25.1. Hydrocarbons http://www.ck12.org
FIGURE 25.3
The geometric isomers of 2-butene are
calledcisandtrans.isomers to exist, there must be a rigid structure in the molecule to prevent free rotation around a bond. If the double
bond in an alkene were capable of rotating, the two geometric isomers above would not be different molecules
because the structures would be free to interconvert. In addition, the two carbon atoms must each be attached to two
different groups for there to be geometric isomers. Propene (Figure25.4) has no geometric isomers because one of
the carbon atoms is bonded to two identical hydrogen atoms.
FIGURE 25.4
Propene.The physical and chemical properties of geometric isomers are generally different. For example,cis-2-butene is
slightly more polar thantrans-2-butene, so it also has a slightly higher boiling point. Heat or irradiation with light
can be used to bring about the conversion of one geometric isomer to another. The input of energy must be large
enough to break the pi bond between the two carbon atoms, which is weaker than the sigma bond. At that point, the
now single bond is free to rotate, and the isomers can interconvert.
Molecules with multiple double bonds are also quite common. Shown below (Figure25.5) is a four-carbon chain
with double bonds between carbons 1 and 2 and between carbons 3 and 4. This molecule is called 1,3-butadiene.
FIGURE 25.5
1,3-butadiene.