Question

3. You have an unknown which may or may not be a hydrocarbon. It gives a negative test with Br/CHCl3 but decolorizes KMnO4. (Both results are correct.) What can you infer about your unknown?

Answer 1

The bromine test is a qualitative test for the presence of unsaturation (carbon-to-carbon double or triple bonds), phenols and anilines. A negative bromine test means there is no unsaturation in the compound. But there are exceptions, electron-withdrawing groups in the vinylic position can slow down bromine addition to the point that a negative test is produced.

The compound gives a positive Baeyer Test (decolorizes KMnO₄). That means the compound have unsaturation in it(contradictory to bromine test result). But a positive Baeyer test also can happen in several cases although there is no unsaturation (carbon-to-carbon double or triple bonds) in the compound. Here are two such listed possibility-

• Easily oxidized compounds give a positive test: a) Most aldehydes give a positive test. b) Formic acid and its esters give a positive test.
• Phenols and aryl amines give a positive test.

So either of the situations can lead to these observations-

1. It can be possible that electron-withdrawing groups present in the vinylic position slow down bromine addition to such an extent that a negative test is produced but gives a positive permanganate (Baeyer) test.

2. It can also be possible that the compound possesses an aldehyde group or formic acid and its esters, otherwise phenols, and arylamines so that it gives a positive permanganate (Baeyer) test but negative bromine test.