Carbanions are anions of carbon, generated by the removal of one of the groups attached to a carbon without removing the bonding electrons. Thus, every carbanion possesses one unshared pair of electrons and three pairs of bonding electrons around the central carbon atom which is hybridized.
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 cabanion 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 carbaion have one non bonding and three bonding elestron pair in four sp3 hybridised orbital .We know that 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. With this picture of carbanion, one can infer that if the three substituents are different, the carboanion 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 structure thus explains the loss of optical activity associated with the asymmetric carbanions.
Formation of carbanions :
(a) When a group or atom departs from a carbon atom without its bonding pairs :
(b) When a negative ion attacks on alkynes
(c) Abstraction of proton by a strong base from carbonyl compounds or nitro compounds.
Effect of electron withdrawing group on formation of carbocation:
As there is little difference between the electro negativities of carbon and hydrogen (2.5 and 2.1, respectively) the polarity of the C—H bond is very small. The heterolytic fission of this covalent bond to form an anion and a proton should then be a very difficult process. In other words, a hydrogen atom bound to an carbon atom shows negligible acidity. However, the presence of electron attracting substituents such as nitro, cyano or carbonyl groups on the same carbon renders the hydrogen relatively acidic. The increase in acidity is not only due to the electron-withdrawing ability of these substituents, but also due to their ability to delocalize the negative charge of the anion. Thus hydrogens on the carbon atom alpha to nitro, cyano or carbonyl groups have acidic character and can be removed as protons leaving resonance stabilized anions.
Stability of carbonions :
all the factor which decrease the negative charge density on carbanion increase its stability.The stability of carbanion may be affected by following factor:
(A) Electronegativity of carbanionic carbon :
Greater the electronegativity of carbanionic carbon more is the stability of carbanion. Electronegativity of carban depend on percentage S character. Carbanion stability, thus, has been found to be in the order:
(B) Inductive effect :
Stability of alkyl carbanions can be explained by inductive effect. Greater the number of alkyl group [+ I effect] attached to the carbon atom bearing negative charge, lesser is the stability. Because there inductive effect increase negative charge density on the carbon.
(c) Effect of resonance:
a carbanion is stabilized by resonance if a double bond is located α to the anionic carbon. Due to delocalisation negative charge diffuse over whole molecule and thus stability increases. This explains the stability of the allylic and benzylic carbanions.
rapidly conversion of triphenylmethane to the triphenylmethyl carbanion can be similarly explained.
(D)Stabilisation by Sulphur and Phosphorous:
Attachment of carbanionic carbon of a sulphur and phosphorus atom causes an increase in carbanion stability. The cause of stability is due to the delocalisation of negative charge of carbanion by vaccant d-orbital (pπ–dπ bonding) of phosphorus and sulphur.
(E) Stabilisation by >C=O, –NO2 and CN groups present on carbanionic carbon :These groups stabilise carbanion by resonance effect
(F) Stability of Aromatic Carbanions :
Anions in which negative charge is present on carbon of aromatic system is known as aromatic carbanions. Aromatic carbanions are most stable carbaions because unshared electron pair of carbaion itself take part in delocalisation. Anions obeying Huckel rule are stable because they are aromatic and there is complete delocalisation of negative charge.
Reactions in which product formation takes place by formation of carbanion as reaction intermediate :
In the following reactions product formation takes place by the formation of carbanion as reaction intermediates :
(i) Condensation reactions of carbonyl compounds, i.e., aldol condensation, Perkin reaction, reformatsky reaction etc.
(ii) Condensation reactions of ester; Claisen condensation.
(iii) Wittig reaction.