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Benzoin Condensation

Reaction Mechanism of Benzoin Condensation

This reaction of aromatic aldehydes, ArCHO, resembles the Cannizzaro reaction in that the initial attack is by an anion (-CN) on the carbonyl carbon atom of one molecule, but instead of hydride transfer it is now converted to carbanion which undergoes addition to the carbonyl carbon atom of the second molecule of ArCHO. The rate law commonly observed is

Rate = k [ArCHO]2 [-CN]

and the Benzoin Condensation is believed to follow the general pathway as

Mechanism of Benzoin Condensation

When aromatic aldehyde is treated with alcoholic KCN, the product is not a cyanohydrin but hydroxy aromatic ketone called benzoin. The product of aromatic aldehydes with KCN is different than aliphatic aldehydes because after the attack of CN-, the intermediate (I) in aromatic aldehyde has sufficient acidity (due to -I effect of Ph) so that intramolecular proton exchange takes place to form a carbanion, which is resonance stabilized. This carbanion then attacks another molecule of aromatic aldehyde, which undergoes intramolecular proton exchange and then ejection of CN- to give final product i.e. benzoin. The rate limiting step of the reaction is attack of carbanion on second molecule of aromatic aldehyde.

Benzoin Condensation

Benzoin Condensation

Example of Benzoin Condensation  

In which of the following substrate, rate of Benzoin condensation will be maximum? 

example of benzoin condensation

Benzoin condensation is due to stability of intermediate (III) when negative charge on C is extensively delocalised in benzene ring, Nitro and CN group. In all other cases, such dispersal is not extensively possible. On the other hand, NO2- is also creating positive charge centre on carbonyl carbon, making it more susceptible to nucleophilic attack of CN-.
Therefor correct answer is (A)

Mechanism of Benzoin Condensation

List of Name Reaction of Organic Chemistry consist of detail Reaction Mechanism of all name reactions of Organic Chemistry.

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