Aldehydes, Ketones and Carboxylic Acids – Complete CBSE Class 12 Chemistry Notes

Unit 8 | 8 marks | CBSE 2026–27

1. Carbonyl Group

Aldehydes and ketones contain the polar carbonyl group, C=O. The carbonyl carbon is electrophilic, which explains nucleophilic addition reactions.

2. Preparation and Properties

Aldehydes can be obtained by controlled oxidation of suitable primary alcohols; ketones by oxidation of suitable secondary alcohols. Further reactions depend on structure and reagent.

3. Nucleophilic Addition

4. Important Reactions

• Oxidation of aldehydes to carboxylic acids.
• Reduction of aldehydes/ketones to alcohols.
• Tollens’ test and Fehling’s test as characteristic reactions of suitable aldehydes.
• Aldol-type reactions for compounds with suitable α-hydrogen.

5. Carboxylic Acids

Carboxylic acids contain –COOH. Their acidic character arises because the carboxylate ion formed after loss of H⁺ is resonance-stabilised.

6. Reactions of Carboxylic Acids

Important reactions include esterification, formation of salts and reduction under suitable conditions.

7. Reactivity Comparison

🧠 Reaction Map

📝 Case-Based Question

An unknown compound gives a positive Tollens’ test and on oxidation forms a carboxylic acid.

1. What functional group is most likely present?
2. Why does Tollens’ reagent respond to suitable aldehydes?
3. What type of product is formed on oxidation?
4. How would reduction change the functional group?

🎯 Board Practice

1. Distinguish aldehydes and ketones using characteristic tests.
2. Write nucleophilic addition steps conceptually.
3. Complete oxidation/reduction conversions.
4. Explain acidity of carboxylic acids.
5. Write esterification reaction and identify its products.

MCQs

1. The carbonyl carbon is electrophilic. True.
2. Suitable aldehydes can give Tollens’ test. True.
3. Ketones are generally more readily oxidised than aldehydes under ordinary mild conditions. False.
4. Carboxylate ion is resonance-stabilised. True.

Carbonyl Reaction Map

Nucleophilic Addition

The polar carbonyl group has an electrophilic carbon. Nucleophiles attack this carbon, followed by proton-transfer steps where appropriate. This helps explain many carbonyl reactions rather than memorising them independently.

Oxidation and Reduction

Aldehydes are readily oxidised to carboxylic acids, while ketones generally resist mild oxidation. Reduction of aldehydes gives primary alcohols; reduction of ketones gives secondary alcohols.

Important Tests

Distinguish aldehydes and ketones using the prescribed chemical tests and understand what observable change indicates a positive result.

Carboxylic Acids

The carboxyl group is acidic because its conjugate base is resonance-stabilised. Electron-withdrawing substituents can increase acidity.

Named/Characteristic Reactions

• Aldol-type condensation where prescribed.
• Cannizzaro reaction for suitable aldehydes lacking α-hydrogen.
• Decarboxylation and esterification reactions.
• Reduction and oxidation conversions.

Common Mistakes

• Confusing aldehyde and ketone oxidation behaviour.
• Forgetting the role of α-hydrogen in aldol/cannizzaro questions.
• Writing incorrect oxidation products.

Practice

1. Predict carbonyl addition products.
2. Distinguish aldehydes and ketones from observations.
3. Complete multi-step organic conversions.
4. Explain acidity of carboxylic acids using resonance.

Aldehydes, Ketones & Carboxylic Acids — Carbonyl Chemistry

Nucleophilic Addition

The carbonyl carbon is electrophilic because oxygen attracts electron density. Nucleophiles therefore attack the carbonyl carbon in appropriate reactions.

Distinguishing Aldehydes and Ketones

Aldehydes are generally more readily oxidised than ketones. Use the prescribed qualitative tests and observations carefully rather than relying only on names.

Carboxylic Acid Acidity

Carboxylate ions are resonance-stabilised, which contributes to the acidic character of carboxylic acids. Substituents can increase or decrease acidity through inductive effects.

Worked Reaction Reasoning

For an aldehyde subjected to a suitable mild oxidation test, identify the aldehyde as the species that is oxidised to the corresponding carboxyl compound while the reagent undergoes the characteristic visible change specified in the syllabus.

CBSE Practice

1. Predict nucleophilic-addition products.
2. Distinguish aldehydes from ketones using prescribed tests.
3. Complete oxidation/reduction conversions.
4. Compare acidity of substituted carboxylic acids.
5. Construct multistep organic conversions.