University of North Carolina Chapel Hill I Inexpensive Spate-Filling

and A. N. Martin Purdue university I Disdav Models . .

West Lafayette, Indiana I

Space-filling models, such as the com- mercially available Stuart-Briegleh models,? have proved useful as a didactic tool. They are particularly helpful in demonstrating such phenomena as stereo- isomerism and conformation among organic molecules. However, the expense of preparing simultaneously several closely related structures (such as disastereo- meric sets) limits the use of such models for teaching.

Hoover and Shriver3 reported the reproduction of prototype Stuart models in plastic by preparing latex rubber molds of the prototype atom for subsequent casting in polyester or epoxy resin. Such castings are connected by pegs to form molecules and are then suitable for supplementing existing sets.

We have found it easy to produce large numbers of molecular models for classroom use and display by duplicating molecular fragments of existing models in plaster via rubber molds. The plaster reproductions are quite inexpensive. They are not flexible and there fore cannot serve the same variety of purposes as the commercially available atom sets, but they are suffi- ciently sturdy if they are handled gently and are there- fore suitable for lecture demonstration. They are

' Present address: Instructional Scientific Equipment Pro- gram, National Science Foundation, Washington, D.C. 20550.

Available from Arthur S. LaPine & Co., Chicago, Ill.

ideal for displays. Figure 1 shows a hand-held model of the boat conformation of cyclohexane. It is made from two casts of the C3H6 fragment. (The chair form is made by connecting the two fragments after rotating one of them 180" with respect to the other from the ar- rangement s h o w n p . ) Such models have been used and displayed in our laboratories for several years.

Figure 1. Model of cyclohemne.

Molds are made from any original atom model or molecular model fragment. The appropriate fragment is placed flat side down on a piece of foil and the ap- propriate mold material applied as described below. Typical fragments used include half of a benzene, acetylene, ethylene, or ethane molecule, an hydroxy group, or a halogen atom. Our most complex single cast is of the CaHs group used to make, among other

374 / lournol of Chemical Education

things, both principal conformers of cyclohexane. ( s e e - ~ i ~ u r e 1.)

Molds may be prepared from either silicone rubber (such as GE or Dow moldin~ silicone rubbers) or latex - (available a t most hobby shops). ~ i~ iconk rubber provides a more rigid mold than latex. It therefore makes a more "accurate" reproduction of the original model, hut a t the expense of durability during re- peated casting and of resistance to splitting during the extraction of large, complex castings. Both materials have been used with success, but we usually limited the silicone rubber to the simpler models with a large open- ing in the mold.

P re~are silicone rubber molds bv sha~ ine the foil . - around the appropriate fragment, flat side down, leaving a space of 5 1 0 mm between the model and the foil. Leave an opening of about 2-3 cm diameter a t the top of the foil cover. Then mix the rubber with the catalyst according to the directions of the manu- facturer and pour over the model, filling the foil shell. Allow curing to proceed for several days a t room tem- perature or for shorter periods with heat, as recom- mended by the manufacturer. Remove the foil care- fully, and extract the original model. Allow the mold to cure for an additional day or two. All surfaces, inside and out, must be exposed to air for several hours to permit the escape of volatile products of the curing before the rubber loses its tackiness.

Figure 2. Hoif-mold fragments for benzene 8. cycloheione.

The procedure of Hoover and Shriver3 may be used to prepare molds from latex. One important modifi- cation is in the thickness of the rubber produced. The heavy plaster mix used, in contrast to the relatively light plastic, will distort a thin-walled mold, producing

unusable casts. Thickness of a t least 3-5 mm is re- quired and often thicker walls are desirable, time per- mitting. Figure 2 shows a half-benzene and a half- cyclohexane fragment coated with latex prior to re- moval of the originals.

Casting is done with a relatively wet plaster mix, containing 7 5 8 0 ml of water per 100 gm of ordinary surgical plaster. Pour the plaster into the mold and flex the mold to eliminate bubbles. In 1 5 3 0 min the plaster will have set sufficiently to make it possible to remove the casting, which then must be allowed to dry a t room temperature for 3-5 days. Then trim the model with a power brush and paint it with or- dinary water colors. Preserve the colors with several coats of plastic spray, and assemble the component parts with glue (such as Elmer's Glue-All). Figure 3 shows a half-benzene mold of silicone, the freshly ejected plaster cast, and the assembled, painted, and finished model of benzene.

Figure 3. Holf-benzene mold, +cted cart , and orrembled, painted model.

To construct molecules more highly substituted than the fragments cast, grind off a hydrogen atom with a power brush leaving a flat face on a carbon atom to which the desired group may be attached. It has been suggested4 that wire imbedded in the castings would improve the structural strength of the models.

Some of the models in use in the laboratory a t North Carolina currently include the 2,2'-diiodohi- phenyl enantiomers, the hemiacetal form of an aldohexose, cis and trans dichloroethylenes, and sev- eral suhstituted cyclohexanes. The silicone molds have been used to estimate a function of molecular volume for chemical-biological correlation^.^

The authors are indebted to the Editor for this suggestion. a Hoomn, W. D., AND SHRIVER, D., J. CAEM. EDUC., 38,295 ~KKELLETT, J. C., AND HITE, C. W., J. Pharm. Sci., 54, 883

(1961). (1965)

Volume 43, Number 7, b l y 1966 / 375