Claisen condensation

The Claisen condensation is a reaction in the field of organic chemistry. It was named after its discoverer Ludwig Claisen. Of the reaction is defined as the base mediated acylation of an ester to a second ester molecule to a β - ketocarboxylic acid ester.

Overview reaction

The Claisen condensation is carried out here by way of example in the reaction of two molecules of the ethyl acetate using ethoxide as the base.

The connection marked in red stands for the newly formed C-C single bond within the resulting β - ketocarboxylic acid ester.

Suitable bases other sodium alkoxides, sodium amide or sodium hydride can be used.

Reaction mechanism

The following mechanism of the Claisen condensation is explained on the example reaction of two molecules of ethyl acetate with ethoxide as the base.

First, an ethyl acetate ( 1) in the α - position to the carbonyl group is deprotonated by added sodium ethoxide. It forms the enolate 2 This then reacts with another molecule of ethyl acetate via an intermediate for β - ketocarboxylic acid esters, the ethyl acetoacetate in Example (3). The β - keto carboxylic ester reacts with another alkoxide to a resonance stabilized anion 4 after aqueous acidic workup ultimately the desired β - keto carboxylic esters 3 The last step is the only irreversible step in the overall mechanism. Since ethanol is a stronger acid than the CH-acid ester 1, the balance of 1-2 on the side of the first contrast, the β - ketocarboxylic acid ester 3 is a stronger acid than ethanol. Therefore, the balance of 3-4 on the side of the resonance-stabilized anion 4 and requires stoichiometric amounts of the base - that is, the ethanolate. Wherein the β - ketocarboxylic acid ester 3 is removed to form 4 in the last step of the reaction mixture and thus enables the Claisen condensation. Protonation at the work-up of 4 then provides the β - keto carboxylic esters 3

Cyclic intramolecular Claisen condensation is known as Dieckmann condensation.