The alkenes, sometimes called olefins, are hydrocarbons that contain one or more double bonds per molecule. Their names are parallel to the names of the alkanes, except that the family ending is -ene, rather than -ane. Thus, the four smallest molecule alkenes containing two, three, four, and five carbon atoms are ethene (also called ethylene), propene (also called propylene), butene (also called butylene), and pentene. (There can be no "methene," because there must be at least two carbon atoms to form a double bond.) A number preceding the name indicates the location of the double bond by counting the carbon atoms from the nearest end of the chain. For example, 2-pentene is the five-carbon alkene with the structure
The locations of branches are similarly indicated by numbers. For example, 3-ethyl 2-pentene has the structure
The lightest three alkenes, ethylene, propylene, and butene, are gases at room temperature; from there on, they are liquids that boil at higher and higher temperatures. The chemical formula of an alkene containing only one double bond per molecule can be obtained from the number of carbon atoms in its molecules: if n is the number of carbon atoms, the formula is CnH2n. Thus, the formula for pentene is C5H10.
Alkenes are called unsaturated hydrocarbons; if there is more than one double bond in an alkene molecule it is said to be polyunsaturated. In principle, two more hydrogen atoms could be added to each double bond to "saturate" the compound, and in fact this does happen quite easily when hydrogen gas is added to an alkene in the presence of a catalyst. This process is called hydrogenation.
Other elements, such as the halogens and hydrogen halides, can also be added easily to the double bonds in alkenes. The resulting halogenated hydrocarbons are very useful but are often toxic or environmentally damaging. Trichloroethylene is a useful solvent, chlorinated hydrocarbons have been used as insecticides, and chlorofluorocarbons (CFCs or Freons) are used as refrigerants but have been shown to damage the earth's ozone layer.