Explain why phenol is acidic
Use resonance delocalisation in the phenoxide ion to explain why phenol can donate H⁺ more readily than an alcohol.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 32. Distinguish phenol (–OH directly on an aromatic ring) from an alcohol (–OH on an aliphatic carbon). Use the specified reagents, temperatures and products; phenol reactions are limited to those listed in the syllabus. Written practice is self-checked
Before you begin
- Recognise alcohols and phenols, aromatic substitution, acids and bases, and diazonium ions from the preceding organic topics.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What stabilises the phenoxide ion?
What you will learn
- Describe phenol ionising to phenoxide
- Explain phenoxide stability using delocalisation
- Connect conjugate-base stability with acidity
Phenol can lose the O–H proton
Phenol acts as a weak acid in water, establishing an equilibrium in which it donates H⁺ to water and forms phenoxide and hydronium ions.
The key comparison is the conjugate base. Removing H⁺ leaves a negative charge on oxygen in phenoxide; this charge can be delocalised into the aromatic π system through resonance.
Delocalisation stabilises phenoxide
A charge spread over more than one atom is more stable than the same charge confined to one atom. Delocalisation stabilises phenoxide, so the equilibrium loss of H⁺ is more favourable than for an ordinary alcohol whose alkoxide charge is localised mainly on oxygen.
This explains acidity qualitatively. Do not say the negative charge is equally shared across the whole ring; oxygen remains an important contributor, while resonance spreads some electron density into the ring.
Pause and explain
Why does a more stable conjugate base correspond to a stronger acid?
Worked example
Explain why phenol is more acidic than ethanol.
Show the worked solution
- Both can lose the hydrogen on oxygen.
- Phenol forms phenoxide; ethanol forms ethoxide.
- Phenoxide is resonance-stabilised, whereas ethoxide has a more localised negative charge.
Answer Phenol is more acidic because its conjugate base, phenoxide, is stabilised by delocalisation into the aromatic ring; ethoxide lacks this stabilisation.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare the conjugate bases of phenol and ethanol.
Give me a hint
Name each anion and state where its negative charge is stabilised.
Compare my reasoning
- Phenol forms phenoxide.
- Ethanol forms ethoxide.
- Phenoxide delocalises charge into the ring; ethoxide is more localised on oxygen.
Phenoxide is resonance-stabilised by the aromatic ring, whereas ethoxide's negative charge is mainly localised on oxygen.
Look for these in your work
- Both conjugate bases named
- Phenoxide stabilisation stated
- Ethoxide contrast stated
Use conjugate-base stability to explain the direction of phenol's acid-base equilibrium with water.
Give me a hint
Show the phenoxide formation and its stabilising feature.
Compare my reasoning
- Phenol transfers H⁺ to water.
- Phenoxide and H₃O⁺ form.
- Delocalisation stabilises phenoxide and supports ionisation more than an unstabilised alkoxide.
The equilibrium can form phenoxide because its negative charge is delocalised into the ring, stabilising the conjugate base and making phenol weakly acidic.
Look for these in your work
- Proton transfer stated
- Products identified
- Stability linked to acidity
A learner says phenol is acidic because the aromatic ring directly breaks the O–H bond. Correct the causal explanation.
Give me a hint
Focus on the species after H⁺ leaves.
Compare my reasoning
- Phenol loses H⁺ to form phenoxide.
- The O–H bond breaks during proton transfer, but that alone does not explain acidity.
- The phenoxide conjugate base is stabilised by delocalisation.
The important explanation is that phenoxide, formed after O–H deprotonation, is stabilised by delocalisation; that stabilisation makes proton loss more favourable.
Look for these in your work
- Ion formed identified
- Bond-breaking distinguished
- Conjugate-base reason stated
Common mistakes
- Explaining acidity by O–H bond polarity alone
- Saying the negative charge moves as a physical particle
- Claiming ethanol cannot lose H⁺ under any conditions
- Confusing resonance stabilisation with a stronger O–H bond
What can you explain now?
What stabilises the phenoxide ion?
Compare with the explanation
Delocalisation of negative charge into the aromatic π system
Resonance spreads some electron density beyond oxygen.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How does resonance stabilisation of phenoxide explain phenol's acidity?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.