chlorofluorocarbons


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chlorofluorocarbons

(klōr'əflo͝or`əkär'bənz, klôr'–) (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 FreonFreon
[trade name], any one of a special class of chemical compounds that are used as refrigerants, aerosol propellants, and solvents. These compounds are haloalkanes, i.e., halogen derivatives of saturated hydrocarbons (see alkane).
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, 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 predicted that CFCs could cause ozone depletion in the upper atmosphere; this was later confirmed by ground-based and 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: CF2Cl2 → CF2Cl + Cl. The resulting free chlorine atoms (Cl) decompose ozone (O3) into oxygen (O2), Cl + O3 → ClO + O2, and are regenerated by interaction with free oxygen atoms (O), ClO + O → Cl + O2. 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 (ClONO2). 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 (BrONO2) 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 cancersskin cancer,
malignant tumor of the skin. The most common types of skin cancer are basal cell carcinoma, squamous cell carcinoma, and melanoma. Rarer forms include mycosis fungoides (a type of lymphoma) and Kaposi's sarcoma.
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 could result from penetrating ultraviolet light. In 1987 an international treaty, the Montreal ProtocolMontreal Protocol,
officially the Protocol on Substances That Deplete the Ozone Layer, treaty signed on Sept. 16, 1987, at Montreal by 25 nations; 197 nations are now parties to the accord.
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, 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. HFCs are, however, greenhouse gases, and a reduction in their use has been called for under an amendent to the protocol.

Chlorofluorocarbons

Stable, artificially created chemical compounds containing carbon, chlorine, fluorine, and sometimes hydrogen. Chlorofluorocarbons, used primarily to facilitate cooling in refrigerators and air conditioners, have been found to deplete the stratospheric ozone layer that protects the earth and its inhabitants from excessive ultraviolet radiation.
References in periodicals archive ?
As a result of these properties it can act as a potential candidate for the halogen bonding and here the main focus of this study was to investigate the halogen bonding ability of different halogens (Cl2, Br2 and F2) and chlorofluorocarbons with nitrobenzene theoretically.
The Commission, furthermore, specifies that the sale of metered dose inhalers containing chlorofluorocarbons listed in Annex I of the decision is banned when the competent authority of one of these markets believes that the use of this type of inhaler destined for these markets is not essential.
Hydrogenated chlorofluorocarbons (HCFCs) could potentially be used as dispersion agents.
Paints, wall coverings, and furniture were screened for toxic properties, and the carpets and insulation used were free of formaldehyde and chlorofluorocarbons (CFCs).
Chlorofluorocarbons (CFCs) are greenhouse gases," said SDPI environmentalist Kashif Salik.
Theme for the current years International Day for the Preservation of Ozone Layer is HCFC (Hydro chlorofluorocarbons) Phase-out: A Unique Opportunity.
Indeed, halons are thought to be three to ten times more ozone-unfriendly than chlorofluorocarbons. For this reason the Montreal Protocol obliged developed nations to cease halon production in 1994, and set a 2010 target date for the developing world.
Poland may continue to use chlorofluorocarbons (CFC) in existing military applications, the European Commission said on January 12 when waving through an exemption from EU legislation on ozone-depleting substances for the Polish navy.
A magnetic-cooling system could also be less polluting than current systems because it wouldn't use environmentally harmful chemicals, such as ammonia or chlorofluorocarbons.
- the production levels for chlorofluorocarbons 11,12, 113, 114 and 115 that Phosphoric Fertilizers Industries is authorised to produce should be increased by 1,786 tonnes of CFC-11 and CFC-12 weighted according to the ozone-depleting potential, from January 1 to December 31, 2004;
Mostly human-made chemicals called chlorofluorocarbons (CFCs).