Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering

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harmless, inexpensive, and easily removed or recovered solvents. One interesting approach is to substitute water for organic solvents; another is to use unusual solvents such as supercritical carbon dioxide or even supercritical water. Synthetic transformations often involve adding catalysts, substances that direct chemical reactions along less energetic but more efficient pathways. Solid catalysts that can be simply filtered away, or retained in a reactor as the other components pass through, are also desirable.

Atom economy is a goal only relatively recently understood. If all the atoms in the reactants are found in the desired product, we say that there is excellent atom economy. However, in many chemical reactions additional products are formed containing some of the atoms of the reactants. This is true in displacement and elimination reactions. It is also true if the reaction is not perfectly selective, and additional undesired products are formed. In most cases, the extra chemicals produced in displacement or elimination reactions or in nonselective reactions must be removed, and disposing of them adds cost and the potential for environmental problems (see Chapter 9 for further discussion of related matters).

PROGRESS TO DATE

Few known and thermodynamically feasible molecular structures are presently seen as impossible goals for synthesis. New transformations and effective strategies permit chemists to synthesize highly complex molecules, such as new natural compounds discovered in the continued chemical exploration of the natural world. Again, the point of such work is to develop new chemistry that permits an approach to structures of the type found in nature. This expands the power of chemistry and allows medicinal chemists to synthesize complex structures.

Synthetic efforts have also extended to compounds of theoretical and practical interest. A variety of novel cyclic, bridged, caged, interlocking, knotted, and otherwise topologically unusual structures have been prepared in both the organic and inorganic arenas. These studies have provided new insights into structure and reactivity: for example, new zeolitic materials have valuable properties as catalysts, and highly strained caged organic structures have shown intriguing energetic properties.

An interesting example is the work to synthesize the alkaloid strychnine. Robert Burns Woodward (Nobel Prize, 1965) was the first chemist to synthesize this very complicated structure, which has many places where the detailed spatial orientation of chemical bonds was important. The synthesis was accomplished by a team of students and postdoctoral associates that he directed. The world did not need a new source of highly toxic strychnine, which is widely available in nature, but the synthetic challenge required Woodward to develop new approaches to some synthetic problems. The general approaches—involving new reactions and new strategies—developed for strychnine and other biologically relevant molecules taught chemists how to tackle other challenging synthetic problems and

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