The Use of Hand Models for Assigning Configurational Nomenclature A subject which often produces consternation in a beginning student in organic chemistry is that of sequential nomenclature involving ehiral centers. After having studied the Cahn-Ingold-Prelog rules of nomenclature a student may beasked to examine a structure as shown.
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Even students with limited stereo-perception have little difficulty assigning an R configuration. The situation becomes more cumbersome if the smallest atom attached to the chiral center is attached via a horizontal bond. A survey of a number of recent organic texts as well as earlier papers on the suhjeets ( 2 , 2 )shows that the student is told to view the molecule from the rear or to turn the molecule so that the lowest priority group is away from the viewer (a process which must be done mentally).
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Such a simple task to an instructor who has done this many times becomes a nightmare to the beginning student who has difficulty visualizing the three dimensional structure of methane. For this reason we have developed the "Rule of Wrist" which has proven to be effective in teaching sequential nomenclature. Our teaching conekpt uses the hand as a-model of the c h i d center t o be named. If in the projection or perspective formula of a compound the smallest group is horizontal and to the left, the left hand is used as a model with the left wrist being assigned the lowest priority group. The left index finger, middle finger and thumb are properly assigned as shown:
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Thefingeriand rhumharethrn marked tfrlt t i p p r n s w t h uaahahleinkareusriirl)arc~~rding I
