β-Keto Acid Decarboxylation
β-Keto Acid Decarboxylation: A β-keto acid loses CO₂ when heated, ultimately producing a ketone. Step-by-step mechanism, reagents and practice.
What happens
A β-keto acid loses CO₂ when heated, ultimately producing a ketone. A β-keto acid has a ketone at the β-position relative to the carboxylic acid (R–CO–CH₂–COOH). Ordinary carboxylic acids generally don't lose CO₂ easily; β-keto acids do because they can react through a favorable six-membered cyclic transition state and produce an enol that tautomerizes into a stable carbonyl product. The carbon count decreases by one.
Reaction summary
β-Keto acid + heat → ketone + CO₂ (COOH removed as CO₂)
Reagents and conditions
- Starting material: 3-Oxobutanoic acid (β-keto acid)
- Reagents: Heat (Δ)
- Products: Acetone (ketone); Carbon dioxide (CO₂)
Mechanism
- Six-Membered Transition State: Upon heating, the β-keto acid forms a six-membered cyclic transition state. A proton transfers internally while the bond to the carboxyl carbon begins to break.
- CO₂ Leaves: The carboxylic acid group is released as CO₂. This produces an enol: β-keto acid → enol + CO₂.
- Keto–Enol Tautomerization: The enol undergoes keto–enol tautomerization (C=C–OH → C=O) to form the more stable ketone.
- Product: The ketone is the final answer. The CO₂ has left, and the carbon count has decreased by one.
Common mistakes
- Don't stop at the enol. Immediately after CO₂ loss an enol is produced, but that is usually not the final product: the enol tautomerizes (C=C–OH → C=O), so your final answer should normally be the ketone.