Steric Effects and Solvent Effects on S_N2 Reactions

We present quantum mechanical calculations designed to disentangle the influences of solvent effects and substituent effects on ionic nucleophilic substitution reactions. In particular, we compare the SN2 reactions of Cl- with CH3CH(X)Cl and (CH3)3CCH(X)Cl for X = H and CN in the gas phase and aqueous solution. We find that, for all of these reactions, transition state distortion and dielectric descreening effects are quantitatively larger in magnitude than hydrophobic effects or exchange repulsion, but they also roughly cancel one another so that differential solvation contributes little to differences in the free energies of activation associated with a CH3 versus a (CH3)3C group as a substituent at the reacting position. Differential solvation of the transition-state structures relative to the reactants is less unfavorable for X = H than for X = CN because of the greater charge separations in the X = H case, and this separation places more positive charge on the reacting carbon center. The smaller deceleration associated with aqueous solvation for X = H roughly balances the gas-phase acceleration predicted for X = CN so that the aqueous activation free energies for the substrates are predicted to be similar for these two substituents.