Compare carboxylic-acid, phenol and alcohol acidity
Rank typical carboxylic acids above phenol and alcohols in acidity by comparing the stability of their conjugate bases.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 33. Focus on the named compounds, reagents and conditions in each outcome; apply the specified reaction type rather than extending it to unlisted cases. Written practice is self-checked
Before you begin
- Recall carboxylic acids, esters, phenols, alcohols, halogenoalkanes and electrophilic/nucleophilic reaction ideas from earlier topics.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which order gives typical acidity from strongest to weakest?
What you will learn
- Order the three functional-group classes by typical acidity
- Compare carboxylate, phenoxide and alkoxide stability
- Use resonance to explain the ranking
Compare conjugate-base stability
For the usual examples in this syllabus comparison, carboxylic acids are more acidic than phenols, which are more acidic than alcohols: carboxylic acid > phenol > alcohol.
A carboxylate ion delocalises its negative charge over two oxygen atoms. Phenoxide is resonance-stabilised by delocalisation into the aromatic ring, while an alkoxide’s charge is largely localised on one oxygen.
Connect stability to acid strength
A more stable conjugate base makes loss of H⁺ more favourable. The carboxylate’s charge is shared between two electronegative oxygen atoms, giving the strongest acidity of these classes.
Use this as a qualitative ranking for comparable compounds. Substituents can change acidity; the following lesson treats chlorine-substituted carboxylic acids explicitly.
Pause and explain
What particularly stabilises a carboxylate ion?
Worked example
Order ethanoic acid, phenol and ethanol from strongest to weakest acid.
Show the worked solution
- Ethanoic acid forms a carboxylate ion delocalised over two oxygens.
- Phenol forms a resonance-stabilised phenoxide ion.
- Ethanol forms an alkoxide with charge mainly on oxygen.
Answer Ethanoic acid > phenol > ethanol.
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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.
Order propanoic acid, phenol and propan-1-ol by decreasing acidity.
Give me a hint
Compare their conjugate bases.
Compare my reasoning
- Carboxylate has charge delocalised over two oxygens.
- Phenoxide is resonance-stabilised.
- Propoxide has a largely localised oxygen charge.
Propanoic acid > phenol > propan-1-ol.
Look for these in your work
- Carboxylic acid strongest
- Phenol middle
- Alcohol weakest
Explain why a carboxylic acid is typically more acidic than an alcohol.
Give me a hint
Focus on the stability of the ions after H⁺ loss.
Compare my reasoning
- The carboxylic acid forms a carboxylate.
- Its negative charge is shared over two oxygen atoms.
- An alkoxide has the charge mainly localised on one oxygen.
The carboxylate conjugate base is more stabilised by delocalisation over two oxygens than an alkoxide, so the carboxylic acid is stronger.
Look for these in your work
- Conjugate bases identified
- Delocalisation compared
- Acidity linked
A learner says phenol must be more acidic than a carboxylic acid because phenoxide has resonance forms. Refine the explanation.
Give me a hint
Compare how the charge is distributed in each conjugate base.
Compare my reasoning
- Phenoxide is resonance-stabilised.
- Carboxylate shares charge over two oxygen atoms.
- The usual ranking is carboxylic acid > phenol > alcohol.
Phenoxide is stabilised, but carboxylate delocalises charge over two oxygens and is typically more stable; carboxylic acids are more acidic than phenols.
Look for these in your work
- Phenoxide credit retained
- Carboxylate compared
- Ranking corrected
Common mistakes
- Ranking ethanol above phenol because both contain O–H
- Claiming the negative charge is fully transferred off oxygen
- Ignoring carboxylate delocalisation over two oxygens
- Treating the ranking as independent of substituents
What can you explain now?
Which order gives typical acidity from strongest to weakest?
Compare with the explanation
Carboxylic acid > phenol > alcohol
Their conjugate-base stabilisation decreases in this order.
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Make it stick
How does the conjugate-base comparison explain carboxylic acid > phenol > alcohol?
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.