Substituent effects on the kinetics of pyridine-catalysed hydrolysis of aromatic sulphonyl chlorides; Brønsted and Hammett correlations

Abstract

The rates of hydrolysis of substituted benzenesulphonyl chlorides, catalysed by substituted pyridines, have been determined. Substituent effects in acid chloride and pyridine are correlated by Hammett and Brønsted equations, respectively. The Hammett plots are curved because of an abnormal low rectivity of m- and p-nitrobenzenesulphonyl chloride. The sensitivity parameters, β and ρ(excluding the nitro-compounds), obtained from the free-energy relations, are dependent upon the reactivity of the system. Thus β, which varies from 0·406 for p-methoxy- to 0·557 for p-nitro-benzenesulphonyl chloride, are linearly related to the σ values for the sulphonyl chloride substituents. Likewise the ρ values are linearly related to pK_a of the pyridines. These results show that bond formation in the transition state increase with electron-attracting substituents in the acid chloride, whereas such substituents in the nucleophile leads to increased bond stretching relative to bond formation.