Standard free energies of transfer, from methanol to aqueous methanol and to water, of solutes and transition states have been divided into a nonelectrostatic contribution, ΔG_n°, and an electrostatic contribution, ΔG_e°. The value of ΔG_e° for an uncharged transition state is considered to be related directly to the degree of charge separation (δ±) in the transition state. It is shown that uncharged transition states in solvolyses thought to proceed by mechanism S_N1 are characterised by high values of δ±(0·8), whereas uncharged transition states in presumed S_N2 solvolyses are characterised by low values of δ±(ca. 0·3); it is suggested that the values of ΔG_e° and of δ± for uncharged transition states in nucleophilic solvolyses can be used as a criterion of reaction mechanism.

Values of δ± for transition states in the electrophilic substitution of tetra-alkyltins by mercury(II) chloride are quite large, averaging ca. 0·55 for substitution of tetramethyltin and tetra-n-propyltin, and ca. 0·65 for the substitution of tetraethyltin. It is suggested that these values of δ± are compatible with mechanism S_E2(open) for these substitutions, but not with mechanism S_E2(cyclic).