The alpha-effect in gas-phase SN2 reactions: existence and the origin of the effect.

The origin of enhanced reactivity of alpha-nucleophiles in SN2 reactions was examined on the basis of computational results at the high level G2(+) method for 22 gas-phase reactions: Nu- + RCl --> RNu + Cl- [R = Et and i-Pr; Nu- = HO-, CH3O-, HS-, Cl-, Br-, NH2O-, HOO-, FO-, HSO-, ClO-, and BrO-]. The results clearly indicate the existence of the alpha-effect, whose size varies depending on the R group and the identity of the alpha-atom. The alpha-effect is larger for i-PrCl than EtCl, and for an alpha-nucleophile with a harder alpha-atom. Analyses of the present results, together with previously reported ones for MeF and MeCl reactions, reveal that several rationales so far presented to explain the alpha-effect, such as thermodynamic product stability, transition state (TS) tightness, electrostatic interaction, ET rationale, and polarizability, cannot explain the observed size of the alpha-effect. The importance of deformation energy on going from the reactant to the TS is presented.