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LearnYCH11WCH15 · Unit 5: Transition Metals and Organic Nitrogen ChemistryArenesPhenol
Arenes · Subtopic 2 of 2

Phenol

Phenol C₆H₅OH is a weak acid, stronger than water and ethanol but weaker than carboxylic acids: the phenoxide ion is stabilised by delocalisation of the O⁻ charge into the ring. It reacts with Na and NaOH but not with carbonates. A lone pair on oxygen also delocalises into the ring, activating it: phenol decolourises bromine water at room temperature without a catalyst, giving a white precipitate of 2,4,6-tribromophenol, and nitrates with dilute HNO₃. Halogenoarenes (e.g. chlorobenzene) are unreactive to nucleophiles because the halogen lone pair delocalises into the ring, strengthening the C–X bond.

Cambridge 9701 Paper 4Edexcel YCH11 WCH15AQA 7405 Paper 2≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Compare the acidity of phenol, water, ethanol and carboxylic acids.
  • Describe reactions of phenol with Na, NaOH and Br₂(aq).
  • Explain ring activation.
  • Explain why halogenoarenes are unreactive.
Pause and think

Phenol and ethanol both have an –OH. Why does only phenol react with NaOH?

Section 2 of 14

Core idea

Start hereThe antiseptic that changed surgery

Lister's carbolic acid was phenol. It kills bacteria, is weakly acidic, and reacts with bromine water instantly.

The big idea

The oxygen lone pair in phenol delocalises into the ring. This stabilises the phenoxide ion (more acidic than ethanol) and activates the ring towards electrophiles.

1. Acidity

Telling acids apart

Use NaOH and Na₂CO₃.

Worked exampleBromination yield

0.940 g phenol reacts fully with bromine water. Mass of 2,4,6-tribromophenol (MrM_r 331)?

Goal
Mass.
Strategy
1 : 1 ratio.
  1. n=0.94094.0=0.0100n=\frac{0.940}{94.0}=0.0100
    Moles of phenol.
  2. m=0.0100×331=3.31 gm=0.0100\times331=3.31\ \text{g}
    Mass of product.
Answer3.31 g.
✓ Check3 Br₂ consumed per phenol ✓.
Edge caseHalogenoarenes In chlorobenzene the Cl lone pair overlaps with the ring, giving the C–Cl bond partial double-bond character. It resists nucleophilic substitution.

Summary

  • Weak acid between water and RCOOH.
  • NaOH yes, Na₂CO₃ no.
  • Bromine water → white ppt, no catalyst.
  • Chlorobenzene unreactive.
Key words
Phenoxide ion
C₆H₅O⁻, conjugate base of phenol.
Activated ring
Benzene ring with increased electron density, more reactive to electrophiles.
HistoryLister's antiseptic. In 1867 Joseph Lister used phenol (carbolic acid) to sterilise surgical instruments and wounds, dramatically cutting deaths after surgery.
Fun factTCP. TCP antiseptic gets its name from trichlorophenol; the smell of phenols is the classic "hospital" smell.
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