kindly explain me hypercongugation??

Akhilesh Gandhi, 13 years ago

Grade:11

2 Answers

AKASH GOYAL AskiitiansExpert-IITD

420 Points

13 years ago

Dear Akhilesh

Dear Ocean

In organic chemistry, hyperconjugation is the interaction of the electrons in a sigma bond (usually C–H or C–C) with an adjacent empty (or partially filled) non-bonding p-orbital or antibonding π orbital or filled π orbital, to give an extended molecular orbital that increases the stability of the system. Only electrons in bonds that are β to the positively charged carbon can stabilize a carbocation by hyperconjugation.

Hyperconjugation is a factor in explaining why increasing the number of alkyl substituents on a carbocation or radical centre leads to an increase in stability.

Let's consider how a methyl group is involved in hyperconjugation with a carbocation centre.
First we need to draw it to show the C-H sigma-bonds. Note that the empty p orbital associated with the positive charge at the carbocation centre is in the same plane (i.e. coplanar) with one of the C-H sigma-bonds (shown in blue.)
This geometry means the electrons in the sigma-bond can be stabilised by an interaction with the empty p-orbital of the carbocation centre. (this diagram shows the similarity with resonance and the structure on the right has the "double bond - no bond" character)

The stabilisation arises because the orbital interaction leads to the electrons being in a lower energy orbital

orbital interaction diagram

Of course, the C-C sigma-bond is free to rotate, and as it does so, each of the C-H sigma-bonds in turn undergoes the stabilising interaction.
So the ethyl cation has 3 C-H sigma-bonds that can be involved in hyperconjugation.
The more hyperconjuagtion there is, the greater the stabilisation of the system.
So for example, the t-butyl cation has 9 C-H sigma-bonds that can be involved in hyperconjugation. Hence (CH₃)₃C+ is more stable than CH₃CH₂+
The effect is not limited to C-H s-bonds, appropriate C-C sigma-bonds can also be involved in hyperconjugation.

All the best.

AKASH GOYAL

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