Hyperconjugation
Hyperconjugation is the stabilising interaction that results from the interaction of the electrons in a σ-bond (usually C-H) with an adjacent empty or partially filled p-orbital or a π-orbital to give an extended molecular orbital that increases the stability of the system. The hyperconjugative effect was first of all observed by Baker and Nathan (1935). Hyperconjugation is possible in Carbocations, free radicals and alkenes.
Conditions for hyperconjugation
Main condition is sp3 hybridized carbon should be connected to carbocation, free radical or alkene. This sp3 hybridized carbon should have at least one α–hydrogen.
Hyperconjugation in Carbocations
This type of hyperconjugation takes place in alkyl carbocations.
Structure (I) is more stable. Many people explain it by using inductive effect, but it can be best explained by the concept of hyperconjugation.
There are basically two approaches to understand the hyperconjugation.
i) σ-bond – p-orbital overlap
ii) No Bond Resonance
σ-bond – p-orbital (vacant) overlap
No Bond Resonance
This is the main approach to understand the concept of hyperconjugation.
03 No Bond Resonating Structures
Number of No Bond Resonating Structure = Number of α-H
Stability of Carbocations
Stability of carbocations depends upon the number of no bond resonance or hyperconjugating structures; more the number of hyperconjugating structures greater will be stability in the carbocation.
Hyperconjugation in Free Radicals
This type of conjugation occurs in alkyl free radicals.
σ-p (Incomplete) overlap
In alkyl free radicals overlap takes place between molecular orbital of C-H bond and incomplete orbital of the adjacent carbon atom.
No Bond Resonance
03 No Bond Resonating Structures
Stability of Free Radicals
Stability order of different types of alkyl free radicals is as follows:
Thus stability of tertiary is more than the secondary which is more than the primary free radical.
Hyperconjugation in Alkenes
This type of conjugation occurs in alkenes and alkyl substituted aromatic compounds.
Sigma bond – π bond overlap
No bond Resonance
Stability of Alkenes
We know that the greater is the number of resonating structures, the more will be stability of the system. Thus tetrasubstituted alkene is the most stable and ethylene is the least stable.
Carbon-carbon double bond length in Alkenes
Due to the hyperconjugation single carbon-carbon bond of an alkane acquires some double bond character and carbon-carbon double bond acquires some single bond character. Thus due to the hyperconjugation carbon-carbon double bond length of substituted alkenes is always more than the ethylene.
Electron donating power of alkyl group in alkylbenzenes
Alkyl group of alkyl benzene is ortho-para directing group and activating group for electrophilic aromatic substitution reactions. This property of alkyl group can be explained by hyperconjugation.
The electron-donating power of alkyl group depends on the number of alpha hydrogens of the alkyl group. The electron-donating power of some alkyl groups in decreasing order is as follows:
Reverse Hyperconjugation
Phenomenon of the hyperconjugation is also observed in the system given below:
Where X = Halogen
In the given system, the effect operates in the reverse direction. The electron interaction is directed from the pi bond to the sigma bond rather than from sigma to pi. Hence, the conjugation in such system is known as reverse hyperconjugation.
The meta directing influence of CX3 group in C6H5-CX3 for electrophilic aromatic substitution reaction can be explained by this effect.
