Reactions · Claisen-Schmidt 1881
Claisen-Schmidt aldol: benzaldehyde + acetone → benzalacetone
Overview
Ph–CHO + CH3–C(=O)–CH3 → Ph–CH=CH–C(=O)–CH3 + H2O
Textbook aldol condensation. Acetone's α-carbon (deprotonated by aqueous NaOH) attacks benzaldehyde, then β-elimination dehydrates the aldol intermediate to give the E-enone benzalacetone (4-phenyl-3-buten-2-one). The 5-fold acetone excess is a kinetic control measure — without it, double-condensation to dibenzylideneacetone (DBA) competes.
Conditions: Dissolve NaOH (0.2 equiv) in 95 % EtOH/H2O (~3 mL per mmol benzaldehyde). Cool to 0–10 °C in an ice bath. Add benzaldehyde (1.0 equiv) followed by acetone (5.0 equiv) over 5 min with stirring. Warm to RT and stir 2–4 h until pale-yellow solid crystallises. Filter; wash crystals with cold water; recrystallise from ethanol. Yellow needles, mp 41–42 °C. Quench excess base with dilute HCl if filtrate is needed for analysis.
- Expected yield
- 70–85 %
- Expected duration
- 2–4 h at RT after 0–10 °C addition
Reagents
| Reagent | Role | Equiv |
|---|---|---|
|
67-64-1
67-64-1 · C3H6O
Acetone — large excess (5 equiv) to suppress double-condensation to dibenzylideneacetone (DBA). Also serves as a co-solvent. |
reactant | 5.0 |
|
Benzaldehyde
100-52-7 · C7H6O
Benzaldehyde — limiting reagent. Freshly distilled or wash with 5 % NaHCO3 then water to remove benzoic acid (white film indicates oxidation). |
reactant | 1.0 |
|
Sodium Hydroxide
1310-73-2 · HNaO
Sodium hydroxide — catalytic base; deprotonates acetone's α-carbon. Aqueous, dissolved into the EtOH/H2O reaction medium. |
catalyst | 0.2 |
|
Ethanol
64-17-5 · C2H6O
Ethanol (95 % aqueous) — primary reaction medium. The 5 % water is part of the kinetic control: anhydrous EtOH overpowers the elimination step. |
solvent | — |
|
122-57-6
122-57-6 · C10H10O
Benzalacetone (4-phenyl-3-buten-2-one, E-isomer) — pale-yellow needles; mp 41–42 °C; recrystallises from EtOH; mild floral odour. Skin sensitiser — gloves. |
product | 1.0 |
Actions
- Open in Reaction Scale Calculator →
- Or jump straight to Benzaldehyde at: 1 mmol 10 mmol 100 mmol
- Mechanism / source ↗