Chlorofluorocarbons Perfluorocarbons

CFCs, chlorofluorocarbon gases PFCs, perfluorocarbon gases. Source From Robertson, G. P (2004). Scope 62, The Global Carbon Cycle, Island Press... 

A carbon atom can be bonded to as many as four halogen atoms, so an enormous number of organic halides can exist. Completely fluorinated compounds are known as fluorocarbons or sometimes perfluorocarbons. The fluorocarbons are even less reactive than hydrocarbons. Saturated compounds in which all H atoms have been replaced by some combination of Cl and F atoms are called chlorofluorocarbons or sometimes freons. These compounds were widely used as refrigerants and as propellants in aerosol cans. However, the release of chlorofluorocarbons into the atmosphere has been shown to be quite damaging to the earth s ozone layer. Since January 1978, the use of chlorofluorocarbons in aerosol cans in the United States has been banned, and efforts to develop both controls for existing chlorofluorocarbons and suitable replacements continue. The production and sale of freons have been banned in many countries. 

Interest in the use of reducing agents to activate C-F bonds was revived when systems that enabled selective removal of fluoride from perfluorocarbons were discovered in contrast to the complete (destructive) removal of halides as in sodium fusion reactions employed in methods for halogen analysis [31]. (Interest in this latter area has also had resurgence in the search for methods capable of destroying chlorofluorocarbons [32].) The key breakthrough was the report by MacNicol and Robertson that hexakis(thiophenoxy)napthalene can be prepared by treatment of perfluorodecalin with excess sodium phenylthiolate for 10 days at 70°C in DMF [33]. 

The paramagnetic absorption of alkane radical cations is critically dependent on their conformation. In neat n-alkane crystals, alkane molecules are in the extended all-trans conformation (see Fig. 5.1) and FDMR spectroscopy unequivocally shows that alkane radical cations retain that conformation in such systems. The extended all-trans conformation is also the preferred conformation of many n-alkane radical cations in chlorofluorocarbon and perfluorocarbon matrices. In this conformation, the unpaired electron occupies the planar cr molecular orbital and delocalizes over the entire extended chain. Only two C-H bonds (both chain-end, one on each side) are in the planar ct molecular frame in the extended structure and high unpaired-electron and positive-hole density appears only on these in-plane protons. Alkane radical cations in the extended conformation (as well as in other conformations) are thus fj-delocalized paramagnetic species. The associated hyperfme interaction with the two (equivalent) in-plane chain-end protons results in a 1 2 1 three-line (triplet) EPR spectrum. The fact that the hyperfme in-... 

The composition of the Earth s atmosphere, given in Table IV, is controlled by biological processes, with additional influences from photochemistry and human activities. The abundances in Table IV are taken from (75), with updated abundances for the chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrochlorocarbons (HCCs), and perfluorocarbons (PFCs) (76). 

The mass flow or reflow methods are suited for high-volume manufacturing. The entire board is heated and large numbers of components on the board are soldered simultaneously. The two most common of these methods are oven reflow soldering and wave soldering. A third technique, vapor phase reflow soldering, has dwindled in popularity due to environmental concerns regarding the use of the chlorofluorocarbon-based solvents that were key to this process. Now, however, perfluorocarbons are substituted and the technique is still in use. 

Application of kilns to incinerate and decompose harmful substances and ozone depleting chemicals such as HFCs (Hydrofluorocarbons), CFCs (Chlorofluorocarbons) and PFCs (Perfluorocarbons). 

Stones are protected from deleterious effects of water and pollution by application of amorphous fluoropolymer solution." Solvent is selected from the group consisting of acetone, methyl-ethyl ketone, ethyl acetate, t-butyl acetate, hydrochlorofluorocarbons, chlorofluorocarbons, hydro fluorocarbons and perfluorocarbons." Another method uses epoxy-modified silane in hydrophilic solvent." ... 

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