Substitution at saturated carbon. Part XII. Calculation of the electrostatic contribution to free energies of transfer from methanol to aqueous methanol and to water of solutes and transition states. A new method for the examination of transition states

Abstract

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).