Dichlorodifluoromethane

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).

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 USP-approved salbutamol, 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. That being said it still listed 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 .

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 an insignificant ozone depletion potential. Automobile manufacturers began phasing in R-134a around 1993. 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:

Gallery

File:Monthly means of CFC-12.png|CFC-12 measured by the Advanced Global Atmospheric Gases Experiment (AGAGE) in the lower atmosphere (troposphere) at stations around the world. Abundances are given as pollution free monthly mean mole fractions in parts-per-trillion. File:Hats f12 global.png|Hemispheric and global mean CFC-12 concentrations (NOAA/ESRL) Image:AYool CFC-12 history.png|Time-series of atmospheric concentrations of CFC-12 (Walker et al., 2000) File:GLODAP sea-surf CFC12 AYool.png|1990s sea surface CFC-12 concentration File:GLODAP invt CFC12 AYool.png|1990s CFC-12 oceanic vertical inventory File:CFC-12 CFC-11 H-1211 SF6 vertical profiles Fairbanks Alaska 1976.png|CFC-12, CFC-11, H-1211 and SF6 vertical profiles

References

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