Hyperconjugation is a key concept in organic chemistry that helps explain the stability of carbocations, alkenes, and free radicals. It involves the interaction between sigma bonds (usually C-H or C-C) and an adjacent empty or partially filled p-orbital or π-orbital. This overlap leads to delocalization of electrons, which increases the stability of the molecule. Often called no bond resonance hyperconjugation plays a crucial role in understanding the behaviour of molecules during chemical reactions. In this guide, we’ll explore how hyperconjugation works, the conditions needed for it to occur, and its practical applications in real life chemical systems. 

Hyperconjugation

Hyperconjugation is the delocalization of sigma electrons or lone pairs of electrons into nearby empty or partially filled p- or pi-orbitals. It is caused by the overlapping of a sigma-bonding orbital or an orbital containing a lone pair with a neighbouring pi-orbital or p-orbital. It is also often referred to as the “Baker-Nathan effect” or “no bond resonance.”

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For hyperconjugation to happen, there are few conditions. The molecule should have a hydrogen atom on a carbon that is next to a carbon with a double bond or positive charge (like in carbocations). This hydrogen should be bonded to a carbon with single bonds only (sp3 hybridized). These types of hydrogens are called α-hydrogens, and they are essential for hyperconjugation to take place. 

Electron delocalization may also take place via parallel overlap of p orbitals with hybridized orbitals involved in sigma bonds during the process of hyperconjugation. Hence, it is also known as no-bond resonance and a variation of resonance theory. Now, take ethyl carbocation as an example.

In structure 2, the vacant p orbital on C1 and the sp3 hybridized orbital on C2 involved in the C2—H1 bond are almost parallel, permitting parallel overlap. This lowers the electron deficit at C1 but increases the electron deficiency at H1. 

The free rotation around the C1—C2 bond cannot be entirely prevented by this overlap. As a result, the vacant p orbital on C1 also shares electrons with the C2—H2 bond and the C2—H3 bond. Therefore, the delocalization of sigma electrons in ethyl carbocation can be depicted as The overall structure of the carbocation represents the sharing of net +1 charge by all the four atoms including one carbon atom and three hydrogen atoms.

The given example also explains the stability of carbocations. When there are more alkyl groups attached to the positively charged carbocation, the effect of hyperconjugation would be more. In other words, there will be greater delocalization of sigma electrons. As a result, the carbocation with a greater number of alkyl groups attached to it will be the most stable.

Hyperconjugation helps in identifying the stability of alkenes, carbocations, free radicals, etc. These applications of hyperconjugation are discussed as follows:

Alkene Stability

Due to hyperconjugation, alkenes with more alkyl groups on their doubly bonded carbon are more stable. It is caused by an increase in the number of contributing no-bond resonance structures.

Carbocation Stability

As the number of contributing structures to hyperconjugation rises, the stability of carbonium ions also increases. This is because there are more alkyl groups (carrying hydrogen) connected to the positively charged carbon.

Stability of Free Radicals

Hyperconjugation directly affects the stability of free radicals just like carbocations. The sigma-electrons of the -C-H bond may delocalize into the odd-electron-carrying p-orbital of carbon. The presence of more alkyl groups contributes to the increasing stability of free radicals.

Reactivity of electrophilic substitution on the benzene ring

In toluene, the methyl group releases electrons toward the benzene ring primarily as a result of hyperconjugation. Therefore, the ring becomes more reactive to electrophilic substitution, which targets the ortho and para positions of the methyl group. Hence, the ortho and para locations of the benzene ring have the highest electron densities for electrophilic substitution.

Difference Between Hyperconjugation and Resonance

The key differences between resonance and hyperconjugation are listed below:

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