Kinetics and mechanism of decomposition of benzyldimethylsulphonium salts in chloroform

Abstract

The decomposition of benzyldimethylsulphonium halides has been studied with chloroform as solvent, in which the reaction goes to completion, yielding the benzyl halide and dimethyl sulphide as the only products. The kinetics show an exact first-order relation with respect to the conductivity of the solution rather than the molar concentration of the salt. The first-order rate constant is controlled by the nucleophilicity of the anion. Addition of small amounts of ammonium salt causes a sharp decrease in rate constant which becomes more gradual and linear at higher concentrations of added salt. This salt effect is completely insensitive to the nature of the anion of the added ammonium salt. Activation parameters and the α-deuterium isotope effect are reported. A mechanism is proposed involving a unimolecular decomposition of a triple ion consisting of two sulphonium cations and one halide ion.