Learning Objectives
- Understand classification, nomenclature, and preparation of alcohols, phenols, and ethers
- Study reactions of alcohols and phenols
- Learn about acidity of alcohols and phenols
- Understand ether preparation and reactions
- Study commercially important alcohols and phenols
Key Concepts
Alcohols (R-OH)
Classification: 1° (RCH₂OH), 2° (R₂CHOH), 3° (R₃COH). Methanol (CH₃OH), Ethanol (C₂H₅OH).
Preparation:
- Hydration of alkenes: CH₂=CH₂ + H₂O → CH₃CH₂OH (H₃PO₄ catalyst, Markovnikov).
- Hydroboration-oxidation: anti-Markovnikov addition. R-CH=CH₂ → RCH₂CH₂OH.
- Grignard reaction: RMgX + R'CHO → R'CH(OH)R (after hydrolysis).
- Reduction of aldehydes/ketones: RCHO + NaBH₄/LiAlH₄ → RCH₂OH.
- Fermentation of sugars: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (yeast).
Reactions:
- With Na: 2ROH + 2Na → 2RONa + H₂ (test for active hydrogen).
- Esterification: ROH + R'COOH ⇌ R'COOR + H₂O (acid catalyst). Fischer esterification.
- Dehydration: ROH → alkene (H₂SO₄, 443K). Zaitsev's rule. 1° alcohols need higher temperature.
- Oxidation: 1° → aldehyde → carboxylic acid. 2° → ketone. 3° → resistant (require strong oxidant → cleavage).
- PCC (pyridinium chlorochromate): selective, stops at aldehyde.
- Lucas test: ROH + conc. HCl/ZnCl₂. 3° immediate turbidity, 2° in 5 min, 1° no reaction at RT.
- Victor Meyer's test: Distinguishes 1°, 2°, 3° alcohols by colour with HNO₂ and NaOH.
Phenols (ArOH)
Preparation: From cumene (cumene hydroperoxide process): C₆H₅CH(CH₃)₂ + O₂ → C₆H₅OH + CH₃COCH₃.
From diazonium salt: ArN₂⁺ + H₂O → ArOH + N₂ + HCl (warm).
Acidity: Phenol (pKₐ ≈ 10) is more acidic than alcohols (pKₐ ≈ 16-18) due to resonance stabilisation of phenoxide ion. Electron-withdrawing groups increase acidity (p-nitrophenol > phenol). Electron-donating groups decrease acidity.
Acidity order: p-nitrophenol > phenol > ethanol > water.
Reactions:
- Kolbe's reaction: C₆H₅ONa + CO₂ → o-hydroxybenzoic acid (salicylic acid). High T and P.
- Reimer-Tiemann reaction: Phenol + CHCl₃ + NaOH → salicylaldehyde (o-hydroxybenzaldehyde).
- EAS (o/p directing, activating): Nitration, halogenation (Br₂/H₂O gives 2,4,6-tribromophenol white precipitate), Friedel-Crafts.
- FeCl₃ test: Phenol gives violet/blue colour with FeCl₃ (characteristic test).
Ethers (R-O-R')
Preparation: Williamson synthesis: R-ONa + R'-X → R-O-R' + NaX (best method; use 1° alkyl halide to avoid elimination).
Dehydration of alcohols at 413K with H₂SO₄: 2C₂H₅OH → C₂H₅OC₂H₅ + H₂O.
Reactions:
- Cleavage by HI: R-O-R' + HI → RI + R'OH (excess HI → RI + R'I + H₂O). Weaker C-O bond cleaves.
- Ethers are relatively unreactive (good solvents).
Summary
Alcohols are prepared by hydration, reduction, or Grignard reactions and undergo dehydration, oxidation, and esterification. Phenols are more acidic than alcohols due to resonance and give specific reactions (Kolbe, Reimer-Tiemann). Ethers are prepared by Williamson synthesis and are cleaved by HI. These compounds are widely used as solvents, intermediates, and in industry.
Important Terms
- Dehydration: Removal of water to form alkene
- Esterification: Reaction of alcohol with acid to form ester
- Williamson Synthesis: Ether preparation from alkoxide and alkyl halide
- Kolbe's Reaction: Carboxylation of phenol
- Reimer-Tiemann: Formylation of phenol using CHCl₃/NaOH
Quick Revision
- Acidity: ArOH > H₂O > ROH (phenol > water > alcohol)
- Lucas test: 3° immediate, 2° in 5 min, 1° no reaction at RT
- Oxidation: 1° → RCHO → RCOOH; 2° → R₂CO; 3° → resistant
- Phenol + Br₂/H₂O → 2,4,6-tribromophenol (white ppt)
- Williamson: RONa + R'X → ROR' (use 1° RX)
- Ether + HI → alcohol + alkyl iodide