A carbocation is a species where a carbon atom bonds to three carbon atoms and
has a positive charge. Carbocations are electron
deficient species and therefore very
reactive and unstable. Anything
which donates electron density to the electron-deficient center will help to
stabilize them.
Factors that
stabilize them are the following:
- Neighboring carbon atoms (inductive effect)
- Neighboring carbon-carbon multiple bonds (resonance effect)
- Neighboring atoms with lone pairs (resonance effect)
How carbocations are stabilized by
neighboring carbons atoms?
The stability of
carbocations decreases as the number of carbons attached to the C+ decreases.
That means that tertiary carbocations are more stable than secondary that in
turn are more stable than primary (Fig. 1).
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| Fig. 1: Carbocation stability increases as methyl substitution increases around the electron deficient carbon C+. The methyl groups (-CH3) are electron donating and therefore stabilize the positive charge (inductive effect) |
An explanation for this is that the methyl
group (-CH3) acts as an electron-donor and therefore stabilizes the positively
charged cation. Remember that the C atom has an electronegativity of 2.5 and
that H 2.2.
A better explanation is that electrons are
donated from the C-H bonds to the empty p orbital of the C+ therefore
stabilizing the carbocation through hyperconjugation
(the more the - CH3 groups attached to the C+ the more stable the
carbocation becomes).
How carbocations are stabilized by carbon-carbon
multiple bonds (resonance)?
Carbocations
where the C+ is adjacent to another carbon atom that has a double or triple
bond have extra stability because of the overlap of the empty p orbital of the
carbocation with the p orbitals of the π bond (pi bond). This overlap of the orbitals allows the
charge to be shared between multiple atoms – delocalization of the charge
- and therefore stabilizes the
carbocation.
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Fig. 2: Carbocation stabilization by multiple bonds
adjacent to the C+ atom through
p-orbital overlap
|
This effect is
called charge delocalization and is shown by drawing resonance structures where
the charge moves from atom to atom. It greatly stabilizes even primary
carbocations – normally very unstable – that are adjacent to a carbon-carbon
multiple bond.
 |
Fig. 3: Carbocation stabilization by multiple bonds adjacent to the C+ atom. |
How carbocations are stabilized by adjacent
atoms with lone pairs?
Adjacent atoms
with lone pairs act as electron donors to the electron-poor carbocation. This
results in forming a double bond (π
bond) and the charge is delocalized to the atom donating the
electron pair (π donation - pi donation).
Nitrogen and
oxygen atoms are the most powerful π
donors (pi donors). However, even halogen atoms stabilize carbocations
through donation of a lone pair.
Fig. 4: Stabilization of the carbocation by lone pair
donation. The O atom donates an electron pair to the C+ atom and a double bond
is formed. The positive charge is delocalized to the oxygen atom providing
extra stability.
Similarly, a N atom – or even a halogen atom - may
donate an electron pair to the C+ atom and disperse the + charge stabilizing
the carbocation.
 |
|
Fig. 5: Stabilization of the carbocation by lone pair
donation. The N atom donates an electron pair to the C+ atom and a double bond
is formed. The positive charge is delocalized to the nitrogen atom providing
extra stability.
|
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