Drawing Different Views of the Chair Form of Substituted Cyclohexanes W. S. Richardson Auburn University at Montgomery. Montgomery, AL 36193 One of the more difficult topics for students in the chemistry curriculum is stereochemistry. A major problem with the subject is the necessity to visualize three-dimensional molecules mentally and in turn represent them on a two-dimensional surface such as paper or a chalkboard. Drawing reasonable stereochemistry structures is demanding for most of us, but the problem is particularly acute for students of organic chemistry since stereochemistry is an integral part of the suhject. One of the three-dimensional drawings that organic students must master is the chair form of cyclohexane. Specifically, exercises requiring the comparison of substituted cyclohexane structures in order to determine their stereochemical relationship represents a difficult problem. The task is especially difficult when drawings representing the structures must be "rotated" ahout the central axis of the structure, the axis that is parallel to the axial positions. The most common structure in this category is one that represents the view of the molecule after a 180' rotation; fortunately it is one of the easier ones to draw. Less common hut much more difficult to visualize or draw is a representation of the molecule after a 60' rotation that displaces the carbon atoms in the ring by one position or a 120' rotation that displaces the atoms by two positions. Questions by students in my organic chemistry classes related to the superimposability and interconvertibility of the mirror images of disubstituted cyclohexane derivatives led me to develop a set of rules to instruct students easily in a method for drawing chair structures after rotation about the central axis. Drawine the chair structures after rotation is accomplished b i a simpleset of rules illustrated in Figures 1 and 2. In the case of rotating the structure by an odd multiple of 60' in which the carbon atoms in the ring are displaced by an odd m u l t i ~ l eof atoms, the oriainal ring structure without the axial and equatorial groups is drawn as the flipped, alternate chair conformation. The axial and equatorial groups are then drawn on the new ring after displacement by the appropriate number of carbon atoms, but their orienta-
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Journal of Chemical Education
tion is not changed. That is, an equatorial group is moved to the new position on the ring hut remains equatorial. Figure 1 represents drawing the clockwise rotation of a chair conformation of a disuhstituted cyclohexane molecule by an odd multiple of 60°, that is, by 1X 60°, or 60". When drawine a structure that has been rotated hv an even multiple or600 in which the carbon atoms in the;ing are dis~lacedbv an even m u l t i ~ l eof atoms, the original ring structure, again without the axial or equatorial groups, simply redrawn in its original chair conformation, not the flipped conformation, and the axial and equatorial groups are drawn on the new ring after displacement by the appropriate number of carbon atoms; their orientation, as before, is not changed. Figure 2 represents drawing the clockwise rotation of a chair conformation of a disubstituted cyclohexane molecule by an even multiple of 60°, that is, by 2 X 60°, or 120'.
Figure 1. Drawing a cyclohexane structure afler rotating llm molecule clock-
wise by an odd multiple of 60'-by
60°.
Figure 2. Drawing a cyclohexane srmcture after rotating the molecule clockwise by an even muiiiple of 6 O ' d y 120°.
