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CondensationIntermediate
Aldol Condensation
Also known as: Aldol Reaction, Aldol Addition
Discovered by Charles-Adolphe Wurtz (1872)
General Equation
2 RCHO → RCH(OH)CH₂CHO (aldol) → RCH=CHCHO + H₂O (condensation)
Description
An enolate (or enol) of one carbonyl compound attacks the carbonyl of another. The aldol addition product is a β-hydroxy carbonyl compound. Heating causes dehydration to the α,β-unsaturated carbonyl (aldol condensation). This forms a new C-C bond.
Reaction Scheme
Reactants:
- • Aldehyde or Ketone with α-hydrogens
Products:
- • β-Hydroxy carbonyl (aldol) or α,β-Unsaturated carbonyl (condensation)
Reagents:
- • NaOH or LDA
Conditions:
- • Base, H₂O or THF; heat for condensation
Mechanism (3 steps)
1
Enolate formation: Base (OH⁻ or LDA) removes an α-hydrogen to form the resonance-stabilized enolate anion.
Arrow: curvedEnolate (RC⁻=CHO ↔ RC=CHO⁻)
α-Hydrogen is slightly acidic (pKa ~20)
2
Nucleophilic addition: The enolate carbon attacks the carbonyl carbon of a second molecule, forming the aldol (β-hydroxy carbonyl).
Arrow: curvedβ-Hydroxy aldehyde/ketone
New C-C bond formed
3
Dehydration (condensation): With heat, the β-hydroxy group eliminates water through an E1cb mechanism to form the α,β-unsaturated carbonyl.
Arrow: curved
Only under heating — gives conjugated enone/enal
Key Points to Remember
- ✓Forms C-C bond between α-carbon and carbonyl carbon
- ✓Aldol addition (low T) vs aldol condensation (high T, with -H₂O)
- ✓Crossed aldol: mix two different carbonyls (selectivity challenge)
- ✓Directed aldol: use LDA at -78°C for kinetic enolate
- ✓Intramolecular aldol: makes 5- and 6-membered rings
- ✓Retro-aldol: reverse reaction (breaks C-C bond)
- ✓Very common in biosynthesis (aldolase enzymes)
Examples
Reactant:
2 Acetaldehyde (CH₃CHO)
Reagent:
NaOH (dilute), 5°C
Product:
3-Hydroxybutanal (aldol product)
Yield: 70%
Reactant:
Benzaldehyde + Acetone
Reagent:
NaOH, EtOH, heat
Product:
Dibenzalacetone
Yield: 85% (crossed aldol condensation)
Synthetic Utility
One of the most powerful C-C bond-forming reactions. Builds β-hydroxy carbonyls and enones. Foundational in total synthesis and industrial chemistry.
Limitations
- ⚠Self-condensation competes with crossed aldol
- ⚠Crossed aldol requires one partner with no α-H (or use LDA)
- ⚠Ketone aldols are less favorable (equilibrium)
- ⚠Over-condensation possible with multiple α-H's
#C-C bond#enolate#condensation#aldol#retrosynthesis