Symmetry analysis in mechanistic studies of nucleophilic substitution and β-elimination reactions

Abstract

A mechanistic study of the bimolecular nucleophilic substitution (S N2) reaction for halomethane CH 3X (X = Cl, Br, or I) is approached by using symmetry principles and molecular orbital theory. The electrophilicity of the functionalized sp3-carbon is attributable to a 2p-orbital-based antibonding MO along the C-X bond. This antibonding MO, upon accepting an electron pair from a nucleophile, gives rise to dissociation of the C-X bond and formation of a new Nuc-C bond. Correlations are made between the molecular orbitals of reactants (Nuc - and CH 3X) and products (NucCH 3 and X -). Similar symmetry analysis has been applied to mechanistic study of the bimolecular β-elimination (E2) reactions of haloalkanes. It well explains the necessity of an anti-coplanar arrangement of the C α-X and C β-H bonds for an E2 reaction (anti-elimination). Having this structural arrangement, the bonding C α-X (σ C-X) and antibonding C β-H (σ C-X *) orbitals become symmetry-match. They can partially overlap resulting in increase in electron density in σ C-H *, which weakens and polarizes the C β-H bond making the β-H acidic. An E2 reaction can readily take place in the presence of a base. The applications of symmetry analysis to the S N2 and E2 reactions represent a new approach to studying organic mechanisms. © 2010 by the authors.