Stereoelectronic Effects in Elimination Reactions

The chirality of the reactant and the product allows us to distinguish the E2 and El mechanisms. The stereochemical consequences of the two mechanisms differ because of the different structures of the intermediates and transition states in these mechanisms. 

The E2 reaction is most favorable when the hydrogen on the (l-carbon and the halogen are in an frperiplanar conformation. This is the case in cM-l-bromo-4-rifrr-butylcyclohexane. A Newman projection structure shows this favorable conformation. 

The stereoelectronic effect of the E2 reactions can also be established with conformationally flexible, open chain molecules, if they have chiral centers, for example, the dehydrobromination of (l/ ,2/ )-l,2-dibromo-l,2-diphenylethane. A specific sta ered conformation is required to properly align the sp hybrid orbitals of the C—H and C—Br bonds. This alignment can yield only the Z isomer in a concerted E2 reaction. 

There is no special geometric requirement for an El reaction. Once the carbocation forms, any of the hydrogen atoms on carbon atoms adjacent to the positive center can be lost. 

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