All revision notes topics

Phenol and bromine waterEdexcel International A Level Chemistry: Revision notes

Section 1

The reaction of phenol with bromine water

Phenol, C₆H₅OH, has an OH group attached directly to a benzene ring. When bromine water is added to aqueous phenol at room temperature, with no catalyst, the orange colour disappears and a white precipitate of 2,4,6-tribromophenol forms:

C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr

This is electrophilic substitution: three hydrogen atoms on the ring, at positions 2, 4 and 6, are replaced by bromine atoms. The ring is retained.

Key termsphenol2,4,6-tribromophenol
Common mistake

Calling the reaction addition because the bromine water is decolourised. It is substitution and HBr is formed.

Section 2

Why phenol is more reactive than benzene

Benzene does not react with bromine water. It reacts only with Br₂ and an anhydrous FeBr₃ catalyst.

In phenol, a lone pair of electrons on the oxygen atom in a p orbital overlaps with the delocalised π system of the ring. This increases the electron density of the ring, especially at positions 2, 4 and 6.

The more electron-rich ring polarises Br₂ more strongly, so phenol is more susceptible to electrophilic attack and reacts at room temperature without a catalyst.

Key termslone pairelectron density
Exam tip

Mention both the lone pair on oxygen and the overlap with the π system; one alone does not explain the activation.

Section 3

Comparing phenol, benzene and cyclohexene

  • Cyclohexene: decolourises bromine water by addition; the localised π bond has high electron density.
  • Benzene: no reaction with bromine water; delocalised electron density is too low to polarise Br₂ enough.
  • Phenol: decolourises bromine water and forms a white precipitate by substitution; the ring has increased electron density.
Key termsadditionsubstitution

Section 4

Worked example: percentage yield

0.470 g of phenol (M = 94.0) gives 1.24 g of 2,4,6-tribromophenol (M = 330.7).

Amount of phenol = 0.470 ÷ 94.0 = 5.00 × 10⁻³ mol

Theoretical mass = 5.00 × 10⁻³ × 330.7 = 1.65 g

Percentage yield = 1.24 ÷ 1.65 × 100 = 75.0%

The ratio of phenol to product is 1 : 1, because 3 moles of Br₂ react per mole of phenol but only one product forms.

Key termspercentage yield

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Phenol and bromine water

  1. A student adds bromine water to aqueous phenol, C₆H₅OH, at room temperature. The orange colour of the bromine water disappears and a white precipitate forms.
    Write an equation for the reaction and name its type.2 marks
  2. Benzene does not react with bromine water. It reacts with bromine only when a catalyst of anhydrous iron(III) bromide is present. Phenol, however, reacts with aqueous bromine at room temperature with no catalyst.
    Explain why phenol reacts without a catalyst but benzene needs one.2 marks
  3. A student treats 0.470 g of phenol, C₆H₅OH, with excess bromine water at room temperature and obtains 1.24 g of dry 2,4,6-tribromophenol, C₆H₂Br₃OH. The OH group in phenol is attached directly to the benzene ring.
    Explain why phenol reacts with bromine water more readily than benzene.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).