Effects of Alkyl Groups in Nucleophilic Substitution

Abstract

THIS communication refers mainly to the polar effects of alkyl groups in S_N2 reactions, in structural circumstances in which steric factors are absent. Rate comparisons showing the effects of methyl and tert.-butyl substituents are summarized in the accompanying table. Reactions (1) to (7) and (9) are of the type ŌH or ŌR (the dots represent electrons). Reactions (l)–(6) are of the second order, being of the first order with respect to each of the reactants specified. In the solvolytic process (7), the reagent (ethanol) is in large excess and the reaction is of the first order. It is probable, however, that this change is also substantially bimolecular¹. Reaction (9) is a first-order replacement at a saturated carbon atom under conditions especially conducive to a unimolecular mechanism². It was included in order to obtain a comparison of the effects relating to the two main substitution mechanisms, S_N2 and S_N1. Reaction (8) is a second-order nucleophilic replacement; but it should be noted that, in contrast to the other changes, it occurs at an aromatic carbon atom.