Carbonyl Compounds for JEE Main: Aldehydes & Ketones

Carbonyl compounds (aldehydes and ketones) sit at the heart of JEE Main Organic Chemistry, contributing four to six questions every session. The chapter tests nucleophilic addition reactions, the distinguishing tests between aldehydes and ketones, and name reactions (Aldol, Cannizzaro, Clemmensen, Wolff-Kishner). Once you understand why the carbonyl carbon is electrophilic, every reaction in the chapter follows a coherent pattern. This guide builds that pattern recognition.

Nucleophilic Addition Reactions

The carbonyl carbon is electrophilic because the electronegative oxygen pulls electron density away, leaving C with a partial positive charge. Nucleophiles attack the carbonyl carbon, breaking the C=O π bond. Key nucleophilic additions: (1) HCN → cyanohydrin (useful because it introduces a new C–C bond, increasing chain length by one); (2) sodium bisulphite → bisulphite addition product (a solid, used for purification — aldehydes and methyl ketones react, but bulky ketones do not); (3) Grignard reagent RMgX → alcohol after hydrolysis; (4) NH₂OH (hydroxylamine) → oxime; (5) C₆H₅NHNH₂ (phenylhydrazine) → phenylhydrazone; (6) H₂NNH₂ (hydrazine) → hydrazone.

2,4-DNP (2,4-dinitrophenylhydrazine) gives a yellow or orange precipitate with both aldehydes and ketones — a qualitative test confirming the C=O group. The precipitate can be filtered and its melting point determined to identify the specific compound. Take a free carbonyl chemistry mock to check your reaction identification speed.

Distinguishing Tests: Aldehydes vs Ketones

The key distinction is that aldehydes are easily oxidised (they have a H on the carbonyl carbon), while ketones are not oxidised under mild conditions. Tests that distinguish them: Tollens' test (ammoniacal AgNO₃ — silver mirror with aldehyde, no reaction with ketone); Fehling's test (Cu²⁺ to brick-red Cu₂O with aliphatic aldehydes, no reaction with ketones or aromatic aldehydes like benzaldehyde); Schiff's test (Schiff's reagent turns pink with aldehydes, no reaction with ketones). Benzaldehyde specifically: it gives Tollens' test positive but Fehling's test negative — this exception is a routine JEE trap question.

The iodoform test (NaOH + I₂) gives a yellow precipitate of CHI₃ with: methyl ketones (CH₃COR), acetaldehyde (CH₃CHO), and secondary alcohols bearing a methyl group adjacent to –OH (ethanol also gives it). The pattern: any compound that can be oxidised to a methyl ketone or is itself CH₃CHO gives iodoform. This test is used both to identify methyl ketones and to distinguish between acetaldehyde and other aldehydes.

Name Reactions

Aldol condensation: aldehydes (or ketones) with α-H react with dilute NaOH to give β-hydroxy aldehyde (or ketone) — the aldol product. On heating, dehydration gives the α,β-unsaturated carbonyl compound. Cannizzaro reaction: aldehydes without α-H (formaldehyde, benzaldehyde) undergo disproportionation with concentrated NaOH: one molecule is oxidised to the carboxylate, another is reduced to the alcohol. This is the go-to reaction when the aldehyde has no α-H. Clemmensen reduction (Zn-Hg/HCl) reduces C=O to CH₂. Wolff-Kishner reduction (NH₂NH₂/KOH) also reduces C=O to CH₂ but under basic conditions. The two reductions are equivalent in result but used in different chemical contexts (acid-sensitive vs base-sensitive substrates).

Crossed aldol condensation (one aldehyde with no α-H + one aldehyde with α-H) gives a defined product: the formaldehyde or benzaldehyde acts only as the electrophile (no self-condensation), and the other aldehyde acts as the nucleophile. JEE uses crossed aldol questions to test mechanistic understanding. For the full organic chain, see our nitrogen compounds guide and our alcohol and phenol guide.