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The quaternization reactions of substituted (Z)-benzyl (X)-benzenesulfonates with substituted (Y)pyridines were investigated in acetonitrile at 35°C. The magnitudes of the Hammett reaction constants r X , r Y and r Z indicate that a stronger nucleophile leads to a lesser degree of bond breaking. In addition, a better leaving group is accompanied by a lesser degree of bond formation. Application of multi-Hammett interactions, j YZ j >j XY j >j XZ j, predicts that these Menschutkin-type reactions are dissociative S N 2 reactions. In particular, the reaction of strongly activated benzyl derivatives with tertiary amines in acetonitrile reveals a more advanced bond breaking like S N 1 reactions. The predicted mechanism for the benzylation of pyridines with benzylic systems is evident from More O'Ferrall-Jencks diagram and the semi-empirical MO calculations with the AM1 method.

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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 kobs = k1 + k2[Nu]. The first‐order rate constant, k1, is unaffected by the nature of the nucleophile, whereas the second‐order rate constant, k2, increased with increasing nucleophilicity. This was ascribed to the simultaneous occurrence of SN1 and SN2 reactions without an intermediate mechanism. These results are taken as evidence for the duality of SN1 and SN2 mechanisms in the Menschutkin reaction in the non‐solvolyzing solvent acetonitrile. Copyright © 2001 John Wiley & Sons, Ltd.