Sales Toll Free No: 1-855-666-7446

# Claisen Condensation

Top
 Sub Topics The base catalysed condensation of two moles of an ester to give a $\beta$-keto ester. Claisen condensations are more favorable with thioesters. This reaction is of great importance in the biosynthesis of fatty acids and polyketides.  Claisen condensation is a reaction forming carbon-carbon bonds that take place between two or one ester and carbonyl compounds, this happens in the with a strong base which results in a $\beta$-keto ester or a $\beta$-diketone. This reaction is named after Rainer Ludwig Claisen, who worked on this reaction and published in 1887. Esters contain a-hydrogens, they can undergo condensation reaction similar to aldol reaction, this reaction is known as Claisen condensation. In this reaction, one ester acts as a nucleophile while a second ester acts as a the electrophile, which reacts to form a carbon-carbon bond resulting in the formation of $\beta$-keto ester.

## Mechanism

The mechanism of the claisen condensation is similar to that of the aldol condensation and involves the nucleophilic addition of an ester enolate ion to the carbonyl group of a second ester molecule. The only difference between the aldol condensation of an aldehyde or ketone and the Claisen condensation of an ester involves the fate of the initially formed intermediate. The alkoxide intermediate in the aldol reaction is protonated to give an alcohol product exactly the behavior previously seen for aldehydes and ketones. The alkoxide intermediate in the claisen reaction however exples an alkoxide leaving group to yield a nucleophilic acyl substitution product exactly the behavior seen for esters.

In the classic condensation there is a self-condensation between two molecules of a compound containing an enolizable ester.

The steps of Claisen condensataion mechanism are as follows.

1. Enolate formation
2. Nucleophilic attack

3. Removal of leaving group

## Mixed Claisen Condensation

The mixed claisen condensation of two different esters is similar to the mixed aldol condensation of two different aldehydes or ketones. Mixed claisen reactions are successful only when one of the two ester components has no $\alpha$ hydrogens and thus can't form an enolate ion. For example, ethyl benzoate and ethyl formate can't form enolate ions and thus can't serve as donors. They can however act as the electrophile acceptor components in reactions with other ester anions to give mixed $\beta$-keto ester products.