Introduction:
A carbon atom or radical bearing a negative charge is termed as the carbanion.
Acidity of α-Hydrogen:
In carbonyl compounds, α- hydrogen is the hydrogen linked to the carbon atom bearing the carbonyl group.
In the study of aldehydes and ketones, the carbonyl group mostly finds out the chemistry by giving a site at which nucleophilic addition can occur. Though one other significant role of the carbonyl group not as a functional group however as a substituent is its capability to strengthen the acidity of the hydrogen atoms linked to the α-carbon and by doing this, gives mount to an entire set of chemical reactions.
If the α-hydrogen is ionized, it yields a carbanion (I) that is resonance hybrid of two structures.
Fig: Acidity of α-Hydrogen
These resonance structures are just possible via the participation through the carbonyl group. Resonance of this type is not possible for carbanion made by the ionization of β-hydrogen, γ- hydrogen and so on from saturated carbonyl compounds.
The carbonyl group influences the acidity of α-hydrogen by helping to accommodate the negative change of the anion which develops as an outcome of attacks.
Fig: Carbonyl group affects acidity of α-hydrogen
Reactions Involving Carbanions:
The carbonyl group wherever it is, forms any α-hydrogen acidic and therefore helps in the formation of carbanions. The carbanions are greatly basic and exceedingly reactive particles. In their reactions they act as nucleophiles.
As nucleophiles, they attack carbon and in so doing form the carbon-carbon bonds.
The given are some reactions comprising carbanions.
Halogenation of ketone:
This can occur by using an acid or base to speed up the reaction.
Fig: Halogenation of ketone
For illustration: base catalyzed halogenations:
Ketones, for illustration react with bromine to form bromo propanone in the presence of the base catalyst (example: hydroxyl ion, acetate ion and so on).
Fig: Base catalyzed halogenations
Nucleophilic addition to carbonyl compounds:
a) Aldol condensation:
In the influence of dilute base or dilute acid, two molecules of an aldehyde or a ketone might combine to form a β-hydroxy aldehyde or β-hydroxy ketone. This reaction is termed as the Aldol condensation.
Fig: Nucleophilic addition-Aldol condensation
The product formed yields from the addition of one molecule of aldehyde (or ketone) to a second molecule in such a manner that the α-carbon of the first becomes linked to the carbonyl carbon of the second.
If the aldehyde or ketone doesn't have an α-hydrogen, a simple Aldol condensation can't occur.
Mechanism:
The hydroxide ion donates a lone pair to one of the α-hydrogen to make a water molecule and a carbanion, the later functioning as the nucleophile towards the unionized carbonyl molecule.
Fig: Mechanism of Aldol condensation:
Aldol condensation is extremely helpful in the preparation of unsaturated alcohols from an α, β - unsaturated aldehydes or ketones
b) Cross Aldol condensation:
This is the Aldol condensation among the two different compounds.
For illustration:
Fig: Cross Aldol condensation
c) Witting reaction:
This reaction comprises the nucleophilic attack on carbonyl carbon through an ylide to form a betaine that after spontaneously undergoes elimination to outcome the product.
Fig: Witting reaction
Nucleophilic Acyl substitution:
a) Claisen condensation: Formation of β-Keto esters.
The α-hydrogen in an ester, similar to α hydrogen in an aldehyde or ketone is weakly acidic and for the similar reason, however resonance, the carbonyl group assists in accommodating the negative charge of the carbanion.
Claisen condensation comprises nucleophilic attack through a carbanion on an election deficient carbonyl carbon. It is for esters and it is the correct counterpart of the Aldol condensation.
In Aldol condensation, the nucleophilic attack leads to addition, the typical reaction of aldehydes and ketones whereas in Claisen condensation, nucleophilic attack leads to substitution, the typical reaction of the acyl compounds.
The preparation of ethyl acetoacetate- a β-keto acid explains the reaction termed as the Claisen condensation. Whenever ethyl acetate is treated by sodium ethoxide and the resultant mixture is acidified, there is obtained ethyl-β- ketobutyrate (ethyl 3-oxobutanoate), usually termed as ethyl acetoacetate or acetoacetic ester.
Fig: Claisen condensation
Whenever complex esters are involved, it yields the products resultant from ionization of an α-hydrogen of the ester. This is always the α-carbon of one molecule which becomes linked to the carbonyl carbon of the other.
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