<scp>MNDO</scp> Study of reaction pathways for <i>S</i>_N2 reactions. Menschutkin reaction potential energy surfaces

Abstract

Abstract MNDO molecular orbital calculations have been employed to investigate limited reaction pathways and potential energy surfaces for a series of S N 2 reactions. Model calculations for X − + CH 3 X ( X = H, F, OH, OCH 3 , and CN) indicate that the MNDO method gives qualitative agreement with ab initio studies except for the hydride–CH 4 exchange. Studies involving alkylation of pyridine (Menschutkin reaction) were also carried out. For the reaction of pyridine with CH 3 Cl, which involves charge separation, our MNDO studies (which do not include solvation effects) do not produce a characteristic S N 2 pathway. For the reaction of pyridine with trimethyloxonium cation [(CH 3 ) 3 O + ] as the alkylating agent, a well defined S N 2 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.