A re-examination of the effects of added solvent on the activation parameters for solvolysis of t-butyl chloride in water

Abstract

Kinetic data for the solvolysis of t-butyl chloride in water and in aqueous mixtures are re-analysed in terms of the Albery–Robinson mechanism which relates the measured rate constant k(obs) to two kinetic parameters k1 and α by k(obs)=k1/(1 +α). The calculated dependence of k1 and α on temperature together with related enthalpy terms reproduce satisfactorily the parameters previously calculated assuming a single-stge reaction. The quantity α depends on temperature, passing through unity at some temperature Tα which depends on the added solvent. The latter dependence, together with the relationship between Tα and the range of temperatures over which the kinetic data were obtained, play an important part in determining, for example, the dependence of heat capacity of activation on composition calculated assuming that the reaction is single-stage and that ΔC≠p is independent of temperature. Combination of the enthalpy of activation ΔH≠1 with published partial molar enthalpies of t-butyl chloride in ethyl alcohol and water mixtures show that the partial molar enthalpies of the transition state for the first step of the Albery and Robinson scheme are markedly dependent on mole fraction of co-solvent.