Hyperconjugation - Effects, Definition, Examples, Applications, FAQs

The hyperconjugation effect is a permanent effect in which sigma electrons of an alkyl group’s C−Hbond are directly connected to an understood system atom or an atom with an unshared p orbital. One of three C−H bonds of the methyl group can align in the plane of empty p orbital, and the electrons comprising the C−Hbond in this plane can then be delocalized into empty p orbital.

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This Story also Contains

1. Hyperconjugation
2. Some Solved Examples
3. Conclusion

In this article, we will cover the topic (Hyperconjugation). This topic falls under the broader category of (Some Basic Principles of Organic Chemistry), which is a crucial chapter in (Class 11 Chemistry). It is not only essential for board exams but also for competitive exams like the Joint Entrance Examination (JEE Main), National Eligibility Entrance Test (NEET), and other entrance exams such as SRMJEE, BITSAT, WBJEE, BCECE, and more

Definition and Mechanism

• Hyperconjugation: Also known as no-bond resonance, hyperconjugation is the stabilizing interaction that results from the overlap of sigma (σ) bonds with an adjacent empty or partially filled p-orbital or π-orbital.Hyperconjugation is an important concept in organic chemistry that describes the delocalization of electrons in a molecule. This delocalization occurs through the interaction between sigma bonds (usually C−H or C−C) and an adjacent empty or partially filled p-orbital, π-orbital, or a positively charged center. Hyperconjugation helps to stabilize the molecule by dispersing charge over multiple atoms.
• Mechanism: Involves the interaction of electrons in a sigma bond (usually C−H or C−C) with an adjacent empty or partially filled p-orbital or π-orbital, leading to the delocalization of electrons.

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Hyperconjugation

Hyperconjugation is a general stabilizing interaction. It involves the delocalization of σ electrons of C−H bond of an alkyl group directly attached to an atom of an unsaturated system or to an atom with an unshared p orbital. The σ electrons of C−H bond of the alkyl group enter into partial conjugation with the attached unsaturated system or with the unshared p orbital. Hyperconjugation is a permanent effect.
To understand the hyperconjugation effect, let us take an example ofCH3CH2+(ethyl cation) 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 theC−H bond in the plane with this p orbital can then be delocalized into the empty p orbital as shown in the figure given below:

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

In general, the greater the number of alkyl groups attached to a positively charged carbon atom, the greater the hyperconjugation interaction and stabilization of the cation. Thus, we have the following relative stability of carbocations:

Hyperconjugation is also possible in free radicals, alkenes, and alkyl arenes.

In general, greater the number of hyperconjugative structures, greater is the stability.

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Some Solved Examples

Q.1 Hyperconjugation is also known as :

(1) No bond resonance

(2) sp3−p conjugation

(3) σ−bond resonance

(4) All of these

Solution:

As we have learned

Hyperconjugation, σ-π resonance, or Baker and Nathan effect -

The conjugation between αC−Pn the sigma bond and adjacent multiple bond or carbocation is called hyperconjugation.

It is a permanent effect.

Since there is no definite bond between the carbon atom & one of the hydrogen atoms in the hyperconjugation forms, hyperconjugation is also called no-bond resonance.

Hence, the answer is the option (4).

Q.2 Which one of the following carbocations is most stable?

(1)

(2)

(3)

(4)

Solution:

As we have learned

Application of Hyperconjugation -

More the no of Hyperconjugation structure more the stability of carbocation

In (CH3)3C◻ , carbocation has 9α-hydrogen in all other option carbocation has less than 9α−H .

So (CH3)3C⊕ is most stable due to more number of hyperconjugative structures.

Hence, the correct answer is Option (3)

Conclusion

The delocalization of electrons from a single link between hydrogen and another atom in the molecule is known as hyperconjugation. The electrons in the bond have been delocalized. Furthermore, the hydrogen atom has no link with the other atom. As a result, the broken connection might be held responsible for the conjugation’s feasibility. Hyperconjugation is also known as no-bond resonance since there is no bond between the hydrogen and the other atom.

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NCERT Chemistry Notes:

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