Natural and Unnatural Models for Illustrating Chirality at Two Centers Paul M. Nave Arkansas State University, State University, AR 72467
Stereochemistry, particularly that of compounds with more than one chiral center, is a difficult concept to grasp initiallv. Some macrosco~icobiects have been used to i1lustra"te the principles.' ' Since chemists use a natural obiect. the human hand, to illustrate the relationship between molecules with a single chiral center. it seems fittinp to use natural models to illustrate the stereoisomerism of molecules with two or more chiral centers. We suggest animals with special color markings (Fig. 1). Ordinary ball-and-stick molecular models of stereoisomers are actually used in the illustration, but their resem-
Figure 1. Illustration of stereoisomers using animals.
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Journal of Chemical Education
blance to animals is exploited to provide familiar analogs to the stereoisomers. Aclassic example of a compound with chiralitv a t two centers is 2.3-dichloronentane, Ball-andstick models of the four stere&omers are easily assembled (omitting hydropen atoms except on the two chiral centers). When roraied t ~ > h cconformath)n in which thealkylgroups are eclipsed, the models rcsernble four poodles that differ in the color patterns of their feet (Fig. 2). When viewed a s poodles, it is quite clear that there are indeed four different versions which can be divided into two pairs of mirror images, thus illustrating the two pairs of enantiomers. Pairs of poodles" that are not mirror images clearly illustrate the relationship between diastereomers. An added benefit of the noodle analow is its illustration of erythro and threo termihology. In ~ i & r e2, the top pair are erythro, and the bottom are threo.
Figure 2. Ball-and-stick models of the stereoisomers of 2 3 dichloropentane.
Figure 4. Unnatural model for compounds which contains two like chiral centers.
Figure 3. Ball-and-stick models of the stereoisomers of 3,4dichlorohexane. To illustrate the stereochemistry of a compound with two similar chiral centers, models of the compound 3,4dichlorohexane are used. These are made by adding one carbon atom to each of the models previously assembled (Fig. 3). These models are like poodles with a head a t each
end of their bodies. (These unnatural creatures may be compared with LoRing's2 tictional "Dr. Dolittle" animal, the "Pushmi-Pullyu", as seen in Fig. 4). I t is easily seen that two of the four models are identical, representing the same "animal" and thus the same compound (the meso compound, Fig. 3, top). As before, the enantiomeric and diastereomeric relationships of the three remaining strnctures can be seen clearly.
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Feldman, M. R . J. Chem. Educ. 1988, 65, 580. Feldman, M. R. J. Chem. Educ. 1984, 61,1050. Lofting,H. The Storyof Doctor Dolittle; Stokes: New York, 1920.
Volume 68 Number 12 December 1991
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