Aromaticity
In some carbanions, the lone pair of electrons of the negative charge is involved in delocalization
to add on to the aromatic character of the molecule which gives them extra stability. For example,
in cyclopentadienyl anion there are 6 π electron and thus it obeys Huckel rule, (4n+2) π electron.
It has pKa value 15. This anion is stabilized by aromatization.
Cyclooctatetraene on reaction with potassium gets converted to cyclooctatetraenyldianion
potassium salt. This is 10 π electron system which is stable due to aromaticity.
Reactivity of Carbanion
Carbonations are strong Lewis acids while carbanions are strong bases (Lewis and Bronsted
bases). Carbanions are part of most of the common reaction types such as displacement,
elimination, condensation, addition, rearrangement, polymerization etc.
Displacement Reaction
Carbanion can combine with positive species in a displacement reaction.
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Elimination
In a Conjugate Base Elimination reaction (E1cb) the C-H bond breaks with formation of
carbanion as intermediate. The developed negative charge on carbon assist in the loss of leaving
group, leading to the formation of alkene
Condensation reactions
Any of a class of reactions in which two molecules combine, usually in the presence of a catalyst,
with elimination of water or some other simple molecule.
Aldol condensation: Reaction of aldehydes or ketones to give β-hydroxy carbonyl compounds is
known as aldol reaction. The reaction can be base or acid catalyzed. The base catalyzed reaction
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is more common. In a base catalyzed reaction, The carbonyl compound is deprotonated at the αcarbon by base (alkali hydroxide) to give carbanion which is resonance stabilized with the enolate
anion.
The next step is the nucleophilic attack of the carbanion to the carbonyl group of another,
nonenolized, aldehyde molecule. The product which is obtained after workup is a β–
hydroxyaldehyde or ketone.
Claisen condensation:
Formation of β-keto esters from carboxylic esters is known as Clasien ester condensation.
The mechanism involves the formation of carbanion from the ester (ethyl acetoacetate) by reaction
with base.
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The carbanion generated can add to another ester molecule. The resulting anionic species results
into β-keto ester by loss of an alkoxide anion.
Dieckmann condensation:
It is intramolecular condensation of diesters to give cyclic βketo ester. The base abstracts α-proton
to one of the ester group to form a carbanion. The carbanion then under goes nucleophilic attack
at the carbonyl carbon of other ester group present within the same molecule. The product
obtained is a cyclic ketone.
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Addition Reaction
Michael reaction: Addition of a carbanion to α, β-unsaturated carbonyl compounds is called the
Michael reaction or Michael addition.
Rearrangement reactions
Favorskii rearrangement: The rearrangement of cyclopropanones and α-halo ketones in the
presence of base to carboxylic acid derivatives is known as Favorskii rearrangement.
For example, in cyclic α-halo ketones, reaction involve abstraction of alpha proton by base to give
a carbanion intermediate, which is resonance stabilized with its enolate form. This carbanion or
enolate cyclize to a cyclopropanone intermediate, which is then attacked by the hydroxide
nucleophile to for carboxylate derivative.
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Wittig reaction: In a Wittig reaction, alkenes or olefins are formed by the reaction of
phosphoniumylides with aldehydes or ketones. The ylides have carbanion character.
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