edited by: RON DELORENZO Middle Georgia College
Cochran. Georgia 31014
Models for Illustrating Chirality at Two Centers Marlin R. Feldman Howard University. WashBgton, DC 20059 Common ohiects which are chiral (ex.. doves. shoes. scis. sors) are often;sed to introduce the concept of enantiomerism. This note suggests common objects which can be used as models to illustrate chirality at two centers, and to introduce the c o n c e ~ t sof diastereomerism and the meso configura. tion. Ordinary eyeglasses have a right and left lens and temple ~. arranged symmetrically with respect to a central plane. set of four "stereoisomeric" chiral eyeglasses can he made from ordinary glasses in the following way. Two pair of glasses which have frames of one color are cut through the n w bridge and the right and left sides separated. The same procedure is repeated with two more pair which have frames of a contrastine color. This is most easilv accomolished with inexpensive-children's sunglasses which have t i i n plastic, brightly colored frames. The senarated halves are now cemented together so that each new-pair consists of different color halves. ~
The four new sets of glasses are shown in the figure. The models labeled 1and 2 are enantiomers, as are 3 and 4, because they are nonsuperposahle mirror images of each other. On the other hand. 1 (or 2) is not the mirror image of 3 (or 4). and therefore l a n d 3 a;e diastereomers. The models 1 and'2 resemble molecules which have the cis configuration, and 3 and 4 resemble the trans configuration. The four stereoisomers are analogous to molecules which have two non-identical chiral centers, e.g., 3-chloro-2-butanol. If the same operations are carried out mine onlv elasses with frames of the same rolor, then models 1 and 5 wili 6e identical. This corresponds w the meso ronfiguration. The set will now consist of oily three stereoisomersr3 and 4 are enantiomers, to and 1is a diastereomer of 3 or 4. The situation is analwous molecules which have two identical chiral centers, e.g., 2,3butanediol. A lab coat serves as another model for two chiral centers. The advantage of this model is the ease with which inversion of configuration can be demonstrated. An ordinary lab coat has a right and a left sleeve, and, if buttons, buttonholes, and pockets are ignored, the coat is meso. If the left. sleeve is pulled inside out (i.e., configuration is inverted) the coat will now have two rieht sleeves. This is demonstrated hv wearine the coat in the usual way, open in front, and showingthat o n 6 the rieht ann will fit orooerlv in either sleeve. Inversion of either &ht sleeve returns t h i meso form, and inversion of a right sleeve again - -gives the final "isomer" with two left sleeves. It can he seen easily that the non-wearable forms which have both sleeves of the same chirality are diastereomeric with the wearable coat, and that two right sleeves is the enantiomer of two left sleeves, as was seen with the eyeglasses (models 3 and 4). What has been demonstrated is that one (or an odd number) inversion converts any form to its diastereomer, hut two (or an even number) inversionsconvert to the enantiomer. The meso form may he changed to chiral coats to produce the com~leteset of four "isomers!' most easilv hv cuttine"~ off one s1ee;e near the elbow. ~here'willthen be two pairs of enantiomers: left shortfieft long with right shortlright long, and left shortlright long with left longlright short. They are interconverted by the appropriate inversions a t each sleeve.
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Set Of stereoisomericeyeglasses. Models 1 and 2 are conslmcted by joining
a right and a len half togelher. Model 3 is conshucted from two len halves. and 1-
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4 from two right halves.
Journal of Chemical Education
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