Competitive dynamics of E2 and S N 2 reaction driven by collision energy and leaving group.

The competition between E2 and S N 2 reactions is essential in organic chemistry. In this paper, the reaction mechanism of F - + CH 3 CH 2 Cl is investigated utilizing direct dynamics simulations, and unravel how the collision energy ( E coll ) and the leaving group affect the competition between S N 2 and E2 in the F - + CH 3 CH 2 Y (Y = Cl and Br) reactions. Simulation results for F - + CH 3 CH 2 Cl 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 N 2 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 - + CH 3 CH 2 Cl are further compared with the F - + CH 3 CH 2 Br 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.