Reactions of the alkenes

The alkenes are hydrocarbons which contain one or more carbon-carbon double bonds. Cyclohexene is a convenient alkene to experiment on. It is a highly flammable liquid with a fairly unpleasant smell.

Watch the videos and make observations. Check your answers using the link at the end of the page. When attempting to draw the structures of the product molecules it is worth bearing in mind that alkenes generally react by addition.

cyclohexene

1. Combustion

Watch the video and describe the combustion that you see. Try to explain your observations. Assuming the normal products for complete combustion write a balanced equation for the combustion of cyclohexene.

Always remove the combustion spoon from the Bunsen flame once the liquid has ignited. Students often leave it in the Bunsen and this makes it difficult to see how it is burning.

Video - the combustion of cyclohexene

2. Oxidation

We use a solution of potassium manganate(VII), well acidified with dilute sulfuric acid. Describe the reaction which takes place. How does this compare with the reaction for the alkanes? Suggest a formula for the product from cyclohexene. The answer is not obvious. It may help you to know that a molecule of water is involved in the addition reaction along with something from the oxidizing agent.

Video - reaction of acidified manganate(VII) ions with an alkene

3. Action of bromine

This is the standard test for unsaturation (presence of double/triple bonds), which we have seen before. Confirm your expected observation from the video. Draw out the displayed formula for the product for cyclohexene.

Video - reaction of bromine water with an alkene

4. Polymerization

This reaction is an example of addition polymerization. It is started using a free radical initiator called didodecanoyl peroxide. In this reaction the monomer methyl 2-methylpropenoate is polymerized to give the polymer poly(methyl 2-methylpropenoate), otherwise known as Perspex. Having checked the theory, try to draw out the structure of the polymer showing two complete monomer units.

Structure of methyl 2-methylpropenoate monomer

Video - the polymerization of methyl 2-methylpropenoate

You may notice in the video that the bottle of methyl 2-methylpropenoate is labeled methyl methacrylate. This is the old non-systematic for the chemical.

Results and equations