Oxidise a primary alcohol to a carboxylic acid by reflux
Reflux a primary alcohol with acidified oxidising agent to allow further oxidation of the aldehyde.
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
Which condition favors oxidation of a primary alcohol to a carboxylic acid?
What you will learn
- Choose reflux for carboxylic-acid production
- Write the primary alcohol to acid change
- Explain the aldehyde intermediate
Keep the mixture reacting to completion
Refluxing a primary alcohol with acidified K₂Cr₂O₇ or acidified KMnO₄ allows the initially formed aldehyde to be oxidised further to a carboxylic acid.
Ethanol → ethanoic acid: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O. The aldehyde ethanal is an intermediate, not the isolated final product under the reflux condition.
Choose the apparatus for the product
Distillation removes the aldehyde to limit further oxidation; reflux condenses volatile material back into the flask so oxidation can continue.
Use reflux for the carboxylic acid target. State acidified oxidising reagent and avoid describing the difference as a different carbon skeleton or oxidant identity alone.
Pause and explain
What is the oxidation product of ethanol under reflux with acidified dichromate?
Worked example
How can butanoic acid be prepared from butan-1-ol by oxidation?
Show the worked solution
- Use acidified K₂Cr₂O₇ or acidified KMnO₄.
- Reflux so the aldehyde intermediate remains in the mixture.
- Oxidise terminal –CH₂OH to –COOH.
Answer CH₃CH₂CH₂CH₂OH + 2[O] → CH₃CH₂CH₂COOH + H₂O; acidified oxidising agent, reflux.
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.
Write the overall equation using [O] for oxidation of propan-1-ol to propanoic acid.
Give me a hint
A primary alcohol to acid uses two [O] overall.
Compare my reasoning
- The chain has three carbon atoms.
- Change terminal –CH₂OH to –COOH.
- Balance oxygen and hydrogen with water.
CH₃CH₂CH₂OH + 2[O] → CH₃CH₂COOH + H₂O.
Look for these in your work
- Acid product correct
- Two [O] used
- Equation balanced
A student distils the mixture while trying to make ethanoic acid from ethanol. Explain the likely issue.
Give me a hint
The volatile aldehyde may leave before further oxidation.
Compare my reasoning
- Distillation removes ethanal from the oxidising mixture.
- The aldehyde can therefore avoid further oxidation.
- Reflux should be used to make the acid.
Distillation favors removal of ethanal; use reflux so it remains available for oxidation to ethanoic acid.
Look for these in your work
- Intermediate identified
- Apparatus effect explained
- Condition corrected
Compare distillation and reflux for the same primary alcohol oxidation.
Give me a hint
Relate the apparatus to aldehyde residence time.
Compare my reasoning
- Distillation removes volatile aldehyde as it forms.
- Reflux returns vapour and allows continued oxidation.
- Thus distil for aldehyde and reflux for acid.
Distil to obtain aldehyde; reflux to continue oxidation to carboxylic acid.
Look for these in your work
- Both methods described
- Residence time linked
- Products matched
Common mistakes
- Distilling the aldehyde away when the acid is required
- Writing an aldehyde as the final reflux product
- Changing the carbon count
What can you explain now?
Which condition favors oxidation of a primary alcohol to a carboxylic acid?
Compare with the explanation
Reflux
Reflux allows the aldehyde intermediate to be further oxidised.
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Make it stick
How does reflux help a primary alcohol oxidation reach a carboxylic acid?
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.