# Aldehydes and Ketones

> Aldehydes and ketones for JEE Chemistry: nucleophilic addition, Tollens and Fehling tests, aldol and Cannizzaro reactions and named reductions.

- Canonical URL: https://prepelephant.com/topics/jee/chemistry/aldehydes-and-ketones
- Exam / course: JEE · Subject: Chemistry
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Aldehydes and Ketones", PrepElephant, https://prepelephant.com/topics/jee/chemistry/aldehydes-and-ketones

## 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.