Rudolph M. Anker Albany Med~calCollege Albany, New York
Constru~tionof Molecular Models
I n the continuing search for more perfect molecular models, commercial sets have become ever more elaborate and costly. The set described in this communication compares favorably in utility with many of the more expensive models, yet it can be constructed cheaply and without the aid of elaborate technical resources which are available only under special circums t a n c e ~ . ~Models of greater simplicity and lower cost have been described,$ but these lack the flexibility and ruggedness of the models to be described here. The atoms comprising this model are sponge rubber balls of various sizes and colors, which are available in retail stores a t small cost. The bonds are made from $/,inch wooden dowels and pieces of a a/s-inch screen door retaining spring. The other raw materials required are a strong rubber cement, such as "Pliobond" (Goodyear), rubber tubing of via-inch i.d., l/,inch o.d., and plaster of Paris. A plaster cast serves as a jig to hold the rubber halls, and also as a template to mark the location of the bonds. The plaster cast is prepared by immersing a rubber ball to a little more
' SUBLUSKEY, L. A., J. CHEM. EDUC., 35, 26-29 TANAKA, J., J. CHEM.EDUC.,34, 603 (1957).
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Journal o f Chemical Education
(1958).
than half its diameter into the freshly mixed plaster investment, and allowing the latter to set; e.g., a Z1/r-inch ball is immersed to a depth of 11/$inches. When the plaster is set, the rubber ball can he removed easily from the cast. Owing to the fact that the cast encloses somewhat more than one-half of the sphere, the rubber balls are held tightly, and they may be inserted and removed from the cast as often as desired for the purpose of marking and subsequent drilling of the holes. For carbon atom models the positions of the tetrahedral angles are marked on the cast by placing a cardboard equilateral triangle in the spherical hollow of the cast in an approximately horizontal position and marking the points where the apices of the triangle touch the walLa The cast is clamped by its top and bottom surfaces into an adjustable drill press vise, and it is aligned for drilling by placing a drop of mercury (or a small head) in the hollow and adjusting the position of the cast so that the mercury (or bead) comes to rest on one of the three marks. This alignment 'The length of the side of the triangle is given by the expsion 1 = 0 817 X d, where I = length of side of triangle and d diameter of rubber mhere.
=
Figure 1 Figure 1. Figure 2.
Figure 2
Drilling tetrahedrolly spaced holes in plaster cart. Mounting ball in cart for drilling.
is necessary to ensure the accurate placement of the marks on the rubber spheres subsequently. A hole is then drilled vertically through the cast at the mark. A drill bit with an extended shaft may have to he used to permit the chuck to clear the cast (Fig. 1). Two holes are drilled at the other two marks in the same manner. Locating Tetrahedral Bonds
The finished plaster jig is now used to make atom models with tetrahedral bonds. A rubber ball is iuserted into the jig and marked a t the sites of the three holes by means of a refill unit from a ball point pen. The fourth mark is placed on the ball by readjusting its position in the cast so that two marks coincide with two holes and the third mark comes to lie a t the upper pole of the ball. The exact equidistant positioning of the four marks on the ball should he checked with a compass. Only a small correction should be required, if any. The ball is reinserted into the cast so that one of the marks comes to lie exactly a t the top. This can be checked by observing the mark at eye level while rotating the cast and sphere on a level surface and by observing whether the mark describes a point locus. If the mark is off-center, it will describe
Figure 3. Typical models showing voriely of rtructvrol representations. Top left: cyclopropone derivative; top right: o trmnr-fused perhydro indene; lower left: benzene; lower right: acetylene.
a circle instead. When a satisfactory adjustment has been made, a hole is drilled a t the mark down to the center of the sphere (Fig. 2), e.g., for a Z1/&ch sphere down to a depth of 11/#inches. A '/le-inch drill bit is used a t the highest speed obtainable. Occasionally a piece of sponge rubber is not disintegrated by the bit and this causes vibration of the ball and cast. Such a piece should be torn off so that the drilling will continue smoothly. Three more holes are drilled a t the other marks in the same manner. Finally, pieces of rubber tubing cut to a length inch less than the radius of the sphere, are cemented into the holes in order to protect the sponge rubber from excessive vear. The elements from carbon to fluorine are represeuted by spheres of different colors, but of the same size, permitting one jig t,o be used to make models of all these atoms. Models of nitrogen are constructed in exactly the same manner as the carbon modele. Normally only three of the four valences will be used; in models representing quateruary nitrogen or nitro groups all four valences will be required. The same plaster cast is used also as the jig for the oxygen model, but holes are drilled a t only two of t,he four marks. The tetrahedral angle (109'28') can be used as the valence angle a t the oxygen atom, since the C-0-C bond angle in dimethyl ether is lllO. If oxonium compounds are to be depicted, a third hole is added midway between the other two marks on the sphere. For the elements from silicon to chlorine, spheres of a larger size and a larger jig are used than for the element,s of the first period. Bromine and iodine require still larger spheres and jigs. All these jigs are equipped with holes to give tetrahedral bond angles and provide for the representation of the univalent halogens, of phosphates, sulfates, and of the halogen oxyacids. Since the bond angles in derivatives of divalent sulfur, trivalent phosphorus, and other elements of the second and higher periods differ substantially from the tetrahedral angle, separate sets of holes have to be added to the jigs in order to make models of such atoms. Hydrogen atoms are represented by smaller spheres than carbon. They need not be made of an elastic material. If a hole is drilled through the sphere along its diameter, only one end of the resulting channel is used for a singly bonded hydrogen atom. A hydrogen bond may be depicted by inserting dowels a t both ends of the channel. If the balls representing the carbon atoms are 21/4 inches in diameter, the relative lengths of single, double, and triple bonds are reproduced most faithfully by wooden dowels 2'/2 inches long, springs Z1/2 inches long, and springs 3l/* inches long, respectively. For ease of insertion the dowels and springs are luhricated with glycerol. The models described have an advantage over most others insofar as only one type of carbon atom is required for the construction of all kinds of strained molecules, including three member rings, and the same carbon models serve, in conjunction with springs, to depict double and triple bonds and aromatic structures. Yet the resulting models are sufficiently rigid to maintain the distinction between "boat" and "chair" forms of cyclohexane rings, and models of complex organic molecules such as polypeptides, steroids, and porphyrins can he constructed readily. Volume 36, Number 3, March 1959
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