Contents

Chlorofluorocarbon

Like simpler alkanes, carbon in the CFC's and the HCFC's is tetrahedral. The fluorine and chlorine atoms differ greatly in size from hydrogen and from each other. Consequently for the methane derivatives, the various CFC's deviate from the perfect tetrahedral symmetry.^[1]

The physical properties of the CFC's and HCFC's are tunable by changes in the number and identity of the halogen atoms. In general they are volatile, but less so than parent alkane. The decreased volatility is attributed to the polarizability of halides, which induces intermolecular van der Waals interactions. Thus, methane boils at -161 °C whereas the fluoromethanes boil between -51.7 (CF₂H₂) and - 128 °C (CF₄). The CFCs have still higher boiling points because the chloride is even more polarizable than fluoride. The polarity of the CFCs makes them useful as solvents. The CFCs are far less flammable than methane, in part because they contain fewer C-H bonds and in part because the released halides quench the free radicals that sustain flames.

The densities of CFC's are invariably higher than the corresponding alkanes. In general the density of these compounds correlates with the number of chlorides.

CFC's and HCFC's are usually produced by halogen exchange starting from chlorinated methanes and ethanes. Illustrative is the synthesis of chlorodifluoromethane from chloroform:

HCCl₃ + 2 HF → HCF₂Cl + 2 HCl

The brominated derivatives are generated by free-radical reactions of the chlorofluorocarbons, replacing C-H bonds with C-Br bonds. The production of the anesthetic 2-bromo-2-chloro-1,1,1-trifluoroethane ("halothane") is illustrative:

CF₃CH₂Cl + Br₂ → CF₃CHBrCl + HBr

Reactions

The most important reaction of the CFC's is the photo-induced scission of a C-Cl bond:

CF₃Cl → CF₃ + Cl^.

The chlorine atom, written often as Cl^., behaves very differently from the chlorine molecule (Cl₂), which is not a radical. The radical Cl^. is long-lived in the upper atmosphere where it catalyzes the conversion of ozone into O₂. Ozone absorbs UV-radiation better than does O₂, so its depletion causes more of this high energy radiation to reach the surface. Bromine atoms are even more efficient catalysts, hence brominated CFC's are also regulated.

Applications

Applications exploit the low toxicity and low flammability of the CFC's, as well as their relatively tunable volatilities. Uses include refrigerants, blowing agents, propellants in medicinal applications, and degreasing solvents. The large-scalle applications require that the CFC's be replaced (see table below).

Classes of compounds

Every permutation of fluorine, chlorine, bromine, and hydrogen have been examined for one and two carbon atoms, and many examples are known for higher numbers of carbon for fluorine, chlorine, and hydrogen. The main classes of of compounds are:

• Chlorofluorocarbons derived from methane have the formula CCl_mF_nH_4-m-n, where n and m are > 0.
• Chlorofluorocarbons derived from ethane have the formula C₂Cl_xF_yH_4-x-y, where x and y are > 0.
• Bromochlorofluorocarbons and bromofluorocarbons are also well studied as they are superior fire suppressants.

The phase-out of the CFCs has led to much work on compounds containing only carbon, hydrogen, and fluorine.

History

Carbon tetrachloride was used in fire extinguishers and glass "anti-fire grenades" from the late nineteenth century until around the end of World War II. Experimentation with chloroalkanes for fire suppression on military aircraft began at least as early as the 1920s. Freon is a trade name for a group of CFCs which are used primarily as refrigerants, but also have uses in fire-fighting and as propellants in aerosol cans. Bromomethane is widely used as a fumigant. Dichloromethane is a versatile industrial solvent.

The Belgian scientist Frédéric Swarts pioneered the synthesis of CFC's in the 1890s. He developed an effective exchange agent to replace chloride in carbon tetrachloride with fluoride to synthesize CFC-11 (CCl₃F) and CFC-12 (CCl₂F₂).

In the late 1920s, Thomas Midgley improved the process of synthesis and led the effort to use CFC as refrigerant to replace ammonia (NH₃), chloromethane (CH₃Cl), and sulfur dioxide (SO₂), which are toxic but were in common use. In searching for a new refrigerant, requirements for the compound were: low boiling point, low toxicity, and to be generally non-reactive. In a demonstration for the American Chemical Society, Midgley flamboyantly demonstrated all these properties by inhaling a breath of the gas and using it to blow out a candle.^[2]

Commercial development and use of CFCs and related compounds

During World War II, various chloroalkanes were in standard use in military aircraft, although these early halons suffered from excessive toxicity. Nevertheless, after the war they slowly became more common in civil aviation as well. In the 1960s, fluoroalkanes and bromofluoroalkanes became available and were quickly recognized as being highly effective fire-fighting materials. Much early research with Halon 1301 was conducted under the auspices of the US Armed Forces, while Halon 1211 was, initially, mainly developed in the UK. By the late 1960s they were standard in many applications where water and dry-powder extinguishers posed a threat of damage to the protected property, including computer rooms, telecommunications switches, laboratories, museums and art collections. Beginning with warships, in the 1970s, bromofluoroalkanes also progressively came to be associated with rapid knockdown of severe fires in confined spaces with minimal risk to personnel.

By the early 1980s, bromofluoroalkanes were in common use on aircraft, ships, and large vehicles as well as in computer facilities and galleries. However, concern was beginning to be felt about the impact of chloroalkanes and bromoalkanes on the ozone layer. The Vienna Convention on Ozone Layer Protection did not cover bromofluoroalkanes as it was thought, at the time, that emergency discharge of extinguishing systems was too small in volume to produce a significant impact, and too important to human safety for restriction.

Regulation

Since the late 1970s, the use of CFCs has been heavily regulated because of their destructive effects on the ozone layer. After the development of his electron capture detector, James Lovelock was the first to detect the widespread presence of CFCs in the air, finding a concentration of 60 parts per trillion of CFC-11 over Ireland. In a self-funded research expedition ending in 1973, Lovelock went on to measure the concentration of CFC-11 in both the Arctic and Antarctic, finding the presence of the gas in each of 50 air samples collected, but incorrectly concluding that CFCs are not hazardous to the environment. The experiment did however provide the first useful data on the presence of CFCs in the atmosphere. The damage caused by CFCs discovered by Sherry Rowland and Mario Molina who, after hearing a lecture on the subject of Lovelock's work, embarked on research resulting in the first publication suggesting the connection in 1974. It turns out that one of CFCs' most attractive features—their low reactivity— is key to their most destructive effects. CFCs' lack of reactivity gives them a lifespan that can exceed 100 years, giving them time to diffuse into the upper stratosphere. Once in the stratosphere, the sun's ultraviolet radiation is strong enough to cause the homolytic cleavage of the C-Cl bond.

By 1987, in response to a dramatic seasonal depletion of the ozone layer over Antarctica, diplomats in Montreal 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.

Because the only CFCs available countries adhering to the treaty is from recycling, their prices have increased considerably. A worldwide end to production should also terminate the smuggling of this material, such as from Mexico to the United States.

By the time of the Montreal Protocol it was realised that deliberate and accidental discharges during system tests and maintenance accounted for substantially larger volumes than emergency discharges, and consequently halons were brought into the treaty, albeit with many exceptions.

Licensing in the US

In the United States, purchase and use of freon gases is regulated by the US EPA, and substantial fines have been levied for their careless venting. Also, licenses, good for life, are required to buy or use these chemicals. The EPA website discusses these rules in great detail, and also lists numerous private companies that are approved to give examinations for these certificates.

There are two kinds of licenses. Obtaining a "Section 609" license to use CFCs to recharge old (pre-1993 model year) car air conditioners is fairly easy and requires only an online multiple choice test offered by several companies. Companies that use unlicensed technicians for CFC recharge operations are subject to a US$15,000 fine per technician by the EPA.

The "Section 608" license, needed to recharge CFC-using stationary and non-automobile mobile units, is also multiple choice but more difficult. A general knowledge test is required, plus separate exams for small size (such as home refrigerator) units, and for high and low pressure systems. These are respectively called Parts I, II, and III. A person who takes and passes all tests receives a "Universal" license; otherwise, one that is endorsed only for the respectively passed Parts. While the general knowledge and Part I exams can be taken online, taking them before a proctor (which has to be done for Parts II and III) lets the applicant pass these tests with lower scores.

Phase out of CFC's

Use of certain chloroalkanes as solvents for large scale application, such as dry cleaning, have been phased out, for example, by the IPPC directive on greenhouse gases in 1994 and by the Volatile Organic Compounds (VOC) directive of the EU in 1997. Permitted chlorofluoroalkane uses are medicinal only.

Bromofluoroalkanes have been largely phased out and the possession of such equipment is prohibited in some countries like the Netherlands and Belgium, from 1 January 2004, based on the Montreal Protocol and guidelines of the European Union.

Production of new stocks ceased in most (probably all) countries as of 1994. However many countries still require aircraft to be fitted with halon fire suppression systems because no safe and completely satisfactory alternative has been discovered for this application. There are also a few other, highly specialized uses. These programs recycle halon through "halon banks" coordinated by the Halon Recycling Corporation^[3] to ensure that discharge to the atmosphere occurs only in a genuine emergency and to conserve remaining stocks.

In the U.S. technicians and others who buy or work with CFC or HCFC gases must pass licensing examinations set by the Environmental Protection Agency. There is one test for the Part 609 license, which allows a person to work on automobile air conditioners. This can be taken on line. There are four examinations for a full Part 608 license, which allows the holder to work on all other types of refrigeration and air conditioning equipment. These tests are given by private groups approved by the US EPA.^[4] The venting of freon, failure to be licensed, or not using approved recovery equipment, can result in substantial fines.

On September 21, 2007, approximately 200 countries agreed to accelerate the elimination of hydrochlorofluorocarbons entirely by 2020 in a United Nations-sponsored Montreal summit. Developing nations were given until 2030. Many nations, such as the United States and China, who had previously resisted such efforts, agreed with the accelerated phase out schedule.^[5]

Development of alternatives for CFC's

Work on alternatives for chlorofluorocarbons in refrigerants began in the late 1970s after the first warnings of damage to stratospheric ozone were published. The hydrochlorofluorocarbons (HCFCs) are less stable in the lower atmosphere, enabling them to break down before reaching the ozone layer. Nevertheless, a significant fraction of the HCFCs do break down in the stratosphere and they have contributed to more chlorine buildup there than originally predicted. Later alternatives lacking the chlorine, the hydrofluorocarbons (HFCs) have an even shorter lifetimes in the lower atmosphere. One of these compounds, HFC-134a, is now used in place of CFC-12 in automobile air conditioners.

Various other solvents and methods have replaced the use of CFCs in laboratory analytics.^[6]

References

External links

• Gas conversion table
• Nomenclature FAQ
• Numbering Scheme for Ozone-Depleting Substances and their Substitutes
• Class I Ozone-Depleting Substances
• CFC Smuggling
• CFC Illegal Trade
• Numbering Scheme for Ozone-Depleting Substances and their Substitutes
• Class I Ozone-Depleting Substances
• Class II Ozone-Depleting Substances (HCFC's)
• History of Halon use by the US Navy
• Ozone Loss: The Chemical Culprits
• Process using pyrolysis in an ultra high temperature plasma arc for the elimination of CFCs
• [1] Environmental Investigation Agency
• [2] Environmental Investigation Agency in the USA