Grignard reagents react with carbon dioxide to yield acid salts, which, upon acidification, produce carboxylic acids.
Acetoacetic ester, an ester formed by the self‐condensation of ethyl acetate via a Claisen condensation, has the following structure:
The hydrogens on the methylene unit located between the two carbonyl functional groups are acidic due to the electron withdrawing effects of the carbonyl groups. Either or both of these hydrogens can be removed by reaction with strong bases.
The resulting carbanions can participate in typical S N reactions that allow the placement of alkyl groups on the chain.
Hydrolysis of the resulting product with concentrated sodium hydroxide solution liberates the sodium salt of the substituted acid.
Addition of aqueous acid liberates the substituted acid.
The second hydrogen on the methylene unit of acetoacetic ester can also be replaced by an alkyl group, creating a disubstituted acid. To accomplish this conversion, the reaction product in step 2 above would be reacted with a very strong base to create a carbanion.
This carbanion can participate in a typical S N reaction, allowing the placement of a second alkyl group on the chain.
Hydrolysis using concentrated aqueous sodium hydroxide leads to the formation of the sodium salt of the disubstituted acid.
Addition of aqueous acid liberates the disubstituted acid.
The acid formed has a methyl and an ethyl group in place of two hydrogens of acetic acid and is therefore often referred to as a disubstituted acetic acid.
If dilute sodium hydroxide were used instead of concentrated, the product formed would be a methyl ketone. This ketone occurs because dilute sodium hydroxide has sufficient strength to hydrolyze the ester functional group but insufficient strength to hydrolyze the ketone functional group. Concentrated sodium hydroxide is strong enough to hydrolyze both the ester functional group and the ketone functional group and, therefore, forms the substituted acid rather than the ketone.
A reaction between a disubstituted acetoacetic ester and dilute sodium hydroxide forms the following products:
Upon heating, the β ketoacid becomes unstable and decarboxylates, leading to the formation of the methyl ketone.
A Claisen condensation of ethyl acetate prepares acetoacetic ester.
The Claisen condensation reaction occurs by a nucleophilic addition to an ester carboxyl group, which follows these steps: