Solvent network at the transition state in the solvolysis of hindered sulfonyl compounds

Abstract

Alcoholysis rates of unhindered benzenesulfonyl chlorides (X‐ArSO2Cl, X = H‐; 4‐Br‐; 4‐Me‐) are similar in methanol; the same behavior is also observed in ethanol, whereas the reactivity order in iso‐propanol is 4 Me‐ < H‐ < 4‐Br‐. On the other hand, alcoholysis of sterically hindered arenesulfonyl chlorides (X‐ArSO2Cl) (X = 2,4,6‐Me3‐3‐NO2‐; 2,6‐Me2‐4‐tBu‐; 2,4,6‐Me3‐; 2,3,5,6‐Me4‐; 2,4,6‐iPr3‐; 2,4‐Me2‐; 2,4,6‐(OMe)3‐) in all studied alcohols show a significant increase in reactivity, the so‐called positive steric effect.Most of the substrates showed a reaction order b ~ 2 with respect to the nucleophile in methanol and ethanol, and b ~ 3 in iso‐propanol. The correlation between reactivity and the Kirkwood function (1/ξ) gives negative sensitivity (U) for all systems. All substrates showed high sensitivity to media nucleophilicity that depends on ΣσX.Obtained results suggest the alcoholysis of benzenesulfonyl chlorides proceeds through SN2 mechanism where the transition state (TS) involves the participation of 2–3 alcohol molecules; such a TS can be cyclic, in the case of unbranched alcohols, or linear, for alcohols with bulkier hydrocarbon groups like iso‐propanol. To include the number of alcohol molecules playing such a role in the TS, the following terminology is proposed: cSN2sn for SN2 reactions involving n solvent molecules in a cyclic (c) TS, where “s” stands for the solvent and “n” is either the closest integer or half‐integer to the reaction order relative to the solvent or, in computational studies, the proposed number of solvent molecules taking part in the TS, whereas SN2sn is proposed when the TS is not cyclic. Copyright © 2016 John Wiley & Sons, Ltd.