Hot KMnO₄ vs Cold KMnO₄
The effect of temperature and pH on the oxidizing power of Permanganate.
By chemca Team • Updated Jan 2026
Potassium Permanganate ($KMnO_4$) is a versatile oxidizing agent. Its action depends heavily on the reaction conditions. Cold, dilute, alkaline KMnO₄ acts as a mild oxidizer, while Hot, concentrated, acidic/alkaline KMnO₄ acts as a vigorous oxidizer capable of breaking carbon-carbon bonds.
1. Cold Dilute Alkaline KMnO₄ (Baeyer's Reagent)
Syn-Hydroxylation (Test for Unsaturation)
Conditions: 1% Alkaline KMnO4 solution at low temperature (273 K).
Action: Adds two -OH groups across the double bond without breaking the carbon chain (formation of Vicinal Diols).
$$ R-CH=CH-R + H_2O + [O] \xrightarrow{\text{Cold, Alk. KMnO}_4} \underset{\text{Vicinal Glycol}}{R-CH(OH)-CH(OH)-R} $$
Stereochemistry: Syn-Addition (Both -OH groups add from the same side).
Example: Cis-2-butene $\rightarrow$ Meso-2,3-butanediol.
Example: Cis-2-butene $\rightarrow$ Meso-2,3-butanediol.
Observation: The deep purple color of KMnO4 fades, and a brown precipitate of $MnO_2$ is formed. This is a classic test for alkenes and alkynes.
2. Hot Acidic or Alkaline KMnO₄
Oxidative Cleavage
Conditions: Concentrated KMnO4, Heat ($\Delta$), Acidic ($H^+$) or Alkaline ($OH^-$ followed by $H^+$).
Action: It breaks the Carbon-Carbon double/triple bonds completely. The products depend on the structure of the alkene.
Rule 1: Terminal $=CH_2$ group converts to $CO_2 + H_2O$.
$$ R-CH=CH_2 \xrightarrow{[O], \Delta} R-COOH + CO_2 + H_2O $$
Rule 2: Monosubstituted $=CH-R$ group converts to Carboxylic Acid ($R-COOH$).
$$ R-CH=CH-R \xrightarrow{[O], \Delta} 2 R-COOH $$
Rule 3: Disubstituted $=CR_2$ group converts to Ketone ($R_2C=O$). Ketones resist further oxidation.
$$ (CH_3)_2C=CH-CH_3 \xrightarrow{[O], \Delta} \underset{\text{Acetone}}{CH_3-CO-CH_3} + \underset{\text{Acetic Acid}}{CH_3COOH} $$
3. Oxidation of Alkyl Benzenes
Hot Alkaline KMnO₄
Vigorous oxidation of alkyl benzenes oxidizes the entire alkyl side chain to a carboxyl group ($-COOH$), irrespective of the chain length.
$$ C_6H_5-R \xrightarrow{1. KMnO_4, KOH, \Delta} C_6H_5-COOK \xrightarrow{2. H_3O^+} \underset{\text{Benzoic Acid}}{C_6H_5-COOH} $$
Crucial Condition: The alkyl group must have at least one Benzylic Hydrogen ($sp^3$ C attached to ring).
- Toluene ($Ph-CH_3$) $\rightarrow$ Benzoic Acid.
- Ethylbenzene ($Ph-CH_2CH_3$) $\rightarrow$ Benzoic Acid.
- Tert-butylbenzene ($Ph-C(CH_3)_3$) $\rightarrow$ No Reaction (No benzylic H).
4. Quick Comparison
| Substrate | Cold Dilute KMnO₄ | Hot Conc. KMnO₄ |
|---|---|---|
| Ethene ($CH_2=CH_2$) | Ethane-1,2-diol (Glycol) | $2 CO_2 + 2 H_2O$ |
| Propene ($CH_3-CH=CH_2$) | Propane-1,2-diol | Ethanoic Acid + $CO_2$ + $H_2O$ |
| 2-Butene ($CH_3-CH=CH-CH_3$) | Butane-2,3-diol | 2 moles of Ethanoic Acid |
| 2-Methylpropene | 2-Methylpropane-1,2-diol | Acetone + $CO_2$ + $H_2O$ |
| 2-Butyne ($CH_3-C \equiv C-CH_3$) | Butane-2,3-dione (Diketone) | 2 moles of Ethanoic Acid |
Knowledge Check
Test your understanding of KMnO4 Reactions
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