Louis F. Fieser Harvard University Cambridge, Massachusetts
Plastic Dreiding Models
In little more than a decade, organic chemistry has taken on a new and exciting three-dimensional look. Classical stereochemistry has been expanded by introduction of the concepts of axial and equatorial bonds, conformational analysis, chair, boat, and twist forms of cyclohexane, nonbonded interactions in medium size rings, steric coutrol of asymmetric synthesis, and steric control of reaction mechanism. An up-to-date teacher trying to get these concepts across has been pressed to find models for use in lecture demonstrations. Models which the student himself can use have been either costly and not very good or adequate but prohibitively expensive. This article describes superior models of a new type which will be available at a cost so low that beginning students can acquire models either by purchase or on loan from the storeroom. The key piece is a half tetrahedron (Fig. I ) , two of which fit together in only one way. The model is as-
fit is snug enough so that the model will stay oriented in a specific conformation when desired.* The C-H and C-C distances are ip the ratio 1.09 A:1.54 A and the scale is 1 cm = 0.2 A. The models lend themselves well to the construction of rings containing five or more carbon atoms. Fred Kaplan suggested that, in an initial consideration of the
Figure 1.
sembled (Fig. 2 ) by fitting an aluminum tube permanently onto the shalt rod of a black plastic piece and cementing together t ~ oidentical units. In this model of methane, carbon is represented by the black tetrahedral piece at the center and hydrogen atoms are assumed to be centered a t the ends of each rod or tube. Figure 3b.
chair form of cyclohexane, one can assemble the atoms so that black plastic rods represent the axial bonds and shiny aluminum tubes represent the equatorial ones; 3b). The a ring flip reverses the situation (Fig. 3a ring flip of Figure 3a is accomplished gracefully by grasping the lw and 5a bonds (metal) and pressing 3a against the chest. The circular pressure labels for numbering atoms can be removed and reused repeatedly. One can demonstrate also the ready conversion of one boat form into another and then stop halfway between the two and preserve the twist form (Fig. 4a, b, c). An interesting assignment is to measure 1,2-, 1,3- and bow-stern H : H distances and estimate the relative stability of the chair, boat, and twist f o m . A con-
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Figure 2.
When a plastic rod of one carbon is inserted into the tube of another and the two are snapped together, a button near the end of the rod engages with an indentation of the tube and holds the carbons together until they are pulled apart. The carbon atoms of ethane are free to rotate about the C-C bond, but the
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venient measuring device can be made from a 10-cm clear plastic ruler either by taping a paper clip over the end to form a resting socket or by securing a 12/24 brass nut with an epoxy resin to the under side with the hole centered a t the zero point. With ten tetrahedrons the student can make models of cis- and trans-decalin, adamantane, twistane, and norbornane. For construction of cyclopropane and cyclobutane, students should be advised not to try to bend the bonds but to use sections of '/=-in. rubber tubing for connecting unmodified tetrahedrons. Amodel of ethylene made in the same way shows clearly that the members of the double bond are in a plane perpendicular to the plane of the C-H bonds. Rubber-connected models of ethylene, cyclopropane, and cyclobutane on being flipped open afford an amusing demonstration of the Baeyer strain theory. Three connectom are used to form acetylene; a larger piece of tubing can be used to form a carbonyl group.
difference in size. The student thus can make models of the lactic acids, the tartaric acids, the aldotetroses. He also can satisfy himself as to the relative stability of different conformations of 1,2-dichloroethane, do conformational analysis on dl- and meso-stilbene dibromide, etc. Selection of appropriate balls and use of rubber tubing connectors to form double bonds and C3-Cc cycloalkanes permits construction of numerous interesting n's and trans olehs, optically active allenes, and small ring-compounds-in some of which optical isomerism is superimposed on geometrical isomerism. Models of maleic and fumaric acid in which the substituents on the double bond are represented by large and small balls bring out the greater symmetry of the trans isomer and suggest that the cis isomer is destabilized by nonbonded interaction between the two large groups. For application of the Cram-Prelog rnle of steric control of asymmetric synthesis, balls of three sizes clearly represent large, medium and small groups. Models which represent a C=C hond by two tetrahedrons joined by rubber connectors and which form a C=O bond by a loop of rubber tubing can be used to present interesting problems. For example, would the CHX
Figure 5.
A mature user wishing to construct permanent models of cyclopropane and cyclobutane can do so after softening the plastic by gently heating with a small flame from a microburner (Fig. 5). Direct the flame for a moment to the outer flat tetrahedral surface to the right, then direct it to the surface to the left; repeat the operations and then heat more strongly on the sharp inside edge, and bend. The set includes four white plastic balls clearly differentiated in size and further identifiable by one, two, three, or four h e beads running around the equator. Each ball has one hole to fit a rod and another to fit a tube. The substituents in compounds such as C H r CH(OH)C02H,C8H6CHBrCHBrCsH6,H02CCH(OH)CH(0H)CHO are clearly differentiated in size and are appropriately represented by balls of corresponding
Figure 40.
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Figure
4b.
CHs
CHa
CH,
hydrogenation of 9-methyl-A4-octalin (I) be expected to be stereospecific? Yes, the hydrocarbon lies on the catalyst on its flat rear side and hydrogen emerges from the catalyst to attack from the rear and give the 10adihydride (11). What is the expected product of borohydride reduction of trans-4-keto-9-methyldecalin? The reagent approaches from the less hindered rear side, opens the rear member of the carbonyl group, and gives the 4palcohol (IV). Although considerable chemistry can be studied with merely tetrahedrons, balls, and rubber tubing, the new set includes the following additional pieces: an oxygen atom (red plastic), a nitrogen atom (blue plastic), and an aromatic double bond (corresponding to 1.39 A), which also serves well enough for isolated and conjugated double bonds. In the interest of economy the oxygen and nitrogen atoms are cast in the mold for the half-tetrahedron, since the bond angles of 110" and 109' are very close to the tetrahedral angle of 109°28'.
Figure 4s.
by summation of appropriate sections of two C-H bonds, and Mathus has applied for a patent covering t1.e additional invention of his design. The first plastic pieces were turned out with an experimental one-cavity mold. The first tubes were stamped out of sheet aluminum, since this process would lead to a lower production cost than use of predrawn tubing. The assembled tetrahedron seemed very promising, particularly since the estimated production cost was under 10 cents, and samples were sent for evaluation to 556 universities and colleges. The experimental mold wore out before requests for samples from some 130 additional colleges and high schools could be met, but prodnction models will he supplied to these schools later as samples. Evaluation of the models and estimates of the volume of sale from colleagues were very helpful. The first samples had faults which were easily corrected: weak stamped tubes were replaced by strong predrawn tubes; brittle polystyrene was replaced by a stronger, more elastic formulation; the fit of rod to tube was made snugger; the balls were adjusted to easily differentiated sizes. Everyone agreed that low-cost plastic models would open up a vast student market not touched by the 95 cent models and that they would not interfere with the use of the precise, many-piece Buchi models in research. A point of particular importance stressed by many commentators was that the models would be purchased widely by students a t all levels only if the price could be kept low, about $2 for a set sufficient for a beginner in organic chemistry.
If an additional oxygen is needed, it can be made easily by painting red the plastic part of a half tetrahedron. A nitrogen atom can be made from an assembled tetrahedron by cutting off one of the bonds carrying a tube and painting the plastic blue. The double bond piece has the correct trigonal carbon angle of 120" and serves nicely for forming benzene rings. The need for two specially molded parts adds to the cost of this piece, but it has the advantage over conventional double bonds in having two rods to connect the carbon atoms. By using wire-cutting pliers to cut a double bond in half, one can produce two carbonyl groups.
Figure
6.
In case a few double bonds are required, other than those using rubber connectors, a dexterous member of the group can make them by applying the heating and bending technique described above for the production of C8and C4cycloalkanes; the result is shown in Figure 6 at the left. The model of acetylene (right) was made the same way. The model representing one resonance structure of benzene was made from three double bonds after spreading the remaining bonds from the tetrahedral angle (logo 28') to the trigonal angle (120"). After forming the ring, the peripheral C-H bonds were straightened.'
Distribution
The cost of production did not preclude meeting this goal and a reasonable royalty agreement could be expected. The cost of production molds and tools was met by drawing on a special annual grant from Research Corporation in support of personal research and projects. There remained the markup to a retailer. Ordinarily a distributor sets as his list price a figure about twice the cost of the item to him and thus is able to meet the cost of handling and advertising as well as realize a modest profit. In the college field the models require little advertising because of the distribution of samples noted above. Further saving will be achieved by offering all pieces in lots of 100 only. For the benefit of those who want to achieve all possible economy, tetrahedrons will be offered uncemented (but with the tubes attached) a t a special price. The student needs only to apply methyl ethyl ketone to the two surfaces with a paint brush and press them together for a full 30 sec. A further discount will be allowed on orders accompanied by cash. Student clubs or societies may serve as order-takers. To promote use of the models in high schools, I have written a small book entitled "Chemistry in Three Dimensions." It presupposes no previous knowledge of chemistry and develops principles of the structure and stereochemistry of organic compounds largely by prompting the student to discover these principles by study of models of his own construction.
What has been described is the result of a non-profit project supported by a grant from the NSF. Funds from this source cannot he used for the purchase of production molds and tools, but they have been very useful in meeting the cost of experimental molds and tools, for incidental expenses, and for informing teachers of the progress of the project and seeking evaluation and g u i d a n ~ e . ~Morningstar Corporation undertook to copy in plastic the stainless steel model invented by Andr6 Dreiding-manufactured by W. Buchi, Glasapparatefabrik, Flawil, Switzerland, under Dreiding's U. S. patent of 1961, and currently sold in the U. S. for $0.95 per tetrahedron. Gregory Mathus of Morningstar designed the plastic Dreiding model described above. This model embodies the principle, covered by a claim in the Dreidiug patent, of forming a C-C bond The models shown in Figures 5 and 6 were made by David H. Sachs, a Harvard s e ~ o r . Preliminary announcement was made in THIS JOURNAL, 40, 62 (1963). A steady stream of requests indicates interest in all parts of the world.
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