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Scheme 1. Metal-Mediated Reaction of an Aldehyde and Allyl Halide in Water carbohydrate feedstocks such as soy17 and corn18 are found in consumer products like automobiles and food packag- ing. Microbial fermentation has been used to convert glucose to a biodegradable polymer.19 Solvents. The design of environmentally benign solvents and solventless systems has been one of the most active areas of green chemistry over the past 10 years. Solvents are highly regulated and used in large quantities. Organic solvents pose a particular concern to the chemical indus- try because of the sheer volume used in synthesis, processing, and separations. Many are classified as volatile organic compounds (VOCs) or hazardous air pollutants (HAPs) and are flammable, toxic, or carcinogenic. Breakthroughs in the use of supercritical fluids such as carbon dioxide have met with success in the research laboratory as well as commercially. Supercritical fluids offer a number of benefits, such as the potential to combine reaction and separation processes and the ability to tune the solvent through variations in temperature and pressure. In the supercritical fluids area, CO2 has received the most attention20-26 because its critical temperature and pressure (Tc ) 31.1 °C, Pc ) 74 bar) are more accessible than those of other solvents (water, for example, has Tc ) 374 °C and Pc ) 221 bar). CO2 offers numerous advantages as a benign solvent: it is nontoxic, nonflammable, and inexpensive, and can be separated from the product by simple depressurization. Applications of supercritical CO2 are found in the dry cleaning industry, where CO2 replaces perchloroethylene as a solvent;27,28 in semiconductor manufacturing, where the low surface tension of super- critical CO2 avoids the damage caused by water in conventional processing;29 and in chemical processing.30 The use of supercritical CO2 as a reaction medium in organic synthesis provides an excellent example of the evolution from fundamental academic research into a commercial process. In collaboration with Thomas Swan & Co. Ltd,31 researchers at the University of Nottingham32 developed synthetic methodologies in supercritical CO2 that are being employed in a new supercritical fluid plant in the U.K., with a capacity up to 1000 tons per year. Conventional solvents are replaced with supercritical fluids in such key technologies as hydrogenation, Friedel- Crafts alkylations and acylations, hydroformylations, and etherification. The use of water as a solvent in ways previously not realized has been an active area of research in green chemistry (Scheme 1).33,34 A number of classic organic reactions, traditionally run in organic solvents, can be carried out in water with the proper design of catalysts and reaction conditions. Even variants of the Grignard reaction, notoriously sensitive to water, can be run in an aqueous solvent using a variety of metals, such as indium35 and zinc.36 The use of an obviously benign and inexpen- sive solvent like water could yield significant green chemistry benefits if challenges of energy and separations can be met. Ionic liquids, a relatively new area of solvent investiga- tion, are attractive because of their negligible vapor pressure and their use in polar systems to generate new chemistries.37-40 A plethora of ionic liquids can be pro- duced by varying the cations and anions, permitting the synthesis of ionic liquids tailored for specific applications. While questions of intrinsic hazard must still be answered for this class of solvents, the potential for the design of next generation ionic liquids holds significant promise for improved environmental benefits. Fluorous solvent systems have demonstrated particular advantages in synthetic systems.41-43 Fluorous systems are particularly appealing in fluorous biphasic catalysis in which the homogeneous catalyst and the product reside in separate phases, thereby eliminating the need for energy-intensive separations. In addition to efficiency, fluorous biphasic systems may reduce accident potential by eliminating the possibility of runaway exothermic reactions. Catalysis. The area of catalysis is sometimes referred to as a “foundational pillar” of green chemistry.44 Catalytic reactions45-51 often reduce energy requirements and decrease separations due to increased selectivity; they may permit the use of renewable feedstocks or minimize the quantities of reagents needed. There is little doubt that the 2001 Nobel Prize-winning work of Sharpless, Noyori, and Knowles met many green chemistry goals.52 Their research on catalytic asymmetric synthesis has been crucial in producing single enantiomer compounds, par- ticularly for the pharmaceutical industry. Catalysis often permits the use of less toxic reagents, as in the case of oxidations using hydrogen peroxide in place of traditional heavy metal catalysts.53 Renewable resources, such as soya sterols54 (Scheme 2) and glucose,55 serve as feedstocks when catalytic methods are employed. Recently, water has been split into oxygen and hydrogen using a photocatalyst that absorbs light in the visible range.56 While still at the research stage, this technology has the potential to provide an efficient source of hydro- gen for use in fuel cells. Hydrogen fuel cells in cars would greatly reduce air pollution, as the oxidation product Origins, Status, and Challenges of Green Chemistry Anastas and Kirchhoff Scheme 2. Synthesis of Bisnoraldehyde from a Renewable Feedstock 688 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 35, NO. 9, 2002PDF Image | Origins, Current Status, and Future Challenges of Green Chemistry
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