Oxidise alcohols and aldehydes to carboxylic acids
Reflux a primary alcohol or aldehyde with acidified dichromate(VI) or manganate(VII) to make a carboxylic acid.
Section 18 · Carboxylic acids and derivatives
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Track the carbon skeleton, name reagents and conditions, and show products and observations. Written practice is self-checked
Before you begin
- Use alcohol oxidation and carbonyl chemistry from Topics 16–17, functional-group names, acid–base equations and balanced reaction formulae.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which apparatus condition helps oxidise a primary alcohol fully to an acid?
What you will learn
- Recall both starting functional groups
- Name the acidified oxidant
- Use reflux to continue oxidation
Reflux promotes complete oxidation
Heat a primary alcohol or aldehyde with acidified K₂Cr₂O₇ or acidified KMnO₄ under reflux to produce a carboxylic acid. Reflux returns volatile reactants and intermediates to the flask so oxidation can continue.
A primary alcohol first forms an aldehyde; continued oxidation converts –CHO into –COOH. An aldehyde can also be oxidised directly to the acid with the same acidified oxidant and reflux.
Follow the carbon skeleton
Butan-1-ol and butanal both form butanoic acid: the number of carbon atoms is retained.
If the aldehyde is the target, distil it as it forms. If the acid is the target, reflux to continue oxidation. Do not confuse the two apparatus choices.
Pause and explain
Which oxidant is suitable for the syllabus route to a carboxylic acid?
Worked example
Give reagents and conditions to convert propan-1-ol into propanoic acid.
Show the worked solution
- Propan-1-ol is a primary alcohol.
- Use acidified K₂Cr₂O₇ or acidified KMnO₄.
- Reflux so the intermediate aldehyde continues to oxidise.
Answer Heat with acidified K₂Cr₂O₇ or KMnO₄ under reflux: propan-1-ol → propanoic acid.
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.
Convert butanal into butanoic acid. State reagent and condition.
Give me a hint
Continue aldehyde oxidation rather than collecting the aldehyde.
Compare my reasoning
- Use an acidified oxidising agent.
- Heat under reflux.
- Retain all four carbon atoms.
Acidified K₂Cr₂O₇ or KMnO₄, heat under reflux: butanal → butanoic acid.
Look for these in your work
- Oxidant named
- Reflux stated
- Product skeleton retained
A student distils a primary alcohol and collects an aldehyde, but the required product is the acid. What should change?
Give me a hint
Compare the intended product with what distillation does.
Compare my reasoning
- Distillation removes the aldehyde.
- The acid target needs continued oxidation.
- Use reflux with acidified oxidant.
Reflux the primary alcohol with acidified K₂Cr₂O₇ or KMnO₄ so its aldehyde intermediate is oxidised further.
Look for these in your work
- Product difference identified
- Condition changed
- Reason explained
What is the oxidation product of pentan-1-ol under reflux with acidified dichromate?
Give me a hint
Classify the alcohol before naming the product.
Compare my reasoning
- Pentan-1-ol is primary.
- Reflux permits further oxidation.
- Keep five carbons and replace the terminal group with –COOH.
Pentanoic acid.
Look for these in your work
- Starting class used
- Product class correct
- Carbon count retained
Common mistakes
- Distilling when the acid is the target
- Starting from a secondary alcohol
- Changing the chain length
What can you explain now?
Which apparatus condition helps oxidise a primary alcohol fully to an acid?
Compare with the explanation
Reflux
Reflux returns volatile aldehyde so it can be oxidised further.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do starting group and apparatus distinguish aldehyde preparation from carboxylic-acid preparation?
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.