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),
butene, pentene, and hexene. Ethene, propene, and butene are commonly
known as ethylene, propylene, and butylene.