Solvent effects on kinetics and reaction mechanisms. The formation of sulphonium and selenonium salts

Abstract

Second-order rate constants and activation parameters for the methylation of methyl phenyl sulphide and methyl phenyl selenide by dimethyl sulphate have been measured in 14 aprotic solvents covering a wide range of dielectric constants. The same reactivity order is observed with both substrates in the various solvents. The analysis of the medium effects on the reactivity was performed by the Koppel–Palm multi-parameter approach. The results indicate that the polarization and electrophilicity of the solvents are responsible for the rates of both reactions. The higher reactivity of methyl phenyl selenide has been ascribed to the higher polarizability and softness of the selenium atom in comparison with the sulphur atom.