Stereoelectronic Effects with Donor and Acceptor Separated by a Vinyl Bridge

In the previous chapter, we concentrated on conformational changes enabled by rotation around a single bond and presented just a few selected examples of reactivity. For substituents at a double bond, the conformational changes are less important (with the exception of imines and similar compounds). Because the conformational equilibrium is usually frozen by the double bond rigidity, the stereoelectronic effects are more often observed as effects on reactivity and relative stability. 

Stereoelectronic Effects A Bridge Between Structure and Reactivity, First Edition. Igor V. Alabugin. 2016 John Wiley Sons, Ltd. Published 2016 by John Wiley Sons, Ltd. 

The cis-conformational preference erodes for the heavier halogens. For example, the cis-isomer of 1,2-dichloroethene is also more stable than the trans-isomer but the difference is smaller (only 0.4kcal/mol). Furthermore, the experimentally determined difference in stability of the two isomers of 1,2-dibromoethene is negligible. 

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Stereoelectronic Effects with Donor and Acceptor Separated by a Vinyl Bridge... 

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