Solvent influences on initial and transition states in solvolysis of 2-chloroethyl methyl sulfide


Nucleophilic substitution reactions of 2-(alkylthio)ethyl derivatives have become important in understanding basic principles of physical organic chemistry. In the present work the solvent effects of 2-chloroethyl methyl sulfide (1-Cl) solvolysis in methanol, methanol-water mixtures, and water are analyzed to provide a dissection of initial-state (IS) and transition-state (TS) effects. The reduction in activation energy accompanying replacement of methanol by water is shown to be due to significant IS destabilization and TS stabilization. Further analysis permits dissection of the free energies of transfer of the TS into electrostatic and nonelectrostatic components. Comparison of the these electrostatic components for 1-Cl and several model substrates shows that charge development in the TS for 1-Cl is approximately 0.6 units of electronic charge. This conclusion is supported by estimation of TS structure from isotope effects, but it is also in apparent conflict with a TS structure deduced from tosylate-chloride rate ratios. An analysis of the tosylate-chloride ratio is presented, and it is proposed that the ratio for 1-Cl is dominated by steric effects at the reactive site, and possibly by IS solvation differences between tosylates and chlorides, rather than by the extent of bond cleavage to the leaving group in the TS.

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