Retro-ene-type elimination
Decarboxylation of a β-keto acid
How the Decarboxylation of a β-keto acid works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: β-Keto acid (e.g. acetoacetic acid). Conditions: Mild heat (50–100 °C), no catalyst. Bonds formed: CO₂; enol C=C; then ketone C=O; bonds broken: C–C (α to CO₂H); O–H. Stereochemistry: Planar enol; α-stereocentre is lost. Rate and selectivity: Low barrier from cyclic six-membered TS; ΔS‡ favourable.
At a glance
Step 1: Six-membered cyclic arrangement
A β-keto acid's carboxylic acid proton and its own carbonyl oxygen are positioned close enough to form a six-membered cyclic array with the C–C bond that's about to break.
Step 2: Concerted retro-ene-type fragmentation
In one concerted step (mechanistically related to a retro-ene reaction), the O–H bond transfers its proton to the ketone oxygen while the C–C bond breaks and CO₂ is released — no discrete carbanion intermediate ever forms.
Step 3: Enol tautomerises to the ketone
The product is an enol, which rapidly tautomerizes to the ketone — this cyclic, low-barrier mechanism is why β-keto acids (unlike most carboxylic acids) decarboxylate readily just on mild heating.
Abbreviations and terms
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
- Δ
- Heat. Reaction is run at elevated temperature.
How it compares with related reactions
| Decarboxylation of a β-keto acid | Aldol condensation | Cope elimination | |
|---|---|---|---|
| Reagents | β-Keto acid (e.g. acetoacetic acid) | NaOH or KOH (cat.) | H₂O₂ or mCPBA (oxidation); then heat |
| Conditions | Mild heat (50–100 °C), no catalyst | H₂O/EtOH, heat 50–100 °C | Oxidation 0–25 °C; elimination 80–160 °C (milder in DMSO/THF) |
| Stereochemistry | Planar enol; α-stereocentre is lost | Mostly E-enal (thermodynamic) | Syn elimination via planar 5-membered TS |
| Rate / selectivity | Low barrier from cyclic six-membered TS; ΔS‡ favourable | E1cB; loss of HO⁻ from the enolate is rate-determining | Intramolecular, first order; faster in polar aprotic solvents |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Decarboxylation of a β-keto acid?
β-Keto acid (e.g. acetoacetic acid). Conditions: Mild heat (50–100 °C), no catalyst.
Which bonds are formed and which are broken?
Formed: CO₂; enol C=C; then ketone C=O. Broken: C–C (α to CO₂H); O–H.
What is the stereochemical outcome?
Planar enol; α-stereocentre is lost
What controls the rate or the selectivity?
Low barrier from cyclic six-membered TS; ΔS‡ favourable
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go