Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are chemicals that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F). These substances are created as volatile chemicals from methane, ethane, and propane.

A common CFC is dichlorodifluoromethane, also known as R-12 or Freon. R-12 is used as a refrigerant. Many CFCs have been used as refrigerants, propellants in aerosol sprays, fire suppression systems, and solvents. Because CFCs harm the ozone layer in the upper atmosphere, their production has been stopped under the Montreal Protocol. They are now being replaced by other chemicals, such as hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs), including R-410A, R-134a, and R-1234yf.

Structure, properties and production

In CFCs, carbon atoms form bonds with a tetrahedral shape, similar to simpler alkanes. However, fluorine and chlorine atoms are much larger and have different charges compared to hydrogen and each other. This causes methane-based CFCs to not have a perfectly tetrahedral shape.

The physical properties of CFCs and HCFCs depend on the number and type of halogen atoms (fluorine, chlorine, or bromine) in their molecules. These compounds are generally volatile, but less so than the alkanes they are based on. The reduced volatility happens because halogen atoms make molecules polar, which increases forces between molecules. For example, methane boils at −161 °C, while fluoromethanes boil between −51.7 °C (CF₂H₂) and −128 °C (CF₄). CFCs have higher boiling points than fluoromethanes because chlorine atoms are more polarizable than fluorine. This polarity makes CFCs useful as solvents, and their boiling points make them suitable for refrigeration. CFCs are much less flammable than methane because they have fewer C–H bonds, and the halogen atoms in chlorides and bromides help stop chemical reactions that cause flames.

CFCs are denser than their corresponding alkanes. Usually, the density of these compounds increases as the number of chlorine atoms in the molecule increases.

CFCs and HCFCs are often made through a process called halogen exchange, which starts with chlorinated methanes and ethanes. An example of this process is the synthesis of chlorodifluoromethane from chloroform.

Brominated compounds are created by replacing hydrogen atoms with bromine in hydrochlorofluorocarbons through free-radical reactions. An example of this is the production of the anesthetic 2-bromo-2-chloro-1,1,1-trifluoroethane ("halothane").

Applications

CFCs and HCFCs are used in many products because they are not easily harmful, do not react quickly with other substances, and are not easily flammable. Scientists have studied all possible combinations of fluorine, chlorine, and hydrogen based on methane and ethane. Most of these combinations are now used in products. Many compounds with more carbon atoms and those that include bromine are also known. These substances are used as refrigerants, blowing agents, aerosol propellants in medicine, and degreasing solvents.

Billions of kilograms of chlorodifluoromethane are made each year. This substance is used to create tetrafluoroethylene, which is the starting material for making Teflon.

Classes of compounds and Numbering System

• Chlorofluorocarbons (CFCs): when made from methane and ethane, these compounds have the formulas CClₘF₄−ₘ and C₂ClₘF₆−ₘ, where m is a number greater than zero.
• Hydrochlorofluorocarbons (HCFCs): when made from methane and ethane, these compounds have the formulas CClₘFₙH₄−ₘ−ₙ and C₂ClₓFᵧH₆−ₓ−ᵧ, where m, n, x, and y are numbers greater than zero.
• Bromofluorocarbons (BFCs): have formulas similar to CFCs and HCFCs, but also include bromine.
• Hydrofluorocarbons (HFCs): when made from methane, ethane, propane, and butane, these compounds have the formulas CFₘH₄−ₘ, C₂FₘH₆−ₘ, C₃FₘH₈−ₘ, and C₄FₘH₁₀−ₘ, where m is a number greater than zero.

A special numbering system is used for fluorinated alkanes, such as Freon-, R-, CFC-, and HCFC-. The rightmost number shows how many fluorine atoms are present. The next number to the left is the number of hydrogen atoms plus one. The next number to the left is the number of carbon atoms minus one (zeroes are not written). The remaining atoms are chlorine.

For example, Freon-12 indicates a methane derivative (only two numbers) with two fluorine atoms (the second number is 2) and no hydrogen atoms (1 minus 1 equals 0). This means the formula is CCl₂F₂.

Another method to find the molecular formula of CFC/R/Freon compounds is to add 90 to the numbering. The result gives the number of carbon atoms as the first digit, the number of hydrogen atoms as the second digit, and the number of fluorine atoms as the third digit. Any remaining bonds are filled with chlorine atoms. This method always produces a three-digit number. For example, CFC-12: 90 + 12 = 102 → 1 carbon, 0 hydrogens, 2 fluorine atoms, and 2 chlorine atoms, resulting in CCl₂F₂. This method is helpful because it directly shows the number of carbon atoms in the molecule.

Freons that include bromine are labeled with four numbers. Isomers, which are common in ethane and propane derivatives, are shown by letters added after the numbers.

Reactions

The reaction of CFCs that causes ozone depletion involves the splitting of a C-Cl bond when exposed to light. The chlorine atom, written as Cl, acts very differently from the chlorine molecule (Cl₂). The Cl radical remains in the upper atmosphere for a long time and helps change ozone into oxygen gas (O₂). Ozone absorbs UV-B radiation, which is harmful. When ozone decreases, more UV-B radiation reaches Earth's surface. Bromine atoms are even more effective at this process, so CFCs containing bromine are also regulated.

Impact as greenhouse gases

CFCs were no longer used because of the Montreal Protocol, which aimed to stop their harmful effects on the ozone layer.

CFCs affect the atmosphere in ways beyond damaging the ozone layer. They absorb heat in specific wavelengths, preventing it from escaping Earth. CFCs absorb most strongly in the range of 7.8–15.3 μm, a part of the atmosphere where heat usually passes through easily, called the "atmospheric window."

The strength of CFCs' heat-trapping abilities and the way the atmosphere responds to these wavelengths create a very strong greenhouse effect. This includes CFCs and other gases like perfluorocarbons, HFCs, HCFCs, bromofluorocarbons, SF₆, and NF₃. These gases absorb heat in the atmospheric window, and because they are present in small amounts, their impact grows more quickly as their levels increase. In contrast, CO₂ has high concentrations and fewer heat-trapping abilities, so its effect on warming is slower and less sensitive to changes in its levels.

People are working to remove old CFCs from the environment to reduce their harm.

According to NASA in 2018, the ozone hole is beginning to heal because of CFC bans. However, a 2019 study found a sudden rise in CFC levels, suggesting some uncontrolled use of these chemicals in China.

History

Before and during the 1920s, refrigerators used dangerous gases like ammonia, sulfur dioxide, and chloromethane as cooling agents. After several deadly accidents caused by leaking chloromethane from refrigerators in the 1920s, companies such as Frigidaire, General Motors, and DuPont worked together to create a safer, non-toxic alternative. Thomas Midgley Jr. of General Motors developed the first chlorofluorocarbons (CFCs). Frigidaire received the first patent for CFCs, number 1,886,339, on December 31, 1928. In 1930, Midgley demonstrated the safety of CFCs to the American Chemical Society by breathing in the gas and using it to blow out a candle.

By 1930, General Motors and DuPont formed the Kinetic Chemical Company to produce Freon, a type of CFC. By 1935, over 8 million refrigerators using R-12, a type of CFC, were sold by Frigidaire and its competitors. In 1932, Carrier used R-11, another CFC, in the world’s first self-contained home air conditioning unit called the "atmospheric cabinet." Because CFCs were not toxic, they quickly became the preferred coolant for large air-conditioning systems. City health rules later changed to allow only CFCs as refrigerants in public buildings.

CFC use grew rapidly in the decades that followed, with annual sales reaching over 1 billion USD and more than 1 million metric tonnes produced each year. In 1974, scientists F. Sherwood Rowland and Mario Molina discovered that CFCs were harming the Earth’s ozone layer. This finding led to the creation of the Montreal Protocol, an international agreement to reduce CFC use.

During World War II, chloroalkanes were commonly used in military aircraft, though these early halons were very toxic. After the war, they became more common in civil aviation. In the 1960s, fluoroalkanes and bromofluoroalkanes were developed and used as fire-fighting tools. The U.S. military tested Halon 1301, while the UK developed Halon 1211. By the late 1960s, these substances were used in places like computer rooms, museums, and laboratories, where water or dry powder extinguishers could damage sensitive equipment. In the 1970s, bromofluoroalkanes were used on ships, aircraft, and large vehicles to quickly control fires in enclosed spaces with little risk to people.

By the early 1980s, bromofluoroalkanes were widely used in aircraft, ships, and computer facilities. However, concerns grew about their impact on the ozone layer. The Vienna Convention for the Protection of the Ozone Layer did not restrict bromofluoroalkanes as much, because emergency use was thought to be too small to harm the ozone layer and too important for safety. Instead, their use was limited to 1986 levels.

Since the late 1970s, CFCs have been heavily regulated due to their harmful effects on the ozone layer. James Lovelock, after developing an electron capture detector, found CFC-11 in the air over Ireland at a level of 60 parts per trillion. In a self-funded research trip ending in 1973, he measured CFC-11 in the Arctic and Antarctic, finding the gas in 50 air samples. This was the first useful data about CFCs in the atmosphere. Later, Rowland and Molina discovered that CFCs damage the ozone layer because they are not reactive and can last over 100 years. In the stratosphere, sunlight breaks the C-Cl bond in CFCs, releasing chlorine that destroys ozone. In 1976, the EPA banned CFCs and aerosol propellants under the Toxic Substances Control Act. This was later replaced by the Clean Air Act amendments in 1990.

In 1987, diplomats met in Montreal to create the Montreal Protocol, which aimed to greatly reduce CFC production. In 1989, 12 European countries agreed to stop producing CFCs by the end of the century. In 1990, the treaty was strengthened to eliminate CFCs completely by 2000. By 2010, CFCs should have been fully phased out in developing countries.

Because countries following the treaty can only use recycled CFCs, their prices have risen. A global stop to production should end the illegal trade of CFCs. However, smuggling remains a problem. A 2006 UNEP report estimated that 16,000–38,000 tonnes of CFCs were smuggled in the mid-1990s, with 7,000–14,000 tonnes entering developing countries annually. Asian countries, including China, India, and South Korea, accounted for about 70% of global CFC production in 2007, though South Korea banned CFC production in 2010. Smuggling continues because older refrigeration systems designed for CFCs are still in use, and replacing them is sometimes cheaper than using ozone-friendly alternatives. In 2018, a UNEP report revealed that 13,000 metric tonnes of CFCs were illegally produced in East Asia each year since 2012.

By the time of the Montreal Protocol, it was clear that accidental or intentional releases

Tracer of ocean circulation

The history of CFC concentrations in the atmosphere is well understood, which helps scientists study ocean circulation. CFCs dissolve in seawater at the ocean surface and move into the deeper ocean. Since CFCs do not react with other substances, their concentration in the ocean reflects the combination of how their levels changed in the atmosphere over time and how the ocean circulates and mixes.

CFCs are valuable tools for tracking how water moves through the ocean. However, after production limits were introduced in the 1980s, the amounts of CFC-11 and CFC-12 in the atmosphere stopped increasing. This caused the ratio of CFC-11 to CFC-12 in the atmosphere to decrease steadily, making it harder to estimate the age of water masses. Meanwhile, sulfur hexafluoride (SF6) has been released into the atmosphere in much larger amounts since the 1970s. Like CFCs, SF6 does not react with ocean chemicals or life, so using both CFCs and SF6 together helps scientists better determine the age of water.

Using CFCs or SF6 as tracers allows scientists to calculate how quickly ocean processes occur. The time since a water mass last touched the atmosphere is called the "tracer-derived age." This age can be estimated by comparing the partial pressure of a specific compound in water to its partial pressure in the atmosphere. For example, the age of a water sample can be calculated using the partial pressure of CFCs (pCFC) or SF6 (pSF6).

The pCFC age of a water sample is calculated using the formula:
pCFC = [CFC] / F,
where [CFC] is the measured CFC concentration (in pmol/kg) and F is the solubility of CFC gas in seawater, which depends on temperature and salinity. The solubility of CFCs is measured in units of 10–12 atmospheres or parts-per-trillion (ppt). Previous studies by Warner and Weiss, Bu and Warner, Wanninkhof et al., and Bullister et al. have measured the solubility of CFC-11, CFC-12, and SF6. These solubility values are expressed as:
F = a1 + a2/T + a3T + b1S + b2S/T + b3S*T,
where F is solubility (in mol/l or mol/kg/atm), T is absolute temperature, S is salinity (in parts per thousand), and a1, a2, a3, b1, b2, and b3 are constants determined from solubility measurements. This equation is based on the Van 't Hoff equation and the logarithmic Setchenow salinity dependence.

It is important to note that the solubility of CFCs increases by about 1% for every degree Celsius decrease in temperature.

Once the partial pressure of CFCs or SF6 is determined, it is compared to historical records of their atmospheric concentrations. The difference between the year corresponding to the partial pressure and the year the seawater sample was collected gives the average age of the water parcel. The age can also be calculated using the ratio of two CFC partial pressures or the ratio of SF6 partial pressure to a CFC partial pressure.

Safety

According to their material safety data sheets, CFCs and HCFCs are colorless, easily evaporated, non-toxic liquids and gases with a mild sweet smell. Exposure to high levels (11% or more) can lead to dizziness, trouble focusing, problems with the brain, and heart issues. Vapor can push out air, making it hard to breathe in small areas. Some CFCs can enter the body through the skin, but very little. When inhaled, CFCs quickly enter the blood, reaching high levels in 15 seconds, and levels even out after 20 minutes. When swallowed, CFCs are absorbed only 1/35 to 1/48 as much as when inhaled. Although these substances do not burn, burning them creates hydrofluoric acid and similar chemicals. Normal work exposure is 0.07%, which is not harmful.