Chapter 11 — Alcohols, Phenols and Ethers — covers three important classes of oxygen-containing organic compounds. This chapter carries 6-8 marks in Board exams and is rich in reactions and conversions. Master the distinction tests, oxidation reactions, and the acidity comparison between alcohols and phenols.
Key Concepts
Alcohols (R−OH)
Classification and Preparation
2. From Grignard Reagent:
HCHO + RMgX → 1° alcohol
RCHO + R’MgX → 2° alcohol
R₂CO + R’MgX → 3° alcohol
3. Reduction of Aldehydes/Ketones: RCHO + H₂/Ni → RCH₂OH (or NaBH₄, LiAlH₄)
4. Hydrolysis of Haloalkanes: RX + aq. KOH → ROH + KX
Reactions of Alcohols
| Reaction | Reagent | Product |
|---|---|---|
| Dehydration | Conc. H₂SO₄, 443 K | Alkene (Saytzeff’s rule) |
| Oxidation (1° alcohol) | PCC or mild oxidant | Aldehyde |
| Oxidation (1° alcohol) | KMnO₄ or K₂Cr₂O₇/H⁺ | Carboxylic acid |
| Oxidation (2° alcohol) | K₂Cr₂O₇/H⁺ | Ketone |
| Oxidation (3° alcohol) | Strong oxidant | Cleaves to smaller molecules |
| Esterification | R’COOH + H₂SO₄ | Ester (R’COOR) + H₂O |
| With Na metal | Na | Alkoxide (R−ONa) + H₂↑ |
| With HBr | HBr | R−Br + H₂O |
3° → immediate turbidity | 2° → turbidity in 5 min | 1° → no turbidity at room temperature
Dehydration Order
Temperature: 1° needs 443 K, 2° needs ~400 K, 3° needs ~350 K
Phenols (ArOH)
Preparation
C₆H₅CH(CH₃)₂ + O₂ → C₆H₅C(CH₃)₂OOH →(H₂SO₄) C₆H₅OH + (CH₃)₂CO
(Phenol + Acetone — both useful products!)
2. From Diazonium Salt: ArN₂⁺Cl⁻ + H₂O →(warm) ArOH + N₂ + HCl
3. Dow Process: C₆H₅Cl + NaOH →(623 K, 300 atm) C₆H₅ONa →(HCl) C₆H₅OH
Acidity of Phenols
Acidity order: Water (pKa 15.7) < Ethanol (15.9) < Phenol (10.0)
Phenol reacts with NaOH but NOT with NaHCO₃ (unlike carboxylic acids)
Reactions of Phenols
Reimer-Tiemann Reaction: C₆H₅OH + CHCl₃ + NaOH → salicylaldehyde (ortho-hydroxybenzaldehyde)
Coupling with Diazonium Salt: C₆H₅OH + C₆H₅N₂⁺Cl⁻ → p-hydroxyazobenzene (orange dye)
Phenol undergoes electrophilic substitution easily (−OH is activating, ortho-para directing):
- Bromination: C₆H₅OH + 3Br₂(aq) → 2,4,6-tribromophenol↓ (white ppt) — used as a test!
- Nitration: With dilute HNO₃ → o- and p-nitrophenol
Ethers (R−O−R’)
Preparation — Williamson Synthesis
Best with primary alkyl halides (SN2). Tertiary halides give elimination (alkene) instead!
Reactions of Ethers
(HI is used because I⁻ is the best nucleophile)
For ArOCH₃ (anisole): ArOCH₃ + HI → ArOH + CH₃I
(C−O bond of aryl group doesn’t break — phenol is formed, not aryl halide)
Important Definitions
| Term | Definition |
|---|---|
| Alcohol | Organic compound with −OH group on saturated carbon |
| Phenol | Organic compound with −OH group directly on benzene ring |
| Ether | Organic compound with C−O−C linkage |
| Dehydration | Removal of water from alcohol to form alkene |
| Williamson Synthesis | Preparation of ether from alkoxide and alkyl halide |
| Kolbe Reaction | Carboxylation of sodium phenoxide to form salicylic acid |
Solved Examples — NCERT Based
Example 1: Grignard Synthesis
Q: How would you prepare propan-1-ol using a Grignard reagent?
Solution:
React ethanal (CH₃CHO) with methylmagnesium bromide (CH₃MgBr):
CH₃CHO + CH₃MgBr → CH₃CH(OMgBr)CH₃ →(H₃O⁺) CH₃CH(OH)CH₃
Wait — this gives propan-2-ol (secondary). For propan-1-ol:
Use HCHO + C₂H₅MgBr → C₂H₅CH₂OH (propan-1-ol, primary)
Example 2: Acidity Comparison
Q: Arrange in increasing acidity: ethanol, phenol, p-nitrophenol, p-methylphenol
Solution:
Electron-withdrawing groups (−NO₂) increase acidity; electron-donating groups (−CH₃) decrease it.
Ethanol < p-methylphenol < phenol < p-nitrophenol
(Most acidic is p-nitrophenol — −NO₂ stabilises the phenoxide ion by delocalisation)
Example 3: Distinction Test
Q: How will you distinguish between ethanol and phenol?
Solution:
Test 1 — FeCl₃ test: Phenol gives violet/purple colour with neutral FeCl₃. Ethanol does not.
Test 2 — Bromine water: Phenol gives white precipitate of 2,4,6-tribromophenol. Ethanol does not react.
Example 4: Williamson Synthesis
Q: Prepare methyl tert-butyl ether (MTBE) using Williamson synthesis. Which alkoxide and which halide should you use?
Solution:
Use sodium methoxide + tert-butyl chloride? NO — 3° halide will give elimination!
Correct method: (CH₃)₃CONa + CH₃I → (CH₃)₃COCH₃
Use tert-butoxide (bulky is fine for alkoxide) + methyl iodide (primary halide → SN2)
Important Questions for Board Exams
1 Mark Questions
- Why is phenol more acidic than ethanol?
- What is Lucas reagent?
- Write the product of Kolbe’s reaction.
- What happens when phenol reacts with bromine water?
2 Mark Questions
- Write the mechanism of acid-catalysed dehydration of ethanol.
- How will you distinguish between 1°, 2° and 3° alcohols?
- What is Williamson ether synthesis? Why should tertiary halides not be used?
- Give the Reimer-Tiemann reaction with equation.
3 Mark Questions
- Explain the preparation of phenol by cumene process. What is the co-product?
- How do you convert: (a) ethanol → ethene (b) phenol → salicylic acid (c) phenol → aspirin?
- Compare the acidity of ethanol, phenol, and p-nitrophenol with reasons.
5 Mark Questions
- Discuss the preparation and chemical reactions of alcohols. Include oxidation, dehydration, esterification, and reaction with Na.
- What are phenols? How are they prepared? Discuss their electrophilic substitution reactions and the Kolbe and Reimer-Tiemann reactions.
Quick Revision Points
- Alcohols: 1° → aldehyde → acid; 2° → ketone; 3° → resistant to oxidation
- Dehydration ease: 3° > 2° > 1° (conc. H₂SO₄)
- Lucas test: 3° immediate, 2° in 5 min, 1° no reaction at RT
- Phenol: acidic (resonance-stabilised phenoxide), reacts with NaOH but not NaHCO₃
- Cumene process: benzene → cumene → phenol + acetone (industrial)
- Kolbe: PhONa + CO₂ → salicylic acid
- Reimer-Tiemann: PhOH + CHCl₃/NaOH → salicylaldehyde
- Williamson: RONa + R’X → ROR’ (use 1° halide to avoid elimination)
- Ether cleavage: R−O−R + HI → 2R−I
- Distinction: phenol → violet with FeCl₃, white ppt with Br₂ water
Chapter Navigation
Previous: Haloalkanes and Haloarenes Class 12 Notes
Next: Aldehydes, Ketones and Carboxylic Acids Class 12 Notes
Related Chapters in Class 12 Chemistry
- Aldehydes, Ketones and Carboxylic Acids Class 12 Notes
- Haloalkanes and Haloarenes Class 12 Notes
- Amines Class 12 Notes
Practice What You Learned
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