Is Aldehydes, Ketones and Carboxylic Acids important for NEET & JEE?

Yes. Aldehydes, Ketones and Carboxylic Acids is part of the Chemistry Class 12 NCERT syllabus and is directly tested in NEET and JEE examinations. StudyHub provides structured notes, diagrams, and practice questions covering all exam-level subtopics.

How can I practice Aldehydes, Ketones and Carboxylic Acids questions on StudyHub?

Open StudyHub and select Chemistry → Aldehydes, Ketones and Carboxylic Acids. Choose Easy, Medium, or Hard difficulty. Hard-tier questions are at NEET & JEE level with full step-by-step explanations.

🧪 Chemistry · Class 12 · NEET & JEE

Aldehydes, Ketones and Carboxylic Acids

Aldehydes, ketones, carboxylic acids, and their derivatives (esters, amides, acid chlorides). Key reactions include nucleophilic addition, Aldol condensation, Cannizzaro, and Claisen. Very heavily tested in JEE and NEET.

Practice Aldehydes, Ketones and Carboxylic Acids Quiz — 100% Free →

Reading time~6 min

Revision time~2 min

Last updated2026-07-19

1 Read the chapter ~6 min

🎯 Key Points

Aldehydes have ≥1 H on carbonyl carbon; ketones have 2 alkyl/aryl groups — aldehydes are generally more reactive to nucleophilic addition (less steric hindrance, more electrophilic carbon)
Tollens'/Fehling's test: positive ONLY for aldehydes (silver mirror / brick-red precipitate respectively)
Iodoform test: positive for any CH₃CO- group (methyl ketones) or ethanol/acetaldehyde-type structures, not just aldehydes generally
Cannizzaro reaction needs NO alpha-H (non-enolisable aldehyde) — self redox: one molecule oxidised to acid, another reduced to alcohol
Carboxylic acids are stronger acids than phenols/alcohols due to greater resonance stabilisation of the carboxylate ion (charge spread over 2 oxygens equally)
Clemmensen (Zn(Hg)/HCl) and Wolff-Kishner (N₂H₄/KOH) both reduce C=O fully to CH₂, useful when other reducible groups must survive each respective acidic/basic condition

Aldehydes (R-CHO) and Ketones (R-CO-R')

Contain the carbonyl group (C=O). Aldehydes have at least one H on the carbonyl carbon; ketones have two alkyl or aryl groups.

Distinguishing Tests

Tollens' test (Ag mirror): positive for aldehydes only
Fehling's test (brick-red ppt): positive for aldehydes only (not aromatic)
Iodoform test (yellow ppt with I₂/NaOH): positive for CH₃CO- group (methyl ketones and acetaldehyde)

Nucleophilic addition to a carbonyl: a nucleophile attacks the electrophilic carbon of an aldehyde or ketone C=O, the pi electrons shift onto oxygen, giving a tetrahedral alkoxide intermediate.

The carbonyl carbon of an aldehyde or ketone is electron-poor (δ+) because oxygen draws the π electrons toward itself. A nucleophile attacks this carbon, the C=O π bond breaks onto oxygen, and a tetrahedral alkoxide intermediate forms — the first step of nucleophilic addition. Image: Unknown author, CC BY 4.0, via Wikimedia Commons.

Carboxylic Acids (R-COOH)

Stronger acids than alcohols and phenols due to resonance in carboxylate ion (RCOO⁻)
Reactions: esterification, amide formation, reduction (LiAlH₄ to 1° alcohol)
Derivatives: acid chlorides (RCOCl), acid anhydrides, esters (RCOOR'), amides (RCONH₂)
Decarboxylation: RCOOH → RH + CO₂ (heating with soda lime)

Nomenclature

Aldehydes: replace -e of alkane with -al (methanal/HCHO, ethanal/CH₃CHO); -CHO carbon is always C-1
Ketones: replace -e with -one, number to give the C=O the lowest locant (propanone, pentan-2-one)
Carboxylic acids: replace -e with -oic acid (methanoic/formic, ethanoic/acetic acid); -COOH carbon is C-1
Common names retained: acetaldehyde, acetone, benzaldehyde, acetophenone, formic/acetic/benzoic acid

Preparation of Aldehydes and Ketones

Rosenmund reduction: acyl chloride + H₂ over Pd/BaSO₄ → aldehyde (BaSO₄ poisons the catalyst to stop over-reduction to alcohol)
Stephen reaction: nitrile + SnCl₂/HCl then H₃O⁺ → aldehyde (via imine); alternatively DIBAL-H reduces nitriles/esters to aldehydes
Gattermann-Koch reaction: benzene + CO + HCl (anhyd. AlCl₃/CuCl) → benzaldehyde
Etard reaction: toluene + CrO₂Cl₂ → benzaldehyde (via chromium complex, then hydrolysis)
Ozonolysis of alkenes and hydration of alkynes (Markovnikov, dil. H₂SO₄/HgSO₄ → ketone; ethyne → acetaldehyde)
Ketones from acyl chlorides with dialkylcadmium (R₂Cd), or aromatic ketones by Friedel-Crafts acylation

Aldol and Cross-Aldol Condensation

Aldol: aldehydes/ketones with α-hydrogen self-condense in dilute base to a β-hydroxy carbonyl (aldol), which on heating dehydrates to an α,β-unsaturated carbonyl
Cross-aldol: between two different carbonyl compounds both bearing α-H gives a mixture of four products (of limited use)
A useful cross-aldol uses one partner with NO α-H (e.g. benzaldehyde, HCHO) so it can only act as the electrophile, giving a clean product
Contrast with Cannizzaro: only for aldehydes with NO α-H

Preparation and Special Reactions of Carboxylic Acids

Preparation: oxidation of 1° alcohols/aldehydes, hydrolysis of nitriles/esters/amides, from Grignard reagent + CO₂ (dry ice) then H₃O⁺, oxidation of alkylbenzenes to benzoic acid
Hell-Volhard-Zelinsky (HVZ) reaction: carboxylic acid with α-H + Cl₂/Br₂ in presence of red P → α-halo carboxylic acid (a key route to α-amino/α-hydroxy acids)
Formation of derivatives: acid chlorides (SOCl₂/PCl₅), anhydrides, esters (Fischer esterification), amides
Reduction: LiAlH₄ or B₂H₆ reduces -COOH to 1° alcohol (not reduced by NaBH₄)
HVZ does not work on formic acid or aromatic acids like benzoic acid (no α-H)

🚀 JEE Advanced Edge

Relative reactivity of carbonyl compounds to nucleophilic addition: HCHO > other aldehydes > ketones, governed by both steric factors (smaller groups = less hindrance to nucleophile approach) and electronic factors (alkyl groups donate electron density via +I effect, reducing the carbonyl carbon's electrophilicity). Aromatic aldehydes/ketones are LESS reactive than aliphatic ones because the ring donates electron density into the carbonyl via resonance.

Acidity order of substituted acetic acids: Cl-CH₂-COOH > CH₃COOH, and trichloroacetic acid > dichloroacetic acid > chloroacetic acid > acetic acid — each additional electron-withdrawing Cl atom further stabilises the carboxylate anion via induction, making the acid progressively stronger.

Worked problem (Cannizzaro): Benzaldehyde (C₆H₅CHO, no alpha-H) is treated with concentrated NaOH. Identify the products. Approach: Since benzaldehyde has no alpha hydrogen, it cannot undergo Aldol condensation, so it undergoes Cannizzaro instead: 2 C₆H₅CHO + NaOH → C₆H₅COONa (sodium benzoate) + C₆H₅CH₂OH (benzyl alcohol) — one molecule is oxidised, the other reduced, in a 1:1 ratio.

2 Revise ~2 min before the exam

Key Reactions

Curved arrows show the nucleophile attacking the carbonyl carbon while the C=O pi electrons move onto oxygen, giving a tetrahedral alkoxide intermediate.

Nucleophilic addition: HCN, NaHSO₃, alcohol (acetal/ketal), NH₂OH (oxime), hydrazine
Aldol condensation: Two aldehyde molecules combine in base to give a beta-hydroxyaldehyde, which dehydrates to an alpha-beta-unsaturated aldehyde
Cannizzaro reaction: Non-enolisable aldehydes (like HCHO, PhCHO) disproportionate in conc. NaOH: one is oxidised to acid, the other is reduced to alcohol
Clemmensen reduction: C=O to CH₂ using Zn(Hg)/conc. HCl
Wolff-Kishner reduction: C=O to CH₂ using N₂H₄/KOH

🔑 Key Reactions

Nucleophilic addition: the carbonyl C=O is attacked by nucleophiles (HCN → cyanohydrin, NaHSO₃ → bisulfite adduct)
Reduction: aldehyde → 1° alcohol; ketone → 2° alcohol (via NaBH₄ or LiAlH₄)
Oxidation: aldehydes oxidise easily to acids; ketones resist
Tollens' test: aldehydes give a silver mirror; ketones do not (distinguishes the two)
Fehling's test: aldehydes give a red Cu₂O precipitate
Aldol condensation: two carbonyls with α-H combine (needs α-hydrogen)
Cannizzaro reaction: aldehydes without α-H disproportionate (e.g. HCHO, benzaldehyde)
Acid strength: carboxylic acids > phenols > alcohols; −I groups (Cl) increase acidity

3 Practice apply it

✍️ Worked Examples

Example 1 — Distinguishing an aldehyde from a ketone
Q: How does Tollens' reagent distinguish propanal from propanone?
Step 1 — Tollens' reagent is ammoniacal silver nitrate, a mild oxidiser.
Step 2 — Propanal, an aldehyde, is oxidised to the acid while Ag⁺ is reduced to metallic silver — a shiny mirror forms.
Step 3 — Propanone, a ketone, cannot be oxidised this way, so no mirror appears.
Answer: propanal gives a silver mirror; propanone does not. Key idea: the ease of aldehyde oxidation is the basis of this and Fehling's test.

Example 2 — Which can undergo aldol condensation
Q: Can ethanal and benzaldehyde each undergo aldol condensation?
Step 1 — Aldol condensation requires an α-hydrogen (a hydrogen on the carbon next to the carbonyl).
Step 2 — Ethanal (CH₃CHO) has three α-hydrogens ⇒ it undergoes aldol condensation.
Step 3 — Benzaldehyde (C₆H₅CHO) has no α-hydrogen ⇒ it cannot, and instead undergoes Cannizzaro.
Answer: ethanal yes, benzaldehyde no. Key idea: the presence or absence of an α-H decides aldol vs Cannizzaro.

Example 3 — Effect of a substituent on acid strength
Q: Which is a stronger acid: acetic acid or chloroacetic acid?
Step 1 — Acidity is greater when the carboxylate anion is more stable.
Step 2 — Chlorine's −I effect pulls electron density away, spreading and stabilising the negative charge.
Step 3 — Acetic acid has no such stabilising group, so its anion is less stable.
Answer: chloroacetic acid is the stronger acid. Note: more chlorines give even stronger acids — trichloroacetic acid is stronger still.