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Hyperconjugation originally describes the conjugation of a $\sigma$-bond with a double or triple bond. Hyperconjugation is the stabilizing interaction that results from the interaction of the electrons in a $\sigma$-bond with an adjacent empty p orbital or an $\pi$ orbital to give an extended molecular orbital that increases the stability of the system.

This concept was introduced by Baker and Nathan. It is called no-bond resonance or $\sigma$-$\pi$ conjugation. The magnitude of effect of hyper conjugation is smaller than resonance, so it is also called secondary resonance.

What is Hyperconjugation?

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Hyperconjugation Definition

"Hyperconjugation is defined as the (usually stabilizing) interaction of an occupied $\sigma$-bonding with a vacant orbital (usually a p- or $\pi$-orbital)."

Negative hyper conjugation which is far less common and whose existence was controversial for many years, is the interaction of a high lying occupied orbital with a vacant $\sigma^{*}$-anti bonding orbital.

Hyperconjugation is defined as the conjugation ability of sigma ($\sigma$) electrons of $\alpha$-hydrogen atom with unsaturated system when a H-C bond is attached to an unsaturated system.

Hyperconjugation Effect

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Hyperconjugation describes the orbital interactions between the $\pi$-systems and adjacent $\sigma$-bonds of substituent groups in organic compounds. This effect arises due to the partial overlap of a sp3-s(C-H bond) with the empty p orbital of an adjacent + vely charged carbon atom.

"When an alkyl group is attached to an unsaturated system such as a double bond or a benzene ring, the order of inductive effect is actually reversed. This effect is called hyper conjugation effect or Baker Nathan effect or an-chimeric assistance."
  • Let us consider ethyl cation in which + vely charged carbon atom has an empty p orbital. One of the C-H bond of methyl group can align in plane of this empty p orbital and electrons of C-H bond in plane with this p-orbital can then be delocalized into this empty p orbitals as shown below.
Delocalization of p Orbital
  • In general greater the number of alkyl groups attached to + vely charged C atom greater is the hyper conjugation an greater will be stabilization of cation.
Stability of Carbocation
  • It is also called no bond resonance shown with the structures of ethyl cation below.
Ethyl Cation
  • Propene is more stable than ethene because of hyper conjugation. Hyperconjugation in propene is shown below.
Hyperconjugation in Propene
  • Hyper conjugation is also called bond sacrificial resonance. Greater the number of alkyl groups attached o doubly bonded C atoms, greater is the number of contributing structures and greater is the stability. The order of stability of some alkenes is shown below.
Stability of Alkenes
  • It also affects the physical properties of compounds. The shortening of C-C bond adjacent to triple bond is due to the contribution of ionic forms.
Cyano Compound
  • Conjugated dienes like 1,3-Butadiene are more stable than simple alkenes due to the possibility of more delocalization in conjugated than in simple alkenes.
Conjugated Dienes

Delocalized Electron

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Hyperconjugation refers to the delocalization of electrons in a $\sigma$ bond through a system of overlapping orbitals. Hyperconjugation involves electron delocalization from a filled bonding orbital to an adjacent unfilled orbital. In the case of a carbocation, the unfilled orbital is the vacant p orbital of the carbocation and the filled orbitals are C-H or C-C sigma bonds at the carbons adjacent to the p orbital of the carbocation.

Sharing of electron density from adjacent C-H or C-C sigma bonds with the carbocation p orbital delocalizes the positive charge. Any time a charge can be dispersed or delocalized a system will be stabilized. A representation pf hyper conjugation between a sigma bonding orbital and an adjacent carbocation p orbital is
Delocalized Electron
The net result of hyper conjugation is an increase of electron density on the cationic carbon thereby delocalizing the positive charge onto the adjacent alkyl groups. As more alkyl groups are bonded to a cationic carbon the hyper conjugation effect becomes stronger and the carbocation becomes more stable.

Hyperconjugation Examples

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Hyper conjugation like +I effect releases electron however the magnitude is in opposite direction. The magnitude of effect of hyper conjugation is smaller than resonance so it is also called secondary resonance. Hyperconjugation increases the stability in alkenes by decreasing heat of hydrogenation.

  1. Pent-2-ene is more stable than pent-1-ene
  2. But-2-ene or isobutene is more stable than Butene-1