Halofluorocarbons

Confirmation of the destmetion of ozone by chlorine and bromine from halofluorocarbons has led to international efforts to reduce emissions of ozone-destroying CPCs and Halons into the atmosphere. The 1987 Montreal Protocol on Substances That Deplete the Ozone Layer (150) (and its 1990 and 1992 revisions) calls for an end to the production of Halons in 1994 and CPCs, carbon tetrachloride, and methylchloroform byjanuary 1, 1996. In 1993, worldwide production of CPCs was reduced to 50% of 1986 levels of 1.13 x 10 and decreases in growth rates of CPC-11 and CPC-12 have been observed (151). 

An efficient catalyst for thermal isomenzations of halofluorocarbons [6, 7, 8, 9] IS prepared by treatment of alumina with dichlorodifluoromethane at 200-300 °C [9] or aluminum chloride with chlorofluorocarbons in the presence of metals [W] or palladium on alumina [II These catalysts are far more efficient than aluminum halides themselves (equations 1 and 2)... 

The introduction of the chlorofluorocarbon (CFC) fluids in the early 1930s marked the incepuon of the halofluorocarbon industry. Both in terms of tonnage produced and product value, the CFCs have dominated the organofluorocarbon industry and have provided the feedstocks for the development of other fluorocarbon products such as poly(tetrafluoroethylene). 

Perfluorocarbons are chemically inert, are nonflammable and have low toxici-ties. Unlike the halofluorocarbons, perfluorocarbons are not ozone-depleting and have been used as CFC replacements [7], However, perfluorocarbons have very long lifetimes in the atmosphere (>2000 years) [8] and this has consequences for their use as solvents since any process has to take into account the fact that any loss of solvent to the atmosphere is extremely undesirable. 

Abel MW, Abel AE, Heyduk, Mouk RW. Methods for the elimination of cyanides in dehalogenation of halofluorocarbons. US Patent 5,602,295, 1997. 

Another use of halofluorocarbons, especially the bromofluorocarbons (also called halons), is as fire retardants. As is seen for a sampling of halofluorocarbons in Table 9, these compounds are nonflammable and nontoxic they are also heavier than air, which is particularly important for functioning as fire retardants. CF3Br, for example, is used for fires in totally enclosed spaces such as offices, airplanes and military tanks. 

Halogenated organics are found on a very large scale in our environment. Not all classes of these pose a serious health threat for instance, an attractive feature of the halofluorocarbons is indeed their lack of toxicity. However, some individual members of certain categories of halogenated organics, such as the halogenated ethylenes, epoxides and aromatic hydrocarbons, have been identified as health risks. 

Despite or, better, because of their extreme chemical stability perfluorocarbons and halofluorocarbons have a dramatic impact on the global environment this was nearly impossible to predict when the substances were first introduced into industrial mass-production and ubiquitous use. 

Because of their extreme stability against all kinds of aggressive chemical agent, for example radicals, perfluorocarbons and halofluorocarbons are not degraded in the lower layers of the atmosphere as are other pollutants. After several years, or even decades, they finally reach the stratosphere at altitudes of 20 to 40 km [26, 27]. In this layer, under the influence of short-wave UV irradiation, ozone is formed continuously (Scheme 1.2). This stratospheric ozone plays an essential role in preserving life on earth by absorbing the short-wavelength UV which would otherwise lead to an increase of photochemically induced mutations in most life-forms. For humans, over-exposure to short-wave UV irradiation results in a dramatically increased risk of skin cancer. Many crops and other plants also react rather sensitively towards an increase of UV exposure. 

Table 4.1 Properties and technical applications of some (hydro)halofluorocarbons [1] (b.p. in °C).

Halofluorocarbons and highly fluorinated ethers have been used as inhalation anesthetics for many years [91], The first anesthetic of this type was Halothane which was introduced clinically in 1956 [92], An excellent anesthetic with few unwanted side-effects, Halothane has the particular advantages of high chemical stability and non-flammability. Several alternative heavily fluorinated inhalation anesthetics have subsequently been introduced [93] (Scheme 4.42). 

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