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Chlorofluorocarbon

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A chlorofluorocarbon is an organic compound that contains carbon, chlorine, and fluorine. Most commonly, the term refers to a family of volatile derivatives of methane and ethane. Representative is chlorodifluoromethane, a precursor to tetrafluoroethylene. Many of these compounds have been widely used as refrigerants, propellants (in aerosol applications), and solvents. The manufacture of such compounds are being phased out by the Montreal Protcol because they contribute to ozone depletion.

Structure. properties, production

Like methane itself, the CFC's are tetrahedral molecules. The fluorine and chlorine atoms differ greatly in size from hydrogen and from each other. Consequently, the various CFC's deviate from the perfect tetrahedral symmetry of methane.[1]

The physical properties of the CFC's are tunable by changes in the number and identity of the halogen atoms. In general they are volatile but less so than methane because of their polarizability of the halides, especially chloride. The polarizability of the halides and the polarity of the molecules makes them useful as solvents. The CFC's are far less flammable than methane.

CFC's are usually produced by halogen exchange. Illustrative is the synthesis of chlorodifluoromethane from chloroform:

HCCl3 + 2 HF → HCF2Cl + 2 HCl

Original development of halomethanes

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 chlorofluorocarbons are used primarily as refrigerants, but also have uses in fire-fighting and as propellants in aersol cans. Bromomethane is widely used as a fumigant. Dichloromethane is a versatile industrial solvent.

The Belgian scientist Frederic Swarts pioneered the synthesis of chlorofluorocarbons (CFC) in the 1890s. He developed an effective exchange agent to replace chlorine in carbon tetrachloride with fluorine to synthesize CFC-11 (CCl3F) and CFC-12 (CCl2F2). In the late 1920s American engineer 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 at the time as refrigerants. The new compound had to have a low boiling point and be non-toxic and 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]

One of the attractive features of this group is that there exists a whole family of the compounds, each having unique properties to suit different applications. In addition to their use as refrigerants, CFCs have been used as propellants in aerosol cans, cleaning solvents for circuit boards, and blowing agents for making expanded plastics used as packaging materials.

During World War II, various chloroalkanes were in standard use in military aircraft by some combatants, but 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.

Work on alternatives for chlorofluorocarbons in refrigerants began in the late 1970s after the first warnings of damage to stratospheric ozone were published in the journal Nature in 1974 by Molina and Rowland (who shared the 1995 Nobel Prize in Chemistry for their work). Adding hydrogen and thus creating hydrochlorofluorocarbons (HCFC), chemists made the compounds less stable in the lower atmosphere, enabling them to break down before reaching the ozone layer. Later alternatives dispense with the chlorine, creating hydrofluorocarbons (HFC) with even shorter lifetimes in the lower atmosphere.

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 unreactivity—has been instrumental in making them one of the most significant pollutants. 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 break off, or homolytically cleave off the chlorine atom. This a highly reactive free radical catalyzes the break up of ozone into oxygen by means of a variety of mechanisms

By 1985, scientists observed a dramatic seasonal depletion of the ozone layer over Antarctica. Diplomats in Montreal in 1987 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.

Ozone-depleting gas trends

Because the only available CFC gases in countries adhering to the treaty is from recycling, their prices have gone up 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.

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 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

A number of substitutes for CFCs have been introduced. Hydrochlorofluorocarbons (HCFCs) are much more reactive than CFCs, so a large fraction of the HCFCs emitted break down in the troposphere, and hence are removed before they have a chance to affect 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. Development of non-chlorine based chemical compounds as a substitute for CFCs and HCFCs continues. One such class are the hydrofluorocarbons (HFCs), which contain only hydrogen and fluorine. One of these compounds, HFC-134a, is now used in place of CFC-12 in automobile air conditioners; which itself may contribute to global warming (see HFC-134a).

There is concern that halons are being broken down in the atmosphere to halogens, which react with ozone, leading to depletion of the ozone layer (this is similar to the case of chlorofluorocarbons such as freon). These issues are complicated: the kinds of fires that require halon extinguishers to be put out will typically cause more damage to the ozone layer than the halon itself, not to mention human and property damage. However, fire extinguisher systems must be tested regularly, and these tests may lead to damage. As a result, some regulatory measures have been taken, and halons are being phased out in most of the world.

In the United States, purchase and use of freon gases is regulated by the Environmental Protection Agency, 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.

PhostrEx is a fire suppression agent developed for use in aviation applications to replace halon. It was developed by Eclipse Aviation for use aboard their Eclipse 500 very light jets as an engine fire suppression system, and is now being marketed to other aviation manufacturers.

PhostrEx meets the requirements of both the Montreal Protocol and the Clean Air Act, and is the first commercially viable Federal Aviation Authority & United States Environmental Protection Agency‎ certified halon replacement fire extinguishing agent. It reacts very quickly with atmospheric moisture, breaking down into phosphoric acid and hydrogen bromide.

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


Principal CFC's
Systematic name Common/Trivial
name(s) 		Code Chem. formula
Trichlorofluoromethane Freon-11, R-11 CFC-11 CCl3F
Dichlorodifluoromethane Freon-12, R-12 CFC-12 CCl2F2
Chlorotrifluoromethane CFC-13 CClF3
Chlorodifluoromethane R-22 HCFC-22 CHClF2
Chlorofluoromethane Freon 31 CH2ClF
Bromochlorodifluoromethane BCF, Halon 1211 BCF, or Freon 12B1 Halon 1211 CBrClF2
1,1,2-Trichloro-1,2,2-trifluoroethane Trichlorotrifluoroethane CFC-113 Cl2FC-CClF2
1,1,1-trichloro-2,2,2-trifluoroethane CFC-113a Cl3C-CF3
1,2-Dichloro-1,1,2,2-tetrafluoroethane Dichlorotetrafluoroethane ClF2C-CClF2
1-Chloro-1,1,2,2,2-pentafluoroethane Chloropentafluoroethane ClF2C-CF3
2-Chloro-1,1,1,2-tetrafluoroethane CHFClCF3
1,1-Dichloro-1-fluoroethane Cl2FC-CH3
1-Chloro-1,1-difluoroethane ClF2C-CH3

References

  1. ^ Günter Siegemund, Werner Schwertfeger, Andrew Feiring, Bruce Smart, Fred Behr, Herward Vogel, Blaine McKusick “Fluorine Compounds, Organic” Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002. doi:10.1002/14356007.a11_349
  2. ^ McNeill, J.R. Something New Under the Sun: An Environmental History of the Twentieth-Century World (2001) New York: Norton, xxvi, 421 pp. (as reviewed in the Journal of Political Ecology)
  3. ^ Welcome to the Halon Corporation
  4. ^ Regulatory Programs | Ozone Depletion | US EPA
  5. ^ HCFC Phaseout Schedule
  6. ^ Use of Ozone Depleting Substances in Laboratories. TemaNord 516/2003


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