Designer chemistry BY A L A N
A
NEWMAN
fundamental change may be taking place in h o w synthetic chemists view their discipline. In industrial, academic, and government laboratories, n e w approaches to the synthesis of commercially important chemicals are being developed that avoid dangerous chemicals, reduce or eliminate hazardous wastes, and even directly design safer chemicals. Many of these new approaches were presented at this August's national American Chemical Society meeting in Washington, DC, u n d e r the banner "Design for the Environment." This n e w paradigm has been labeled "benign-by-design" chemistry. "It is fundamental pollution prevention," says Paul Anastas of EPA's Office of Pollution Prevention and Toxics. "Basic research chemists have quickly changed to this concept," says technology analyst Joel Hirschhorn (Hirschhorn & Associates, Lanham, MD). "The controlling linkage is h o w to apply the technology." Hirschhorn traces the rise of this n e w paradigm to the federal Pollution Prevention Act of 1990, w h i c h , although it lacks regulations, set out the basic principles. In recent years, EPA has used the pollution prevention principles and a small amount of funding to test n e w concepts. According to Mary Ellen Weber, also of EPA's Office of Pollution Prevention and Toxics, the Agency has leveraged funds by forming partnerships. For the benign-by-design initiative, EPA and the National Science Foundation have signed a m e m o r a n d u m of understanding to promote this "green chemistry." Weber also cites partnerships with the Department of Energy, industrial laboratories, and academia. In many ways, the benign-bydesign concept radically restructures industrial chemistry by placing value on chemical processes that use i n n o c u o u s a n d renewable feedstocks; alternative reagents such as non-heavy metals or light-initiated reactions; avoiding organic solvents through the use of, for example, supercritical C 0 2 or aqueous solvents; a n d handling dangerous chemicals
through insitu generation or "justi n - t i m e " production (i.e., items are used as soon as they are received). These safer approaches are woven in with traditional industrial concerns such as the cost of materials and optimizing the yield of the commercial product. In theory, these benign-by-design processes should save industry m o n e y by reducing costs of hazardous waste disposal a n d meeting safety and occupational regulations. For example, Joseph DeSimone of the University of North Carolina at Chapel Hill presented a paper at the meeting showing that supercritical C 0 2 can be used as a
The benign-by-design concept radically restructures industrial chemistry. . . solvent for homogenous a n d heterogenous polymerization reactions. Pietro T u n d o of the Université di Venezia, Italy, reported that the Italian firm Enichem h a d replaced toxic phosgene in carboxylation reactions a n d toxic dimethyl sulfate in methylation reactions w i t h the safer reagent dimethyl carbonate. However, the concept goes beyond restructuring syntheses to produce existing commercially valuable products to developing n e w chemicals that are inherently safer for the environment. " T h i s challenges the basic concept of toxicology and all areas of chemistry," said Roger Garrett of EPA's Office of Pollution Prevention a n d Toxics. "We need to develop a n e w 'hybrid' chemist." These n e w chemists w o u l d use toxicological concepts such as structure—activity relationships to generate new chemicals. An example that Stephen DeVito of the Office of Pollution Pre-
vention and Toxics presented was to replace ethylene glycol ethers (which are metabolized to toxic alkoxy acetic acids) with certain safer propylene glycol ethers. The key is understanding the metabolic pathways and defeating those enzymes that produce toxic chemicals. In other cases, just modifying or blocking certain functional groups, such as aromatic nitro groups or isocyanates, can lead to less toxic chemicals. Another approach for modifying chemicals is through isosterism, w h i c h equates atoms or atom groups containing the same number of atoms and having similar shapes or similar electronic properties. Replacing one group with an isosteric group can give similar chemical properties (or in an industrial sense similar efficacies) but potentially different toxicities. For example, toxic Cr could be replaced with Fe in azo dyes, or a hydrogen atom could be strategically replaced with a fluorine atom in benzo[a]anthracene to disrupt the metabolism to a carcinogen. Isosterism extends to more complex groupings of atoms. Another n e w concept, retrometabolism, uses the knowledge of metabolic pathways to design chemicals that are, for example, easily metabolized to excretable products. This concept began in the pharmaceutical industry, but DeVito suggested that this could work with pesticides. Others researchers are promoting even more radical ideas for the future of industrial synthesis. "We have a 1950s vision of the economy. Molecular m a c h i n e systems already dominate U.S. prod u c t i o n , " said K. Eric Drexler of the Institute for Molecular Manufacturing (Palo Alto, CA). What Drexler advocates is to mimic biology and move to direct manufacturing on a molecular level w i t h such exotic devices as nanomachines and artificially constructed ribosomes. In this world, the raw feedstock for industrial processes would shift from petroleum to glucose.
Alan Newman is an associate editor on the Washington staff of ES&T. Environ. Sci. Technol., Vol. 28, No. 11, 1994
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