CarbAnion!

 Hello Readers,

Evidently, most of the organic reactions occur in multiple steps. Except the final step, the product formed after each step is called reaction intermediate. These intermediates are short lived and cannot be isolated under normal conditions.

Let’s discuss about one such reaction intermediate – “Carbanion”. A carbanion is a nucleophile and its stability is determined by several factors.

Thumb rule for stability of any intermediate:

Remember this!
‘More the charge density, lesser will be the stability and vice versa.’
Factors affecting stability of Carbanion:

1. Inductive Effect:

An alkyl carbanion having greater number of alkyl substituents will be less stable, compared to the one having lesser number of alkyl substituents.

Since the alkyl group donates the electron density via +  (Inductive Effect-Electron releasing), it increases the electron density on the carbanion and decreases the stability of carbanion.

Example:

Determine the more stable carbanion from the given compounds.



The number of alkyl substituents in (A) is 1.

The number of alkyl substituents in (B) is 2.

Thus, carbanion (A) is more stable than carbanion (B).

2. Resonance Effect:

Carbanions can be stabilized by resonance effect. The pair of electrons can be delocalized by resonance in a conjugated system. Delocalization of negative charge decreases the electron density on carbanion and increases the stability of carbanion.

Example:

The benzyl carbanion is stabilized by resonance with the benzene ring. The resonating structures of benzyl anion are shown below.



Therefore, greater the number of phenyl rings attached to the carbanion, more will be the number of resonating structures and more will be the charge delocalization. Increase in charge delocalization increases the stability of carbanion.

3. Mesomeric effect (-M):

The substituents that can accept a pair of electrons show -M effect (electron withdrawing effect). Thus, a carbanion donates the pair of electrons to such species in order to get stabilized.

Example:

In this example, nitro-group behaves as an electron withdrawing group and stabilizes the adjacent carbanion.



4. Hybridization:

Stability order:

sp hybridized carbanion > sp2 hybridized carbanion > sp3 hybridized carbanion

The hybridized orbitals with higher ‘s ’ character are more electronegative and therefore can hold the negative charge better.



Example: