In the Classroom
A Modeling Exercise for the Organic Classroom Christine R. Whitlock Department of Chemistry, Georgia Southern University, Statesboro, Georgia 30460-8064
[email protected] Basic conformational analysis is a challenging topic for beginning organic students. They must view two-dimensional chalkboard structures as three-dimensional molecules. They must also be able to correctly name these molecules and mentally manipulate them into different conformations. The introduction of stereochemistry further complicates this concept. Taught in the first semester of organic chemistry, conformational analysis is often the first challenging, new topic students face. A variety of approaches, including handheld models (1, 2), computer models (3, 4), animations (5), overhead transparencies (6), electronic databases (7), and polar maps (8), have been developed to aid in the visualization and drawing of organic molecules. Traditionally, exercises utilizing handheld models provide the names of molecules and ask students to build the appropriate models. Recently, Pellegrinet and Mata (2) reported a model exercise that was conducted in reverse. In their exercise, each student in a group of four students was given a box containing one of four possible compounds. The student was also given written questions to answer about his or her model and asked to build new models. At the end of the class, each group presented the answers to their classmates. The entire exercise took approximately 1 h. Model Exercise I now report a modification of Pellegrinet's successful exercise. In this model exercise, the class is self-assigned into groups of four, and each group is given a box containing two molecular models. One model is a cyclic alkane (cis-1,3-dichlorocyclohexane), and one model is an alkene [(Z)-(S)-4-chloro2-heptene]. The students are told that black and gray atoms represent carbon and that green atoms represent chlorine. They are then asked to answer eight written questions about the models (available in the supporting information). To begin, the students are told to examine and manipulate the cyclic molecule without breaking any bonds. They are asked to orient the six-membered ring into a boat conformation and locate the “flagpole” hydrogens. They are then asked to provide an IUPAC name for the molecule. Finally, the students are to orient the model into the most stable chair conformation and draw it. In the second half of the exercise, students consider the alkene. The molecule contains a stereocenter, and the students are asked to provide the correct absolute configuration for the stereocenter and name the compound. After removing the chlorine, they are asked to draw and name the resulting alkene. Next, the students are instructed to remove the gray double bond unit and examine butane. They are to orient the C2-C3 bond into anti, gauche, eclipsed, and totally eclipsed conformations and to draw the least stable conformation.
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For fun, the students are told to open the six-membered alkane ring, remove the chlorine atoms, and connect it to the butane chain to produce n-decane. They are challenged to arrange every carbon-carbon bond into its anti conformation and make an observation about the carbon backbone of the model (which is a zig-zag much like the ones they draw on paper). Conclusion This model exercise was completed during a 50 min class period with 45 students. It proved to be an effective way to review nomenclature, absolute configuration, and conformational analysis. Student attitude toward this approach was overwhelmingly positive, and several students shared that “seeing” the molecules already built helped them to understand these topics. The class average for this exercise was 94.5%. Cooperative learning is an effective method for organic chemistry (9-15) and proved effective for this exercise. Acknowledgment I thank the Department of Chemistry at Georgia Southern University for financial support by purchasing 15 Molecular Visions model kits for this exercise. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Murphy, W. S. J. Chem. Educ. 1981, 58, 504. Pellegrinet, S. C.; Mata, E. G. J. Chem. Educ. 2005, 82, 73–74. Fitzgerald, J. P. J. Chem. Educ. 1993, 70, 988–990. Montes, I.; Prieto, J. A.; García, M. Chem. Educator 2002, 7, 293–296. Nelson, J. E.; Williamson, S. A.; Steffen, L. K. Chem. Educator 1996, 1 (6), 1–9. Silverman, L. P.; Barbaro, J. J. Chem. Educ. 1989, 76, 630. Bateman, R. C.; Booth, D.; Sirochman, R.; Richardson, J.; Richardson, D. J. Chem. Educ. 2002, 79, 551–552. Ounsworth, J. P.; Weiler, L. J. Chem. Educ. 1987, 64, 568–572. Dinan, F. J. J. Chem. Educ. 1995, 72 (5), 429–431. Dougherty, R. C. J. Chem. Educ. 1997, 74 (6), 722–726. Harvey, L. C.; Hodges, L. C. Chem. Educator 1999, 4, 89–93. Browne, L. M.; Blackburn, E. V. J. Chem. Educ. 1999, 76 (8), 1104–1107. Paulson, D. R. J. Chem. Educ. 1999, 76 (8), 1136–1140. Hagen, J. P. J. Chem. Educ. 2000, 77 (11), 1441–1444. Hodges, L. C.; Harvey, L. C. Chem. Educator 2003, 8, 346–351.
Supporting Information Available Questionnaire for the modeling exercise. This material is available via the Internet at http://pubs.acs.org.
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Vol. 87 No. 12 December 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100405j Published on Web 09/17/2010
