Chapter 12 — Aldehydes, Ketones and Carboxylic Acids — is one of the most reaction-rich and important chapters for 8-10 marks in Board exams. This chapter covers the carbonyl group (C=O), its reactions, and the chemistry of carboxylic acids. Focus on nucleophilic addition, named reactions (Aldol, Cannizzaro, Clemmensen), and distinction tests.
Key Concepts
Aldehydes and Ketones
Preparation
1° alcohol + PCC → Aldehyde (mild, stops at aldehyde)
1° alcohol + K₂Cr₂O₇/H⁺ → Carboxylic acid (strong, goes all the way)
2° alcohol + K₂Cr₂O₇/H⁺ → Ketone
2. Ozonolysis of Alkenes: R₂C=CR₂ + O₃ → ozonide →(Zn/H₂O) aldehydes + ketones
3. Rosenmund Reduction: RCOCl + H₂/Pd-BaSO₄ → RCHO + HCl (acyl chloride → aldehyde)
4. Stephen Reduction: RCN + SnCl₂/HCl → RCHO (nitrile → aldehyde)
5. Gattermann-Koch: C₆H₆ + CO + HCl →(AlCl₃) C₆H₅CHO (benzaldehyde)
6. Friedel-Crafts Acylation: C₆H₆ + RCOCl →(AlCl₃) C₆H₅COR (aromatic ketone)
Nucleophilic Addition Reactions
The C=O group is polar (C^δ⁺=O^δ⁻). Nucleophiles attack the electrophilic carbon.
| Nucleophile | Reagent | Product |
|---|---|---|
| HCN | HCN/KCN | Cyanohydrin (R−CH(OH)CN) |
| NaHSO₃ | Sodium bisulphite | Bisulphite adduct (used for purification) |
| NH₂OH | Hydroxylamine | Oxime (C=NOH) |
| NH₂−NH₂ | Hydrazine | Hydrazone (C=N−NH₂) |
| C₆H₅NH−NH₂ | Phenylhydrazine | Phenylhydrazone |
| 2,4-DNP | 2,4-dinitrophenylhydrazine | 2,4-DNP derivative (yellow/orange ppt — TEST!) |
| NH₂−CO−NH−NH₂ | Semicarbazide | Semicarbazone |
HCHO > CH₃CHO > C₂H₅CHO > (CH₃)₂CO
(More alkyl groups → more steric hindrance + electron donation → less electrophilic carbon → slower)
Oxidation Reactions
(Silver mirror deposited — works for aldehydes ONLY, not ketones)
Fehling’s Test: RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O↓(red ppt) + 3H₂O
(Works for aliphatic aldehydes; aromatic aldehydes don’t respond)
Reduction Reactions
Clemmensen Reduction: R−CO−R’ + Zn-Hg/conc. HCl → R−CH₂−R’ (C=O → CH₂, acidic)
Wolff-Kishner Reduction: R−CO−R’ + NH₂NH₂/KOH/ethylene glycol → R−CH₂−R’ (C=O → CH₂, basic)
Important Named Reactions
On heating: → CH₃CH=CHCHO + H₂O (α,β-unsaturated aldehyde)
Requires: α-hydrogen on the aldehyde/ketone
Cross Aldol: HCHO + CH₃CHO → HOCH₂CH₂CHO
Cannizzaro Reaction: 2HCHO →(conc. NaOH) HCOO⁻Na⁺ + CH₃OH
(Self-oxidation-reduction — one molecule oxidised to acid, one reduced to alcohol)
Requires: NO α-hydrogen (HCHO, C₆H₅CHO, (CH₃)₃CCHO)
Key Rule: If α-H present → Aldol; If no α-H → Cannizzaro
With I₂/NaOH: gives yellow precipitate of CHI₃ (iodoform) — Iodoform Test
Works for: CH₃CHO, methyl ketones (CH₃COR), ethanol, 2° alcohols (CH₃CHOH−R)
Carboxylic Acids (R−COOH)
Preparation
2. From Grignard: RMgX + CO₂ →(dry ether) RCOOMgX →(H₃O⁺) RCOOH
3. Hydrolysis of Nitriles: RCN + H₃O⁺ → RCOOH + NH₄⁺
4. Hydrolysis of Esters: RCOOR’ + NaOH → RCOONa + R’OH (saponification)
Acidity of Carboxylic Acids
Reason: RCOO⁻ is stabilised by resonance — negative charge delocalised over two equivalent oxygen atoms. This makes it exceptionally stable compared to alkoxide or phenoxide.
Test: Carboxylic acids react with NaHCO₃ → CO₂↑ (effervescence)
Phenols do NOT give this test!
Effect of Substituents on Acidity
- Electron-withdrawing groups (−Cl, −NO₂, −CF₃): Increase acidity (stabilise COO⁻)
- Electron-donating groups (−CH₃, −OCH₃, −NH₂): Decrease acidity (destabilise COO⁻)
- Cl₃CCOOH > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH (more Cl → more acidic)
Reactions of Carboxylic Acids
With NH₃: RCOOH + NH₃ → RCOONH₄ →(Δ) RCONH₂ (amide)
Esterification: RCOOH + R’OH →(H₂SO₄) RCOOR’ + H₂O
Decarboxylation: RCOONa + NaOH →(CaO, Δ) RH + Na₂CO₃ (loses CO₂)
Hell-Volhard-Zelinsky (HVZ): RCOOH + Br₂/P → α-bromoacid (halogenation at α-carbon)
Important Definitions
| Term | Definition |
|---|---|
| Nucleophilic Addition | Addition of a nucleophile to the electrophilic C of C=O |
| Aldol Condensation | Self-condensation of aldehydes/ketones with α-H in dilute base |
| Cannizzaro Reaction | Self-redox of aldehydes without α-H in conc. NaOH |
| Iodoform Test | Yellow ppt of CHI₃ with I₂/NaOH — tests for CH₃CO− or CH₃CHOH− |
| Tollen’s Test | Silver mirror test for aldehydes using ammoniacal AgNO₃ |
| Saponification | Alkaline hydrolysis of ester to give soap (sodium carboxylate) |
Solved Examples — NCERT Based
Example 1: Distinguishing Aldehyde from Ketone
Q: How do you distinguish between ethanal and propanone?
Solution:
Tollen’s test: Ethanal (CH₃CHO) gives silver mirror. Propanone (CH₃COCH₃) does not.
Fehling’s test: Ethanal gives red precipitate of Cu₂O. Propanone does not.
Iodoform test: Both give positive iodoform test (both have CH₃CO−). So this test cannot distinguish them.
Example 2: Aldol Condensation
Q: Write the products of aldol condensation of acetaldehyde (CH₃CHO).
Solution:
2CH₃CHO →(dil. NaOH) CH₃CH(OH)CH₂CHO (3-hydroxybutanal — aldol)
On heating: CH₃CH(OH)CH₂CHO → CH₃CH=CHCHO + H₂O (crotonaldehyde)
Example 3: Acidity Order
Q: Arrange in increasing acidity: CH₃COOH, HCOOH, ClCH₂COOH, FCH₂COOH
Solution:
−I effect: F > Cl (F is more electronegative). HCOOH has no alkyl group → more acidic than CH₃COOH.
Order: CH₃COOH < HCOOH < ClCH₂COOH < FCH₂COOH
Example 4: Identifying Reactions
Q: Which reaction will formaldehyde (HCHO) undergo: Aldol or Cannizzaro? Write the products.
Solution:
HCHO has no α-hydrogen → undergoes Cannizzaro reaction.
2HCHO + conc. NaOH → HCOONa (sodium formate) + CH₃OH (methanol)
One molecule is oxidised to formate, the other is reduced to methanol.
Important Questions for Board Exams
1 Mark Questions
- What is the reagent for Tollen’s test?
- Which compound gives both Aldol and Cannizzaro? (None — they’re mutually exclusive!)
- Name the product when acetic acid reacts with SOCl₂.
- Why is HCOOH stronger acid than CH₃COOH?
2 Mark Questions
- Explain why aldehydes are more reactive than ketones towards nucleophilic addition.
- Write the mechanism of Cannizzaro reaction of formaldehyde.
- Give the iodoform test. What structural feature does it detect?
- How do you convert: (a) benzaldehyde → benzoic acid (b) acetone → 2-propanol?
3 Mark Questions
- What is aldol condensation? Give its mechanism. Can ketones undergo aldol condensation?
- Distinguish between: (a) ethanal and propanal (b) acetaldehyde and benzaldehyde (c) pentan-2-one and pentan-3-one.
- Compare the acidity of acetic acid, formic acid, chloroacetic acid and phenol with reasons.
5 Mark Questions
- Discuss the nucleophilic addition reactions of aldehydes and ketones. Why are aldehydes more reactive? Explain with 5 reactions.
- What are carboxylic acids? Discuss their preparation (any 3 methods), acidic character, and important reactions (esterification, SOCl₂, HVZ).
Quick Revision Points
- Nucleophilic addition: HCHO > other aldehydes > ketones (steric + electronic)
- Tollen’s test: Ag mirror (aldehyde only); Fehling’s: Cu₂O red ppt (aliphatic aldehyde only)
- 2,4-DNP test: confirms C=O group (both aldehyde and ketone)
- Iodoform: CH₃CHO, CH₃COR, CH₃CHOH-R give yellow CHI₃
- α-H present → Aldol; no α-H → Cannizzaro
- Clemmensen: Zn-Hg/HCl (acidic) | Wolff-Kishner: NH₂NH₂/KOH (basic) — both reduce C=O to CH₂
- Acidity: RCOOH > PhOH > H₂O > ROH
- EWG increases acidity (−Cl, −NO₂), EDG decreases (−CH₃)
- NaHCO₃ test: carboxylic acids give CO₂, phenols don’t
- HVZ: α-halogenation of carboxylic acids using Br₂/P
Chapter Navigation
Previous: Alcohols, Phenols and Ethers Class 12 Notes
Next: Amines Class 12 Notes
Related Chapters in Class 12 Chemistry
- Alcohols, Phenols and Ethers Class 12 Notes
- Amines Class 12 Notes
- Haloalkanes and Haloarenes Class 12 Notes
Practice What You Learned
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