CARBANIONS
• A carbanion is an anion in which carbon is tetravalent (forms three bonds) and bears a formal
negative charge in at least one significant mesomeric contributor (resonance form).
• Absent π delocalization, carbanions assume a trigonal pyramidal, bent, or linear geometry
when the carbanionic carbon is bound to three (e.g., methyl anion), two (e.g., phenyl anion),
or one (e.g., acetylide anion) substituents, respectively.
- Formally, a carbanion is the conjugate base of a carbon acid: R C-H + B− → R C− + HB
(where B stands for the base)
Characteristic of Carbanions:
(i) Hybridisation and geometry: Alkyl carbanion has three bond pairs and one lone pair.
Thus,hybridisation is sp3 and geometry is pryamidal. [Note: Geometry of allyl and benzyl
carbanion is almost planar and hybridisation is sp2].
(ii) There are eight electrons in the outermost orbit of carbanionic carbon hence its octet is
complete.
(iii) It behaves as charged nucleophile.
(iv) It is diamagnetic in character because all eight electrons are paired.
(v) It is formed by heterolytic bond fission.
(vi) It reacts with electrophiles.
Structure of Carbanion:
• As carbanion have one non-bonding and three bonding electron pair in four sp3 hybridised
orbital.
• The non-bonding electron pair repels the bonding pair more than the bonding pairs repel each
other and because of this, there is reduction of the angle between bonding pairs to a value
slightly lesser than the tetrahedral value of 109.5°.
• The configuration of simple carbanions such as the methyl anion thus appears to be pyramidal
just like that of ammonia with which the methyl anion is isoelectronic.
If the three substituents of carbanion are different, then the carbanion should be asymmetric and
consequently there should be retention of configuration in the reactions involving a carbanion
intermediate. However, it could not be demonstrated experimentally as the unshared pair and the central
carbon rapidly oscillate from one side of the plane to the other. This rapid equilibrium between
enantiomeric pyramidal structures thus explains the loss of optical activity associated with the
asymmetric carbanions.
