All condensed-ring hydrocarbons share a disturbing property. They cause cancer. Benzapyrene is the most dangerous in this respect.
Condensed-ring hydrocarbons are released when organic materials are heated to high temperatures. This happens, for example, in the carbonization of coal, when it is burned at very high temperatures. As a result, these compounds have been linked to especially high incidences of skin cancers among those who have worked with coal tar.
Cigarette smokers also suffer the effects of poisoning from condensed-ring compounds. These compounds are in the tar deposited in
smokers' lungs. Scientists have long ago drawn positive links between heavy smoking and the incidence of lung cancer.
Theoretically, sheets of condensed-ring hydrocarbons could stretch to infinity. Such compounds would be virtually identical with the large sheets of carbon rings that exist in graphite. Graphite is a fairly common form of natural carbon. The fused rings are held together tightly. But the planes of atoms tend to slip over one another, making graphite a "slippery" substance. For this reason, it is sometimes used as a solid lubricant.
Tobacco smokecontains hundreds of different chemical compounds, many of which are poisonous. Probably the most noxious of these substances are condensed-ring aromatic compounds. These are thought to be responsible for causing lung cancer because they have been found in high concentrations in tar in smokers' lungs.
The well-known "pitch lake"at La Brea, Trinidad, is a naturally occurring deposit of asphalt. Asphalt is a substance that contains a large proportion of condensed-ring aromatic compounds. It is formed as a result of volatile hydrocarbons evaporating from petroleum. The residue then becomes partly oxidized. Asphalt is used mainly for making road surfaces and as a waterproofing material.
It is comparatively easy to introduce halogens (chlorine, bromine, fluorine, and iodine) into various organic compounds. This accounts for the vast number and range of organic halides (halogen compounds) that have been synthesized or exist in nature. Some have found specific uses, for instance as pesticides or starting materials for plastics. Organohalides, however, are of most value because of their use in a wide range of synthesis reactions during the production of a variety of compounds.
Usually, organic compounds can be halogenated (combined with halogens) by treatment with reagents, such as hypohalous acids (for example, hypochlorous acid), or the halogen itself. A reagent is a substance that detects the presence of other substances by the chemical reactions it causes. Alkenes (olefins) are among the easiest compounds to halogenate.
The crystal(above left) is paradichlorobenzene. This is a halogenated aromatic hydrocarbon consisting of two chlorine atoms attached to a benzene ring. This substance has insecticidal properties. It is particularly effective against moths, such as the brown house moth (Hof-mannophila pseudospre-tella), (above right.) This type of moth infests carpets and fabrics.