Carbocations: Factors affecting their Stability

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. 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 (πbond) and the charge is delocalized to the atom donating the electron pair (π donation).

Nitrogen and oxygen atoms are the most powerful πdonors. 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.

 

  Please watch the following informative video regarding carbocation stabilization: