Chirality, or handedness, is a pervasive element in our perception of the universe. From the obvious differentiation of left- and right-handed gloves to the hardwon understanding of different physiological effects resulting from mirror-image molecular stereoisomers, chirality significantly influences human life. It is interesting to consider that there is often an apparent difference in the chirality of a gross structure versus the building blocks from which it is composed. Take people as examples. We exhibit bilateral symmetry in the whole, having a mirror plane which can generate our left side from our right (or vice versa). Thus, we have a left half and a right half which cancel each other, leaving each person achiral. However, on the molecular level, enzymes, which control the chemical reactions leading to body construction and function, are composed of only the levorotary stereoisomers of the amino acids. This chapter deals with the similar relationship between the chirality of molecules and the nature of the forms they attain in the solid (crystalline) state, particularly as it relates to organic compounds utilized as drugs.

Stereoisomerism

The development of the theory of molecular stereoisomerism arose from observations of the chirality of observable structures. A series of important discoveries in France were critical to this process. Hemihydrism, the existence of nonsuper-imposable crystal forms, was noticed in quartz crystals by the mineralogist Haüy in 1801 (Fig. 6.1) (Haüy 1801). Shortly thereafter (1809) plane-polarized light was discovered by Malus, a physicist.