13.2 Carbanions

  General Organic Chemistry

An ion in which the carbon bears the negative charge is called as carbon anion or carbanion.

They are formed by heterolytic cleavage when a group departs from a molecule without the bonding pair of electrons.

Formation of carbanions

(a) When a group or atom departs from a carbon atom without its bonding pair

(b) When a negative ion attacks on a multiple bond

Reactions of carbanions: They undergo two types of rections,

1. Displacement reaction
2. Addition reaction

1. Displacement reaction

2. Addition reaction

Geometric shape: Since in the formation of carbanion, only an atom or a group departs without the pair of electrons, the unbonded electron pair continues to remain in the same orbital as in the molecule, i.e., the hybridization of the carbanionic carbon is unaltered.

Stabilities of carbanion and carbocation: Ions are stabilized by delocalization of the charge on other carbon atom or other elements.

Inductive effect and hyperconjugation also stabilize the carbocation by dispersal of the charge. The effects are additive, hence the stability order of carbocation is $3^\circ > 2^\circ > 1^\circ > \mathop {\text{C}}\limits^{\text{ + }} {{\text{H}}_{\text{3}}}$

The order is reversed in case of carbanions due to repulsion between like charges.

As resonance effect is stronger than other effects, allyl and benzyl carbocations are more stable than $3^\circ $ carbocations.

Change in the hybridization of the carbon bearing the charge on going $sp \to s{p^2} \to s{p^3}$ progressively decreases the stability of carbanions. This is due to the greater $s$ character (50%) in $sp$ orbital and least (25%) in $sp^3$ orbitals, i.e. the electron of $sp$ orbitals are more firmly held by positive nucleus than those of $sp^3$ orbital with consequent stability.

Hence the stability order of alkyl, vinyl and acetylide carbanions is,