Loss of Carboxy Group as CO₂: Decarboxylation of Malonic Acid Derivatives

Just like β-keto acids—which upon thermal decarboxylation form ketones—β-dicarboxylic acids undergo decarboxylation to generate monocarboxylic acids with the liberation of carbon dioxide.

The mechanism initiates with an internal electronic redistribution, resulting in a cyclic six-membered transition state. This is followed by a C–C bond cleavage to produce an enol by releasing carbon dioxide gas. Next, the enol rapidly tautomerizes under the acidic conditions to yield a more stable monocarboxylic acid.

In malonic ester synthesis, the key step is the decarboxylation of malonic acid, wherein alkyl halides are converted to monocarboxylic acids.