Definition of Hyper conjugation
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 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,
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.