How many of the following statments are correct regarding Claisen condensation reaction (when an ester is treated with a strong base)?
1. A proton is removed from the $α$ -carbon to form a resonance stabilised carbanion of the ester.
2. Carbanion acts as a nucleophile in a nucleophilic acyl substitution reaction with another ester molecule.
3. A new $C - C$ bond is formed.
4. $α, β$ unsaturated product is obtained

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Solution

Ester undergoes self condensation in presence of strong base to give $β$ -ketoester. This reaction is called Claisen condensation reaction.

Mechanism of Claisen condensation reaction:
1. Enolate formation: The strong base will abstract the acidic $α$ -hydrogen to form the enolate ion which is in resonance with the ester group.

2. Nucleophile attack: Formed enolate will attack the carbonyl carbon of another ester molecule and leaving group (-OR) will leave the compound to form $β -$ keto ester. The reaction leads to new $C - C$ bond between the two ester substrate.

Hence, option (1), (2) and (3) are correct.

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