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Introduction to the Chemistry of Alkyl Halides

Haloalkanes (Alkyl Halides) in Industry and Agriculture

In industry, halogen-containing hydrocarbons represent important raw materials and solvents. Dichloromethane (CH2Cl2, methylene dichloride), trichloromethane (CHCl3, chloroform) and trichloroethylene (Cl2CCHCl), for example, are used as solvents and degreasing agents. However, their environmental compatibility is questionable because they also dissolve fats in the skin and can accumulate in fatty tissue. Therefore, they are suspected of causing dermatitis.

In agriculture, haloalkanes are applied as herbicides (e.g. 2,4-D) and fungicides (e.g. captan). The insecticide DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane) is effectively used to control infectious insects in countries which are plaqued by malaria, typhus, and cholera. In Germany and many other industrial countries, the use of DDT has been outlawed since the early 1960s. Since DDT accumulates in fatty tissue of warm-blooded animals (and humans), it was suspected to be carcinogenic. This assumption has been proven not to be correct.

Haloalkanes in agriculture

At present, more than 15,000 halogenated organic compounds are produced for industrial purposes. They are used as feedstock for the production of PVC, industrial lubricants, pesticides, insecticides, herbicides, solvents and many others. Many of these compounds are either not at all or only barely biodegradable. Gammexane (lindane) and chlordane, as well as PCBs (polychlorinated biphenyls), even cause hereditary injuries.

Decachlorobiphenyl, a PCB

Until the mid-1980s, chlorofluorocarbons (CFCs) were produced in large quantities. Freons such as trichlorofluoromethane (freon-11), tetrafluoromethane (freon-14) and trichlorotrifluoroethane (freon-113), for example, are relatively simple CFCs in which all hydrogen atoms have been substituted by chlorine or fluor. CFCs are produced at a reasonable price and are stable, non-toxic, non-inflammable, and non-corrosive. Therefore, CFCs were considered perfect materials for usage in many practical applications, including propellants, coolants in refrigerators and fire-extinguishing agents. Unfortunately, due to their high chemical stability, CFCs have been identified as a severe environmental liability. They are a main cause of ozonolysis in the stratosphere and are largely responsible for the ozone hole. As a result, the production of CFCs is prohibited worldwide. However, due to their high stability, the CFC concentration in the stratosphere will not considerably decrease to a safe level until the year 2050.

Examples of chlorofluorocarbons


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