Halogenated CFCs

In 1976 the United States banned the use of CFCs as aerosol propellants. No further steps were taken until 1987 when the United States and some 50 other countries adopted the Montreal Protocol, specifing a 50% reduction of fully halogenated CFCs by 1999. In 1990, an agreement was reached among 93 nations to accelerate the discontinuation of CFCs and completely eliminate production by the year 2000. The 1990 Clean Air Act Amendments contain a phaseout schedule for CFCs, halons, carbon tetrachloride, and methylchloroform. Such steps should stop the iacrease of CFCs ia the atmosphere but, because of the long lifetimes, CFCs will remain ia the atmosphere for centuries. 

Although inert in the lower atmosphere (troposphere), the hilly halogenated CFCs and Halons diffuse into the upper stratosphere where they are photodissociated, ie, photolyzed, by the intense ultraviolet radiation. 

The phasing-out of fully halogenated CFCs is a success story of international cooperation in ameliorating an environmental hazard created by anthropogenic activities. In part, this success was achieved because chemical manufacturers found ready substitutes for the fully halogenated CFCs, the partially halogenated hydrochloro-fluorocarbons (HCFCs), for example HCFC-22,... 

Fully halogenated CFCs were eliminated in polystyrene foam food packaging and containers. Substitute blowing agents used are either no threat to the ozone or are a 95% improvement over fully halogenated CFCs. Action has been taken such as where PS foam cups now are 100% CFC-free, etc. 

Unlike the simple fully halogenated CFC s which can only be made in a single step, there are many potentially viable routes to the alternatives, several of which involve a catalytic hydrogenation step [4]. 

The primary atmospheric removal processes for halocarbons are photolysis and reaction with tropospheric hydroxyl radicals (OH). For the fully halogenated CFCs and halons, photolysis is the only important sink and their atmospheric lifetimes are dependent on their absorption cross-sections, the solar flux, and the surface to stratosphere transport time. As a general rule, the greater the number of Cl, Br, or I atoms on any one carbon atom, the larger the cross-section and the shorter the lifetime. For example, the lifetimes of CCIF3 (CFC-13), CCI2F2... 

As long as there were no C-H bonds in the CFC molecule, the eluting peak showed good recovery. With C-H bonds present the CFC was decomposed. The selectivity of aluminum oxide for CFC s is very high, as shown in Fig. 7-46. Due to the decomposition of the partly halogenated CFC s the quantitative analysis for these CFC s is very difficult. 

In order to enable the fast discontinuation of the production and use of fully halogenated CFCs, hydro-chlorofluorocarbons were introduced in industry as a transition stage. The atmospheric fate and impact of these hydrochlorofluorocarbons and chlorinated solvents are described in [387]. The authors come to the conclusion, that these compounds, with the exception of 1,1,1-trichIoroethane, make a small or insignificant contribution to the stratospheric ozone depletion, global warming, photochemical smog , acid rain or chloride and fluoride levels in precipitations. The ozone depletion potentials are 10 to 50 times lower than that of CFCll or CFC12, mainly as a consequence of their shorter atmospheric lifetime—some months to 10 years—due to destruction in the atmosphere. 

The haloalkanes (also called alkyl halides) are alkanes in which at least one hydrogen atom has been replaced by a halogen atom. Although they have important uses, many haloalkanes are highly toxic and a threat to the environment. The haloalkane 1,2-dichlorofluoroethane, CHC1FCH2C1, is an example of a chlorofluorocarbon (CFC), one of the compounds held responsible for the depletion of the ozone layer (see Box 13.3). Many pesticides are aromatic compounds with several halogen atoms. 

Evidence that many of these compounds can have adverse effects on the immune, endocrine and nervous systems and that some are carcinogenic has grown during the last decade. The role of chlorofluorocarbons (CFCs) and of methyl bromide in the ozone layer depletion is well established (ref. 3).It is therefore not surprising that many halogenated derivatives are cast as environmental and health villains by various concerned groups who call for total phase out of chlorine and chlorinated hydrocarbons. 

CaO was activated by the addition of Ni (acetylacetonate)2 followed by calcining at 973 K. This resulted in the production of nickel on the carbonate matrix, and was effective in decomposing dichlorodifluoromethane (CFC-12) to CO2 at 723 K—with low amounts of CO—while the halogen was combined as CaFCl (Tamai et al. 2006). 

Sonier DN, NLDuran, GB Smith (1994) Dechlorination of trichlorofluoromethane (CFC-11) by sulfate-reducing bacteria from an aquifer contaminated with halogenated aliphatic compounds. Appl Environ Microbiol 60 4567-4572. 

The halogenated hydrocarbons known as chlorofluorocarbons are a second class of materials that comprise water-repellent coatings. In particular, the substance known as l,l,2-trichloro-l,2,2-trifluoroethane or CFC-113 (Fig. 7.5.2) is... 

Volatile organic compounds (VOC) contribute to the formation of tropospheric ozone (summer smog). Certain halogenated hydrocarbons (e.g. CFCs) also destroy the stratospheric ozone layer. Chlorinated solvents are hazardous to water and, if disposed of incorrectly (e.g. burning), may emit highly toxic substances (e.g. dioxins). 

Chemical Family a group of elements that share similar chemical properties and share the same column in the periodic table, for example, halogens, alkali earth Chirality condition that describes the handedness of a molecule or whether a molecule exists in forms that can be superimposed on each other Chlorofluorocarbons also called CFCs, compounds consisting of chorine, fluorine, and carbon that are responsible for stratospheric ozone destruction Coagulation precipitation or separation from a dispersed state Coefficient of Thermal Expansion measure of the rate at which a substance will expand when heated... 

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