How a Solvent Molecule Affects Competing Elimination and Substitution Dynamics. Insight into Mechanism Evolution with Increased Solvation

Competiting S_N2 substitution and E2 elimination reactions are of central importance in preparative organic synthesis. Here, we unravel how individual solvent molecules may affect underlying S_N2/E2 atomistic dynamics, which remains largely unclear with respective to their effects on reactivity. Results are presented for a prototype microsolvated case of fluoride anion reacting with ethyl bromide. Reaction dynamics simulations reproduce experimental findings at near thermal energies and show that the E2 mechanism dominates over S_N2 for solvent-free reaction. This is energetically quite unexpected and results from dynamical effects. Adding one solvating methanol molecule introduces strikingly distinct dynamical behaviors that largely promote the S_N2 reaction, a feature which attributes to a differential solute-solvent interaction at the central barrier that more strongly stabilizes the transition state for substitution. Upon further solvation, this enhanced stabilization of the S_N2 mechanism becomes more pronounced, concomitant with drastic suppression of the E2 route. This work highlights the interplay between energetics and dynamics in determining mechanistic selectivity and provides insight into the impact of solvent molecules on a general transition from elimination to substitution for chemical reactions proceeding from gas-to solution-phase environments.