Hydrocarbons and their classes
Hydrocarbons are compounds that contain only carbon and hydrogen and are divided into two main classes: aliphatic hydrocarbons and aromatic hydrocarbons. This classification dates from the nineteenth century, when organic chemistry was almost exclusively devoted to the study of materials from natural sources, and terms were coined that reflected a substance’s origin. Two sources were fats and oils, and the word aliphatic was derived from the Greek word aleiphar (“fat”). Aromatic hydrocarbons, irrespective of their own odor, were typically obtained by chemical treatment of pleasant-smelling plant extracts. Aliphatic hydrocarbons include three major groups: alkanes, alkenes, and alkynes.
Alkanes are hydrocarbons in which all the bonds are single bonds, alkenes contain a carbon–carbon double bond, and alkynes contain a carbon–carbon triple bond. Examples of the three classes of aliphatic hydrocarbons are the two-carbon compounds ethane, ethylene, and acetylene.
Another name for aromatic hydrocarbons is arenes. Arenes have properties that are much different from alkanes, alkenes, and alkynes. The most important aromatic hydrocarbon is benzene.
Many of the principles of organic chemistry can be developed by examining the series of hydrocarbons in the order: alkanes, alkenes, alkynes, and arenes.