Hydrocarbons — Saturated, Unsaturated and Aromatic

Abstract

Hydrocarbons are compounds that are composed solely of carbon and hydrogen atoms. Saturated, unsaturated, and aromatic hydrocarbons are the three principal classes of naturally occurring hydrocarbons. These classes differ in their chemical and metabolic* activities and relative abundances in organisms and sedimentary deposits. Saturated hydrocarbons are called paraffins, a name derived from the Latin parum affinis, which means slight affinity. Paraffins or alkanes are trace constituents of biological lipids, but alkanes are the most stable and abundant hydrocarbon constituents of terrestrial rocks. Unsaturated hydrocarbons are commonly referred to as olefins because ethylene, a gaseous unsaturated hydrocarbon, was found to react with chlorine and bromine to yield oily products. Its trivial name “olefiant gas” or oil-forming gas gave rise to the term olefin which has come to denote compounds related to ethylene. Olefins are more abundant than alkanes in biological lipids. Squalene, a polyolefinic triterpene, is an anabolic * intermediate of plant and animal steroids which may be the precursors of some aromatic hydrocarbons in terrestrial rocks [1]. Aromatic hydrocarbons contain at least one six-carbon ring structure with alternate single and double bonds, a benzenoid ring system, and they owe their family name to the fact that certain of these compounds have pleasant odors. Aromatic hydrocarbons are far more abundant than olefins but less abundant than alkanes in sedimentary deposits. The concentrations of aromatic hydrocarbons in organisms are negligible. Most aromatic hydrocarbons are metabolically and chemically less active than olefins.