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).
|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.
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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