Hyperconjugation - Effects, Definition, Examples, Applications, FAQs

Hyperconjugation

Hyperconjugation is a general stabilising interaction. It involves the delocalisation of σ electrons of the 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 the 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 of $\mathrm{CH}_3 \mathrm{CH}_2^{+}$ (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 the C−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:

Application of Hyperconjugation

Hyperconjugation is useful in explaining the stability:

Stability of Alkenes

Alkenes with more alkyl substituents on the double bond are more stable due to hyperconjugation.

Stability order:
Tetra $>$ Tri $>$ Di $>$ Mono $>$ Ethylene
Example:

$\left(\mathrm{CH}_3\right)_2 \mathrm{C}=\mathrm{CH}_2>\mathrm{CH}_3 \mathrm{CH}=\mathrm{CH}_2>\mathrm{CH}_2=\mathrm{CH}_2$

Reason: More $\boldsymbol{\alpha}$-hydrogen atoms → more hyperconjugation structures → more stability

Alkyl Carbocation Stability

Carbocations are stabilised by hyperconjugation from adjacent $\mathrm{C}-\mathrm{H}$ bonds.

$3^{\circ}>2^{\circ}>1^{\circ}>\mathrm{CH}_3^{+}$

Example:

$\left(\mathrm{CH}_3\right)_3 \mathrm{C}^{+}>\left(\mathrm{CH}_3\right)_2 \mathrm{CH}^{+}>\mathrm{CH}_3 \mathrm{CH}_2^{+}>\mathrm{CH}_3^{+}$

Alkyl Free Radicals stability

Alkyl radicals are stabilised through hyperconjugation.

Order:

$3^{\circ}>2^{\circ}>1^{\circ}>\mathrm{CH}_3 .$

The overlapping stabilises the carbocation in such a way that electron density from the adjacent σ bond helps in dispersing the positive charge.

In general, greater the number of alkyl groups attached to a positively charged carbon atom, the greater is 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, the greater the number of hyperconjugative structures, the greater the stability.

Types of Hyperconjugation

Positive hyperconjugation: when an empty p orbital interacts with the adjacent sigma bond.

Negative hyperconjugation: when the filled p or pi orbital interacts with an adjacent antibonding sigma orbital is said to negate the type of hyperconjugation.

Sacrificial Hyperconjugation

In the sacrificial Hyperconjugation, the canonical forms involve no bond resonance. The main form has no charge distribution. These types of hyperconjugation occur least, but the isovalent type of hyperconjugation, which occurs in free radicals and carbonation, occurs readily.

Also read -

Some Solved problems

Question 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 and 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 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 -

The more the no of Hyperconjugation structures more the stability of carbocation

In (CH₃)₃C^◻ , the carbocation has 9α-hydrogen; in all other options 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)

Question 3: Which of the following is not an application of hyperconjugation?

1)Stability of Carbocations

2)Stability of carbon-free radicals

3)Stability of Alkenes

4) (correct)Stability of Carbanions

Solution

Hyperconjugation affects the stability of Carbocations, Carbon free radicals and alkenes.

It has no effect on the stability of the carbanions.

Hence, the answer is option (4).

Question 4: Given below are two statements :
Statement I : Hyperconjugation is not a permanent effect.
Statement II : In general, greater the number of alkyl groups attached to a positively charged C -atom, greater is the hyperconjugation interaction and stabilization of the cation.
In the light of the above statements, choose the correct answer from the options given below

1) Statement I is true but Statement II is false

2) Both Statement I and Statement II are false

3) Statement I is false but Statement II is true

4) Both Statement I and Statement II are true

Solution:

Hyper conjugation is permanent effect because external reagent is not required, so Statement-I is false and Statement-II is true. because more alkyl group, more $\alpha-\mathrm{H}$, so more hyperconjugation which results more stability of carbocation.

Hence, the correct answer is option (3).

NCERT Chemistry Notes: