Side-chain oxidation of alkylbenzenes template

Toluene, ethylbenzene and isopropylbenzene all give benzoic acid with hot alkaline KMnO₄ (then acid), p-xylene gives terephthalic acid, and tert-butylbenzene, with no benzylic H, is not oxidised.

Mechanism step by step

Hot KMnO₄ cuts any alkyl side chain that has a benzylic H down to a COOH on the ring

1. Benzylic C–H attack – First step (simplified): permanganate takes a benzylic H atom; the benzyl radical is stabilised by the ring. Further oxidation to COOH is not shown
2. Longer chains – The rest of the chain is cut off: ethylbenzene still gives benzoic acid (the CH₃ ends up as CO₂)
3. Two side chains – Each chain with a benzylic H becomes COOH: p-xylene gives terephthalic acid (for polyesters)
4. No benzylic H: no reaction – tert-Butylbenzene has no H on the benzylic carbon, so the chain survives

Key points

• The reaction needs at least one benzylic C–H: attack starts there by H-atom abstraction (the benzyl radical is stabilised by the ring); the later steps are complex and not drawn.
• However long the chain, only the carbon attached to the ring is kept, as COOH; the benzene ring itself is not oxidised.
• Hot acidic Na₂Cr₂O₇ does the same; in alkaline KMnO₄ the product is the carboxylate, freed by acidifying (brown MnO₂ forms).
• Useful in synthesis: an alkyl group (o,p-directing, activating) becomes COOH (meta-directing, deactivating).

Drawn with 101 atoms, 9 arrows, 8 shapes – every one editable in MolDraw.

Sources

• Wade, Organic Chemistry, 8th ed., Pearson 2013, ch. 17 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 19 Textbook