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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

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Part 1 of 5The Reaction

5.4.4 Phenol Bromination
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.

Part 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.
FeatureBenzenePhenol
ReagentBr₂ with FeBr₃Br₂(aq)
ConditionsAnhydrous, catalystRoom temperature
ProductBromobenzene2,4,6-tribromophenol

Part 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.

Part 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.

  1. Moles of phenol = 0.940 / 94.0 = 0.0100 mol.
  2. The ratio phenoltribromophenol is 1 : 1, so the theoretical mass = 0.0100 x 330.8 = 3.31 g.
  3. Actual mass = 3.31 x 0.800 = 2.65 g.

Answer 2.65 g

Part 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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