Phenol and Bromine Water
A concise revision guide to phenol: how the oxygen lone pair is delocalised into the ring, the reaction with bromine water to give 2,4,6-tribromophenol and why phenol reacts more readily than benzene.
Why Phenol Reacts Faster Than Benzene
Choose a step to watch the oxygen lone pair join the ring, bromine react with phenol without a catalyst, and benzene fail to react.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Before You Start
Phenol is a benzene ring carrying an OH group. Its chemistry combines what you know about arenes with what you know about the OH group.
- The three-step mechanism of electrophilic substitution and the horseshoe intermediate: Electrophilic Substitution in Benzene.
- Benzene needs a catalyst to react with bromine, and only one H is replaced: Halogenation of Benzene.
- Alcohols contain an OH group on a saturated carbon: Topic 6 Organic Chemistry I.
Phenol: an OH Group on the Ring
Phenol, C₆H₅OH, has a hydroxy group bonded directly to a carbon atom of the benzene ring. Any compound with an OH bonded straight to an aromatic ring is a phenol.
- Phenol is a white crystalline solid, melting at 43 °C. It is only slightly soluble in water and is corrosive to skin.
- Phenylmethanol, C₆H₅CH₂OH, is not a phenol. Its OH is on a CH₂ carbon, so it is an alcohol and reacts like one.
- Because the oxygen lone pair interacts with the ring, phenol does not behave like an alcohol: it is not oxidised and its OH is not easily substituted.
- Uses: antiseptics and disinfectants, and the manufacture of plastics and resins.
| Phenol, C₆H₅OH | Phenylmethanol, C₆H₅CH₂OH | |
|---|---|---|
| OH bonded to | a ring carbon | a CH₂ carbon outside the ring |
| class | phenol | primary alcohol |
| oxidised by acidified K₂Cr₂O₇? | no | yes, to benzaldehyde then benzoic acid |
| reacts with bromine water? | yes, white precipitate | no |
Remember: Look at the carbon that carries the OH. On the ring: a phenol. One carbon away from the ring: an alcohol.
Why the Ring Is Activated
One of the two lone pairs on the oxygen atom of phenol sits in a p orbital. That p orbital lies parallel to the p orbitals of the ring, so it overlaps sideways with the π system.
- The lone pair is partly delocalised into the ring, so the electron density of the ring increases.
- The increase is greatest at positions 2, 4 and 6, the carbons next to and opposite the OH group.
- An electron-rich ring attracts electrophiles more strongly than benzene does. The ring is said to be activated.
- The oxygen is left slightly electron deficient (δ+) and the ring slightly electron rich (δ−).
The oxygen p orbital overlaps with the π system above and below the ring. Electron density rises at carbons 2, 4 and 6.
Exam focus: Two phrases earn the marks: “a lone pair on the oxygen is delocalised into the ring” and “the electron density of the ring increases”. Give both.
Check: The Structure of Phenol
Pick out phenols from alcohols and explain, with the lone pair, why the ring in phenol is electron rich.
Reaction with Bromine Water
Shake phenol with bromine water at room temperature. No catalyst is needed and the reaction is immediate.
- Observations: the orange bromine water is decolourised and a white precipitate forms. The mixture has an antiseptic smell.
- The precipitate is 2,4,6-tribromophenol, C₆H₂Br₃OH.
- Equation: C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr.
- Three hydrogen atoms are replaced, at the 2, 4 and 6 positions. This is multiple electrophilic substitution.
- An alkene also decolourises bromine water, but by addition and with no precipitate.
Worked example: the equation
Phenol with three molecules of bromine gives 2,4,6-tribromophenol and three molecules of hydrogen bromide.
Before and after: orange bromine water becomes colourless and a white solid settles. A benzene control stays orange.
Remember: Name the product in full: 2,4,6-tribromophenol. The carbon carrying the OH is carbon 1, and the three Br atoms are at 2, 4 and 6.
Quick Check: How Much Bromine Is Used?
Work out how many moles of Br₂ each new compound uses, then rank them.
Phenol Compared with Benzene
Benzene does not react with bromine water at all, and needs a catalyst to react with liquid bromine. Phenol reacts with bromine water at once, three times over. The difference is the oxygen lone pair.
- The lone pair on the oxygen is delocalised into the ring.
- The electron density of the ring in phenol is therefore higher than in benzene.
- The electron-rich ring polarises an approaching Br₂ molecule, inducing a dipole: Br δ+ – Br δ−.
- The ring attacks the δ+ bromine without a catalyst. Benzene cannot polarise Br₂ enough, so it needs Br⁺ made by a catalyst.
- The activated ring reacts at all three favoured positions, so substitution does not stop at one Br.
| Phenol | Benzene | |
|---|---|---|
| reagent | bromine water, Br₂(aq) | liquid bromine, Br₂ |
| catalyst | none | AlBr₃ or FeBr₃ needed |
| conditions | room temperature, shake | room temperature to warm, anhydrous |
| Br atoms substituted | three | one |
| product | 2,4,6-tribromophenol + 3HBr | bromobenzene + HBr |
| seen | decolourised; white precipitate | no change with bromine water |
Reagent, catalyst, conditions, product and observation side by side. The bottom strip gives the three-step explanation.
Exam focus: Three ideas score: the lone pair is delocalised into the ring, the electron density is higher, and Br₂ is polarised so no catalyst is needed.
Check: Phenol and Bromine Water
Describe and explain the bromine water test, and account for the difference between phenol and benzene.
Check: Phenol Against Benzene
Compare the ring in phenol with the ring in benzene and apply the lone-pair explanation to new examples.
Common Exam Points
- A phenol has the OH bonded directly to the ring. Phenylmethanol, C₆H₅CH₂OH, is an alcohol.
- The lone pair on oxygen is in a p orbital and is delocalised into the ring, raising the electron density, most at positions 2, 4 and 6.
- Bromine water: decolourised, white precipitate of 2,4,6-tribromophenol, no catalyst, room temperature.
- C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr: three substitutions, three HBr.
- Benzene does not react with bromine water and needs a catalyst for one substitution, because its lower electron density cannot polarise Br₂.
FAQs
Use these quick answers to check why phenol is activated, the bromine water test and the comparison with benzene.
Why does phenol react with bromine water when benzene does not?
A lone pair on the oxygen is delocalised into the ring, raising the electron density, especially at the 2, 4 and 6 positions. The ring can now polarise Br₂ and attack it without a catalyst, whereas benzene needs pure bromine and a catalyst.
What do I observe when phenol reacts with bromine water?
Shake phenol with bromine water at room temperature: the orange colour disappears and a white precipitate of 2,4,6-tribromophenol forms, with an antiseptic smell. C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr. Three bromine atoms substitute at once.
Is phenol an alcohol?
No. In phenol the OH group is bonded directly to a ring carbon, whereas in an alcohol such as phenylmethanol, C₆H₅CH₂OH, it is on a carbon outside the ring.
The interaction of the lone pair with the ring makes phenol behave differently from alcohols.
How does the bromination of phenol compare with that of benzene?
Benzene needs pure bromine, a catalyst and warming, and gives one substitution, bromobenzene. Phenol needs only dilute bromine water at room temperature and gives three substitutions. The difference comes from the extra electron density that the oxygen lone pair supplies.
Why do the bromine atoms go to the 2, 4 and 6 positions?
The oxygen lone pair is delocalised mainly onto carbons 2, 4 and 6, so those carbons have the highest electron density and attract the electrophile. Carbons 2 and 6 are next to the OH carbon and 4 is opposite it, so three bromine atoms substitute.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
Keep this note — free
Save your progress across every Edexcel topic. A free account remembers which topics you have covered, saves your question scores, and syncs across your phone and laptop.
- Track every topic you have finished
- Keep your practice-question scores
- No payment, no card, free forever
Already registered? Log in