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Which reagent (Baeyer's reagent) is used to test for the presence of an olefinic carbon-carbon double bond?

1. acidified KMnO4
2. aqueous KMnO4
3. 1% alkaline KMnO4 solution
4. KMnO4 in benzene

Correct answer: 1% alkaline KMnO4 solution

Solution

Baeyer's reagent is cold, dilute (about 1%) alkaline KMnO4; alkenes decolourise it, forming vicinal diols.

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