Effect of a Phase Transfer Catalyst on the Dynamics of an S_N2 Reaction. A Molecular Dynamics Study

The effect of a tetramethylammonium cation phase transfer catalyst on the benchmark Cl − + CH 3 Cl reaction at the water/chloroform liquid/liquid interface is investigated by a molecular dynamics-empirical valence bond (EVB) model. The effect of the catalyst on the reaction free energy profile at different interface locations and in bulk chloroform is examined. We find that, because of significant water “pollution”, activation of the nucleophilic attack is limited to the bulk organic region. The barrier height at the interface is equal to or slightly larger than the barrier in bulk water and is unaffected by the presence of the catalyst. In bulk chloroform, our calculations suggest that the barrier height, which is much lower than in bulk water, moderately increases when a few water molecules interact with the system and when the catalyst forms an ion-pair with the nucleophile. Thus, the catalyst is most effective if its role is limited to bringing the nucleophile to the bulk organic phase.