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

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 results 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 SN2-type mechanism which can shift towards an SN1 or SAN 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.