Testing for alkenes

1. Reaction with bromine water

Alkenes are unsaturated hydrocarbons which contain one or more carbon-carbon double bonds. You might also meet the alkynes which contain a carbon-carbon triple bond. Unsaturated compounds are much more reactive than the corresponding saturated alkanes. This is the principle behind the first two tests used to identify unsaturated compounds. The most common test involves shaking the compound with some bromine water. With an alkene we see rapid decolourisation of the original orange solution.

Video - testing for alkenes with bromine water

A bottle of bromine water

2. Reaction with acidified potassium manganate(VII) solution

A similar test involves shaking the compound with potassium manganate(VII) solution acidified with dilute sulfuric acid. The initially purple solution is decolourised by alkenes. It is important to ensure that the potassium manganate(VII) solution is well acidified or a brown colouration may result. A 50:50 mixture of 0.02M potassium manganate(VII) and 1M sulfuric acid will work well.

Potassium manganate(VII) bottle showing solution colour in inset

Video - testing for alkenes with acidified potassium manganate(VII) solution

In both of the last tests it is important to avoid using too much solution. This is particularly important where the alkene being tested is a gas and there may not be much in the test tube. 1 centimetre depth of solution is easily enough to see any colour change. The colour changes are the results of chemical reactions. If you half fill your test tube with bromine water, there may be too much bromine for the alkene to react with. The reaction will be incomplete and the solution will not decolourise.

Also bear in mind that acidified potassium manganate(VII) is an oxidizing agent and would be decolourised on reaction with easily oxidized materials like aldehydes and some alcohols. However, this reaction is usually much slower than the reaction with alkenes and will usually require heating.

3. Combustion

Another indication of unsaturation can be found when the substance is burnt. Unsaturated compounds tend to burn with sooty, luminous flames. This is perhaps not surprising as unsaturation has the effect of decreasing the number of hydrogen atoms in the compound. This increases the proportion of carbon, and it is the carbon which creates the sooty, luminous flame. This effect becomes more significant as the degree of unsaturation increases.

Video - combustion of an unsaturated compound

alkene burning with a sooty flame