Oxidation of diphenylmethanol by bromamine T. A kinetic and mechanistic study

Abstract

Bromamine T (BAT) oxidises diphenylmethanol and substituted diphenylmethanols smoothly and quantitatively to the respective benzophenones in aquoeus acetic acid. Mineral acids catalyse the reaction and the rate law is found to be –d[BAT]/dt=k[BAT][Alcohol][H⁺]. The reaction is characterised by the absence of any kinetic isotope effect and a break in the Hammett plot with ρ values of –4.2 and –0.4 for the two limbs. Three alternative mechanistic pathways have been proposed involving (i) diphenylmethyl cation, (ii) a ‘phenonium ion’ type intermediate from the decomposition of the alkyl hypobromite ester, or (iii)ipso substitution at the benzene ring followed by dehydrobromination and aromatization.