A Mechanistic Study of S_N2 Reaction in a Diol Solvent

We present calculations for the mechanism of S_N2 reactions in ethylene glycol, focusing on the role of two protic functional groups (−OH) in the solvent molecule. We find that some hydroxyl groups act as Lewis base to interact with the cation, whereas others affect the reaction as Lewis acid to the nucleophile. We predict that the nucleophile (F⁻) reacts as an ion pair rather than as a solvent-separated ion when metal cation (Cs⁺) is used as a counterion. The overall influence of ethylene glycol manifests itself as the reaction barrier (E^⧧ = 20.0, G_353K^⧧ = 21.5 kcal/mol) that is a bit smaller than that in tert-butyl alcohol, which proved to be a very efficient solvent for S_N2 reactions [Kim, D. W., et al. J. Am. Chem. Soc. 2006, 128, 16394]. We therefore show that a small protic solvent such as ethylene glycol may be as efficient as a bulky alcohol for S_N2 reactions.