The second homologous series is the alkenes. Their names all end in –ene, for example ethene.
Alkenes all contain a carbon to carbon double bond which makes them more reactive than the alkanes. The alkenes have the general formula .
The alkenes are unsaturated. This means that they have a carbon to carbon double bond. The alkanes are saturated because they only contain single bonds. An unsaturated hydrocarbon decolourises bromine water quickly. Alkanes do this slowly. Alkenes can be used as fuels, solvents and starting materials for everyday products such as plastics and alcohol.
Naming alkenes has the same rules as alkanes.
However, the position of the carbon to carbon double bond must be identified.
Note that the functional group (in the case of alkenes, the double bond) is given the lowest possible number first, before any branches are numbered.