MNDO Study of reaction pathways for SN2 reactions. Menschutkin reaction potential energy surfaces

MNDO molecular orbital calculations have been employed to investigate limited reaction pathways and potential energy surfaces for a series of S_N2 reactions. Model calculations for X^? + CH₃X (X = H, F, OH, OCH₃, and CN) indicate that the MNDO method gives qualitative agreement with ab initio studies except for the hydride–CH₄ exchange. Studies involving alkylation of pyridine (Menschutkin reaction) were also carried out. For the reaction of pyridine with CH₃Cl, which involves charge separation, our MNDO studies (which do not include solvation effects) do not produce a characteristic S_N2 pathway. For the reaction of pyridine with trimethyloxonium cation (CH₃)₃O⁺] as the alkylating agent, a well defined S_N2 reaction pathway was obtained; this reaction involves charge transfer. A potential energy surface for the pyridine–trimethyloxonium cation reaction shows the presence of a saddle point transition state that resembles starting materials, in agreement with the Hammond postulate for this exothermic reaction.