CHEM20018 · Chemistry: Reactions And Synthesis
Aldol, Claisen & Dieckmann Condensations
Here the enolate of the previous chapter meets a carbonyl electrophile to forge a new C–C bond. An aldol reaction joins an enolate to an aldehyde or ketone to give a β-hydroxy carbonyl; with heat or base the product can dehydrate (E1cb) in an aldol condensation to an α,β-unsaturated carbonyl. The Claisen condensation is the ester analogue — an ester enolate attacks a second ester to give a β-keto ester — and its intramolecular version, the Dieckmann, builds rings. Control of crossed (mixed) reactions, by using a non-enolisable partner, is the recurring exam skill.
What this chapter covers
- 01Aldol reaction → β-hydroxy carbonyl; aldol condensation → α,β-unsaturated carbonyl (loss of H₂O, E1cb)
- 02Aldehydes are better electrophiles than ketones (less steric, more δ+)
- 03Crossed/mixed aldol control: use a non-enolisable partner (benzaldehyde, formaldehyde) as electrophile only
- 04Intramolecular aldol to form rings (favoured 5- and 6-membered)
- 05Claisen condensation: ester enolate + ester → β-keto ester (+ alkoxide)
- 06Dieckmann condensation: intramolecular Claisen → cyclic β-keto ester
- 07Match the alkoxide base to the ester's alkoxy group to avoid transesterification
- 08Retrosynthesis: disconnect the Cα–Cβ bond of a β-hydroxy / α,β-unsaturated carbonyl or a β-keto ester
Crossed aldol condensation + a Claisen self-condensation
- 2 marks — identify non-enolisable partner as electrophile(a) Benzaldehyde has no α-hydrogen, so it cannot form an enolate — it can act only as the electrophile. Propanal can enolise, so it supplies the nucleophilic enolate. This pairing removes the usual 'mixture of four products' problem of a crossed aldol.
- 2 marks — aldol then E1cb dehydration with reasonThe propanal enolate adds to the benzaldehyde carbonyl to give the β-hydroxy aldehyde, which under warm base dehydrates (E1cb) because the resulting alkene is conjugated with both the carbonyl and the aromatic ring — a strong thermodynamic driving force.
- 1 mark — correct condensation product + geometry(a) Product: (E)-2-methyl-3-phenylprop-2-enal (the α-methyl cinnamaldehyde-type enal). The E-alkene is favoured for steric reasons.
- 2 marks — name Claisen + give β-keto ester product(b) With only one ester present, NaOEt deprotonates its α-carbon and that ester enolate attacks a second molecule of ethyl propanoate — a Claisen self-condensation. The tetrahedral intermediate ejects ethoxide to give a β-keto ester; the matched NaOEt/OEt base prevents transesterification.
Key terms
- Aldol reaction
- Addition of an enolate to the carbonyl of an aldehyde or ketone, giving a β-hydroxy carbonyl ('aldol') compound.
- Aldol condensation
- The aldol product loses water on heating (E1cb elimination) to give a conjugated α,β-unsaturated carbonyl.
- Crossed (mixed) aldol
- An aldol between two different carbonyls; controlled cleanly by pairing one enolisable carbonyl with one non-enolisable electrophile.
- Claisen condensation
- Reaction of an ester enolate with a second ester to give a β-keto ester, expelling an alkoxide.
- Dieckmann condensation
- An intramolecular Claisen condensation of a diester that produces a cyclic β-keto ester.
Aldol, Claisen & Dieckmann Condensations FAQ
What's the difference between an aldol reaction and an aldol condensation?
The aldol reaction stops at the β-hydroxy carbonyl. The aldol condensation goes one step further: under heat or base the β-hydroxy carbonyl dehydrates (E1cb) to the conjugated α,β-unsaturated carbonyl. Watch the conditions in the question — 'warm' or 'Δ' usually signals condensation.
Why does a Claisen need a full equivalent of base while an aldol is base-catalytic?
The β-keto ester product is acidic (pKa ≈ 11), so it is deprotonated by the alkoxide as it forms. That final deprotonation is what drives an otherwise unfavourable equilibrium forward, so a full equivalent of base is consumed; you acidify in a separate workup step to recover the neutral β-keto ester.
How do I avoid transesterification in a Claisen?
Match the alkoxide base to the ester's alkoxy group — use NaOEt with ethyl esters, NaOMe with methyl esters. A mismatched alkoxide would swap into the ester (transesterification) and scramble your product.
Exam move
Build a comparison table: aldol vs Claisen — nucleophile, electrophile, product (β-hydroxy/α,β-unsaturated carbonyl vs β-keto ester), and the role of base. The single highest-yield skill is retrosynthesis: train yourself to look at a β-hydroxy carbonyl, an α,β-unsaturated carbonyl or a β-keto ester and immediately mark the Cα–Cβ disconnection that reveals the two starting carbonyls. Practise crossed reactions by always identifying the non-enolisable partner. Then do intramolecular versions (aldol ring closure, Dieckmann) by counting ring size — 5 and 6 are favoured. These reactions are the backbone of Section A's multi-step synthesis questions, so drill them until forward and reverse are equally fast.
