1. Phenols: classification, names and preparation
Mono-, di-, trihydric phenols: phenol; cresols (methylphenols); catechol (1,2-), resorcinol (1,3-), quinol (1,4-diol); pyrogallol (1,2,3-triol).
Preparation: (1) ; (2) Dow: C₆H₅Cl + NaOH, 623 K, 300 atm, then acid; (3) C₆H₅SO₃Na fused with NaOH (573 K), then acid; (4) cumene process: benzene + propene → cumene → (O₂) cumene hydroperoxide → (H⁺) phenol + propanone.
| Method | Key condition |
|---|---|
| Diazonium salt | Warm with water |
| Dow | 623 K, 300 atm |
| Sulphonate fusion | NaOH, 573 K |
| Cumene | O₂, then H⁺ |
Formulas
- Cumene hydroperoxide cleavage
Key terms
- Cumene
Isopropylbenzene, C₆H₅CH(CH₃)₂, made from benzene and propene.
2. Physical properties, acidity and reactions of the –OH group
Phenol: colourless solid (m.p. 316 K), corrosive, turns pink in air; H-bonding → high b.p.; moderately soluble in water. 2-Nitrophenol: intramolecular H-bond → more volatile (steam distillable) than 4-nitrophenol.
Acidity: pKa 10.0 (ethanol 15.9) — phenoxide resonance. o/p-NO₂ increase acidity (4-nitro 7.1; picric acid 0.4); –CH₃, –OCH₃ decrease. Dissolves in NaOH, not in NaHCO₃.
Reactions: NaOH, Na → C₆H₅ONa; Zn dust → C₆H₆; CH₃COCl or (CH₃CO)₂O → phenyl ethanoate; PCl₅ → poor yield of C₆H₅Cl (C–O partial double bond), mainly (C₆H₅O)₃PO.
Formulas
- Phenol with NaOH
Key terms
- Phenoxide ion
C₆H₅O⁻, stabilised by delocalisation of its negative charge into the ring.
- Intramolecular hydrogen bond
A hydrogen bond within one molecule, e.g. in 2-nitrophenol.
3. Electrophilic substitution, named reactions, tests and uses
–OH activates (+R), o/p directing. Br₂(aq) → 2,4,6-tribromophenol (white ppt); Br₂/CS₂, 273 K → 4- (and 2-) bromophenol. Dilute HNO₃ → 2- and 4-nitrophenol; conc. HNO₃ → picric acid. Conc. H₂SO₄: 2-sulphonic acid at about 290 K, 4- at about 373 K.
Kolbe: C₆H₅ONa + CO₂ (400 K, 4–7 atm), H⁺ → salicylic acid. Reimer–Tiemann: CHCl₃ + NaOH (via :CCl₂), H⁺ → salicylaldehyde.
Tests: neutral FeCl₃ → violet; azo dye with benzenediazonium chloride (alkaline, cold) → orange 4-hydroxyazobenzene; bromine water → white ppt. Uses: antiseptics, Bakelite, aspirin, dyes, picric acid.
| Test | Phenol |
|---|---|
| FeCl₃ (neutral) | Violet |
| Br₂(aq) | White ppt |
| Diazonium salt, alkaline | Orange dye |
Formulas
- Bromine water
- Reimer–Tiemann
Key terms
- Kolbe reaction
Carboxylation of sodium phenoxide with CO₂ to give salicylic acid.
- Reimer–Tiemann reaction
Formylation of phenol with CHCl₃ and NaOH to give salicylaldehyde.
4. Aliphatic ethers
O sp³; C–O–C ≈ 111.7°; named as alkoxyalkanes (2-ethoxypropane). Williamson: RONa + primary R′X (SN2); tertiary groups go on the alkoxide (tertiary halides eliminate). Also 2ROH → ROR (conc. H₂SO₄, 413 K).
Physical: no O–H → b.p. close to alkanes; accept H-bonds → lower ethers soluble like alcohols; volatile, flammable solvents.
Reactions: Cl₂ (dark: α-chloro; light: perchloro); O₂ (air, light) → explosive peroxides (test FeSO₄/KSCN; remove with acidified FeSO₄); PCl₅ → 2RCl + POCl₃; HI: cold 1 mol → smaller (or tertiary) group becomes RI + alcohol; hot excess → 2RI.
Formulas
- Hot excess HI
Key terms
- Williamson synthesis
Preparation of an ether from an alkoxide (or phenoxide) and a primary alkyl halide.
- Ether peroxides
Explosive compounds formed slowly when ethers stand in air and light.
5. Anisole
Preparation: C₆H₅ONa + CH₃I → C₆H₅OCH₃ + NaI. –OCH₃ activates (+R), o/p directing, para major: Br₂ in ethanoic acid (no catalyst) → 4-bromoanisole; HNO₃/H₂SO₄ → 4-nitroanisole; CH₃Cl/AlCl₃ → 4-methylanisole; CH₃COCl/AlCl₃ → 4-methoxyacetophenone. HI → phenol + CH₃I (aryl C–O does not break).
Uses of ethers: ethoxyethane — solvent, Grignard reactions, early anaesthetic; methoxymethane — refrigerant, propellant; anisole — perfumes.
| Reagent | Major product |
|---|---|
| Br₂/CH₃COOH | 4-Bromoanisole |
| HNO₃/H₂SO₄ | 4-Nitroanisole |
| CH₃COCl/AlCl₃ | 4-Methoxyacetophenone |
| HI | Phenol + CH₃I |
Formulas
- Cleavage of anisole
Key terms
- Anisole
Methoxybenzene, C₆H₅OCH₃.
Exam tips
Common mistakes
- Saying phenol reacts with NaHCO₃.
- Giving monobromophenol for bromine water (it is 2,4,6-tribromophenol).
- Using a tertiary halide in Williamson’s synthesis.
- Writing iodobenzene as a product of anisole + HI.
How marks are usually awarded
- Acidity (2 marks): resonance structures of phenoxide; substituent effects.
- Named reactions (1–2 marks): reagents, conditions and product names.
- Ether cleavage (1–2 marks): products with the mechanism that decides them.