Dichlorodifluoromethane

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Dichlorodifluoromethane

OVERVIEW

Dichlorodifluoromethane (DIE-klor-oh-DIE-floor-oh-METH-ane) belongs to a family of compounds called the chlorofluorocarbons (CFCs) that consist of carbon, chlorine, and fluorine atoms in varying arrangements. They can be considered as derivatives of alkanes, organic compounds consisting of carbon and hydro gen only, in which all of the hydrogen atoms have been replaced by chlorine and/or fluorine atoms. Chlorofluorocarbons are also referred to by their trade names, assigned to them by the DuPont corporation, which holds patents for their production. Dichlorodifluoromethane, for example, is also known as Freon12®. The numbers in each compound's Freon name specify the carbon atoms on which the chlorine and/or fluorine atoms are located. Freon12® is also known as refrigerant 12, propellant 12, and halon 122. The term halon refers to a related family of compounds consisting of carbon, chlorine, fluorine, bromine, and/or iodine.

KEY FACTS

OTHER NAMES:

Difluorodichloro methane; Freon 12®

FORMULA:

CCl₂F₂

ELEMENTS:

Carbon, chlorine, fluorine

COMPOUND TYPE:

Halogenated hydrocarbon; alkyl halide (organic)

STATE:

Gas

MOLECULAR WEIGHT:

102.91 g/mol

MELTING POINT:

−158°C (−252°F)

BOLLING POINT:

−29.8°C (−21.6°F)

SOLUBILITY:

Very slightly soluble in water; soluble in alcohol and ether

Dichlorodifluoromethane is the most widely used of all Freons®. It is a colorless, nonflammable gas that smells like ether in high concentrations.

During the late 1800s and early 1900s, ammonia (NH₃) and sulfur dioxide (SO₂) were the gases most commonly used in refrigeration systems. These substances were toxic and flammable, however, and their use resulted in many deaths and injuries. Three U.S. companies, DuPont, General Motors, and Frigidaire, began searching for a nonflammable, nontoxic alternative to ammonia and sulfur dioxide in their refrigeration products. In the late 1920s, Thomas Midgley (1889–1944), a researcher at General Motors, discovered such a compound, dichlorodifluoromethane. It enabled cooling systems to operate more efficiently and more safely. DuPont, which had joined forces with General Motors in the search for a new coolant, began manufacturing Freon in 1930.

For several decades, Freon® was widely used in refrigeration and for a number of other industrial applications. In the 1970s, two scientists at the University of California, F. Sherwood Rowland (1927–) and Mario Molina (1943–) discovered that CFCs escaping from appliances on the Earth's surface drifted upward into the stratosphere, where they were exposed to intense ultraviolet radiation from the Sun. That radiation caused the decomposition of CFCs into a number of products, one of which was chlorine. The chlorine produced in this breakdown, in turn, attacked ozone molecules (O₃) in the stratosphere and converted them into normal oxygen molecules (O₂).

Once recognized, this series of reactions became a matter of serious concern to scientists. The ozone layer absorbs ultraviolet radiation from the Sun, reducing the risk of skin cancer and other health problems produced by ultraviolet radiation. In 1987, twenty-seven nations had signed the Montreal Protocol on Substances that Deplete the Ozone Layer, which called for a 50-percent reduction in CFC production by the year 2000. An amendment to the protocol adopted in 1990 went even further, banning production of CFCs altogether in developed countries. Eventually, 148 countries were to sign the Montreal Protocol. By 1996, CFC production had come essentially to a halt except for a few developing nations and for use in certain specific applications, such as asthma inhalers. A decade later, scientists reported evidence that the ozone layer had begun to show signs of recovery.

Since the ban on CFC production has been adopted, manufacturers are replacing Freon® products with two related groups of compounds, the hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). These hydrogen-containing compounds are less energy efficient than the CFCs, but they are considered safer (although not entirely safe) for the environment.

HOW IT IS MADE

Dichlorodifluoromethane is made by reacting carbon tetrachloride (CCl₄) with hydrogen fluoride gas (H₂F₂) in the presence of a catalyst, usually antimony pentafluoride (SbF₅). In this reaction, two of the chlorines present in carbon tetrachloride are replaced by two fluorines from hydrogen fluoride, producing dichlorodifluoromethane.

COMMON USES AND POTENTIAL HAZARDS

Dichlorodifluoromethane and other Freons® were once used for many commercial applications: as refrigerants for air conditioners and refrigerators; as an aerosol propellant for hair sprays, insecticides, paints, adhesives, and cleaners; and as a foaming agent in the manufacture of shipping plastics. Its relative nontoxicity made these materials useful for food preservation and even for chilling cocktail glasses. Some other former uses of the Freons® include the following:

As a rocket propellant;
In solvents used in the manufacture of paints and varnishes;
In the preparation of frozen sections of tissue;
For the freezing of foods as a method of preservation;
In water purification systems;
As an agent for detecting leaks.

Companies favored Freons® for so many uses because they are both nontoxic and nonreactive in the troposphere. This property turned out to be a disadvantage also since it means that Freons® remain in the atmosphere for such long periods of time that they can eventually reach the stratosphere, where they pose a serious threat to the ozone layer.

Interesting Facts

In 1930, Thomas Midgley demonstrated the safety of Freon® at a meeting of the American Chemical Society by breathing in the gas and then breathing it out to extinguish a candle flame.
The production of dichlorodifluoromethane worldwide rose from about 150 million kilograms annually in 1960 to more than 750 million kilograms (1.65 billion pounds) annually in 1990. By 1996, because of the Montreal Protocol, it had dropped to less than 50 million kilograms (110 million pounds) annually.
One chlorine atom produced in the stratosphere by the decomposition of a CFC molecule can destroy 100,000 molecules of ozone.

Freons® may pose a health hazard if inhaled in large amounts. They displace oxygen in the air, causing dizziness, drowsiness, irregular heartbeat, cardiac arrest, unconscious-ness, and death by suffocation. Sprayed on the skin, Freons® can cause frostbite, damage produced by exposure to freezing temperatures. The ignition of Freons® also poses hazards to human health because of the release of toxic gases such as hydrogen chloride (HCl), hydrogen fluoride (H₂F₂), and phosgene (COCl₂).

Words to Know

CATALYST: A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.
STRATOSPHERE: The layer of the Earth's atmosphere extending from about 15 kilometers (8 miles) to about 50 kilometers (30 miles) above the Earth's surface.
TROPOSPHERE: The lowest layer of the Earth's atmosphere, reaching to about 8 miles (15 kilometers) above the Earth's surface.

FOR FURTHER INFORMTION

Baker, Linda. "The Hole in the Sky (Ozone Layer)." E (November 2000): 34.

Cagin, Seth, and Philip Dray. Between Earth and Sky: How CFCs Changed Our World and Endangered the Ozone Layer. New York: Pantheon Books, 1993.

"Dichlorodifluoromethane." NIOSH Pocket Guide to Chemical Hazards. http://www.cdc.gov/niosh/npg/npgd0192.html (accessed on October 7, 2005).

Elkins, James W. "Chlorofluorocarbons (CFCs)." National Oceanic and Atmospheric Administration (NOAA). http://www.cmdl.noaa.gov/noah/publictn/elkins/cfcs.html (accessed on October 7, 2005).

"Gas Data." Air Liquide. http://www.airliquide.com/en/business/products/gases/gasdata/index.asp?GasID=22 (accessed on October 7, 2005).

Chemical Compounds