Stereoscopic Projection in Organic Chemistry Bridging the Gap between Two and Three Dimensions Arlene A. Rouelle and Stuart M. Rosenfeld' Smith College, Northampton, MA 01063 One of the most useful and enduring things that we can teach students of organic chemistry is an appreciation of the three-dimensional character of molecule^.^ The ability to visualize molecules in three dimensions and to represent them meaningfully in two dimensions empowers the student in her nursuit of an understanding of structure and chemical reactions. In fact, since both 2-D and 3-D representations embodv our ideas about molecules, one might say that in organic chemistry the model is the message.As the increasindv - .subtle features of molecular structure are introduced in the early months of an organic chemistry course, the instructor has an opportunity to motivate students by helpins to use molecular models and two-dimensional re- a~ them ~-~~~ presentations as tools that reward the user with the satisfaction inherent in the skillful use of anv tool. Moreover, this period provides an excellent opportunity to inspire an apureciation of the aesthetics of molecular architecture. Students come to organic chemistry with widely varying abilities in visualizing molecular structure in three dimensions. Most students have some difficulty in this area and some find this aspect of organic chemistry to be especially frustrating. This problem is compounded by the abstract nature of two-dimensional representations, many of which are meaningful only to one who has an appreciation of the corresponding three-dimensional representation. Though molecular helo the student overcome this harrier, - ~ - - ~ models ~ many students exhibit a reluctance to extensive use of models and some do not use models a t all. Stereoscopic proiection of pictures of molecular models is a powerf"l tiaching tool and an effective vehicle for motivating students to use molecular models. Other authors have discussed various aspects of the use of stereoscopic images in teaching chemistry including the use of pre-exi>tinp .stereo drawings in texthooks and in the primary literature for use with stereoscopic viewers (I),projection of stereo drawings (especially those generated by the ORTEP computer program but also ones created with adesk calculator and plotter (2))using two slide projectors in the conventional polarization method ( 3 , 4 ) ,use of a single projector with stereo pairs created through use of a special prism (5) or the use of anaglyphic pairs on a single slide ~~
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frame (.6.. ) . or anaelvohic "". double overlavs on an overhead projector (7), and, making stereoscopic siide photographs of molecular models for use with simple hand-held viewers (8). I t appears, however, that stereo projection of photos of molecular models is rarely used in teaching organic chemistry. We now describe a very positive experience with the use of full-color ~roiectionof sterea photos of molecular models using the EonGentional polariza&on method (see inset). We have used stereo slide projection in presenting the following topics to agronp of about 100 first-semester organic chemistry students a t Smith College: How models and two-dimensional representations embody selected aspects of structure.Introduction to several distinct types of molecular models (hall-and-stick,space-filling,framework). 2) Fundamentals of using the specific model set required for the eourse.3 The range of geometries around carbon in simple compounds. 3) Conformational isomerism including cyclohexane. 4) Chiralitv. Large fill-color 3-D images are visually impressive. They convev the idea that aeometrv is important in our current conception of molecules and that molkcnlar architecture has a certain aesthetic appeal. Student attention appears to be firmly fixed on the& images and the instruitor has the pleasure of providing a guided tour of the subtleties of each particular structure. Under these conditions, for example, one might present several views of ethane in eclipsed and staggered arrangements, including a view down the carbon1)
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'At a recent symposium on teaching organic chemistry (8th Biennial Conference on Chemical EdJcation. University 01 Connecticut. August 5-10, 1984)several panelists expressed the view that organic textbooks uniformly overemphasizedescriptive chemistry though students tend not to retain this material beyond the duration of the course. We use the Allyn and Bacon Organic Chemistry Model Set because it allows construction of either balland-stick or space-filling models.
carhon hond axis, to illustrate the relationship between the model and a Newman projection of ethane. The demonstration can he reinforced as one examines models of cyclohexane conformers where the Newrnan projections include two carhon-carhon hond axes and are significantly more difficult for students to appreciate and to draw. These slide lectures also provide many opportunities for interaction, ranging from sight recognition questions (e.g., what molecule is represented by this model?) to more probing questions requirmg visual comparisons between two models. The area of stereochemistry is rich with situations where visual comparison of 3-D pictures provides a link that allows students to move toward making the same judgements using 2-D representations. For example, we have found that most students are able, without previous experience, to compare 3-D pictures of two models side hy side and easily decide whether they are identical, enantiomeric, or diastereomeric. Many students are initially deceived when they look at two models of a meso compound arranged so that one mirrors the other and most have difficulty when they first view two enantiomeric alleues together. Those that do not make all the connections in a given class are left with the injunction to work through the same exercise with their own models.
Many students have offered comments on these slide presentations. All comments have been positive and have eenerally focused on the experience as useful, morivaring, and fun. The techniques for preparing and projecring stereo slides are straiphtfnru,ard tinset! and allow the instructor to. railor .-... . each-presentation to the needs of the students. Although preparation for a class that includes stereo nroiection ioitinlly rrquires lead time for making slides, one can easily asselnble an extensive personal slidr librarv in a matter of davs. In any case, the rewards for both student and instructor greatly outweight the effort involved.
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Acknowledgment
SMR gratefully acknowledges inspiration provided by Kenneth J. Miller.
Volume 62 Number 12 December 1985
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