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 substitution reaction of m ‐fluorophenyltriethyltin, 1, with mercuric chloride in order to obtain information about the polarity of the transition state. Second‐order rate constants have been determined for the reaction of 1 with mercuric chloride in methanol and methanol–water mixtures at 25·0°C, allowing determination of δΔ G ‡. Molar standard free energies of transfer (Δ G   t 0 ) for the same solvent mixtures have also been determined for 1 at 25·0°C. Combination of our δΔ G ‡ and δΔ G   t 0values with literature data for Δ G   t 0of HgCl 2 yield values of Δ G   t 0 (TS), the standard free energy of transfer of the corresponding transition state (TS) in the reaction from methanol to aqueous methanol. It is shown that the reduction in activation energy accompanying replacement of methanol by water is due to initial state destabilization and not to TS stabilization. In fact, the TS is destabilized as methanol is replaced by water. Further analysis permits dissection of the free energies of transfer of the TS into electrostatic and non‐electrostatic components. Comparison of these electrostatic components for the 1/HgCl 2 system with some model reactions shows that charge development in TS of 1/HgCl 2 is approximately 0·5 units of electronic charge.