Using the QCPE Holdings in Chemical Education Molecular Models in the Organic Chemistry Laboratory Kenny Llpkowltz Indiana-Purdue University at Indianapolis, Indianapolis, IN 46223 Organic chemists rely heavily on information gleaned from molecular models. We insist our students use models in their course work. and i t is not uncommon to find lahoratoniexercises where structural problems encountered in organic chemistrv are addressed and analyzed with the aid of mechanical models. One of the problems with hand-held models is that they are mechanical; they are limited in scope and can give misleading structural information. For example, all mechanical models indicate 1,4-cyclohexadiene and related molecules will exist in a boat conformation. This is incorrect.' While encouraging students to use mechanical models we should also make them aware of their pitfalls, and we should point out that not all models are mechanical. In fact, some of the better models I have used in research and teaching have been computer models. In this paper I wish to discuss a successfully implemented laboratory experiment that compares the strengths and weaknesses of mechanical and computer models. This laboratorv exercise was develooed for an undermaduate oreanic cheiistry lab and represe& a unique hyhgd of theory and exoeriment. I t is a special topics experiment that lasts two weeks and is offered to our first-semester students. The Models Molecular structure calculations mav be done Quantum mechanically or with an empirical force held as incorporated in molecular mechanic^.^ We use the latter method because the theoretical concepts are more readily understood by students. Like mechanical models that are inexpensive and readily available, computer models can be purchased through the Quantum Chemistry Program Exchange at a modest They usually run on a wide variety of mainframe computers, are well documented, and come with test decks to insure the program is working properly. The program we use is MMI-.MMPI by N. L. A l l i ~ ~ g e r . ~ It isabsolutely necessary tostress that the programnot he cunsidrred as u black box. It must he repetitively emphasized what thr inherent limitations of the computational mothod are and examples of how one could abuse thr program shuuld he provided. Without a sound explanation of the theory and a caution to students about the black hox approach there is little intellectual involvement for them and even less confidence in the data that is produced. Consequently, we begin the lahoratorywitha40min lecture on molecular mechanics' followed by a 20-min description of the cornouter output and how to interpret it."The studenw, in groups of four; then compare computer and mechanical
' Lipkowitz. K. B., Rabideau. P. W., Raber, D. J.. Hardee, L. E.,
Schleyer, P. v. R., Kos, A. J., and Kahn, R. A., J. Org. Chem.,47, 1002 11982). . .
'Boyd, D. B., and Lipkowitz, K. B., J. CHEM.EDUC., 59. 269
..---,Information can be obtained by wntacting Richard Counts, Quantum
119821.
Chemistry Program Exchange. Department of Chemistry, Indiana University. Bloomington. IN 47405. Ailinger, N. L., et al., O P E , 11.318 (1976). The laboratwy handout is available on request from the author.
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NMR s+mchnmof 2hlorocyclahsxanone 10%by wlum In benzene. Top: 0.5 ppm sweep widm; bottom: 10.0 ppm sweep width.
models for two exercises. Plastic models and input decks of cards are provided hy the instructor. The latwrauky write-ups are done individually and are in the format of an American Chemical Society publication. Exerclse I. Molecular Deformations Induced by Steric Effects.
A comparison of o- and p-di-tert-hutylhenzene is made with the two models. The students are asked to estimate the energy difference between the two isomers and, based on their mechanical model, asked to predict the shape of the ortho isomer. Then, using a set of cards provided by the instructor they are to compute the energies and structures of the isomers. A literature reference is provided for students to gauge the reliability of the theoretical findings. The ortho isomer is calculated to he 23.5 kcal mol-' less stable than the isomer. This arises from steric compression of the tert-butyl groups. T o reduce this severe nonhonded interaction one can anticipate several reasonable modes of deformation 1) the tertiary butyl groups move out-of-plane,in opposite direc-
tions maintaining a planar benzene ring. the tertiary butyl groups move out-of-plane,in oppoaitedirections with a eoncommitmentwardne of the benzene rine 3) the tertiary hutyl groups are shoiedapart in the plane the benzene ring. 2)
of
In all cases we anticipate large Me-C-Me and H-C-H angle distortions on the tertiary hutyl groups. Interestingly, the molecular structure is computed and found to have a planar benzene ring with the tertiary hutyl groups distorted Volume 61 Number 12 December 1984
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in-plane. It becomes clear that one is handicapped if molecular structure determination is based solely on mechanical models. Exercise II. Conformational Analysis The conformational properties of 2-chlorocyclohexanone are investigated. This molecule is unique because the ratio [axial]/[equatorial] >1.00 in solvents with a low dielectric constant but
