182 REPORT— 1902. 



and a subsequent modification of the experimental conditions led to the 

 discovery of benzenediazonium nitrate, 



H HO : 



•3 



H, + m ' '■ \no. 



V03 



These three substances, the eso-anhydride, the diazoamine, and the 

 diazonium salt, each containing a diazo-complex, N2, attached to one 

 aromatic nucleus, are typical examples of three important classes of 

 aromatic diazo-compounds. 



The process of converting the salt of a primary aromatic amine into 

 the corresponding diazonium derivative is termed diazotisation, and this 

 important operation may be suitably considered under a separate heading. 



L TheP 



^reparation and Practical Application of Diaxo-compounds, 

 A. The Diazo-reaction. 



Benzenediazonium nitrate, the substance formed by the action of 

 titrous fumes on a cold aqueous solution of aniline nitrate, was found 

 to be a highly explosive salt readily soluble in water or alcohol. On 

 account of these properties the isolation of the salt and the corresponding 

 chloride and sulphate is a somewhat difficult and dangerous operation, 

 and for many years the properties of these substances were studied in 

 the solutions obtained by the action of the alkali nitrites on the 

 primary ai'omatic base dissolved in excess of cold dilute acid. It has 

 been found by colorimetric and electrolytic determinations of the velocity 

 of this reaction that, in the absence of disturbing influences, all the 

 aromatic amines arc diazotised at appi'oximately the same rate. ' 



It is however sometimes necessary to operate with the dry diazonium 

 salts, and these may be prepared by the action of amyl nitrite on solutions 

 of the salts of the amines in absolute alcohol,'- or glacial acetic acid.-' 



Excess of acid should be avoided in these preparations, for the 

 diazonium chlorides derived from the halogen -substituted anilines combine 

 with hydrogen chloride, forming acid salts of the types RNgCl, HCl and 

 3RN2CI, HCl,' these products being less stable than the normal salts 

 themselves. 



Bamberger showed that pure benzenediazonium chloride, when 

 dissolved in water, gives a neutral reaction- and differs in this respect 

 from aniline hydrochloride, the solution of which is acid, owing to the 

 hydrolytic dissociation of the salt. This result indicates that the strength 

 of the diazonium base is much greater than that of the corresponding 

 primary aromatic amine. 



Griess found that the introduction of a diazonium salt into the 

 alkaline solution of a phenol resulted in the formation of an intensely 



' Hantzsch and Schumann, Ber., 1899, 32. 1691 ; 1900, 33, 527. 

 " Knoevenagel, Ber., 1890, 23, 2094 ; Bamberger, Ber,, 1890., 29, 446. 

 " Hantzsch, Ber., 1897, 30, 92 ; 1901, 34, 3337. 

 * Ber., 1897, 30, 1148 and 1153. 



