Phenol
Chemistry· Section 10.2 Arenes· 12 min read
1. Structure and Bonding of Phenol★★☆☆☆⏱ 3 min
Phenol consists of a benzene ring covalently bonded to a single hydroxyl (-OH) functional group. Unlike aliphatic alcohols where the hydroxyl group is attached to an sp³ alkyl carbon, the oxygen atom of phenol is adjacent to the sp² hybridised benzene ring, allowing overlap between the lone pair of electrons on oxygen and the delocalised pi system of the aromatic ring.
Activated aromatic ring
A benzene ring with a substituent that donates electron density to the delocalised pi system, increasing its reactivity towards electrophiles relative to unsubstituted benzene.
State one difference in bonding between a phenol molecule and a cyclohexanol molecule.
- 1
First, identify the key structural difference: phenol has the -OH group bonded directly to a benzene ring, cyclohexanol has -OH bonded to a saturated cyclohexane ring.
- 2
Next, describe the bonding consequence: the lone pair on the oxygen of phenol delocalises into the benzene pi system, which cannot happen in cyclohexanol as there is no delocalised pi ring.
- 3
Final answer: The oxygen lone pair in phenol overlaps with the benzene delocalised pi system, increasing electron density of the aromatic ring.
Confirm your understanding of phenol structure:
What is the hybridisation of the carbon atom bonded to the -OH group in phenol?
sp
sp²
sp³
sp⁴
Reveal answer
sp² —All carbon atoms in the benzene ring of phenol are sp² hybridised, enabling the delocalised pi system.
2. Acidity of Phenol★★★☆☆⏱ 3 min
Phenol is significantly more acidic than aliphatic alcohols, but less acidic than carboxylic acids. When phenol dissociates, it forms a phenoxide ion, and the negative charge on the oxygen atom of this ion can delocalise across the 2, 4 and 6 positions of the benzene ring, spreading the charge and stabilising the anion. This stabilisation is not possible for alkoxide ions formed from aliphatic alcohols, where the negative charge is localised entirely on the oxygen atom.
Explain why phenol is more acidic than ethanol.
- 1
Step 1: Write the dissociation reaction for both compounds to form their respective anions.
- 2
Step 2: Compare stability of the anions: the phenoxide ion has its negative charge delocalised across the benzene ring, while the ethoxide ion has localised negative charge on oxygen, destabilised by the positive inductive effect of the ethyl group.
- 3
Step 3: Conclude that a more stable conjugate anion means phenol dissociates more readily than ethanol, making it a stronger acid.
3. Electrophilic Substitution of Phenol★★★☆☆⏱ 4 min
The hydroxyl group on phenol is a 2,4-directing activating group, meaning incoming electrophiles will almost exclusively attack the ortho (position 2 and 6) and para (position 4) positions on the benzene ring. The increased electron density of the ring means phenol undergoes electrophilic substitution far faster than unsubstituted benzene, under much milder reaction conditions.
Reaction | Conditions | Major Products |
|---|---|---|
Bromination | Bromine water, room temperature, no catalyst | 2,4,6-tribromophenol (white precipitate), HBr |
Nitration | Dilute nitric acid, room temperature | Mixture of 2-nitrophenol and 4-nitrophenol |
Sulfonation | Concentrated sulfuric acid, 20°C | 4-phenolsulfonic acid as major product |
Describe what you observe when excess bromine water is added to an aqueous solution of phenol.
- 1
Step 1: Note the initial colour of bromine water is orange/brown.
- 2
Step 2: State the first observation: the orange colour of the bromine water decolourises immediately.
- 3
Step 3: State the second observation: a dense white precipitate of 2,4,6-tribromophenol forms in the solution.
4. Qualitative Identification Tests for Phenol★★☆☆☆⏱ 2 min
There are two standard CIE tests used to confirm the presence of a phenol group in an unknown organic compound. The first is the reaction with neutral iron(III) chloride solution, which forms a characteristic violet coloured iron-phenoxide complex. The second is the reaction with dilute nitric acid to form nitrophenol isomers, but the ferric chloride test is the standard practical assessment test.
Ferric chloride test
Qualitative test for phenol groups that produces an intense violet colour when a few drops of neutral iron(III) chloride solution are added to an aqueous phenol sample.
Suggest one simple chemical test to distinguish phenol from benzoic acid.
- 1
Step 1: Select a reagent that reacts differently with the two compounds: sodium carbonate solution.
- 2
Step 2: State the observation for benzoic acid: as a stronger acid, it will react to produce colourless carbon dioxide gas bubbles.
- 3
Step 3: State the observation for phenol: it is not acidic enough to react with sodium carbonate, so no visible change occurs.
5. Common Pitfalls
Wrong move:
Stating phenol is more acidic than carboxylic acid
Why:
You forget the carboxylate ion has negative charge delocalised across two equivalent oxygen atoms, making it far more stable than the phenoxide ion where charge is spread across the carbon ring
Correct move:
Always use the standard ranking: Carboxylic acid > Phenol > Water > Aliphatic alcohol
Wrong move:
Adding AlBr₃ catalyst for phenol bromination
Why:
The -OH group activates the phenol ring so strongly no Lewis acid is required to generate the electrophile
Correct move:
Use bromine water at room temperature, no catalyst, for full tri-substitution
Wrong move:
Stating the product of phenol bromination is a monobromophenol
Why:
The ring is so activated all three available ortho and para positions get substituted at room temperature
Correct move:
Identify the product as 2,4,6-tribromophenol, a white insoluble precipitate
Wrong move:
Explaining phenol acidity using the positive inductive effect of the -OH group
Why:
Inductive effects are negligible here, the dominant factor is stabilisation of the phenoxide ion via delocalisation of negative charge into the benzene ring
Correct move:
Always reference phenoxide ion stability as the primary reason for phenol's increased acidity
Wrong move:
Stating phenol is fully miscible with cold water
Why:
The large non-polar benzene ring dominates the physical properties, limiting hydrogen bonding with water
Correct move:
Note phenol is only slightly soluble in cold water, and forms a separate oily layer if added in large quantities
6. Quick Reference Cheatsheet
Property | Phenol | Aliphatic Alcohol | Benzene |
|---|---|---|---|
pKa at 298K | ~10 | ~16 | ~40 |
Bromination conditions | Bromine water, RTP, no catalyst | No reaction | Br₂ + AlBr₃, heat |
Bromination product | 2,4,6-tribromophenol | Bromobenzene | |
Ferric chloride test | Violet colour complex | No colour change | No colour change |
Reaction with Na₂CO₃ | No gas produced | No reaction | No reaction |
7. Frequently Asked
Why is no Lewis acid catalyst needed for phenol bromination?
The lone pair of electrons on the oxygen of the hydroxyl group delocalises into the benzene pi system, significantly increasing electron density of the ring, making it far more nucleophilic than benzene, so it can polarise bromine molecules directly without an activating catalyst.
Is phenol classified as a strong acid?
No, phenol is a weak acid, far weaker than carboxylic acids, and only partially dissociates in aqueous solution. It will not produce CO₂ gas when reacted with sodium carbonate, a key distinguishing test from carboxylic acids.
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2024 · Paper 4
Phenol acidity comparison question
- 2023 · Paper 3
Phenol practical identification test
- 2022 · Paper 2
Bromination of phenol reaction outline
What's Next
Mastering phenol properties and reactions builds directly on your prior knowledge of benzene substitution and alcohol acidity, and is a core prerequisite for upcoming arenes topics including aromatic amines and diazonium salt coupling reactions. Phenol reactions are frequently tested alongside carboxylic acid acidity comparisons in A2 structured questions, and appear in almost every CIE A-level practical paper as an unknown organic identification test. You will also use phenol as a starting material for synthesis of high-value organic compounds like aspirin and paracetamol in later organic synthesis modules. Practice applying the 2,4-directing group rules you learned here to other substituted benzene derivatives to reinforce your understanding.