Reactions of aldehydes and ketones

The essentials

The typical reaction of aldehydes and ketones is nucleophilic addition: a nucleophile attacks the δ+ carbonyl carbon, the carbon changes from sp² to sp³, and the oxygen picks up a proton. Aldehydes are generally more reactive than ketones because they have less steric crowding and only one electron-releasing alkyl group reducing the positive charge on carbon. Aldehydes are also easily oxidised, which forms the basis of tests to tell them from ketones. Compounds with α-hydrogens show reactions such as aldol condensation, because the α-hydrogen is acidic.

Key points

• Addition of HCN gives cyanohydrins; addition of NaHSO₃ gives crystalline bisulphite adducts (used for separation).
• Grignard reagents add to give alcohols after hydrolysis. Alcohols add to aldehydes (dry HCl) to give hemiacetals and acetals; ketones with ethylene glycol give ketals.
• Ammonia derivatives (Z–NH₂) give C=N–Z: hydroxylamine → oxime, hydrazine → hydrazone, 2,4-DNP → 2,4-dinitrophenylhydrazone, semicarbazide → semicarbazone.
• Reduction: NaBH₄ or LiAlH₄ gives alcohols. Clemmensen (Zn–Hg/conc. HCl) and Wolff–Kishner (NH₂NH₂, then KOH in ethylene glycol on heating) reduce C=O to CH₂.
• Tollens' reagent (ammoniacal AgNO₃) gives a silver mirror with aldehydes. Fehling's solution gives a red-brown Cu₂O precipitate with aliphatic aldehydes but not with aromatic aldehydes.
• Haloform reaction: compounds with a CH₃CO– group (methyl ketones, ethanal) and alcohols that can be oxidised to them, with NaOI, give yellow iodoform, CHI₃.
• Aldol condensation: aldehydes/ketones with an α-H in dilute alkali give β-hydroxy aldehydes/ketones, which lose water to give α,β-unsaturated compounds.
• Cannizzaro reaction: aldehydes without α-H (HCHO, C₆H₅CHO) in concentrated alkali undergo self oxidation–reduction to an alcohol and a carboxylate salt.
• The ring of aromatic carbonyl compounds undergoes electrophilic substitution at the meta position.

Exam tips

• Check for α-hydrogen first: present → aldol; absent → Cannizzaro.
• In each Tollens' test, one mole of RCHO deposits two moles of silver.

Common mistakes

• Assuming all ketones give the iodoform test; only those containing CH₃CO– do.