Disproportionation — non-enolizable aldehyde
Cannizzaro reaction of benzaldehyde
How the Cannizzaro reaction of benzaldehyde works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Conc. NaOH or KOH (50 %). Conditions: Aqueous/alcoholic, 25–80 °C; aldehyde lacks α-H. Bonds formed: C–O (carboxylate), C–H (hydride delivered); bonds broken: C–H (hydride donor), C=O π (twice). Stereochemistry: Not applicable (no new stereocentre). Rate and selectivity: Second order in aldehyde, first order in HO⁻ (rate-limiting hydride transfer).
At a glance
Step 1: Hydroxide adds to the aldehyde
Benzaldehyde has no alpha hydrogens, so it can't form an enolate. Instead, hydroxide attacks its carbonyl carbon directly, forming a tetrahedral alkoxide.
Step 2: Hydride transfer to a second aldehyde
This intermediate acts as a hydride donor: its C–H electrons transfer as a hydride onto the carbonyl carbon of a second benzaldehyde molecule, reducing it.
Step 3: Acid and alcohol (after proton transfer)
One molecule is oxidized to benzoate, the other reduced to benzyl alcohol — a self-redox disproportionation, only possible because there's no competing alpha-hydrogen chemistry.
Abbreviations and terms
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
- KOH
- Potassium hydroxide. Strong aqueous/alcoholic base.
How it compares with related reactions
| Cannizzaro reaction of benzaldehyde | Corey-Fuchs reaction | Tishchenko reaction | |
|---|---|---|---|
| Reagents | Conc. NaOH or KOH (50 %) | 1) CBr₄, PPh₃ 2) 2 equiv n-BuLi, then H₂O | 2 RCHO, cat. Al(OR)₃ (e.g. Al(OEt)₃ or Al(OiPr)₃) |
| Conditions | Aqueous/alcoholic, 25–80 °C; aldehyde lacks α-H | CH₂Cl₂, 0 °C; then THF, –78 °C | Neat or toluene, 0–25 °C |
| Stereochemistry | Not applicable (no new stereocentre) | No stereochemistry (alkyne is linear) | None (acetaldehyde example) |
| Rate / selectivity | Second order in aldehyde, first order in HO⁻ (rate-limiting hydride transfer) | Wittig step fast; Li–Br exchange very fast at –78 °C | 1,5-hydride shift in a six-membered Al-bound cycle is turnover-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Cannizzaro reaction of benzaldehyde?
Conc. NaOH or KOH (50 %). Conditions: Aqueous/alcoholic, 25–80 °C; aldehyde lacks α-H.
Which bonds are formed and which are broken?
Formed: C–O (carboxylate), C–H (hydride delivered). Broken: C–H (hydride donor), C=O π (twice).
What is the stereochemical outcome?
Not applicable (no new stereocentre)
What controls the rate or the selectivity?
Second order in aldehyde, first order in HO⁻ (rate-limiting hydride transfer)
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go