Aldehydes and Ketones: MDCAT Notes and MCQs

Nomenclature & structure

• Aldehydes (–CHO): suffix -al — HCHO methanal (formaldehyde), CH₃CHO ethanal (acetaldehyde). Ketones (R–CO–R'): suffix -one — CH₃COCH₃ propanone (acetone), butanone.
• Carbonyl carbon is sp², trigonal planar (≈ 120°); C=O is polar (C^δ+, O^δ−) → nucleophiles attack carbon.
• Formalin = 37–40% aqueous formaldehyde (preservative).

Preparation

• Oxidation of alcohols with K₂Cr₂O₇/H₂SO₄: 1° → aldehyde (distil off quickly to avoid further oxidation), 2° → ketone.
• Industrial: methanol + air over Ag/Cu at 500 °C → HCHO; ethene oxidation (Wacker) → CH₃CHO; hydration of alkynes; dry distillation of calcium salts of acids (calcium acetate → acetone).

Reactivity — aldehydes > ketones

Ketones have two electron-donating alkyl groups (reduce δ+ on carbon) and greater steric hindrance. Order: HCHO > CH₃CHO > CH₃COCH₃.

Nucleophilic addition

• Base-catalysed: base generates a strong nucleophile which attacks C=O first, then protonation. E.g. HCN (with base, CN⁻) → cyanohydrin; NaHSO₃ → bisulphite adduct; aldol condensation (aldehydes/ketones with α-H, dil. NaOH → β-hydroxy aldehyde/ketone); Cannizzaro reaction (aldehydes without α-H like HCHO, benzaldehyde, conc. NaOH → alcohol + salt of acid).
• Acid-catalysed: H⁺ protonates carbonyl oxygen first, making carbon more electrophilic for weak nucleophiles. E.g. addition of water/alcohols (hemiacetals, acetals), and addition–elimination with ammonia derivatives: 2,4-DNPH → orange-yellow 2,4-dinitrophenylhydrazone (test for C=O), hydroxylamine → oxime, hydrazine → hydrazone.

Reduction

With NaBH₄ or LiAlH₄ (or H₂/Ni): aldehydes → 1° alcohols; ketones → 2° alcohols. Clemmensen (Zn-Hg/HCl) or Wolff–Kishner reduction → alkanes.

Oxidation

• Aldehydes are easily oxidized to carboxylic acids (strong reducing agents) — even by mild oxidants:
  • Tollens' reagent (ammoniacal AgNO₃) → silver mirror.
  • Fehling's / Benedict's solution (Cu²⁺) → brick-red ppt of Cu₂O (aliphatic aldehydes only; benzaldehyde negative to Fehling's).
• Ketones resist mild oxidation; strong oxidants break C–C bonds giving a mixture of acids.
• Iodoform test (I₂/NaOH → yellow CHI₃): methyl ketones (CH₃CO–) and ethanal.