Last updated: 23 August 2026
Phenol looks deceptively simple. At first glance, you might imagine that it is just another alcohol because it contains an –OH group. But moving that –OH group from an alkyl carbon onto a benzene ring changes the chemistry dramatically.
The reason lies in electron delocalisation. The oxygen atom does not behave independently of the aromatic ring: one of its lone pairs can interact with the ring's delocalised π electrons. Once you see this connection, two important properties of phenol become much easier to understand: phenol is unusually acidic for a hydroxy compound, and its benzene ring is unusually reactive towards electrophiles.
Try these five questions before reading the chapter. Look carefully at how the –OH group is connected to the benzene ring and think about how this affects acidity and electrophilic substitution.
- 5 diagnostic questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
What makes phenol different from an alcohol?
Phenol has the molecular formula C₆H₅OH. Its defining feature is not simply the presence of an –OH group, but the fact that the oxygen atom is bonded directly to an aromatic carbon atom.
In ethanol, the oxygen is attached to an sp³-hybridised carbon of an alkyl group. In phenol, oxygen is attached directly to the benzene ring. That apparently small structural difference allows one of the oxygen lone pairs to interact with the aromatic π system.
Structure controls reactivity: because the –OH group in phenol is directly conjugated with the aromatic ring, the oxygen atom and the benzene ring influence one another electronically.
Delocalisation between oxygen and the benzene ring
The oxygen atom in phenol contains lone pairs of electrons. One of these lone pairs can overlap with the p orbitals involved in the delocalised π system of the benzene ring.
Electron density can therefore be donated from oxygen into the aromatic ring. This interaction has two particularly important consequences.
Effect on the –OH group: the conjugate base formed after loss of H⁺ is stabilised by delocalisation, making phenol more acidic than ordinary alcohols.
Effect on the benzene ring: donation of electron density from oxygen activates the ring towards electrophilic substitution.
The C–O bond has partial double-bond character
Because the oxygen lone pair interacts with the aromatic π system, the bond between oxygen and the aromatic carbon has some partial double-bond character.
The important examination idea is not merely to memorise that statement. The useful underlying picture is that the oxygen lone pair is electronically connected to the benzene ring.
When explaining the chemistry of phenol, do not stop at saying that “phenol contains a benzene ring”. The useful reasoning is that a lone pair on oxygen interacts with the aromatic π system. That single idea helps explain both the acidity of phenol and the increased reactivity of its ring.
Why is phenol acidic?
According to the Brønsted–Lowry definition, an acid is a proton donor. Phenol can donate the hydrogen atom of its –OH group as H⁺, producing a phenoxide ion.
Phenol is still a weak acid, so only a small proportion of phenol molecules are ionised in water. Nevertheless, it is significantly more acidic than alcohols such as ethanol.
The stability of the phenoxide ion
The best way to understand this difference is to compare the species formed after the acidic proton has been removed.
In the phenoxide ion, the negative charge associated with oxygen can be delocalised into the aromatic π system. The negative charge is therefore not confined entirely to one oxygen atom.
Delocalisation spreads the charge over a larger system and stabilises the phenoxide ion. Because the conjugate base is relatively stable, loss of H⁺ from phenol is more favourable.
When comparing acid strength, compare conjugate-base stability. The more stable the species produced after loss of H⁺, the greater the tendency of the original species to behave as an acid.
Why is ethanol less acidic?
When ethanol loses H⁺, it forms the ethoxide ion, CH₃CH₂O⁻. There is no adjacent aromatic π system into which the negative charge can be delocalised, so the charge remains largely localised on oxygen.
The ethyl group is also electron-donating. It pushes electron density towards an oxygen atom that is already negatively charged, which destabilises the ethoxide ion.
| Species | Conjugate base | Reason for relative acidity |
|---|---|---|
| Phenol | Phenoxide, C₆H₅O⁻ | Negative charge can be delocalised into the aromatic ring, stabilising the ion. |
| Water | Hydroxide, OH⁻ | No electron-donating alkyl group is attached to oxygen. |
| Ethanol | Ethoxide, CH₃CH₂O⁻ | The charge remains localised and the ethyl group donates electron density towards O⁻. |
Acidity order: phenol > water > ethanol.
Central reason: phenoxide is stabilised by delocalisation, whereas ethoxide is destabilised by the electron-donating alkyl group.
Phenol reacting as an acid
The greater acidity of phenol compared with alcohols becomes especially useful when predicting its reactions. Phenol reacts with both sodium metal and aqueous sodium hydroxide.
Reaction with sodium metal
Like alcohols, phenol reacts with sodium metal. The hydrogen atom of the –OH group is replaced by sodium, producing sodium phenoxide and hydrogen gas.
The visible observation is effervescence as hydrogen gas is evolved.
However, this reaction cannot be used to distinguish phenol from ethanol, because ethanol also reacts with sodium metal and releases hydrogen.
Reaction with aqueous sodium hydroxide
A useful chemical distinction between phenol and an ordinary alcohol comes from sodium hydroxide. OH⁻ is sufficiently basic to remove H⁺ from phenol.
Sodium phenoxide is ionic and much more soluble in water than neutral phenol. Ethanol does not undergo the corresponding acid–base reaction with aqueous NaOH to an appreciable extent because ethanol is much less acidic.
This difference is useful chemically, but it is not always the best qualitative test because the visible observation may depend on the physical form and amount of phenol present.
Why phenol does not give the carbonate test
Phenol is more acidic than an alcohol, but it is still much weaker than a carboxylic acid. It therefore does not liberate carbon dioxide from aqueous sodium carbonate or sodium hydrogencarbonate under the usual qualitative test conditions.
This gives an important distinction between phenols and carboxylic acids. A carboxylic acid produces effervescence because CO₂ is formed; phenol does not.
Useful acid-reaction pattern: phenol reacts with Na and NaOH, but it does not give the characteristic CO₂-producing reaction of carboxylic acids with Na₂CO₃ or NaHCO₃.
Testing for phenol with neutral iron(III) chloride
A useful qualitative test for a phenolic –OH group is neutral iron(III) chloride solution, FeCl₃(aq). Phenol gives a characteristic violet or purple colouration.
The colour appears because Fe³⁺ ions form a coloured complex with the phenoxide-type ligand derived from phenol. Ordinary alcohols such as ethanol do not give this characteristic violet or purple colouration.
Test for phenol: add neutral FeCl₃(aq). A violet or purple colouration is a positive test for a phenolic –OH group.
Why neutral FeCl₃ is useful for distinguishing phenol from ethanol
Sodium metal is not suitable because both phenol and ethanol release hydrogen. Aqueous NaOH shows an important chemical difference, but may not always provide the clearest visible test.
Neutral FeCl₃ is more direct: phenol gives a conspicuous violet or purple colouration, whereas ethanol shows no corresponding characteristic colour change.
If an examination question asks for a reagent and an observation to distinguish phenol from ethanol, neutral FeCl₃ is particularly useful: phenol gives a violet or purple colouration, while ethanol does not.
Why is the benzene ring in phenol so reactive?
The same interaction that affects the acidity of phenol also changes the chemistry of its aromatic ring. A lone pair on oxygen is donated into the benzene π system, increasing electron density in the ring.
Electrophiles are attracted to electron-rich regions. The benzene ring in phenol is therefore activated towards electrophilic substitution compared with benzene itself.
The –OH group is activating and 2,4-directing. Electron donation from oxygen increases electron density particularly at the carbon atoms in the 2-, 4- and 6-positions relative to –OH.
Bromination of phenol
Benzene requires a halogen carrier such as FeBr₃ to react readily with bromine. Phenol is much more reactive. It reacts rapidly with aqueous bromine at room temperature without a catalyst.
The orange-brown colour of bromine is decolourised, and a white precipitate of 2,4,6-tribromophenol forms.
Three bromine atoms enter the ring because activation by the –OH group is strong enough for substitution to occur readily at both ortho positions and the para position.
Benzene + Br₂: requires a halogen carrier such as FeBr₃ because benzene is comparatively resistant to electrophilic attack.
Phenol + Br₂(aq): reacts readily at room temperature without a catalyst because the –OH group has strongly activated the ring.
Nitration of phenol
The activated ring also reacts with nitric acid more readily than benzene. Phenol can be nitrated using dilute nitric acid at room temperature.
The main products are a mixture of 2-nitrophenol and 4-nitrophenol.
In contrast, nitration of benzene requires much harsher conditions, typically concentrated nitric acid together with concentrated sulfuric acid.
What does 2,4-directing mean?
Number the carbon bearing the –OH group as carbon 1. The positions adjacent to it are carbon 2 and carbon 6, while the carbon opposite it is carbon 4. Electron donation from oxygen increases electron density particularly at these positions.
The –OH group therefore directs incoming electrophiles preferentially towards the 2- and 4-positions. Position 6 is equivalent to position 2 in an otherwise unsubstituted phenol molecule.
Do not memorise bromination and nitration as two unrelated reactions. Both follow from the same electronic effect: the oxygen lone pair donates electron density into the aromatic ring, making the ring more reactive and favouring substitution at the 2-, 4- and 6-positions.
Using reactions to distinguish phenol from related compounds
Examination questions often ask you to distinguish phenol from an alcohol, a carboxylic acid or benzene. Rather than memorising isolated tests, connect each observation to the structural reason behind it.
| Test | Phenol | What the result tells you |
|---|---|---|
| Neutral FeCl₃(aq) | Violet or purple colouration | Provides a characteristic qualitative test for a phenolic –OH group. |
| NaOH(aq) | Reacts to form sodium phenoxide | Shows that phenol is more acidic than ordinary alcohols such as ethanol. |
| Na₂CO₃(aq) or NaHCO₃(aq) | No CO₂ effervescence | Distinguishes phenol from carboxylic acids. |
| Br₂(aq), room temperature | Decolourises bromine and forms a white precipitate | Shows the strongly activated aromatic ring of phenol. |
Choose a distinguishing reagent that gives a clear difference in observation. Sodium metal is not useful for distinguishing phenol from ethanol because both release H₂. Neutral FeCl₃ is much more suitable because phenol gives a characteristic violet or purple colouration.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Treating phenol as an ordinary alcohol | The –OH group is attached directly to an aromatic ring, so its lone pair interacts with the π system. | Always check whether oxygen is bonded directly to an aromatic carbon before predicting reactions. |
| Saying phenol is more acidic simply because it contains benzene | The presence of a ring alone does not explain the acidity. | Explain that the phenoxide ion is stabilised by delocalisation of the negative charge. |
| Predicting CO₂ when phenol is added to Na₂CO₃ | Phenol is not sufficiently acidic to give the usual carbonate effervescence test. | Reserve CO₂ effervescence with carbonate for stronger acids such as carboxylic acids. |
| Using sodium metal to distinguish phenol from ethanol | Both phenol and ethanol react with sodium and evolve H₂. | Use a test with a genuinely different observation, such as neutral FeCl₃. |
| Using benzene bromination conditions for phenol | The –OH group strongly activates the aromatic ring, so FeBr₃ is unnecessary. | Use Br₂(aq) at room temperature and expect 2,4,6-tribromophenol. |
What you should be able to do
Recognise phenol
Identify a structure in which an –OH group is bonded directly to an aromatic ring.
Explain the acidity of phenol
Use delocalisation and conjugate-base stability to explain why phenol is more acidic than water and ethanol.
Predict acid reactions
Write equations for reactions of phenol with sodium and sodium hydroxide and recognise the absence of carbonate effervescence.
Test for phenol
Recall that neutral FeCl₃ gives a violet or purple colouration with a phenolic –OH group.
Explain ring activation
Relate donation of an oxygen lone pair into the aromatic π system to increased susceptibility towards electrophilic attack.
Recall bromination and nitration
State the reagents, conditions, observations and products for electrophilic substitution of phenol.
These ten questions test whether you can connect the structure of phenol with acidity, qualitative tests and electrophilic substitution.
- 10 mastery questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
Frequently asked questions
Questions students often ask about phenol
Phenol brings together acidity, delocalisation, qualitative analysis and aromatic chemistry, so the same few questions often appear when students first connect these ideas.
Is phenol an alcohol?
No. Although phenol contains an –OH group, the oxygen is bonded directly to an aromatic carbon atom. In an alcohol, the –OH group is bonded to an sp³-hybridised carbon atom. This structural difference produces very different chemical behaviour.
Why is phenol more acidic than ethanol?
Loss of H⁺ from phenol produces the phenoxide ion. Its negative charge can be delocalised into the aromatic π system, which stabilises the ion. In ethoxide, the negative charge remains largely localised on oxygen and is further destabilised by the electron-donating ethyl group.
Why does phenol react with NaOH but ethanol does not?
Phenol is sufficiently acidic for OH⁻ to remove its phenolic H⁺, producing sodium phenoxide and water. Ethanol is less acidic, so aqueous hydroxide does not deprotonate it to an appreciable extent.
What is the neutral FeCl₃ test for phenol?
Phenol gives a violet or purple colouration when neutral iron(III) chloride solution is added. The colour is due to formation of a coloured iron(III) complex involving the phenolic species. Ordinary alcohols such as ethanol do not give this characteristic colouration.
Why does phenol react with bromine without FeBr₃?
A lone pair on oxygen is donated into the aromatic π system, increasing electron density in the ring. The ring therefore attracts electrophiles much more strongly than benzene does, allowing bromination with Br₂(aq) at room temperature without a halogen carrier.
Phenol becomes much easier to understand when its reactions are traced back to one central idea: the oxygen lone pair interacts with the aromatic π system. That interaction stabilises phenoxide, making phenol relatively acidic, while simultaneously donating electron density to the ring and making electrophilic substitution unusually easy. Its characteristic violet or purple reaction with neutral FeCl₃ then provides a useful practical way of recognising a phenolic –OH group.