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

Abstract

AbstractMNDO molecular orbital calculations have been employed to investigate limited reaction pathways and potential energy surfaces for a series of SN2 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 SN2 pathway. For the reaction of pyridine with trimethyloxonium cation [(CH3)3O+] as the alkylating agent, a well defined SN2 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.