33 No. 3
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Because of their peculiar characteristics, halogens are widely used by all sectors of the chemical industry to produce solvents, catalysts, building blocks, additives, and drugs. In addition, halogens are contained in several commodities that we all use daily (e.g., chlorine is contained in PVC, one of the most widely used plastic materials). More than 20 million tonnes of chlorine and coproducts caustic soda and hydrogen are produced each year at about 80 plants across Europe, mostly (about 95 percent) via electrolysis-based techniques (chlor-alkali industry); the sector directly employs about 40 000 people in 20 countries (data: www.eurochlor.org).
Since the Industrial Revolution, the halogen chlorine has been “an iconic molecule” for industrial chemical production. Even though its production by the electrolysis of sodium chloride is really energy intensive, it still is used, since it allows the manufacture of chlorinated derivatives in a very easy way, because of its high energy and reactivity; for example, AlCl3, SnCl4, TiCl4, SiCl4, ZnCl2, PCl3, PCl5, POCl3, COCl2, etc. Thus, chlorinated derivatives have many applications. Examples of their use include plastics, solvents for dry cleaning and metal degreasing, textiles, agrochemicals and pharmaceuticals, insecticides, dyestuffs, household cleaning products, and disinfectants. Chlorine is used extensively in organic and inorganic chemistry as an oxidizing agent and as a leaving group in substitution and elimination reactions. In addition, chlorine compounds find use as intermediates in the production of a number of important commercial products that do not contain chlorine. Examples include polycarbonates, polyurethanes, silicones, polytetrafluoroethylene, carboxymethyl cellulose, and propylene oxide. Through a chain of chemical derivatives and relatively easily made compounds and intermediates, such molecules have utilized the intrinsic energy available through the use of chlorine primarily produced via electrolysis.
The substitution of compounds where “chlorine is used in the making” means that we will avoid such a primary energetic source; this, however, makes chemistry “without chlorine” considerably more difficult and illustrates why it has not been adopted before. The environmental constraints and the growing need for efficient energy usage force us to take advantage of available high technologies to develop a new chemical strategy. Because of the negative impacts of chlorine and other halogens on global environment and health (e.g., toxicity and ecotoxicity, ozone layer depletion, energy consumption, and climate change, etc.), significant contributions to alternative solutions may be provided by research devoted to the systematic substitution of halogens (whenever feasible), which adopt a holistic and proactive approach. The substitution of halogens requires a look at the whole picture, rather than approaching the problem through one elemental aspect of halogen usage at a time.
This special issue of the IUPAC journal Pure and Applied Chemistry (PAC) intends to contribute to these aims, and deals with the following topics: chlorine-free reagents, chlorine-free catalysts, phosgene replacement, chlorine-free solvents, thionyl chloride substitution, and metrics for chlorine-free reactions.
last modified 6 May 2011.
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