Hyperconjugation

Hyperconjugation is electron delocalization through overlap of a sigma bond with an adjacent empty or partially filled orbital. It helps explain molecular stability, structure, and reactivity.

Sigma conjugation: Electron delocalization involving sigma bonds rather than adjacent pi bonds. Hyperconjugation is commonly described as sigma conjugation between a bond and a neighboring orbital.
Carbocation: An organic ion in which a carbon atom bears a positive charge and has an electron-deficient valence shell. Adjacent C–H and C–C bonds donate electron density into its vacant p orbital.
Valence bond theory: A bonding model that describes molecules using localized atomic orbitals and their overlaps. Its localized bonds provide the sigma-bond donor orbitals used in hyperconjugation descriptions.
Conjugation: Electron delocalization across adjacent p orbitals in a chain or ring of atoms. It is the familiar pi-system counterpart to sigma-bond delocalization.
Orbital interaction: The influence of one atomic or molecular orbital on another through overlap and energy coupling. Hyperconjugation is one specific stabilizing interaction between neighboring orbitals.
Alkene stability: The relative thermodynamic favorability of alkenes, often compared by substitution and molecular energy. Hyperconjugative donation from neighboring alkyl bonds helps stabilize more substituted alkenes.
Molecular orbital theory: A theory of chemical bonding in which electrons occupy orbitals extending over entire molecules. It describes hyperconjugation as mixing and delocalization among molecular orbitals.
Negative hyperconjugation: Electron donation from a filled lone-pair or pi orbital into an adjacent sigma antibonding orbital. It reverses the usual donor–acceptor direction associated with conventional hyperconjugation.
Donor–acceptor interaction: An interaction in which electron density from a donor orbital is shared with an acceptor orbital. A filled sigma bond can donate electron density into a nearby vacant or partly occupied orbital.
Bond dissociation energy: The energy required to break a specified bond homolytically in the gas phase. Hyperconjugation can stabilize the radical formed after cleavage and lower the required energy.

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