Hydrocarbons are organic compounds that contain only carbon (C) and hydrogen (H) atoms. They can be straight-chain, branched chain, or cyclic molecules. Carbon tends to form four bonds in a tetrahedral geometry.
Hydrocarbons are classified as aliphatic or aromatic. Aliphatic hydrocarbons are not linked together to form a ring. Compounds include the alkanes, alkenes, and alkynes, and substances derived from them by replacing one or more hydrogen atoms by atoms of other elements or groups of atoms. Aromatic hydrocarbons have elements linked together in rings. Because of their relatively nonpolarity, all hydrocarbons are insoluble in water. When hydrocarbons burn in sufficient oxygen, carbon dioxide and water are the sole products.
Polycyclic aromatic hydrocarbons (PAHs) are a family of chemical compounds made up of carbon and hydrogen atoms, with a molecular structure consisting of at least two fused aromatic rings, each with five or six carbon atoms. The PAH family includes about 100 substances, differing in the number and position of their rings.
PAHs are generally formed from the incomplete combustion of organic matter.
Sources of Hydrocarbons
Almost all usuable supplies of hydrocarbons are obtained from fossil fuels--coal, petroleum, and natural gas. Through distillation, crude oil is boiled and condensed over several fractions to give the desired mixture of compounds. Gasoline, for example, is a fraction boiling roughly between 40 and 200°C. The vapors that are condensed in this fraction are mostly alkanes and have between 5 to 10 carbon atoms.
Environmental Effects
Most motor vehicles and engines are powered by hydrocarbon-based fuels such as gasoline and diesel. Hydrocarbon pollution results when unburned or partially burned fuel is emitted from the engine as exhaust, and also when fuel evaporates directly into the atmosphere. Hydrocarbons include many toxic compounds that cause cancer and other adverse health effects. Hydrocarbons also react with nitrogen oxides in the presence of sunlight to form ozone. Hydrocarbons, which may take the form of gases, tiny particles, or droplets, come from a great variety of industrial and natural processes. In typical urban areas, a very significant fraction comes from cars, buses, trucks, and nonroad mobile sources such as construction vehicles and boats.
|