5.4.4Unit 5. Transition Metals and Organic Nitrogen Chemistry
Phenol Bromination — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 30 of 43 Unit quiz
Section 1 of 5The Reaction
Reagent
- Bromine water, or bromine in a polar solvent, reacts with phenol at room temperature without a catalyst.
Observation
- The orange colour of bromine disappears and a white precipitate forms.
Product
- 2,4,6-tribromophenol is produced and hydrogen bromide is also formed.
Equation
- C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr.
Section 2 of 5Why Phenol Is More Reactive
Why Phenol Is More Reactive
Lone pair
- A lone pair on the oxygen atom overlaps with the pi system of the ring and becomes partly delocalised into it.
Electron density
- The ring has increased electron density, particularly at the 2, 4 and 6 positions.
Polarisation
- The raised electron density can polarise Br₂, so no halogen carrier is needed.
Direction
- The oxygen atom directs substitution into the 2, 4 and 6 positions, giving all three substituted at once.
Mechanism
- The reaction is still electrophilic substitution, but the activated ring is attacked by a weakly polarised Br₂ without needing a Lewis acid.
| Feature | Benzene | Phenol |
|---|---|---|
| Reagent | Br₂ with FeBr₃ | Br₂(aq) |
| Conditions | Anhydrous, catalyst | Room temperature |
| Product | Bromobenzene | 2,4,6-tribromophenol |
Section 3 of 5Related Reactions
1
Phenol is nitrated by dilute nitric acid at room temperature, whereas benzene needs a concentrated mixture.
2
Phenol is a weak acid and reacts with sodium hydroxide to form sodium phenoxide, but not with sodium carbonate.
3
The O-H group makes phenol soluble in water to a small extent through hydrogen bonding.
4
The white precipitate of 2,4,6-tribromophenol can be used as a test for phenol.
Section 4 of 5Worked Example
Question Calculate the mass of 2,4,6-tribromophenol (Mr 330.8) made from 0.940 g of phenol (Mr 94.0) at 80.0% yield.
- Moles of phenol = 0.940 / 94.0 = 0.0100 mol.
- The ratio phenoltribromophenol is 1 : 1, so the theoretical mass = 0.0100 x 330.8 = 3.31 g.
- Actual mass = 3.31 x 0.800 = 2.65 g.
Answer 2.65 g
Section 5 of 5Exam Checklist & Tips
5.4.4 – Key Points for the Exam
- Phenol An aromatic compound with an OH group on the ring, C₆H₅OH.
- Tribromophenol The white precipitate formed with bromine water.
- Activating group A substituent that increases ring reactivity.
- Directing effect The preferred positions of substitution.
Common Mistakes
- Saying phenol needs FeBr₃ for bromination.
- Forgetting that three bromine atoms are substituted.
- Describing the precipitate as yellow.
- Writing the product with Br at position 3.
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