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sir,can u plz xpalain me hyperconjugation in detail..and some of its applications??

6 years ago

Answers : (1)

AKASH GOYAL AskiitiansExpert-IITD
419 Points

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.  interaction of a methyl group
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.)
look at the sigma bonds
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)

electrons in the sigma bond interact with the empty p orbital

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


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6 years ago
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