ON OUR PRESENT KNOWLEDGE OF AROMATIC DIAZO-COMPOUNDS. 183 



coloured substance containing the diazo-complex N : N, attached to 

 a second aromatic nucleus, 



CeH^N^Cl + HC.H.OH = CgHsK^.CeH.OH + HCI. 



These products, which are called azo-compounds, are also produced 

 either by the reduction and coalescence of two molecules of a nitro- 

 compound, or by tlie condensation of the nitroso-compounds with the 

 primary amines.^ 



On account of their condensation to form azo-compounds and their 

 ready reduction to hydrazines, the diazonium salts were for many years 

 assumed to have the constitution R.N : N.Cl, a formula which was 

 suggested by Kekule, although Strecker, Erlenmeyer, and Blomstrand had 

 at different times advocated the claims of the symbol 



R.KCl. 



N 



According to the former view, which generally prevailed until 1894, 

 the diazonium bases are substances of the oxime type RN : NOH, and 

 this formula, which satisfactorily accounts for the fact, first noticed by 

 Griess, that benzenediazonium hydroxide forms a potassium derivative, is 

 not opposed to the general behaviour of these bases and their salts when 

 employed in the various synthetical operations which engrossed the 

 attention of investigators in this field until the close of the period now 

 under review. 



The synthetical application of the diazonium salts will be considered 

 in the next section, since it does not necessarily involve any discussion as 

 to their precise configuration. 



B. Diazo-compounds as Agents in Chemical Synthesis. 



1. Replacement of NH^ by CI, Br, I, CN, CNO, or CNS.^ 

 Griess's researches on the production of the halogen derivatives of the 

 aromatic hydrocarbons are now only of historical interest, since the methods 

 commonly employed at the present time are due to Sandmeyer - and to 

 Gattermann.3 The latter of these investigators also extended the scope 

 of this synthetical process by demonstrating that the cyano (CNO) and 

 thiocyano (CNS) radicles could also be introduced into the aromatic 

 nucleus by the use of the appropriate diazonium salt. 



The course of the reaction between diazonium derivatives and cuprous 

 salts (Sandmeyer) or copper powder (Gattermann) has been recently 

 reviewed by Hantzech,-' whose conclusions may be summarised as 

 follows : — 



The course of the Sandmeyer reaction is not a simple one, and the 

 final result is due to the simultaneous effect of three concurrent actions : 

 (i) the formation of a labile diazonium cuprous double salt, which sub- 

 sequently undergoes decomposition in such a manner that the radicle 

 originally attached to copper migrates to the aromatic nucleus ; (ii) a 

 catalytic action, which becomes the main reaction when copper powder is 

 employed, whereby nitrogen is eliminated from the diazo-salt, so that the 



' C. Mills, Trans., 1895, 67, 925. ^ Bvv., 1884, 17, 2S50. 



» Bcr., 1890, 23, 1218 ; 1892, 25, 1074. •" Bcr., 1900, 33, 2544. 



