Question

Propose a mechanism for the crossed Claisen condensation between ethyl acetate and ethyl benzoate.

Short answer

A crossed Claisen condensation is accomplished by adding the ester without

alpha-hydrogens, e.g., methyl benzoate, methyl formate, dimethyl carbonate etc., to a solution of the alkoxide base. The ester with alpha-hydrogens, e.g., ethyl acetate, is slowly added to this solution, where it forms an enolate and condenses.

Step-by-step solution

Crossed Claisen condensation

A crossed Claisen condensation is accomplished by adding the ester without

alpha-hydrogens, e.g., methyl benzoate, methyl formate, dimethyl carbonate etc., to a solution of the alkoxide base. The ester with alpha-hydrogens, e.g., ethyl acetate, is slowly added to this solution, where it forms an enolate and condenses.

Crossed Claisen condensation between ethyl acetate and ethyl benzoate

Ethylacetate has alpha-hydrogens. So when reacted with ethoxide it forms an enolate which is resonance stabilized.This enolate acts as a nucleophile and attack the carbonyl carbon of ethyl benzoate to giveethyl 3-oxo-3-phenylpropanoate with the elimination of an ethoxide ion.

In the presence of ethoxide ion, ethyl 3-oxo-3-phenylpropanoate is deprotonated to give its enolate. This exothermic deprotonation helps to drive the reaction to completion. When the reaction is complete, the enolate ion is reprotonated to give the desired product ethyl 3-oxo-3-phenylpropanoate.

Formation of ethyl 3-oxo-3-phenylpropanoate