Oxidise a primary alcohol to an aldehyde by distillation
Use acidified dichromate(VI) or manganate(VII) and distil the aldehyde as it forms.
Section 16 · Hydroxy compounds
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
How is an aldehyde obtained from a primary alcohol before it oxidises further?
What you will learn
- Recognise primary alcohol oxidation
- Choose distillation to obtain an aldehyde
- State acidified oxidising reagents
Distillation limits further oxidation
A primary alcohol oxidised with acidified K₂Cr₂O₇ or acidified KMnO₄ can form an aldehyde. Distil the aldehyde from the reaction mixture as it forms to reduce further oxidation.
Ethanol → ethanal: CH₃CH₂OH + [O] → CH₃CHO + H₂O. The carbon skeleton is retained and the terminal –CH₂OH group becomes –CHO.
Use one oxidation equivalent
The syllabus permits [O] as a representation of an oxygen atom supplied by the oxidising agent in organic equations. Include water when writing the balanced oxidation equation.
Refluxing a primary alcohol instead keeps it in contact with the oxidising mixture and favors the carboxylic acid, taught separately. State the apparatus condition as well as the reagent.
Pause and explain
Which oxidising reagent is suitable for primary alcohol oxidation in the syllabus?
Worked example
Describe how to obtain propanal from propan-1-ol.
Show the worked solution
- Use acidified K₂Cr₂O₇ or acidified KMnO₄.
- Heat while distilling the product from the mixture.
- Oxidise the terminal –CH₂OH to –CHO.
Answer CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O; acidified K₂Cr₂O₇ (or KMnO₄), heat and distil the propanal.
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.
Give conditions and an equation for making ethanal from ethanol.
Give me a hint
Distil the carbonyl compound as it forms.
Compare my reasoning
- Use an acidified oxidising agent.
- Oxidise CH₃CH₂OH to CH₃CHO.
- Show water as the other product.
CH₃CH₂OH + [O] → CH₃CHO + H₂O; acidified K₂Cr₂O₇ or KMnO₄, heat and distil.
Look for these in your work
- Correct product
- Reagent stated
- Distillation stated
A student refluxes propan-1-ol but wants propanal as the main product. What should change?
Give me a hint
Compare distillation with reflux.
Compare my reasoning
- Reflux leaves the primary alcohol and aldehyde in the oxidising mixture.
- Further oxidation to propanoic acid can occur.
- Distil the aldehyde from the mixture as it forms.
Use distillation rather than reflux to obtain propanal and limit further oxidation.
Look for these in your work
- Reflux consequence explained
- Product pathway stated
- Apparatus corrected
Why should the product carbon count not change during oxidation of propan-1-ol to propanal?
Give me a hint
Oxidation changes the functional group, not the carbon framework.
Compare my reasoning
- The terminal C–OH carbon becomes the aldehyde carbon.
- No carbon–carbon bond is added or removed.
- Both compounds contain three carbons.
The carbon skeleton is retained; the terminal alcohol carbon becomes –CHO, so both molecules have three carbons.
Look for these in your work
- Site tracked
- Skeleton retention stated
- Count verified
Common mistakes
- Refluxing when the target is the aldehyde
- Oxidising the primary alcohol directly to a ketone
- Omitting acidification from the oxidising reagent
What can you explain now?
How is an aldehyde obtained from a primary alcohol before it oxidises further?
Compare with the explanation
Distil it from the reaction mixture
Removing the aldehyde as it forms limits further oxidation.
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Make it stick
Why is an aldehyde distilled from an oxidising mixture rather than refluxed when it is the target?
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.