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# Hyperconjugation

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 Sub Topics 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?

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

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.
• 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.
• It is also called no bond resonance shown with the structures of ethyl cation below.
• Propene is more stable than ethene because of hyper conjugation. Hyperconjugation in propene is shown below.
• 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.

• 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.

• 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.

## Delocalized Electron

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.