Side-chain oxidation: practice and mechanism
Hot KMnO₄ oxidises any alkyl chain with a benzylic hydrogen to –COOH on the ring; the ring stays.
Example
Hot potassium permanganate oxidises an alkyl side chain with a benzylic hydrogen all the way to the carboxylic acid. The aromatic ring itself is not attacked.
Mechanism step by step
- 1
KMnO₄ attacks the benzylic C–H bond, which is weakened by the ring.
- 2
Via alcohol and aldehyde the benzylic carbon is oxidised to the carboxylic acid; the rest of the chain is cleaved off.
- 3
Benzoic acid always remains (–COOH directly on the ring), however long the chain was.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Hot KMnO₄ does not stop at the benzyl alcohol; it oxidises all the way to the carboxylic acid.
- Hot KMnO₄ does not stop at the aldehyde either: you end up with benzoic acid.
Often confused with: Benzylic bromination · Nitration (SEAr)
Frequently asked questions
What happens with tert-butylbenzene?
Nothing: without a benzylic hydrogen there is no point of attack and the side chain stays.
What is it useful for?
To turn alkyl groups into COOH, e.g. p-xylene → terephthalic acid (for PET).