Halogens—ghlorine, bromine, fluorine, and iodine-can be introduced into various organic compounds. This accounts for the vast number and range of organic halides (halogen compounds) that have been synthesized. Some have found specific uses as pesticides, for example, or as starting materials for plastics. Organohalides, however, are the most valuable because of their use in a wide range of synthesis reactions during the production of a variety of compounds.
Usually, organic compounds can be halo-genated (combined with halogens) by treatment with reagents. (A reagent is a substance that plays an essential role in chemical reactions.) Alkenes (olefins) are among the easiest compounds to halogenate. For example, eth-ene (ethylene) reacts with hypochlorous acid (a very weak chlorine acid) to form chlorohy-drin. Alkanes, cycloalkanes, and aromatic compounds are also easy to halogenate. For example, bromine reacts with benzene to give bromobenzene.
Reactions of organic halogen compounds
Halogens can often be displaced from organic molecules by substances that are more reactive, such as the hydroxide ion (an electrically charged group of atoms containing hydrogen and oxygen), ammonia, and the hydrosulfide ion (an electrically charged group of atoms containing hydrogen and sulfur). These types of conversions in organic molecules are called substitution reactions.
In elimination reactions, a complete molecule is displaced from the reagent being treated without anything taking its place. In this way, alkyl halides yield alkenes and alkynes, two valuable unsaturated hydrocarbons used in synthetic chemistry. Use of a strong base under vigorous conditions also enables substitution of halogens in aromatic rings.
Organic halides are generally converted into other intermediate substances, one of which is used to make vinyl chloride, the building block for polyvinyl chloride. Polyvinyl chloride, noted for its hardness and resistance to chemicals, is used in making floor coverings, pipes, and fabrics. Another important halogenated hydrocarbon is methyl chloroform,
made from vinyl chloride and used for degreasing, cleaning drains, and dissolving other substances.
Natural gas provides the starting material for the manufacture of halogenated hydrocarbons. Simple examples include trichloro-methane (chloroform) and tetrachlorometh-ane (carbon tetrachloride). These two substances are used mainly as starting materials for the manufacture of freons—fluorocarbon refrigerants, used in refrigerators and air conditioners. Teflon, used to coat the insides of cooking equipment, is also derived from a hydrocarbon containing methane, carbon, and fluorine. And the synthetic rubber Neoprene is produced from halogen-hydrocarbon compounds.
In addition to their use as refrigerants, chlo-rofluorocarbons (CFCs) were used as propellants in aerosol spray cans. In the mid-1970s, evidence suggested that discharge of CFCs into the atmosphere might deplete the ozone layer, which filters out harmful ultraviolet rays from the sun. This led to a ban on the use of CFCs as propellants in 1978.
Flydrocarbons combined with chlorine are among the most effective insecticides known. The most familiar of this group is DDT (dichlo-rodiphenyl-trichloroethane). DDT is termed a “hard” insecticide because it can exist in soil and water systems for months or even years. Other hard insecticides include Aldrin, Diel-drin, and Lindane. Today, the use of these pesticides is severely restricted. Although they successfully eradicate disease-transmitting insects and crop predators, they are hazardous to humans, in addition, many herbicides are halogenated hydrocarbons.