Overall reaction
2 CH₃COOC₂H₅ → (NaOEt, then H₃O⁺) CH₃COCH₂COOC₂H₅ (ethyl acetoacetate) + C₂H₅OH
Ester enolate attacks a second ester; methoxide is expelled.
Name reaction · mechanism
Claisen Condensation
The Claisen condensation joins two ester molecules into a β-keto ester. An alkoxide removes an α-hydrogen to form an ester enolate, which adds to the carbonyl of a second ester; the tetrahedral intermediate expels alkoxide. Deprotonation of the doubly activated β-keto ester drives the reaction, and acid work-up gives the neutral product.
Mechanism step by step
- Enolate formation. Base removes an α-H
- Addition. The enolate attacks the second ester
- Elimination. The intermediate expels methoxide
Each step is checked electron by electron: the curly arrows give exactly the structures drawn.
Key points
- The base matches the ester (NaOMe for methyl esters) to avoid exchange.
- Deprotonation of the β-keto ester drives the equilibrium.
- Intramolecular version: the Dieckmann cyclisation.
- Acid work-up gives the neutral β-keto ester.
For JEE and NEET
- The ester needs two α-hydrogens for the final deprotonation that drives the equilibrium.
- Ethyl acetate → ethyl acetoacetate (acetoacetic ester).
- Intramolecular version: Dieckmann condensation (five- and six-membered rings).
- Crossed Claisen works when one ester has no α-H (e.g. ethyl benzoate, ethyl formate).
FAQ
Why is a full equivalent of base needed?
The β-keto ester (pKa ≈ 11) is deprotonated by the alkoxide; this irreversible-looking step pulls the unfavourable equilibria over.
Why must the base match the ester?
A different alkoxide would cause transesterification and give mixtures of esters.
Claisen vs aldol?
Aldol: enolate adds to an aldehyde/ketone (addition). Claisen: enolate adds to an ester and an alkoxide leaves (substitution).
Draw it in MolDraw
The button above opens the complete figure — overall reaction, numbered mechanism and key points — as ordinary editable structures. Change substituents, recolour atoms, or export a PNG, SVG or chemfig code for notes and slides. The same figure is in the Templates panel under Name reactions.