Transition state enthalpies of transfer from propanol to acetonitrile in the reaction of imidazole with benzoyl and benzenesulphonyl chlorides

Abstract

Rates, activation parameters, and heats of solution of the reactants have been measured for the reaction of imidazole with benzoyl and benzenesulphonyl chlorides in propanol and acetonitrile. Enthalpies of transfer from propanol to acetonitrile of the transition state for the two reactions have been calculated. Approximate values for transfer from methanol are also given. Transition state enthalpies of transfer from propanol and methanol for substitution on carbonyl carbon are ca.–4 to +7 kJ mol^–1 whereas corresponding values for substitution at sulphonyl sulphur are ca.–15 to –25 kJ mol^–1. This is taken to mean that transition states for substitution at sulphonyl sulphur are polar and polarizable structures with little localized negative charge, poor hydrogen bond acceptors, and well solvated by dipolar aprotic solvents. Corresponding transition states for substitution at carbonyl carbon resemble the tetrahedral intermediate, are good hydrogen bond acceptors, and well solvated by protic solvents.