Computer Assisted Instruction in Stereochemical Configuration Analysis Students in organic chemistry usually have difficulty in visualizing molecules in three dimensions. Drill with the Cahn-Ingold-Prelag' RIS system of stereochemical nomenclature can assist their development of three-dimensional intuition. and general chemistry..' Computers have already been utilized successfully for drill in organic hemi is try,^ qualitltive analy~is,~ We have developed an interactive Basic computer program to help students master the concepts of chirality, R/S configuration, and meso structure. The program is presently running in Fordham's DEC 20 time-sharing environment. A full listing is available from the first author. The program is designed for molecules with either one or two central carbons. Each time the program is run the substituents are selected a t random from the following list: H, D, CH3, C2H5,t-C4Ho,CHIOH, CHO, COOH, OH, CI, Br, and I. The moleeulesare drawn on a terminal and theanalysis begins by comparing the substituent groups on the same carbon. The one-central-carbon compounds are classified as being either achiral or c h i d ; the two-eentral-carbon compounds are classified as having the upper central carbon chiral and the lower central carbon achiral, the upper central carbon achiral and the lower central carbon c h i d , both carbons chiral, or both carbons achiral. In the ease of an achiral compound there is no need far further analysis; chiral compounds are tested for their RIS configuration. If two chiral centers are present the possibility of a meso configuration is also examined. Before the RIS configuration can be determined for molecules with two central carbons, it is necessary to have determined the order of priority of the four suhstituents around each central carbon. Three of the substituents are placed there directly and thusdo not present a problem. However, the fourthgroupconsists of theother central carbon and its three substituents. This necessitates comparative analysis on an atom to atom basis. The double bond between C and 0 in CHO and COOH is resolved and considered as a duplication of atoms, 0 and O-C. This procedure results in the addition of two more "groups" t o the original list of twelve. These are given priority numbers between COOH and OH and between OH and CI, respectively. The groups must now be ordered to determine the RIS configuration. They are renamed as B(1),B(2),KG), and B(4) as shown in Figure 1.This diagram is used far both the one central and two central eases. B(4) represents the lower central carbon and its groups when the upper central carbon is being tested and B(1) represents the upper central carbon and its groups when the lower central carbon is being tested. We set B(1) > B(2) > B(3) > B(4) as the standard reference priority order. Clearly, this arrangement has the x x. S configuration. Each time two groups are interchanged (e.g. one position is I----z 7-switched) from the standard. the configb(i, ~uration inverts. An odd number of rearB,,, *-z -x Y YXrangements leads to an inverted configuration, R, whereas an even number of reX Z z arrangements leaves the configuration 0 14) unchanged, S. To keep track of the numFigure 1. Standard A R c ber of interchanges we have adopted the Figure 2. Meso test shuctures. sorting program presented by Rerkm5 reference structure. In the case of a molecule with two c h i d centers a meso test is done. This is based upon the specificarrangement of similar groups on corresponding carbons (Fig. 2). If the groups are aligned so that the drawn molecule has groups that are symmetrical about an axis perpendicular to the C-C bond as in A, the molecule is meso. If none of the groups aie reflected about the axis as in B, the molecule is also meso. If the groups are switched so that only one set of groups is reflected about the axis as in C, the molecule is not meso. Once all the needed information is obtained by the program, it is conveyed to corresponding PRINT statements. Questions are orinted and thestudent is reouired to rive theanswer. If the answer is rieht. it is recoenized as correct. If the ;tniutr i i nnmy, the C I ~ P C answer I is given. The Bmic itring manipulative t a ~ ~ l i t xllw ~ e r the pr( y w m to ~ U LUI I I ~ P I llte y i c e u f a dijluy usirng the tkalenr'i name. Sw h "pwwnal" fraturec cuuplvd tu i t t d t nl itwn-t i t , r.,mpmrrc inrremr %ltal~nt
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Kaloustian, a n d k i e h a r d ~ r a n c kfor a number of suggestions which have improved our program.
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Morrison, R. T., and Boyd, R. N., "Organic Chemistry," Allyn and Bacon, Inc., Boston, 1973, p. 130-3. Rodewald, L. B., Culp, G. H., and Lagowski, J. J., J. CHEM. EDUC., 47,134 (1970); Smith, S. G., J. CHEM. EDUC., 48,727 (1971);Clark, H. A,, Marshall, J. C., and Isenhour, T. L., J. CHEM. EDUC., 50,645 (1973);Feldman, M., and Bishop, M., J. CHEM. EDUC., 53,91 (1976). Bishop, M., J. CHEM. EDUC., 54,689 (1977). 'Chabay, R., and Smith, S. G:, J. CHEM. EDUC., 54,745 (1977). Rerko, A. J., Kilobaud, 34 (April, 1977).
The College at Lincoln Center Fordham University 60th S t r e e t and Columbus Avenue New York, New Yark 10023
318 1 Journal of Chemical Education
Marvin Bishop Maria Nowak
