The hydrolytic reactions of sulfonate esters have previously been considered to occur by concerted mechanisms. We now report the observation of a break in a Brønsted correlation for the alkaline hydrolysis of aryl benzenesulfonates. On either side of a break-point, β(leaving group) values of -0.27 (pK(a) < 8.5) and -0.97 (pK(a) > 8.5) are measured. These data are consistent with a two-step mechanism involving a pentavalent intermediate that is also supported by QM/MM calculations. The emerging scenario can be explained by the combined effect of a strong nucleophile with a poor leaving group that compel a usually concerted reaction to favour a stepwise process.
