CHEMISTRY THE CENTRAL SCIENCE

2 ATOMS, MOLECULES, AND IONS

2.9 SOME SIMPLE ORGANIC COMPOUNDS

The study of compounds of carbon is called organic chemistry, and as noted earlier, compounds that contain carbon and hydrogen, often in combination with oxygen, nitrogen, or other elements, are called organic compounds. We will examine organic compounds in Chapter 24, but here we present a brief introduction to some of the simplest organic compounds.

Alkanes

Compounds that contain only carbon and hydrogen are called hydrocarbons. In the simplest class of hydrocarbons, alkanes, each carbon is bonded to four other atoms. The three smallest alkanes are methane (CH4), ethane (C2H6), and propane (C3H8). The structural formulas of these three alkanes are as follows:

Although hydrocarbons are binary molecular compounds, they are not named like the binary inorganic compounds discussed in Section 2.8. Instead, each alkane has a name that ends in -ane. The alkane with four carbons is called butane. For alkanes with five or more carbons, the names are derived from prefixes like those in Table 2.6. An alkane with eight carbon atoms, for example, is octane (C8H18), where the octa- prefix for eight is combined with the -ane ending for an alkane.

Some Derivatives of Alkanes

Other classes of organic compounds are obtained when one or more hydrogen atoms in an alkane are replaced with functional groups, which are specific groups of atoms. An alcohol, for example, is obtained by replacing an H atom of an alkane with an —OH group. The name of the alcohol is derived from that of the alkane by adding an -ol ending:

Alcohols have properties that are very different from the properties of the alkanes from which the alcohols are obtained. For example, methane, ethane, and propane are all colorless gases under normal conditions, whereas methanol, ethanol, and propanol are colorless liquids. We will discuss the reasons for these differences in Chapter 11.

The prefix “1” in the name 1-propanol indicates that the replacement of H with OH has occurred at one of the “outer” carbon atoms rather than the “middle” carbon atom. A different compound, called either 2-propanol or isopropyl alcohol, is obtained when the OH functional group is attached to the middle carbon atom (FIGURE 2.27).

Compounds with the same molecular formula but different arrangements of atoms are called isomers. There are many different kinds of isomers, as we will discover later in this book. What we have here with 1-propanol and 2-propanol are structural isomers, compounds having the same molecular formula but different structural formulas.

Figure 2.27 The two forms (isomers) of propanol.

GIVE IT SOME THOUGHT

Draw the structural formulas of the two isomers of butane, C4H10.

Much of the richness of organic chemistry is possible because organic compounds can form long chains of carbon-carbon bonds. The series of alkanes that begins with methane, ethane, and propane and the series of alcohols that begins with methanol, ethanol, and propanol can both be extended for as long as we desire, in principle. The properties of alkanes and alcohols change as the chains get longer. Octanes, which are alkanes with eight carbon atoms, are liquids under normal conditions. If the alkane series is extended to tens of thousands of carbon atoms, we obtainpolyethylene, a solid substance that is used to make thousands of plastic products, such as plastic bags, food containers, and laboratory equipment.

SAMPLE EXERCISE 2.16 Writing Structural and Molecular Formulas for Hydrocarbons

Assuming the carbon atoms in pentane are in a linear chain, write (a) the structural formula and (b) the molecular formula for this alkane.

SOLUTION

(a) Alkanes contain only carbon and hydrogen, and each carbon is attached to four other atoms. The name pentane contains the prefix penta- for five (Table 2.6), and we are told that the carbons are in a linear chain. If we then add enough hydrogen atoms to make four bonds to each carbon, we obtain the structural formula

This form of pentane is often called n-pentane, where the n- stands for “normal” because all five carbon atoms are in one line in the structural formula.

(b) Once the structural formula is written, we determine the molecular formula by counting the atoms present. Thus, n-pentane has the molecular formula C5H12.

PRACTICE EXERCISE

(a) What is the molecular formula of butane, the alkane with four carbons? (b) What are the name and molecular formula of an alcohol derived from butane?

Answers: (a) C4H10(b) butanol, C4H10O or C4H9OH