Molecular Structure and Chirality David J. Brand' University of Minnesota, Minneapolis, MN 55455 Jed Fisher Lipids Research. The Upjohn Company. Kalamazoo, MI 49001 The concept of stereochemistry as an essential attribute of molecular structure dates back to the origins of modern . organic chemistry over a century ago. Despite this apparent maturity, stereochemistry remains an active and changing field of inquiry. Indeed, the relevance of stereochemistry continues to grow, paralleling the increasing sophistication with which molecules are assembled and analyzed. This growth does, however, necessitate a retrospective look at the appropriate definitions of the terms associated with the dayto-day applications of stereochemistry. These terms have been to a laree extent emoiricallv defined in resoonse to continuing observation and'thought. ~ v e the n term that is at the verv heart of stereochemistrv. chiralitv. has not escaoed ambig;ity.~~hefocus of this short essay is-; recent approach to chirality (I) that clarifies thisambiguity, and inso doing it seeks to call attention to the fact that current usage of this term is often inconsistent with its proper relationship to molecular s t r u ~ t u r e . ~ Chirality Chiralityl is 5 geometrical attribute. An object is c h i d (pronounced Ki' ral, as in "chiropractic") when i t is not superimposahle upon its mirror image and achiral when i t is suoerimoosahle. Two common~laceexamoles of chiral objects ar; an individual's right and left hands and a pair of counterclockwise- and clockwise-threaded screws. In extending chirality to molecules, most introductory texts strictly adhere to its geometric origins, and so define a chiral molecule as "one which is not su
