Chemistry Podcast

Saturday, 25 June 2016

What is Hyperconjugation

Definition of Hyper conjugation

What is Hyperconjugation
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.
        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.
definition of hyper conjugation
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”.
       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.
"heterovalent hyperconjugation" or "sacrificial hyperconjugation"
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.
Hyper-Conjugation exists in the carbocations, free radicals and alkenes and arenes

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.


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