Hyperconjugation chemistry

Hyperconjugation is the stabilizing interaction that results from action of the electrons in a sigma bond (usually C- H or with an adjacent empty (or partially filled) non-bonding π-Orbital or antibonding p-orbital to give an extended molecular orbital that increases the stability of the system.

Hyperconjugation chemistry

Let us take the example of CH3, -CR2 in which the positively charged carbon atom has an empty p-orbital. One of the C-H bonds of the methyl group can align in the plane of this empty p-orbital and the electrons constituting the C – H bond in plane with a p-orbital can then be delocalised into the empty p-orbital.

This type of overlap stabilizes the carbocation because density from the adjacent σ bond helps in dispersing the positive charge.

Hyperconjugation is delocalisation of  σ electrons. Inductive effect of alkyl groups on a saturated hydrocarbon chain follows the order

(CH3),3 C-> (CH3)2  CH-> CH3CH2>-CH3

(tertiary butyl > isopropyl > ethyl > methyl)

Whereas if an alkyl group is attached to an unsaturated system. the order of inductive effect is just reversed. This effect is called hyperconjugation effect or Baker Nathan effect. It is a permanent effect. In fact hyperconjugation is an extension of resonance. Resonance effect involves delocalization of π electrons of two or more conjugated double bonds while hyperconjugation involves delocalization of σ electrons. Hyperconjugation can be described as “double bond – no bond resonance”.

Thus, when the C – H bond is attached to an unsaturated carbon atom, the σ -electrons of the C – H bond becomes less localized by entering into partial conjugation with the attached unsaturated system. i. e σ, π conjugation.

This type of conjugation between the  σ electron of  single bond and π electron of multiple bond is known as hyperconjugation it is permanent effect which stabilizes the molecules.

Significant of the Hyperconjugation Effect

Hyperconjugation effect is much weaker than the resonance.  it is useful in explaining some of the following properties of organic molecule.
1. Directive influence of alkyl groups:
The o, p directive influence of -CH3 and other alkyl groups can be easily explained on the basis of hyperconjugation.
Six more structures are possible because of two other alpha hydrogen atom. As a result of hyperconjugation (C-H  σ bond hyperconjugation), the electron density at- O and -p position with respect to CH3 group increase and hence toluene is said to be o and p _ directing group.
2. Shortening of carbon – carbon single bond:
Carbon – carbon single bond distance decreases due to adjacent multiple bond.
C2 – C3 bond distance decrease from 1.54 A to 1.49 A because of hyperconjugation of C-H bond C3 carbon.
3. Relative stability  of alkenes :
Heat of hydrogenation ( H ) clearly shows that greater the number of alkyl groups attached to double Bonded carbon atom’s greater is the stability of the alkene , i. e more stable alkene less will be heat of hydrogenation for example
stability of alkene can be explained on the basis of hyper conjugation. more the number of hydrogen atoms at the adjustment carbon of Doubly  bonded carbon atom more  will be the hyperconjugating structure and thus our alkene will be more stable and will have less heat of hydrogenation.

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