Chlorofluorocarbons and the Ozone Layer

RGAM1ZATI0N A major organizational change from the third to fourth edition has been the conversion and expansion of the topics previously found in Chapter 16 (Air The Precious Canopy) into three separate chapters. This material, now Chapter 4 (The Air We Breathe), Chapter 6 (Carbon Dioxide and the Greenhouse Effect), and Chapter 7 (Chlorofluorocarbons and the Ozone Layer), has been moved to a more prominent place early in the text. The very limited treatment of acid rain in Chapter 16 of the third edition has... 

After the usual sections on the history of the discoveries of and the application of the network to the halogens, special sections on (1) the oxoacids and their salts and (2) the interhalogens follow. The section on reactions and compounds of practical importance comes next and finally the selected topic in depth, which concerns the threat posed by chlorofluorocarbons to the ozone layer. 

The other global environmental problem, stratospheric ozone depletion, was less controversial and more imminent. The U.S. Senate Committee Report supporting the Clean Air Act Amendments of 1990 states, Destruction of the ozone layer is caused primarily by the release into the atmosphere of chlorofluorocarbons (CFCs) and similar manufactured substances—persistent chemicals that rise into the stratosphere where they catalyze the destruction of stratospheric ozone. A decrease in stratospheric ozone will allow more ultraviolet (UV) radiation to reach Earth, resulting in increased rates of disease in humans, including increased incidence of skin cancer, cataracts, and, potentially, suppression of the immune system. Increased UV radiation has also been shown to damage crops and marine resources."... 

As you know, most countries are phasing out certain refrigerants to lessen damage to the ozone layer. The chemicals being phased out are chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Replacements are hydrofluorocarbons (HFCs) and certain blends. The DuPont web site (www.dupont.com) gives the handy Table I of recommended replacement refrigerants for various applications. 

Another area of potential interest is in refrigerator liners. The move away from the ozone-layer-damaging chlorofluorocarbons (CFCs) to HCFCs in the USA and pentane/cyclopentane blends in Europe has not been without problems. These newer materials have an adverse effect on ABS whereas the nitrile resin appears satisfactory, if more expensive. 

During the 1980s, it became recognized that chlorofluorocarbons (CFCs), widely used as aerosol propellants, are damaging the ozone layer, and aerosol manufacturers were asked to use other propellants. Some... 

The formed mixture is composed of trichlorofluoromethane (Freon-11) and dichlorodifluoromethane (Freon-12). These compounds are used as aerosols and as refrigerants. Due to the depleting effect of chlorofluorocarbons (CFCs) on the ozone layer, the production of these compounds may be reduced appreciably. 

Mario Molina and Sherwood Rowland used Crutzen s work and other data in 1974 to build a model of the stratosphere that explained how chlorofluorocarbons could threaten the ozone layer. In 1985, ozone levels over Antarctica were indeed found to be decreasing and had dropped to the lowest ever observed by the year 2000, the hole had reached Chile. These losses are now known to be global in extent and it has been postulated that they may be contributing to global warming in the Southern Hemisphere. 

The haloalkanes (also called alkyl halides) are alkanes in which at least one hydrogen atom has been replaced by a halogen atom. Although they have important uses, many haloalkanes are highly toxic and a threat to the environment. The haloalkane 1,2-dichlorofluoroethane, CHC1FCH2C1, is an example of a chlorofluorocarbon (CFC), one of the compounds held responsible for the depletion of the ozone layer (see Box 13.3). Many pesticides are aromatic compounds with several halogen atoms. 

Evidence that many of these compounds can have adverse effects on the immune, endocrine and nervous systems and that some are carcinogenic has grown during the last decade. The role of chlorofluorocarbons (CFCs) and of methyl bromide in the ozone layer depletion is well established (ref. 3).It is therefore not surprising that many halogenated derivatives are cast as environmental and health villains by various concerned groups who call for total phase out of chlorine and chlorinated hydrocarbons. 

A typical example of the interaction between hypothesis and experiment is the story of the work that resulted in worldwide concern over the depletion of the ozone layer in the stratosphere. These studies led to the awarding of the 1995 Nobel Prize for Chemistry to Paul Crutzen, Mario Molina, and F. Sherwood Rowland. Figure FT provides a schematic view of how this prize-winning research advanced. It began in 1971 when experiments revealed that chlorofluorocarbons, or CFCs, had appeared in the Earth s atmosphere. At the time, these CFCs were widely used as refrigerants and as aerosol propellants. Rowland wondered what eventually would happen to these gaseous compounds. He carried out a theoretical analysis, from which he concluded that CFCs are very durable and could persist in the atmosphere for many years. 

The chlorine atoms in the upper atmosphere come from the breakdown of CF2 CI2 and other similar chlorofluorocarbons (CFCs), known commercially as Freons. Production of these compounds was more than one million tons in 1988, largely for use in relrigerators and air conditioners. Once released into the atmosphere, CFCs diffuse slowly upward in the atmosphere until they reach the ozone layer. There, ultraviolet light Irom the sun splits off chlorine atoms. These react with ozone, with dramatic results. Annual ozone decreases have exceeded 50% above Antarctica. The background photo shows the Antarctic hole (red-violet) on September 24, 2003. 

Over the next two years, Midgley and his group discovered and patented other chlorofluorocarbons and the halons, a class of bromofluoro-carbon compounds that are the world s best Are fighters. At the time, their remarkable stability seemed like an advantage. In the 1970s scientists were able to determine that CFCs and halons—which are so stable that they remain in the atmosphere for long periods of time—deplete the ozone layer 15 miles above Earth. 

The chlorofluorocarbon effect on the ozone layer illustrates another chemical concern—the special problem that can arise when materials released into the environment are able to act as catalysts. If every chlorine atom generated in the upper atmosphere simply destroyed one ozone molecule, the effect would be minimal. But chemists have elucidated the catalytic cycle by which each chlorine atom destroys thousands of ozone molecules. It is particularly important for chemists to study and understand which substances can have such catalytic effects— and to learn how to prevent the release of such substances into the environment. 

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