Class 12 Chemistry · NCERT Chapter 8 (Part 1)

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Aldehydes, Ketones and Carboxylic Acids Class 12 Chemistry Notes

This chapter is built on the carbonyl group: aldehydes and ketones react by nucleophilic addition, while carboxylic acids bring acidity and derivative chemistry. It covers preparations, aldol and Cannizzaro reactions, Tollen's/Fehling's/iodoform tests, and the reactions of the −COOH group.

Class:12Subject:ChemistryCovers:Aldol · Cannizzaro · Iodoform testExams:CBSE · CUET · NEET

6 Key Formulas2 Solved Examples40 Practice MCQs

DWritten byDeep Narayan

UpdatedOctober 2026

Key Concept Summary

How would you distinguish acetaldehyde from acetone?

Add Tollen's reagent (ammoniacal AgNO₃): acetaldehyde (an aldehyde) gives a silver mirror; acetone gives none. Both give a yellow precipitate with 2,4-DNP and both give the iodoform test (acetaldehyde has the CH₃CO– group and acetone is a methyl ketone), so silver mirror is the discriminating test you must quote.

Nomenclature and Structure of the Carbonyls

Aldehydes have the C=O on a terminal carbon (−CHO), ketones have it inside the chain (>C=O), and carboxylic acids carry a terminal –COOH. The carbonyl group is planar with a polarised C=O bond: O pulls electrons, making carbon the electrophilic centre.

IUPAC: aldehydes → alkanal (ethanal), ketones → alkanone with locant (propanone for acetone, butan-2-one), acids → alkanoic acid (ethanoic).
Common names stay in exam vocabulary: acetaldehyde, acetone, benzaldehyde, acetic acid.
Aromatic: benzaldehyde, acetophenone (C₆H₅COCH₃); the ring makes properties differ from pure aliphatic.

Priority rules

In a molecule with both groups, -C(=O)H trumps ketone: 4-oxopentanal is named as an aldehyde with ketone as oxo prefix. Acids outrank both.

MCQ Questions

The IUPAC name of CH₃COCH₂CH₃ is:

In the carbonyl group C=O, the carbon acts as:

Preparation of Aldehydes and Ketones

Aldehydes: oxidation of 1° alcohol (PCC/CrO₃·pyridine stops at aldehyde); Rosenmund reduction of acid chloride (Pd+BaSO₄ poisoned); ozonolysis of alkenes; from nitriles (Stephen, SnCl₂/HCl); from methylbenzene with chromyl chloride (Etard reaction).
Ketones: oxidation of 2° alcohol; Friedel–Crafts acylation of arenes (acetophenone from benzene + CH₃COCl/AlCl₃); hydration of alkynes (Kucherov, Hg²⁺/H⁺); from acid chlorides with dialkylcadmium (stops at ketone, no 3° alcohol).

Friedel–Crafts acylation builds benzophenone/acetophenone

\ce C 6 H 6 + C H 3 C O C l - > [A l C l 3] C 6 H 5 C O C H 3 + H C l

Stay at the aldehyde

Common oxidants go all the way to acid; PCC (pyridinium chlorochromate) or chromyl chloride in dichloromethane stop cleanly at the aldehyde, a favourite in 'choose the right reagent' questions.

MCQ Questions

Rosenmund reduction of an acid chloride gives:

Kucherov reaction adds water to ethyne to give:

Preparation of Carboxylic Acids

From 1° alcohols/aldehydes: strong oxidation (KMnO₄/H⁺, acid dichromate) → acid.
From alkenes: hot KMnO₄ oxidatively cleaves the double bond to carboxylic acids.
From nitriles: hydrolysis (H⁺ or OH⁻) → acid + ammonia.
From Grignard: RMgX + CO₂ → RCOOH (extend the chain by one carbon).
From acyl derivatives: acid chlorides, anhydrides, esters and amides all hydrolyse to acids.
Aromatic acids: side-chain oxidation of alkylbenzenes (toluene → benzoic acid).

Carboxylation by Grignard adds one carbon

\ce R - M g X + C O 2 - > [H 3 O +] R C O O H

Lab vs exam

The nitrile route (1 carbon up) and the Grignard route (1 carbon up) are the two 'ascend the series' tricks. Both are standard in CBSE 3-markers.

MCQ Questions

CH₃CN on acid hydrolysis gives:

Toluene is oxidised to:

Physical Properties and Nucleophilic Addition

Boiling points: acids > alcohols > aldehydes/ketones; aldehydes > alkanes; acids dimerise via H-bonding (2 molecules).
Solubility: small aldehydes/ketones/acids dissolve in water via H-bonding; formic and acetic acid are completely miscible.
Aldehydes/ketones do not hydrogen-bond between themselves (no O–H), so they boil lower than the alcohols.

Definition

Nucleophilic addition

The carbonyl carbon (electrophile) accepts a nucleophile, then a proton adds to oxygen: C=O → C(OH)Nu. Products: cyanohydrin (with HCN), alcohols (with RMgX), bisulphite adducts, and acetal/ketal with alcohols under acid.

Faster with fewer R groups

HCHO > RCHO > R₂C=O: each alkyl group donates electron density (electronic) and blocks the approach from the side (steric). Both effects cut reactivity.

MCQ Questions

HCN adds to ethanal giving:

The most reactive toward nucleophilic addition is:

Chemical Tests: Tollen's, Fehling's and Iodoform

Tollen's reagent (ammoniacal silver nitrate): aldehydes give a silver mirror; ketones do not — KEY distinction.
Fehling's solution (Cu²⁺/tartrate, alkaline): aliphatic aldehydes give red Cu₂O; aromatic aldehydes and ketones do not.
Benedict's solution similar to Fehling's (Cu²⁺ citrate).
2,4-Dinitrophenylhydrazine (Brady's reagent): yellow/orange ppt with BOTH aldehydes and ketones (identifies C=O but not the class).
Iodoform test (I₂/NaOH): yellow CHI₃ ppt for the group CH₃CO– (acetone, acetaldehyde) and for CH₃CH₂–OH (ethanol), which is oxidised to acetaldehyde first.
Schiff's reagent distinguishes aldehydes (restores pink).

Silver mirror — the aldehyde fingerprint

\ce R C H O + 2 [A g (N H 3) 2] + + 3 O H - - > R C O O - + 2 A g + 2 H 2 O + 2 N H 3

Which test for which

Only aldehyde: Tollen's mirror. Only aliphatic aldehyde: Fehling's. Both: 2,4-DNP. Only −COCH₃/CH₃CH₂OH: iodoform. Blend these four into every identify-the-compound answer.

MCQ Questions

Acetone gives a positive test with:

Q10

Iodoform test is positive for compounds containing the group:

Aldol, Cannizzaro and Acid Reactions

Definition

Aldol condensation

Base removes an α-hydrogen forming an enolate, which adds to another carbonyl; mild acid work-up gives a β-hydroxy aldehyde/ketone (the aldol), and heating dehydrates it to an α,β-unsaturated carbonyl. Only carbonyls with α-H take part.

Definition

Cannizzaro reaction

Concentrated NaOH disproportionates aldehydes with NO α-hydrogen: HCHO → CH₃OH + HCOONa; benzaldehyde → benzyl alcohol + benzoate. (With both HCHO and MeCHO, crossed Cannizzaro: HCHO is oxidised.)

Carboxylic acids: esterification (ROH/H⁺), acid chloride (PCl₅/SOCl₂/PCl₃), amides (NH₃, heat), reduction (LiAlH₄ → 1° alcohol), HVZ α-halogenation, decarboxylation (NaOH/CaO).
Acidity: acetic pKa ≈ 4.8; EWG raises (Cl₃C > Cl₂CH > ClCH > CH₃ acid), EDG lowers; resonance stabilised carboxylate is the reason.

Aromatic acids special

Benzoic acid does no aldol (no α-H) but does Cannizzaro-like reactions; with PCl₅ it gives benzoyl chloride; its acidity is stronger than phenol but weaker than formic because of resonance/inductive balance.

MCQ Questions

Q11

Which compound undergoes the Cannizzaro reaction?

Q12

The strongest acid among the following is:

NCERT Exercise Questions

From the textbook

NCERT questions for this chapter, in textbook order. Answers are given at a glance below each set.

Exercises (NCERT Q1–Q3)

1
Give a simple chemical test to distinguish between aldehydes and ketones.
2
Why do carboxylic acids have higher boiling points than alcohols of comparable molecular mass?
3
Write the reaction of acetic acid with phosphorus pentachloride.

Answers at a glance

1
Tollens’ reagent (AgNH3)2+ gives a silver mirror with aldehydes only.
2
Due to dimerisation via two hydrogen bonds, carboxylic acids have higher boiling points than comparable alcohols.
3
Acetyl chloride, phosphoryl chloride and hydrogen chloride are formed.

CBSE Pattern Questions

How CBSE asks this chapter

CBSE typically asks 2-3 questions from Aldehydes, Ketones and Carboxylic Acids, focusing on nomenclature, preparation, nucleophilic addition reactions, aldol condensation, Cannizzaro reaction and distinction tests.

MCQ Questions

Q13

Which of the following reagents does not react with both aldehydes and ketones?

Q14

The IUPAC name of CH3COCH2CH2CH3 is:

Q15

Which of the following aldehydes is most reactive towards nucleophilic addition?

Q16

Carboxylic acids do not give which of the following reactions?

1 Mark Questions

1
What is the functional group of aldehydes?
2
Write the chemical equation for the Cannizzaro reaction of formaldehyde.
3
State the Hell‑Volhard‑Zelinsky reaction.

2 Mark Questions

1
Arrange the following in increasing order of reactivity towards nucleophilic addition: acetone, acetaldehyde, formaldehyde, benzaldehyde.
2
How will you obtain benzoic acid from toluene?

3 Mark Questions

1
Explain why aldehydes are generally more reactive than ketones towards nucleophilic addition reactions.
2
Describe the mechanism of aldol condensation.

5 Mark Questions

1
An organic compound (A) with molecular formula C8H8O gives a positive 2,4‑DNP test. It gives a precipitate with ammoniacal silver nitrate and does not reduce Fehling’s solution. Identify A.
2
Explain the following: (i) Why does acetic acid not undergo Friedel‑Crafts reaction? (ii) How will you distinguish between acetaldehyde and benzaldehyde?

PDF: Assignments & Tests

Printable study material

Download topic-wise assignments and full CBSE-pattern tests with answer keys for this chapter.

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Class 12 Chemistry — downloadable assignments and CBSE-pattern tests (PDF).

Assignment 1 — Topic-wise Practice Set A

Topic-wise practice covering the first half of the chapter, with answer key.

aldehydes-ketones-and-carboxylic-acids-class-12-chemistry-notes-assignment-1.pdf

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Assignment 2 — Topic-wise Practice Set B

Topic-wise practice covering the second half of the chapter, with answer key.

aldehydes-ketones-and-carboxylic-acids-class-12-chemistry-notes-assignment-2.pdf

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CBSE Pattern Test 1

Full CBSE-pattern test (90 minutes, 40 marks) with answer key.

aldehydes-ketones-and-carboxylic-acids-class-12-chemistry-notes-test-1.pdf

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CBSE Pattern Test 2

Second full CBSE-pattern test (90 minutes, 40 marks) with answer key.

aldehydes-ketones-and-carboxylic-acids-class-12-chemistry-notes-test-2.pdf

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Quick Revision

Key formulas at a glance

Memorise these equations — direct application numericals and derivations in CBSE & JEE frequently hinge on these.

Carbonyl group

\ce R C H O, \ce R 2 C O, \ce R C O O H

Aldehyde, ketone, carboxylic acid functional units.

Nucleophilic addition

\ce R 2 C = O + N u H - > [H +] R 2 C (O H) N u

Carbonyl carbon is electrophilic.

Aldol condensation

2 \ce C H 3 C H O - > [d i l . N a O H] C H 3 C H (O H) C H 2 C H O

Requires an α-hydrogen.

Cannizzaro reaction

2 \ce H C H O - > [co n c . N a O H] C H 3 O H + H C O O N a

Only aldehydes without α-H.

Iodoform test

\ce C H 3 C O - R - > [I 2/ N a O H] C H I 3 + R C O O N a

For −COCH₃ and CH₃CH₂OH.

Acidity of acetic acid

\ce C H 3 C O O H <=> C H 3 C O O - + H + (K_{a} \approx 1.8 \times 1 0^{- 5})

Acidity directed by the stabilised carboxylate.

Exam Strategy

How this chapter is asked

High-yield question patterns observed across CBSE boards, JEE Main & Advanced, and NEET.

Nomenclature of aldehydes, ketones, acids (alkanals, 2-ones, alkanoic acids) and their common names.
Preparation of aldehydes/ketones: oxidation of alcohols, Friedel–Crafts acylation, ozonolysis, hydration of alkynes (Kucherov).
Nucleophilic addition mechanism and the reactivity order HCHO > RCHO > R₂C=O.
Grignard addition to aldehydes/ketones to make 1° and 2° alcohols (and 3° with esters).
Tollen's (silver mirror), Fehling's (red Cu₂O), Benedict's, 2,4-DNP (yellow ppt) — which positive for what.
Iodoform test for the −COCH₃ group; also positive for ethanol (oxidised to acetaldehyde).
Aldol condensation with α-H; Cannizzaro for HCHO and benzaldehyde.
Rosenmund, Stephen and Etard preparations of aldehydes.
Carboxylic acids: acidity, esterification, acid chloride, amide formation, HVZ α-halogenation, decarboxylation.
Compare acidity: electron-withdrawing groups (Cl₃CCOOH > Cl₂CHCOOH > ClCH₂COOH) and conjugate base stability.

Practice Numericals

Step-by-step worked examples

Standard exam-level problems solved with clear conceptual breakdowns.

Reactivity order in addition

Problem StatementWhy is HCHO more reactive than CH₃CHO than acetone toward nucleophilic addition?

Step-by-Step Solution

1Nucleophile attacks the carbonyl carbon; alkyl groups are electron donating.
2Each CH₃ raises electron density on C=O and shields it sterically.
3Fewer alkyl groups → more positive carbon → faster addition.

Final Answer

HCHO > CH₃CHO > (CH₃)₂CO — both electronic and steric factors.

Iodoform test result

Problem StatementWhich of propanone, propanal, ethanol gives positive iodoform and why?

Step-by-Step Solution

1Iodoform needs the group CH₃CO– (or CH₃CH₂OH oxidisable to it).
2Propanone has CH₃CO–CH₃ → CHI₃ + CH₃COONa.
3Ethanol oxidises to acetaldehyde (CH₃CO–) → positive. Propanal gives CH₃CH₂COOH → no CHI₃.

Final Answer

Propanone and ethanol are positive; propanal is not.

FAQ

Frequently asked questions

What is the difference between aldol and Cannizzaro reactions?

Both are condensation-style reactions of carbonyls in base. Aldol happens only when the aldehyde/ketone has an α-hydrogen: base removes the α-H, the enolate adds to another carbonyl, giving a β-hydroxy aldehyde/ketone (then dehydration→α,β-unsaturated). Cannizzaro happens when there is no α-H (formaldehyde, benzaldehyde): in concentrated NaOH one molecule is oxidised to the acid and another reduced to the alcohol in a single disproportionation.

Why is the carbonyl carbon electrophilic?

Oxygen is more electronegative and pulls the C=O π-electron density toward itself, leaving the carbon electron-poor; and the carbon has no lone pair or excess electron density. A nucleophile therefore attacks the carbon while a proton (or acid) adds to the oxygen. Electron-withdrawing substituents on the carbon make it more electrophilic, alkyl groups make it less.

How does the Grignard reagent convert a carbonyl into an alcohol?

R-MgX delivers its carbanion R⁻ to the carbonyl carbon. With formaldehyde you get a 1° alcohol, with an aldehyde a 2° alcohol, and with a ketone or ester a 3° alcohol. After the addition, acidic work-up protonates the alkoxide. This chain makes the reaction the standard 'name the product' single-step problem.

Why are carboxylic acids more acidic than phenols?

Compare the conjugate bases: carboxylate delocalises the negative charge over two oxygen atoms, while phenoxide spreads it over one oxygen and the ring. The carboxylate's two equivalent resonance forms give it greater stabilisation, lowering the energy of the conjugate base and pushing the equilibrium toward dissociation. This is why acids such as acetic (pKa ~4.8) are far stronger than phenol (~10).

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