Overall reaction
2 C₆H₅CHO + NaOH (conc.) → C₆H₅CH₂OH + C₆H₅COO⁻Na⁺
An aldehyde with no α-H disproportionates in strong base: one is oxidised, one reduced.
Name reaction · mechanism
Cannizzaro Reaction
In the Cannizzaro reaction an aldehyde without α-hydrogens (HCHO, benzaldehyde, pivalaldehyde) disproportionates in concentrated base: one molecule is oxidised to the carboxylate and another is reduced to the alcohol. Hydroxide adds to one aldehyde, and the tetrahedral anion transfers a hydride to a second aldehyde.
Mechanism step by step
- Hydroxide adds. Tetrahedral intermediate
- Hydride transfer. C–H moves as H⁻ to a second aldehyde
- Proton transfer. The acid protonates the alkoxide
Each step is checked electron by electron: the curly arrows give exactly the structures drawn.
Key points
- Only aldehydes without α-H (HCHO, PhCHO, (CH₃)₃CCHO): others undergo aldol reactions.
- Rate depends on [aldehyde]² and [OH⁻]: hydride transfer is the slow step.
- Crossed Cannizzaro: formaldehyde is oxidised, reducing the other aldehyde.
- Acidifying the mixture turns benzoate into benzoic acid.
For JEE and NEET
- Only aldehydes with no α-H; others undergo aldol reactions.
- Hydride transfer (from the tetrahedral intermediate) is the key and slow step.
- Crossed Cannizzaro: HCHO is oxidised to formate and reduces the other aldehyde to its alcohol.
- Intramolecular Cannizzaro: glyoxal → glycolate.
FAQ
Why do aldehydes with α-hydrogens not give the Cannizzaro reaction?
Base removes the α-hydrogen first, so they undergo aldol addition/condensation instead.
What is crossed Cannizzaro?
A mixture of formaldehyde and another non-enolisable aldehyde: formaldehyde (the better hydride donor after OH⁻ addition) becomes formate, and the other aldehyde becomes the alcohol.
Is the hydride free in solution?
No. It moves directly from the carbon of the tetrahedral anion to the carbonyl carbon of the second aldehyde.
Draw it in MolDraw
The button above opens the complete figure — overall reaction, numbered mechanism and key points — as ordinary editable structures. Change substituents, recolour atoms, or export a PNG, SVG or chemfig code for notes and slides. The same figure is in the Templates panel under Name reactions.