Computational study of SN2 reactions promoted by crown ether: Contact ion pair versus solvent-separated ion pair mechanism

초록

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 CH3CN 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.

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