Application The Ozone Layer and CFCs

Chlorination at C2 occurs at the stereogenic center. Abstraction of a hydrogen atom at C2 forms a trigonal planar sp hybridized radical that is now achiral. This achiral radical then reacts with CI2 from either side to form a new stereogenic center, resulting in an equal amount of two enantiomers—a racemic mixture. 

four isomers are formed by chlorination of (R)-2-bromobutane at C2 and C3. Attack at the stereogenic center (C2) gives a product with one stereogenic center, resulting in a mixture of enantiomers. Attack at C3 forms a new stereogenic center, giving a mixture of diastereomers. 

Problem 15.1 7 what products are formed from monochlorination of (R)-2-bromobutane at C1 and C4 Assign R and S designations to each stereogenic center. 

Problem 15.18 Draw the monochlorination products formed when each compound is heated with CIp. Include the stereochemistry at any stereogenic centers. 

Ozone is formed in the upper atmosphere by reaction of oxygen molecules with oxygen atoms. Ozone is also decomposed with sunlight back to these same two species. The overall result of these reactions is to convert high-energy ultraviolet light into heat. 

The 1995 Nobel Prize In Chemistry was awarded to Mario Molina, Paul Crutzen, and F. Sherwood Rowland for their work In elucidating the Interaction of ozone with CFCs. What began as a fundamental research project turned out to have Important Implications In the practical world. 

Propane and butane are now used as propellants in spray cans in place of CFCs. 

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As you know, most countries are phasing out certain refrigerants to lessen damage to the ozone layer. The chemicals being phased out are chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Replacements are hydrofluorocarbons (HFCs) and certain blends. The DuPont web site (www.dupont.com) gives the handy Table I of recommended replacement refrigerants for various applications. 

Table 1 lA presents tabulations of the safety of important refrigerants, but this list does not include aU available refrigerants. Table 11-5 summarizes a limited list of comparative hazards to life of refrigerant gas and vapor. The current more applicable refrigerants from the m or manufacturers of the CFC and HCFC refrigerants and their azeotropes/ blends/mrxtures are included, but the list excludes the pure hydrocarbons such as propane, chlorinated hydrocarbons such as methyl chloride and others, inorganics, ammonia, carbon dioxide, etc. See Table 11-6. The CFC compounds have a longer and more serious ozone depletion potential than the HCFC compounds, because these decompose at a much lower atmospheric level and have relatively short atmospheric lifetimes therefore, they do less damage to the ozone layer. Table 11-7 summarizes alternate refrigerants of the same classes as discussed previously. Table 11-8 correlates DuPont s SUVA refrigerant numbers to the corresponding ASHRAE numbers.

The cost of raw materials for use in phenolic foams is low, but since this foam has the weak points listed above, it cannot exceed polyurethane foam in cost performance. Resol-type foams and benzylic ether-type foams use chlorofluorocarbon (CFC) as a foamimg agent. It is necessary to develop a new foaming agent which does not destroy the ozone layer in the atmosphere. When these problems are solved phenolic foam applications will make further rapid progress. 

It is impossible to overstate the importance of the role that CFCs have played in making refrigeration, air-conditioning, and, to some extent, aerosols such essential facets of modern existence. Life, as we know it, would be impossible in some parts of the world without the means for cooling food and houses. Of course, the very property that made the CFCs so useful, their stability, allows them to reach the stratosphere and there release chlorine atoms, which interact with the ozone layer. There will be a drastic decline in the use of CFCs in most of their high-volume applications of the heavier CFCs and the volumes will be inconsequential compared to the 2.5 billion pounds of volatile CFCs produced worldwide in 1986. 

Hydrochlorofluorocarbons (HCFCs) have been proposed and used as interim replacements for CFCs in refrigerant applications. Because they are not as stable as CFCs, FlCFCs in the atmosphere generally break down before they reach the stratosphere. Hence they pose less of a threat to the ozone layer. An example would be CHCI2F. In this molecule, which bond(s) are the most polar Which bond(s) are the least polar ... 

Chlorofluorocarbons Approximately 46000 tons of chlorine were used in 1995 to produce CFC-11 through CFC-13. All the CFCs were phased out in 1996, although small amounts are still used as polymer precursors, and in specialized applications. The HCFC category of alternate CFCs deplete the ozone layer to a much smaller degree than CFCs, and are transitional substitutes for CFCs until they are phased out between 2015 and 2030. HFCs, containing no chlorine and made using TCE and PCE, are not subject to being phased out. 

Measurements over the last several decades have indicated a rapid decrease of stratospheric ozone. This decrease has been most drastic over Antarctica, where the ozone layer is now almost completely absent (Figure 11.17). Stratospheric ozone over the rest of the planet has decreased at a rate of about 6% each year. Each year, we are exposed to increasing levels of harmful UV radiation that are linked with skin cancer and other health issues. While many factors contribute to ozone depletion, it is believed that the main culprits are compounds called chlorofluorocarbons (CFCs). CFCs are compounds containing only carbon, chlorine, and fluorine. In the past they were heavily used for a wide variety of commercial applications, including as refrigerants, propellants, in the production of foam insulation, as fire-flghting materials, and many other useful applications. They were sold under the trade name Freons. ... 

Enough countries have agreed to eliminate CFCs from their products that scientists fully expect the ozone layer to recover. Enough CFCs are in the atmosphere already that recovery will take several decades, but ozone levels will recover. This has been an excellent example of the application of good science to a global problem and to the problem s solution. 

All other scorecard items stem directly from lifecycle analysis assessment theory (32). For some impact types, quantitative norms can be stated. For stratospheric ozone layer depletion components, the emission norm is zero. This means in practice that components such as CFCs and SF6 should not be used at all, not even in contained technical applications, because containment over the lifecycle cannot be ensured. 

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