Atomistic dynamics of elimination and substitution driven by entrance channel.

E2 elimination and SN2 substitution reactions are of central importance in preparative organic synthesis due to their stereospecificity. Herein, atomistic dynamics of a prototype reaction of ethyl chloride with hydroxide ion are uncovered that show strikingly distinct features from the case with fluoride anion. Chemical dynamics simulations reproduce the experimental reaction rate and reveal that the E2 proceeding through a direct elimination mechanism dominates over SN2 for the hydroxide ion reaction. This unexpected finding of a pronounced contribution of direct reaction dynamics, even at a near-thermal energy, is in strong contrast to the complex-mediated indirect mechanism for the fluoride case that characterizes the low-energy ion-molecule reactions. The entrance channel structures are found to be crucial and the differences are attributed to subtle changes in the hydrogen-bonding interaction of the approaching reactants. This effect presents in E2/SN2 reactions of different bases and alkyl halides and might play a role in complex chemical networks and environments.