Mechanism of aromatic bromination by hypobromous acid in aqueous perchloric acid. Kinetic evidence against the prior formation of ‘positive bromine’

Abstract

Results of kinetic studies are used to show that the acid-catalysed mechanism of aromatic bromination by hypobromous acid cannot always involve the prior formation of a positively charged brominating agent. The argument is based on a stepwise comparison of the reactivity of a range of aromatic compounds varying in reactivity from mesitylene to the 2,6-dimethylpyridinium ion. These results, together with the solvent isotope effect, lead to the suggestion of a mechanism for aromatic bromination at low acidities in which the effective electrophile in the rate-limiting step is the hypobromous acidium ion, but where this electrophile is formed by the protonation of a complex of the aromatic compound with hypobromous acid in a pre-equilibrium step. The proposed mechanism has many similarities to that suggested on thermodynamic grounds by Shilov, Vainshtein, and Yasnikov.