Aldehydes and Ketones
8.0 Claisen Condensation
8.0 Claisen Condensation
When two molecules of ester undergo a condensation reaction, the reaction is called Claisen Condensation. The product of the Claisen condensation is a $\beta $-keto ester.
After nucleophilic attack, the aldol addition and the Claisen condensation differ. In the Claisen condensation, the negatively charged oxygen reforms the carbon oxygen $\pi $-bond and eliminates the $OR$ group.
The elimination step in the Claisen condensation is a reversible reaction since the alkoxide ion can readily attack the keto group of the $\beta $-keto ester. The reaction can be driven to completion by removing a proton from the $\beta $-keto ester. This is easily to do because, since the central $\alpha $-carbon of the $\beta $-keto ester is flanked by two carbonyl groups and its hydrogens are more acidic than the $\alpha $-hydrogens of the reacting ester. Consequently, a successful Claisen condensation requires an ester with two $\alpha $-hdyrogens and an equivalent amount of base rather than a catalytic amount of base. When the reaction is over, addition of acid to the reaction mixture reprotonates the $\beta $-keto ester anion.
