Aldehydes & Ketones

Concept overview

The carbonyl group C=O is the most reactive functional group in JEE organic chemistry. The carbon is electrophilic (partially positive), the oxygen is nucleophilic (partially negative), and the entire double bond is polarised. Nucleophiles attack the carbon; electrophiles (usually H⁺) attack the oxygen.

The archetypal reaction is nucleophilic addition. HCN gives a cyanohydrin. NaHSO₃ gives a bisulphite adduct (used to purify aldehydes). Grignard reagents give tertiary alcohols after hydrolysis. NH₂-Z compounds — amines, hydroxylamine, hydrazine, semicarbazide — condense to form imines, oximes, hydrazones and semicarbazones respectively.

Reactivity toward nucleophiles depends on steric and electronic effects. Aldehydes are more reactive than ketones because a ketone has two electron-donating alkyl groups stabilising the partial positive charge on carbon; an aldehyde has only one. Aromatic aldehydes are less reactive than aliphatic because the ring donates electron density through resonance.

Named reactions dominate the JEE profile of this chapter. Aldol condensation (α-hydrogen containing aldehyde/ketone + base → β-hydroxy carbonyl → dehydration to α,β-unsaturated carbonyl). Cannizzaro (aldehyde without α-H + strong base → disproportionation to alcohol + carboxylate). Wolff-Kishner and Clemmensen (carbonyl → methylene: two different reagent pairs for reducing C=O to CH₂). Perkin, Benzoin, Wittig round out the list. Master 8–10 named reactions and you have most JEE Main aldehyde/ketone questions covered.

Key reactions & principles

• Reactivity toward Nu: HCHO > R–CHO > R–CO–R' (aldehydes > ketones)
• Grignard on carbonyl: R₂C=O + R'MgX → R₂C(OMgX)R' → R₂C(OH)R' (tertiary alcohol from ketone)
• Aldol condensation: 2 R–CHO (with α-H) + NaOH → β-hydroxy aldehyde → α,β-unsaturated aldehyde
• Cannizzaro reaction: 2 Ar–CHO (no α-H) + NaOH → Ar–CH₂OH + Ar–COONa
• Clemmensen reduction: C=O + Zn(Hg)/HCl → CH₂ (works in acidic conditions)
• Wolff-Kishner reduction: C=O + NH₂NH₂ / KOH / Δ → CH₂ (works in basic conditions)
• Tollens' test: Ag(NH₃)₂⁺ + R–CHO → Ag mirror + R–COO⁻ (identifies aldehyde)

Common mistakes to avoid

• Attempting an aldol condensation on formaldehyde or benzaldehyde — neither has α-H, so aldol is not possible. Those substrates undergo Cannizzaro instead.
• Mixing up Clemmensen (acidic Zn/Hg HCl) and Wolff-Kishner (basic NH₂NH₂/KOH) — use Clemmensen for base-sensitive substrates and Wolff-Kishner for acid-sensitive substrates.
• Predicting Grignard products without accounting for the electrophile — formaldehyde gives 1° alcohols, other aldehydes give 2° alcohols, and ketones give 3° alcohols.
• Assuming ketones give a positive Tollens' test — only aldehydes do; Tollens' cleanly distinguishes them from ketones.

JEE Main relevance

JEE Main asks 2 aldehyde/ketone questions every paper. Typically one named-reaction identification (which reagents give which product) and one product-prediction problem. The named-reaction pool is small — memorise 8–10 and you cover almost every JEE Main permutation.

JEE Advanced relevance

Advanced problems chain multiple named reactions — for instance, aldol condensation followed by Michael addition, or Cannizzaro followed by esterification. Retrosynthesis problems asking you to construct a complex α,β-unsaturated ester from simpler carbonyls are also frequent Advanced patterns.

Previous-year questions (PYQ)

Filter the PYQ bank by 'Aldehydes and Ketones' and drill named reactions in a single dedicated session — the archetypes repeat almost verbatim across years.