What is hyperconjugation?

1 Answer
Dec 6, 2014

Hyperconjugation is the overlap of a #σ# bond with an adjacent empty or partially filled #p# or #π# orbital to give a delocalized molecular orbital.

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Only electrons in bonds that are #β# to the positively charged carbon can take part in in hyperconjugation.

This overlap of orbitals and delocalization of electrons increases the stability of the system

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Hyperconjugation explains the relative stability of carbocations:

(CH₃)₃C⁺ > (CH₃)₂CH⁺ > (CH₃)CH₂⁺ > CH₃⁺

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The C–C #σ# bond adjacent to the cationic carbon is free to rotate.

As it does so, the three C–H #σ# bonds of the methyl group in turn overlap with the vacant #p# orbital.

The more adjacent C-H bonds there are, the larger the hyperconjugation stabilization is.

An ethyl cation has three C-H #σ# bonds; an isopropyl cation has six; and a t-butyl cation has nine C-H #σ# bonds.

This agrees with the observed order of stability.

Here’s a video that explains hyperconjugation.