A molecular electron density theory study of the bimolecular nucleophilic substitution reactions on monosubstituted methyl compounds

The nucleophilic substitution reactions involving methyl monosubstituted compounds have been studied within the Molecular Electron Density Theory (MEDT) at the omega B97X-D/6-311+G(d,p) computational level in DMSO. This study aims to characterize the electronic nature of the transition state structures (TSs) involved in the so-called SN2 and SNi reactions. Both electron localization function and atom-in-molecules topological analyses indicate that the TSs involved in these nucleophilic substitutions can be described as a central methyl CH3+ carbocation, which is strongly stabilized by the presence of two neighbouring nucleophilic species through electron density transfer. This MEDT study establishes a significant electronic similarity between the so-called SN1 and SN2 reactions. Due to the weak electrophilic character of the methyl tetrahedral carbons, the departure of the leaving group should be expected with the approach of the nucleophile. However, while along the SN1 reactions, the strong stabilization of the tertiary carbocation does not demand the participation of the nucleophile, along the SN2 and SNi reactions involving primary tetrahedral carbons, the nucleophiles should participate in the reaction to stabilize the unstable methyl carbocation. ELF and AIM topological analyses of the electron density in the transition state structures of the SN2 reactions of monosubstituted methyl compounds reveal a central CH3+ carbocation strongly stabilized by the two adjacent nucleophilic species.