Claisen-Schmidt aldol: benzaldehyde + acetone → benzalacetone
Ready to scaleCondensationC-C bond formation
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.
| Compound | Role | Equiv | MW (g/mol) | ρ (g/mL) | Safety |
|---|---|---|---|---|---|
| Benzaldehyde100-52-7 · C7H6OBenzaldehyde — limiting reagent. Freshly distilled or wash with 5 % NaHCO3 then water to remove benzoic acid (white film indicates oxidation). | reactant | 1.0 | 106.12 | 1.050 | WarningSDS |
| Acetone67-64-1 · C3H6OAcetone — large excess (5 equiv) to suppress double-condensation to dibenzylideneacetone (DBA). Also serves as a co-solvent. | reactant | 5.0 | 58.08 | 0.790 | DangerSDS |
| Sodium hydroxide1310-73-2 · HNaOSodium hydroxide — catalytic base; deprotonates acetone's α-carbon. Aqueous, dissolved into the EtOH/H2O reaction medium. | catalyst20 mol% | 0.2 | 40.00 | — | DangerSDS |
| Ethanol64-17-5 · C2H6OEthanol (95 % aqueous) — primary reaction medium. The 5 % water is part of the kinetic control: anhydrous EtOH overpowers the elimination step. | solvent | — | 46.07 | 0.790 | DangerSDS |
| 4-Phenyl-3-buten-2-one122-57-6 · C10H10OBenzalacetone (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 | — | 146.19 | — | Warning |