Semiempirical study of the solvent effect on the Menshutkin reaction

Abstract

The solvent effect on the Menshutkin S_N2 reaction of CH₃X (X = Cl, Br, I) has been studied using multicavity self-consistent reaction field (MCa SCRF). Two dielectric media, corresponding to hexane and water, have been modelled. The linear and nonlinear structures of the reaction complexes have been used to model the reaction path. The reaction field due to the solvent stabilizes significantly both the transition state and the local minimum of the product of the reaction. The structure of the complexes corresponding to other minima in the reaction path is also discussed. Our results indicate that the reactions of ammonia with methyl chloride, methyl bromide and methyl iodide are exothermic (except methyl iodide in hexane) even in the gas phase, due to the formation of different products (complex between the electroneutral HX and CH₃NH₂). In hexane, the ion pair CH₃NH₃⁺X^– is formed, whereas in water the reaction results in free ions. A full scheme of the reaction in different media is presented.