Aldehydes and Ketones
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Direct answer
Aldehydes and ketones are carbonyl compounds, and their chemistry is nucleophilic addition at the polarised C=O: the carbon is electrophilic, and reactivity falls as alkyl groups crowd and donate (HCHO > other aldehydes > ketones). Aldehydes additionally oxidise easily — the basis of the Tollens and Fehling tests — while both classes form derivatives with ammonia reagents and undergo condensation or disproportionation depending on alpha hydrogens. The named reactions here are among the most tested in JEE organic chemistry.
What you must remember
- Preparations: aldehydes by Rosenmund reduction (acid chloride + H2 over poisoned Pd-BaSO4) and ozonolysis of alkenes; ketones from 2° alcohols or nitriles plus Grignard reagents.
- Nucleophilic addition: HCN/KCN give cyanohydrins (one-carbon chain extension); NaHSO3 gives crystalline bisulphite adducts used for purification; Grignards give alcohols after hydrolysis.
- Carbonyl derivatives: NH2OH gives oximes, NH2NH2 hydrazones, semicarbazide semicarbazones; 2,4-DNP gives the orange precipitate that announces a carbonyl.
- Tollens test: aldehydes give the silver mirror; Fehling solution is reduced to red Cu2O by aliphatic aldehydes — benzaldehyde fails, a classic discriminator.
- Iodoform reaction: methyl ketones and acetaldehyde (plus their alcohol precursors) give yellow CHI3 with I2 and NaOH.
- Aldol condensation: dilute NaOH, needs alpha hydrogen — a beta-hydroxy carbonyl forms and dehydrates on heating (2 CH3CHO → CH3CH=CHCHO).
- Cannizzaro reaction: concentrated alkali, needs no alpha hydrogen — 2 HCHO + NaOH → CH3OH + HCOONa; benzaldehyde gives benzyl alcohol and benzoate.
- Reductions of C=O to CH2: Clemmensen (Zn-Hg, concentrated HCl, acidic) and Wolff-Kishner (NH2NH2, KOH, glycol, basic) — chosen by what else the molecule must survive.
Common confusion
The recurring error is misapplying the oxidation tests: students expect every aldehyde to reduce Fehling solution, but aromatic aldehydes fail while still passing Tollens. The second confusion is aldol versus Cannizzaro — the sole criterion is the alpha hydrogen: with it, dilute alkali gives aldol; without it, concentrated alkali forces disproportionation. Clemmensen and Wolff-Kishner both deliver CH2, and choosing by the substrate's acid or base sensitivity is the decision JEE loves to examine.
Exam-focused takeaway
JEE Main tests named reactions as product identification: aldol product, Cannizzaro products, 2,4-DNP, Tollens, iodoform substrates. JEE Advanced builds multi-step synthesis and deduction — identify a compound from its test results, run cross-aldol selectivity, choose the reduction route that spares a nitro group or ester. Write the alpha-hydrogen count first: it instantly sorts condensation behaviour and enolate chemistry for every carbonyl in the question.
Frequently asked questions
Why are aldehydes more reactive than ketones?
Ketones carry two electron-donating, space-filling alkyl groups; aldehydes have one, and formaldehyde none — so the carbonyl carbon stays more exposed and electrophilic.
What is the difference between Tollens and Fehling tests?
Tollens (ammoniacal silver nitrate) is reduced by all aldehydes to a silver mirror; Fehling only by aliphatic aldehydes — benzaldehyde stays clear.
When does a carbonyl give Cannizzaro instead of aldol?
When it has no alpha hydrogen: concentrated alkali disproportionates two molecules into an alcohol and a carboxylate.
What do Clemmensen and Wolff-Kishner reductions share?
Both convert C=O to CH2: Clemmensen in acid, Wolff-Kishner in base — pick by what else in the molecule survives the medium.
Which compounds give a positive iodoform test?
Methyl ketones, acetaldehyde, and alcohols oxidisable to them (ethanol, isopropanol) — the yellow precipitate is CHI3.
What does the 2,4-DNP test detect?
Any aldehyde or ketone: the orange hydrazone precipitate is the standard quick confirmation of a carbonyl group.