Computational study of S N 2 reactions promoted by crown ether: Contact ion pair versus solvent‐separated ion pair mechanism

We examined by quantum chemical methods the mechanism of S N 2 reaction using metal bromide MBr (M = Na, K, Cs) and KX (X= F, Cl) in CH 3 CN promoted by crown ether (18‐crown‐6). We focus on whether the metal salts react as a contact ion pair (CIP; M + and X – in close contact) or as a solvent‐separated ion pair (SSIP; M + and X – at large distance). In SSIP mechanism, X – is removed far enough from M + for the metal salt to be considered as “separated” by the effects of the crown ether and the solvent. In the CIP picture, conversely, the coordination of 18‐crown‐6 to M + is not sufficient to overcome the powerful Coulombic interactions between M + and X – . We find that the CIP route is favored for S N 2 bromination processes using MBr (M = Na, K, Cs). For S N 2 reaction using KF, the feasibility of the two pathways is essentially equal, whereas for S N 2 chlorination by KCl the SSIP route is predicted to be favored.