Mechanism of benzidine and semidine rearrangements. Part XXII. Kinetics and products of the acid conversion of 2,2′-di-iodo- and 4,4′-di-iodo-hydrazobenzene

Abstract

The acid conversion of 2,2′-di-iodohydrazobenzene is wholly of first order in hydrogen ions at concentrations of acid up to 1·6N in ‘60 %’ aqueous dioxan. As to absolute rate, the measured reaction is the slowest in like conditions of any of the dihalogeno-hydrazobenzenes investigated. The second-order rate constant of this one-proton mechanism is the smallest of any known for one-proton hydrazo-conversions in comparable conditions. The undetected two-proton reaction must be slower by a factor of at least 10⁷ than the two-proton reaction of hydrazobenzene. This is the most extreme known case of the raising to prominence of the one-proton mechanism by a deactivating substituent. As to products, the reaction is wholly one of rearrangement to a single product, 3,3′-di-iodobenzidine: no disproportionation was detected.

The acid conversion of 4,4′-di-iodohydrazobenzene, in contrast, goes by the two-proton mechanism, at an only moderately slow rate similar to those of the conversions of the 4,4′-dichloro- and 4,4′-dibromo-analogues. The reaction is wholly one of disproportionation : no rearrangement was detected.