Explain why alcohols are weaker acids than water
Compare O–H ionisation and use the electron-releasing alkyl group to explain relative acidity.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State reagents and conditions, follow the stated reaction pathway, and distinguish each alcohol class and oxidation product. Written practice is self-checked
Before you begin
- Use functional-group formulae and reaction types from Topic 13, alkene reactions from Topic 14, and halogenoalkane substitution from Topic 15.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What bond supplies the proton when an alcohol acts as an acid?
What you will learn
- Represent alcohol ionisation
- Compare alcohols with water
- Explain the alkyl inductive effect
Alcohols can donate an O–H proton
An alcohol is a weak Brønsted acid: ROH ⇌ H⁺ + RO⁻. In water, the equilibrium lies strongly towards the undissociated alcohol; ionisation is limited.
Water also ionises weakly. In the usual comparison, simple alcohols are less acidic than water, so they donate H⁺ less readily and their conjugate bases RO⁻ are stronger bases than OH⁻.
An alkyl group pushes electron density towards oxygen
The alkyl group has an electron-releasing inductive effect through σ bonds. It increases electron density near O and makes formation of the negatively charged alkoxide less favorable than formation of hydroxide from water.
This is a relative, qualitative explanation. Do not say alcohols are not acidic at all, and do not confuse acidity with the vigorous reaction of alcohols with sodium metal.
Pause and explain
How does the electron-releasing alkyl group affect alcohol acidity relative to water?
Worked example
Compare ethanol's acidity with water and explain the direction using an inductive effect.
Show the worked solution
- Write the ionisation products: ethoxide and H⁺.
- An ethyl group exerts an electron-releasing inductive effect.
- This makes ethoxide formation less favorable than hydroxide formation from water.
Answer Ethanol is a weaker acid than water because the electron-releasing ethyl group makes formation of C₂H₅O⁻ less favorable than formation of OH⁻.
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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.
Write the ionisation equilibrium for propanol and identify its conjugate base.
Give me a hint
Remove H⁺ from the O–H group.
Compare my reasoning
- Start with CH₃CH₂CH₂OH.
- Removing H⁺ leaves CH₃CH₂CH₂O⁻.
- Show the equilibrium arrow for a weak acid.
CH₃CH₂CH₂OH ⇌ H⁺ + CH₃CH₂CH₂O⁻; propoxide is the conjugate base.
Look for these in your work
- O–H proton removed
- Charge correct
- Equilibrium shown
Why is hydroxide less basic than ethoxide in the qualitative acidity comparison?
Give me a hint
Compare the stability of the conjugate bases.
Compare my reasoning
- Water is the stronger acid than ethanol in the stated comparison.
- Its conjugate base OH⁻ is correspondingly the weaker base.
- The ethyl group's +I effect makes ethoxide more electron-rich.
Ethoxide is the stronger base; its electron-releasing ethyl group increases electron density compared with OH⁻.
Look for these in your work
- Conjugate pair logic used
- Base ranking correct
- Inductive effect linked
A student says ethanol is a strong acid because it reacts with sodium. Correct the inference.
Give me a hint
A reaction with a reactive metal does not rank acidity against water by itself.
Compare my reasoning
- Alcohols are weak acids and only partially ionise.
- Sodium can react with the O–H hydrogen, releasing H₂.
- Relative acidity is explained by ionisation and conjugate-base stability.
Ethanol is weakly acidic, but sodium still reacts with its O–H proton; the metal reaction alone does not show ethanol is more acidic than water.
Look for these in your work
- Weak-acid meaning stated
- Metal reaction interpreted
- Comparison corrected
Common mistakes
- Claiming ethanol cannot donate H⁺
- Saying the alkyl group withdraws electron density
- Using the sodium reaction as proof that alcohol is more acidic than water
What can you explain now?
What bond supplies the proton when an alcohol acts as an acid?
Compare with the explanation
The O–H bond
Alcohol acidity involves loss of the O–H proton.
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Make it stick
How does an alkyl group's inductive effect explain why an alcohol is less acidic than water?
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.