Intermediate Topic 867 of 1102 Organic Chemistry
Baeyer's Reagent
Cold dilute potassium permanganate tests for double bonds by decolorizing.
In-depth explanation of Baeyer's Reagent
Baeyer's reagent is cold, dilute, alkaline potassium permanganate, and it is used as a test for carbon-carbon multiple bonds. The purple permanganate is reduced by alkenes, which are oxidized to cis-diols, while the manganese is reduced to brown manganese dioxide, MnO2. The sign of a positive test is that the purple colour disappears and a brown precipitate forms. Alkanes do not react with the reagent under these mild conditions, so the test separates unsaturated compounds from saturated ones.
Going deeper: Baeyer's Reagent
Key ideas: Cold dilute alkaline KMnO4 (purple) tests for unsaturation: alkenes are converted to cis-1,2-diols by syn dihydroxylation, with purple fading to brown MnO2 precipitate. Alkynes also decolourise. Hot concentrated KMnO4 cleaves the double bond into carbonyl compounds or acids. Saturated alkanes, benzene and aromatic rings are unreactive. Aldehydes and alcohols also reduce permanganate, so the test is not specific.
Common mistakes and connections: Students claim the product of cold KMnO4 is a trans diol, or that the test is specific for alkenes. Another error is forgetting the colour change, which is purple to brown, not purple to colourless as in acid. Links to Br2 test, ozonolysis and syn addition with OsO4.
Visualization of Baeyer's Reagent
Cold dilute alkaline KMnO4 adds two OH groups across C=C of an alkene like ethene, turning purple to brown, a test for unsaturation. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Ethene decolourizes Baeyer's reagent, forming a brown precipitate of manganese dioxide.
Frequently asked questions about Baeyer's Reagent
What is Baeyer's Reagent?
Cold dilute potassium permanganate tests for double bonds by decolorizing.
Is Baeyer's Reagent a beginner, intermediate or advanced topic?
Baeyer's Reagent is a intermediate topic in Organic Chemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Baeyer's Reagent?
Study these earlier topics in Organic Chemistry first: Acidity of Terminal Alkynes, Keto-Enol Tautomerism, Alpha Hydrogen Acidity.
What comes after Baeyer's Reagent?
The next topics in Organic Chemistry are Williamson vs Elimination and Reduction of Carbonyls.
Can I practise Baeyer's Reagent with quizzes?
Yes. The interactive lesson includes a self-check question on Baeyer's Reagent, and the Organic Chemistry quiz draws on every topic in this area.
Where Baeyer's Reagent fits in Organic Chemistry
Baeyer's Reagent is topic 139 of 148 in Organic Chemistry.