SOLVENT EFFECTS ON INITIAL AND TRANSITION-STATES IN THE REACTION OF M-FLUOROPHENYLTRIETHYLTIN WITH MERCURIC-CHLORIDE IN METHANOL AND METHANOL-WATER MIXTURES

We have investigated solvent effects on the aromatic electrophilic sub stitution reaction of m-fluorophenyltriethyltin, 1, with mercuric chlo ride in order to obtain information about the polarity of the transiti on state. Second-order rate constants have been determined for the rea ction of 1 with mercuric chloride in methanol and methanol-water mixtu res at 25.0 degrees C, allowing determination of delta Delta G(double dagger) . Molar standard free energies of transfer (Delta G(1)(0)) for the same solvent mixtures have also been determined for 1 at 25.0 deg rees C. Combination of our delta Delta G(double dagger) and Delta G(1) (0) values with literature data for Delta G(1)(0) of HgCl2 yield value s of Delta G(1)(0)(TS), the standard free energy of transfer of the co rresponding transition state (TS) in the reaction from methanol to aqu eous methanol. It is shown that the reduction in activation energy acc ompanying replacement of methanol by water is due to initial state des tabilization and not to TS stabilization. In fact, the TS is destabili zed as methanol is replaced by water. Further analysis permits dissect ion of the free energies of transfer of the TS into electrostatic and non-electrostatic components. Comparison of these electrostatic compon ents for the 1/HgCl2 system with some model reactions shows that charg e development in TS of 1/HgCl2 is approximately 0.5 units of electroni c charge.