Oxidise an alkyl side chain to benzoic acid
Use hot alkaline potassium manganate(VII), then dilute acid, to oxidise an arene side chain completely to –COOH.
Section 30 · Hydrocarbons
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 30. Follow the listed reagents, catalysts, temperatures and reaction scope for benzene and methylbenzene. Directing effects are limited to –NH₂, –OH, –R, –NO₂, –COOH and –COR. Written practice is self-checked
Before you begin
- Use the Topic 29 model of benzene as a planar aromatic ring with a delocalised π system, and recognise electrophiles, nucleophiles and organic functional groups.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the final product of complete side-chain oxidation of methylbenzene?
What you will learn
- Recall the two-stage reagent sequence
- Predict benzoic acid from methylbenzene side-chain oxidation
- Distinguish side-chain oxidation from ring oxidation
Oxidise the side chain in two stages
Heat methylbenzene with alkaline KMnO₄ to oxidise the side chain; then add dilute acid. The product is benzoic acid. State both the hot alkaline oxidation and the subsequent acidification.
The syllabus describes complete oxidation of the side chain. The ring remains attached to the carbon that becomes the carboxyl carbon; the methyl carbon is not simply removed.
Use the transformation pattern
For methylbenzene, C₆H₅CH₃ → C₆H₅COOH. A longer alkyl side chain with a suitable benzylic carbon can also be oxidised to the carboxyl group on the ring, but the syllabus's named example is methylbenzene.
Do not claim this reagent is oxidising the aromatic ring or every side-chain carbon into a longer acid. The final aromatic carboxylic-acid product is benzoic acid in the specified complete side-chain oxidation pattern.
Pause and explain
Which sequence matches the syllabus oxidation conditions?
Worked example
Predict the product when methylbenzene is heated with alkaline KMnO₄ and then acidified.
Show the worked solution
- The methyl side chain is oxidised completely.
- The carbon attached directly to the ring becomes the carboxyl carbon.
- The product is benzoic acid.
Answer Methylbenzene forms benzoic acid, C₆H₅COOH, after hot alkaline KMnO₄ followed by dilute acid.
Try a topic test
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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 the product of complete side-chain oxidation of methylbenzene and the reagents in order.
Give me a hint
Keep the aromatic ring and convert the side chain to –COOH.
Compare my reasoning
- Use hot alkaline KMnO₄.
- The side-chain carbon attached to the ring becomes the carboxyl carbon.
- Acidify with dilute acid.
Benzoic acid forms using hot alkaline KMnO₄ followed by dilute acid.
Look for these in your work
- Product identified
- Reagents in order
- Side-chain change explained
Why is benzyl alcohol not the final product in the stated complete oxidation?
Give me a hint
Read the word complete and the full oxidation conditions.
Compare my reasoning
- The side chain is oxidised beyond an alcohol intermediate.
- The specified final group is –COOH.
- The product is benzoic acid after acidification.
The reaction is complete side-chain oxidation, so it proceeds to the carboxyl group and gives benzoic acid rather than stopping at benzyl alcohol.
Look for these in your work
- Complete oxidation applied
- Target group stated
- Intermediate distinguished
A student writes C₆H₅CH₃ → C₆H₅COOH but claims the ring was oxidised. Correct the explanation.
Give me a hint
Identify which atoms changed and which framework stayed.
Compare my reasoning
- The methyl side chain is the part oxidised.
- Its carbon becomes the carboxyl carbon.
- The aromatic ring framework remains.
The side chain is oxidised: its carbon becomes the carbon of –COOH, while the benzene ring remains in the benzoic-acid product.
Look for these in your work
- Reaction site corrected
- Carbon fate traced
- Ring framework retained
Common mistakes
- Omitting the acidification step
- Saying the ring is oxidised to an open-chain product
- Stopping at benzyl alcohol or an aldehyde
- Removing the side-chain carbon rather than converting it to –COOH
What can you explain now?
What is the final product of complete side-chain oxidation of methylbenzene?
Compare with the explanation
Benzoic acid
The side chain is converted to a carboxyl group on the ring.
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Make it stick
What does hot alkaline KMnO₄ followed by acid do to the side chain of methylbenzene?
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