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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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

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

Xiaoping Sun

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

Published online: 20 August 2003

Abstract. 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 CLG bond (LG is a leaving group). This antibonding MO, upon accepting an electron pair from a nucleophile, gives rise to dissociation of the CLG bond and formation of a new NucC 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.

Key Words:  In the Classroom; organic chemistry; orbitals

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Article in PDF format (425 KB) HTML format

Supporting Materials:


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.

Issue date: October 1, 2003

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