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Cannizzaro Reaction: Why Aldehydes Without Alpha-Hydrogen Disproportionate

Chemistry · Aldehydes, Ketones And Carboxylic Acid · NEET

The Cannizzaro reaction happens only in aldehydes that have NO alpha-hydrogen (like HCHO and C6H5CHO). When you heat them with strong concentrated alkali (conc. NaOH/KOH), two molecules react together: one aldehyde is reduced to an alcohol and the other is oxidised to a carboxylic acid salt (carboxylate). This "self oxidation-reduction" is called disproportionation. Memory hook: "No alpha-H, no aldol — so it disproportionates instead: one goes UP to acid, one goes DOWN to alcohol."
Cannizzaro Reaction (no alpha-H) — Disproportionation2 C6H5CHO(benzaldehyde, no alpha-H)conc. NaOHheatC6H5CH2OH (REDUCED)benzyl alcohol — gains HC6H5COONa (OXIDISED)sodium benzoate — loses HRule check:alpha-C has NO Hso aldol is blocked
Two benzaldehyde molecules (no alpha-hydrogen) heated with conc. NaOH disproportionate: one is reduced to benzyl alcohol, the other is oxidised to sodium benzoate.

Your doubts, answered

Why must the aldehyde have NO alpha-hydrogen for Cannizzaro?

An alpha-hydrogen is a hydrogen on the carbon right next to the C=O group. If that hydrogen exists, the base pulls it off and the molecule reacts by aldol condensation instead (the faster path). Cannizzaro only wins when there is no alpha-hydrogen, so aldol is impossible. Then the only way two aldehyde molecules can react with base is by passing a hydride (H with its electrons) from one to the other — that is Cannizzaro. So 'no alpha-H' is a rule, not a coincidence: it blocks the competing aldol reaction.

What exactly are the two products, and which is which?

You always get TWO different products from two molecules of the same aldehyde. One molecule GAINS hydrogen and becomes a primary alcohol (this molecule is reduced). The other molecule LOSES hydrogen and becomes a carboxylic acid salt / carboxylate (this molecule is oxidised). Example: 2 HCHO + conc. NaOH gives CH3OH (methanol) + HCOONa (sodium formate). For benzaldehyde: 2 C6H5CHO + conc. NaOH gives C6H5CH2OH (benzyl alcohol) + C6H5COONa (sodium benzoate). Remember: acid comes out as its sodium SALT because strong alkali is present.

What is disproportionation? Why is Cannizzaro called self oxidation-reduction?

Disproportionation means the SAME substance is both oxidised and reduced at the same time. In Cannizzaro, both product-making molecules started identical (both are the same aldehyde), but one ends up oxidised (to acid) and one ends up reduced (to alcohol). No outside oxidising or reducing agent is added — the aldehyde does it to itself. That is why NCERT calls it 'self oxidation and reduction (disproportionation)'.

How is Cannizzaro different from aldol condensation?

They are decided by the alpha-hydrogen. Aldol needs AT LEAST ONE alpha-hydrogen and gives a beta-hydroxy carbonyl (aldol/ketol), later an alpha,beta-unsaturated product on heating. Cannizzaro needs NO alpha-hydrogen and gives one alcohol + one carboxylate. They are mutually exclusive for a single aldehyde: HCHO and benzaldehyde do Cannizzaro; acetaldehyde and acetone (they have alpha-H) do aldol. This either/or split is a favourite NEET question.

What is a CROSS Cannizzaro reaction?

When TWO different aldehydes (both without alpha-H) are used, it is cross Cannizzaro. The most useful case uses formaldehyde (HCHO) with another no-alpha-H aldehyde. HCHO is the best hydride donor, so HCHO is almost always the one oxidised (to sodium formate, HCOONa) while the OTHER aldehyde is reduced to its alcohol. Example: HCHO + C6H5CHO + conc. NaOH gives C6H5CH2OH + HCOONa. So HCHO 'sacrifices itself' and pushes the other aldehyde fully to alcohol.

Does acetaldehyde or acetone give Cannizzaro?

No. Acetaldehyde (CH3CHO) has three alpha-hydrogens and acetone (CH3COCH3) has six alpha-hydrogens. Because alpha-H is present, both undergo aldol condensation, not Cannizzaro. A quick NEET check: if the carbon next to C=O carries any H, rule out Cannizzaro immediately. Only aldehydes like HCHO, C6H5CHO, (CH3)3C-CHO (trimethylacetaldehyde) and other alpha-H-free aldehydes give Cannizzaro.

⚠️ The NEET trap
✗ Benzaldehyde + acetophenone with dilute NaOH gives a cross Cannizzaro reaction because benzaldehyde has no alpha-hydrogen.
✓ It is a CROSS ALDOL condensation. Acetophenone (CH3COC6H5) HAS alpha-hydrogens on its CH3, so its alpha-carbon adds to benzaldehyde's C=O and, after losing water, gives the alpha,beta-unsaturated ketone (chalcone). Cross Cannizzaro would need BOTH partners to have no alpha-H.
🧠 Cannizzaro needs BOTH aldehydes alpha-H-free. If even one partner has an alpha-H, it is aldol, not Cannizzaro.

Real NEET questions

NEET 2020

The reaction between benzaldehyde and acetophenone in the presence of dilute NaOH is known as:

A · Cross Cannizzaro's reaction
B · Cross aldol condensation ✓
C · Aldol condensation
D · Cannizzaro's reaction
Solution: Benzaldehyde (C6H5CHO) has NO alpha-hydrogen, but acetophenone (CH3COC6H5) HAS alpha-hydrogens on its methyl group. Because one partner supplies an alpha-H, aldol chemistry takes over: the alpha-carbon of acetophenone adds to the C=O of benzaldehyde and, on losing water, gives the alpha,beta-unsaturated ketone C6H5CH=CH-CO-C6H5 (chalcone). Since two DIFFERENT carbonyl compounds react, it is a CROSS (mixed) aldol condensation. It is NOT Cannizzaro, because Cannizzaro requires BOTH partners to lack an alpha-hydrogen. Answer: (B).
NEET 2016 Phase 1

The correct statement regarding a carbonyl compound with a hydrogen atom on its alpha-carbon is:

A · It never equilibrates with its corresponding enol.
B · It rapidly equilibrates with its enol; this is called aldehyde-ketone equilibration.
C · It rapidly equilibrates with its enol; this is called carbonylation.
D · It rapidly equilibrates with its enol; this is called keto-enol tautomerism. ✓
Solution: This PYQ shows what an alpha-hydrogen enables. A carbonyl WITH an alpha-H can shift that H to the carbonyl oxygen, giving the enol form (C=C-OH); this rapid interconversion is keto-enol tautomerism (D). The same alpha-H is also what powers aldol condensation. So aldehydes that HAVE alpha-H tautomerise and do aldol, while aldehydes with NO alpha-H cannot do either and are forced into Cannizzaro instead. The alpha-hydrogen is the single switch that decides the pathway. Answer: (D).

Solved Aldehydes, Ketones And Carboxylic Acid NEET PYQs

Try the real previous-year questions from this chapter — each with the answer and a full solution.

See all 44 Aldehydes, Ketones And Carboxylic Acid NEET PYQs ›
Next concept: Grignard Reagents with Aldehydes, Ketones and CO2 →Keep learning — 2 minFeeling ready? Solve the Aldehydes, Ketones And Carboxylic Acid NEET PYQs ›Or practice on your phone — get the free MedicNEET app ›

Frequently asked

Why does formaldehyde (HCHO) undergo Cannizzaro but acetaldehyde does not?

HCHO has no carbon next to C=O that carries a hydrogen (no alpha-H), so it cannot do aldol and undergoes Cannizzaro. Acetaldehyde (CH3CHO) has alpha-hydrogens, so it does aldol condensation instead.

What alkali is used in the Cannizzaro reaction?

A strong CONCENTRATED alkali such as concentrated NaOH or KOH, usually with heating. The high hydroxide concentration is needed to attack the carbonyl and transfer hydride between the two aldehyde molecules.

Are the two products formed in equal amounts?

Yes, in a simple (self) Cannizzaro. Two identical aldehyde molecules react, so you get equal moles of the alcohol and the carboxylic acid salt (about a 1:1 ratio).

Why does the acid appear as a salt (carboxylate) and not as free COOH?

The reaction runs in strong concentrated alkali (NaOH). Any carboxylic acid formed is immediately neutralised by the base, so it stays as the sodium carboxylate (e.g. HCOONa, C6H5COONa). You get the free acid only after adding acid on work-up.

How do I quickly decide aldol vs Cannizzaro in an exam?

Look at the carbon next to C=O. If it has at least one hydrogen (alpha-H present) it is aldol. If it has NO hydrogen (alpha-H absent, like HCHO, benzaldehyde) it is Cannizzaro. This one check answers most NEET questions on this topic.

Related concepts:
Aldol and Cross Aldol Condensation ReactionsAlpha-Hydrogen Acidity and Keto-Enol TautomerismOxidation of Aldehydes: Tollens and Fehling TestsGrignard Reagents with Aldehydes, Ketones and CO2Reducing Carbonyls to Alcohols and Hydrocarbons
Practise Chemistry for NEET:
Chemistry MCQs (free, chapter-wise)Aldehydes, Ketones And Carboxylic Acid PYQs