Criterion of Mechanism based on Optical Rotation and Rearrangement in Alkyl Groups during Formation of Alkyl Halides

Abstract

THE geometrical and electronic requirements of replacement reactions designated S_N2, S_N1 and S_Ni were described by Ingold and Hughes on the basis of kinetics and changes in optical rotation¹. Recent work^2,3, however, has shown the need for caution in the practice which afterwards became fashionable of assigning these classifications on the basis of the optical result, unsupported by kinetic data, in other types of reaction, such as replacement of hydroxyl by halogen. The only unequivocal decision in such a reaction would seem to be the assignment of the exclusive S_N2 process to account for the isolation of, for example, optically pure (−)-2-chloro-octane from (+)-2-octanol. Isolation of the optically pure (+)-halide, retained configuration, does not necessarily point to the S_Ni, internal, mechanism, since this result could be due to a double S_N2 process, the first one being independent of halogen. Predominant inversion but with some loss in optical purity could be due to (S_N2 + S_N1 + S_Ni) or (S_N2 + S_N1) or (S_N2 + S_Ni), whereas predominant retention could be due to (S_N2 + S_Ni + S_N1) or (S_N1 + S_Ni) or (S_N2 + S_Ni), bearing in mind that the effect ascribed to S_Ni could be due entirely or partly to the double S_N2.