Hydrocarbons that have double or triple bonds between carbon atoms are called unsaturated hydrocarbons; they are unsaturated in the sense that more hydrogen atoms can be added when H2 reacts across the double or triple bonds ( right ).

Virtually free rotation exists about a carbon-carbon single bond. A methyl group (CH3-) can spin like a top about the single bond joining it to another atom.

In contrast, a molecule such as ethylene cannot be twisted about one of its double bonds without breaking the second bond of the double bond, as we saw in Chapter 9. Double bonds are important in defining the geometry of many biologically important molecules, and in helping to make them rigid.

As was mentioned previously, saturated hydrocarbons are called alkanes, and identified by the suffix "ane" in the series methane, ethane, propane, butane, pentane, and hexane, which have one through six carbon atoms, respectively.

Unsaturated hydrocarbons, which have double bonds, are called alkenes and have similar names ending with the suffix "-ene", as in ethene (C2H4), propene (C3H6), butene, pentene, and hexene. Ethene, propene, and butene are commonly known as ethylene, propylene, and butylene.