A Simple Hand Method for Cahn-lngold-Prelog ~ssignmentof R and S Configuration to Chiri Carbons Maltin P. Aalund The University of Texas at Austin, Austin, TX 78712 James A. Pincock Dalhousie University, Halifax, Nova Scotia, Canada B3H A13
The Cahn-Ingold-Prelog rules for assigning priority t o groups on a chiral carbon atom are straightforward, and very few beginners have difficulty learning them. Application of these priorities for assigning a carbon as R or S usually causes no problem when the group of lowest priority is oriented behind the carbon in question as in 1. However, if the chiral carbon is drawn in other orientations, students often have difficulty visualizing the three-dimensional orientation of the tetrahedron when asked t o look from "behind the blackboard", 2, or "from the bottom, lying on the floor", 3. Sometimes students are taught t o leave the group of lowest priority oriented in front of the carbon as in 2 but to reverse the assignment of the observed circle. This method will not, of course, help for 3. The rule of interchanges is also taught as an aid: an odd number of interchanges gives the enantiomericcarbon,andaneven numbergiv& the identical carbon hnck. This involves drawing additional wuctures and may
lead to confusion. For instance, does one interchange a t each of two chiral carbons give the identical structure back, or does i t give the enantiomer of the original? Obviously, the latter is correct. Particularly troublesome to students are chiral carbons in chair cvclohexanes (i.e.. 4 and 5). Newman projections, 6, or chiral carlwns as in Fischar projectims, 7. Clearlv. an unamt,irnous method of assienine R or S without manipktion of thestructure would be usefui. A recent paper in THIS JOURNAL^ outlines a convenient method for reorienting tetrahedra using the arm and hand as a molecular model. This allows for simple assignment of configuration but does not make the connection between the chirality of the carbon in question and the chirality of the probe. The choice of left or right is based on whether the
' Beauchamp, P. S. J. Chem. Educ. 1984, 61,666. CI
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