Mechanism of the oxidation of phenylmercaptoacetates by potassium hexacyanoferrate(III): structural and substituent effects

Abstract

Mechanistic investigations of the oxidation of phenylmercaptoacetates by potassium hexacyano-ferrate(III) in aqueous sodium hydroxide reveal that the oxidation follows first-order kinetics in each of the oxidant, substrate and OH^– at constant ionic strength. While added radical scavenger, acryl-amide, enhances the rate, potassium hexacyanoferrate(II) retards it. Studies with different substrates indicate that the substrate must possess an acidic proton. It has also been realised that d-orbitals of sulphur are important in stabilizing the intermediate by dπ–pπ bonding. Rate studies with substituted phenylmercaptoacetates give an excellent Hammett correlation with a positive reaction constant (ρ 1.31). On the basis of kinetic evidence, a mechanism which involves an initial reversible proton abstraction followed by another reversible electron transfer step has been postulated.