Class 12 Chemistry Chapter 12 Notes: Aldehydes Ketones and Carboxylic Acids | Important Questions & PYQs

 Class 12 Chemistry – Chapter 12: Aldehydes, Ketones & Carboxylic Acids Notes

🔹 12.1 Carbonyl Group (–C=O)

✔ Structure & Nature

• Carbonyl carbon is sp² hybridised

• Shape: Trigonal planar

• Bond is polar → C (δ⁺), O (δ⁻)

• Responsible for nucleophilic addition reactions

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🔹 12.2 Preparation of Aldehydes & Ketones (Key Methods)

Aldehydes

• Oxidation of 1° alcohols (PCC)

• Rosenmund reduction (acid chloride → aldehyde)

• Ozonolysis of alkenes

Ketones

• Oxidation of 2° alcohols

• Friedel–Crafts acylation

• Ozonolysis of alkenes

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🔹 12.3 Physical Properties (Important Order)

✔ Boiling Point Order (NCERT Intext)

CH₃CH₂CH₃ < CH₃OCH₃ < CH₃CHO < CH₃CH₂OH

Reason:

• Alcohol → H-bonding (highest b.p.)

• Aldehyde → dipole–dipole

• Ether → weak polarity

• Alkane → van der Waals only

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🔹 12.4 Chemical Reactions of Aldehydes & Ketones

🔸 Nucleophilic Addition (Very Important)

Reactivity Order

(a) Aliphatic compounds

Butanone < Propanone < Propanal < Ethanal

Reason:

• Aldehydes > Ketones

• Less steric hindrance

• Less +I effect

(b) Aromatic compounds

Acetophenone < p-Tolualdehyde < Benzaldehyde < p-Nitrobenzaldehyde

Reason:

• –CH₃ → +I effect → decreases reactivity

• –NO₂ → –I, –R effect → increases reactivity

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🔸 Important Name Reactions

✔ Aldol Condensation

• Compound must have α-hydrogen

• Gives β-hydroxy aldehyde/ketone

• On heating → α,β-unsaturated compound

✔ Cannizzaro Reaction

• Aldehydes without α-H

• Concentrated alkali

• One molecule oxidised, one reduced

✔ Clemmensen Reduction

• Zn–Hg / conc. HCl

• Carbonyl → alkane (acidic)

✔ Wolff–Kishner Reduction

• NH₂NH₂ / KOH

• Carbonyl → alkane (basic)

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🔹 12.5 Uses of Aldehydes & Ketones

• Formaldehyde → preservative

• Acetone → solvent

• Benzaldehyde → flavouring agent

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🔹 12.6 Carboxyl Group (–COOH)

✔ Structure

• Combination of –C=O and –OH

• Highly polar

• Strong intermolecular H-bonding

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🔹 12.7 Preparation of Carboxylic Acids

• Oxidation of 1° alcohols / aldehydes

• Hydrolysis of nitriles

• Side chain oxidation of alkyl benzene → benzoic acid

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🔹 12.8 Acidity of Carboxylic Acids (Very Important)

✔ Factors Affecting Acidity

1. –I effect increases acidity

2. Closer EWG to –COOH → stronger acid

3. More electronegative atom → stronger acid

✔ NCERT Intext Examples

• CH₂FCOOH > CH₃COOH

• CH₂FCOOH > CH₂ClCOOH

• CH₃CHFCH₂COOH > CH₂FCH₂CH₂COOH

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🔹 12.9 Chemical Reactions of Carboxylic Acids

✔ Esterification

• Acid + Alcohol → Ester + Water

• Reversible reaction

• Remove water → shift equilibrium forward

✔ Decarboxylation

• Sodium salt + soda lime

• CO₂ removed → alkane

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🔹 12.10 Uses of Carboxylic Acids

• Acetic acid → vinegar

• Benzoic acid → preservative

• Fatty acids → soaps, detergents

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🔹 IMPORTANT DEFINITIONS (Direct Questions)

• Cyanohydrin: –OH and –CN on same carbon

• Acetal: Two –OR groups on same carbon

• Hemiacetal: –OR and –OH on same carbon

• Oxime: Carbonyl + NH₂OH

• Semicarbazone: Carbonyl + semicarbazide

• Schiff’s base: Aldehyde/ketone + primary amine

• Ketal: Ketone + dihydric alcohol

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🔹 Tests & Identification (High Weightage)

Test	Observation	Conclusion
Tollens	Silver mirror	Aldehyde
Fehling	Red ppt	Aliphatic aldehyde
Iodoform	Yellow ppt	CH₃CO– group
NaHSO₃	Addition compound	Aldehyde / methyl ketone

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🔹 Why Questions (PYQ Favourite)

✔ Carboxylic acids > Phenols (Acidity)

• Carboxylate ion → better resonance stabilisation

• Negative charge delocalised on two O atoms

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🔹 HOT EXAM ZONE 🔥

✔ Aldol vs Cannizzaro
✔ Reactivity order (aldehyde vs ketone)
✔ Acidity order (–I effect)
✔ Tollens / Iodoform tests
✔ Reason based questions

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What is a carbonyl group?
   The functional group >C=O present in aldehydes and ketones.

2. What is the hybridisation of carbonyl carbon?
   sp² hybridised with trigonal planar geometry.

3. Why are aldehydes more reactive than ketones towards nucleophilic addition?
   Due to less steric hindrance and weaker +I effect.

4. Name a reagent used to oxidise primary alcohols to aldehydes.
   PCC (Pyridinium chlorochromate).

5. Which reaction converts aldehydes and ketones into alkanes in acidic medium?
   Clemmensen reduction.

6. What is Cannizzaro reaction?
   Disproportionation of aldehydes without α-hydrogen in presence of strong alkali.

7. Which compounds give Tollens’ test?
   Aldehydes.

8. What is iodoform test used for?
   To detect CH₃CO– group (methyl ketone).

9. Why do carboxylic acids have high boiling points?
   Due to strong intermolecular hydrogen bonding.

10. Why are carboxylic acids stronger acids than phenols?
    Because carboxylate ion is more resonance-stabilised.

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📝 Long Answer Questions

1. Explain nucleophilic addition reactions of aldehydes and ketones.

Aldehydes and ketones undergo nucleophilic addition due to polar C=O bond.

Reactivity order:

(a) Aliphatic compounds
Butanone < Propanone < Propanal < Ethanal

Reason:

• Aldehydes have less steric hindrance

• Ketones have two alkyl groups with +I effect

(b) Aromatic compounds
Acetophenone < p-Tolualdehyde < Benzaldehyde < p-Nitrobenzaldehyde

Reason:

• Electron-withdrawing groups (–NO₂) increase reactivity

• Electron-donating groups (–CH₃) decrease reactivity

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2. Explain Aldol and Cannizzaro reactions with conditions.

Aldol Condensation

• Aldehydes/ketones having α-hydrogen

• In dilute alkali

• Forms β-hydroxy aldehyde/ketone

• On heating → α,β-unsaturated compound

Cannizzaro Reaction

• Aldehydes without α-hydrogen

• In concentrated alkali

• One molecule oxidised to acid

• One molecule reduced to alcohol

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3. Explain acidity of carboxylic acids.

Carboxylic acids are acidic due to formation of stable carboxylate ion.

Factors affecting acidity:

• –I effect: Electron-withdrawing groups increase acidity

• Distance: Closer EWG → stronger acid

• Electronegativity: More electronegative atom → higher acidity

Examples:

• CH₂FCOOH > CH₃COOH

• CH₂FCOOH > CH₂ClCOOH

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❓ FAQs (Concept Clarity)

1. Why do aldehydes give Tollens’ test but ketones do not?
   Because aldehydes are easily oxidised, ketones are not.

2. Why is acetone used as a solvent?
   Because it dissolves many organic compounds and is volatile.

3. Why does Cannizzaro reaction not occur in aldehydes with α-hydrogen?
   Because they undergo aldol condensation instead.

4. Why is esterification a reversible reaction?
   Because ester and water can react back to form acid and alcohol.

5. Why does –NO₂ group increase acidity of carboxylic acids?
   Due to strong –I and –R effects stabilising the carboxylate ion.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

🧪 NCERT Class 12 Chemistry Notes (Updated)
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