http://www.ck12.org Chapter 3. Chemical Interactions
FIGURE 3.63
How Many Isomers?
Butane has only two isomers and pentane has just three, but some hydrocarbons have many more isomers than
these. As you increase the number of carbon atoms in a hydrocarbon, the number of isomers quickly increases. For
example, heptane, with seven carbon atoms, has nine isomers; and dodecane, with twelve carbon atoms, has 355
isomers. Some hydrocarbons with many more carbon atoms have billions of isomers!
Q:Why does the number of carbon atoms in a hydrocarbon determine how many isomers it has?
A:The more carbon atoms there are, the greater the number of possible arrangements of carbon atoms.
Properties of Isomers
Because isomers are different compounds, they have different properties. Generally, branched-chain isomers have
lower boiling and melting points than straight-chain isomers. For example, the boiling and melting points ofiso-
butane are -12 °C and -160 °C, respectively, compared with 0 °C and -138 °C forn-butane. The more branching
there is, the lower the boiling and melting points are.
Q:The boiling point ofn-pentane is 36 °C. Predict the boiling points ofiso-pentane andneo-pentane.
A:The boiling point ofiso-pentane is 28 °C, and the boiling point ofneo-pentane is 10 °C.
Summary
- Hydrocarbons with the same numbers of atoms but different shapes form different compounds called isomers.
- Hydrocarbons with four or more carbon atoms have isomers. The more carbon atoms a hydrocarbon has, the
greater the number of isomers.