Substitution at saturated carbon. Part 21. Effect of alcoholic solvents on the enthalpy and entropy of the tetraethyltin–mercury(II) chloride transition state and of the t-butyl chloride solvolysis transition state; comparison with 1 : 1 electrolytes

Abstract

Heats of solution are reported for tetraethyltin and mercury(II) chloride in methanol, methanol–t-butyl alcohol mixtures, and t-butyl alcohol. With previous data on free energies, these measurements lead to entropies of transfer as well as enthalpies of transfer from methanol to the other solvents of tetraethyltin, mercury(II) chloride, and the tetraethyltin–mercury(II) chloride transition state. It is shown that the very large variation in ΔS^‡ for the reaction between tetraethyltin and mercury(II) chloride is almost entirely due to initial state effects. Heats of solution of t-butyl chloride in a number of pure alcohols have been determined and lead similarly to enthalpies and entropies of transfer of the solvolysis transition state from methanol to the other alcohols. Variations of the entropy of the [Et₄Sn–HgCl₂]^‡ and [Bu^tCl]^‡ transition states with change of alcoholic solvent are quite small, whereas the entropy of dissociated species such as (Pr₄N⁺+ I^–) or (Me₄N⁺+ Cl^–) is considerably lower in the less polar alcohols than in methanol. It is suggested that these dissociated species are poor models for the particular transition states discussed, and that in general thermodynamic properties of polar, electrically neutral, transition states are not comparable quantitatively with those of dissociated species, as far as variation with solvent is concerned.