Mechanism of benzidine and semidine rearrangements. Part XXV. The acid-catalysed disproportionation of hydrazobenzenes

Abstract

Acid treatment of 3,3′,5,5′-tetrabromohydrazobenzene resulted in rearrangement (78%) accompanied by disproportionation to azo-compound and fission amine (22%). The proportions of the latter process and the rate of disappearance of the substrate were unaffected by deuteriation of the aromatic rings or by variation in the concentrationof the substrate. Similar results were obtained for the disproportionations of 4,4′-dichloro- and 4-acetylamino-substituted hydrazobenzenes. For the former, tracer experiments showed that the azo-compound was formed by oxidation of the substrate rather than by intermolecular recombination of anilino or nitrene fragments, and the percentage of such disproportionation was markedly reduced when the viscosity of the medium was increased by the addition of glycerol.

Rearrangement and disproportionation are considered to be concomitant reactions of different intermediates rather than alternative reactions of a common intermediate. Experiments with e.s.r. and chemically induced dynamic nuclear polarisation (CIDNP) techniques and scavengers suggest that these intermediates are quinonoids.