Kinetics and mechanism of oxidation of dimethyl and diphenyl sulfoxides by tetrabutylammonium tribromide

The kinetics of oxidative conversion of dimethyl and diphenyl sulfoxides to corresponding sulfones with environmentally benign tetrabutylammonium tribromide was investigated in 50% v/v aqueous acetic acid medium. The reactions were carried out under pseudo first-order conditions keeping an excess of sulfoxides over that of the oxidant. The tribromide ion, , was found to be the active species of the oxidant. The electrophilic attack of the ion on the nucleophilic sulfur of the sulfoxide leads to the formation of a complex between the reactants in a prior equilibrium. The complex formed undergoes rate-determining hydrolysis to yield sulfone as the product. The complex formation was kinetically verified by the Michealis—Menten plot. The formation constants for the complexes and the rate constant for the slow hydrolysis step were determined by studying the reaction at four different temperatures. The values of observed rate constant and the complex formation constant for dimethyl sulfoxide were found to be comparatively greater than that of diphenyl sulfoxide due to the positive inductive effect of methyl groups. The thermodynamic parameters with respect to the slow step of the reaction were determined and the values support the proposed mechanism.