Class 12 Chemistry Chapter 8: Aldehydes, Ketones and Carboxylic Acids | NCERT Notes

Chapter 8: Aldehydes, Ketones and Carboxylic Acids

Class: 12 | Subject: Chemistry | Syllabus: NCERT Rationalised 2025–26

Introduction

Aldehydes, ketones and carboxylic acids are among the most important classes of organic compounds. They contain the carbonyl group (>C=O), which is responsible for their characteristic physical and chemical properties. These compounds occur widely in nature and are essential in biological systems, industrial chemistry, pharmaceuticals, polymers and perfumery.

Understanding this chapter is crucial because it connects several key concepts of organic chemistry, such as nucleophilic addition reactions, oxidation–reduction reactions and acidity trends.

Carbonyl Compounds: Classification

Aldehydes (R–CHO): Carbonyl carbon bonded to at least one hydrogen atom.
Ketones (R–CO–R′): Carbonyl carbon bonded to two carbon atoms.
Carboxylic Acids (R–COOH): Carbonyl group attached to a hydroxyl group.

Nature and Structure of the Carbonyl Group

Hybridisation and Geometry

The carbon atom of the carbonyl group is sp²-hybridised. It forms three sigma (σ) bonds lying in the same plane with bond angles close to 120°, giving a trigonal planar geometry.

Fig 8.1: Orbital overlap and planar geometry of the carbonyl group (NCERT style).

Polarity of Carbonyl Group

Oxygen is more electronegative than carbon. As a result, the C=O bond is polarised, with carbon carrying a partial positive charge and oxygen a partial negative charge.

>C^δ+ = O^δ−

Nomenclature

Aldehydes

In IUPAC nomenclature, the suffix “-al” is used. The carbonyl carbon is always numbered as C-1.

Ketones

Ketones are named using the suffix “-one”, and numbering is done to give the carbonyl group the lowest possible number.

Compound	Common Name	IUPAC Name
HCHO	Formaldehyde	Methanal
CH₃CHO	Acetaldehyde	Ethanal
CH₃COCH₃	Acetone	Propanone
C₆H₅CHO	Benzaldehyde	Benzenecarbaldehyde
C₆H₅COCH₃	Acetophenone	1-Phenylethanone
C₆H₅COC₆H₅	Benzophenone	Diphenylmethanone

STEP-1 जारी है… अगला भाग: Methods of Preparation (Complete NCERT)

⬆ Back to Top

Methods of Preparation of Aldehydes and Ketones

Aldehydes and ketones are prepared by a variety of methods depending upon the nature of the starting compound. These methods are broadly classified into:

General methods (for both aldehydes and ketones)
Methods for aldehydes only
Methods for ketones only

General Methods of Preparation

Oxidation of Alcohols

The oxidation of alcohols is one of the most common laboratory methods for preparing aldehydes and ketones.

Primary alcohols give aldehydes with mild oxidising agents.
Secondary alcohols give ketones.
Tertiary alcohols resist oxidation.

R–CH₂OH + [O] ⟶ R–CHO (using PCC)
R–CH(OH)–R′ + [O] ⟶ R–CO–R′

Dehydrogenation of Alcohols

Alcohol vapours passed over heated copper at about 573 K undergo dehydrogenation.

R–CH₂OH ⟶ R–CHO + H₂
R–CH(OH)–R′ ⟶ R–CO–R′ + H₂

Ozonolysis of Alkenes

Alkenes react with ozone to form ozonides which on reductive hydrolysis yield aldehydes or ketones.

R–CH=CH–R′ ⟶[O₃/Zn,H₂O] R–CHO + R′–CHO

Hydration of Alkynes

Addition of water to alkynes in the presence of dilute sulphuric acid and mercuric sulphate gives carbonyl compounds.

CH≡CH ⟶[HgSO₄/H₂SO₄] CH₃CHO (Acetaldehyde)
R–C≡CH ⟶ Ketone

Preparation of Aldehydes Only

Rosenmund Reduction

Acid chlorides are reduced to aldehydes by hydrogen in the presence of palladium supported on barium sulphate.

R–COCl + H₂ ⟶[Pd/BaSO₄] R–CHO + HCl

Stephen Reaction

Nitriles are reduced with stannous chloride and hydrochloric acid to form imines, which on hydrolysis give aldehydes.

R–CN ⟶[SnCl₂/HCl] R–CH=NH ⟶[H₂O] R–CHO

Etard Reaction

Toluene is oxidised with chromyl chloride to form benzaldehyde.

C₆H₅CH₃ ⟶[CrO₂Cl₂] C₆H₅CHO

Gattermann–Koch Reaction

Benzene reacts with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride to form benzaldehyde.

C₆H₆ + CO + HCl ⟶[AlCl₃/CuCl] C₆H₅CHO

Preparation of Ketones Only

From Acid Chlorides

Acid chlorides react with dialkyl cadmium to give ketones.

R–COCl + R′₂Cd ⟶ R–CO–R′

Friedel–Crafts Acylation

Benzene reacts with acid chlorides in the presence of anhydrous aluminium chloride.

C₆H₆ + R–COCl ⟶[AlCl₃] C₆H₅COR

Physical Properties of Aldehydes and Ketones

They are polar compounds due to the presence of the carbonyl group.
They have higher boiling points than hydrocarbons and ethers of similar molecular mass.
Lower aldehydes and ketones are soluble in water due to hydrogen bonding with water.

Chemical Reactions of Aldehydes and Ketones

Nucleophilic Addition Reactions

The carbonyl group undergoes nucleophilic addition reactions due to the electrophilic nature of the carbonyl carbon.

Addition of Hydrogen Cyanide

R–CHO + HCN ⟶ R–CH(OH)CN

Addition of Sodium Bisulphite

R–CHO + NaHSO₃ ⟶ Bisulphite addition compound

Addition of Grignard Reagent

R–CHO + RMgX ⟶ Alcohol

Addition of Alcohols (Acetals and Ketals)

Aldehydes react with alcohols to form acetals, while ketones form ketals. This reaction is used to protect the carbonyl group.

R–CHO + 2R′OH ⇌ R–CH(OR′)₂ + H₂O

Addition of Ammonia and Its Derivatives

Reagent	Product
NH₂OH	Oxime
NH₂NH₂	Hydrazone
2,4-DNP	2,4-Dinitrophenylhydrazone
Semicarbazide	Semicarbazone

Reduction Reactions

Clemmensen Reduction: R–CO–R′ ⟶[Zn–Hg/HCl] Hydrocarbon
Wolff–Kishner Reduction: R–CO–R′ ⟶[NH₂NH₂/KOH] Hydrocarbon

Oxidation and Distinction Tests

Tollens’ Test: Silver mirror formation (aldehydes).
Fehling’s Test: Red precipitate of Cu₂O (aliphatic aldehydes).
Iodoform Test: Yellow precipitate of CHI₃ (methyl ketones).

Reactions Due to α-Hydrogen

Aldol Condensation

2 CH₃CHO ⟶[NaOH] Aldol ⟶ Dehydration product

Cannizzaro Reaction

2 HCHO ⟶[Conc. KOH] CH₃OH + HCOOK

Carboxylic Acids

Carboxylic acids contain the carboxyl group (–COOH). They are stronger acids than alcohols and phenols due to resonance stabilisation of the carboxylate ion.

Preparation of Carboxylic Acids

R–CH₂OH ⟶[KMnO₄] R–COOH
C₆H₅CH₃ ⟶[KMnO₄] C₆H₅COOH
R–MgX + CO₂ ⟶ R–COOH

Chemical Properties

Acidic Nature

R–COOH + NaHCO₃ ⟶ R–COONa + CO₂ + H₂O

Hell–Volhard–Zelinsky Reaction

R–CH₂–COOH ⟶[X₂/P] R–CHX–COOH

Formation of Anhydrides

2 R–COOH ⟶ (R–CO)₂O + H₂O

Uses

Formaldehyde: Preservation of biological specimens.
Acetone: Industrial solvent.
Acetic acid: Vinegar and chemical manufacture.
Benzoic acid: Food preservative.

⬆ Back to Top

Previous Years’ Board Questions (PYQs: 2016–2025)

The following questions have been selected from CBSE, RBSE, and State Board examinations from the last 10 years. They cover High Order Thinking Skills (HOTS) and frequent concepts.

A. Very Short Answer Questions (1 Mark)

(CBSE 2018) Write the IUPAC name of the compound: CH₃–CH(CH₃)–CH₂–CHO.
(RBSE 2019) Why is carboxylic acid a stronger acid than phenol?
(CBSE 2020) Name the reagent used in the Etard reaction.
(CBSE 2017) Predict the product: Cyclohexanone + HCN.
(All India 2016) Which aldehyde does not undergo Cannizzaro reaction?

B. Short Answer Questions (Reasoning & Conversions) (2–3 Marks)

(CBSE 2019) Arrange the following in increasing order of their boiling points:
CH₃CHO, CH₃CH₂OH, CH₃OCH₃, CH₃CH₂CH₃.
(RBSE 2020) Distinguish between the following pairs using a chemical test:
(a) Propanal and Propanone
(b) Benzaldehyde and Acetophenone
(CBSE 2022) How will you convert:
(a) Ethanal to But-2-enal
(b) Toluene to Benzoic acid
(Delhi 2015) Explain why:
(a) Aldehydes are more reactive than ketones towards nucleophilic addition.
(b) The boiling points of aldehydes are lower than those of corresponding alcohols.
(CBSE 2023) Write the mechanism of nucleophilic addition of HCN to acetone.

C. Long Answer Questions (Word Problems) (5 Marks)

(Classic Board Problem) An organic compound ‘A’ (C₈H₁₆O₂) was hydrolysed with dilute sulphuric acid to give a carboxylic acid ‘B’ and an alcohol ‘C’. Oxidation of ‘C’ with chromic acid produced ‘B’. ‘C’ on dehydration gives But-1-ene. Identify A, B, and C.
(CBSE 2024 Sample)
(a) Write the chemical equation for Hell-Volhard-Zelinsky (HVZ) reaction.
(b) Write the structure of 2,4-DNP derivative of benzaldehyde.
(c) Explain Decarboxylation with an example.

Mock Test Paper: Chapter 8

Aldehydes, Ketones and Carboxylic Acids

Time: 3 Hours | Max Marks: 70

Section A: Multiple Choice Questions (1 × 10 = 10 Marks)

Which compound undergoes Cannizzaro reaction?
(a) Ethanal (b) Propanone (c) Benzaldehyde (d) Acetophenone
The reagent used in Clemmensen reduction is:
(a) Zn-Hg/HCl (b) NH₂NH₂/KOH (c) LiAlH₄ (d) NaBH₄
Which is the strongest acid?
(a) CH₃COOH (b) ClCH₂COOH (c) FCH₂COOH (d) NO₂CH₂COOH
Reaction of acetaldehyde with HCN followed by hydrolysis gives:
(a) Lactic acid (b) Acetic acid (c) Formic acid (d) Propanoic acid
Which of the following does not give Fehling’s test?
(a) Ethanal (b) Propanal (c) Benzaldehyde (d) Methanal
Formalin is a 40% aqueous solution of:
(a) Methanal (b) Ethanal (c) Methanoic acid (d) Ethanoic acid
Iodoform test is given by compounds containing:
(a) –OH group (b) –COOH group (c) CH₃–CO– group (d) –CHO group
Reduction of nitriles using SnCl₂/HCl is called:
(a) Rosenmund reduction (b) Stephen reaction (c) Etard reaction (d) Clemmensen reduction
Hybridisation of carboxyl carbon is:
(a) sp (b) sp² (c) sp³ (d) dsp²
Aldol condensation is characteristic of:
(a) Aldehydes with α-H (b) Aldehydes without α-H (c) Carboxylic acids (d) Alcohols

Section B: Very Short Answer (2 × 5 = 10 Marks)

Why is boiling point of carboxylic acids higher than alcohols?
Write the structure of the product formed when acetone reacts with 2,4-DNP.
Why are carboxylic acids stronger acids than phenols?
What is the function of BaSO₄ in Rosenmund reduction?
Name the electrophile produced in Gatterman-Koch reaction.

Section C: Short Answer (3 × 8 = 24 Marks)

Explain the mechanism of nucleophilic addition of HCN to a carbonyl group.
Distinguish between: (a) Pentan-2-one and Pentan-3-one (b) Benzoic acid and Ethyl benzoate.
Write chemical equations for: (a) Cross Aldol Condensation (b) Wolff-Kishner Reduction.
Arrange in increasing order of reactivity towards nucleophilic addition:
Ethanal, Propanal, Propanone, Butanone. Give reason.
Complete the reaction: Toluene + CrO₂Cl₂/CS₂ ⟶ ? ⟶ ?
An organic compound 'A' (C₃H₆O) does not reduce Tollens' reagent but forms an addition compound with sodium hydrogen sulphite. Identify 'A' and write the reaction.
Explain the effect of (a) Electron withdrawing groups (b) Electron donating groups on the acidity of carboxylic acids.
How will you convert: (a) Benzene to Benzaldehyde (b) Ethanol to Acetone?

Section D: Long Answer (5 × 4 = 20 Marks)

(a) Describe Cannizzaro reaction with mechanism.
(b) Describe HVZ reaction.
(c) Draw the structure of Semicarbazone of Ethanal.
Identify A, B, C:
An organic compound 'A' (C₈H₈O) gives positive Iodoform test. On vigorous oxidation, it gives a carboxylic acid 'B' (C₇H₆O₂). 'B' on reaction with Ammonia and heating gives 'C'. Identify A, B, C and write reactions.
(a) Why is Benzaldehyde less reactive than Propanal in nucleophilic addition?
(b) Explain why carboxylic acids do not give reactions of carbonyl group.
(c) Write the reaction of Ethanoic acid with PCl₅.
(a) Describe the preparation of Aldehydes from (i) Nitriles (ii) Esters.
(b) Explain the cleaning action of soaps (Related concept/or replace with Ozonolysis).

End of Question Paper

Answer Key (Brief Hints)

Section A (MCQs):

(c) Benzaldehyde
(a) Zn-Hg/HCl
(d) NO₂CH₂COOH
(a) Lactic acid
(c) Benzaldehyde
(a) Methanal
(c) CH₃-CO- group
(b) Stephen reaction
(b) sp²
(a) Aldehydes with α-H

Section B (Hints):

Q11: Due to extensive intermolecular hydrogen bonding and dimer formation.
Q13: Resonance stabilisation of carboxylate ion involves two electronegative oxygens, phenoxide ion involves carbon.
Q14: Acts as catalytic poison to prevent further reduction to alcohol.

Section C (Hints):

Q21: Pentan-2-one gives Iodoform test; Pentan-3-one does not. Benzoic acid gives NaHCO₃ test.
Q23: Order: Propanone < Butanone < Propanal < Ethanal. (Steric + Electronic reasons).
Q25: A is Propanone (Acetone).

Section D (Hints):

Q27 (Word Problem):
A: Acetophenone (C₆H₅COCH₃)
B: Benzoic Acid (C₆H₅COOH)
C: Benzamide (C₆H₅CONH₂)

Downloadable Resources

This chapter is designed as an ultimate pillar resource for Class 12 students. To facilitate offline revision, we provide a clean, print-friendly version.

📄 PDF Download:
[Click Here] Download Chapter 8 – Aldehydes, Ketones & Carboxylic Acids (PDF)

Related NCERT Chemistry Chapters

To master Organic Chemistry for Board Exams & NEET/JEE, link this chapter with:

Frequently Asked Questions (FAQ)

Why are aldehydes more reactive than ketones towards nucleophilic addition?

Aldehydes are more reactive due to two reasons: (1) Steric Effect: Less hindrance in aldehydes. (2) Electronic Effect: Ketones have two electron-releasing alkyl groups which reduce the electrophilicity of carbonyl carbon.

What are the most important Named Reactions in Chapter 8?

The 5 most critical named reactions for board exams are: Aldol Condensation, Cannizzaro Reaction, Rosenmund Reduction, Clemmensen Reduction, and Hell-Volhard-Zelinsky (HVZ) Reaction.

Which test is used to distinguish between Propanal and Propanone?

Tollens' Test or Fehling's Test. Propanal (Aldehyde) gives a positive silver mirror or red precipitate, while Propanone (Ketone) does not react.

Why is Benzoic acid stronger than Acetic acid?

Due to the resonance effect of the phenyl ring (though weak), but mainly because the conjugate base (benzoate ion) is resonance stabilised. Note: Acidity orders often depend on solvent, but generally, sp² carbon of phenyl ring is more electronegative than sp³ carbon of methyl group.

Save this summary chart for last-minute revision.

Blogger Post Settings (Copy-Paste):
Labels: Class 12 Chemistry, NCERT Notes, Organic Chemistry, RBSE Board Exam, CBSE Class 12
Search Description: Complete NCERT Notes for Chapter 8 Aldehydes, Ketones and Carboxylic Acids. Includes Structure, Preparation, Named Reactions (Aldol, Cannizzaro), Acidity Trends, PYQs and Test Paper.
Permalink: aldehydes-ketones-carboxylic-acids-class-12-ncert-notes

Ad

Class 12 Chemistry Chapter 8: Aldehydes, Ketones and Carboxylic Acids – NCERT Notes, PYQs & Test