The Hammett equation applied to the nucleophilic displacement of ions and ion pairs on substituted benzenesulphonates

Nucleophilic substitution on meta- and para-substituted methyl benzenesulphonates was studied with two chloride salts with different structures: NBu4Cl or KCl-Kryptofix 2,2,2. Treating the results with the Acree equation shows that the reaction proceeds by two reaction paths, one involving the chloride ion and the other, slower one, involving the ion pairs. Treating the results with the Hammett equation gives consistent data, and shows that rho is positive and nearly the same for the two reaction paths (rho approximate to +2). The reactivity of methyl p-nitrobenzenesulphonate was compared with that of the corresponding ethyl derivative. and it is shown that the methyl derivative reacts faster than the ethyl derivative in both paths. The results are interpreted based on the assumption that in both paths a negative charge is developed on the leaving group in the transition state, and that the activated complex is linear. Copyright (C) 2001 John Wiley and Sons, Ltd.