A theoretical study of the mechanism of selective fluorination of saturated hydrocarbons by molecular fluorine. Participation of CHCl3 solvent molecules in the ionic process.
The fluorination reaction of methane and isobutane by molecular F(2) with and without CHCl(3) solvent was studied with the ab initio and ONIOM theoretical calculations. The electrophilic pathway for the RH + F(2) reaction in general is a two-step process, consisting of hydride abstraction leading to an intermediate of the type R(+)(delta)...HF...F(-)(delta), followed by complicated rearrangement to give the electrophilic substitution product, RF + HF. In the case of methane, the overall barrier for this reaction is too high for the reaction to take place under mild conditions even in the presence of CHCl(3) solvent molecules. In the case of isobutane without CHCl(3), the electrophilic pathway has a high rate-determining barrier of 25 kcal/mol, and is not likely to take place; the radical process forming t-Bu* + HF + F* may be preferred. However, the t-Bu(+)(delta)...HF...F(-)(delta) intermediate and, in particular, the transition state TS2 for rearrangement of the intermediate are highly ionic, and are stabilized dramatically when a few CHCl(3) solvent molecules form a solvation cage. The electrophilic reaction for isobutane + F(2) has a low overall barrier when at least three CHCl(3) molecules are present and can take place under mild conditions with full retention of configuration.
Fukaya
Haruhiko
H
Institute for Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan.
Morokuma
Keiji
K