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INEXPENSIVE MOLECULAR MODELS FOR USE IN THE LABORATORY JOHN TANAKA South Dakota State College, Brookings
A RECENT trend in the teachiug of organic chemistry has involved the wider application of molecular models. Besides the use of models in lecture demonstrations, the construction of molecular models has started to find its way into laboratory manuals as exercises for the student.' This is an excellent idea since the kinesthetic learning helps beginning students in recognizing the many different two-dimensional structural representations possible for organic compounds. However, a serious objection to an experiment of this kind is the cost of supplying each student or pair of students with commercial molecular models. I t has been found that satisfactory low-cost models can be made by using a wax such as Ceraweld Amber or Ceratack 155. These waxes are made by the Bareco Oil Company of Tulsa, Oklahoma, and are obtainable in 80-pound lots at approximately 20 cents a pound. Since the wax is used by some art departments for sculpturing purposes, it is possible that the local hookstores already carry some in stock. The wax is light amber in color with a reported melting point of 155°F. Although fairly hard, it can I-e cut with a knife and is plastic enough to he molded with ease. Models can be made by making balls of 10 to 15 mm. in diameter and connecting them with bouds made by cutting wooden applicator sticks to the appropriate lengths of about 15 to 20 mm. Molecules as large as hexane with an actual model length of 14 cm. for the coufiguration of maximum length can he made which are sturdy enough to be twisted about the various carbon-carbon bonds to illustrate the different shapes possible. This is a decided advantage over t.he use of molding clay, or Plastacene, which is satisfactory for models of methane and possibly ethane hut structurally too weak for larger molecules. The use of wax for models is also suuerior to the cork and toothpick method sometimes k e d because of the greater ease of construction and adaptability exhibited by the wax. Several of the disadvantages of not using commercial models which have tetrahedrally drilled holes and varying colored atoms can be overcome by designing an appropriate experiment. Methane, constructed as the first exercise, can be checked easily by t,he labora-
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CAMPAIGNE. A N D R. D. SCHUETZ. ''1,aboratom , E.., H. HART. Manual for a Short Course in Organic Chemistry,' Houghton Mifflin Co., Boston, 1953.
VOLUME 34, NO. 12, DECEMBER, 1957
tory instructor for acceptability. If the molecule htls not been constructed tetrahedrally, the corrections can be made easily. This model can then be used as a criterion for evaluating the correctness of the bond angles of subsequent models. I n molecules such as cyclohexane, the conformation of the hydrogens can be used to check the general accuracy of bond angles. In the absence of color coding, the different atoms may be distinguished by making the atoms in distinctly different sizes. Halomethanes, made by using a large ball for halogen, a medium sized ball for carbon, and asmall ball for hydrogen, present no difficulties. Where a wider variety is desirable, different shapes such as cuhes and cones can be used. Since spring bonds to illustrate carbon-carbon double bonds are impractical for wax models, recourse can be made to a more accurate representation of the sigma and pi bonds involved. An applicator stick is cut to an appropriate length so that it will pierce a wax hall leaving about 1 cm. projecting from each pole. The three sigma bonds are then inserted trigonally on the equator of the wax ball. The l-cm. projections from the poles thus represent the pi orbitals. Overlap to form the pi bond can be represented by slipping rubber bands over the sticks representing the pi orbitals. The actual laboratory use of models of this type has demonstrated at least one distinct advantage. Because of the low costs involved, each student is instructed to take the materials home so that the stereochemical concepts can be visualized when these are presented later in the course. Each student needs 15-25 g. of wax and bonds which he can quickly and easily cut from about four applicator sticks with diagonal cutters. The cost is about 16 per student, an inexnensive ex~erimentunder anv standards.
Low-Cost Molesulor Models Mmde Ethylene.
from W a x L e f t , H e x a m Right, Rubber Bands I l l u ~ t r e t ePi Orbital Overlap t o Form a Pi
Bond
€03
