Question

What is true about the following Claisen condensation reaction?

• A. The reaction proceeds through a carbanion intermediate.
• B. If ethyl propanoate is taken in place of ethyl acetate, condensation would occur at faster rate.
• C. The condensation reaction could also be carried out in presence of acid catalyst.
• D. All of the above.

Answer 1

The correct option is A The reaction proceeds through a carbanion intermediate.

Claisen condensation reaction is self condensation of ester in presence of strong base to give $\beta$-ketoester.
Mechanism of Claisen condensation reaction:
1. Enolate formation : The strong base will abstract the acidic $\alpha$-hydrogen to form the enolate ion (carbanion).

2. Nucleophile attack: Formed enolate will attack the carbonyl carbon of another ester molecule and leaving group (-OR) will leave the compound to form $\beta -$ ketoester.

In ethyl propanoate, the alkyl group attached to ester group has more +I effect compare to ethyl acetate, thus, electrophilicity of ester carbon is lower in ethyl propanoate. Hence, ethyl propanoate will occur at slower rate compared to ethyl acetate.

Hence, option (a) is correct.