Chlorofluorocarbon

Chlorofluorocarbons (CFCs ), chemical nomenclature according to IUPAC: chlorine fluorocarbons ( HCFCs ), are an extensive chemical group of low molecular weight organic compounds, which are used as propellants, refrigerants or solvents. CFCs are hydrocarbons in which hydrogen atoms have been replaced by the halogens chlorine and fluorine, they are a subset of the halocarbons. CFCs contain only single bonds, is called saturated CFCs. In the compound contain no more hydrogen, as it is called chlorofluorocarbons. Throughout the 1970s and 1980s, it was found that the release of CFCs into the atmosphere to a large extent is responsible for the degradation of the ozone layer in the stratosphere ( ozone hole ), which is why the use of CFCs is now banned in many application areas.

Under HCFCs are understood partially halogenated chlorofluorocarbons, whose hydrogen atoms are partially replaced by chlorine and fluorine atoms. They have a much lower ozone depletion potential than CFCs, their global warming potential is also far below that of the CFCs. In addition, the HCFCs are already degraded in the troposphere and reach only partially into the stratosphere.

Properties

CFCs are very stable, non-flammable, odorless, transparent ( colorless) and are often toxic or have low toxicity. The CFC of methane and ethane series have a low boiling point and can be easily liquefied by compression. CFCs have a high residence time in the atmosphere because of their inertness. They, therefore, rise up into the stratosphere where they are broken down by the UV rays. In this case, chlorine or fluorine radicals are released which will react with the ozone of the ozone layer, and damage it. In 2007, three CFCs with atypical features - very reactive and toxic - detected in the atmosphere.

Production

A direct fluorination of alkanes is difficult to perform, since the highly exothermic reaction usually explosive runs and almost always leads to a mixture of perfluorinated compounds. Technically you win chlorofluoroalkanes by fluorination of the corresponding chloro alkanes with anhydrous hydrogen fluoride at fixed bed catalysts made ​​of aluminum or chromium fluorides. Also possible is the use of an antimony ( V) chloride catalyst.

• Example 1: Reaction of carbon tetrachloride with anhydrous hydrogen fluoride in a mixture of dichlorodifluoromethane, trichlorofluoromethane and hydrogen chloride:

• Example 2: Reaction of chloroform with anhydrous hydrogen fluoride in chlorodifluoromethane (Freon 22). Here is used as the catalyst of antimony (V ) chloride for application.

It is also possible electrofluorination Simons. The anodic fluorination is carried out in anhydrous hydrogen fluoride at a voltage not sufficient to release elemental fluorine.

History and Use

End of the 19th century, the first halogenated hydrocarbons by direct fluorination ( Moissan ) and electrophilic catalyzed halogen exchange ( Swarts ) were prepared. The first CFC ( CFCl3 and CF2Cl2 ) were synthesized in 1929 by Thomas Midgley at General Motors. In 1930, CFCs were produced industrially and are increasingly used as refrigerants in refrigeration, as propellant for aerosol cans, as blowing agents for foams, as cleaners and solvents. Its use as a refrigerant in refrigerators has been banned since 1995 because CFCs contribute to ozone depletion.

Brominated CFCs have been used as fire extinguishing agents and are also known as halons.

Before the use of CFCs was first warned in 1974, but this was not taken seriously. The discovery of the ozone hole in 1985 caused a change of opinion. The Montreal Protocol of 16 September 1987, many states committed to a drastic reduction in the production of CFCs. On 29 June 1990, the International Conference on the Protection of the Ozone Layer in London decided (see also London Conference (1990)), the production and use of CFRP and CFCs from 2000 to ban or at least severely restrict. The agreement stipulated to 1995 to reduce CFC use by 50 % by 1997 by 85%. The chemical stability makes these gases in the atmosphere poorly degradable ( mean residence time depending on the product between 44-180 years).

The main CFC refrigerants:

Environmental impact

The low molecular weight, hydrogen-free carbon fiber reach due to their chemical stability and their great volatility in the stratosphere and react with the ozone layer. example:

Here, a photon of appropriate frequency, and a chlorine radical.

The chlorine radical builds ozone from oxygen to biatomarem. The enzyme bound to the oxygen, chlorine is free again, with molecular chlorine is produced. By a photon of appropriate energy chlorine radicals are released from it again, so the cycle can start again:

Thus, the ozone layer is destroyed. Without their protective effect may be harsh UV rays penetrate to the Earth's surface and cause damage to plants, animals and humans.

CFC also absorb solar radiation (stronger than CO2) in the infrared region and contribute according to their respective global warming potential (in CO2 equivalent) differently to global warming. Some CFCs exceed the global warming potential of carbon dioxide by ten thousand times.

Alternatives

Alternatives to CFC- based propellants for aerosol atomization thereof include HFA -134a, which does not affect the ozone layer, but promotes the greenhouse effect. Usually, however, an easy-to liquefying pressure alkane mixture of propane and butane is used, which is why these spray cans carry the hazard symbol extremely flammable. In the refrigerants to propane, butane, pentane, ammonia, 2,3,3,3 - Tetrafluoropropene or carbon dioxide and the chlorine-free refrigerant such as R134a, R404a, etc. as alternatives offer, bearing in mind that the first three substances flammable, ammonia and 2,3,3,3 - tetrafluoropropene are corrosive and toxic.

As an alternative to CFCs in the electronics industry in the manufacture of flat panel displays, solar cells and microcircuits nitrogen trifluoride was recommended, which has since been used. New measurement method reported after 2008 its concentration in the atmosphere and the significant climate damage.

Improper recycling of Altkühlschränken still get large amounts of CFCs in the atmosphere.