Hyperconjugation (No-bond Resonance)

 

Hyperconjugation (No-bond Resonance)

There is another type of delocalization, which involves σ-electrons called Hyperconjugation.

It may be regarded as a σ-π orbital overlap analogous to the π-π orbital overlap.

In hyperconjugation, the sigma C-H bond on the alpha carbon is delocalized with the empty p-orbital of a C= or a carbocation. As a result, H⁺ does not change its position.

In the canonical form(s), there is no bond between the Carbon and Hydrogen atoms. This type of resonance is called no-bond resonance, and this is a system of conjugation where the sigma electrons of the C-H bond are in conjugation with π-electrons of the unsaturated system or with the p-orbital is known as hyperconjugation.

Alpha Carbon:

The carbon atom adjacent to the Functional group (positively charged carbon (carbocation) or to a double bond (C=C)) is called the alpha carbon.

α Hydrogen:

The hydrogen atom attaches to the α – carbon is α – hydrogen.

No Bond Resonance:

In hyperconjugation, the bonding pair of electrons between the alpha carbon and alpha hydrogen gets partially delocalized over the adjacent system (like a carbocation or a double bond), stabilizing the molecule through "no-bond resonance." This is the core idea of sigma-π conjugation, where the sigma electrons behave similarly to the delocalized π-electrons in resonance structures.

Requirements for Hyperconjugation:

1. Interaction between hybridized sigma σ bond and empty or partially filled π orbital.
2. Must have α – carbon.
3. Must have conjugation.
4. H⁺ position remain same.

Hyper conjugation is possible in Compounds

• Alkene
• Alkynes
• Carbocations
• Free Radical

Other examples are:

Ter. Carbocation > Sec. Carbocation > Pri. Carbocation

General Rule:

The number of hyperconjugation structures is equal to the number of alpha hydrogens. If a molecule has n alpha hydrogens, it can form n hyper-conjugating structures.

Example

Consider a tert-butyl cation:

(CH₃)₃C⁺

1. There are three methyl groups attached to the central carbon atom.
2. Each methyl group (CH₃) has 3 alpha hydrogens.
3. Since there are 3 methyl groups, there are a total of 3×3=9 alpha hydrogens.
4. Therefore, there will be 9 hyperconjugating structures for the tert-butyl cation.