Compare the acidities of water, phenol and ethanol
Use the stability of each conjugate base to place phenol above water and ethanol in acidity.
Section 32 · Hydroxy compounds
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
Which is strongest among water, phenol and ethanol?
What you will learn
- Order water, phenol and ethanol by acid strength
- Compare phenoxide with hydroxide and ethoxide
- Explain the ranking using charge stabilisation
Phenol is the strongest acid in this comparison
The syllabus asks for the relative acidities of water, phenol and ethanol. Phenol is more acidic than both water and ethanol because phenoxide is stabilised by delocalisation into the aromatic ring.
Water and ethanol are much closer in acidity; on the usual aqueous pKa comparison, water is slightly more acidic than ethanol. State the course ordering as phenol > water > ethanol when a single ranking is requested.
Compare the conjugate bases
Water forms hydroxide, phenol forms phenoxide and ethanol forms ethoxide. Phenoxide is stabilised by resonance; hydroxide and ethoxide have charge mainly on oxygen.
Between hydroxide and ethoxide, the ethyl group donates electron density toward oxygen, which destabilises the negative charge slightly relative to hydroxide. This makes ethanol slightly less acidic than water in the stated comparison.
Pause and explain
What is the usual order from stronger to weaker acid?
Worked example
Order water, phenol and ethanol from strongest to weakest acid.
Show the worked solution
- Phenoxide gains resonance stabilisation.
- Hydroxide and ethoxide lack comparable aromatic delocalisation.
- Water is slightly more acidic than ethanol in the usual aqueous comparison.
Answer Phenol > water > 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 the acids and name each conjugate base.
Give me a hint
Compare phenoxide, hydroxide and ethoxide stability.
Compare my reasoning
- Phenol forms phenoxide.
- Water forms hydroxide; ethanol forms ethoxide.
- Phenol > water > ethanol in acidity.
Phenol > water > ethanol; their conjugate bases are phenoxide, hydroxide and ethoxide, respectively.
Look for these in your work
- Ranking correct
- All bases named
- Conjugate pairs matched
Why is water slightly more acidic than ethanol in the usual aqueous comparison?
Give me a hint
Consider the electron-donating effect of the ethyl group in ethoxide.
Compare my reasoning
- Hydroxide has negative charge on oxygen.
- The ethyl group donates electron density toward the oxygen in ethoxide.
- This slightly destabilises ethoxide relative to hydroxide, making ethanol slightly weaker as an acid.
The ethyl group pushes electron density toward O⁻ in ethoxide, making it slightly less stable than hydroxide; therefore ethanol is slightly less acidic than water.
Look for these in your work
- Conjugate bases compared
- Inductive effect stated
- Relative acidity concluded
A student gives phenol < water < ethanol. Identify the reversed comparison and supply the correct order.
Give me a hint
Use phenoxide resonance and the hydroxide/ethoxide distinction.
Compare my reasoning
- Phenol is strongest because phenoxide is delocalised.
- Water is slightly stronger than ethanol.
- Write the order from strongest to weakest.
The order is reversed. From strongest to weakest: phenol > water > ethanol.
Look for these in your work
- Phenol placement corrected
- Water/ethanol order corrected
- Full ranking given
Common mistakes
- Putting ethanol above phenol because it has an O–H group
- Ignoring conjugate-base stability
- Calling the small water/ethanol difference a large one
- Claiming phenoxide charge is fully transferred to carbon
What can you explain now?
Which is strongest among water, phenol and ethanol?
Compare with the explanation
Phenol
Phenoxide is stabilised by delocalisation.
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Make it stick
How do the three conjugate bases explain the order phenol > water > ethanol?
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.