Substituent effects on transition-state structures for dissociative and associative SN2 reactions

Abstract

α-Deuterium secondary kinetic isotope effects (KIEs) have been investigated for the reactions of anilines with benzyl, methyl and ethyl benzenesulfonates involving deuteriated aniline nucleophiles and substrates. The secondary KIEs observed are normal, k_H/k_D > 1.0, only for the deuteriated benzyl system and are of an inverse type, k_H/k_D < 1.0, for the deuteriated methyl and ethyl systems as well as for all the deuteriated nucleophiles. These results suggest that the relief of steric strain by bond breaking of the leaving group prevails over the steric congestion incurred by bond formation of the nucleophile in the reactions of the benzyl compound (dissociative S_N2 mechanism), in contrast to the reverse trend, i.e., the strain incurred by bond formation of the nucleophile is greater than the relief of steric congestion by bond breaking of the leaving group in the reactions of alkyl compounds (associative S_N2 mechanism). The effects of substituents in the nucleophile (X) and leaving group (Z) on the secondary KIEs are in complete agreement with the transition-state variation predicted by the sign and magnitude of the cross-interaction constant, ρ_xz.