Cross interaction constants as a measure of transition state structure. Part 7. Aminolysis of alkyl benzenesulphonates

Abstract

Kinetic studies of the reactions of methyl and ethyl benzenesulphonates with anilines and benzylamines in methanol and acetonitrile at 65.0 °C have been reported. The magnitudes of crossinteraction constants between substituents in the nucleophile (X) and the leaving group (Z), ρ_xz and β_xz, were found to be greater for the ethyl series which indicates a tighter transition state for ethyl rather than methyl derivatives. This unexpected trend has been rationalized by making the assumption that the small electron-donating polar effect, of the α-methyl substituent in the ethyl compounds, requires a tighter transition-state structure in addition to the major effect of steric repulsion on the activation barrier which is present in all S_N2 reactions taking place at a carbon centre.