Drawing the Chair Conformers of Cyclohexane Conformational relationships of ryrlohexane and its derivntiver are an important topic in organic chemistry. The rubject as presented in must introductory courses has several aspects including morphology, energetics, and reactivity. While three-dimensional models are valuable tools both for classroom demonstration and individual student use, students also need to develop the ability to draw realistic twodimensional representations, especially of the chair conformers. Unfortunatelv. manv students (not t o mention their mentors and other practicine chemists) ~~.havLdiffi<v makink accurate freehand sketches of chair c d o h e a me. Sumetimes the innrruroeies merely detract irom tncesthetrer of the sketch, hut not infrrqurntly student sketchesare iupoor that it beromesfruitratingl~diffirulrto transfer to paper information clearly seen in a model. I have found that much of the difficulty in sketching the chair conformers of cyclohexane stems from the sequence in which the lines forming the chair are drawn. The usual procedure1 involves first drawing two parallel lines of equal length with ends slightly offset to he the "seat" of the chair. The chair is then completed by adding points above and below the plane defined by the first two lines. The difficulty, of course, is getting the two points t o come out equal and symmetric. Below I list a set of instructions for an alternate approach that I and most of my students have found gives neater and more uniform results. More importantly, this approach greatly reduces the fraction of student sketches that cannot be interpreted. Each step is illustrated with a freehand example in the figure a t the right. 1. Draw two horizontal line segments of equal length with a space of the same length between them. 2. Draw two more line segments with the same length as those in step 1.The new lines extend a t about 45' above and below the horizontal plane from the inner ends of the first two lines. 3. Ccmplete the ring by connecting the outer ends of rhr horizontal liner uith the free ends of the 45' lines. 4. Vertices [hat point up have axial hunds going up; w r t i r ~ sthat point duwn have axial bonds going down. 3. The \,errires at the left and right of the skevh have equatorial bonds in the "laneof the mper (lines iS uniform thickness). The two fmward vertices (lower) ha!,e kauatoria~binds comine out (wedges). The two rear vertices (upper) have equatorial bonds going back (dashes). ~~~
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With this approach and only a little practice it is quite easy to produce a clear symmetric sketch of chair cyclohexane. To produce the other chair conformer, step 2 is modified so that the right-hand oblique line isdirected up while the left is directed down. 'For example see McMurry, J. Organic Chemistry; Brooks Cole: Monterey, CA, 1988; p 107; or Wade, L. G. Organic Chemistry; Prentiee-Hall: Englewood Cliffs, NJ, 1987; p 109. David G. Shaw University of Alaska Fairbanks, AK 99775
Volume 65
Number 7
July 1988
587
