Mechanism of benzidine and semidine rearrangements. Part XVIII. Kinetics and products of the acid conversion of 4-acetamidohydrazobenzene, products from 4-aminohydrazobenzene, normality of p-semidine formation, and crossed redox analogues of disproportionation

Abstract

4-Acetamidohydrazobenzene is converted by 0·007–0·1N-perchloric acid in ‘60%’ aqueous dioxan to give 30% of rearrangement and 70% of disproportionation. The kinetics are of first order in hydrazo-compound and first order in hydrogen ions. Each type of process must individually have these kinetics. The ratio in which the two processes occur remains constant when the acidity is raised to 4N, but in 2–4N-acid additional isomers appear in small amount in the mixture of rearrangement products. The most important of these is the p-semidine, formed in 4N-acid to the extent of 5% of the total conversion. The conversion of 4-aminohydrazobenzene by dilute acid in the same solvent gives 5% of rearrangement and 95% of disproportionation; and the p-semidine is a main component of the rearrangement products.

The evidence on which has been based a long-standing preconception that p-semidines are not produced in the homogeneous acid rearrangements of hydrazoarenes, except through the intervention of heavy metal ions, has been experimentally revised. Both the evidence, and the inference drawn from it, are shown to be illusory.

4-Acetamidohydrazobenzene and hydrazobenzene were converted with acid together in such conditions that, separately, the former would be converted completely and the latter not at all. In the mixed conversion, the latter was oxidised to azobenzene. The bearing of this and other such observations on the mechanism of disproportionation is discussed.