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To study about hyperconjugation we have to know how hyperconjugation is formed due to electron displacement. Electrons are concentrated between specific atoms in a bond, however if double or single bonds alternately exist in a complex, it is called a conjugated system.
Effects of electron displacement
The induction effect
Polarity is observed at the carbon atom and the other group when any electron withdrawing or electron releasing group is present in a carbon chain. Electron displacement due to differences in electronegativities causes permanent polarity. This is known as the inductive effect, or I-effect.
The electronegativity of the substituent determines the inductive effect. Sigma bonds transmit the inductive effect, which weakens as the distance between the substituent and the reactive center grows. As a result, the effect is strongest for the adjacent bond, which can be weakly left further away.
The presence of an electron releasing group causes a positive inductive effect. On the chain, it develops a negative charge.
As the electron releasing groups enhance electron density, basic nature increases while naturally acidic nature decreases. As a result, alkyl groups weaken acids.
Conjugation
It is a term used in organic chemistry to describe the process of joining two molecules together.
Normally, electrons are concentrated between specific atoms in a bond, however if double or single bonds alternately exist in a complex, it is called a conjugated system.
Conjugation Chemistry
The presence of double or triple bonds in an organic substance is required for conjugation chemistry.
The p orbitals involved in bonding within the pi bond system are the most basic requirement for the existence of a conjugated system.
The conjugation formed in that moment is lost if a position in the chain excludes p orbitals or if the geometry is out of alignment.
Hyperconjugation
Consider the case where an alkyl group is attached to a saturated system, as discussed earlier in the +I effect. In such cases, the alkyl group releases electrons, but this is not the same as the inductive effect.
Hyperconjugation is the release of electrons by an alkyl group attached to a saturated group.
Although the hydrogen atom has a positive charge, there is no link between it and the carbon atom. As a result, this type of resonance is known as no-bond resonance.
Examples of hyperconjugation:
At least one H-atom should be present at alpha carbon.
The more C-H bonds there are at alpha carbon in an unsaturated solution, the more electrons are released and the hyperconjugation effect increases.
By sketching the following hyperconjugation structures of toluene, the electron donating effect of a methyl group in toluene may also be described on the basis of hyperconjugation.
Structures of hyperconjugation
Hyperconjugation interaction is feasible in any primary radical, such as the ethyl radical, but not in the simple methyl radical. As a result, the ethyl radical (1°) has a higher stability than the methyl radical. Similarly, the 2° alkyl radical is more stable than the 1o alkyl radical, while the 3°alkyl radical is more stable than the 2o alkyl radical.
CH 3° > 2° > 1° > CH3
Hyperconjugation in alkene
Delocalization of electrons, this time by overlap between a pi orbital and a sigma orbital of the alkyl group, has been attributed to the same underlying reason as stabilisation by a second double bond: delocalization of electrons.
Alkene hyperconjugation
Hyperconjugation in an alkene is illustrated by the overlap of a sigma and pi orbital in the diagram above.
Individual electrons can bind four nuclei together to some extent because of this overlap. We call this type of delocalization, which involves a sigma bond orbital, hyperconjugation.
Reverse Hyperconjugation
A system in which an electron contact is directed from the pi bond to the sigma bond rather than from sigma to pi bond is known as reverse hyperconjugation or negative hyperconjugation. In other words, electrons shift from the pi bond to the sigma bond.
Applications of hyperconjugation
Alkene stability: hyperconjugation explains why some alkenes are more stable than others. Alkene stability is proportional to the number of alpha hydrogens in the molecule, which is proportional to resonating structures.
The number of resonant structures and the amount of alpha hydrogens are directly proportional to the stability of alkyl carbocations.
The size of the carbon-carbon double bond in an alkene is: The single bond character will increase as the number of resonant structures increases.
R’s electron-releasing power in alkyl benzene: CH3 is the +R group, which is utilised for electrophilic aromatic substitution due to hyperconjugation.
Free radical stability: hyperconjugation explains the stability of free radicals.
Conclusion
We conclude that Increases in the amount of alkyl substituents on a carbocation or radical center lead to an increase in stability, which is explained by hyperconjugation. Hyperconjugation is caused by two basic factors. The existence of a hydrogen atom in a certain position is one of them, while the presence of a lone pair of electrons in a specific position is the other.
