13.16 : CO_2 としてのカルボキシ基の損失: β-ケト酸の脱炭酸

Carboxylic acids bearing a keto group at the β position to the carboxyl group are called β-keto acids. When heated, these acids can readily undergo decarboxylation to lose CO₂ gas while simultaneously generating a ketone.

The decarboxylation mechanism begins with the intramolecular hydrogen bonding between the β-keto oxygen atom and the hydrogen atom of the carboxyl group, which generates a reactive conformation.

As the partially protonated keto-oxygen is highly electrophilic, it leads to an internal electron transfer, proceeding through a six-membered cyclic transition state.

Consequently, a C–C bond cleavage results in the formation of an enol, along with the evolution of CO₂ as gas.

Finally, the enol undergoes facile keto-enol tautomerism to yield more stable ketones.