Nucleophilic substitution at sulphonyl sulphur. Part 4. Hydrolysis of substituted thiophenesulphonyl halides in water–acetone mixtures

Abstract

Hydrolysis of substituted thiophene-2-sulphonyl bromides and chlorides have been studied in water and in water–acetone mixtures at 25°C. Curved Hammett plots with an upward concavity are obtained in water and in water-acetone with up to 40% acetone. For solvents richer in acetone (>80%) the Hammett plots become straight lines with positive slopes for both chlorides and bromides. The result are rationalized in terms of different timing between nucleophile–sulphur bond making and sulphur–halogen bond-breaking on changing substituent, nucleofuge, and solvent. The application of the More O'Ferrall potential energy surface model is consistent with an S_N2-type mechanism which can shift towards an S_N1 or S_AN process depending on different factors. On the other hand, the VBCM model, recently proposed by Pross, makes predictions which are in conflict with the experimental results. A comparison between some solvolysis reactions of benzoyl chlorides and thiophene-2-sulphonyl chlorides indicate that there is close analogy between these two electrophilic centres.