Hyperconjugation

Hyperconjugation is considered as a special case of resonance. Hyperconjugation involves delocalisation of σ electrons of C−H bond of any alkyl group which is directly attached to an atom having a π bond (i.e, a multiple bond) or to an atom with unshared p-orbital. Hyperconjugation is a permanent effect.

Let us take an example of ethyl carbocation (CH₃CH₂⁺) to understand hyperconjugation.

In ethyl carbocation, the positively charged carbon atom has an empty p-orbital. One of the C−H bonds of the methyl group can lie in the plane of this empty orbital and the electrons responsible for the establishment of this C−H bond can then be delocalised into the empty p orbital as shown in the figure :

Due to delocalization of electrons, the positive charge gets dispersed into the group of atoms that results in better stability of carbocation.

Structures a, b and c are known as hyperconjugation structures. It may be noted that those free protons are not completely detached and, therefore, not free to move.

Hyperconjugation effect is also seen in some alkenes and alkylarenes. Hyperconjugation structures of propene are given below :

Hyperconjugation is also known as no bond resonance. You may note that resonance involves delocalisation of π electrons but its the σ electrons that are delocalised in hyperconjugation.

Significance of Hyperconjugation

Shortening of carbon-carbon single bonds adjacent to multiple bonds : The normal carbon-carbon single bond is 1.54 Å. However, in some compounds such as propene, it is a little shorter. The reason being the presence of some double bond character in C−C single bond due to hyperconjugation (hyperconjugation structures of propene are given above).

Stability of Carbocation : The stability of carbocation increases with increase in number of alkyl groups attached to the positively charged carbon. Hence, stability of carbocation follows the order :

Reason : The number of hyperconjugation structures of carbocations decreases in the order : (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂CH₂⁺ > CH₃⁺ because the number of C−H bonds on alkyl groups attached to C⁺ decreases in the same order. In fact, CH₃⁺ carbocation does not have any hyperconjugation structures. Since hyperconjugation effect helps in stabilizing molecules, the more the hyperconjugation structures, the more stable is the carbocation. Because of the same reason, the stability of free radicals also follows the order : tertiary > secondary > primary.

Relative Stability of Alkenes : Alkenes containing greater number of alkyl groups on doubly bonded carbon are more stable due to hyperconjugation.