KINETICS AND MECHANISM OF OXIDATION OF (ARYLTHIO)ACETIC ACIDS BY PYRIDINIUM HYDROBROMIDE PERBROMIDE

摘要

Oxidation of several monosubstituted (phenylthio)acetic acids (PTAA) by pyridinium hydrobromide perbromide (PHPB) was studied in aqueous acetic acid. The reaction is first order with respect to PHPB. Michaelis-Menten type kinetics are observed with respect to (arylthio)acetic acid. The effect of solvent composition indicates that the transition state is more polar than the reactants. The formation constants of the intermediate substrate-PHPB complexes and the rates of their decomposition were determined at different temperatures. The rates of oxidation of para and meta-substituted (phenylthio)acetic acids were correlated with Hammett's substituent constants. The ρ value is -1.60 at 35℃. The rates of oxidation of ortho substituted compounds are correlated with Charton's triparametric equation. A mechanism involving the decomposition of the intermediate complex in the slow rate-determining step affording a sulphonium ion which hydrolyses in a subsequent fast step to the sulphoxide is proposed.