s8pe-31102-ca
12/9/05
5:03 PM
The prefix iso- means
“equal,” and the suffix -mer
means “part.”
MAZER
Page 350
Isomers
There is another reason why there are so many carbon-based molecules.
Carbon can form different molecules with the same atoms. The atoms
in these molecules are in different places. The molecules have different
structures. Because the atoms are arranged differently, they are actually two different substances. Compounds that contain the same atoms,
but in different places, are called isomers.
The formulas below show a pair of isomers, butane and isobutane.
Both molecules contain four carbon atoms and ten hydrogen atoms.
However, butane molecules are straight chains of carbon atoms. Isobutane molecules are branched chains of carbon atoms. Even though
both butane and isobutane contain the same atoms, the structures of the
molecules are different, so they are isomers. These two isomers are
very similar and are often mixed with one another. Isobutane, however,
does have a slightly lower boiling point than regular butane.
Isobutane
Butane
CH 3
CH 3
CH 2
CH 2
CH 3
Butane contains four carbon atoms
and ten hydrogen atoms. It has a
straight chain structure.
CH 3
CH
CH 3
Isobutane also contains four carbon
atoms and ten hydrogen atoms. It has
a branched chain structure.
Some carbon-based molecules can shift from one isomer to another.
For example, isomers of a molecule called retinal are necessary for
your eyesight. When light strikes retinal, its structure changes from
one isomer to another. The new isomer of retinal starts a process that
sends a signal from the eye to the brain. After the retinal isomer gives
the signal, the molecule shifts back to its original structure.
check your reading
If two substances are isomers of each other, how are they
the same? different?
KEY CONCEPTS
CRITICAL THINKING
1. Why were carbon-based compounds first called organic? How
has the understanding of organic compounds changed? (8.6.b)
4. Infer Could the last carbon
atom in a carbon chain make
bonds with four hydrogen
atoms? Why or why not?
2. How is the way in which carbon
atoms bond to each other important for the number of carbon-based compounds? (8.6.a)
5. Synthesize Do you think
molecules based on carbon
rings can have isomers? Why
or why not?
3. Describe three structures of
carbon-based molecules. (8.6.a)
350 Unit 3: Chemical Interactions
CHALLENGE
6. Communicate A molecule
called naphthalene consists of
ten carbon atoms and eight
hydrogen atoms in two linked
benzene rings. Draw a diagram
of a molecule that could be
naphthalene. Be sure to
include the atoms and the
bonds between the atoms.
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