Carbocations with Hyperconjugation

In organic chemistry, hyperconjugation is a stabilising reaction in which the localisation of σ electrons of the C-H bond takes place. This C-H bond is directly attached to an atom that has an unshared p orbital. An example of hyperconjugation is the interaction between the electrons of p and s bonds of a substituent group in an organic compound. This article elaborates the concept of hyperconjugation, the meaning of hyperconjugation, and hyperconjugation its examples.

What is hyperconjugation?

Hyperconjugation is a stabilising reaction that is the outcome of the interaction between electrons of a σ-bond with an adjacent empty or filled p-orbital or a π orbital. This creates an extended or prolonged molecular orbital that boosts the stability of the system. 

Concept of hyperconjugation:

Hyperconjugation is, in a way, similar to the electromeric effect; the only difference is that hyperconjugation has a permanent effect, while the electromeric effect is temporary. Localisation of electrons of an alkali group’s C-H bond directly attached to either an unsaturated system’s atom or an unshared p orbitals atom occurs in this effect. 

The reason for the stabilisation of glucose, which allows the positive charge to spread, is due to hyperconjugation. The greater the number of alkyl groups attached to a positively charged carbon atom, the stronger is the carbonation’s stabilisation and hyperconjugation interaction.

Applications of hyperconjugation:

There are several applications for hyperconjugation. Most commonly, it is used to explain a variety of chemical phenomena, such as the gauche effect, the beta-silicon effect, and the anomeric effect.  

Hyperconjugation is also used to explain the rotational barrier of ethane, the relative stability of substituted carbocations and substituted carbon-centred radicals, the vibrational frequency of exocyclic carbonyl groups, and the thermodynamic Zaitsev’s rule for alkene stability.

Types of hyperconjugation effects:

Hyperconjugation can be divided into two broad categories.

Isovalent Hyperconjugation:

Isovalent hyperconjugation takes place in free radicals and carbocations where the canonical form displays no charge separation. 

Sacrificial Hyperconjugation:

In this type of hyperconjugation, the canonical form displays no bond resonance, and the main form has no charge distribution. 

Solved Hyperconjugation Examples:

Question 1:

Among the following, which one shows the Baker-Nathan effect?

1. Mesomeric effect
2. Inductive effect
3. Hyperconjugation
4. Electromeric effect

Solution: 

The Baker-Nathan effect is another name for hyperconjugation. The sustaining interaction that focuses on the interaction of electrons in a σ-bond (usually C-H or C-C) with an adjacent empty or partially filled p-orbital or a π-orbital to give an elongated molecular orbital that improves the system’s stability is known as hyperconjugation.

Question 2:

Hyperconjugation uses delocalisation of?

1.  σ bond orbital
2.  π bond orbital
3.  Both σ and π bond orbital
4. None of these

Solution:

The delocalization of the bond orbital is involved in hyperconjugation. It is the steadying interaction that involves electrons in a σ-bond (usually C-H or C-C) with an adjacent empty or partially filled p-orbital or a π-orbital to give a prolonged molecular orbital that improves the system’s stability.

Question 3:

Bigger the number of hyperconjugation structures, the stability of the free radicals

1. Increase
2. Decrease
3. Remains the same
4. None of these

Solution:

Higher the number of hyperconjugation structures, greater is the stability of free radicals. 

Question 4:

Among the situations listed below, which properly describes the Baker-Nathan Effect?

• It aids in clarifying the direct impact of alkyl groups in aromatic alkyl benzene.

• The relative stability of alkenes can be explained by it.

• The relative stabilities of alkyl carbocations can be explained by it.

• All of the above

Solution:

The Baker-Nathan effect explains the direct influence of alkyl groups in aromatic alkyl benzene. It is useful in explaining the relative stability of alkenes as well as the relative stability of alkyl carbocations.

Question 5:

Ethene has no alpha hydrogen. Therefore, no hyperconjugation is possible. 

1. True
2. False

Solution:

The statement is true. Since ethene lacks alpha hydrogen, hyperconjugation is not possible. The presence of alpha hydrogen is required for hyperconjugation.

Question 6:

If the contributing structure has the same number of two-electron bonds as the standard Lewis formula, it will be called:

1. Heterovalent Hyperconjugation
2. Sacrificial hyperconjugation
3. Isovalent hyperconjugation
4. All of these

Solution:

Isovalent hyperconjugation occurs when the contributing structure contains the same number of two-electron bonds as the standard Lewis formula.

Question 7:

If we increase the number of α-hydrogens, the number of hyperconjugation structures will:

1. Decrease
2. Increase
3. Remains same
4. None of these

Solution:

The answer is option b. The number of hyperconjugation structures grows as the number of α-hydrogens grows. The number of α-hydrogens determines the degree of hyperconjugation. The hydrogen atom that is attached to the carbon that is attached to a functional group is known as α-hydrogen.

Conclusion

This article elaborated the concept of hyperconjugation, the meaning of hyperconjugation and its examples. Hyperconjugation is a permanent effect that deals with the localisation of σ electrons of C-H bonds of adjacent molecules in an organic compound. There are two types of hyperconjugation: isovalent and sacrificial hyperconjugation.