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

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

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
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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MDPI and ACS Style

Sun, X. Symmetry Analysis in Mechanistic Studies of Nucleophilic Substitution and β-Elimination Reactions. Symmetry 2010, 2, 201-212.

AMA Style

Sun X. Symmetry Analysis in Mechanistic Studies of Nucleophilic Substitution and β-Elimination Reactions. Symmetry. 2010; 2(1):201-212.

Chicago/Turabian Style

Sun, Xiaoping. 2010. "Symmetry Analysis in Mechanistic Studies of Nucleophilic Substitution and β-Elimination Reactions." Symmetry 2, no. 1: 201-212.


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