Solvent-induced dual nucleophiles and the α-effect in the SN2 versus E2 competition

We have quantum chemically investigated how microsolvation affects the various E2 and S(N)2 pathways, their mutual competition, and the alpha-effect of the model reaction system HOO-(H2O)(n) + CH3CH2Cl, at the CCSD(T) level. Interestingly, we identify the dual nature of the alpha-nucleophile HOO- which, upon solvation, is in equilibrium with HO-. This solvent-induced dual appearance gives rise to a rich network of competing reaction channels. Among both nucleophiles, S(N)2 is always favored over E2, and this preference increases upon increasing microsolvation. Furthermore, we found a pronounced alpha-effect, not only for S(N)2 substitution but also for E2 elimination, i.e., HOO- is more reactive than HO- in both cases. Our activation strain and quantitative molecular orbital analyses reveal the physical mechanisms behind the various computed trends. In particular, we demonstrate that two recently proposed criteria, required for solvent-free nucleophiles to display the alpha-effect, must also be satisfied by microsolvated HOO-(H2O)(n) nucleophiles.