Oxidation of Alkenes with KMnO₄: Cold vs. Hot
Potassium Permanganate ($KMnO_4$) behaves very differently with alkenes depending on the reaction conditions. With Cold, Dilute, Alkaline KMnO₄, it performs hydroxylation. However, with Hot, Concentrated, Acidic/Alkaline KMnO₄, it causes vigorous oxidative cleavage of the double bond.
1. Reaction with Cold Dilute Alkaline KMnO₄ (Baeyer's Reagent)
A 1% alkaline solution of cold potassium permanganate is known as Baeyer's Reagent. This reaction involves Syn-Hydroxylation (addition of two -OH groups to the same face) to form Vicinal Diols (Glycols).
Test for Unsaturation
The purple color of $KMnO_4$ disappears, and a brown precipitate of Manganese Dioxide ($MnO_2$) forms. This color change (Purple $\to$ Colorless/Brown) is a classic laboratory test for the presence of double or triple bonds.
Mechanism & Stereochemistry
The reaction proceeds via a cyclic manganate ester intermediate.
- The permanganate ion adds to the alkene face in a concerted manner, forming a five-membered cyclic ester.
- Hydrolysis cleaves the Mn-O bonds, leaving two -OH groups on the same side of the plane.
- Result: Syn-diol formation. Example: Cis-2-butene $\to$ Meso-2,3-butanediol.
2. Reaction with Hot Concentrated KMnO₄
Under vigorous conditions (Hot, Concentrated, usually Acidic), the carbon-carbon double bond is completely cleaved (broken). The fragments are oxidized to their highest oxidation states (Ketones or Carboxylic Acids).
Rules for Cleavage Products
The product depends on the substitution pattern of the alkene carbons:
- Terminal $=CH_2$: Oxidizes to $CO_2 + H_2O$.
- Monosubstituted $=CH-R$: Oxidizes to Carboxylic Acid ($R-COOH$).
- Disubstituted $=CR_2$: Oxidizes to Ketone ($R-CO-R$). Ketones are resistant to further oxidation.
3. Examples of Oxidative Cleavage
A. 2-Butene
$$ CH_3-CH=CH-CH_3 \xrightarrow{\text{Hot } KMnO_4} 2 \ CH_3COOH \text{ (Acetic Acid)} $$B. 2-Methyl-2-butene
Contains one disubstituted carbon and one monosubstituted carbon.
$$ (CH_3)_2C=CH-CH_3 \xrightarrow{\text{Hot } KMnO_4} \underbrace{(CH_3)_2C=O}_{\text{Acetone}} + \underbrace{CH_3COOH}_{\text{Acetic Acid}} $$C. 1-Butene (Terminal Alkene)
$$ CH_3-CH_2-CH=CH_2 \xrightarrow{\text{Hot } KMnO_4} \underbrace{CH_3-CH_2-COOH}_{\text{Propanoic Acid}} + \underbrace{CO_2 + H_2O}_{\text{From } =CH_2} $$4. Comparison: KMnO₄ vs Ozonolysis
| Substrate Group | Reductive Ozonolysis ($O_3, Zn$) | Hot $KMnO_4$ / Oxidative Ozonolysis |
|---|---|---|
| $=CH-R$ | Aldehyde ($R-CHO$) | Carboxylic Acid ($R-COOH$) |
| $=CR_2$ | Ketone ($R_2CO$) | Ketone ($R_2CO$) |
| $=CH_2$ (Terminal) | Formaldehyde ($HCHO$) | $CO_2 + H_2O$ |
5. Oxidation of Cyclic Alkenes
Hot KMnO₄ opens the ring to form dicarboxylic acids or keto-acids.
1-Methylcyclohexene $\xrightarrow{\text{Hot } KMnO_4}$ 6-Oxoheptanoic acid
KMnO4 Oxidation Quiz
Test your knowledge on Baeyer's reagent and Oxidative cleavage. 10 MCQs.
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