Chlorofluorocarbons (CFC)
Chlorofluorocarbons (CFCs) are synthetic compounds that were widely utilized in refrigeration, aerosol propellants, and solvents due to their stable, odorless, and nonflammable properties. However, these compounds became known for their detrimental impact on the ozone layer, leading to the adoption of the Montreal Protocol in 1987, which aimed to phase out their production and use. CFCs do not occur naturally; instead, they are created by replacing hydrogen in hydrocarbons with halogens like chlorine and fluorine. The most common CFCs, such as trichlorofluoromethane (R-11) and dichlorodifluoromethane (R-12), were essential in many industrial applications until their environmental consequences became evident.
Research in the 1970s linked CFC emissions to ozone depletion, a critical issue highlighted by the discovery of the ozone hole over Antarctica. This depletion allows increased ultraviolet radiation to reach the Earth's surface, contributing to health risks like skin cancer and environmental damage. In response, international agreements have led to the gradual elimination of CFCs and the introduction of alternatives, including hydrofluorocarbons (HFCs) and natural refrigerants. Despite progress, concerns remain over the greenhouse gas potential of some substitutes, emphasizing the ongoing need for sustainable solutions in chemical production and usage.
Subject Terms
Chlorofluorocarbons (CFC)
Summary: Chlorofluorocarbons have been widely used in refrigerants, propellants, and solvents. The manufacture of these compounds began to be phased out in 1987 after the adoption of the Montreal Protocol, because they contribute to depletion of Earth’s protective ozone layer.
Chlorofluorocarbons (CFCs) are very stable, odorless, colorless, nontoxic, volatile, and nonflammable organic compounds, which belong to the halogenated hydrocarbons. Originally these properties made them popular and useful for technical applications, but that was prior to the discovery of their destructive impact on Earth’s ozone layer. They do not occur naturally but are synthesized from hydrocarbons, such as methane and ethane, by substituting hydrogen atoms through halogens, such as chlorine, fluorine, and bromine. When they additionally contain hydrogen, they are called hydrochlorofluorocarbons (HCFCs). The most usual representatives, before adoption of the Montreal Protocol, were trichlorofluoromethane (CFCl3 or R-11) and dichlorodifluoromethane (CF2Cl2 or R-12). The company DuPont launched the brand name Freon for CFCs, HCFCs, and related compounds. HCFCs that have solely single bonds are called saturated HCFCs.
Chemistry
At the end of the 19th century, the chemists Henri Moissan and Frédéric Swarts, independently of each other, synthesized the first halogenated hydrocarbons. In 1929, the Belgian Thomas Midgley developed a technical process for General Motors and introduced CFCs as refrigerants. After 1930, CFCs were produced industrially and increasingly used as refrigerants and propellants in aerosol cans and foam.
All CFCs and HCFCs can be modified regarding their physical properties, regulated by the number and kind of halogen atoms. Because CFCs are polarizable, they are useful solvents and detergents for textiles and sensitive electronic parts. Because they are nonflammable, they have been widely used as fire-extinguishing agents. Chlorodifluoromethane is still produced as a precursor to tetrafluorethylene, a chemical precursor for Teflon.
Fluorinated alkanes are coded in a numbering system, prefixed with Freon-, R-, CFC-, or HCFC-. One method to get the molecular formula is to add 90 to the numbering in the code. The three-digit sum gives the number of carbons as the first digit, the second digit gives the number of hydrogen atoms, and the third digit gives the number of fluorine atoms. Unaccounted carbon bonds are occupied by chlorine atoms. For example, CFC-12 is a code for CCl2F2: 90 + 12 = 102 (that is, 1 carbon, 0 hydrogen, 2 fluorine atoms, and hence 2 chlorine atoms).
Ozone Hole
In the stratosphere, under the influence of ultraviolet (UV) radiation, the ozone layer is formed, naturally establishing an equilibrium condition. The ozone layer is a vital shield against UV radiation, and without it life could not exist as it does on Earth. In 1973, James Lovelock reported measurements, taken in the Arctic and the Antarctic, demonstrating significant enrichment of CFCs in the atmosphere. One year later, Sherwood Rowland and Mario Molina discovered that CFCs damage the ozone layer. Because of the protective function of the ozone layer, the UV radiation reaching the troposphere is no longer capable of depleting HCFCs. These chemical compounds have, because of their low reactivity, a very long lifetime of more than 100 years, allowing them to rise up into the stratosphere, about 12 to 30 miles (20 to 50 kilometers) above the Earth’s surface. In the stratosphere, the sun’s ultraviolet radiation is strong enough to split the carbon-chlorine (C–Cl) bonds.
CCl3F g CCl2F· + Cl·
The chlorine radical Cl· acts as catalyst, converting ozone into O2. Oxygen does not absorb as much UV radiation as ozone, so more high-energy radiation can reach the Earth’s surface.
The depletion of (H)CFCs leads eventually to ClO radicals, reacting with nitrogen dioxide (NO2) to form chlorine nitrate (ClONO2) or with nitrogen monoxide (NO) and methane to form salt acid (HCl) and nitric acid (HNO3). HCl and ClONO2 are relatively stable and do not react with ozone.
In 1985, the ozone hole was discovered above the Antarctic. Here the conditions necessary for an intense ozone depletion can be found. They are so special that such processes were not described until the ozone hole was actually observed. Extremely low temperatures of minus 80 degrees Celsius or lower, prevalent during polar nights, enable nitric acid and water to form stratospheric ice clouds. At the surface of such ice clouds, HCl and ClONO2 can react to form nitric acid (HNO3) and molecular chlorine (Cl2), which is not reactive with ozone. Upon strong UV radiation, provided by the polar dawn, however, it can be split into two radicals, attacking ozone, forming ClO. Usually this happens in very high layers of the atmosphere, almost above the ozone layer. Above Antarctica this occurs only 8.6 to 13.6 miles (14 to 22 kilometers) above the Earth’s surface, due to the polar vortex, a continuous cyclone around the pole, transporting ClO downward, where the ozone layer is affected at its core.
A depletion of the ozone layer causes greater amounts of UV radiation to reach the planet’s surface, resulting in increased rates of skin cancer and crop failures. By 1978, the United States had banned the use of CFCs in aerosol cans. Another early regulatory treaty, the Vienna Convention for the Protection of the Ozone Layer, did not cover bromofluoroalkanes. Later, when it was proven that bromine atoms are even more efficient catalysts, brominated CFCs were also restricted.
In addition to the effect of (H)CFCs on the ozone layer, the long lifetime of (H)CFCs in the stratosphere and their capacity to absorb infrared radiation cause an enormous greenhouse potential: 3,000 to 8,000 times higher than carbon dioxide. Their abilities to destroy ozone and to act as greenhouse gases are, in principle, independent features: the first a chemical and the latter a physical property.
The Montreal Protocol
In 1987, alarmed by the ozone hole over Antarctica, many countries adopted a treaty, the Montreal Protocol, aiming to reduce the production of CFCs. It regulates gases that may increase the stratospheric concentration of chemical compounds containing chlorine or bromine. Further diplomatic agreements, by the European Community in 1989 and by the London Conference in 1990, led to general prohibitions of production and use of (H)CFCs until 2000 in major economic powers and until 2010 in developing countries.
Certain chloroalkanes used as solvents were phased out by the national Plant Protection Convention (IPPC) directive on greenhouse gases in 1994 and by the volatile organic compounds (VOCs) directive of the European Union (EU) in 1997. Chlorofluoroalkanes or brominated derivates are permitted for medicinal uses only. However, halon fire suppression systems are still used for many aircraft, because no single safe alternative has yet been discovered. Halon has been replaced in some aircraft uses, such as lavatory compartments and portable fire extinguishers, but cargo compartment systems still relied on halon in 2024. Halon recycling is coordinated by the Halon Recycling Corporation.
CFC emissions, when converted into CO2 equivalents, were reduced from 2 gigatons of carbon (GtC) per year in the late 1980s to 0.7 GtC in 2000. Because of the long atmospheric lifetime of CFCs, they remain in the stratosphere for decades.
Substitutes
Substitutes for CFCs are hydrofluorocarbons (HFCs), halocarbon-free fluids (HCFCs), diluted citric acids, and water. These lack chlorine and hence do not destroy ozone. However, increasing amounts of HFCs, as measured in 2008, demonstrated increasing atmospheric concentration and negative effects on climate. HFCs are potent greenhouse gases, up to 1,500 times more effective than carbon dioxide. HCFCs include carbon, chlorine, fluorine, and hydrogen atoms. HCFCs have a shorter atmospheric lifetime because the hydrogen reacts with tropsheric hydroxyl. Whereas CFC-12 has an atmospheric lifetime of one hundred years, HCFC-22 has an atmospheric lifetime of about twelve years. Often natural refrigerants, such as propane, butane, pentane, ammonia, and carbon dioxide, although also having negative features, are used as substitutes for CFCs. In the electronics industry, for example, nitrogen trifluoride has been chosen as a substitute in the production of flat screens and solar cells.
With the adoption of the Kyoto Protocol and the European Community Act 842/2006, the requirements for alternative compounds of HCFCs were extended, and their potential as greenhouse gases was also taken into account. In 2007, about 200 countries, including the United States and China, agreed in a United Nations-sponsored Montreal summit to strengthen the elimination of hydrochlorofluorocarbons with the goal of eliminating them entirely by 2020 in developed nations and by 2030 in developing nations. However, researchers found increases of five CFCs between 2010 and 2020. They concluded that emissions of three--CFC-113a, CFC-114a, and CFC-115--likely occur during production of HFC, but could not account for increasing emissions of CFC-13 and CFC-112a.
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