chlorofluorocarbons

chlorofluorocarbons (CFCs), organic compounds that contain carbon, chlorine, and fluorine atoms. CFCs are highly effective refrigerants that were developed in response to the pressing need to eliminate toxic and flammable substances, such as sulfur dioxide and ammonia, in refrigeration units and air conditioners. The most common commercial CFCs, marketed under the trade name Freon , are trichlorofluoromethane (CFC-11) and dichlorodifluoromethane (CFC-12). Commercial CFCs are nonflammable, noncorrosive, nontoxic, and odorless, and their vapor pressures and heats of vaporization made them very suitable for refrigeration applications. They were also widely used as aerosol propellants, cleansing agents for electrical and electronic components, and foaming agents in shipping-plastics manufacturing.

In the mid-1970s, scientists at the Univ. of California, Irvine identified CFCs as the major cause of ozone depletion in the upper atmosphere; this was later confirmed by satellite studies. When CFCs are released into the atmosphere, they move via air currents to altitudes ranging from 15 to 25 mi (25-40 km). There, they are dissociated by ultraviolet light as given by the reaction: CF ₂ Cl ₂  → CF ₂ Cl + Cl. The resulting free chlorine atoms (Cl) decompose ozone (O ₃ ) into oxygen (O ₂ ), Cl + O ₃  → ClO + O ₂ , and are regenerated by interaction with free oxygen atoms (O), ClO + O → Cl + O ₂ . When chlorine is regenerated, it is free to continue to break down other ozone molecules. This process continues for the atmospheric lifetime of the chlorine atom (one to two years), during which it destroys an average of 100,000 ozone molecules. Chlorine radicals are removed from the stratosphere after forming two compounds that are relatively resistant to dissociation by ultraviolet light: hydrogen chloride (HCl) and chlorine nitrate (ClONO ₂ ). Dissociation is slow enough so that these compounds can diffuse down to the troposphere, where they react with water vapor and are removed in rain.

Bromine radicals react like chlorine radicals to remove ozone from the stratosphere and sometimes react in concert with chlorine. Bromine is much more destructive than chlorine because the compounds hydrogen bromide (HBr) and bromine nitrate (BrONO ₂ ) are much more susceptible to dissociation by ultraviolet light; thus, many more ozone molecules are destroyed before the bromine molecules can diffuse downward. Fluorine radicals combine to form hydrogen fluoride (HF) and other stable compounds that do not affect the ozone layer.

Ozone is vital to human and animal survival because it is responsible for the absorption of the sun's ultraviolet light. Without this protection, blindness and skin cancers could result from penetrating ultraviolet light. In 1987 an international treaty, the Montreal Protocol , called for reducing CFC use by 50% by 2000. A 1992 amendment to the treaty called for the end of CFC production in industrial countries by 1996, and by 1993 CFC emissions had dropped dramatically.

Halons are organic compounds that are similar to CFCs. They contain carbon, fluorine, and bromine and may contain chlorine. Halons have been used primarily as propellants in fire extinguishers. Because of their bromine content they are even more destructive to ozone than CFCs, and an amendment to the Montreal Protocol banned their use by 1994.

Hydrochlorofluorocarbons (HCFCs) are organic compounds that are similar to CFCs but less destructive to ozone. HCFCs consist of carbon, hydrogen, chlorine and fluorine. They are used as replacements for CFCs, but are to be phased out by the year 2020, as specified by the Montreal Protocol as amended, when they are expected to be replaced by hydrofluorocarbons (HFCs). HFCs are organic compounds that contain hydrogen, carbon and fluorine. HFCs, which do not contain chlorine, do not have any potential for the destruction of ozone, and so are suitable replacements for CFCs.

chlorofluorocarbons (CFCs) Compounds obtained by replacing the hydrogen atoms of hydrocarbons by chlorine and fluorine atoms. Their high stability to temperature makes them suitable for a variety of uses, including aerosol propellants, oils, polymers, etc. They are often known as freons. Their widespread use in aerosols and refrigerator coolants has led to increased concentrations in the upper atmosphere, where photochemical reactions cause them to break down and react with ozone, which results in depletion of the ozone layer. For this reason CFCs are now being replaced with less damaging alternatives. See also pollution.

chlorofluorocarbon (CFC) Chemical compound in which hydrogen atoms of a hydrocarbon, such as an alkane, are replaced by atoms of fluorine, chlorine, carbon and sometimes bromine. CFCs are inert, stable at high temperatures and are odourless, colourless, nontoxic, noncorrosive and nonflammable. Under the trade name of Freons, CFCs were widely used in aerosols, fire-extinguishers, refrigerators, and in the manufacture of foam plastics. The two most common are Freon 11 (trichlorofluoromethane, CFCl₃) and Freon 12 (dichlorodifluoromethane, CF₂Cl₂). CFCs slowly drift into the stratosphere and are broken down by the Sun's ultraviolet radiation into chlorine atoms that destroy the ozone layer. It often takes more than 100 years for CFCs to disappear from the atmosphere. Growing environmental concern led to an international agreement in 1990 to reduce and eventually phase out the use of CFCs, and to develop safe substitutes.

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