"Absolutely" Simple Stereochemistry Philip S. Beauchamp California State Polytechnic University. Pomona, CA 91768
It has been my experience that beginning organic chemistry students encounter major problems trying to visualize three-dimensional stereochemical representations a t even one chiral center and, of course, have evengreater difficulty with two or more chiral centers. There does not seem to be much problem with recognizing where the chiral centers are (four different suhstituents) or providing a priority order to the suhstituents (highest atomic number according to the CahnIngold-Prelog sequence rules). Fischer projections are understood easily, hut there are twelve pwsihle representations for each enantiomer of a molecule with a single chiral center (24 total for both enantiomers). The mental gymnastics required to rotate chiral centers in space, either on paper or mentally, can he very frustrating t o the student. Models are helpful if availahle, hut the process of nuttine them toeether in iust the right wav is so time consuming fir the be$nning &dents that the students seldom resort to such a methwl. When structures with multide chiral centers are confined to specific Fischer projections(usually the longest carbon chain in the vertical position) the lowest priority group (often hydrogen) is toward the student. This is the opposite direction which the student needs to assign ahsolute configuration. Trying t o redraw several atoms in a molecule with multiple chiral centers is often hopelessly frustrating. From my own early frustrations I have developed what appears to he a very simple and effective method for determining absolute configurations.At the same time, the students develop 3-D visualization skills equivalent to much practice with models. T o use this method, the students need to recognize where the chiral centers are and he able to assign proper priorities. They also need the use of either arm and the thumb and first two fingers of that hand. I usually begin with single chiral center molecules and rapidly progress to more complicated molecules (two or more chiral centers). The student effectively uses hisher arms and hands and fineers as an ever-nresent molecular model kit. llsing a tetrahidral shape with numhers for relative priorities instead of atoms the method can he illustrated easilv. When the lowest priority group is already away &om the viewer as in I or 11,
LOWEST PRIORITY GROUP TOWARD VIEWER Occasionally the student is told to leave the lowest priority group toward himher and invert hisher assignment, hut obtaining the absolute configuration in this manner results in no positive visualization of the chiral center. When the lowest priority group is coward the student it is usually on the left or right side. With the method described here, if the low priority group is on the left side the student is told to use hisher left arm as the low priority group (the arm,right or left, is always assigned as the low priority group). The thumb and first two fingers comprise the rest of the tetrahedral structure. Thus, molecule 111 (above) is developed as follows:
LOW PRIORITY GROUP IS NOW AWAY FROM VIEWER With the arm fourth in priority, the thumb and first two fingers are rapidly assigned their proper priorities. I t is now a simple twist of the arm to rotate the low priority group away from the viewer to visualize the proper absolute configuration. The arm and fingers held in the fashion indicated are very close to the tetrahedral shape so the student gains 3-D visualization exercise in this process. With more than one chiral center, this method proves more valuable. A complicated system like D-glucose (four chiral centers) becomes four simple, single-center problems.
LOWEST PRIORITY GROUP AWAY FROM VIEWER there is no ~ r o h l e min assirmine absolute confiauration. One simply traces the circles th;ou~hsuhstituents 11, and Ill; if the circle traces to the right, the absolute confiauration is R, and if the circle traces to-the left, the absolute configuration is S. However, if the lowest priority group is toward the viewer an in 111 or IV, the entire molecule must be rotated.
