In the Classroom edited by
Overhead Projector Demonstrations
Doris K. Kolb Bradley University Peoria, IL 61625
Illustrating Newman Projections by Using Overhead Transparencies L. Phillip Silverman and John Barbaro*† Department of Chemistry, The University of Georgia, Athens, GA 30602; *
[email protected] A major stumbling block for many undergraduate organic chemistry students is learning to look at and manipulate the two-dimensional structures that organic chemists use to represent three-dimensional molecules. Confusion often arises when conformational analysis is discussed. An undergraduate’s first exposure to conformational analysis typically involves using Newman projections to represent various conformations of an alkane. When asked to draw a Newman projection of a molecule, most students have little problem looking down a given bond and attaching the appropriate groups to the front and the back carbon atoms of the Newman projection. Difficulties arise when they are asked to take the Newman projection and rotate about the σ-bond in question to obtain different conformations of the molecule. This confusion primarily comes from the inability to picture in their heads the three-dimensional changes that are occurring in the molecule during the rotation. Rather than just show drawings of various conformations on the board in class, it is more informative to use a ball-andstick model or some other hand-held representation of the compound in question to illustrate to the class how rotating about a bond changes the relative positions of the molecule’s groups in space. While these types of demonstrations work well in a small class, as the size of the class increases it becomes more difficult for the students toward the back of the class to see the model clearly and grasp the significance of the conformational changes occurring during the rotation. A better approach for illustrating the rotations about a bond in a Newman projection to a large class uses a simple demonstration on an overhead projector. This demonstration requires a thumbtack and two overhead transparencies—one to represent each of the two carbon atoms of the Newman projection. Combining these two overhead transparencies with the thumbtack on the overhead projector gives a movable Newman projection. Using ethane as the example molecule, prepare the two transparencies beforehand by drawing the CH3 of the front carbon atom of the Newman projection on one transparency and the CH3 of the back carbon atom of the Newman projection on the other transparency as shown. (This is easily done either freehand or by using a drawing program such as ChemDraw.)
H
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Now make a hole in each transparency by running the thumbtack through the center of the picture of the front carbon atom (i.e., through the point where the three bonds intersect) and through the center of the circle drawn on the transparency with the back carbon depicted on it. To use the Newman projection in the classroom, lay the thumbtack on the overhead projector and place the two transparencies on top of each other on the overhead projector, giving the Newman projection as shown. The pin of the thumbtack acts as the spindle about which the rotations about the bond in the Newman projection will occur.
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thumbtack through the two transparencies
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Simply hold the back transparency in place on the projector and rotate the front transparency about the spindle formed by the thumbtack. As the front transparency moves about the spindle, the hydrogens move in a circle, illustrating the different conformations of the ethane molecule.
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eclipsed
I have used this demonstration in my large lecture classes the last two years with good success and with an increased student understanding of using Newman projections for conformational analysis.
H Make a thumbtack hole
†
Current address: Depar tment of Chemistry, The University of Alabama at Birmingham, Birmingham, AL 35294.
Journal of Chemical Education • Vol. 76 No. 5 May 1999 • JChemEd.chem.wisc.edu
