Hyper-conjugation:
When a H — C bond is attached to an unsaturated system such as double bond or a benzene ring, the sigma (σ) electrons of the H — C bond interact or enter into conjugation with the unsaturated system. The interactions between the electrons of π- systems (multiple bonds) and the adjacent σ bonds (single H—C bonds) of the substituent groups in organic compounds is called hyper conjugation.
According to this concept, if an alkyl group carrying at least one hydrogen atom is attached to a unsaturated carbon atom, it releases electrons of carbon – hydrogen single bond towards the multiple bonds. For example, the hyper conjugation in propene is shown below:
The various hyperconjugation forms of propene are called contributing structures. In fact hyperconjugation effect is similar to resonance effect. Since there is no bond between the α-carbon atom and one of the hydrogen atoms, the hyperconjugation is also called no-bond resonance. Since the hyperconjugation depends upon the presence of H atoms at α -carbon atom, it is expected to be more due to CH3 — group than due to CH3 CH2 — group because the latter has only two H atoms attached to the α – carbon atom. In a similar manner, the hyperconjugation effect will be less if (CH3)2 CH — group is attached to unsaturated system and there will be no hyperconjugation effect if (CH3)3C — group is attached to α -carbon of unsaturated system.
Thus, the order of hyperconjugation is:
CH3— > CH3 CH2−> (CH3)2 CH— > (CH3)3C—