Competitive dynamics of E2 and SN2 reaction driven by collision energy and leaving group

Abstract

The competition between E2 and S_N2 reactions is essential in organic chemistry. In this paper, the reaction mechanism of F⁻ + CH₃CH₂Cl is investigated utilizing direct dynamics simulations, and unravel how the collision energy (E_coll) and the leaving group affect the competition between S_N2 and E2 in the F⁻ + CH₃CH₂Y (Y = Cl and Br) reactions. Simulation results for F⁻ + CH₃CH₂Cl reaction show that the anti-E2 channel is dominant, but with the increase of E_coll from 0.04 to 1.9 eV the branching ratio of the anti-E2 pathway significantly decreases by 21%, and the S_N2 pathway becomes more important. A transition from indirect to direct reaction has been revealed when E_coll is increased from 0.04 to 1.90 eV. At lower E_coll, a large ratio of indirect events occurs via a long-lived hydrogen-bonded complex, and as the collision energy is increased, the lifetimes of the hydrogen-bonded complexes are shortened, due to an initial faster relative velocity. The simulation results of F⁻ + CH₃CH₂Cl are further compared with the F⁻ + CH₃CH₂Br reaction at E_coll of 0.04 eV. Changing the leaving group from Cl to Br drastically suppresses the indirect events of anti-E2 with a branching ratio decreasing from 0.46 to 0.36 due to the mass effect, and promotes direct rebound mechanism resulting from a looser transition state geometry caused by varied electronegativity.