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.
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.
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.
- 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.
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.
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