 
 dilution, and the increase ceases at a point of very moderate 
 dilution ; the salts of silver and thallium behave similarly, as 
 do also salts of complex ammonium bases, such as mono-, di-, 
 tri-, and tetra-alkylammonium and phenyltrimethylam- 
 monium, but not phenylammonium, the ion of the aniline 
 salts. Now the diazonium salts behave in exactly a similar 
 manner,* so that there was strong presumptive evidence that 
 diazonium was constituted similarly to the complex 
 ammoniums, f 
 
 2. Relation between diazonium compounds and normal 
 or s^m-diazo-cornpounds. Hantzsch's theory that the syn- 
 diazo-compounds are those which ' couple ' with phenols, &c., 
 to yield azo-dyestuffs led him to explain that these syn-com- 
 pounds were formed as intermediate products in the ordinary 
 reactions of the diazo-salts. The coupling process was thus 
 represented 
 
 CJBL R 
 
 The formation of the diazo-metallic salts, &c., was expressed 
 as follows 
 
 OK S0 3 K C i N 
 
 C 6 H 6 OK (S0 3 K), (ON) 
 
 + KNO 3 , &c. 
 * Ber. t 1895, 28, 1737. t Ber., 1895, 28, 1740; 1898, 31, 1612. 
 
136 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 and the decomposition of the diazonium salts was explained 
 similarly 
 
 OH C 6 H 5 \ /OH C 6 H 5 OH* 
 
 : N + | -> N=N -* N 2 
 
 H Cl fl C1H 
 
 Diazosulphanilic acid, when treated with one molecule of 
 alkali, passes into the mono- and with two molecules of alkali 
 into the di-alkali salt, thus 
 
 C 6 H 4 . N i N S0 3 Na.C 6 H 4 . N S0 3 Na.C 6 H 4 . N 
 
 S0 2 O HO.N NaO.N 
 
 This view of the intermediate formation of syrc-diazo-com- 
 pounds in the reactions of the diazonium salts received con- 
 firmation from the experiments of Hantzsch and Gerilowski,f 
 who showed that whereas free diazosulphanilic acid is fairly 
 stable in aqueous solution, the primary alkali salt obtained 
 by the action of one molecule of alkali, under the same condi- 
 tions, loses practically all its nitrogen, thus 
 
 S0 3 Na.C 6 H 4 .N S0 3 Na.C 6 H 4 N 
 
 HO! - ie + U 
 
 It is evident that the nature of the decomposition of the 
 diazo-salts is different from that of the ammonium salts, for 
 if the two resembled each other, one would expect that as the 
 group attached to the diazonium radical becomes less nega- 
 tive the stability of the salt would decrease just as 
 NH 4 C1, (NH 4 ) 2 CO 3 , and NH 4 . OH decrease in stability. 
 
 This is, however, not the case, as solutions of diazonium 
 carbonates are comparatively stable. Moreover, one would 
 expect also that diazonium halogen salts, if they decomposed 
 in aqueous solution analogously to the ammonium salts, would 
 yield halogen-substituted benzenes and not phenols. 
 
 Hantzsch explained this difference by assuming the inter- 
 
 * Ber., 1895, 28, 1734 ; 1900, 33, 2517. 
 t Her., 1896, 29, 1063. 
 
CONSTITUTION OF DIAZO-SALTS AFTER 1894 137 
 
 mediate formation of s^/ti-diazobenzene hydroxide, which then 
 could decompose into nitrogen and phenol 
 
 C 6 H 6 .N C 6 H 5 N 
 
 C 6 H 5 . N Ci:N + H 2 -* ^ -> ^ + J 
 
 The action of alcohol was explained in this way : 
 (1) Formation of ethers 
 
 Ar OEt 
 
 Ar OEt 
 
 Ar.OEt 
 
 I 1 
 
 I | 
 
 
 NiN + - 
 
 N=N 
 
 -> NiN 
 
 A, i 
 
 C1H 
 
 C1H 
 
 (2) Formation of hydrocarbons 
 
 Ar H Ar H 
 
 -* N:N 
 
 H 4 
 
 3. Double salts of diazouinm halides and metallic salts. 
 
 Double salts of diazo-halides with platinum and gold 
 chlorides have been known since the days of Griess and a 
 large number of others have since been prepared. 
 
 Hantzsch has further shown * that two kinds exist, namely, 
 colourless, stable, diazonium halogen double salts and coloured, 
 labile, s2/7i-diazo-halogen double salts. 
 
 For example we have 
 
 C 6 H 6 . N.C1, HgCl 2 C 6 H 5 . N, Co^CI, 
 
 and oil 
 
 Certain of the diazo-halides can also unite with halogen 
 acids to form additive compounds of formulae 
 
 Ar.N 2 Cl, HC1 and 3Ar.N 2 . 01, H01,f 
 the constitution of the former being represented by Hantzsch 
 as 
 
 Ar.N.Cl 
 
 Ber., 1895, 28, 1736. 
 
 t Hantzsch, Ber. s 1897, 30, 1153. 
 
138 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 and the latter being regarded as compounds of two molecules 
 of the neutral and one of the acid salt. 
 
 4. Diazoninm halides and 87/tt-diazo-halides. Hantzsch 
 found that all diazo-bromides, iodides, and thiocyanates which 
 belong to the same series as colourless diazonium chlorides 
 and salts of oxygen acids are coloured when in the solid state ; 
 with increase of colour is noticed an increase in the explosi- 
 bility. Thus we have 
 
 Diazo-chloride. Diazo-bromide. Diazo-thiocyanate. 
 
 Colourless. Slightly coloured. Strongly coloured. 
 
 Hardly explosive. Slightly explosive. Very explosive. 
 
 Diazo-iodide. 
 
 Intensely coloured. 
 Extremely explosive. 
 
 As diazonium salts should be, like the corresponding alkali 
 and ammonium salts, colourless, and as, on the other hand, syn- 
 diazo-halides, from analogy to the coloured s^/n-diazo-cyanides, 
 should be coloured, and also, as being compounds of the type 
 of nitrogen iodide, would be expected to be explosive, 
 Hantzsch * concluded that the properties of the above series 
 of compounds were only to be explained by the assumption 
 that they consist of an equilibrium mixture f of colourless 
 diazonium halides and coloured s^w-diazo-halides, thus 
 
 Ar.N(Br, SON, I) Ar.N 
 
 N (Br, SON, I) N 
 
 the chlorides belonging entirely to the diazonium series, and 
 the cyanides to the syn-series. 
 
 The proportion of s^/n-diazo-compound in the mixture 
 becomes less with a lowered temperature,! for at 60 many 
 diazo- halides are nearly colourless and become more intensely 
 coloured with rise of temperature. In the colourless aqueous 
 solutions, of course, the s^Ti-compound has become entirely 
 transformed into the diazonium isomeride. 
 
 5. Diazoninm perhalides. Griess found that two bromine 
 atoms in diazobenzene perbromide are more loosely combined 
 
 * Ber., 1897, 33, 2179. t Ber., 1900, 33, 2179. 
 
 } Euler, Bet-., 1895, 31, 4168. 
 
CONSTITUTION OF DIAZO-SALTS AFTER 1894 139 
 
 than the third; the compound was therefore regarded as 
 having the constitution C 6 H 5 . N : NBr, Br 2 . 
 
 Kekule' looked upon this as a tribromohydrazine, 
 
 C 6 H 5 .NBr.NBr 2> 
 and Erlenmeyer wrote it as 
 
 C 6 H 5 .NBr 
 
 III 
 NBr 2 
 
 Hantzsch has prepared a large number of these perhalides,* 
 thus 
 
 ArN 8 .ClaBr ArN 2 .Br. ArN 2 .I 3 
 
 ArN 2 .Cl 2 I ArN 2 .Br 2 Cl ArN 2 .I 2 Cl 
 
 ArN 2 .BrJ ArN 2 .I 2 Br 
 
 ArN 2 .ClBrI 
 
 and regards them as analogous to potassium tri-iodide, caesium 
 tri-iodide, and the trihalides of the quaternary ammonium 
 series. The exact arrangement of the three halogen atoms 
 in the molecule is not known, and no isomerism such as 
 (Ar.N 2 .Cl + Br2) and (Ar.N 2 Br + BrCl) exists. 
 
 6. Relation between syn- and aw ^-compounds. The rate 
 of isomerization of syn- to <mi-diazo-compounds depends 
 largely on the substituents present in the aromatic nucleus. 
 Methyl groups hinder the rate, whilst halogen atoms increase 
 it. Thus the transformation is very difficult to bring about 
 in the case of trimethyl- and methoxy-benzenediazo-oxides, 
 whilst in the case of the unsubstituted benzenediazo-oxide, 
 C 6 H 5 . N 2 . OK, it proceeds quickly above 100. The >-bromo- 
 derivative, on the other hand, is isomerized at boiling-point, 
 the p-sulpho-derivative, SO 3 K . C 6 H 5 . N 2 . OK, slowly at the 
 ordinary temperature, and the tribromo- and >-nitro-deriva- 
 tives, C 6 H 2 Br 3 .N 2 .OK and N0 2 .C 6 H 4 .N 2 . OK, instanta- 
 neously, so that the syn-salt cannot be isolated. 
 
 In the case of the diazo-sulphonates it is the alkylated 
 derivatives which isomerize more quickly than the parent 
 substance, whilst the p- and o-halogen substituted derivatives 
 of the syn series are relatively stable.* 
 
 * Ber., 1895, 28, 2754. 
 
 t Hantzsch and Schmiedel, Ber. } 1894, 27, 3071, 3530. 
 
140 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 Similarly among the syn-di&zo-cy&mdes the o- andp-halogen 
 substituted derivatives are fairly stable, and the parent sub- 
 stance, C 6 H 5 . N 2 . CN, has not been isolated. 
 
 The presence of the nitro-group greatly increases the rate of 
 isomerization in all the above series of s^-compounds. 
 
 7. The isomeric diazo-sulphonates and diazo-cyanides. 
 Having explained Hantzsch's views as to the constitution of 
 the diazonium salts, we can now resume the discussion of the 
 constitution of the diazo-sulphonates and diazo-cyanides which 
 were described on pp. 128, 130. 
 
 The formulae given by Hantzsch to these compounds 
 C 6 H 6 .N " C 6 H 5 .N 
 
 S0 3 K.N N.S0 3 K 
 
 Labile salt (syn). Stable salt (anti). 
 
 C 6 H 6 .N C 6 H 5 .N 
 
 CN.N N.CN 
 
 Labile (syn). Stable (anti). 
 
 were objected to by Bamberger* and Blomstrand,t who did 
 not accept the stereochemical hypothesis. These chemists 
 assigned the following formulae to the above substances 
 C 6 H 6 .N(S0 3 K)iN C 6 H 6 .N:N.S0 3 K 
 
 Labile (normal). Stable (iso). 
 
 C 6 H 5 . N(CN) i N C 6 H 5 . N : N.CN. 
 
 Labile (normal). Stable (iso). 
 
 the formulae for the two sulphonates having already been 
 suggested by V. Meyer and Jacobson, J to which these authors 
 have adhered in succeeding editions of their book. 
 
 Bamberger regarded the quinquevalency of nitrogen in the 
 diazonium salts as being dependent on the negative character 
 of the group with which the diazonium radical was united. 
 Thus when these groups were Cl, NO 3 , SO 4 H, the nitrogen 
 atom was necessarily quinquevalent. This condition was still 
 maintained by the S0 3 K and CN groups, but owing to their 
 
 * Ber., 1895, 28, 242, 447, 834. 
 
 t J. pr. Chem., 1896 [ii], 53, 169 ; 1897, 55, 481. 
 
 I Lehrbuch der org. Chem., II. 303. 
 
CONSTITUTION OF DIAZO-SALTS AFTER 1894 141 
 
 small negative character the labile salts could pass into the 
 stable salts (the nitrogen atom becoming tervalent) with 
 extreme ease. 
 
 Hantzsch * reiterated his objection to this view of the consti- 
 tution of the diazo-sulphonates from the fact that these salts 
 are only dissociated into two ions, namely, Ar.N 2 . SO 3 and K, 
 whilst if they possessed the diazonium constitution they would } 
 according to him, be expected to yield three ions, namely, 
 Ar.N 2 , S0 3 , and K, corresponding to the behaviour of potassium 
 sulphite, which gives the ions K, K, and S0 3 .f Further, he 
 pointed out that the colour of the normal diazo-sulphonate 
 (red) was another argument against Bamberger's view, as 
 benzenediazonium salts with colourless anions (as S0 3 ) were 
 colourless. 
 
 A further argument against the stereochemical view of the 
 isomerism of the diazo-sulphonates was adduced by von Pech- 
 mann. 
 
 He pointed out the fact that both the groups S0 3 H and CN 
 themselves could give rise to isomerism. In order to find a 
 group free from this objection, von Pechmann J selected the 
 diazo-salts of benzenesulphinic acid, C 6 H 6 . N:N.S0 2 . C 6 H 5 , 
 which had been prepared by Koenigs. 
 
 In whatever manner this salt was prepared, it was impos- 
 sible to discover the existence of an isomeride ; this was also 
 true of the p-nitro-derivative, N0 2 . C 6 H 4 . N:N.SO 2 . C 6 H 5 , and 
 von Pechmaim concluded that these facts militated against 
 Hantzsch's theory. 
 
 Hantzsch and Singer || also prepared a number of these 
 additive compounds, but were unable to detect the existence 
 of isomerism. The supposed isomerism of the diazo-thio- 
 sulphonates ^f was shown by Dybowski and Hantzsch ** to 
 have no foundation in fact. ft 
 
 * Per., 1895, 28, 676. 
 
 t See however Ostwald's opinion, p. 130. 
 
 t Ber., 1895, 28, 861. Ber., 1877, 10, 1531. 
 
 || Ber., 1897, 3O, 312. 
 
 II Troger and Ewers, J. pr. Chem., 1900 [ii], 62, 369. 
 * Ber., 1902, 35, 268. 
 
 ft Compare also Hantzsch and Glogauer, Ber., 1897, 30, 2548; Hantzsch, 
 Ber., 1898, 31, 636. 
 
142 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 With regard to the constitution of the diazo-cyanides, 
 Hantzsch * insisted that a diazonium cyanide, Ar.N(CN) N, 
 must be similar to an alkali cyanide, but as the normal diazo- 
 cyanides were coloured, sparingly soluble in water, and soluble 
 in organic solvents, they could not have the constitution attri- 
 buted to them by Bamberger and Blomstrand. 
 
 Finally, Hantzsch and Danzigerf succeeded in preparing 
 a third series of cyanides by treating a diazonium chloride 
 with a suspension of silver cyanide. The insoluble yellow 
 S2/7i-diazo-cyanide is also formed in this reaction, but the filtrate 
 contains a soluble double cyanide with silver cyanide, which is 
 considered by Hantzsch to be a true diazonium derivative. 
 These substances are soluble in water and colourless and 
 resemble the alkali cyanides, and are therefore diazonium 
 cyanides. 
 
 The formation of these double diazonium cyanides led 
 Hantzsch to the hypothesis that the sparingly soluble syn-di&zo- 
 cyanides may exist in solution in a state of equilibrium with 
 the isomeric diazonium salt, and a study of the diazo-cyanides 
 derived from jo-anisidine confirmed this idea.J ^-Methoxy- 
 benzenediazonium bromide and chloride with potassium 
 cyanide in alcoholic solution yield the s^-diazo-cyanide 
 
 MeO.C 6 H 4 .N 
 
 CN.N 
 
 an orange-red, insoluble substance, melting at 51, and coupling 
 with )3-naphthol. This changes slowly into the anti-salt 
 MeO.C 6 H 4 .N 
 
 which is brownish red, melts at 121, and does not couple with 
 /3-naphthol. 
 
 (Certain s^-cyanides are difficult to convert into the anti- 
 modification. Thus that derived from 2:4: 6-tribromoaniline 
 must be combined with benzenesulphinic acid to form the 
 additive product C 6 H 2 Br 3 . NH.N(CN).S0 2 . C 6 H 6 , which on 
 treatment with alkalis yields the anti-cy&mde.) 
 
 * Ber., 1895, 28, 668. t Ber., 1897, 30, 2529. 
 
 J Ber., 1900, 33, 2161 ; 1901, 34, 4166. 
 
CONSTITUTION OF DIAZO-SALTS AFTER 1894 143 
 
 When, however, an aqueous solution of >-methoxybenzene- 
 diazonium hydroxide is evaporated with excess of hydrogen 
 cyanide at the ordinary temperature, a colourless crystalline 
 substance is obtained which has the composition 
 MeO . C 6 H 4 . N 2 . CN, HCN, 2 H 2 O. 
 
 This possesses all the properties of a true metallic salt, it is 
 very soluble, and its solution is an electrolyte. It couples with 
 -naphthol, and is converted into the s?/7i-diazo-cyamde by the 
 action of alkaline solutions. 
 
 There can thus be prepared from ^-anisidine three different 
 diazo-cyanides, namely 
 
 MeO.C 6 H 4 . N 1 N MeO.C 6 H 4 . N MeO.C 6 H 4 . N 
 
 CN CN.N N.CH 
 
 Colourless, soluble Labile, coloured Stable, coloured 
 
 electrolyte. non-electrolyte. non-electrolyte. 
 
 The isolation of these three isomerides was regarded by 
 Hantzsch as a very strong proof of his stereochemical theory, 
 as Bamberger's theory could only account for two of them.* 
 
 It is, however, highly significant that in the two series of 
 isomeric diazo-compounds, the cyanides and the sulphonates, 
 both groups attached to the diazo-nucleus, should themselves 
 be capable of giving rise to isomerism. As regards the 
 cyanides, it has indeed been suggested f that Hantzsch's 
 S2/7i-compound has the constitution Ar.N : N.NC, and the anti- 
 compound Ar.N : N.CN. (See also p. 141.) 
 
 This view would seem to be confirmed by the observation 
 of Hantzsch and Schultze J that both series give the same 
 (anti) diazobenzenecarboxylic acid, Ar.N:N.CO 2 H, for the 
 former compound would be expected to undergo transforma- 
 tion into the latter. According to the stereochemical theory 
 the labile s^/Ti-compound would pass into the more stable anti- 
 cyanide before hydrolysis. Moreover, Hantzsch has offered 
 no proof against this obvious view; he contented himself 
 with stating that neither of these compounds was an iso- 
 cyanide. 
 
 * Another way of accounting for these is explained on p. 168. 
 t Orton, Trans., 1903, 83, 805. J Ber., 1895, 88, 2073. 
 
144 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 8. Constitution of the metallic diazo-oxides. At the 
 
 beginning of 1895 Bamberger's views on the constitution of 
 diazo-compounds up to this time had been as follows: The 
 diazo-salts were to be represented by the Blomstrand formula 
 
 C 6 H 6 .NC1:N. 
 
 The normal, labile diazo-compounds (coupling with phenols) 
 had the constitution C 6 H 5 . N : N.OX. 
 
 The ^so-diazo-compounds (nitrosoamines, not coupling with 
 phenols) were C 6 H 5 .NH.NO or C 6 H 5 .NX.NO, X being a 
 metal such as K, Na, &c., but a little later he represented 
 them as being divided into two groups, namely, (1) normal 
 diazo-compounds (of diazonium type, C 6 H 5 . NCI : N) ; (2) iso- 
 diazo-compounds (of azo-type, C 6 H 6 . N : N.OH). 
 
 Bamberger was led to this change of view by his work on 
 the interaction of nitrosobenzene and hydroxylamine,* from 
 which he supposed that the stable form of diazobenzene 
 hydroxide was formed according to the equation 
 
 C 6 H 5 . NO + H 2 : N.OH = C 6 H 5 . N : N.OH + H 2 O. 
 
 but, as we have shown (p. 133), in reality the normal or labile 
 modification is produced. 
 
 The controversy existing in the years 1895 to 1897 be- 
 tween Hantzsch and Bamberger mainly resolved itself into 
 a discussion of the constitution of the metallic diazo-salts. 
 On the one hand Hantzsch strove to prove that they were 
 stereoisomeric by means of physical measurements (electrical 
 conductivity, &c.), whilst, on the other, Bamberger maintained 
 that their differing chemical characteristics were sufficient 
 evidence that they differed in constitution. 
 
 In 1895 Hantzsch and Gerilowski f prepared a labile form 
 of the sodium salt of diazobenzenesulphonic acid, 
 
 NaO.N 2 . C 6 H 4 . S0 3 Na, 4H 2 O, 
 
 the stable isomeride having been already obtained by Bam- 
 berger. This new labile form is obtained by treating the 
 diazotized sulphanilic acid mixed with water with concen- 
 trated aqueous sodium hydroxide at 0. It forms white, silky 
 
 * Ber., 1895, 28, 1218. t Ber., 1895, 28, 2002. 
 
CONSTITUTION OF DIAZO-SALTS AFTER 1894 145 
 
 needles, has a strongly alkaline reaction, and couples instantly 
 with /8-naphthol. It becomes changed into the stable iso- 
 meride (which contains no water and does not couple with 
 /9-naphthol) by heating with water. The labile salt in aqueous 
 solution forms three ions, as does the stable salt ; according 
 to Hantzsch, if it were a diazonium compound it should form 
 four ions. 
 
 In the following year Bamberger asserted that Hantzsch's 
 conclusions as to the stereoisomerism of these two salts could 
 not be maintained, as they were based on inaccurate observa- 
 tions of their behaviour.* 
 
 Further work was, however, done by Hantzsch. The deter- 
 mination of the electrical conductivity of the two salts f 
 showed that at moderate dilution (v 16 -v 64 ) the conductivity 
 was the same in each case. Whilst, however, the increase in 
 the conductivity of the stable salt from v 32 to v 1024 corresponds 
 with the theory for sodium salts of dibasic acids not hydroly- 
 tically dissociated in aqueous solution,! a fact which shows 
 that the stable salt is not hydrolysed, the conductivity of the 
 labile salt from v 128 increases very rapidly, thus showing that 
 the labile salt has become hydrolysed, forming 
 
 NaS0 3 .C 6 H 4 .N 2 .OH and NaOH. 
 
 The solution also has an alkaline reaction, whilst that of the 
 stable salt is neutral. The conclusion is, therefore, that both 
 diazo-complexes possess acid properties, that of the labile salt 
 being the weaker. The difference between the two salts is 
 thus only a gradual one, and consequently Hantzsch con- 
 sidered that they were stereoisomeric, assigning to them the 
 constitution 
 
 NaS0 3 . C 6 H 4 . N NaS0 3 . C 6 H 4 . N 
 
 NaO.N N.ONa 
 
 Similarly, the cryoscopic researches of Goldschmidt showed 
 that both the normal (syri) and the iso (anti) potassium ben- 
 zenediazo-oxides possessed the same number of ions in aqueous 
 
 * Ber., 1896, 29, 564. t Ber., 1896, 29, 743. 
 
 t Zeitsch. physikal. Chem., 1894, 13, 222. 
 Ber., 1895, 28, 2020. 
 
146 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 solution, and this was likewise considered to be a proof of 
 Hantzsch's view of their constitution. 
 
 Bamberger,* on the other hand, maintained his view that 
 the two compounds were to be formulated 
 
 C B H 5 .N.OK 
 
 C 6 H 6 .N:N.OK 
 
 I 
 
 Labile (normal). Stable (iso). 
 
 To Hantzsch's criticism that there existed no alkali metal 
 the hydroxide of which possessed acid properties, Bamberger 
 denied that diazonium was a compound alkali metal, and held 
 that the hydroxide was neither comparable with tetramethyl- 
 ammonium hydroxide or with potassium hydroxide. 
 
 Bamberger's examples of chemical differences between the 
 normal and the ^so-salts, namely, that the '^so-salt was reduced 
 by sodium amalgam to phenylhydrazine and the normal not,f 
 and that the ^so-salt was converted into the normal salt when 
 treated with benzoyl chloride, whilst the normal salt gave 
 nitrosobenzanilidej were both shown by Hantzsch to be 
 based on error, as he obtained both phenylhydrazine and 
 nitrosobenzanilide || in equal amounts in the two cases. 
 
 Later, however, Bamberger If became convinced that a 
 diazonium hydroxide could not act as an acid, and gave up 
 the diazonium configuration for the normal metallic salts. 
 He now regarded the metallic diazo-salts as existing in the 
 two forms: (1) normal metallic diazo-salts (or diazotates), 
 Ar(N 2 OK), of unknown constitution; the normal diazo- 
 hydroxides, however, were, according to him, 
 
 Ar.N : NH Ar.N N.H 
 
 A or Y I 
 
 and (2) ieo-diazotates, Ar.NiN.OK. 
 
 9. Diazo-ethers. As has been explained (p. 98), von 
 Pechmann and Frobenius ** discovered that the sodium salt of 
 
 * Ber., 1896, 29, 457. 
 
 t Ber., 1896, 29, 473. j Ber., 1897, 30, 211. 
 
 Ber., 1897, 3O, 339. 
 
 || Ber., 1897, 3O, 621 ; 1899, 32, 1718. 
 
 f Annalen, 1900, 313, 97. ** Ber., 1894, 27, 672. 
 
CONSTITUTION OF DIAZO-SALTS AFTER 1894 147 
 
 p-nitrobenzenediazo-oxide (iso-compound of Schraube and 
 Schmidt) gave with methyl iodide a nitrogen ether, 
 
 N0 2 .C 6 H 4 .N(CH 3 ).NO 
 but that the silver salt yielded an oxygen ether 
 
 NO 2 .C 6 H 4 .N:N.O.CH 3 . 
 
 On this account they considered that nitro-iso-diazobenzene 
 hydroxide was a tautomeric substance 
 
 N0 2 . C 6 H 4 . NH.NO or N0 2 . C 6 H 4 . N : N.Ofl. 
 They regarded the oxygen ether, therefore, as a normal diazo- 
 compound (although it was derived from the -iso-salt), as it 
 combined with phenols like diazo-salts, and the nitrogen 
 ether as the ^so-compound. 
 
 Hantzsch * regarded the oxygen ether as an cm^-compound, 
 but experiments by Bambergerf and von Pechmann and 
 Frobenius J confirmed the resemblance of this compound to 
 the normal diazo-salts and its difference from the anti-corn- 
 pounds. Moreover, a large number of similar ethers were 
 prepared by Bamberger, and these were also found to react 
 as normal compounds ; on hydrolysis with alkalis they yielded 
 normal metallic derivatives. Shortly after, Bamberger [| con- 
 sidered that the diazo- ether ought to be regarded as an ^so- 
 compound, and Hantzsch and Wechsler IF found that>-bromo- 
 diazobenzene ethyl ether yielded the anti-oxide on hydrolysis. 
 
 Some time later, as the conflicting views on this subject had 
 not been entirely reconciled, Euler ** investigated the matter 
 afresh. By careful experiment he found that the product of 
 hydrolysis of diazobenzene methyl ether, as well as >-bromo- 
 diazobenzene methyl ether, reacted, as did a normal diazo- 
 compound, but Hantzsch was able to show that the coupling 
 with a-naphthol, on which these experiments were based, was 
 due to a secondary reaction, and that, in fact, the iso (anti) 
 compounds were produced on hydrolysis. It appears, there- 
 fore, that von Pechmann's oxygen ether belongs to the iso- or 
 o^i-series.ff 
 
 * Ser., 1894, 27, 1865, 2968. t Ber., 1894, 27, 3412. 
 
 J Ber., 1895, 28, 170. Ber., 1895, 28, 225. 
 
 || Ber., 1895, 28, 829. IF Annalen, 1902, 325, 226. 
 
 ** Ber., 1903, 36, 2503. 
 
 tt Hantzsch, Ber., 1903, 36, 3097, 4361 ; 1904, 37, 3030 ; Euler, Ber., 
 1903, 36, 3835. 
 
 L 2, 
 
148 CHEMISTRY OF THE DIAZO- COMPOUNDS 
 
 10. Diazo-anhydrides. In 1896 Bamberger * discovered 
 that when normal metallic diazo-salts are treated with cold 
 dilute acetic acid, extremely explosive, yellow diazo-anhydrides 
 are formed. These cannot be obtained from the ^so-salts, 
 which yield colourless hydroxides under similar conditions, 
 and this difference was considered by Bamberger to be 
 another proof of the structural difference of the two. 
 
 The diazo-anhydrides may also be prepared in some cases 
 by treating a diazonium salt with a normal metallic diazo- 
 salt. They couple slowly with phenols, yield oxygen ethers 
 with the alcohols, and react explosively with benzene, yielding 
 diphenyl derivatives. With alkalis they yield the correspond- 
 ing normal salt, and mineral acids convert them into diazonium 
 salts. 
 
 With amines, diazoamino-compounds are obtained; with 
 ammonia, bisdiazoamino-compounds ; and with bromine, diazo- 
 perbromides. 
 
 Bamberger was of the opinion that their constitution was 
 to be represented by 
 
 E.N.O.N.R 
 
 1 1 
 
 but Hantzschf considered that they were more probably 
 represented by R.N : N.O.N : N.R. He found later J that 
 the diazo-anhydrides readily yield s^-diazo-cyanides on 
 treatment with hydrogen cyanide, and pointed out that the 
 anhydrides dissolve very slowly in hydrochloric acid to form 
 diazonium chlorides facts which confirmed his theory of 
 their azo-constitution. 
 
 Bamberger later suggested the formula 
 R.N.O.N : N.R 
 
 i 
 
 but this was rejected by Hantzsch on the ground that, the 
 ST/w-diazo-hydroxide being an extremely weak acid, such a 
 
 * Per., 1896, 29, 446. 
 
 t Ber., 1896, 29, 1074; 1897, 30, 626. 
 
 t Ber., 1898, 31, 636. Ber., 1898, 31, 2636. 
 
CONSTITUTION OF DIAZO-SALTS AFTER 1894 149 
 
 diazonium diazo-oxide should be instantly decomposed by 
 acids. 
 
 From the fact that the diazo-anhydrides yielded syn- 
 diazo-cyanides with hydrogen cyanide and s^/Ti-diazo-sulpho- 
 nates with potassium sulphite, Hantzsch adopted the syn- 
 
 formula 
 
 N.R KN 
 
 II II 
 
 N-0 N 
 
 11. Diazo-hydroxides. Up to the year 1898, although 
 the existence of isomeric metallic diazo-oxides was without 
 doubt, the free diazo-hydroxides corresponding to these had 
 not been prepared. 
 
 From the great similarity of the diazonium salts to the 
 ammonium salts, Hantzsch drew the conclusion that a corre- 
 sponding diazonium hydroxide should be capable of existence, 
 which would of course make a third isomeric hydroxide, 
 having the constitution C 6 H 5 . N(OH) \ N. He succeeded in 
 obtaining an aqueous solution of this by treating diazobenzene 
 chloride with silver oxide (see p. 100). Determination of the 
 electric conductivity of the solution * showed that the affinity 
 constant of the base at is seventy times greater than that 
 of ammonium hydroxide, and is a little greater than that of 
 piperidine. The affinity constants of methoxybenzenedi- 
 azonium hydroxide and ^-cumenediazonium hydroxide are 
 even greater, and are very close to those of the alkali 
 hydroxides. 
 
 The effect of introducing halogens into the aromatic nucleus 
 is shown in the following table : 
 
 k = velocity constant. 
 
 C 6 H 5 .N 2 .OH 0-123 
 
 Br.C 6 H 4 .N 2 .OH 0-0149 
 
 (2:4)Br 2 :C 6 H 3 .N 2 .OH . . . 0-0136 
 (2:4:6) BrgiCgH..^. OH . . 0-0014 
 
 A comparison of the electrical conductivity experiments 
 with the results obtained in the hydrolysis of ethyl acetate 
 
 * Hantzsch and Davidson, Per., 1896, 31, 1612. 
 
150 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 by benzenediazonium hydroxide indicates that, in 1/128 
 -ZV-solution at 0, about 33 per cent, of the base exists in the 
 ionized condition. The ionization observed in the hydrolysis 
 experiments is greater than that determined by the conduc- 
 tivity experiments, and this shows that the electrolytic 
 dissociation is entirely due to the reaction 
 
 C 6 H 5 .N 2 .OH ^ C 6 H 5 .N|N + OH 
 and not to the electrolysis of a diazonium S2/n-diazo-oxide 
 C 6 H 5 .N.O.N 2 .C 6 H 5 
 
 12. Condition of the non-ionized diazonium hydroxide. 
 
 The solution of benzenediazonium hydroxide, when treated 
 with alkali hydroxides, generates an appreciable amount of 
 heat, and thus behaves as a weak acid. This reaction is also 
 indicated by determinations of the electrical conductivity of 
 the diazonium hydroxide solutions when treated with one, 
 two, or more molecular proportions of sodium hydroxide. 
 
 Hantzsch and Davidson explain this by assuming that the 
 non-ionized part of the diazonium hydroxide exists in solution 
 in a hydrated form, thus 
 
 C 6 H 5 .N.OH 
 
 HO.N.H 
 
 which, with alkali hydroxide, loses water, giving the syn- 
 diazo-hydroxide 
 
 C 6 H 5 .N 
 
 and this then furnishes the sodium salt 
 
 C 6 H 5 .N 
 
 NaO.N 
 
 Diazonium hydroxides are consequently known only in solu- 
 tion, and the existence of st/Ti-diazo-hydroxides is doubtful. 
 
 13. Constitntion of iso (anti) diazo-hydr oxides. In 
 1899 Hantzsch enunciated his theory of pseudo-acids, a term 
 
CONSTITUTION OF DIAZO-SALTS AFTER 1894 151 
 
 applied to neutral compounds which, under the influence of 
 alkalis, yield stable salts. Thus, for example, phenylnitro- 
 methane, C 6 H 6 . CH 2 . N0 2 , is stable, neutral, and a non- 
 electrolyte, but with alkalis it changes to the isomeric form 
 C 6 H 5 . CH : NO.OH, which forms stable salts, thus 
 
 C 6 H 5 .CH:NO.OK* 
 
 An examination of the properties of the metallic anti-di&zo- 
 oxides showed that the solution obtained by treating them 
 with an equivalent amount of hydrochloric acid has a neutral 
 reaction, and, conversely, when this solution is treated with 
 an equivalent quantity of alkali, the product is neutral. The 
 substance obtained, therefore, by treating the diazo-salt with 
 acid has the properties of a pseudo-acid,f and is best repre- 
 sented as being a primary nitrosoamine 
 
 R.N [R.N "1 
 
 -* || -> R.NH.NO 
 
 [.OK L N -5j Stable nitrosoamine 
 
 an^-diazo-oxide. Labile (acid). (pseudo-acid). 
 
 In this, it will be noticed, Hantzsch adopts the older view, so 
 long combated by him, of the tautomeric form of the iso- 
 diazo-hydroxide, with the exception that he adheres to the 
 cm^-configuration for the labile form. 
 
 Hantzsch and Pohl % claimed to have prepared these anti- 
 diazo-hydroxides and also the nitrosoamines in the solid 
 condition, and stated that e the nature of the so-called iso- 
 diazo-hydroxides is now definitely elucidated*. This work, 
 however, cannot be regarded as having any bearing on the 
 point, as one example dealt with by Hantzsch and Pohl, 
 namely, the conversion of the metallic anti-salt of 2 : 4 : 6-tri- 
 bromodiazobenzene into the nitrosoamine, was shown by 
 Orton to be quite inaccurate. Orton found that the sub- 
 stance described as the nitrosoamine by Hantzsch and Pohl 
 was in reality a mixture of the quinonediazide, 
 
 * Ber., 1899, 32, 575. 
 
 t Hantzsch, Schumann, and Engler, Ber., 1899, 32, 1703. 
 
 1 Ber., 1902, 35, 2964. 
 
 Proc. Boy. Soc., 1902, 71, 153 ; Trans., 1903, 83, 796. 
 
152 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 Br 
 (see p. 67), and a hydroxyazo-compound.* 
 
 This proof, of course, must be held to throw grave doubt 
 on the correctness of the other cases mentioned by Hantzsch 
 and Pohl, especially as Hantzsch has admitted that 2:4: 6-tri- 
 bromophenylnitrosoamine is unstable and cannot be isolated 
 free from other substances in an analysable condition. Orton's 
 work thus shows that no nitrosoamine is formed under the 
 conditions used by Hantzsch. 
 
 * Compare also Hantzsch, Ber., 1903, 36, 2069 ; Orton, Trans.. 1905, 
 87,99. 
 
CHAPTEK XIX 
 
 OTHER VIEWS OF THE CONSTITUTION OF 
 THE DIAZO-COMPOUNDS FROM 1895 
 
 1. Constitution of the coloured diazo-salts of Jacobson. 
 
 In 1895 Jacobson * examined the diazo-salts of ^-amino- 
 diphenylamine which had been first prepared by Ikuta.f 
 These diazo-salts are distinguished by their great stability 
 and by their yellow colour, in consequence of which Jacobson 
 assigned to them the constitution 
 
 C 6 H,.N.C 6 H 4 .N,HX 
 
 \N/- 
 
 Hantzsch investigated the reactions of these compounds, 
 pointing out that other coloured diazo-salts were known 
 which had undoubtedly the normal constitution, namely, the 
 diazo-salts of di-iodobenzene, diazofluoren, diazophenanthrene, 
 &c. 
 
 He showed that Jacobson's diazo-salts had a neutral reaction 
 and were thus similar to the ordinary diazo-salts, whilst 
 a compound of the above formula would be expected to 
 undergo hydrolytic dissociation and therefore show an acid 
 reaction. The salts were therefore considered by Hantzsch to 
 possess the constitution C 6 H 5 . NH.C 6 H 4 . NX N. 
 
 By the action of potassium hydroxide, however, no corre- 
 sponding metallic salt was formed, but an explosive compound 
 insoluble in water, having the formula C 12 H 9 N 3 , which was 
 evidently an anhydride of the diazo-hydroxide, 
 
 C 6 H 5 .NH.C 6 H 4 .N 2 .OH 
 and to which Hantzsch gave the formula 
 
 C 6 H 6 .N:C 6 H 4 <j| 
 * Annalen, 1895, 287, 131. + Annalen, 1893, 272, 282. 
 
154 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 corresponding to Wolffs formula * for the quinonediazides 
 
 0:C 6 H/ 
 
 2. Constitution of diazo-salts according to Walther. 
 A formula for diazobenzene chloride was proposed by Walther 
 in 1895,f but has not hitherto found acceptance. 
 
 Walther, in endeavouring to explain the fact that the same 
 product is formed by the interaction of diazobenzene chloride 
 and bromoaniline, on the one hand, and bromodiazobenzene 
 chloride and aniline, on the other, suggested that nitrous acid 
 might be supposed to contain quinquevalent nitrogen, and 
 represented the formation of diazobenzene chloride by the 
 equation 
 
 C 6 H 6 . NH 8 C1 + N=0 = C 6 H 5 . 
 \H 
 
 = C 6 H 5 . NHC1 : NH : O + H 2 
 
 this representing diazobenzene chloride only in aqueous solu- 
 tion. The hydroxide would hence be C 6 H 6 . N : NH : O, and 
 diazoaminobenzene C 6 H 6 . N : NH : N . C 6 H 6 , thus providing an 
 explanation of the fact referred to above. 
 
 3. Constitution of diazo-compounds according to Briihl. 
 The question of the constitution of the diazo-compounds 
 has been attacked by Briihl from the point of view of their 
 refractive powers.^ 
 
 It was found that the refraction of the N 2 group in the 
 diazo-compounds is 8-41, or about 3-4 higher than that of the 
 same group in the primary hydrazines. 
 
 In azoxy benzene, the value for the N 2 O group is 11-9, whilst 
 that calculated on the assumption of a single linking between 
 the nitrogen atoms is 7-5, so that Briihl regards azoxy- 
 benzene as a compound of the structure 
 
 ,N.C 6 H 5 C 6 H 5 .N:N.C 6 H 5 
 
 C 6 H 6 
 
 ft 
 
 * Annalen, 1900, 312, 126. 
 
 t J. pr. Chem., 1895 [ii], 51, 528, 581. 
 
 J Zeitsch. physikal. Chem., 1898, 25, 577, 26, 47. 
 
VARIOUS THEORIES OF CONSTITUTION 155 
 
 The normal diazo-oxides are looked upon as being constituted 
 similarly to the nitrosoacyl-compounds, and the formation of 
 diazobenzene from nitrosoacetanilide is written 
 
 CH 5V 3$ C 6 H 5 
 
 >N< | +NaOH = CBL. CO s Na + 
 CHa.CO' 
 
 Normal diazo- 
 benzene. 
 
 C 6 H 5 .N:N.ONa 
 
 Na 
 
 Normal diazo-oxide. tso-diazo-oxide. 
 
 Diazonium salts, however, have the Blomstrand formula 
 C 6 H 5 .NC1|N. >-Nitrodiazobenzene methyl ether (compare 
 p. 147) has the constitution NO 2 . C 6 H 4 . N : N.OMe and is an 
 iso-compound. 
 
 Finally, benzenediazoic acid is regarded as possessing the 
 nitroamine constitution 
 
 O 
 
 4. Constitution of the diazo-componnds according to 
 Bobbie and Tinkler. Dobbie and Tinkler* attacked the 
 problem of deciding the constitution of the isomeric diazo- 
 compounds by observing their ultraviolet absorption spectra. 
 
 The two forms of benzaldoxime had previously been shown 
 to exhibit identical spectra, so that isomeric substances, differ- 
 ing only as do the benzaldoximes, should also give identical 
 spectra, but distinct ones if they were structurally isomeric. 
 
 Diazo-sulphonates. The potassium benzenediazo-sulpho- 
 nates were found to give identical spectra, which would be 
 expected if the substances had the constitution assigned to 
 them by Hantzsch, namely 
 
 C 6 H 5 .N C 6 H 5 .N 
 
 II and || 
 
 S0 3 K.N N.SO 3 K 
 
 Diazo-cyanides. The diazo-cyanides prepared from >-anisi- 
 dine and p-chloroaniline were examined ; both of these pairs 
 
 * Trans., 1905, 87, 273. 
 
156 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 of isomerides gave almost identical spectra, so that here again 
 the syn- and a-n^-configuration would account for this. 
 
 The solution of the diazonium cyanide, OMe.C 6 H 4 . N(CN) j N, 
 gave an entirely different spectrum. This compound is there- 
 fore structurally isomeric with the other two. 
 
 Diazo-oxides. The potassium benzenediazo-oxides were 
 found to give quite different spectra, and the conclusion is that 
 they are structurally isomeric. The nitrosoamine formula for 
 the more stable salt would account for this difference, and it was 
 found that the spectrum of this salt and that of phenylmethyl- 
 nitrosoamine, C 6 H 5 . N(CH 3 ).NO, were in complete agreement, 
 a fact which points to the formula C 6 H 5 .NK.NO as the 
 correct one for the stable salt. 
 
 It was further discovered that a very dilute solution of the 
 labile compound had a spectrum agreeing closely with that of 
 diazobenzene chloride, and this appears to indicate that the 
 original compound changes into a third modification, having 
 the constitution of a true diazonium compound 
 C 6 H 6 -N(OK);N. 
 
 Sulphobenzeiiediazo-oxides. The potassium and sodium 
 compounds obtained from diazotized sulphanilic acid by the 
 action of caustic alkali were also examined. These gave 
 similar results, as in the preceding case ; the spectra were 
 different and they are therefore structurally isomeric and not 
 stereoisomeric. 
 
 Applying the reasoning used by Dobbie and Tinkler in the 
 preceding case, these compounds would consequently possess 
 the constitution 
 
 KS0 3 . C 6 H 4 . N 2 . OK KSO 3 . C 6 H 4 . NK.NO 
 
 Labile. Stable. 
 
 5. Constitution of the diazo-eomponnds according to 
 Armstrong and Robertson. Armstrong and Robertson, in 
 1905,* in discussing the question of the relation of colour to 
 constitution, considered that the yellow colour of phenylazo- 
 ethane, C 6 H 6 . N : N.C 2 H 5 , is conditioned by the presence of the 
 group C 6 H 5 . N : N. alone, the ethyl radical not being known 
 
 * Trans., 1905, 87, 1280. 
 
VARIOUS THEORIES OF CONSTITUTION 157 
 
 as a chromogenic centre in any other case. Arguing from 
 this, they concluded that all compounds of the form 
 
 C 6 H 5 .N:N.X 
 
 should be coloured, and consequently that only coloured diazo- 
 compounds can be represented by such a formula. Armstrong 
 and Robertson call the above compound ' phenyldiazoethane ', 
 but it belongs to the azo-group just as much as does azoben- 
 zene, C 6 H 5 . N : N.C 6 H 5 . Moreover, there are great chemical 
 differences between the coloured diazo-compounds 
 
 C 6 H 5 .N 2 .X 
 
 and the azo-compounds, C 6 H 5 .N 2 .R, where X is an acidiq 
 group and R is an inert group, and, to take an example, 
 according to the above reasoning, if coloured diazo-compounds 
 of the formula C 6 H 5 . N : N.X unite readily with phenols, &c.j 
 to form azo-compounds, one should expect all compounds 
 containing the group C 6 H 5 . N : N. to give the same reaction, 
 which of course they do not. It is therefore not correct to 
 compare the two in the way Armstrong and Robertson have 
 done. These authors, from the above reasoning, deny that the 
 syn- and an^-formulae can represent the constitutions of the 
 labile and stable metallic diazo-compounds respectively, and 
 adopt the nitrosoamine formula R.NK.NO for the latter. 
 
 For the colourless diazo-salts the diazonium formula is 
 advocated as being the only alternative one which, at the 
 time, could be devised. The isomeric sulphonates and cyanides 
 are considered to be represented by the formulae 
 R.N.S0 3 K R.N:N.S0 3 K 
 
 Stable. 
 
 Labile. 
 
 and R.N.CN R.N : N.CN 
 
 Stable. 
 
 Labile. 
 
 In the case of the cyanides, Hantzsch's syn -compound is> 
 regarded as a mixture of the diazonium salt and the anti-salt. 
 
 In a similar way the labile metallic compounds are assigned 
 the diazonium configuration, whilst the stable compounds, 
 
158 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 which, being colourless, could not be written R.N : N.OH 
 according to the authors' reasoning, are considered to be 
 further hydrated, R.N(OH).NH(OH) or R.NH.N(OH) 2 , and 
 these diazo-hydrates, on dehydration, would give rise to the 
 isodynamic nitrosoamines, thus 
 
 R.N(OH).NH(OH) -> R.N.NH 
 
 R.NH.N(OH) 2 -* R.NH.NO. 
 
 Which of these is the parent substance of the iso-compounds 
 is considered to depend on the colour or non-colour of pure 
 nitrosoamines. 
 
 These views provoked a vigorous criticism by Hantzsch,* 
 who pointed out that both coloured and colourless azo-eom- 
 pounds exist in the aliphatic series, for example, the deep red 
 azo-dicarboxylic ester, C0 2 R.N : N.CO 2 R, and the colourless 
 azo-^o-butyric acid derivatives, CRMe 2 . N : N.CRMe 2 ,f and in 
 the aromatic series, the nitrodiazo-ester 
 
 N0 2 .C 6 H 4 .N:N.O.CH 3 t 
 is quite colourless. 
 
 He maintained, therefore, that the presence of the group 
 .N : N. was no reason why a compound should be coloured. 
 
 The diazonium formula for the normal diazo-oxides, and 
 Armstrong and Robertson's proposed formulae for the iso- 
 diazo-oxides had previously been shown to be unsatisfactory 
 and, as the s^-diazo-cyanides and sulphonates are more 
 intensely coloured than the cm^-forms, the former could not 
 consist of a mixture of the latter with a colourless diazonium 
 salt. 
 
 * Proc., 1905, 21, 289. t Thiele, Annalen, 1896, 200, 1. 
 
 J von Pechmann, Ber. t 1894, 27, 672. 
 
CHAPTEE XX 
 
 A REVIEW OF THE VARIOUS THEORIES OF THE 
 DIAZO-COMPOUNDS TO 1907 
 
 IT is evidently an impossible task to reconcile all the 
 conflicting theories of the constitution of diazo- compounds, 
 and although some of them may be dismissed at once, others 
 must receive careful consideration. 
 
 In spite of the immense amount of work done in this field 
 of research by Hantzsch and his pupils, it cannot be said that 
 the stereochemical theory is generally accepted there are 
 many evidences in chemical literature which point to this 
 conclusion. On the other hand, the alternative view of 
 structural isomerism has several exponents, but no common 
 ground has apparently been reached. 
 
 We shall, therefore, endeavour to sum up the principal 
 points connected with the diazo-compounds, which are im- 
 portant in arriving at a theory of the constitution of these 
 compounds. 
 
 1. Constitution of the diazo-salts (diazoninm salts). 
 
 There is fairly general agreement that the formula of 
 Blomstrand represents the constitution of the diazo-salts 
 better than that proposed by Kekule'. The existence of a 
 salt-forming nitrogen atom in the diazo-complex makes it 
 necessary to assume that one nitrogen at least is quinquevalent. 
 Moreover, it would appear most probable that this nitrogen 
 atom is the one attached directly to the aromatic nucleus, for 
 otherwise we should arrive at the formula C 6 H 5 . N j NCI, 
 which, postulating as it does a quadruple linking between 
 the two nitrogen atoms, is unlikely. We thus arrive at the 
 conclusion that in diazobenzene chloride there is a union 
 between the phenyl group and a quinquevalent nitrogen atom, 
 which is linked to a univalent chlorine atom and united with 
 
160 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 a second nitrogen atom. This union may be indicated by 
 a dotted line thus 
 
 C 6 H 5 N-C1 
 
 Nm 
 
 It is very important to observe, however, that the facts 
 referred to do not prove anything more than this ; that is to 
 say, they do not indicate the number of bonds between phenyl 
 and quinquevalent nitrogen or between quinquevalent and 
 tervalent nitrogen. It has, however, been generally considered 
 that the union between phenyl and quinquevalent nitrogen is 
 one linking, whilst that between the two nitrogen atoms is 
 three, thus 
 
 C 6 H 5 -N-01 
 
 N 
 
 There is, however, another and, in the opinion of the 
 author, a better way of arranging these linkings (see p. 163). 
 
 2. The labile and stable isomeric diazo-componnds. In 
 
 studying the properties of these compounds it is impossible 
 to avoid the conclusion that the labile or normal compounds 
 resemble the diazonium salts most closely. The similar 
 behaviour with regard to the formation of azo-compounds, 
 the instability, the probability that, in many cases, equilibrium 
 mixtures or solid solutions of the two exist, and the re- 
 semblance of the two absorption spectra all point to this 
 direction. 
 
 The constitution of the labile salts should therefore be more 
 closely allied to that of the diazonium salts than to that of 
 the stable salts. Probably for this reason V. Meyer and 
 Jacobson, Blomstrand and others, regarded the labile com- 
 pounds actually as diazonium compounds. 
 
 It seems, however, probable that Hantzsch's reasoning 
 against this view as regards the metallic compounds (p. 135) 
 is correct, that is to say, they are not diazonium derivatives. 
 
 In this connexion the views of Briihl as to the close 
 relationship between the constitution of the normal metallic 
 
REVIEW OF THEORIES OF CONSTITUTION 161 
 
 diazo- compounds and the nitrosoacyl-compounds are to be 
 noted, and it seems likely that the true constitution of the 
 former may be similar to that of the latter, although not 
 assuming the form assigned to them by Briihl, and that the 
 formulae of the nitrosoacyl-compounds may be tautomeric 
 with those of the corresponding diazo-acetates. Such a new 
 formulation of the nitrosoacyl-compounds, however, cannot be 
 suggested up to the present. 
 
 We arrive, therefore, at the conclusion that neither the 
 actual diazonium formula C 6 H 5 . N(OK) j N nor the syn-di&zo- 
 formula 
 
 C 6 H 5 .N 
 KO.N 
 
 really represents the constitutions of these compounds. 
 
 The labile sulphonates are also probably not diazonium 
 compounds, but the arguments against the possibility of them 
 being sulphites are not so strong, and this is one of the 
 reasons why the stereochemical theory is not accepted with 
 regard to this case. 
 
 Hantzsch's argument against the sulphite constitution is 
 principally that a diazo-sulphite would form three ions whilst 
 the sulphonates give rise to only two ions. This argument 
 has been recorded in many textbooks without, however, the 
 important fact being added that no less an authority than 
 Ostwald has stated (p. 130) that a diazonium sulphite would 
 give rise only to two ions. Here, therefore, the possibility 
 of the normal sulphonates being really sulphites cannot be 
 regarded as excluded. 
 
 In the case of the labile diazo-cyanides we are met with 
 a similar uncertainty as in the case of the sulphonates, namely, 
 the possibility of isomeric change in the added group. So 
 long as it is not proved that the labile diazo-cyanides cannot 
 be isocyanides it cannot be maintained that they are syn- 
 compounds. 
 
 Turning now to the stable or ^so-compounds, most of the 
 work done points to the nitrosoamine formula for the metallic 
 compounds, C 6 H 5 . NK.NO, and there seems to be a consensus 
 
 M 
 
162 CHEMISTRY OF THE DIAZO-COMPOTJNDS 
 
 of opinion that the 'iso-diazo-hydroxides, diazo-cyanides, and 
 
 sulphonates have the azo-constitution 
 
 C 6 H 5 . N : N.OH C 6 H 5 . N : N.CN C 6 H 5 . N : N.S0 3 H. 
 
 From what has been said previously it will be evident that 
 the existence of a special aTi^-configuration of these compounds 
 depends on the simultaneous existence of the corresponding 
 s^/Ti-compounds, which, as has been shown, cannot be regarded 
 as having been definitely proved to possess this constitution. 
 
APPENDIX 
 
 A NEW THEORY OF THE CONSTITUTION OF 
 THE DIAZO-COMPOUNDS 
 
 IN 1907 the author of this book put forward a new theory 
 of the constitution of diazo-compounds,* 1 which it is con- 
 sidered will not only explain the reactions of the diazo- 
 compounds more readily than any of its predecessors, but also 
 serve to throw light on some phenomena hitherto left unsolved. 
 Perhaps the most striking reaction of the diazo-salts (diazonium 
 salts) is the readiness with which the whole of the diazo-nitrogen 
 is eliminated. There are no examples in the literature of singly- 
 linked nitrogen being otherwise than firmly attached to the 
 benzene nucleus and requiring energetic treatment for its 
 liberation. 
 
 There are, however, cases where nitrogen, when attached to 
 the benzene nucleus by two bonds, is most readily eliminated, 
 one of the most striking being that of quinonechloroimide 
 
 :NC1 
 
 Here, as in the case of the diazo-salts, the nitrogen is split off 
 simply by heating the compound with water to 100. 
 
 This reaction showing the great difference in behaviour of 
 nitrogen attached by one and two linkings respectively to the 
 aromatic nucleus, is obviously of much importance in arriving 
 at a decision as to the manner in which nitrogen is united with 
 the aromatic nucleus in diazo-salts. 
 
 It appears almost certain, from this analogy, that an atom 
 of nitrogen in these salts is attached to the aromatic nucleus 
 by two linkings. This idea at once leads us to a quinonoid 
 configuration of the diazo-salts, thus 
 
 C1N:N 
 
 H' 
 
 * Trans., 1907, 91, 1049. 
 
164 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 which conform to the requirements of these salts in that the 
 nitrogen attached to the benzene ring is quinquevalent, 
 and, of course, it explains more satisfactorily than does the 
 Blomstrand formula the ready elimination of diazo-nitrogen. 
 An obvious criticism, and indeed one which has been privately 
 advanced against this formula, is that, with a single linking 
 between the tervalent nitrogen atom and the para-carbon 
 atom, one should expect that, on reduction, the double linking 
 between the nitrogen atom would break and a para-diamine 
 result. This objection would be a weighty one were the 
 tervalent nitrogen united to a carbon atom simply (as in the 
 case of aniline) instead of to the CH group. This fact is of 
 much importance, for that a great difference in stability exists 
 in the two cases has been proved by E. Buchner. In his 
 researches on the action of diazoacetic ester on unsaturated 
 acid esters * this chemist found that the group 
 
 always became converted into the group : C : N.NH, and it 
 may be concluded therefore that the latter group is more stable 
 than the former. 
 
 But we have in the new formula for diazo-salts (I) the 
 same group (II) 
 
 C1.N : N 
 
 /\ 
 
 :C:N:N.CH. 
 
 I II 
 
 in which, from the above work by Buchner, we may reason- 
 ably conclude that the linking : N.CH is more unstable than 
 the linking : C : N, and would be the first to be ruptured in 
 any reaction tending to destroy the configuration. 
 
 The first stage, therefore, in such a reaction can be repre- 
 sented by 
 
 * Ber., 1894, 27, 868, 877, 879 ; see also Curtius, Ber., 1896, 29, 767. 
 
APPENDIX 165 
 
 C1N : N GIN : N- 
 
 and the quinonoid formation having been thus disturbed, the 
 ordinary configuration is resumed when the reaction proceeds 
 to the next stage (reduction, formation of azo-compounds, 
 elimination of nitrogen, &c.). 
 
 The ordinary reactions of the diazo-salts are thus satisfac- 
 torily explained. 
 
 All the work of Hantzsch and his collaborators on the nature 
 of the radical 'diazonium', as has been shown, indicates 
 that this acts like a compound alkali metal, the acid radical 
 attaching itself to the quinquevalent nitrogen atom. 
 
 It is quite obvious that in this respect no difference can be 
 detected between 
 
 V V 
 
 C 6 H 5 .N;N and C 6 H 5 :N:N 
 
 so that Hantzsch's results are equally applicable to the new 
 formulation of ' diazoniuin '. 
 
 We shall now consider some phenomena in diazo-chemistry 
 which have hitherto remained unexplained by any of the 
 former theories. When we compare ^-phenylenediamine and 
 benzidine 
 
 ~ " NH 
 
 we find a great difference in their behaviour towards nitrous 
 acid. The former is converted into'the tetrazo-compound only 
 with difficulty and under special conditions, but the latter 
 changes with perfect readiness. According to the Blomstrand 
 or Kekule' formula this cannot be explained, but light is 
 thrown on the mechanism of the reaction by the following 
 considerations. Benzidine, when tetrazotized, becomes 
 
 C1N: 
 
166 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 
 but the first stage in diazotizing p-phenylenediamine must 
 give a compound of formula 
 
 :NC1 
 
 We now obtain a compound containing an amino-group, 
 which is in the para-position with respect to a carbon atom, 
 all of whose affinities are satisfied, and therefore it cannot 
 link up with a second nitrogen atom. This explains why the 
 tetrazotization does not proceed normally. Under the special 
 conditions necessary, however (see p. 21), the linking between 
 the aminic carbon atom and the tervalent diazo-nitrogen atom 
 is broken, thus 
 
 NH 2 
 
 :NC1 
 
 and now the amino-group can be diazotized, for its added 
 nitrogen atom unites with the corresponding nitrogen atom 
 of the first diazo-group 
 
 C1N:< >:NC1 
 
 . JLJ1J 
 
 There are also several ^-diamines in which only one amino- 
 group can be diazotized, thus 
 
 NH, 
 
 Here the same explanation as that given for the case of 
 p-phenylenediamine probably holds good, but the para-linking 
 may be rendered more stable by the presence of the acidic 
 groups, and hence it does not break to allow the diazotization 
 of the second amino-group to take place. This is, however 
 broken when an azo-compound is formed, so that the second 
 amino-group may now be readily diazotized. 
 
APPENDIX 
 
 167 
 
 It is evident that, according to this theory, diazo-salts cannot 
 be formed where a quinonoid configuration is precluded, so 
 that we can now explain why the compounds 
 H 
 
 NH 2 
 
 and 
 
 H 
 
 do not give diazo-salts, whilst the compounds 
 NH 2 H NH 
 
 ^ 
 
 and 
 
 are readily diazotized. 
 
 In this connexion, also, we see that it is impossible for 
 aliphatic amines to yield diazo-salts, thus 
 
 CH 2 . NH 2 
 C0 2 Et 
 
 gives 
 
 / 
 
 CH< 
 
 I xi 
 
 C0 2 Et 
 
 and not 
 
 CH 2 .N 2 C1 
 
 This new formula for diazo-salts has received confirmation 
 by the work of Morgan and Hird.* 
 
 Isomeric diazo-compounds. The hydroxide corresponding 
 with diazobenzene chloride would, on the above formulation, 
 have the constitution 
 
 H> 
 
 v :N.OH 
 
 It is evident that the hydroxyl group may migrate to the 
 other nitrogen atom now that the quinquevalency of the first 
 nitrogen atom is not supported by the presence of an acidic 
 group. We thus arrive at the formula 
 
 * Trans., 1907, 91, 1505 ; compare also Morgan and Wootton, Trans.. 
 1907, 91, 1311. 
 
168 CHEMISTRY OF THE DIAZO-COMPOUNDS 
 H\ 
 
 -N.OH 
 
 which, from its close connexion with the previous one, is an 
 exceedingly probable one for the normal (syn) diazo-compounds 
 (metallic salts, and, supposing that the normal cyanides and 
 sulphonates are not isocyanides and sulphites respectively, 
 for these also). 
 
 The great resemblance existing between the normal diazo- 
 compounds and the diazonium salts is very readily explained 
 by this formula. The more energetic means necessary to 
 produce the iso-diazo-compounds naturally tend to destroy the 
 bicyclic system here shown ; accordingly the change from 
 normal to iso-compounds occurs thus 
 
 V > N 
 
 N(OH)(CN)(S0 3 H) 
 
 -> <^ )> .N : N(OH) (CN) (S0 3 H), 
 
 arriving in a very natural manner at the most probable formula 
 for the -iso-compounds. 
 
 As has been shown, the formula for the ^so-metallic salts 
 can allow tautomerism to take place,, and consequently the 
 most stable condition is assumed Iby these compounds 
 C 6 H 5 . N : N.OK -* C 6 H 5 . NK.NO. 
 
 Finally, as the stereochemical theory appears thus to be 
 rendered unnecessary, it seems best to use the older terms, 
 normal and iso, instead of syn and anii respectively. 
 
 For the diazo-salts with acids, it is, however, most convenient 
 to retain the term ' diazonium ', although in this book, as was 
 explained in the introduction, the term ' diazo ' introduced by 
 Griess, and also used by the Chemical Society, has been 
 retained in order to avoid confusion or misunderstanding before 
 the theoretical explanation had been reached. 
 
SUBJECT INDEX 
 
 Absorption spectra of diazo-compounds, 
 155. 
 
 Acetoxy-group, replacement of diazo- 
 group by, 53. 
 
 Alcohols, action of, on diazo-compounds, 
 38. 
 
 Amines, diazotization of, 14. 
 
 Aminoazo-compounds, 83 et seq. 
 
 Amino-group, replacement of diazo-group 
 by, 52. 
 
 Aminonaphthols, diazotization of, 16. 
 
 Ammonia, action on diazo-compounds, 
 56. 
 
 Amyl diazoacetate, 106. 
 
 Amyl nitrite, use of, 6. 
 
 Aniline, 53. 
 
 Aurin, 30. 
 
 Azoammonium, 117. 
 
 Azobenzene, 82. 
 
 Azo-compounds, 80 et seq. 
 
 Azo-dyes, discovery of, 2, 4. 
 
 Azogen red, 26. 
 
 Azoimino-group, replacement of diazo- 
 group by, 56, 58. 
 
 o-Azonaphthalene, 51. 
 
 Azophor blue D., 26. 
 
 Azophor red P.N., 26. 
 
 Azoxy benzene, 81. 
 
 Azoxy-compounds, 81. 
 
 Barium nitrite, use of, 10. 
 Benzeneazoacetaldehyde, 93. 
 Benzeneazoacetone, 92. 
 Benzeneazodiphenyl, 51, 60, 62. 
 Benzeneazomethane, 91. 
 Benzeneazonitroethane, 91. 
 Benzenediazoic acid, 102, 122, 155. 
 Benzenediazosulphonates, 128, 140, 155. 
 Benzidine, 23. 
 Benzonitrol, 26. 
 
 Benzoyl chloride, action on diazo-com- 
 pounds, 59, 
 Bromobenzene, 45. 
 /3-Bromonaphthalene, 46. 
 
 Calcium nitrite, use of, 9. 
 Chlorobenzene, 43. 
 
 1-Chlorodiazo-jS-naphthalene nitrite, 69. 
 Chlorodibromodiazobenzene bromide, 68. 
 Cuprous chloride, r6le of, 44. 
 Cyanogen, replacement of diazo-group 
 
 by, 49. 
 Cyano-group, replacement of diazo-group 
 
 by, 49. 
 
 Decomposition of diazo-compounds, rate 
 of, 35. 
 
 Diamines, 23. 
 Diazo, meaning of, 1, 117. 
 Diazoacetamide, 106. 
 Diazoacetic ester, 104. 
 Diazoacetophenone, 110. 
 Diazoaminobenzene, 73. 
 Diazoaminobenzoic acid, discovery of, 1, 
 Diazoamino-compounds, 73 et seq., 78. 
 Diazoaminomethane, 111. 
 Diazo-anhydrides, 148. 
 Diazo-azides, 9. 
 Diazobenzene chloride, 7, 17. 
 Diazobenzene hydroxide, 100, 120. 
 Diazobenzeneimide, 57, 58, 114. 
 Diazobenzene nitrate, 6, 11, 27, 28, 70, 
 
 116. 
 
 Diazobenzene picrate, 8. 
 Diazobenzene sulphate, 7, 27. 
 jp-Diazobenzenesulphonic acid,8, 144,156. 
 Biazocamphor, 103. 
 Diazo-carbonates, 9. 
 Diazo-chromates, 8, 9. 
 Diazo-compounds, constitution of, 112 et 
 
 seq. 
 
 Diazo-cyanides, 130, 140, 142, 143, 155. 
 Diazodiphenylamine, 153. 
 Diazo-ethers, 146. 
 Diazo-fluorides, 9. 
 Diazo-group, migration of, 76. 
 Diazo-halides, 138. 
 Diazo-hydroferrioyanides, 9. 
 Diazo-hydroxides, 149, 150, 167. 
 Diazoic acids, 101. 
 Diazomethane, 108. 
 Diazomethanedisulphonic acid, 108. 
 Diazonaphthalenesulphonic acid, 8, 30. 
 Diazo-nitrates, 10. 
 Diazo-nitrites, 9. 
 Diazonium, 133. 
 Diazonium hydroxide, 150. 
 Diazo-oxides, 144, 156. 
 wo-Diazo-oxides, 100. 
 Diazo-perchlorates, 9. 
 Diazo-perhalides, 138, 139. 
 Diazophenols, 9, 11. See also Quinone- 
 
 j>-Diazophenylhydroxylamine chloride, 
 
 Diazo-picrates, 8. 
 Diazoprimuline, 70. 
 Diazo-salts, discovery of, 3. 
 Diazo-sulphides, 56. See also Thiodia- 
 
 zoles. 
 
 Diazo-thiosulphates, 9. 
 Diazotization, 13 et seq., 28. 
 #-Diazotoluene nitrate, 11. 
 Diazo-tungstates, 9. 
 
170 
 
 SUBJECT INDEX 
 
 4 :4'-Dihydroxydiphenyl, 29. 
 
 /3j3-Dinaphthyl, 62. 
 
 3 : 4-Dinitro-o-anisidine, diazotization of, 
 
 63. 
 
 Dinitro-p-anisidine, diazotization of, 63. 
 2 : 2'-Dinitrodiphenyl, 61. 
 Dinitro-p-toluidine, 15. 
 Diphenyl, 60. 
 Diphenyl ether, 41. 
 Dithiosalicylic acid, 50. 
 
 Ethyl diazoacetate, 105, 106, 107. 
 Ethyl wo-diazoacetate, 107. 
 Explosibility of diazo-compounds, 27. 
 
 Fluorobenzene, 46. 
 Formazyl compounds, 94. 
 
 Guaiacol, 31. 
 
 Halogens, replacement of diazo-group 
 by, 43. 
 
 Hydrazine, action on diazo-compounds, 
 57. 
 
 Hydrogen, replacement of diazo-group 
 by, 42, 55. 
 
 Hydrogen sulphide, action on diazo- 
 compounds, 56. 
 
 Hydroxyazo-compounds, 86 et seq. 
 
 Hydroxydiphenyl, 30, 60. 
 
 lodo-compounds, 46. 
 
 Light, action oa diazo-compounds, 70 et 
 
 Methyl diazoacetate, 105. 
 
 Nitrazol C., 26. 
 
 Nitrobenzene, 51, 52. 
 
 2?-Nitro-o-cresol, 31. 
 
 ?n-Nitrodiazobenzene chloride, 28. 
 
 o-Nitrodiazobenzene chloride. 14, 26, 28, 
 96. 
 
 #-Nitrodiazobenzene nitrate, 27. 
 
 Nitroformazyl, 91. 
 
 Nitro-group, replacement of diazo-group 
 by, 51. 
 
 Nitro-p-phenylenediamine, 22. 
 
 jp-Nitrophenylnitrosoamine, 97. 
 
 Nitrosamine red in paste, 26. 
 
 Nitrosoacetanilide, 77. 
 
 Nitrosoamines, 151. 
 
 Nitrosoanilides, 121, 155. 
 
 Nitrosodiazo-derivatives, 17. 
 
 Nitroso-group, replacement of diazo- 
 group by, 52. 
 
 Nitrosulphonic acid, use of, 10. 
 Nitrosyl bromide, use of, 10. 
 Nitrosyl chloride, use of, 10. 
 
 Oxidation of diazo-compounds, 101. 
 
 Pentabromoaniline, 15. 
 Phenols, formation of, 29. 
 Phenylcarbimide, 49. 
 Phenyldiazomethane, 109. 
 Phenylenediamines, action of nitrous 
 
 acid on, 19. 
 
 Phenylethyltriazen, 73. 
 Phenylhydrazine, constitution of, 119. 
 Phenylmethyltriazen, 78. 
 Phenyl sulphide, 50, 51, 62. 
 Phenylthiocarbimide, 49. 
 Potassium benzenediazo-oxides, 99. 
 Potassium benzyldiazo-oxide, 110. 
 Potassium methyldiazo-oxide, 110. 
 
 Quinonediazides, 11, 12, 64, 67, 100, 126, 
 152. See also Diazophenols. 
 
 Reduction of diazo-salts, 42. 
 Eefraction of diazo-compounds, 154. 
 
 Sandmeyer's reaction, 43 et seq. 
 
 Sodium diazoacetate, 109. 
 
 Stability of diazo-solutions, 34. 
 
 Strecker's salt, 118, 128. 
 
 Sulphinic acid group, replacement of 
 diazo-group by, 55. 
 
 Sulphonic acid group, replacement of 
 diazo-group by, 51. 
 
 Sulphur dioxide, action on diazo-com- 
 pounds, 54. 
 
 Sulphur, replacement of diazo-group by, 
 50. 
 
 syn and anti, 123. 
 
 Thermochemistry of diazo-compounds, 
 27. 
 
 j?-Thiocyanodiazobenzene chloride, 68. 
 
 Thiocyano-group, replacement of diazo- 
 group by, 49. 
 
 Thiodiazoles, 12. 
 
 Thiophenols, 50. 
 
 p-Toluonitrile, 48. 
 
 Triamines, 25. 
 
 Triazolens, 32. 
 
 2:4: 6-Tribromodiazobenzene chloride 
 transformation of, 69. 
 
 2:4: 6-Tribromo-l-nitrobenzene, 52. 
 
 Trinitroaniline, 15. 
 
NAME INDEX 
 
 Abt, 19. 
 
 Altschul, 13, 18. 
 
 Ambuhl, 7, 91. 
 
 Ammelburg, 65. 
 
 Ampola, 91. 
 
 Andresen, 5. 
 
 Andrews, 83. 
 
 Angeli, 44, 103, 110, 111, 
 133. 
 
 Armstrong, 45, 156, 157. 
 
 Auvers, 89, 90. 
 
 Badische Anilin- rind Soda- 
 Fabrik, 67, 68. 
 
 Baeyer, 8, 42, 47, 82. 
 
 Baly, 83, 91. 
 
 Bamberger, 5, 11, 17, 18, 
 27, 32, 52, 56, 61, 67, 74, 
 76, 82, 86, 91, 94, 96 et 
 seq., 108, 120, 121,122,125, 
 127, 128, 129, 132, 133, 
 134, 140, 141, 144 et seq. 
 
 Battegay, 68. 
 
 Bayer & Co., 51. 
 
 Beeson, 41, 42. 
 
 Bennewitz, 10. 
 
 Berger, 61. 
 
 Berlin, 55. 
 
 Bernthsen, 74. 
 
 Berthelot, 27, 28, 70. 
 
 Bevan, 70. 
 
 Beyer, 93. 
 
 Beysen, 15. 
 
 Biehringer, 59. 
 
 Binder, 83. 
 
 Blagden, 47, 52. 
 
 Blomstrand, 116, 117, 120, 
 140, 160. 
 
 Bornstein, 51, 62. 
 
 Boehringer & Sons, 18. 
 
 Borghaus, 20, 21. 
 
 Borsche, 60, 90. 
 
 Bromwell, 40. 
 
 Brtihl, 154, 160. 
 
 Buchner, 164. 
 
 Bulow, 22, 92, 93. 
 
 Buntrock, 35. 
 
 Burdett, 71. 
 
 Busch, A., 59. 
 
 Busch, M., 94. 
 
 Butleroff, 112. 
 
 Cain, 31, 32, 33, 36, 37, 48, 
 163. 
 
 Cameron, 11, 39, 40. 
 
 Cantzler, 48. 
 
 Caro, 2, 8, 25. 
 
 Cassella&Co., 14. 
 
 Castellana, 8, 9. 
 
 Cauffman, 40. 
 
 Chamberlain, 41, 42. 
 
 Chattaway, 62. 
 
 Ciusa, 72. 
 
 Claisen, 93. 
 
 Glaus, 15, 129. 
 
 Coates, 71. 
 
 Graff, 83. 
 
 Corse, 60. 
 
 Grandmougin, 32, 42, 55, 
 
 Cross, 70. 
 
 87. 
 
 Culmann, 42, 61. 
 
 Green, 70. 
 
 Curtius, 4, 96, 103, 106, 107, 
 
 Griess, 1, 2, 3, 4, 6, 8, 12, 
 
 110, 164. 
 
 16, 19, 21, 22, 29, 34, 38, 
 
 D'Angelo, 8, 9. 
 
 39, 43, 55, 56, 57, 58, 60, 
 
 Danziger, 142. 
 
 74, 76, 83, 96, 112, 113, 
 
 Darapsky, 107. 
 
 138. 
 
 Dashiell, 41. 
 
 Griffin, 42. 
 
 Davidson, 11, 149, 150. 
 
 Gruhl, 73. 
 
 Dimroth, 73, 111. 
 
 Haarhaus, 83. 
 
 Dobbie, 155, 156. 
 
 Hailer, 93. 
 
 Duval, 95. 
 
 Haller, 39. 
 
 Dybowski, 141. 
 
 Hantzsch, 5, 7, 9, 11, 15, 17, 
 
 East, 53. 
 
 20, 21, 28, 32, 35, 37, 40. 
 
 Eble, 73. 
 
 47, 52, 54, 56, 68, 69, 76, 
 
 Ehrenpreis, 83. 
 
 89, 98, 100, 101, 102, 104, 
 
 Ehrhardt, 48. 
 
 107, 108, 109, 121, 123 et 
 
 Eibner, 42. 
 
 seq., 133 et seq., 141 et 
 
 Ekbom, 54. 
 
 seq., 158, 160, 161, 165. 
 
 Engler, 151. 
 
 Hasse, 39. 
 
 Epstein, 20. 
 
 Hausknecht, 48. 
 
 Erban, 14. 
 
 Hausser, 35. 
 
 Erdmann, 44. 
 
 Hayduck, 38. 
 
 Erlenmeyer, 112, 116, 139. 
 
 Heinichen, 31. 
 
 Euler, 36, 37, 147. 
 
 Heller, 87. 
 
 Ewers, 141. 
 
 Henderson, 51. 
 
 Eynon, 9, 83, 84. 
 
 Hepburn, 9. 
 
 Eyre, 63, 66. 
 
 Hepp, 17. 
 
 Farmer, 89. 
 
 Heusler, 60. 
 
 Favrel, 92, 94, 95. 
 
 Hewitt, 89, 90. 
 
 Feer, 70. 
 
 Hille, 55. 
 
 Feitler, 44. 
 
 Hird, 167. 
 
 Fierz, 57. 
 
 Hirsch, 7, 30, 34, 37, 60. 
 
 Fischer, E., 18, 25, 38, 55, 
 
 Honigsberger, 90. 
 
 58, 92, 118, 119. 
 
 Hopfner, 93. 
 
 Fischer, O., 11, 17, 25, 38. 
 
 Hofmann, 39, 48, 80, 113. 
 
 Forgan, 61. 
 
 Hofmeister, 41, 86. 
 
 Forster, 57. 
 
 Holleman, 115. 
 
 Fourneaux, 83. 
 
 Holm, 73. 
 
 Fraenkel, 106. 
 
 Hopkins, 40. 
 
 Franke, 55. 
 
 Hunt, 1, 2. 
 
 Freese, 56. 
 
 Ikuta, 153. 
 
 Freimann, 87. 
 
 Jacobson, 12, 89, 90, 140, 
 
 Friedlander, 42, 66. 
 
 153, 160. 
 
 Friese, 91. 
 
 Jager, 68. 
 
 Fritsch, 76. 
 
 Jaeger, 8, 17, 47. 
 
 Frobenius,98, 121, 146, 147. 
 
 Japp, 92, 120. 
 
 Gabriel, 48. 
 
 Jenisch, 93. 
 
 Gaess, 65. 
 
 Jochem, 7, 40. 
 
 Gasiorowski, 42, 61. 
 
 Jourdan, 92. 
 
 Gattermann, 5, 44, 45, 48. 
 
 Jungius, 78. 
 
 Gehren, 46. 
 
 Kalle & Co., 31. 
 
 Gerilowski, 136, 144. 
 
 Karres, 25. 
 
 Gerland, 1, 2. 
 
 Kastle, 10. 
 
 Girard, 10. 
 
 Kaufler, 23, 25. 
 
 Glogauer, 141. 
 
 Kegel, 89. 
 
 Glutz, 55. 
 
 Keiser, 10. 
 
 Goldschmidt, 73, 75, 78, 88, 
 
 Kekule, 57, 113, 114, 115, 
 
 89, 90, 133, 145. 
 
 139. 
 
 Goske, 74. 
 
 Kiefer, 41. 
 
 Graebe, 50. 
 
 Kjellin, 92. 
 
172 
 
 NAME INDEX 
 
 Klason, 50. 
 
 Klingemann, 92, 120. 
 
 Knoevenagel, 6, 27. 
 
 Koenigs, 54, 141. 
 
 Kolbe, 1, 2. 
 
 Koninck, 10. 
 
 Kortright, 40. 
 
 Kostanecki, 89. 
 
 Kraus, 56, 67. 
 
 Krttckeberg, 95. 
 
 Kuchenbecker, 82. 
 
 Kuhling, 61. 
 
 Kuster, 48. 
 
 Kunz, 11. 
 
 Ladenburg, 10, 19. 
 
 Landsteiner, 101. 
 
 Lange, 25. 
 
 Langfurth, 113. 
 
 Lauth, 18. 
 
 Lehmann, 108, 109. 
 
 Lenz, 82. 
 
 Leuchart, 50. 
 
 Liebermann, 89. 
 
 Limpricht, 10. 
 
 Locher, 60. 
 
 Low-Beer, 90. 
 
 Macintyre, 60. 
 
 McPherson, 89. 
 
 Mai, 42, 53. 
 
 Manck, 108. 
 
 Mann, 50. 
 
 Marquardt, 95. 
 
 Martins, 6. 
 
 Mebus, 14. 
 
 Meissen, 103. 
 
 Meldola, 5, 9, 18, 32, 53, 63, 
 
 64, 66, 71, 76, 82, 83, 84, 
 
 89. 
 
 Metcalf, 41. 
 Meyer, C., 108. 
 Meyer, F., 61. 
 Meyer, V., 7, 15, 75, 91, 92, 
 
 93, 120, 140, 160. 
 Michael, 58. 
 Michaelis, 18. 
 Micklethwait, 9, 84. 
 Mills, 82, 83. 
 Mitchell, 90. 
 Mitscherlich, 80. 
 Mixter, 83. 
 Moale, 42. 
 
 Mohlau, 17, 32, 61, 89. 
 Morgan, 8, 9, 69, 84, 85, 167. 
 Muller, E., 107, 110. 
 Muller, F. H. S., 55. 
 Muller, J., 94, 99. 
 Mtinzer, 92, 93. 
 Muller, 35. 
 Nicoll, 36, 37. 
 Niementowski, 61. 
 Nietzki, 21, 83, 89. 
 
 Noelting, 15, 19, 32, 58, 68, 
 
 83. 
 
 Notzel, 87. 
 Norris, 60. 
 
 Oddo, 28, 34, 37, 46, 91. 
 Oehmichen, 32. 
 Oliveri-Tortorici, 17. 
 Orloff, 77. 
 
 Orndorflf, 39, 40, 53, 87. 
 Orton, 5, 12, 52, 68, 71, 143, 
 
 151, 152. 
 Osborne, 109. 
 Ostwald, 130, 161. 
 Pabst, 10. 
 Palmer, 39, 41. 
 Parks, 41. 
 Pechmann, 4, 56, 75, 93, 94, 
 
 98, 107, 108, 120, 121, 125, 
 
 141, 146, 147, 158. 
 Pelet, 25. 
 Perkin, F. M., 76. 
 Pfitzinger, 42. 
 Piria, 1. 
 Pohl, 151. 
 Power, 32. 
 Rabischong, 94. 
 Ray, 87. 
 Redard, 25. 
 Renauld. 108. 
 Remsen, 39, 41. 
 Robertson, 156, 157. 
 Romer, 55, 118. 
 Rugheimer, 11. 
 Ruff, 70. 
 
 Sandmeyer, 5, 43. 
 Saunders, 46. 
 Salkowski, 31. 
 Schaub, 55. 
 Schiff, 58, 103. 
 Schleissing, 68. 
 Schmiedel, 139. 
 Schmidt, 5, 96, 97. 
 Schmitt, O., 18. 
 Schmitt, R., 3, 8, 9, 11, 46, 
 
 55. 
 
 Schraube, 5, 76, 96, 97. 
 Schultze, 130, 143. 
 Schumann, 28, 151. 
 Schwalbe, 14, 35, 36. 
 Seidler, 15. 
 Sheddon, 32. 
 Shober, 41. 
 
 Silberrad, 10, 104, 107. 
 Silberstein, 67. 
 Simpson, Maule & Nichol- 
 son, 86. 
 
 Singer, 54, 141. 
 Smart, 10. 
 Smith, W., 1. 
 Soc. Ohim. des Usines du 
 
 Rh6ne, 31. 
 
 Smythe, 68. 
 Spiegelberg, 113. 
 Stallberg, 48. 
 Stein, 70. 
 Stenhouse, 10. 
 Stephens, 64. 
 Storch, 52, 101. 
 Streatfeild, 66, 76. 
 Strecker, 55, 116, 118, 134. 
 Stiiber, 15. 
 Tauber, 19, 20, 23. 
 Tafel, 91. 
 Thiele, 108, 109. 
 Tichwinsky, 61. 
 Tinkler, 155, 156. 
 Traube, 109. 
 TrOger, 55, 141. 
 Tuck, 83, 90, 91. 
 Uhlmann, 95. 
 Ulatowski, 54. 
 Ullmann, 44, 45, 61. 
 Vagt, 109. 
 Vanino, 93. 
 Vaubel, 77. 
 Veraguth, 90. 
 Vesterling, 55. 
 Vielle, 27, 28, 70. 
 Vignon, 27. 
 Vock, 9, 40. 
 Vorlander, 61. 
 Votocek, 44, 45. 
 Wacker, 53. 
 Waijss, 42. 
 Walder, 19, 20. 
 Wallach, 17. 
 Wallbaum, 15. 
 Walter, J., 45. 
 Walter, L. E., 50. 
 Walther, 154. 
 Wanklyn, 25. 
 Warnecke, 55. 
 Wechsler, 147. 
 Wichelhaus, 28. 
 Wiesinger, 55. 
 Willstatter, 90. 
 Winston, 42. 
 Wislicenus, 111. 
 Witt, 10, 32. 
 Wohl, 58. 
 Wolbring, 94. 
 Wolff, 126. 
 Woolcott, 32, 71. 
 Wootton, 8, 167. 
 Wray, 32, 71. 
 Wroblewski, 32, 38. 
 Wulz, 74. 
 Wurtz, 112. 
 Zander, 38. 
 Zenisek, 45. 
 Zincke, 10, 82, 89. 
 Zinin, 80. 
 
 Oxford : Horace Hart, Printer to the University. 
 
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