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

The Menschutkin reactions of 3,4-methylenedioxybenzyl and 3,4-dimethoxybenz yl bromides and also p-methoxybenzyl bromide with Y-substituted pyridines w ere kinetically studied for a range of amine concentration in acetonitrile. The strongly activated benzyl bromides showed a significant positive inter cept in the plot of the pseudo-first-order rate constants against the conce ntrations of nucleophiles, while less activated benzyl bromides did not giv e 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 cons tant, k(1), is unaffected by the nature of the nucleophile, whereas the sec ond-order rate constant, k(2), increased with increasing nucleophilicity. T his was ascribed to the simultaneous occurrence of S(N)1 and S(N)2 reaction s without an intermediate mechanism. These results an taken as evidence for the duality of S(N)1 and S(N)2 mechanisms in the Menschutkin reaction in t he non-solvolyzing solvent acetonitrile. Copyright (C) 2001 John Wiley & So ns, Ltd.