Cannizzaro disproportionation: 2 benzaldehyde → benzyl alcohol + sodium benzoate
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Textbook redox disproportionation of an α-H-free aldehyde. Concentrated NaOH drives hydride transfer between two benzaldehyde molecules: one is reduced to benzyl alcohol, the other oxidised to sodium benzoate. Mechanism goes through a tetrahedral hydride-donor intermediate; chemists isolate the products by acidifying the aqueous phase to precipitate benzoic acid and ether-extracting the alcohol.
Conditions
Dissolve NaOH (1.1 equiv per pair of aldehyde, ~50 wt% aq.) in water. Add benzaldehyde (2.0 equiv) — exotherm; cool with an ice bath to keep T < 30 °C during addition. Stir vigorously (the system rapidly thickens to a paste as the disproportionation advances). Continue stirring 4–24 h at RT until a single phase forms.
Workup
Dilute with cold water; extract benzyl alcohol into Et2O (3×). Acidify the aqueous phase with conc. HCl to pH < 2 — benzoic acid precipitates as white needles. Filter, wash with cold water, dry. Recrystallise from hot water. α-H-free aldehydes are the substrate scope (benzaldehyde, furfural, formaldehyde): if the aldehyde has α-H, aldol condensation outcompetes Cannizzaro at the same conditions.
| Compound | Role | Equiv | MW (g/mol) | ρ (g/mL) | Safety |
|---|---|---|---|---|---|
| Benzaldehyde100-52-7 · C7H6OBenzaldehyde — both reductant AND oxidant (the same molecule plays both roles in the disproportionation). Freshly distilled; remove benzoic acid (white film on bottle) by 5% NaHCO3 wash before use. | reactant | 2.0 | 106.12 | 1.050 | WarningSDS |
| Sodium hydroxide1310-73-2 · HNaOSodium hydroxide — slight excess (1.1 eq per aldehyde pair) so the reaction goes to completion. Used as 50 wt% aqueous solution (~19 M); concentrated NaOH is corrosive — face shield + chemical-resistant gloves. | reagent | 1.1 | 40.00 | — | DangerSDS |
| Water7732-18-5 · H2OWater — co-solvent / reaction medium. The alkali drives the hydride transfer; water solvates the resulting alkoxide. Dilution kills the rate (Cannizzaro is third-order in [base]). | solvent | — | 18.02 | 1.000 | Not classifiedSDS |
| Benzoic acid65-85-0 · C7H6O2Benzoic acid — the oxidation product; mp 122 °C. Released by acidifying the aqueous phase with conc. HCl. White needles from hot water recrystallisation. | product | — | 122.12 | — | DangerSDS |
| Benzyl alcohol100-51-6 · C7H8OBenzyl alcohol — the reduction product; bp 205 °C. Extract into Et2O after dilution; clean by distillation. | product | — | 108.14 | 1.040 | Warning |