Kinetics of the pyridine-catalysed methanolysis of aromatic sulphonyl chlorides

The kinetics of the methanolysis of benzenesulphonyl chloride in the presence of pyridine and its methyl derivatives, and of some para-substituted benzenesulphonyl chlorides in the presence of pyridine, have been studied. The reaction as studied shows first-order kinetics, but is of the first order in sulphonyl chloride and in pyridine or alkylpyridine. From the kinetics, solvent isotope effect, and the low reactivity of 2-picoline compared with the other alkylpyridines, it is concluded that the pyridines function as nucleophilic catalysts. Structural variations in the pyridine and sulphonyl chloride follow the Bronsted and Hammett equations respectively. Comparison of the present data with those for the corresponding hydrolysis show that the reaction of pyridines with benzenesulphonyl chloride is slower in methanol than in water because of a more negative entropy of activation.