 In this video you will learn how a halogen reacts with an alkene and why the reaction of bromine is a test for the presence of a carbon-carbon double bond and the alkene functional group. Alkenes contain a carbon-carbon double bond, for example ethene, propene and butene. When a halogen such as chlorine, bromine or iodine reacts with this carbon-carbon double bond a halogenation occurs. This is an addition reaction where the halogen is added across the double bond. Fluorine is at the top of group 7 halogens and the reaction of fluorine with ethene reacts violently to form carbon and hydrogen fluoride. This is not a particularly useful reaction. Below fluorine is chlorine, bromine and iodine. These three all react in addition reactions where the double bond breaks and the halogen is added across the carbon-carbon double bond. This animation shows chlorine in an addition reaction with ethene, followed by the reaction of bromine and ethene and here is iodine and ethene. As we move down the group the reactions become slower, chlorine reacts quickly with iodine the slowest. In these examples we have used ethene, but the same chemistry applies for larger alkenes. The products formed are always halogenal alkenes and it would be a good extension task to research the naming of these molecules. If you have watched the other videos in our series you may have noticed that the addition reaction of bromine is used as a test for the presence of alkenes. Bromine is an orange-bran colour and when reacted with an alkene forms the colourless halogenal alkane, therefore showing the presence of the carbon-carbon double bond. Now at the end of this video you should be able to explain how a halogen reacts with an alkene in an addition reaction and how the reaction with bromine with an alkene is the test for the presence of the carbon-carbon double bond.