Question

Part 3: KMnO₄ Test for Unsaturated Hydrocarbons

• Potassium Permanganate, KMnO₄, is also often used to test for alkenes
• Alkenes react with KMnO₄ to produce a compound with 2 hydroxyl groups (a diol)

• The color of KMnO₄solution is purple. The color of the diol product is colorless. The color of the side product, MnO₂, is brown
• Therefore, we can observe whether a reaction has occured by observing the color of the mixture:
  • If the color after mixing stays purple, that means KMnO₄ does not react. Conclusion: alkene is not present
  • If the color after mixing becomes brown, that means KMnO₄ has reacted. Conclusion: alkene is present
• Watch the following video:

• Although the above video is not in English, each reaction is labelled with the reactant, and you can see the observation in the video. Understanding of the audio is not crucial to your observations
• Record the observation in the table below: https://youtu.be/UASzJbQsDSU

Compound Tested	Structure	Observation	Is Alkene Present?
Cyclohexane
Cyclohexene
2-chloro-2-methylpropane

Answer 1

1) Cyclohexane

Cyclohexane is not an unsaturated compound i.e double bond is not present in the following compound.

Hence cyclohexane in $KMnO_4$ remains unreacted, therefore there is no change in colour and the colour remains purple.

2) Cyclohexene

Cyclohexene is an unsaturated compound i.e double bond is present in the following molecule.

Hence cyclohexene in $KMnO_4$ reacts to give diol as product and the color changes from purple to colourless.

3) 2-chloro-2-methylpropane

2-chloro-2-methylpropane is not an unsaturated compound i.e double bond is not present in the following molecule.

Hence 2-chloro-2-methylpropane in $KMnO_4$ remains unreacted, therefore there is no change in colour and the colour remains purple.