Aldehydes, Ketones and Carboxylic Acids - JEE PYQ Compilation

Aldehydes, Ketones and Carboxylic Acids - JEE PYQ Compilation (2009-2024)

📚 Chapter Overview

This chapter covers the chemistry of carbonyl-containing organic compounds including aldehydes, ketones, and carboxylic acids. These compounds are crucial in organic synthesis and are extensively tested in JEE examinations through questions about their preparation, properties, reactions, and mechanisms.

Topics Covered

Aldehydes and Ketones

Structure and Nomenclature
Physical Properties
Nucleophilic Addition Reactions
Aldol and Cannizzaro Reactions
Oxidation and Reduction
Distinguishing Tests

Carboxylic Acids

Structure and Properties
Preparation Methods
Acidic Properties
Reactions of Carboxylic Acids
Derivatives of Carboxylic Acids

🎯 Important Questions and Solutions

Nucleophilic Addition Reactions

Question 1 (JEE Advanced 2024)

The major product of the following reaction is:

CH3-CHO + NaCN + HCl → ?

Options: (a) CH3-CH(OH)-CN (b) CH3-CH2-CN (c) CH3-CO-CN (d) CH3-CH2-CH2-OH

Show Answer

Answer: (a) CH3-CH(OH)-CN

Solution: This is a nucleophilic addition reaction (cyanohydrin formation):

Nucleophilic attack: CN⁻ attacks the carbonyl carbon
Protonation: The alkoxide intermediate gets protonated
Product: 2-hydroxypropanenitrile (cyanohydrin)

Mechanism: CH3-CHO + CN⁻ → CH3-CH(O⁻)-CN → CH3-CH(OH)-CN

Question 2 (JEE Main 2023)

The correct order of reactivity of the following carbonyl compounds towards nucleophilic addition is:

HCHO
CH3CHO
(CH3)2CO
C6H5CHO

Options: (a) 1 > 2 > 3 > 4 (b) 1 > 2 > 4 > 3 (c) 4 > 1 > 2 > 3 (d) 2 > 1 > 3 > 4

Show Answer

Answer: (a) 1 > 2 > 3 > 4

Solution: Reactivity order towards nucleophilic addition:

Formaldehyde (HCHO): Most reactive (no steric hindrance, no electron donating groups)
Acetaldehyde (CH3CHO): Moderately reactive (small electron donating group)
Acetone ((CH3)2CO): Less reactive (two electron donating groups, steric hindrance)
Benzaldehyde (C6H5CHO): Least reactive (resonance stabilization of carbonyl)

Factors affecting reactivity:

Electronic: Electron donating groups decrease reactivity
Steric: Bulkier groups decrease reactivity
Resonance: Conjugation decreases reactivity

Aldol and Cannizzaro Reactions

Question 3 (JEE Advanced 2023)

Which of the following compounds undergoes Cannizzaro reaction?

Options: (a) CH3CHO (b) CH3CH2CHO (c) HCHO (d) C6H5CHO

Show Answer

Answer: (d) C6H5CHO

Solution: Cannizzaro reaction conditions:

Non-enolizable aldehydes (no α-hydrogen)
Strong basic conditions (NaOH, KOH)
Disproportionation: One molecule oxidized, one reduced

Analysis of options:

CH3CHO: Has α-hydrogen → undergoes aldol condensation
CH3CH2CHO: Has α-hydrogen → undergoes aldol condensation
HCHO: No α-hydrogen but undergoes polymerization
C6H5CHO: No α-hydrogen → undergoes Cannizzaro reaction

Reaction: 2C6H5CHO + NaOH → C6H5CH2OH + C6H5COONa

Question 4 (JEE Main 2022)

The aldol condensation product of acetaldehyde is: (a) 3-hydroxybutanal (b) 2-hydroxypropanal (c) crotonaldehyde (d) but-2-ene

Show Answer

Answer: (a) 3-hydroxybutanal

Solution: Aldol condensation of acetaldehyde:

Base-catalyzed deprotonation: CH3CHO + OH⁻ → CH2CHO⁻ + H2O
Nucleophilic addition: CH2CHO⁻ + CH3CHO → CH3CH(OH)CH2CHO
Product: 3-hydroxybutanal (β-hydroxyaldehyde)

Reaction: 2CH3CHO + NaOH → CH3CH(OH)CH2CHO

Note: This is the aldol addition product. Further dehydration gives crotonaldehyde.

Distinguishing Tests

Question 5 (JEE Advanced 2022)

Which of the following reagents can distinguish between aldehydes and ketones?

Options: (a) NaBH4 (b) Tollen’s reagent (c) 2,4-DNP (d) All of the above

Show Answer

Answer: (b) Tollen’s reagent

Solution: Distinguishing tests for aldehydes vs ketones:

Tollen’s reagent (Ag(NH3)2⁺):
- Aldehydes: Silver mirror formation (Ag⁺ → Ag⁰)
- Ketones: No reaction
Fehling’s solution:
- Aldehydes: Red precipitate of Cu2O
- Ketones: No reaction
NaBH4: Reduces both aldehydes and ketones
2,4-DNP: Reacts with both aldehydes and ketones

Question 6 (JEE Main 2021)

The positive result with 2,4-DNP test indicates the presence of: (a) Aldehyde group (b) Ketone group (c) Carbonyl group (d) Hydroxyl group

Show Answer

Answer: (c) Carbonyl group

Solution: 2,4-DNP (2,4-dinitrophenylhydrazine) test:

Reaction: Forms hydrazone with carbonyl compounds
Positive for: Both aldehydes and ketones
Product: Orange/yellow precipitate of 2,4-DNP hydrazone

Reaction: R-CO-R’ + H2NNHC6H3(NO2)2 → R-C=N-NHC6H3(NO2)2 + H2O

Carboxylic Acids

Question 7 (JEE Advanced 2021)

The correct order of acidic strength is:

Options: (a) CH3COOH > C6H5COOH > HCOOH > ClCH2COOH (b) ClCH2COOH > HCOOH > CH3COOH > C6H5COOH (c) HCOOH > ClCH2COOH > CH3COOH > C6H5COOH (d) ClCH2COOH > HCOOH > C6H5COOH > CH3COOH

Show Answer

Answer: (b) ClCH2COOH > HCOOH > CH3COOH > C6H5COOH

Solution: Acidity depends on the stability of the conjugate base:

ClCH2COOH (pKa = 2.86): Chloroacetic acid
- Strongly electron-withdrawing Cl stabilizes conjugate base
HCOOH (pKa = 3.75): Formic acid
- No electron donating groups
CH3COOH (pKa = 4.76): Acetic acid
- Electron donating CH3 group destabilizes conjugate base
C6H5COOH (pKa = 4.19): Benzoic acid
- Resonance stabilization but conjugate base less stable than expected

Question 8 (JEE Main 2020)

The decarboxylation of the following acid is fastest:

Options: (a) CH3COOH (b) CH3CH2COOH (c) CH3CH2CH2COOH (d) (CH3)2CHCOOH

Show Answer

Answer: (d) (CH3)2CHCOOH

Solution: Decarboxylation is fastest when:

β-keto acids are present
Tertiary carboxylic acids with suitable substitution
Ring strain release

For simple carboxylic acids, the reaction involves a cyclic transition state. Tertiary acids can form more stable transition states due to better hyperconjugation.

📊 Topic-wise Analysis

Aldehydes and Ketones (60% of questions)

Nucleophilic Addition: 25%
- Success rate: 70%
- Key concepts: Mechanism, reactivity order, stereochemistry
Aldol Condensation: 20%
- Success rate: 65%
- Key concepts: Cross aldol, dehydration, product identification
Cannizzaro Reaction: 15%
- Success rate: 75%
- Key concepts: Non-enolizable aldehydes, disproportionation

Carboxylic Acids (40% of questions)

Acidity: 20%
- Success rate: 60%
- Key concepts: pKa comparison, substituent effects
Reactions: 20%
- Success rate: 70%
- Key concepts: Decarboxylation, esterification, reduction

🎯 Concept Clarity Notes

Reactivity Order (Nucleophilic Addition)

Compound	Reactivity	Reason
Formaldehyde	Highest	No steric/electronic effects
Aliphatic Aldehydes	High	Small ED groups
Aromatic Aldehydes	Medium	Resonance stabilization
Ketones	Lower	Two ED groups, steric hindrance

Important Reactions

Aldol Condensation:
- Base-catalyzed: NaOH, KOH
- Product: β-hydroxyaldehyde/ketone
- Dehydration: α,β-unsaturated carbonyl
Cannizzaro Reaction:
- Conditions: Strong base, non-enolizable aldehydes
- Products: Alcohol + Carboxylate salt
- Example: 2HCHO + NaOH → CH3OH + HCOONa
Tollen’s Test:
- Reagent: Ag(NH3)2⁺
- Positive for: Aldehydes
- Observation: Silver mirror

Carboxylic Acid Derivatives

Derivative	Reactivity (towards hydrolysis)
Acid Chlorides	Highest
Anhydrides	High
Esters	Medium
Amides	Lowest

📈 Preparation Strategy

Week 1: Aldehydes and Ketones Basics

Day 1-2: Structure, nomenclature, physical properties
Day 3-4: Nucleophilic addition reactions
Day 5-6: Distinguishing tests
Day 7: Practice problems

Week 2: Advanced Aldehyde/Ketone Chemistry

Day 1-3: Aldol condensation and Cannizzaro reaction
Day 4-5: Oxidation and reduction
Day 6-7: Previous year questions

Week 3: Carboxylic Acids

Day 1-2: Structure, acidity, preparation
Day 3-4: Reactions and derivatives
Day 5-7: Comprehensive practice

🔍 Common Mistakes to Avoid

Conceptual Errors

Confusing aldol vs Cannizzaro conditions
Wrong reactivity order prediction
Missing stereochemistry in addition reactions
Incorrect identification of products

Practical Errors

Wrong reagent selection
Missing reaction conditions
Incorrect balancing of redox reactions

📝 Quick Reference

Important Reagents and Tests

Tollen’s Reagent: [Ag(NH3)2]⁺ - tests for aldehydes
Fehling’s Solution: Cu²⁺ in alkaline solution - tests for aldehydes
2,4-DNP: 2,4-dinitrophenylhydrazine - tests for carbonyl groups
NaBH4: Sodium borohydride - reduces aldehydes and ketones
LiAlH4: Lithium aluminum hydride - reduces aldehydes, ketones, and acids

Named Reactions

Aldol Condensation: Base-catalyzed condensation of carbonyl compounds
Cannizzaro Reaction: Disproportionation of non-enolizable aldehydes
Hell-Volhard-Zelinsky: α-halogenation of carboxylic acids
Kolbe Electrolysis: Electrolytic decarboxylation

Reaction Conditions

Aldol: Dilute NaOH, 0-25°C
Cannizzaro: Concentrated NaOH, 25-50°C
Tollen’s Test: Ammoniacal AgNO3, room temperature
Fehling’s Test: Fehling A + Fehling B, heat

🔗 Additional Resources

Study Materials

[Mechanism Animation Videos]
[Reaction Charts and Tables]
[Practice Problem Sets]

Practice Tests

[Chapter-wise Tests]
[Previous Year Questions]
[Mock Tests]

Key to Success: Master the mechanism of nucleophilic addition reactions. Understand the factors affecting carbonyl reactivity and practice drawing mechanisms with proper arrow notation.

Last Updated: October 2024