Our primary source of hydrocarbons is petroleum or crude oil, that thick, black liquid that we find in the earth. Petroleum consists almost entirely of a mixture of alkanes with some alkenes and smaller amounts of aromatic hydrocarbons. When petroleum is distilled at a series of different temperatures, the lowest molecular-weight hydrocarbons boil off at the lowest temperatures and the higher-molecular-weight ones boil off at successively higher temperatures. This process, called fractional distillation, is used to separate the complex mixture of compounds. The table shows the various hydrocarbon mixtures ("fractions") that distill off in various temperature ranges.
In addition to harvesting the hydrocarbons that occur naturally in petroleum, oil refineries use a variety of processes to convert some of them into other more desirable hydrocarbons.
A vast number of synthetic (man-made) organic chemicals, including drugs, plastics, paints, adhesives, fibers, detergents, synthetic rubber, and agricultural chemicals, owe their existence to petrochemicals: chemicals derived from petroleum. The top six petrochemicals produced in the United States are ethylene, propylene, benzene, xylene, butadiene (the four-carbon-atom alkene with two double bonds), and toluene. From these, hundreds of other chemicals are manufactured.