The study of compounds of carbon is called organic chemistry. Compounds that contain carbon and hydrogen, often in combination with oxygen, nitrogen, or other elements, are called organic compounds. We will examine organic compounds and organic chemistry in some detail in Chapter 25. You will see a number of organic compounds throughout this text; many of them have practical applications or are relevant to the chemistry of biological systems. Here we present a very brief introduction to some of the simplest organic compounds so as to provide you with a sense of what these molecules look like and how they are named.

Alkanes

Compounds that contain only carbon and hydrogen are called hydrocarbons. In the most basic class of hydrocarbons, each carbon atom is bonded to four other atoms. These compounds are called alkanes. The three simplest alkanes, which contain one, two, and three carbon atoms, respectively, are methane ethane and propane The structural formulas of these three alkanes are as follows:

Each of the alkanes has a name that ends in -ane. Longer alkanes can be made by adding additional carbon atoms to the "skeleton" of the molecule. For alkanes with five or more carbon atoms, the names are derived from prefixes like those in Table 2.6. An alkane with eight carbon atoms, for example, is called octane where the octa- prefix for eight is combined with the -ane ending for an alkane. Gasoline consists primarily of octanes, as will be discussed in Chapter 25.

Some Derivatives of Alkanes

Other classes of organic compounds are obtained when hydrogen atoms of alkanes 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 alkanes from which they 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 properties in Chapter 11.

The prefix "1" in the name of 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 2-propanol (also known as isopropyl alcohol) is obtained if the OH functional group is attached to the middle carbon atom. Ball-and-stick models of 1-propanol and 2-propanol are presented in Figure 2.28. As you will learn in Chapter 25, the nomenclature of organic compounds provides ways in which we can unambiguously define which atoms are bonded to one another.

Much of the richness of organic chemistry is possible because compounds with long chains of carbon-carbon bonds are found in nature or can be synthesized. The series of alkanes and alcohols that begins with methane, ethane, and propane can 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 obtain polyethylene, a solid substance that is used to make thousands of plastic products, such as plastic bags, food containers, and laboratory equipment. Polyethylene is an example of a polymer, a substance that is made by adding together thousands of smaller molecules. We will discuss polymers in greater detail in Chapter 12.

In all the compounds discussed so far, the carbon atoms in the structural formula are linked to four other atoms by a single line; in later chapters you will learn that the single line represents a single bond between the carbon atom and the other atom. Carbon, however, can also form multiple bonds to itself and to other atoms, such as oxygen and nitrogen. Multiple bonds greatly change the properties of organic molecules and are one of the main reasons that many of you will take a year-long course dedicated entirely to organic chemistry! Some familiar organic substances that contain double bonds to carbon are shown below. In each case, we have given the proper name of the compound, which is derived from the prefix of an alkane, and the "common" name by which you probably know the substance:

Ethylene is an unsaturated hydrocarbon, which is a compound with a carbon-carbon multiple bond. The carbon-carbon double bond makes ethylene much more reactive than alkanes. Acetic acid is a carboxylic acid. It is the characteristic component of vinegar. Acetone is a ketone. Acetone is a common organic solvent that is used in households as a lacquer and nail-polish remover. Figure 2.29 shows space-filling models of acetic acid and acetone. You will encounter other organic molecules throughout the text, and you should note the numbers of carbon atoms involved and the types of other atoms to which carbon is bonded. As noted earlier, we will provide a more complete discussion of organic chemistry in Chapter 25.