Decarboxylation of β-keto acids: practice and mechanism
β-Keto acids and malonic acids lose CO₂ on heating via a six-membered transition state. An enol forms, which tautomerises to the ketone or carboxylic acid.
Example
β-Keto acids and malonic acids lose CO₂ on heating, through a six-membered cyclic transition state. An enol forms first and tautomerises to the ketone or carboxylic acid. Ordinary carboxylic acids do not do this.
Mechanism step by step
- 1
The COOH group and the β-carbonyl group form a six-membered ring through a hydrogen bond.
- 2
In one step three electron pairs shift: CO₂ is released and an enol forms.
- 3
The enol tautomerises to the keto form (ketone or carboxylic acid).
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which reagent achieves this transformation?
Typical mistakes
- It is not the keto group that leaves, but CO₂ from the COOH group. The C=O group in the β-position is exactly what makes the transition state possible.
- The enol is only the intermediate. It tautomerises at once to the more stable keto form.
- The COOH group is not replaced by OH: CO₂ leaves and an H takes its place.
Often confused with: Malonic ester synthesis
Frequently asked questions
Why do ordinary carboxylic acids not decarboxylate?
Without the β-carbonyl group there is no six-membered transition state and no stable enol product.
Where does it appear?
As the last step of the malonic and acetoacetic ester syntheses and in biochemistry (e.g. in the citric acid cycle).