12 min readPathway sheet1-mark MCQ or assertion-reason on which compound gives aldol or Cannizzaro; 2 or 3-mark equations; a 5-mark answer often pairs aldol and Cannizzaro; also a NEET favourite

Reactions due to α-hydrogen, the Cannizzaro reaction and ring substitution

Carbonyl compounds with an α-hydrogen undergo aldol condensation in dilute alkali. Aldehydes without one undergo the Cannizzaro reaction in concentrated alkali.

α-HydrogenAn H on the carbon next to C=O; acidic, because the enolate ion left behind is resonance-stabilised

Aldol condensationDilute NaOH gives a β-hydroxy aldehyde or ketone, which loses water on heating

Cannizzaro reactionAldehyde with no α-H + concentrated alkali: one molecule → alcohol, the other → carboxylate salt

Ring substitution-CHO and -COR groups are deactivating and meta-directing

Why α-hydrogens are acidic

The carbon next to the C=O group is the α-carbon, and a hydrogen on it is an α-hydrogen. α-Hydrogens are acidic for two reasons. First, the C=O group strongly pulls electrons away from the α-carbon. Second, when the H⁺ leaves, the enolate ion formed is stabilised by resonance: its negative charge is shared between the α-carbon and the oxygen. In aldol condensation, OH⁻ removes an α-hydrogen. The enolate ion then acts as a nucleophile and adds to the C=O carbon of a second molecule. CH₃CHO has three α-hydrogens. HCHO, C₆H₅CHO and (CH₃)₃CCHO have none.

Aldol and cross aldol condensation

1★
2CH₃CHO ⇌ (dil. NaOH) CH₃CH(OH)CH₂CHO
3-Hydroxybutanal, an aldol (ald from aldehyde, ol from alcohol)
2
CH₃CH(OH)CH₂CHO → (heat) CH₃CH=CHCHO + H₂O
But-2-enal, an α,β-unsaturated aldehyde: the aldol condensation product
3
2CH₃COCH₃ ⇌ (Ba(OH)₂) (CH₃)₂C(OH)CH₂COCH₃
4-Hydroxy-4-methylpentan-2-one, a ketol
4
(CH₃)₂C(OH)CH₂COCH₃ → (heat) (CH₃)₂C=CHCOCH₃ + H₂O
4-Methylpent-3-en-2-one. The name 'aldol condensation' is used for ketones too
5
Cross aldol: CH₃CHO + CH₃CH₂CHO → (i) dil. NaOH (ii) heat → mixture of four products
Both have α-H. Self: but-2-enal and 2-methylpent-2-enal. Cross: pent-2-enal and 2-methylbut-2-enal
6★
C₆H₅CHO + CH₃COC₆H₅ → (NaOH, 293 K) C₆H₅CH=CHCOC₆H₅ + H₂O
A ketone can be one partner. Benzaldehyde has no α-H, so it only accepts. Major product: 1,3-diphenylprop-2-en-1-one (benzalacetophenone). A cross aldol of an aromatic aldehyde like this is also called the Claisen-Schmidt reaction, the name your lab manual uses

★ key step

Cannizzaro reaction

1★
2HCHO + KOH (conc.) → (heat) CH₃OH + HCOOK
Methanal has no α-H. One molecule is reduced to methanol and the other oxidised to potassium methanoate: self oxidation-reduction (disproportionation)
2
2C₆H₅CHO + NaOH (conc.) → (heat) C₆H₅CH₂OH + C₆H₅COONa
Benzyl alcohol and sodium benzoate
3
C₆H₅COONa + HCl → C₆H₅COOH + NaCl
Acidifying the salt gives the free acid, benzoic acid

★ key step

Aldol vs Cannizzaro

Aldol condensationNeeds at least one α-hydrogen. Dilute alkali. A new C-C bond joins two molecules; product is a β-hydroxy aldehyde or ketone, then an α,β-unsaturated one. Examples: CH₃CHO, CH₃COCH₃, cyclohexanone.

Cannizzaro reactionAldehydes with no α-hydrogen. Concentrated alkali. No new C-C bond: one molecule is oxidised, one reduced. Examples: HCHO, C₆H₅CHO, (CH₃)₃CCHO.

A ketone with no α-H, such as benzophenone, gives neither: no α-H for aldol, and no H on the C=O carbon for Cannizzaro.

Electrophilic substitution in the ring

1
The -CHO (or -COR) group pulls electrons out of the benzene ring, so it is deactivating and meta-directing.
2★
C₆H₅CHO + HNO₃ → (conc. H₂SO₄, 273 to 283 K) m-O₂NC₆H₄CHO + H₂O
Nitration of benzaldehyde gives 3-nitrobenzaldehyde (m-nitrobenzaldehyde)

★ key step

Activity: prepare dibenzalacetone

1
Dissolve 3.15 g of NaOH in a mixture of 25 mL of ethanol and 30 mL of water in a 250 mL beaker. Keep the beaker in an ice bath at about 20 to 25 °C.
Keep the temperature near 20 °C, and use freshly distilled benzaldehyde
2
Mix 3.2 mL of benzaldehyde with 2.3 mL of acetone. Add half of this mixture slowly to the NaOH solution with vigorous stirring, then stir gently for about 15 minutes.
A fluffy pale yellow precipitate appears within 1 to 2 minutes
3
Add the rest of the benzaldehyde and acetone mixture and stir for 30 minutes more.
4★
2C₆H₅CHO + CH₃COCH₃ → (NaOH, ethanol) C₆H₅CH=CHCOCH=CHC₆H₅ + 2H₂O
Dibenzalacetone (dibenzylideneacetone), 1,5-diphenylpenta-1,4-dien-3-one: a cross aldol condensation on each side of acetone, also called the Claisen-Schmidt reaction
5
Filter the pale yellow solid and wash it with cold water. Dry it and recrystallise a small amount from ethanol or ethyl acetate.
6
Record the yield and the melting point (pure dibenzalacetone melts at about 110 °C).

★ key step

In real life

Cinnamaldehyde, the flavour of dalchini, can be made by a cross aldol reaction of benzaldehyde with ethanal. Dibenzalacetone absorbs ultraviolet light because of its long chain of alternating double bonds; many sunscreen UV filters work on the same idea.

Exam angle

2 marks: “Write the aldol condensation of ethanal. Name the aldol and the final product.” 2CH₃CHO ⇌ (dil. NaOH) CH₃CH(OH)CH₂CHO (3-hydroxybutanal) → (heat) CH₃CH=CHCHO (but-2-enal) + H₂O.

3 marks: “Which of these undergo aldol condensation, which the Cannizzaro reaction, and which neither: propanal, 2,2-dimethylpropanal, cyclopentanone, 4-methoxybenzaldehyde, 2,2,4,4-tetramethylpentan-3-one? Give reasons.” Aldol: propanal and cyclopentanone (have α-H). Cannizzaro: 2,2-dimethylpropanal and 4-methoxybenzaldehyde (aldehydes with no α-H). Neither: 2,2,4,4-tetramethylpentan-3-one, (CH₃)₃CCOC(CH₃)₃, a ketone with no α-H.

5 marks: “Write the Cannizzaro reaction of benzaldehyde and the cross aldol reaction of benzaldehyde with acetophenone. Why does benzaldehyde not give a self aldol product, and what conditions does each reaction need?” 2C₆H₅CHO + conc. NaOH → C₆H₅CH₂OH + C₆H₅COONa. C₆H₅CHO + CH₃COC₆H₅ → (NaOH, 293 K) C₆H₅CH=CHCOC₆H₅ + H₂O. Benzaldehyde has no α-H. Cannizzaro: concentrated alkali; aldol: dilute alkali.

Remember it

Has α-H? Aldol, in dilute alkali. No α-H? Cannizzaro, in concentrated alkali: half reduced, half oxidised.