Illustrated Glossary of Organic Chemistry

Stereoelectronic effect: Any effect on structure, reactivity, or other properties of a molecule that is caused by the spatial position of the molecule's orbitals.

The E2 reaction transition state has a stereoelectronic requirement. For an E2 reaction to occur the carbon-leaving group bond (in blue) and the carbon-β-hydrogen bond (in red) must be parallel (either anti-periplanar or syn-periplanar) so that the incipient p orbitals (in red) are also parallel for pi bond formation.

When the carbon-leaving group bond and carbon-β-hydrogen bond are not parallel the E2 reaction cannot occur.