Nucleophilic Substitution at Unactivated Vinylic Carbon. Factors Conducive to the Energetic Preference for the In-Plane SN2 Pathway

The gas-phase nucleophilic bimolecular substitutions at unactivated vinylic carbon (CH 2 CHCl, 1 ) by four nucleophiles (OH -, SH -, Cl -, and Br - ) are investigated theoretically at the G2(+)(MP2) level. The results show that the stronger nucleophiles (OH - and SH - ) substitute by an out-of-plane S N 2 path with retention (S N π) but an in-plane S N 2 path (S N σ) with inversion of configuration is preferred for the substitution by the weaker bases, Cl - and Br - . However, the elimination pathway is much more facile for OH - than any substitution process. We have considerd three factors, (i) the LUMO symmetries (σ*, or π*), (ii) the proximate σ−σ* charge-transfer interactions, and (iii) the electrostatic interactions in the transition state, as possible causes for preferred pathway for each nucleophile. The stability of the S N π transition state is predominantly influenced by the proximate σ−σ* type interactions, whereas electrostatic interactions are the major factor conducive to the energetic preference for the S N σ over the S N π processes for Cl - and Br - . Solvent effect raises the barrier height but the mechanism and preferred path are not affected.