Class 12 Chemistry Important Chapter 12 Aldehydes, Ketones and Carboxylic Acids

Class 12 Chemistry Important Chapter 12 Aldehydes, Ketones and Carboxylic Acids Solutions English Medium As Per The New Syllabus to each chapter is provided in the list so that you can easily browse through different chapters ASSEB Class 12 Chemistry Important Solutions in English and select need one. AHSEC Class 12 Chemistry Additional Notes Download PDF. HS 2nd Year Chemistry Additional Solutions.

Class 12 Chemistry Important Chapter 12 Aldehydes, Ketones and Carboxylic Acids

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Aldehydes, Ketones and Carboxylic Acids

Chapter: 12

PART – II

IMPORTANT QUESTION AND ANSWER

1. Why are aldehydes more reactive than ketones towards nucleophilic addition?

Ans: Due to lesser steric hindrance and greater electrophilic character (only one alkyl group in aldehydes vs. two in ketones).

2. What is an aldol condensation? Write its general reaction.

Ans: When aldehydes or ketones with α-hydrogen react with dilute alkali, they form β-hydroxy aldehydes/ketones, which on dehydration give α,β-unsaturated carbonyl compounds.

3. Explain the Hell-Volhard-Zelinsky (HVZ) reaction.

Ans:Carboxylic acids with α-hydrogen react with Cl₂/Br₂ and red phosphorus to give α-halocarboxylic acids.

4. What happens when acetaldehyde reacts with HCN?

Ans: It forms acetaldehyde cyanohydrin, via nucleophilic addition of CN⁻ and H⁺ to the carbonyl group.

5. Discuss the mechanism of nucleophilic addition to the carbonyl group.

Ans: (i) Nucleophile attacks electrophilic carbon of >C=O.

(ii) This forms a tetrahedral intermediate.

(iii) The intermediate then gets protonated to give the additional product.

Example: RCHO + HCN → RCH(OH)CN

6. Compare the acidity of carboxylic acids and phenols. Explain why carboxylic acids are stronger.

Ans: Carboxylic acids (pKa ~4-5) are stronger than phenols (pKa ~10) because:

(i) Carboxylate ion is resonance-stabilized with charge delocalized over two electronegative oxygens.

(ii) Phenoxide ion delocalizes charge on less electronegative carbon as well.

(iii) Thus, the conjugate base of acid is more stable → stronger acid.

7. Write the reactions to distinguish:

(a) Benzaldehyde and Acetophenone.

Ans: Fehling’s test: Benzaldehyde gives ppt., acetophenone does not.

(b) Propanal and Propanone

Ans: Tollens’ test: Propanal gives silver mirror, propanone does not.

8. Explain the mechanism of nucleophilic addition reaction in aldehydes and ketones. Why are aldehydes more reactive than ketones?

Ans: Mechanism:

(i) The carbonyl carbon in >C=O is electrophilic.

(ii) A nucleophile attacks the carbon, forming a tetrahedral alkoxide intermediate.

(iii) This intermediate gets protonated to form the addition product.

General Reaction:

RCHO + Nu⁻+ H + → RCH(OH)Nu

Example:

With HCN:

CH₃CHO + HCN → CH₃CH(OH)CN (cyanohydrin)

Reactivity Comparison:

(i) Aldehydes have one alkyl group and one hydrogen → less steric hindrance.

(ii) Ketones have two alkyl groups → more steric hindrance and electron-donating effects reduce electrophilicity of carbonyl carbon.

9. Describe the Aldol and Cannizzaro reactions with examples.

Ans: (i) Aldol Condensation:

(a) Occurs in aldehydes/ketones with α-hydrogen.

(b) In basic medium, two molecules react to form β-hydroxy aldehyde or ketone.

Example:

(Product: Crotonaldehyde)

Cannizzaro Reaction:

Aldehydes with no α-hydrogen undergo self oxidation-reduction in presence of strong base.

Example: 

10. Discuss the effect of substituents on the acidity of carboxylic acids with examples.

Ans: (i) Carboxylic acids are acidic because the conjugate base (carboxylate ion) is resonance stabilized.

(ii) Electron Withdrawing Groups (EWG) increase acidity.

(iii) Electron Donating Groups (EDG) decrease acidity.

Order of Acidity:

CF₃COOH > CCl₃COOH > CHCl₂COOH > CH₃COOH > C₂H₅COOH

Reason:

(i) EWG like NO₂, Cl, F withdraw electron density and stabilize the negative charge on carboxylate ion.

(ii) EDG like CH₃ donate electrons, destabilizing the ion.