hyperconjugation

Hyperconjugation is a stabilizing interaction in organic chemistry arising from the delocalization of electrons in sigma bonds, typically C–H or C–C, into an adjacent empty p orbital or into a π system. The sigma orbital of a bond next to an electron-deficient center, a carbon–carbon double bond, or a carbonyl group can overlap with the empty p orbital or with the π system, creating extended molecular orbitals and lowering the overall energy of the system. It is considered a form of resonance involving sigma bonds, though it does not involve a distinct additional bond.

The concept is most familiar in explaining the stability of carbocations. When a positively charged carbon is substituted by alkyl groups, the nearby C–H (or C–C) bonds can donate electron density into the empty p orbital on the cation. The stability increases with more adjacent C–H bonds: tertiary carbocations are more stable than secondary, which are more stable than primary. For example, a tert-butyl carbocation benefits from nine β C–H bonds contributing via hyperconjugation.

Hyperconjugation also plays a role in radical chemistry and in alkenes and carbonyl-containing compounds. In radicals, neighboring C–H bonds can delocalize electron density toward the unpaired electron, stabilizing the radical. In alkenes, alkyl substituents provide hyperconjugative stabilization of the π system, influencing reactivity and physical properties such as heats of hydrogenation. In carbonyl chemistry, adjacent sigma bonds can donate into the carbonyl system, a related effect often described as keto hyperconjugation.

Overall, hyperconjugation is a key explanatory tool for trends in stability and reactivity in many organics, reflecting sigma–p or sigma–π interactions rather than discrete additional bonds.