Acid-catalysed disproportionation and benzidine rearrangement of phenylhydrazinopyridines: reaction pathways, kinetics and mechanism

A kinetic and product analysis study of the reaction of 4-[N'-(4-hydroxyphenylhydrazino)]pyridine (1) and 4-(N'-phenylhydrazino)pyridine (2) in 10-55 wt% aqueous sulfuric acid media is reported. The final products obtained from 1 are the oxidation product, 4-(4-hydroxyphenylazo)pyridine (3), and the two reduction products 4-aminophenol and 4-aminopyridine. This product pattern resembles that of the disproportionation products observed in some cases of the benzidine rearrangement. The reaction of 2 gives a complex product mixture including the disproportionation products 4-(phenylazo)pyridine (4), aniline and 4-aminopyridine, and other benzidine-type products including a p-semidine; Complicating the situation is the concurrent hydroxylation and disproportionation of 4 which take place in these media, giving 3, 4-aminopyridine and 4-aminophenol. The observed pseudo-first-order rate constants for the disproportionations increase steeply with acid concentration without showing any downturn at higher acidities. The disproportionation rate constants for 1 and 2 are much larger than those for the hydroxylation of 4. A reasonable mechanism which explains the observations is a benzidine-type rearrangement with accompanying disproportionation. To our knowledge this work is the first description of this type of reaction in substrates containing pyridinium rings.