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

Nucleophilic substitution on meta- and para-substituted methyl benzenesulph onates was studied with two chloride salts with different structures: NBu4C l or KCl-Kryptofix 2,2,2. Treating the results with the Acree equation show s that the reaction proceeds by two reaction paths, one involving the chlor ide ion and the other, slower one, involving the ion pairs. Treating the re sults with the Hammett equation gives consistent data, and shows that rho i s 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 methy l derivative reacts faster than the ethyl derivative in both paths. The res ults 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, Lt d.