In the Classroom
Showing Enantiomorphous Crystals of Tartaric Acid
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Julio Andrade-Gamboa Área de Química, Centro Regional Universitario Bariloche, Universidad Nacional del Comahue and Centro Atómico Bariloche, Comisión Nacional de Energía Atómica, Av. Bustillo Km 9.500, (8400) San Carlos de Bariloche, Río Negro, Argentina;
[email protected] Chirality is an important and amazing aspect of chemistry. Chirality and related subjects show the relevance of 3D space in chemistry and teaching about them is a beautiful challenge. Enantiomerism is a concept strongly linked to the historical resolution of racemic tartaric acid (1–4) by Pasteur, in which left- and right-handed crystals were mechanically separated. Most of the articles and textbooks that present drawings of enantiomorphous crystals use an inadequate view to show that these are non-superimposable mirror images of one another. For example, to represent the chirality of tartaric acid crystals, usually drawings based on the original Pasteur drawings (Figure 1) (1, 2) are shown (5–7). In these representations, the (100) and (001) faces seem to be at a right angle, and therefore it seems to be possible to get superimposition after rotating one enantiomorph 180⬚ through a vertical axis. However, these crystals belongs to the monoclinic system (8), which is a not orthogonal (the angle between the above mentioned faces for tartaric acid is 100.3⬚; ref 6 ) and thus superimposition is not possible.1 In other words, the horizontal two-fold rotation axis, a necessary condition for chirality in this case, is not evident in the representations in Figure 1. Although any drawing can be complemented with pertinent explanations to demonstrate chirality, it is more powerful to use representations that validate the old adage, a picture is worth a thousand words. For example an alternative view of tartaric acid is shown in Figure 2 in which the “mirror” is placed perpendicular to the plane of paper and the two-fold axis directed with a low deviation to the observer that allows one to see both the mirror relationship and the non-congruence of the crystals. Finally, as 3D models are better than graphical representations, paper models for enantiomorphous crystals of tartaric acid are presented in Figure 3. These models allow one to appreciate crystal enantiomorphism in all its dimensions.
Figure 1. Classic view of enantiomorphous crystals of tartaric acid.
Figure 2. Alternative view of enantiomorphous crystals of tartaric acid (only the visible faces were labeled).
Conclusions Considering that 3D aspects depicted through 2D representations are difficult to visualize, all necessary precautions must be taken when used to explain stereochemistry. In particular if a graphical presentation of crystal chirality is not evident, the main attribute of crystal enantiomorphism can not be understood by the students. Although an explanatory text could be added, it is pedagogically more suitable if drawings show that enantiomorphous crystals are non-superimposable mirror images of one another. W
Supplemental Material
An enlarged version of the paper model shown in Figure 3 is available in this issue of JCE Online. www.JCE.DivCHED.org
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Figure 3. Paper models for enantiomorphous crystals of tartaric acid. Once built compare them with Figure 2. Note that enantiomeric pairs can be obtained starting from a single draw model just refolding the paper on the reverse side.
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Journal of Chemical Education
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In the Classroom
Acknowledgment The author wishes to thank the reviewers for corrections and useful suggestions.
2. Kauffman, G. B.; Myers, R. D. Chem. Educator [electronic version] 1998, 3, S1430–4171(98)06257-9. DOI 10.1333/ s00897980257a. 3. Suh, Il-H.; Park, K. H.; Jensen, W. P.; Lewis, D. E. J. Chem. Educ. 1997, 74, 800–805.
Note 1. Strictly speaking, the essence of monoclinic symmetry does not impose this angle to be different from 90⬚. It is possible that, within the experimental error and for other monoclinic crystals, a right angle could be measured, but the two-fold axis presence re᎑ quires, for example, that faces (101) and (101) belong to different forms; consequently their development will be different.
4. Kostyanovsky, R. G. Mendeleev Commun. 2003, 13, 85–90.
Literature Cited
7. Phillips, F. C. An Introduction to Crystallography, 3rd ed.; Wiley: New York, 1963; p 110.
1. Kauffman, G. B.; Myers, R. D. J. Chem. Educ. 1975, 52, 777– 781.
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5. Vainshtein, B. K. Modern Crystallography I. Symmetry of Crystals. Methods of Structural Crystallography, 1st ed.; SpringerVerlag: New York, 1981; p 81. 6. Waser, J. J. Chem. Phys. 1949, 17, 498–499.
8. Sharma, B. D. J. Chem. Educ. 1982, 59, 742–743.
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