Hydrocarbon is a generic name for a very large number of chemical compounds that consist entirely of hydrogen and carbon atoms. They are further subdivided into aliphatic, aromatic and cyclic. Aliphatic hydrocarbons consist of linear or branched chains and can be saturated (paraffins) or unsaturated (olefins, acetylenes, etc). Aromatic hydrocarbons consist of closed, cyclic, hexagonal rings of a distinctive chemical structure in terms of chemical bonds. The compounds benzene, napthalene and anthracene belong to this group. The name “aromatic” derives from the distinctive odor associated with these compounds. They are in general chemically reactive compounds whose use would have been widespread if it was not for their carcinogenic effects. The third group, cyclic hydrocarbons, consists of compounds that contain three or more carbon atoms in a ring structure. They can be of the saturated (cycloparaffins) or unsaturated (cycloolefins, cycloacetylenes, etc.) variety. Cyclohexane is the most widely used chemical from this group. Hydrocarbons have widespread use as raw materials in the production of other organic compounds and as a primary, global energy source.