The α-Effect in Gas-Phase S_N2 Reactions: Existence and the Origin of the Effect

The origin of enhanced reactivity of α-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 α-effect, whose size varies depending on the R group and the identity of the α-atom. The α-effect is larger for i-PrCl than EtCl, and for an α-nucleophile with a harder α-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 α-effect, such as thermodynamic product stability, transition state (TS) tightness, electrostatic interaction, ET rationale, and polarizability, cannot explain the observed size of the α-effect. The importance of deformation energy on going from the reactant to the TS is presented.