Competing E2 and S_N2 Mechanisms for the F^– + CH₃CH₂I Reaction

Abstract

Anti-E2, syn-E2, inv-, and ret-S_N2 reaction channels for the gas-phase reaction of F^- + CH₃CH₂I were characterized with a variety of electronic structure calculations. Geometrical analysis confirmed synchronous E2-type transition states for the elimination of the current reaction, instead of nonconcerted processes through E1cb-like and E1-like mechanisms. Importantly, the controversy concerning the reactant complex for anti-E2 and inv-S_N2 paths has been clarified in the present work. A positive barrier of +19.2 kcal/mol for ret-S_N2 shows the least feasibility to occur at room temperature. Negative activation energies (-16.9, -16.0, and -4.9 kcal/mol, respectively) for inv-S_N2, anti-E2, and syn-E2 indicate that inv-S_N2 and anti-E2 mechanisms significantly prevail over the eclipsed elimination. Varying the leaving group for a series of reactions F^- + CH₃CH₂Y (Y = F, Cl, Br, and I) leads to monotonically decreasing barriers, which relates to the gradually looser TS structures following the order F > Cl > Br > I. The reactivity of each channel nearly holds unchanged except for the perturbation between anti-E2 and inv-S_N2. RRKM calculation reveals that the reaction of the fluorine ion with ethyl iodide occurs predominately via anti-E2 elimination, and the inv-S_N2 pathway is suppressed, although it is energetically favored. This phenomenon indicates that, in evaluating the competition between E2 and S_N2 processes, the kinetic or dynamical factors may play a significant role. By comparison with benchmark CCSD(T) energies, MP2, CAM-B3LYP, and M06 methods are recommended to perform dynamics simulations of the title reaction.