ℹ️
The lone pair on the –OH oxygen donates electron density into the benzene ring, making it more reactive than benzene toward electrophilic substitution. Substitution occurs at the ortho and para positions.
① Acidic Nature — Reaction with NaOH
1
Phenol is a weak acid (pKₐ ≈ 10). It reacts with NaOH to form sodium phenoxide (soluble salt) and water:
C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O
2
Phenol also reacts with sodium metal:
2C₆H₅OH + 2Na → 2C₆H₅O⁻Na⁺ + H₂↑
3
Unlike carboxylic acids, phenol is too weak an acid to react with Na₂CO₃ or NaHCO₃ — no CO₂ is evolved. This distinguishes it from carboxylic acids.
② Bromination — No Catalyst Required
1
Phenol reacts with bromine water immediately at room temperature — no catalyst needed (unlike benzene which needs AlBr₃/FeBr₃):
C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr
(2,4,6-tribromophenol — white precipitate)
2
Observation: orange-brown bromine water → decolourised + white precipitate of 2,4,6-tribromophenol forms.
3
Substitution occurs at all three ortho/para positions because the –OH group is a very strong activating group.
C₆H₅OH + 3Br₂(aq) → C₆H₂Br₃OH + 3HBr
⚙ Conditions
- Bromine water (Br₂(aq))
- Room temperature
- No catalyst needed
- Product: 2,4,6-tribromophenol (white ppt)
③ Nitration — Mild Conditions
1
Phenol can be nitrated using dilute HNO₃ (unlike benzene which needs concentrated mixed acid). A mixture of ortho- and para-nitrophenol is formed:
C₆H₅OH + HNO₃(dil) → o-NO₂·C₆H₄OH + p-NO₂·C₆H₄OH + H₂O
2
With concentrated mixed acid, further nitration gives 2,4,6-trinitrophenol (picric acid).
C₆H₅OH + HNO₃(dil) → 2-NO₂C₆H₄OH + 4-NO₂C₆H₄OH + H₂O
⚙ Conditions
- Dilute HNO₃ (no H₂SO₄ needed)
- Room temperature or mild warming
- Products: ortho- and para-nitrophenol mixture
④ Esterification — Reaction with Acyl Chlorides / Acid Anhydrides
1
Phenol reacts with acyl chlorides to form phenyl esters. (Phenol does not esterify easily with carboxylic acids alone — acyl chlorides or anhydrides are needed):
C₆H₅OH + CH₃COCl → CH₃COOC₆H₅ + HCl
(phenyl ethanoate)
2
With ethanoic anhydride:
C₆H₅OH + (CH₃CO)₂O → CH₃COOC₆H₅ + CH₃COOH
C₆H₅OH + CH₃COCl → CH₃COOC₆H₅ + HCl
⚙ Conditions
- Acyl chloride or acid anhydride
- Room temperature, no catalyst
- Product: phenyl ester
⑤ Coupling Reaction with Diazonium Salts → Azo Dye
1
Phenol (in alkaline solution as phenoxide C₆H₅O⁻) couples with a diazonium ion at the para position to give an azo dye:
C₆H₅N₂⁺ + C₆H₅OH → C₆H₅–N=N–C₆H₄–OH + H⁺
2
The –N=N– (azo) group is the chromophore responsible for the intense orange/red colour.
C₆H₅N₂⁺ + C₆H₅OH → C₆H₅–N=N–C₆H₄OH + H⁺ (orange/red azo dye)
⚙ Conditions
- Alkaline solution (phenol → phenoxide)
- Cold (0–5°C)
- Coupling at para position
✅
Key comparison with benzene: Phenol reacts with Br₂ without a catalyst (tribromination); benzene needs AlBr₃ and only monosubstitutes. Phenol reacts with dilute HNO₃; benzene needs mixed acid.