Oxidise a primary alcohol to an aldehyde
Distil a primary alcohol with acidified dichromate(VI) or manganate(VII) to obtain an aldehyde.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Distinguish aldehydes from ketones by structure and evidence; state reagent, condition and observation. Written practice is self-checked
Before you begin
- Use carbonyl structures, alcohol oxidation and reduction from Topics 13 and 16, nucleophiles and curly arrows, and the organic reaction patterns from Topic 15.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How should an aldehyde be collected from oxidation of a primary alcohol?
What you will learn
- Recall the acidified oxidising agents
- Use distillation to obtain an aldehyde
- Write a balanced oxidation equation
Distillation removes the aldehyde
A primary alcohol oxidises to an aldehyde when heated with acidified K₂Cr₂O₇ or acidified KMnO₄ and the aldehyde is distilled from the mixture as it forms.
For ethanol: CH₃CH₂OH + [O] → CH₃CHO + H₂O. Removing ethanal limits its further oxidation to ethanoic acid.
The product keeps its carbon skeleton
The terminal –CH₂OH group becomes –CHO; the number and arrangement of carbon atoms stay the same.
Use distillation for the aldehyde target. Reflux keeps volatile material in the oxidising mixture and is used to continue oxidation to a carboxylic acid in Topic 16.
Pause and explain
Which oxidising reagent is listed for primary alcohol oxidation?
Worked example
Give the reagent, apparatus condition and equation for making propanal from propan-1-ol.
Show the worked solution
- Use acidified K₂Cr₂O₇ or acidified KMnO₄.
- Distil propanal as it forms.
- Oxidise the terminal –CH₂OH to –CHO and include water.
Answer CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O; acidified oxidising agent, heat and distil.
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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 oxidation equation and conditions for ethanol to ethanal.
Give me a hint
Distil the aldehyde before it can be oxidised further.
Compare my reasoning
- Ethanol is a primary alcohol.
- Its terminal –CH₂OH becomes –CHO.
- Add [O] and water to balance.
CH₃CH₂OH + [O] → CH₃CHO + H₂O; acidified K₂Cr₂O₇ or KMnO₄, heat and distil.
Look for these in your work
- Aldehyde correct
- Equation balanced
- Distillation stated
A student refluxes ethanol and collects ethanoic acid, but the target was ethanal. What change should be made?
Give me a hint
Compare distillation and reflux.
Compare my reasoning
- Reflux allows continued oxidation of ethanal.
- Distilling ethanal removes it as it forms.
- Use distillation to favor the aldehyde product.
Distil the ethanal from the mixture rather than refluxing to the acid.
Look for these in your work
- Apparatus difference explained
- Product corrected
- Reason stated
Why does oxidation of butan-1-ol to butanal not shorten the carbon chain?
Give me a hint
Oxidation changes the functional group at the end.
Compare my reasoning
- The C1 alcohol carbon becomes the aldehyde carbon.
- No carbon–carbon bond is broken.
- Both molecules have four carbon atoms.
The terminal carbon remains in the chain and changes from –CH₂OH to –CHO; both compounds have four carbons.
Look for these in your work
- Carbon site tracked
- Skeleton retained
- Count checked
Common mistakes
- Refluxing when the aldehyde is the target
- Writing a ketone from a primary alcohol
- Changing the carbon count
What can you explain now?
How should an aldehyde be collected from oxidation of a primary alcohol?
Compare with the explanation
Distil it from the reaction mixture
Distillation removes aldehyde and limits further oxidation.
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Make it stick
How do reagent and distillation conditions produce an aldehyde from a primary alcohol?
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