Abstract
| - The fluorination reaction of methane and isobutane by molecular F2 with and without CHCl3 solventwas studied with the ab initio and ONIOM theoretical calculations. The electrophilic pathway forthe RH + F2 reaction in general is a two-step process, consisting of hydride abstraction leading toan intermediate of the type R+δ···HF···F-δ, followed by complicated rearrangement to give theelectrophilic substitution product, RF + HF. In the case of methane, the overall barrier for thisreaction is too high for the reaction to take place under mild conditions even in the presence ofCHCl3 solvent molecules. In the case of isobutane without CHCl3, the electrophilic pathway has ahigh rate-determining barrier of 25 kcal/mol, and is not likely to take place; the radical processforming t-Bu• + HF + F• may be preferred. However, the t-Bu+δ···HF···F-δ intermediate and, inparticular, the transition state TS2 for rearrangement of the intermediate are highly ionic, andare stabilized dramatically when a few CHCl3 solvent molecules form a solvation cage. Theelectrophilic reaction for isobutane + F2 has a low overall barrier when at least three CHCl3molecules are present and can take place under mild conditions with full retention of configuration.
|