Intermediate Complexes in SN_2 Reaction:[Na~+,F~-,H_2O,CH_3Cl] System

摘要

The unsymmetrical S_N2 reaction [F~- + CH_3X -> CH_3F + X~-] has been studied in the gas phase by several groups to interrogate the mechanism in solution.The F~- anion is one of the most reactive nucleophiles in the gas phase,and the reaction proceeds very fast almost to completion with very small activation barrier of about 2 kcal/mol.This highly simplified version of the S_N2 reaction may not explain why the fluorination reaction has been considered to be quite difficult to occur in the solution phase in the organic chemistry community.The solvent is expected to influence the rate of the reaction profoundly:For example,protic solvent is usually known to be unfavorable for the S_N2 reaction.Studying the reaction mechanism in the solution phase either experimentally or theoretically is,however,difficult for several reasons.First,in the solution phase S_N2 reaction,there exist no discernible intermediate pre- and post-reaction complexes that are considered to be the configurations of the reacting and the produced species in the gas phase.Second,it is very difficult to study the structures of the transition state in the solution phase.Third,it is virtually impossible to examine the effects of infinite number of solvent molecules on the reaction accurately (that is,on the molecular level).Quantum chemical calculations would be very useful in this regard for elucidating the mechanism by adopting a cluster model comprising the reacting species and a number of solvent molecules.