alkenes chemical properties

Higher and foundation tier

The alkenes

The alkenes are a homologous series of hydrocarbon molecules which are similar in some ways to the alkanes, but they are much more reactive than the alkanes. The Alkenes are unsaturated hydrocarbons; this means that they contain carbon atoms which have a double covalent bond between two of the carbon atoms in the hydrocarbon molecule e.g. the diagram below shows the first three alkenes.

The first three alkenes

The first 3 members of the alkene homologous series are ethene, propene and butene. The diagram below shows ball and stick model of these three alkene molecules as well as their displayed and structural formula.

3D models, molecular formula, displayed, structural formulae for the first three alkenes, ethene, propene and butene.

The alkenes form a new homologous series of hydrocarbons. They are unsaturated meaning they contain one C=C, that is a carbon carbon double covalent bond. The number of hydrogen atoms in an alkene is simply double the number of carbon atoms. This gives a general formula for the alkenes of CnH2n.

In GCSE chemistry you need to be familiar with the structure and formulae of the first four members of the alkene homologous series. The image below shows the structure of butene and pentene, the 4th and 5th members of the alkene homologous series. You may notice that the structure of butene shown below is different from the one shown above. The differences are in the placement of the double bond, however it does not matter in GCSE chemistry where you place the double bond in these molecules.

Ball and stick model of butene and penetene, as well as the structural formulae and displayed formula of butene and penetene are shown

Like the alkanes the alkenes combust to form carbon dioxide gas and water vapour but they will burn with a more sooty and dirty flame than the alkanes. They will also release less energy than alkanes when they burn. However alkenes as mentioned above are much more reactive than the alkanes due to the presence of the C=C functional group which makes the alkene molecules react readily with many other substances.

Key Points

Students working in a school chemistry lab

Practice questions

Check your understanding - Questions the alkenes