Oxidative cleavage of alkenes with hot KMnO₄, then H₃O⁺ (heat)

Hot, concentrated permanganate snaps an alkene across the double bond, oxidising both vinylic carbons. Any site that still carries a hydrogen is marched all the way to a carboxylic acid during acidic workup, while fully substituted centers stop at ketones. Terminal =CH₂ fragments are over-oxidized to carbon dioxide. The entry step mirrors the syn manganate addition you see under cold, dilute KMnO₄, but the high-temperature, strongly oxidising conditions force C–C bond cleavage instead of diol isolation. No radicals, no rearrangements—just closed-shell oxidations all the way through.

Quick Summary

• Reagents/conditions: Hot, concentrated KMnO₄ (classically aqueous base, Δ) followed by hot H₃O⁺ to release neutral products; acidified hot KMnO₄ also works.
• Outcome rules:
  • Vinylic carbon with ≥1 H → carboxylic acid after workup.
  • Vinylic carbon with 0 H → ketone.
  • Terminal =CH₂ fragment → CO₂.
• Stereochemistry: Lost after cleavage; rings open to chains bearing carbonyls.
• Comparison: Same product map as ozonolysis with oxidative workup (O₃/H₂O₂).
• Observation: Purple MnO₄⁻ bleaches and often leaves brown MnO₂ precipitate before acidification.

Mechanism (5 Steps)

Class: Closed-shell oxidation via manganate ester, over-oxidation, and C–C bond cleavage.

Two equatorial oxygens on permanganate attack the π bond in a concerted fashion, giving the familiar five-membered cyclic manganate ester. The metal centre accepts electron density from the double bond, setting up syn delivery to both carbons.

The C–O bonds reorganise to the syn diol-like manifold. Under milder (cold) conditions this is where you would stop; here the strongly oxidising environment keeps pushing forward.

Electron flow from the newly formed C–O bonds cleaves the former alkene C–C bond. Each vinylic carbon becomes the carbonyl carbon of a fragment still associated with the manganese centre.

Any fragment that briefly resembles an aldehyde is pushed on to a carboxylate. Hydronium delivers the proton and collapses the O–H bond back onto oxygen, while a terminal =CH₂ fragment is driven all the way to CO₂. Fully substituted carbons remain as ketones.

The acidic workup protonates each carboxylate to its corresponding carboxylic acid. Ketone fragments survive, and CO₂ is released as a gas. The mixture now reflects the familiar permanganate cleavage rules.

Mechanistic Checklist

• Treat the reaction as double-bond cleavage: regiochemistry and stereochemistry of the original alkene do not survive.
• Assign each vinylic carbon by substitution before drawing products (vinylic H → acid, no H → ketone, terminal =CH₂ → CO₂).
• No radicals or carbocations are formed—everything proceeds through closed-shell polar steps.
• Cold, dilute KMnO₄ (or OsO₄) gives syn diols instead; only hot, concentrated permanganate pushes to cleavage.
• Rings will open; watch for symmetry so you do not double-count identical fragments.

Worked Examples

• Example A: 1-methylcyclohex-1-ene → 6-oxoheptanoic acid (one ketone side, one acid side).
• Example B: 4-methylpent-2-ene → 2-methylpropanoic acid + acetic acid.
• Example C: 2,3,4-trimethylpent-2-ene → 3-methylbutan-2-one + propan-2-one.

Multiple Alkenes & Selectivity

• KMnO₄ is unforgiving—given time, every accessible C=C in the molecule will be cleaved.
• Electron-rich and less hindered alkenes usually react first, but the oxidant keeps going; stopping the reaction early is the only way to preserve other alkenes.
• For complex molecules, staged ozonolysis (O₃), followed by a chosen workup, offers better selectivity and temperature control.

Practical Tips & Pitfalls

• Conditions: Hot aqueous KMnO₄ under basic conditions (NaOH/KOH) is classic; acidify hot with H₃O⁺ to obtain neutral carboxylic acids.
• Observation: The deep purple permanganate fades; insoluble brown MnO₂ commonly precipitates—filter before acidification.
• Functional groups: Allylic alcohols, aldehydes, and sulfides are also oxidized; protect or use a milder method if sensitive groups are present.
• Safety: KMnO₄ is a strong oxidizer—add slowly, control exotherms, and handle manganese-containing waste appropriately.

Exam-Style Summary

• Hot KMnO₄ cleaves alkenes. After H₃O⁺/Δ workup: vinylic H → carboxylic acid, no vinylic H → ketone, terminal =CH₂ → CO₂.
• Same product logic as oxidative ozonolysis (O₃/H₂O₂).
• Rings open; stereochemistry of the starting alkene is erased.
• Cold permanganate stops at syn diols—remember which conditions you are using.

Interactive Toolbox

• Use the Mechanism Solver to export the five-step KMnO₄ sequence for any substrate you want to practice.
• Check predicted products with the Reaction Solver; choose the oxidative KMnO₄ mode to see acids vs. ketones vs. CO₂ assignments instantly.
• Pair this lesson with the ozonolysis + H₂O₂ guide to compare oxidative workups side by side.

FAQ / Exam Notes

• Do radicals form? No—permanganate operates through polar, closed-shell pathways in this context.
• Can I halt oxidation at the diol? Only with cold, dilute KMnO₄ (or OsO₄). Hot, concentrated permanganate will march on to cleavage.
• What happens to aldehydes? They do not survive; permanganate over-oxidizes them to carboxylates before the acid workup.
• Why CO₂ from terminal alkenes? A terminal =CH₂ is oxidized from the aldehyde stage directly to carbonate/CO₂ under these harsh conditions.