Dichlorodifluoromethane
Chemical compound
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Through Montreal Protocol, Ozone layer and Solubility
R-12 has the highest potential for ozone destruction of all refrigeration gases.
Preparation
It can be prepared by reacting carbon tetrachloride with hydrogen fluoride in the presence of a catalytic amount of antimony pentachloride:
- CCl4 + 2HF → CCl2F2 + 2HCl
This reaction can also produce trichlorofluoromethane (CCl3F), chlorotrifluoromethane (CClF3) and tetrafluoromethane (CF4).
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History
Charles F. Kettering, vice president of General Motors Research Corporation, was seeking a refrigerant replacement that would be colorless, odorless, tasteless, nontoxic, and nonflammable. He assembled a team that included Thomas Midgley Jr., Albert Leon Henne, and Robert McNary. From 1930 to 1935, they developed dichlorodifluoromethane (CCl2F2 or R12), trichlorofluoromethane (CCl3F or R11), chlorodifluoromethane (CHClF2 or R22), trichlorotrifluoroethane (CCl2FCClF2 or R113), and dichlorotetrafluoroethane (CClF2CClF2 or R114), through Kinetic Chemicals which was a joint venture between DuPont and General Motors.
Use as an aerosol
The use of chlorofluorocarbons as aerosols in medicine, such as metered-dose inhalers, has been phased out by the U.S. Food and Drug Administration. A different propellant known as hydrofluoroalkane, or HFA, which was not known to harm the environment, was chosen to replace it. However, it remains on the FDA's approved food additive list.
Environmental effects
R-12 has the highest ozone depletion potential among chlorocarbons due to the presence of 2 chlorine atoms in the molecule. R-12 also has intense global warming potential (GWP) with the 20yr, 100yr and 500yr GWP being 11400, 11200 and 5100 times greater than CO2.
Retrofitting
R-12 was used in most refrigeration and vehicle air conditioning applications prior to 1994 before being replaced by 1,1,1,2-tetrafluoroethane (R-134a), which has a much lower ozone depletion potential. When older units leak or require repair involving removal of the refrigerant, retrofitment to a refrigerant other than R-12 (most commonly R-134a) is required in some jurisdictions. The United States does not require such conversion. Retrofitment requires a system flush and a new filter/dryer or accumulator, and may also involve the installation of new seals and/or hoses made of materials compatible with the refrigerant being installed. Mineral oil used with R-12 is not compatible with R-134a. Some oils designed for conversion to R-134a are advertised as compatible with residual R-12 mineral oil. Replacements for R-12 include highly flammable hydrocarbon blends such as HC-12a, the flammability of which has caused injuries and deaths.
Dangers
Aside from its environmental impacts, R12, like most chlorofluoroalkanes, forms phosgene gas when exposed to a naked flame. When packaged in cans under high pressure in liquid form, exposure to heat could cause the can to explode.
Properties
Table of thermal and physical properties of saturated liquid refrigerant 12: