Question

Write short notes on the following:
-Claisen condensation
-Benzoin condensation

Answer 1

Hint: Claisen condensation involves self condensation of esters in presence of base. Two compounds having aldehyde functional groups can form benzoin which is called benzoin condensation.

Complete Answer:
-> It is known to you that claisen condensation is a self condensation and benzoin condensation is a reaction between two aldehydes. Let’s see both of the reactions one by one.
-> Claisen condensation:
It involves self-condensation of two molecules of ester containing alpha-hydrogen in the presence of a strong base such as sodium ethoxide to form beta-keto ester. The esters undergo self condensation to produce beta-keto esters. Mixed or crossed claisen condensation also occurs between two different esters or between an ester and a ketone. The reaction occurs in four steps to produce the desired product. Below is an example of this reaction. We can see that a $\beta -$ keto ester is formed as an end product.

- Benzoin condensation:
Two molecules of aromatic aldehyde (such as benzaldehyde), on heating in the presence of ethanolic KCN get condensed to form benzoin. The catalyst cyanide ion acts as a good nucleophilic attacker which can promote nucleophilicity of the intermediate and also act as a good leaving group. Benzoin condensation is a difficult reaction, for the reason that there is possibility of benzaldehyde to be oxidized into benzoic acid. Below is the reaction of two benzaldehyde molecules reacting to give Benzoin

Note: Remember that $\alpha$ hydrogen is a necessary requirement for the ester to undergo claisen condensation reaction. Remember that aliphatic aldehydes cannot undergo benzoin condensation as the aldehyde needs to be aromatic in order to give benzoin as a final product.