Hyperconjugation involves the donation of electrons from a filled orbital (commonly the bonding C-H sigma molecular orbital) into an adjacent/partially occupied p-orbital (for instance an adjacent carbocation) leading to stabilisation. The tag should be applied to questions where hyperconjugation is involved, even if the question does not explicitly refer to it.
Hyperconjugation involves the donation of electrons from a filled orbital (commonly the bonding C-H sigma molecular orbital) into an adjacent/partially occupied p-orbital (for instance an adjacent carbocation) leading to stabilisation.
In organic chemistry, hyperconjugation is used to explain the increased stability of alkyl-substituted radicals/carbocations and has been invoked in explanations of the anomeric and beta-silicon effects.
In the formalism that separates bonds into sigma and pi types, hyperconjugation is the interaction of sigma-bonds (e.g. C–H, C–C, etc.) with a pi network. This interaction is customarily illustrated by contributing structures, e.g. for toluene (below), sometimes said to be an example of 'heterovalent' or 'sacrificial hyperconjugation', so named because the contributing structure contains one two-electron bond less than the normal Lewis formula for toluene:
Applicability of the hyperconjugation tag:
- hyperconjugation will almost always be applied with the organic-chemistry tag as the stabilising effect is observed in organic molecules. molecular-orbital-theory also fits since the hyper conjugative effect is a result of bonding MO’s donating electron density. It should be applied even if the OP does not realise that hyperconjugation is involved (for instance questions blindly asking why certain cations are stabilised relative to others).
- additional tags may be applied depending on the nature of the question, such as carbocation if discussing the way in which hyperconjugation stabilises a carbocation for example. stereoelectronics may also be relevant.
The following tags are related to hyperconjugation, with multiple questions on chemistry.se already tagged with one of more of the following which may be useful as additional tags on questions identified as being related to medicinal chemistry:
Hyperconjugation is quite a narrow topic but is covered by many organic chemistry texts (see organic-chemistry for references).