Interpretation of the temperature dependence of the Arrhenius activation energy in solvolytic reactions

Abstract

Solvolytic reactions do not, in general, follow the Arrhenius equation exactly. The deviations are usually interpreted either by a genuine heat capacity of activation of an one-step reaction or by an apparent ΔC^‡_p caused by the hypothetical ion-pair mechanism of Albery and Robinson. These explanations are compared, taking into account, in addition to S_N solvolyses, the neutral ester hydrolysis where a tetrahedral intermediate is known to exist. It is concluded that the heat capacity of activation and its derivatives are real rate parameters which may include a ‘spurious’ contribution caused by the partitioning of a possible intermediate.