Nucleophilic substitution at four-co-ordinate sulphur. Mobility of the leaving group

Abstract

The leaving group effect has been measured for the reactions of benzenesulphonyl halides with aniline, n-butylamine, and hydroxide ion. The specific rate constants of displacement at 25° with I, Br, Cl, and F, respectively, as leaving groups, are the following: with aniline, 3·55 × 10^–2, 31·2 × 10^–2, 4·27 × 10^–2, and 2·6 × 10^–7; with n-butylamine, 21·9, 103, 42·6, and 1·01 × 10^–2; and with hydroxide ion, 43·6, 28·9, 15·5, and 3·40. The almost identical leaving group mobility of I, Br, and Cl for each nucleophile and the enormous change in relative group mobility of fluorine on changing the pK_a of the nucleophile, point to a mechanism involving an intermediate complex with bond forming or bond breaking as the rate-limiting step according to the substrate. The activation parameters are also reported and agree with this interpretation.