Acidified potassium permanganate solution can be used to differentiate between alkanes and alkenes. Alkenes will react with potassium permanganate solution in the presence of sulfuric acid while alkanes will not. During the reaction with an alkene the permanganate ions (MnO4-) are reduced to manganese (II) ions (Mn2+), resulting in the purple solution becoming colourless, while the alkene is oxidized to a diol.
Preparation of acidified potassium permanganate: Students can prepare their own acidified potassium permanganate solution by adding 2 mL of 0.01 M aqueous potassium permanganate solution to a test tube followed by 1 mL of 2 M sulfuric acid. Then use 1 mL of the acidified potassium permanganate solution to react with 1mL of the alkane or alkene.
Science ASSIST recommendations:
- A site-specific risk assessment should be conducted
- Conduct this activity in an operating fume cupboard or in a well-ventilated room.
- Suitable PPE such as laboratory coat, closed in shoes, gloves and safety glasses should be worn at all times
- Sulfuric acid is corrosive; causes severe skin burns and eye damage.
- Potassium permanganate solution is harmful to aquatic life with long lasting effects
- Alkanes and alkenes are highly flammable; keep away from ignition sources and heat.
- Most commonly used alkanes are hexane and cyclohexane: both are skin irritants and can cause drowsiness or dizziness.
- Most commonly used alkenes are hexene and cyclohexene: avoid contact with skin and it is harmful if swallowed and if inhaled.
- At the end of the activity collect the wastes generated from the two reactions into a non-halogenated organic waste bottle. Store the waste bottle for collection by a licensed waste disposal contractor. It is important to keep track of the substances, which are added to a waste bottle. Attach a blank label onto the back of the waste bottle. Each time waste is added to the bottle, record the names of the substances added on the label. When the bottle is full, then replace the hand-written label with a printed label containing this information.
- For potassium permanganate disposal, see previously answered questions Can Potassium Permanganate waste be neutralised? and potassium permanganate
A better way to test for the presence of an alkane or alkene is to use bromine water. For more information see previously answered question Bromine water
‘Hexane’, Safety Data Sheet, Chem-Supply website, https://www.chemsupply.com.au/documents/HA0181CH33.pdf (February 2016)
‘Cyclohexane’, Safety Data Sheet, Chem-Supply website https://www.chemsupply.com.au/documents/LC1033M.pdf (April 2011)
‘Cyclohexene’, Safety Data Sheet, Chem-Supply website https://www.chemsupply.com.au/documents/CL0731CH2D.pdf (July 2013)
‘Hexene’, Safety Data Sheet, Sigma-Aldrich website http://www.sigmaaldrich.com/MSDS/MSDS/DisplayMSDSPage.do?country=AU&lang... (May 2014)
‘Chemical waste from ester prac’, Question to Science ASSIST website, https://assist.asta.edu.au/question/2741/chemical-waste-ester-prac?search-id=fe93952 (March 2015)
Science Teachers Association of WA. 2014. Exploring chemistry Year 11: Experiments, investigations and problems, STAWA, Perth WA, p 108.