Hyperconjugation

Dr. Afshan Suraiya Professor & Head Department of Chemistry G.B.M. College, Gaya Magadh University, Bodh Gaya

Hyperconjugation is a concept of electron release given by Baker and Nathan. It involves release of electrons present in σ (Sigma) bonds between α carbon and hydrogen; α carbon and hydrogen with respect to Sp 2 carbon. All the hydrogen atoms present on α carbon can get involved in hyperconjugation. Thus greater number of α hydrogen atom greater should be hyperconjugation and its effect. This phenomenon is known as hyperconjugation, no bond effect or Baker and Nathan effect. In terms of molecular orbital theory it can be said that conjugation involves delocalization of electrons present in p orbital.

Whereas in hyperconjugation delocalization takes place between p orbitals of carbon and σ (sigma) molecular orbital of α carbon and hydrogen atoms.

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In tertiary butyl carbonium ion nine α H atoms are present, thus nine hyperconjugative structures can be written. In isopropyl carbonium ion 6 α H atoms are present thus six hyperconjugative structures may be written. Where as in ethyl carbonium ion only three hyperconjugative are possible due to presence of three α hydrogen atoms. And in methyl carbonium ion no hyperconjugative structure is possible due to lack of α H atom. Importance of hyperconjugation The conjugation effect due to hyperconjugation is weaker than resonance, but even then it is very useful in explaining some physical and chemical properties of organic molecules. (a) Stability of carbonium ions and free radial – The order of stability of carbocations and free radicals can be explained on the basis of

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hyperconjugation. The order of stability of carbonium ions (carbocations) and free radicals is 30 > 2 0 > 1 0

Decreasing order of stability of carbocations

Decreasing order of stability of free radicals (b) Stability of alkenes – The alkene which has maximum number of alkyl groups attached to the doubly bonded carbon atoms is most stable. This can be explained on the basis of hyperconjugation.

More the number of alkyl groups, more is the number of hyperconjugation structures that can be written for each one of the alkene. Therefore, greater the number of such structures, more stable is the alkene. (c) Directive influence of alkyl groups – The ortho, para-directing influence of alkyl groups can be explained on the basis of hyperconjugation. For example toluene is depicted as a hybrid of the following contributing forms.

Hyperconjugative structures of toluene Six more structures involving other α–H atoms may be written.

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