One major way to reduce waste in the manufacture of complex organic substances is to reduce the number of steps required from raw materials to the final product. Every intermediate must usually be purified after each step, and nearly all purification processes produce waste. For example, a new ICI route to a fungicide process reduces the number of reaction steps from six to three while using fewer solvents and other chemicals. The total effect decreases the amount of waste to only 10% of that generated in the previous process (Hileman 1992). In addition, reducing steps means decreasing capital and operating costs.
Figure 3.6.2 compares the six-step ibuprofen process with the three-step process. Both processes start with the Friedel-Crafts conversion of sec-butylbenzene to p-sec-butylacetophenone. The six-step process also uses alu-
minum chloride as a catalyst with potential problems of water quench, emulsions, difficult extractions or filtration, and a large volume of water-borne waste. The three-step process uses the cleaner hydrogen fluoride as a catalyst and reduces the need for treatment of the water effluent and eliminates the formation of HCl.
Molecular Design in Basel, Switzerland has reaction retrieval systems that can be used to design improved processes to clients' target compounds (Stinson 1993). These reaction retrieval systems allow the design of a sequence that has only four steps, begins from more easily accessible substituted phthalimides to spirosuccinimides, and eliminates the generation of toxic by-products (see Figure 3.6.3). One of the steps uses trimethylsilyl cyanide which could be added to the company's catalog for sales to others.
In various forms, such computer software has been under development around the country for about twenty-five years. Its purpose is to help chemists identify new syntheses for target molecules from the myriad of potential routes and to suggest novel chemical reactions that might be investigated.
Most of these programs are retrosynthetic—that is they generate syntheses for target molecules by working back-
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