What is aldol condensation? Explain it with the equation for ethanal. How does it differ from the Cannizzaro reaction? Write the Cannizzaro reaction of methanal.

1. 1 · Define. Aldehydes or ketones with at least one α-hydrogen, in dilute alkali, join to give a β-hydroxy aldehyde (aldol) or ketone (ketol), which loses water on heating.
2. 2 · Explain the α-hydrogen. It is acidic: C=O pulls electrons, and the enolate ion is resonance-stabilised. OH⁻ removes it; the enolate attacks the C=O of a second molecule.
3. 3 · Write the ethanal equations. 2CH₃CHO ⇌ (dil. NaOH) CH₃CH(OH)CH₂CHO → (heat) CH₃CH=CHCHO + H₂O.
4. 4 · Draw and label. Draw 3-hydroxybutanal and mark the α and β carbons; then draw but-2-enal with the C=C between them.
5. 5 · Write the Cannizzaro reaction. No α-H, concentrated alkali: 2HCHO + KOH → CH₃OH + HCOOK. One molecule is reduced, one oxidised (disproportionation).
6. 6 · Contrast the two. Aldol: α-H, dilute alkali, new C-C bond. Cannizzaro: no α-H, concentrated alkali, oxidation-reduction.
7. 7 · Close in one line. So the presence or absence of an α-hydrogen decides which reaction a carbonyl compound gives.

Scoring tip: Always state 'dilute' for aldol and 'concentrated' for Cannizzaro, and name both products of the Cannizzaro reaction.