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Department of Natural Science and Mathematics, University of Charleston, 2300 MacCorkle Avenue, S. E., Charleston, WV 25304, USA





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





Autor: Xiaoping Sun

Fuente: http://mdpi.com/



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