Oxidise methylbenzene to benzoic acid
Use hot alkaline potassium manganate(VII), then dilute acid, to oxidise the methyl side chain of methylbenzene to benzoic acid.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 33. Focus on the named compounds, reagents and conditions in each outcome; apply the specified reaction type rather than extending it to unlisted cases. Written practice is self-checked
Before you begin
- Recall carboxylic acids, esters, phenols, alcohols, halogenoalkanes and electrophilic/nucleophilic reaction ideas from earlier topics.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which reagent sequence produces benzoic acid from methylbenzene?
What you will learn
- Recall the reagents and sequence for producing benzoic acid from methylbenzene
- Distinguish side-chain oxidation from aromatic-ring substitution
- Explain why dilute acid follows the alkaline oxidation
Oxidise the side chain
Heat methylbenzene with alkaline KMnO₄. The methyl side chain is fully oxidised to a carboxylate under the hot alkaline conditions; the benzene ring remains intact.
Then add dilute acid to convert the benzoate salt into benzoic acid. The syllabus example is methylbenzene → benzoic acid, using hot alkaline KMnO₄ followed by dilute acid.
Keep the order and product straight
The alkaline oxidation first gives a benzoate species in the reaction mixture; acidification gives the named carboxylic acid product.
Do not confuse this reaction with nitration or halogenation of the ring: the change occurs at the methyl side chain, which becomes –COOH.
Pause and explain
What does the dilute acid do after the alkaline oxidation?
Worked example
Describe how methylbenzene is converted into benzoic acid.
Show the worked solution
- Heat methylbenzene with alkaline KMnO₄.
- Oxidise the methyl side chain to a benzoate species.
- Add dilute acid to obtain benzoic acid.
Answer Heat methylbenzene with alkaline KMnO₄, then add dilute acid to produce benzoic 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.
Write the reagent sequence for methylbenzene → benzoic acid.
Give me a hint
Separate the oxidation conditions from the final work-up.
Compare my reasoning
- Use alkaline KMnO₄.
- Heat the mixture to oxidise the side chain.
- Add dilute acid to obtain benzoic acid.
Hot alkaline KMnO₄, followed by dilute acid.
Look for these in your work
- Correct oxidant
- Hot condition
- Acid work-up
A student stops after heating methylbenzene with alkaline KMnO₄ and names potassium benzoate as the final product. Complete the route.
Give me a hint
Look at the required acid work-up.
Compare my reasoning
- The alkaline mixture contains a benzoate salt.
- Add dilute acid after oxidation.
- Name the resulting carboxylic acid.
Acidify the benzoate mixture with dilute acid to form benzoic acid.
Look for these in your work
- Intermediate recognised
- Work-up added
- Final product named
A methylbenzene derivative is shown with an alkyl side chain and the question specifies the syllabus oxidation conditions. Where should you look for the main structural change?
Give me a hint
Distinguish the side chain from the aromatic ring.
Compare my reasoning
- Hot alkaline KMnO₄ oxidises the alkyl side chain.
- The benzylic side chain is converted to a carboxyl group in the named example.
- Do not replace this with ring substitution.
The side chain is oxidised; in the methylbenzene example it becomes –COOH, while the aromatic ring remains intact.
Look for these in your work
- Correct site selected
- Oxidation identified
- Ring/side-chain distinction
Common mistakes
- Using acidified permanganate as the stated first condition
- Leaving out the hot condition
- Stopping at potassium benzoate instead of acidifying
- Claiming the benzene ring is oxidised to open-chain products
What can you explain now?
Which reagent sequence produces benzoic acid from methylbenzene?
Compare with the explanation
Hot alkaline KMnO₄, then dilute acid
Oxidation occurs in alkaline solution before acidification.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Which two stages and conditions convert methylbenzene to benzoic 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.