ON OUR PRESENT KNOWLEDGE OF AROMATIC DIAZO-COMPOUNDS. 187 



(iii) Another extremely interesting case of molecular rearrangement is 

 the conversion of the bromodiazoniuni chlorides into the isomeric chloro- 

 diazonium bromides.' 



This transformation, which has been studied quantitatively, is a mono- 

 molecular change, subject to the following laws : — 



1. The bromine atoms undergo replacement only when present in the 

 para- or ortho-position with respect to the diazo-radicle, those in the 

 ortho-position being most readily ^removed. Metabromo-derivatives are 

 not aflected. 



2. The ease of transformation increases with the number of bromine 

 atoms present. 



3. Tlie transformation constant, calculated from the equation ic= 

 l/t log a/ (a— x), 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. 



The diazonium salts containing two bromine atoms are stable when 

 dry, but rapidly undergo conversion in ethyl alcohol ; 2 : 4 : 6-tribromo- 

 benzenediazonium chloride becomes transformed even in the dry state. 



(iv) Dibenzoylhydrazines RN(COC6H.5)]Sr(COC6H5)R are obtained by 

 treating diazonium salts with an aqueous suspension of benzoyl chloride 

 and copper powder.^ 



C. Diazo-compounds employed in the Production of Azo-cohuriftg Matters. 



1. Aminoazo-compounds. — The action of a diazonium salt on a primary 

 aromatic amine gives rise either to a diazoamine or an aminoazo- 

 conipound, according as to whether the diazo-radicle remains attached 

 to the aminic nitrogen, or migrates into the aromatic nucleus. Aniline 

 and its homologues and substitution products yield diazoamines, which 

 are often capable of undergoing rearrangement into aminoazo-compounds, 

 this change being effected either by allowing the diazoamine to remain 

 in alcoholic solution or by warming it with a mixture of the parent base 

 and its hydrochloride. The latter mode of transformation has been 

 studied quantitatively by Goldschmidt and his pupils,^ with the following 

 results : — 



The transformation of diazoaminobenzene into aminoazobenzene in 

 an aniline solution containing aniline hydrochloride is a monomolecular 

 reaction, the velocity of transformation, in moderately dilute solution, 

 being proportional to the temperature and to the amount of catalyst 

 (aniline hydrochloride) but independent of the concentration. Benzene- 

 diazoaraino-^>-toluene, when dissolved in aniline containing its hydro- 

 chloride, becomes converted into diazoaminobenzene and ^^-toluidine, the 

 resulting diazoamine then undergoing transformation in accordance with 

 the preceding rules. 



This conversion takes place more slowly when the diazo-radicle shifts 

 into the ortho-position with respect to the amino-group. The trans- 

 formation constant of diazoaminobenzene at 45° is 0'081, whereas the 

 value of this coefficient for diazoamino-jo-toluene is only 0'0095, the 

 solutions employed in both cases being semi-normal. 



' Hantzsch, Bcr., 1897, 30, 2334; Journ. prakt. Chem., 27, 98; and Ber., 1900, 

 33, 505. 



•■' Ber., 1902, 35, 1964. . » Ber., 1896, 29, 1369, 1899. 



