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

TLDR: In this paper, the free energies of transfer, from methanol to aqueous methanoline and to water, of solutes and transition states have been divided into a nonelectrostatic contribution ΔGn° and an electrostatic contribution, ΔGe° for an uncharged transition state.

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, ΔGn°, and an electrostatic contribution, ΔGe° The value of ΔGe° 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 SN1 are characterised by high values of δ±(0·8), whereas uncharged transition states in presumed SN2 solvolyses are characterised by low values of δ±(ca 0·3); it is suggested that the values of ΔGe° and of δ± for uncharged transition states in nucleophilic solvolyses can be used as a criterion of reaction mechanismValues 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 SE2(open) for these substitutions, but not with mechanism SE2(cyclic)