Concurrent S N 1 and S N 2 reactions in the benzylation of pyridines

Abstract The Menschutkin reactions of 3,4‐methylenedioxybenzyl and 3,4‐dimethoxybenzyl bromides and also p ‐methoxybenzyl bromide with Y‐substituted pyridines were kinetically studied for a range of amine concentration in acetonitrile. The strongly activated benzyl bromides showed a significant positive intercept in the plot of the pseudo‐first‐order rate constants against the concentrations of nucleophiles, while less activated benzyl bromides did not give such significant intercepts. The rate data for respective substrates were fitted to the equation k obs  =  k 1  +  k 2 [Nu]. The first‐order rate constant, k 1 , is unaffected by the nature of the nucleophile, whereas the second‐order rate constant, k 2 , increased with increasing nucleophilicity. This was ascribed to the simultaneous occurrence of S N 1 and S N 2 reactions without an intermediate mechanism. These results are taken as evidence for the duality of S N 1 and S N 2 mechanisms in the Menschutkin reaction in the non‐solvolyzing solvent acetonitrile. Copyright © 2001 John Wiley & Sons, Ltd.