Rates, activation parameters, and enthalpies of transfer of transition states for the reaction of imidazole with aromatic sulphonyl chlorides in methanol and acetonitrile

Abstract

Rates of reaction of substituted benzenesulphonyl chlorides with imidazole have been measured in methanol and acetonitrile. The increase in rate on changing solvent from protic to dipolar aprotic is relatively small (ca. 35 times faster in acetonitrile at 25 °C). This is due to compensation of a substantial decrease in the enthalpy of activation in the dipolar aprotic solvent by a concomitant decrease in the entropy of activation. Substituent effects on the rate of reaction as measured by ρ are equal in the two solvents. The solvent effect on the enthalpy of activation has been separated into initial-state and transition state-effects by measurements of heats of solution of the reactants in the two solvents. The lower activation enthalpy in the aprotic solvent (ca. 7 kcal mol^–1) is thus found to be caused by a combination of an endothermic enthalpy of transfer of the nucleophile (ca. 3 kcal mol^–1) and an exothermic enthalpy of transfer of the transition state (ca.–4 kcal mol^–1) from the protic to the aprotic solvent.