%0 Journal Article
%T Symmetry Analysis in Mechanistic Studies of Nucleophilic Substitution and 汕-Elimination Reactions
%A Xiaoping Sun
%J Symmetry
%D 2010
%I MDPI AG
%R 10.3390/sym2010201
%X 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每carbon is attributable to a 2p-orbital-based antibonding MO along the C每X bond. This antibonding MO, upon accepting an electron pair from a nucleophile, gives rise to dissociation of the C每X bond and formation of a new Nuc每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 C汐每X and C汕每H bonds for an E2 reaction (anti-elimination). Having this structural arrangement, the bonding C汐每X (考C-X) and antibonding C汕每H (考C-H*) orbitals become symmetry每match. They can partially overlap resulting in increase in electron density in 考C-H*, which weakens and polarizes the C汕每H bond making the 汕-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.
%K SN2 reaction
%K E2 reaction
%K molecular orbital (MO)
%K symmetry
%K electrophilicity
%U http://www.mdpi.com/2073-8994/2/1/201