Chlorofluorocarbon ( CFC , chemical nomenclature by IUPAC : chloro fluorocarbons , HCFCs , or Freons ) are an extensive chemical group of low-molecular organic compounds used as propellants , refrigerants or solvents are used. CFCs are hydrocarbons in which hydrogen atoms have been replaced by the halogens chlorine and fluorine ; they are a subgroup of the halogenated hydrocarbons . CFCs that contain only single bonds are called saturated CFCs. If the compound no longer contains hydrogen, it is called chlorofluorocarbons . In the course of the 1970s and 1980s it became clear that the release of CFCs into the atmosphere is to a large extent responsible for the depletion of the ozone layer in the stratosphere (“ ozone hole ”), which is why the use of CFCs is now prohibited in many areas of application.
With HCFCs "partially halogenated" chlorofluorocarbon be called, its hydrogen atoms are partially replaced by chlorine and fluorine atoms: They have a much lower ozone depleting potential than CFCs, their "global warming potential" is also well below that of CFCs. In addition, the HCFCs are already broken down in the troposphere and only partially reach the stratosphere.
CFCs are very resistant, non-flammable, odorless, transparent (colorless) and are often non-toxic or have only low toxicity . The CFCs of the methane and ethane series have a low boiling point and can be easily liquefied by compression. Since they can absorb large amounts of heat during evaporation, they are particularly important as a coolant (see below). CFCs have because of their inertness high residence time in the atmosphere . They therefore rise up into the stratosphere and are broken down there by the UV rays. This releases chlorine or fluorine radicals , which react with the ozone in the ozone layer and damage it. In 1981, Veerabhadran Ramanathan described that the very strong greenhouse effect of chlorofluorocarbons alone would warm the earth's atmosphere by a whole degree by the year 2000 if emissions of this gas were not dramatically reduced.
In 2007, three CFCs with atypical properties - very reactive and toxic - were detected in the atmosphere.
Since there are no known natural reactions that release CFCs, spectral analysis will be used in the future to search for CFCs in the exoplanet's atmospheres , as that would mean that there is probably a CFC-making civilization there.
Direct fluorination of alkanes is difficult to carry out because the highly exothermic reaction usually takes place in an explosive manner and almost always leads to a mixture of perfluorinated compounds. Technically, chlorofluoroalkanes are obtained by fluorinating the corresponding chloroalkanes with anhydrous hydrogen fluoride on fixed bed catalysts made from aluminum or chromium fluorides. It is also possible to use an antimony (V) chloride catalyst.
- example 1
- Conversion of carbon tetrachloride with anhydrous hydrogen fluoride into a mixture of dichlorodifluoromethane , trichlorofluoromethane and hydrogen chloride :
- Example 2
- Reaction of chloroform with anhydrous hydrogen fluoride in chlorodifluoromethane (Freon 22). Antimony (V) chloride is used as a catalyst here.
History and use
At the end of the 19th century, the first halogenated hydrocarbons were produced by direct fluorination (Moissan) and electrophilically catalyzed halogen exchange (Swarts). The first CFCs (CFCl 3 and CF 2 Cl 2 ) were synthesized in 1929 by Thomas Midgley at General Motors . From 1930, CFCs were technically produced and increasingly used as a refrigerant in refrigeration machines , as a propellant for spray cans , as a propellant for foams , as a cleaning agent and solvent . Its use as a refrigerant in refrigerators has been banned since 1995, as CFCs contribute to the destruction of the ozone layer .
The chemists Harold D. Johnston, Paul J. Crutzen (actually a meteorologist), Frank Sherwood Rowland and Mario J. Molina discovered the radical mechanism in the early 1970s. a. CFCs converted into reactive radicals that can destroy the ozone in the stratosphere. The use of CFCs was first warned in 1974, but it was not taken seriously. The discovery of the ozone hole in 1985 caused a change of opinion. In the Montreal Protocol of September 16, 1987, many countries pledged to drastically reduce the production of CFCs. On June 29, 1990, the international conference on the protection of the ozone layer in London (see also London conference ) decided to ban or at least severely restrict the production and use of CFRP and CFC from the year 2000. The agreement envisaged reducing CFC use by 50% by 1995 and by 85% by 1997. As of 2018, it was publicly perceived that an unknown source in East Asia had been emitting significant amounts of CFCs of around 13,000 t annually since around 2012. According to research by the Environmental Investigation Agency , this is due to the fact that numerous Chinese building material manufacturers use CFC-11 in the production of insulating foam. The chemical stability makes these gases difficult to degrade in the atmosphere (average residence time between 44 and 180 years, depending on the product).
The main CFC refrigerants:
|designation||Common name||Molecular formula||boiling point|
|Trichlorofluoromethane||Friday 11||CCl 3 F||24.9 ° C|
|Dichlorodifluoromethane||Friday 12||CCl 2 F 2||−30° C|
|Dichlorofluoromethane||Friday 21st||CHCl 2 F||8.9 ° C|
|Chlorodifluoromethane||Frigen 22||CHClF 2||−40.7 ° C|
|1,1,2-trichloro-1,2,2-trifluoroethane||Frigen 113||CClF 2 -CCl 2 F||48° C|
|1,2-dichloro-1,1,2,2-tetrafluoroethane||Frigen 114 or Cryofluoran||CClF 2 - CClF 2||3.5 ° C|
The chlorine radical breaks down ozone into biatomic oxygen. The chlorine bound to the oxygen is released again, creating molecular chlorine. With a photon of suitable energy, chlorine radicals are released again, which means that the cycle can start over:
CFCs also absorb solar radiation in the infrared range (more strongly than CO 2 ) and contribute differently to global warming according to their respective global warming potential (in CO 2 equivalent) . Some CFCs exceed the global warming potential of carbon dioxide by ten thousand times.
Alternatives to CFC-based propellants for aerosol - spraying include. a. HFA-134a , which does not affect the ozone layer, but promotes the greenhouse effect . Most of the time, however, an alkane mixture of propane and butane that is easy to liquefy under pressure is used, which is why these spray cans carry the extremely flammable hazard symbol . When it comes to refrigerants , propane, butane, pentane , ammonia , 2,3,3,3-tetrafluoropropene or carbon dioxide as well as the chlorine-free refrigerants such as R134a , R404a etc. are available as alternatives, whereby it should be noted that the first three substances are flammable, Ammonia and 2,3,3,3-tetrafluoropropene are corrosive and toxic .
Nitrogen trifluoride , which has been used since then, was recommended as an alternative to CFCs in the electronics industry in the manufacture of flat screens, solar cells and microcircuits . In 2008, new measuring methods demonstrated its concentration in the atmosphere and the significant damage to the climate.
Improper recycling of old refrigerators in Germany still releases large amounts of CFCs into the atmosphere (as of 2008), while in Austria far more CFCs are separated during recycling.
- Fluorocarbons (HFC)
- Ulrich Schurath: Fluorocarbons - an environmental risk? In: Chemistry in our time , Volume 11, 1977, No. 6, pp. 181–189, doi : 10.1002 / ciuz.19770110604 .
- Peter Frieß, Andreas Fickers (eds.): Dieter Bärmann and Wolfgang Lohbeck talk about “Advertising against environmental destruction?” The introduction of the first CFC- and HFC-free refrigerator (= TechnikDialog , issue 8). Deutsches Museum / Lemmens, Bonn 1997, ISBN 3-932306-05-8 .
- Vollhardt, K. Peter C .; Schore, Neil E .; Butenschön, Holger: Organic Chemistry. 5th edition New York: John Wiley & Sons, 2011, p. 133.
- Spencer Weart: The Discovery of Global Warming: Other Greenhouse Gases . Center of History at the American Institute of Physics , aip.org
- JC Laube and A. Engel: First atmospheric observations of three chlorofluorocarbons (PDF; 504 kB). in: Atmos. Chem. Phys. Discuss. , 2008, 8, pp. 6683-6695.
- Kai Stoppel: "CFCs would be an indication of a civilization". Retrieved November 27, 2019 .
- H. Beyer, W. Walter, Fluorinated hydrocarbons . Textbook of organic chemistry , 22nd edition, S. Hirzel Verlag, Stuttgart 1991, p. 138ff.
- ozone killer: a prohibited substance in the atmosphere - WORLD. In: welt.de. Retrieved May 18, 2018 .
- Ozone hole-forming chemical emissions increasing and mysterious source in East Asia may be responsible - The Independent. In: independent.co.uk. Retrieved May 18, 2018 .
- CFC: Why China continues to blow the dangerous chemical into the air - WELT. In: welt.de. Retrieved October 10, 2018 .
- Super greenhouse gas discovered in the atmosphere , Scinexx, Springer.
- Güven Purtul: “Problem case in the shredder” , Süddeutsche Zeitung , May 20, 2008, accessed on July 26, 2010.
- Dieter Bärmann, Head of the Refrigeration Product Division, BSH Bosch and Siemens Hausgeräte GmbH, Hersbruck and Wolfgang Lohbeck , senior Greenpeace employee.