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

TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

Table of Contents

Abstract Volume 8 Issue 5 (2003) pp 303-306

A Study of the S_N2 Mechanism by Symmetry Rules and Qualitative Molecular Orbital Theory

Xiaoping Sun

Department of Natural Science and Mathematics, University of Charleston, Charleston, West Virginia 25304, xsun@ucwv.edu
Received May 21, 2003. Accepted July 9, 2003.

Published online: 20 August 2003

Abstract. Traditionally, the S_N2 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 S_N2 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 S_N2 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 S_N2 and related reactions are electronically controlled. This method would be an asset in the senior-level undergraduate and graduate organic chemistry curricula.

Key Words: In the Classroom; organic chemistry; orbitals

(*) Corresponding author. (E-mail: xsun@ucwv.edu)

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Supporting Materials:

A supporting file containing symmetry analysis and ligand group orbitals (lgos) for various functionalized molecules is available in a Zip file (43 KB) 10.1007/ s00897030717a.