(Redirected from Chlorofluorocarbon
Halon, CFC, and Hydrochlorofluorocarbon and added to Alkyl halide.
CFCs (chlorofluorocarbons) are a family of artificial chemical compounds containing chlorine, fluorine and carbon. They were formerly used widely in industry, for example as refrigerants, propellants, and cleaning solvents. Their use has been generally prohibited by the Montreal Protocol, because of fears of their possible destructive effects on the ozone layer (see ozone depletion). Hydrochlorofluorocarbons are now used as CFC substitutes.
CFCs were developed by the American engineer Thomas Midgley in 1928 as a replacement for ammonia (then a common refrigerant). The new compound developed had to have a low boiling point, be non-toxic, and be generally non-reactive. In a demonstration for the American Chemical Association , Midgley flamboyantly demonstrated all these properties by inhaling a breath of the gas and using it to blow out a candle.
Midgley specifically developed CCl2F2 (CFC-12). However, one of the attractive features of CFCs is that there exists a whole family of the compounds, each having a unique boiling point which can suit different applications. In addition to their original application as refrigerants, CFCs have been used as propellants in aerosol cans, cleaning solvents for circuit boards, and as blowing agents for making expanded plastics (such as the expanded polystyrene used in packaging materials and disposable coffee cups).
One major use of CFCs has been as propellants in aerosol inhalers for drugs used to treat asthma. The conversion of these devices and treatments from CFC to halocarbons that do not have the same effect on the ozone layer is well under way. There are some differences between asthma inhalers using CFCs and the newer propellants, but the conversion has not proven difficult. (By contrast, a significant amount of development effort has been required to develop non-CFC alternatives to CFC-based refrigerants, particularly for applications where the refrigeration mechanism cannot be modified or replaced.)
CFCs, HCFCs, and HFCs are named as follows:
- 0 = number of double bonds (omitted if zero)
- 1 = Carbon atoms - 1 (omitted if zero)
- 2 = Hydrogen atoms + 1
- 3 = Fluorine atoms
- 4 = Chlorine atoms replaced by Bromine ("B" prefix added)
- a = letter added to identify isomers, the "normal" isomer in any number has the smallest mass difference on each carbon, and a, b, or c are added as the masses diverge from normal.
CFCs in the atmosphere
There has been a movement since the late 1970s to ban CFCs because of their destructive effect on the ozone layer. This damage was discovered by Sherry Rowland and Mario Molina, who first published a paper suggesting the connection in 1974. It turns out that one of CFCs' most attractive features—their unreactivity—has been instrumental in making them one of the most significant pollutants. CFCs' lack of reactivity gives them a lifespan which can exceed 100 years in some cases. This gives them time to diffuse into the upper stratosphere. Here, the sun's ultraviolet radiation is strong enough to break off the chlorine atom, which on its own is a highly reactive free radical. This catalyses the break up of ozone into oxygen:
- Cl + O3 → ClO + O2
- ClO + O → Cl + O2
CFCs are a problem because the chlorine is regenerated at the end of these reactions, making it able to repeat this reaction millions of times. This disrupts the ionic structure of oxygen atoms, causing their lowest ionisation energy to drop. This reaction is believed to be the cause of the ozone hole observed over the poles and upper latitudes of the Earth. Decreases in stratospheric ozone have been linked to increases in skin cancer.
In 1975, Oregon enacted the world's first ban of CFCs (legislation introduced by Walter F. Brown). By 1985, scientists observed a dramatic seasonal depletion of the ozone layer over Antarctica, which led the United Nations to take action in 1987. International attention to CFCs resulted in a meeting of world diplomats in Montreal in 1987. They forged a treaty, the Montreal Protocol, which called for drastic reductions in the production of CFCs. On March 2, 1989, 12 European Community nations agreed to ban the production of all CFCs by the end of the century. In 1990, diplomats met in London and voted to significantly strengthen the Montreal Protocol by calling for a complete elimination of CFCs by the year 2000. By the year 2010 CFCs should be completely eliminated from developing countries as well.
Last updated: 05-07-2005 13:49:01
Last updated: 05-13-2005 07:56:04