Structure of azo coupling products of 5-nitro-2,1-benzisothiazole-3-diazonium hydrogensulphate with aromatic amines

The reaction of 5-nitro-2,1-benzisothiazole-3-diazonium hydrogensulphate with anilines, N-alkylanilines and diphenylamine gives N-substituted 3-(4-aminophenyldiazenyl)-5-nitro-2,1-benzisothiazoles 2 and isomeric 1-(5-nitro-2,1-benzisothiazol-3-yl)-3-substituted-3-phenyltriazenes 1. The predominant reaction products are triazenes 1, being present up to 95% in reaction products. A larger amount of azo compounds 2 are formed in the reaction of 5-nitro-2,1-benzisothiazole-3-diazonium with diphenylamine (about 45%), with N-alkyl-3-methylanilines and 3-methylaniline (as much as 50%). The triazenes formed are extraordinarily stable in acid medium (the decomposition half-lives in 0.5 mol l−1 H2SO4 in aqueous acetic acid (1:1 v/v) are in the order of hours), which is explained by a different site of their protonation: while 1,3-diaryltriazenes are protonated at the –NH–NN– group, triazenes 1 are protonated at the heterocyclic nitrogen. The triazenes 1 formed by azo coupling reaction with anilines containing primary amino group undergo an acid–base reaction in methanolic solutions, connected with distinct change in colour, which indicates a considerable acidity of the proton in –NH–NN– grouping.