Aldehydes Ketones and Carboxylic Acids — Previous Year Questions (Class 12 Chemistry)
Aldehydes, ketones, and carboxylic acids are crucial functional groups in organic chemistry. Their reactions and syntheses are fundamental to understanding
TL;DR: Aldehydes, ketones, and carboxylic acids are crucial functional groups in organic chemistry. Their reactions and syntheses are fundamental to understa…
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Aldehydes, ketones, and carboxylic acids are crucial functional groups in organic chemistry. Their reactions and syntheses are fundamental to understanding
Aldehydes Ketones and Carboxylic Acids — Previous Year Questions with Solutions
Q (2023, 2 marks): Distinguish between aldehydes and ketones. Give their structural differences and one example each.
Answer: Aldehydes:
- Carbonyl group (C=O) bonded to one alkyl/aryl group and one hydrogen atom.
- General formula: RCHO
- Structure: R-CHO
- Example: Acetaldehyde (CH3CHO)
- Aldehyde group must be at the terminal position (position 1)
Ketones:
- Carbonyl group (C=O) bonded to two alkyl/aryl groups.
- General formula: RCOR'
- Structure: R-CO-R'
- Example: Acetone (CH3COCH3)
- Carbonyl group is internal (not at terminal position)
Key differences:
1. Position: Aldehydes are terminal, ketones are internal
2. Oxidation: Aldehydes are easily oxidized to carboxylic acids, ketones resist oxidation
3. Reactivity: Aldehydes are more reactive due to hydrogen at carbonyl
4. Tests: Aldehydes give positive Tollens' and Fehling's tests; ketones do not
Q (2022, 3 marks): Explain the mechanism of nucleophilic addition reaction in aldehydes and ketones.
Answer: Nucleophilic addition to carbonyl group (C=O):
The carbonyl carbon is electrophilic due to polar C=O bond (oxygen is more electronegative).
Mechanism (using HCN addition as example):
Step 1: Nucleophilic attack
R2C=O + HCN → R2C⁻-O⁺ (nucleophile CN⁻ attacks electrophilic C)
Transition state: partial C-CN and C-O bonds
Step 2: Protonation
R2C⁻-O⁺ + H⁺ → R2C(OH)-CN
(cyanohydrin formed)
Factor determining reactivity:
1. Aldehydes more reactive than ketones:
- Aldehyde has H (smaller group), easier approach for nucleophile
- Ketone has two bulky alkyl groups (steric hindrance)
2. Electron-withdrawing groups increase reactivity
3. More polar carbonyl → faster addition
Common additions:
- Hydrocyanation: RCHO + HCN → RCH(OH)CN
- Hydration: RCHO + H2O → RCH(OH)2
- Addition of Grignard: RCHO + RMgX → RCH(OH)R'
Q (2023, 3 marks): Explain why carboxylic acids are more acidic than phenols. Compare their acidic strength using Ka values and structure.
Answer: Carboxylic acids (pKa ≈ 4-5) are stronger acids than phenols (pKa ≈ 10).
Reason - Stability of conjugate base:
Carboxylic acid (RCOOH):
- Dissociates: RCOOH ⇌ RCOO⁻ + H⁺
- Carboxylate ion (RCOO⁻) is stabilized by resonance:
RCOO⁻ ←→ RC(O⁻)O⁻ (negative charge shared on both oxygens)
- Both C-O bonds are equivalent (1.5 bond order)
- High stability of conjugate base makes acid stronger
Phenol (C6H5OH):
- Dissociates: C6H5OH ⇌ C6H5O⁻ + H⁺
- Phenoxide ion (C6H5O⁻) has resonance, but:
- Negative charge is dispersed on aromatic ring carbons
- However, this disrupts aromaticity (loses resonance stabilization of benzene)
- Less stabilization than carboxylate
- Less stable conjugate base makes acid weaker
Comparison:
Carboxylic acid: Gains stability when deprotonated (better resonance without losing aromaticity)
Phenol: Loses stability when deprotonated (disrupts aromatic stability)
Hence, carboxylic acids are much stronger acids than phenols.
Q (2022, 3 marks): How are aldehydes and ketones oxidized? Write equations for oxidation of acetaldehyde and acetone.
Answer: Aldehydes:
- Very easily oxidized to carboxylic acids
- Oxidizing agents: K2Cr2O7/H⁺, KMnO4, Ag(NH3)2⁺ (Tollens), Cu(OH) (Fehling)
Example - Acetaldehyde oxidation:
2CH3CHO + K2Cr2O7 + H2SO4 → 2CH3COOH + Cr2(SO4)3 + K2SO4 + H2O
Or: CH3CHO + [O] → CH3COOH
Ketones:
- Resistant to normal oxidizing agents
- Require vigorous oxidation (strong oxidant or high temperature)
- C-C bond cleaves, forming carboxylic acids
Example - Acetone oxidation:
CH3COCH3 + O2 (with catalyst and heat) → CH3COOH + CH3COOH
Or: CH3COCH3 + KMnO4 (conc., heat) → CH3COOH + CO2
Keypoint: Aldehydes oxidize easily; ketones oxidize with difficulty, producing smaller carboxylic acids and CO2 when the central C-C bond breaks.
Q (2023, 2 marks): Describe the preparation of carboxylic acids from primary alcohols and aldehydes.
Answer: Method 1: Oxidation of primary alcohols
RCH2OH + oxidizing agent → RCHO + agent (aldehyde, intermediate)
RCH2OH + stronger oxidizing agent → RCOOH
Examples:
- Ethanol to acetic acid:
CH3CH2OH + K2Cr2O7/H2SO4 → CH3COOH (via CH3CHO)
- Methanol to formic acid:
CH3OH + KMnO4 → HCOOH
Method 2: Oxidation of aldehydes
RCHO + [O] → RCOOH
Examples:
- Acetaldehyde to acetic acid:
CH3CHO + K2Cr2O7/H2SO4 → CH3COOH
- Benzaldehyde to benzoic acid:
C6H5CHO + KMnO4 → C6H5COOH
Common oxidizing agents:
- K2Cr2O7 (acidified)
- KMnO4 (acidified or neutral)
- Ag(NH3)2⁺ (Tollens reagent) - milder
- Cu(OH) in Fehling solution
Q (2021, 3 marks): Explain the reaction of carboxylic acids with alcohols (esterification). Write the mechanism.
Answer: Esterification: RCOOH + R'OH ⇌ RCOOR' + H2O (reversible reaction)
Catalyst: H2SO4 (concentrated)
Example: Acetic acid + Ethanol → Ethyl acetate
CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O
Mechanism (Fischer-Speier esterification):
Step 1: Protonation of carbonyl oxygen
RCOOH + H⁺ → RCOO⁺H (carboxylic acid becomes more electrophilic)
Step 2: Nucleophilic attack by alcohol
RCOO⁺H + R'OH → RC(OH)2⁺-OR' (intermediate, oxygen adds to carbonyl C)
Step 3: Proton transfer
RC(OH)2⁺-OR' → RC(OH)(OR')⁺ + H2O (water leaves as leaving group)
Step 4: Deprotonation
RC(OH)(OR')⁺ → RCOOR' + H⁺ (final ester product)
Key points:
1. Reaction is reversible (equilibrium)
2. High yield requires: excess alcohol, acid catalyst, heat, and removal of water
3. The OH from the carboxylic acid is retained in water (via isotope labeling studies)
4. This is a nucleophilic acyl substitution mechanism
Frequently Asked Questions
What is the difference between oxidation of aldehydes and ketones?
Aldehydes are easily oxidized to carboxylic acids by mild oxidizing agents. The C-H bond in the aldehyde is weak and readily attacked. Ketones are very resistant to oxidation by mild oxidizing agents because they lack the reactive C-H. They require vigorous oxidation (strong oxidant, high heat) that breaks the C-C bonds, producing smaller carboxylic acids and CO2.
Why is esterification an equilibrium reaction and how can the yield be increased?
Esterification is reversible because the reverse reaction (hydrolysis) can occur. Esters react with water to reform the alcohol and carboxylic acid. To shift the equilibrium toward ester formation: (1) use excess alcohol (Le Chatelier), (2) remove water (using desiccants or distillation), (3) use concentrated H2SO4 catalyst, (4) apply heat. These measures drive the reaction forward and increase ester yield.
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