Answer:
Substitution.
Explanation:
Claisen reaction was first published in 1887 by a prominent German chemist known as Rainer Ludwig Claisen.
A Claisen reaction is a nucleophilic substitution in which an enolate is the nucleophile. It's typically a reaction of two molecules of an ester to form a β-keto ester, in the presence of an alkoxide base. Thus, a Claisen reaction is simply a characteristic condensation reaction of esters through a nucleophilic carbonyl substitution with an enolate such as a ketone enolate or an ester enolate.
Furthermore, a Claisen reaction results in the formation of a carbon-carbon bond in the presence of a strong base to yield a β-keto ester.
