ON OUR PRESENT KNOWLEDGE OF AROMATIC DIAZO-COMPOUNDS. 191 



gation on diazo-compounds employed in the production of these ' ingrain ' 

 azo-compounds that Schraube and Schmidt first isolated the so-called 

 ' isodiazo-compounds.' ^ This important discovery inaugurated a new- 

 era of investigation on the diazo-compounds, and led to a revival of the 

 discussion regarding their constitution, a question which had been in 

 abeyance for many years. 



II. The Constitution of Diazo-compounds. 

 A. Metallic Diazo-derivatives. 



The alkali isodiazo-oxides, discovered by Schraube and Schmidt, are 

 prepared by adding a solution of diazonium salt to a warm concentrated 

 solution of an alkali hydroxide. The presence of negative radicles in the 

 aromatic nuclus of the diazonium salt causes the transformation to take 

 place more readily ; the conversion of p-nitrobenzenediazonium chloride 

 is effected, even at — 10°, whilst the rearrangement of the unsubstituted 

 diazonium compound requires a very concentrated solution of alkali 

 hydroxide, and a temperature of 130-140°. The more acidic diazo- 

 compounds are transformed even by the action of the alkali carbonates. ^ 

 It was at first supposed that these isodiazo-oxides were metallic derivatives 

 of the primary aromatic nitrosamines, the transformation taking place in 

 accordance with the following equation : 



CgHsNaCl-f 2 KOH = KCl + HgO-f CgHj.NK.NO. 



This view derived support from the fact that these metallic derivatives 

 showed little or no tendency to yield azo-compounds with alkaline solu- 

 tions of the phenols ; moreover, they yielded nitrosamines of the secondary 

 amines on alkylation, 



RNKNO ^ RNCH3.NO. 



That the nitrosamine formula is insufficient to account for all the reactions 

 of the tsodiazo-oxides was soon shown by von Pechmann, who obtained an 

 oxygen ether, NO2C13H4N2OCH3, from silver iso-;j-nitrobenzenediazo- 

 oxide ; this methyl derivative has the properties of a diazonium salt, 

 yields azo-compounds with the phenols and a diazoamine with aniline, and 

 it evolves nitrogen on boiling with water, ^;-nitrophenol being simulta- 

 neously produced. It therefore follows that the hydroxide corresponding 

 with these metallic derivatives exhibits the phenomenon of tautomerism. 

 On neutralising a suspension of potassium iso-p-nitrobenzenediazooxide 

 with a weak acid, a yellow svibstance is produced, which at first shows 

 little tendency to form azo-compounds with ;Q-naphthol, and its disulphonic 

 acid, but when left in contact with dilute mineral acid, the product slowly 

 dissolves and the soluble compound obtained has all the properties of the 

 original diazonium salt. This change may be indicated in the following 

 manner : 



XO2.C6H4NK.NO -> NO2C6H4NH.NO -> N02C6H4N,C1. 



This restoration of the capacity for coupling with phenols after acidifica- 

 tion was at first considered to be characteristic of zVo-diazo-derivatives, 



' Btr., 1894, 27, 520; Badische Anilin u. Soda-Fabrik, D.R.-P., 78874, 80263, 

 81134, 81202. 



'^ Schraube and Schmidt, and Badische Anilin u. Soda Fabrik, lac. cit. 



