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Symmetry 2010, 2(1), 201-212; doi:10.3390/sym2010201

Symmetry Analysis in Mechanistic Studies of Nucleophilic Substitution and β-Elimination Reactions

Department of Natural Science and Mathematics, University of Charleston, 2300 MacCorkle Avenue, S. E., Charleston, WV 25304, USA
Received: 31 January 2010 / Accepted: 20 February 2010 / Published: 25 February 2010
(This article belongs to the Special Issue Feature Papers: Symmetry Concepts and Applications)
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Abstract

A mechanistic study of the bimolecular nucleophilic substitution (SN2) reaction for halomethane CH3X (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 CH3X) and products (NucCH3 and X-). Similar symmetry analysis has been applied to mechanistic study of the bimolecular b-elimination (E2) reactions of haloalkanes. It well explains the necessity of an anti-coplanar arrangement of the Cα–X and Cβ–H bonds for an E2 reaction (anti-elimination). Having this structural arrangement, the bonding Cα–X (σC-X) and antibonding Cβ–H (σC-H*) orbitals become symmetry–match. They can partially overlap resulting in increase in electron density in σC-H*, which weakens and polarizes the Cβ–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 SN2 and E2 reactions represent a new approach to studying organic mechanisms.
Keywords: SN2 reaction; E2 reaction; molecular orbital (MO); symmetry; electrophilicity SN2 reaction; E2 reaction; molecular orbital (MO); symmetry; electrophilicity
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Sun, X. Symmetry Analysis in Mechanistic Studies of Nucleophilic Substitution and β-Elimination Reactions. Symmetry 2010, 2, 201-212.

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