A theoretical comparison of SN1 and SN2 reactions of saturated alkyl chlorides

Abstract

The mechanistic difference between bimolecular (S_N2) and unimolecular (S_N1) nucleophilic substitutions of saturated alkyl chlorides is investigated with ab initio molecular orbital (MO) calculations. The t-butyl substrate undergoes early (almost spontaneous) C–Cl bond scission, which may give double OH^–(reagent) co-ordination on the carbonium cation. However, the symmetrically solvated ion, which has been accepted as a stable intermediate for racemization, is found to be located at the energy maximum of the OH^– exchange reaction. The origin of racemization in nucleophilic substitution is discussed on the basis of the calculated potential surface.