CK-12 Physical Science - For Middle School

http://www.ck12.org Chapter 9. Chemistry of Carbon

Saturated Hydrocarbons

Saturated hydrocarbonscontain only single bonds between carbon atoms. They are the simplest hydrocarbons.
They are called saturated because each carbon atom is bonded to as many hydrogen atoms as possible. In other
words, the carbon atoms are saturated with hydrogen. You can see an example of a saturated hydrocarbon inFigure
9.7. Each carbon atom is bonded to three hydrogen atoms in this compound, which is named ethane.

FIGURE 9.7

Ethane is a saturated hydrocarbon. What is its chemical formula?

Saturated hydrocarbons are given the general name ofalkanes. The name of specific alkanes always ends in -ane.
The first part of the name indicates how many carbon atoms each molecule of the alkane has. The smallest alkane is
methane. It has just one carbon atom. The next largest is ethane, with two carbon atoms. The chemical formulas and
properties of methane, ethane, and several other alkanes are listed inTable9.2. The boiling and melting points of
alkanes are determined mainly by the number of carbon atoms they have. Alkanes with more carbon atoms generally
have higher boiling and melting points.

This table shows only alkanes with relatively few carbon atoms. Some alkanes have many more carbon atoms. What
properties might larger alkanes have? For example, do you think that any of them might be solids?

TABLE9.2:Small Alkanes

Alkane Chemical Formula Boiling Point (°C) Melting Point (°C) State (at 20°C)

Methane CH 4 -162 -183 gas

Ethane C 2 H 6 -89 -172 gas

Propane C 3 H 8 -42 -188 gas

Butane C 4 H 10 0 -138 gas

Pentane C 5 H 12 36 -130 liquid

Hexane C 6 H 14 69 -95 liquid

Heptane C 7 H 16 98 -91 liquid

Octane C 8 H 18 126 -57 liquid

Shapes of Alkanes

Structural formulas are often used to represent hydrocarbon compounds because the molecules can have different
shapes, or structures. Hydrocarbons may form straight chains, branched chains, or rings. Figure9.8 shows an
example of an alkane with each shape.

• In straight-chain molecules, all the carbon atoms are lined up in a row like cars of a train. They form what is
  called the backbone of the molecule.