Baeyer's Test: Detection of Unsaturation | Chemca

Organic Chemistry

Baeyer's Test: Detection of Unsaturation

By Chemca Editorial Team • Last Updated: January 2026 • 7 min read

Baeyer's Test is a simple qualitative test used to detect the presence of unsaturation (carbon-carbon double or triple bonds) in organic compounds. The reagent used is cold, dilute, alkaline Potassium Permanganate ($KMnO_4$).

1. The Reagent

Baeyer's Reagent

A 1% alkaline solution of cold Potassium Permanganate ($KMnO_4$).
Color: Intense Purple / Pink.

2. General Reaction & Observation

When an alkene or alkyne is added to Baeyer's reagent, the purple color of the permanganate fades and is replaced by a brown precipitate of Manganese Dioxide ($MnO_2$).

$$ \underbrace{R-CH=CH-R}_{\text{Alkene}} + \underbrace{H_2O + [O]}_{\text{from } KMnO_4} \xrightarrow{\text{Cold, } OH^-} \underbrace{R-CH(OH)-CH(OH)-R}_{\text{Colorless Glycol}} + \underbrace{MnO_2 \downarrow}_{\text{Brown Ppt}} $$

Observation: Disappearance of Purple Color $\rightarrow$ Formation of Brown Precipitate.

3. Reaction with Alkenes (Syn-Hydroxylation)

Alkenes react to form Vicinal Diols (Glycols). The addition of the two hydroxyl groups occurs on the same face of the double bond (Syn-Addition).

Mechanism

The permanganate ion ($MnO_4^-$) forms a cyclic intermediate with the alkene.

$$ >C=C< + MnO_4^- \rightarrow \text{Cyclic Manganate Ester} $$ $$ \text{Ester} + H_2O \rightarrow \text{Syn-1,2-Diol} + MnO_2 $$

Stereochemistry Example

cis-2-Butene $\xrightarrow{\text{Baeyer's Reagent}}$ meso-2,3-Butanediol.

trans-2-Butene $\xrightarrow{\text{Baeyer's Reagent}}$ racemic-2,3-Butanediol.

4. Reaction with Alkynes

Alkynes are also oxidized, but the final product depends on the position of the triple bond. Initially, diketones are formed, which may cleave under stronger conditions, but in the mild Baeyer's test, they typically form 1,2-Diketones.

$$ R-C \equiv C-R' \xrightarrow{KMnO_4, \ H_2O} R-CO-CO-R' \text{ (Diketone)} $$

Terminal alkynes ($R-C \equiv CH$) may cleave to form carboxylic acids and $CO_2$ even under relatively mild conditions.

5. Comparison: Baeyer's vs. Bromine Water

Test	Reagent	Observation	Stereochemistry
Baeyer's Test	Cold Alkaline $KMnO_4$	Purple $\to$ Brown Ppt	Syn-Hydroxylation
Bromine Water	$Br_2$ in $H_2O/CCl_4$	Orange $\to$ Colorless	Anti-Addition

6. Limitations

Not specific to double bonds: Aldehydes, Formic acid, and Phenols can also reduce KMnO4 and give a positive test (they are oxidized).
Cyclopropanes: Generally do not react, but highly strained rings might.
Benzene: Does NOT give Baeyer's test due to the high stability of its aromatic system.

Baeyer's Test Quiz

Test your concepts on Unsaturation. 10 MCQs with explanations.

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