What is Hyperconjugation?
Hyperconjugation is the stabilizing
interaction that results from the interaction of the electrons in a σ-bond
(usually C-H or C-C) with an adjacent empty or partially filled
p-orbital or a π-orbital to give an extended molecular orbital that increases
the stability of the system.
Hyperconjugation effect is a
permanent effect in which localization of σ electrons of C-H bond of an alkyl
group directly attached to an atom of the unsaturated system or to an atom with
an unshared p orbital takes place.
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| Hyperconjugation |
From the above figure, we
observe that one of the three C-H bonds of the methyl group can align in the
plane of the empty p orbital and the electrons constituting the C-H bond in a
plane with this p orbital can then be delocalized into the empty p orbital.
We also observe that the
hyperconjugation stabilizes the carbocation as it helps in the dispersal of
positive charges. Thus, we can say that the greater the number of alkyl groups
attached to a positively charged carbon atom, the greater is
the hyperconjugation interaction and stabilization of the carbonation. The
relative stability on the basis of hyperconjugation is given as,
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| Hyperconjugation |
Hyperconjugation is a factor
in explaining why increasing the number of alkyl substituents on a carbocation
or radical center leads to an increase in stability.
Let's consider how a methyl
group is involved in hyperconjugation with a carbocation center.
First we need to draw it to
show the C-H σ-bonds.
Note that the empty p orbital associated with the positive charge at the
carbocation center is in the same plane (i.e. coplanar) with one of
the C-H σ-bonds (shown in blue.)
This geometry means the
electrons in the σ-bond can be stabilized by an interaction with the empty
p-orbital of the carbocation center.
(This diagram shows the
similarity with resonance and the structure on the right has the "double
bond - no bond" character)
- The stabilization arises because the orbital interaction leads to the electrons being in a lower energy orbital
- Of course, the C-C σ-bond is free to rotate, and as it does so, each of the C-H σ-bonds in turn undergoes the stabilizing interaction.
- The ethyl cation has 3 C-H σ-bonds that can be involved in hyperconjugation.
- The more hyperconjuagtion there is, the greater the stabilization of the system.
- For example, the t-butyl cation has 9 C-H σ-bonds that can be involved in hyperconjugation.
- Hence (CH3)3C+ is more stable than CH3CH2+
- The effect is not limited to C-H σ-bonds, appropriate C-C σ-bonds can also be involved in hyperconjugation.
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