What is Hyperconjugation

Definition of Hyper conjugation

What is Hyperconjugation?

Hyperconjugation is stabilizing interactions that are results from interaction of electrons in a sigma (σ-bond) (usually C-H or C-C) with an adjacent empty or partially-filled p-orbital or a pi (π) orbital to give an extended molecular orbitals that increases stability of system.

Hyper-conjugation is chemistry terminology and also known as Sigma (σ) electron delocalization.

Also,

        The movement of pi (π) electrons through p-orbital pathway is the Mesomeric-effect and leads to the resonance.
        The movement of sigma (σ) electrons through adjacent pi (π) system or a carbocation is the hyperconjugation. And hyper-conjugation therefore involves the sigma (σ) electron delocalization.

The electrons of sigma (σ) bond between C and H are involved in the delocalization.
In the upper structure there is no bond between C and H due to the migration of sigma (σ) bond. Hence hyperconjugation is also known as “no bond resonance”.
But,

       This does not mean that the H-atom is completely detached from structure.  As it indicates the some degree of ionic character in C-H bond and some single bond character between C-C double bond.

When we see Toluene structure, there is a partial negative-charge on the C-atom bonded to the methyl (-CH₃) group and the methyl C-atom is slightly positive-charge. This is due to hyperconjugation and has proved by the X-Ray diffraction studies.

Toluene is an example of "heterovalent hyperconjugation" or "sacrificial hyperconjugation", because the contributing structure (of toluene in hyperconjugation) contains one two-electron bond less than normal Lewis formula for the toluene.

Hyperconjugation can account for the (I-effect) Inductive effect. In Toluene, methyl group exhibits the +I effect that is responsible for the polarization of the electron (e^-) density.

The interaction between filled pi (π) or p-orbitals and adjacent antibonding sigma (σ *) orbitals gives "negative hyperconjugation", example of negative hyperconjugation is fluoroethyl anion.

The interaction between sigma (σ) bonds and an unfilled or partially filled pi (π) or p-orbital gives "isovalent hyperconjugation", example of isovalent hyperconjugation is tert-butyl cation.

What are the requirements for the Hyper-Conjugation?

Requirements for the Hyper-Conjugation is described below-

• Hyper-Conjugation exists in the carbocations, free radicals and alkenes and arenes.

• The alpha (α) C-atom next to the pi (π) bond (double bond) or C free radical or C+ (carbocation) should be sp3 hybridized with at least one H-atom bonded to it.

What is the effect of hyperconjugation on chemical structure?

• Due to hyperconjugation C-C single bond gains some double bond character while C=C double bond gains some single bond character.

• Therefore C=C double bond length in the substituted alkenes at all times greater than in ehtene.

Based on the valence-bond-model of the bonding, hyper-conjugation can be described as the "double bond- no bond resonance" but really it is not what we would say "normally" resonance.

What is main difference between resonance and hyperconjugation?

Resonance involves pi orbitals but Hyperconjugation involves a sigma orbital, usually a C-H or C-C bond.