Mechanisms of elimination reactions. Part IV. Rates of elimination from substituted phenethyl'onium salts in isopropyl alcohol containing potassium isopropoxide

Abstract

Rate coefficients for elimination from a number of p-substituted phenethyl-dimethylsulphonium and -trimethylammonium ions have been measured in isopropyl alcohol containing potassium isopropoxide. Kinetic deuterium isotope effects were measured using the 2,2-dideuteriophenethyl and phenethyl substrates. Compared with previous results in ethanol, the isotope effects in isopropyl alcohol are larger for both ‘onium salts, suggestive of less proton transfer to the base in the transition state in the more basic medium. The Hammett reaction constant for the sulphonium salt eliminations increases markedly as the solvent is changed from ethanol to isopropyl alcohol, consistent with an increase in the carbanion character of the transition state. In contrast, little change in ρ is noted for the ammonium salts. This observation demonstrates the uncertainty in using the changes in ammonium ion eliminations as representative models for eliminations involving other charged leaving groups.