A Study of the SN2 Mechanism by Symmetry Rules and Qualitative Molecular Orbital Theory

Traditionally, the SN2 mechanism has been studied by qualitative transition state theory. It is intuitive and readily understandable; however, the explanation of the nature of the transition state is vague. The argument on electrophilicity based on a charge analysis is less overwhelming. It does not explain why the SN2 reactions do not occur readily on molecules containing two or more functional groups on the central sp3-hybridized carbon (e.g., CH2Cl2 and CHCl3). In this article, an alternative approach to the study of the SN2 mechanism for the alkyl halides RCH2X (X = Cl, Br, or I; R = H or an alkyl group) using symmetry rules and qualitative molecular orbital theory is presented. The electrophilicity of a functionalized sp3-hybridized carbon is attributable to a 2p-orbital-based antibonding MO along the C–LG bond (LG is a leaving group). This antibonding MO, upon accepting an electron pair from a nucleophile, gives rise to dissociation of the C–LG bond and formation of a new Nuc–C bond. The presence or absence of such an antibonding MO in various functionalized molecules has been established readily by symmetry analysis aided by character tables. This model, together with the traditional method, provides a better picture of how SN2 and related reactions are electronically controlled. This method would be an asset in the senior-level undergraduate and graduate organic chemistry curricula.