Hydrocarbon, an organic chemical compound composed of carbon and hydrogen. Hydrocarbons may be gases, liquids, or solids. In nature, they are found mainly in plants, petroleum, natural gas, coal, and oil shale. Most burn readily and dissolve more readily in alcohol than in water. Some are toxic and some form an explosive mixture with air. Hydrocarbons are used as fuels and solvents and in the manufacture of industrial chemicals, explosives, plastics, synthetic rubber, dyes, drugs, and other products.

More than 100,000 hydrocarbons are known. The main reason there are so many of these compounds is that carbon atoms can unite in many different ways to form complex chain or ring frameworks. Different arrangements of atoms yield different molecules. For example, the formula C30 H62 (30 carbon atoms and 62 hydrogen atoms) represents 4,111,846,763 possible isomers—compounds whose molecules differ only in structure.

In chemistry, compounds closely related in structure to hydrocarbons are called hydrocarbon derivatives. Such compounds consist of carbon, hydrogen, and other elements, usually oxygen, nitrogen, chlorine, phosphorus, or sulfur. They are often named for the hydrocarbons to which they are most closely related. For example, the organic chloride compound most closely related to ethane is called ethyl chloride. Hydrocarbons and their derivatives form a large part of the subject matter of organic chemistry.

Classification of Hydrocarbons

Hydrocarbons are divided into two main groups according to structure—aliphatic (or acyclic) hydrocarbons and cyclic hydrocarbons. They are further divided into groups of compounds having similar chemical properties. Members within a group are called homologs, and each complete group is called a homologous series. Neighboring members of a homologous series differ from each other by one carbon atom and two hydrogen atoms.

The alkane homologous series, for example, begins with the following compounds: methane (formula: CH4 ); ethane (C2 H6 ); propane (C3 H8 ); butane (C4 H10 ). When a CH2 unit is added to methane, the result is C2 H6 , or ethane; if a CH2 unit is taken away from butane, the result is C3 H8 , or propane. Although homologs have similar chemical properties, their physical properties (such as boiling point) vary in a regular pattern.

Saturated hydrocarbons contain carbon atoms joined by single chemical bonds. Such compounds contain the maximum number of hydrogen atoms. Unsaturated hydrocarbons contain one or more pairs of carbon atoms joined by double or triple chemical bonds.

Aliphatic (or Acyclic) Hydrocarbons

are those whose molecules contain carbon atoms that are joined to form straight or branched chains. Aliphatic compounds burn readily and are toxic.

The alkane, or paraffin, series contains saturated compounds. This series is also called the methane series for the simplest member, methane (CH4 ).

The alkene, or olefin, series contains unsaturated compounds having one or more double bonds. This series is also called the ethylene series for the simplest member, ethylene (H2 CCH2 , or C2 H4 ).

The alkyne, or acetylene, series contains unsaturated compounds having one or more triple bonds. The simplest is acetylene (HCCH, or C2 H2 ).

Cyclic Hydrocarbons

are those whose molecules contain carbon atoms that are joined to form one or more rings or various other structures.

Alicyclic hydrocarbons are saturated or unsaturated ring compounds having aliphatic properties. The simplest saturated member is cyclopropane (C3 H6 ); unsaturated; cyclopropene (C3 H4 ).

Aromatic hydrocarbons contain benzene rings—six carbon atoms joined in a ring characteristic of benzene (C6 H6 ), which is the simplest member of the group. Most aromatic hydrocarbons have a penetrating odor.

Nonbenzenoid aromatic hydrocarbons are compounds that have carbon-carbon bonds characteristic of benzene but do not contain a benzene ring. These compounds may have ringlike structures or be cube-, prism-, or diamond-shaped. Most are prepared synthetically.