decarboxylation
Summary
Decarboxylation is the loss of a carboxyl group as CO₂, leaving a C–H bond in its place. It occurs readily for β-keto acids and malonic acids on gentle heating, through a cyclic six-membered transition state and an enol, while most simple carboxylic acids resist it.
Detail
Meaning: Decarboxylation replaces a C–COOH bond with a C–H bond and releases carbon dioxide. It is easy when the carbon next to the carboxyl can stabilize the electrons left behind. β-Keto acids and malonic acids lose CO₂ on gentle heating through a cyclic six-membered transition state: the acidic hydrogen transfers to the ketone oxygen as the C–C bond breaks, giving an enol that tautomerizes to a ketone. Escape of gaseous CO₂ also favors the reaction by entropy.
Equation: RCOCH₂COOH → RCOCH₃ + CO₂ (heat), where R is an alkyl or aryl group and CO₂ is carbon dioxide gas. No SI unit applies to this scheme.
On the MCAT: A passage heats a β-keto acid or a substituted malonic acid (as in malonic ester synthesis) and asks for the product, which is a ketone or carboxylic acid with one fewer carbon. You may also be asked to identify the enol intermediate, or to spot where CO₂ leaves in pyruvate dehydrogenase and the citric acid cycle.
Common confusion: Students assume any carboxylic acid decarboxylates when heated. Simple acids such as acetic acid resist it; the β-carbonyl group is what makes the reaction easy. Also, the thermal loss of CO₂ from a β-keto acid is not a redox step, whereas biological oxidative decarboxylations couple CO₂ loss to NAD⁺ reduction.
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
- Organic Chemistry (Clayden)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.