Nucleophilic substitution at sulphonyl sulphur. Part 1. Reactivity of thiophen-2-sulphonyl halides in water and methanol-acetonitrile

The reaction kinetics of substituted thiophen-2-sulphonyl chlorides and fluorides (5-OMe, 5-Me, H, 5-Cl, 4-NO2, 5-NO2) with anionic and neutral nucleophiles were studied in water at 25°C. For the reactions of chlorides with H2O, AcO-, and N3 - and for the hydrolysis of fluorides U shaped Hammett plots were observed. For the reaction of sulphonyl chlorides with aniline, pyridine, imidazole, and OH- an approximately linear Hammett correlation is found. Common chloride ion effects on the hydrolysis rate constants appear to be absent. Nucleophilic substitution reactions of substituted thiophen-2-sulphonyl chlorides were also studied in MeOH-MeCN, where the Hammett equation is obeyed. The data appear consistent with an SN2-type mechanism which can shift toward an SN1 or an SAN process depending on the nucleophile, on the ring substituent, and on the leaving group ability. The application of the More O'Ferrall and Thornton approaches for the prediction of substituent effects on the transition state structure seems to support the above interpretation.