Claisen condensation reaction – Examples and Mechanism

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Claisen condensation reaction

Condensation between two molecules of esters having α-hydrogen atom in the presence of strong base like sodium ethoxide and an acid to form β-keto ester is known as Claisen condensation reaction. Eg.

Claisen condensation reaction

Mechanism of Claisen condensation reaction

Taking ethyl acetate (ethyl ethanoate) as an example, Claisen condensation reaction involves following steps:

Step-I: In this step, ethoxide ion removes a proton from α-carbon of one molecule of ethyl acetate to form a carbocation.

  • This step is similar to aldol condensation reaction.

Mechanism of Claisen condensation reaction

Step-II: In this step, the carbanion (i.e. nucleophile) attacks the carbonyl carbon of second ester molecule to displace ethoxide ion and forms the keto ester.

Mechanism of Claisen condensation reaction

  • The claisen condensation differs from aldol condensation at this point. In the aldol condensation, nucleophilic attack leads to addition and in the claisen condensation reaction it leads to substitution.

Step-III: In this step, there is an acid base reaction between ethoxide ion and β-keto ester to give β-keto ester anion and ethanol.

Step-IV: In this step, an acid is added to the reaction mixture which gives proton to the β-keto ester anion to form the β-keto ester which is in equilibrium with its enol form.

Mechanism of Claisen condensation reaction

Note:

Keto-enol tautomerism :

Keto-enol tautomerism is a special type of isomerism in which two isomeric forms, keto and enol exist in dynamic equilibrium with each other. There is transfer of a α- hydrogen to the oxygen atom of carbonyl group and vice-versa.

Keto-enol tautomerism

References