Models for Demonstrating Organic Structures to a Large Audience Sheldon C. Crane and Robert S. H. Llu University of Hawaii, Honolulu, HI 96822 One nagging problem to an organic instructor is the absence of a molecular model set that is small enough for easy manipulation yet large enough to be seen by medium-tolarge audiences. Recently, because of the extensive use of molecular models in seeking solutions to the problems of the primary process of vision1 and the photocycles of bacteriorhodopsin2,one of us urgently needed such a tool. This paper describes an inexpensive solution to the dilemma. It involves the construction of colored transparent molecular fragments to be used in conjunction with existing commercial model sets, and an overhead projector. Atoms
Visually distinctive atoms were made from variously colored sheets of %-in. and 'Is-in. transparent Lucite. The thinner material was used for H atoms only. Disks, 11.0 mm in diameter were cut from the stock using a special "cooky cutter". Alternatively, small squares of Lucite were cement-
Double Bond
Clear Lucite
Triple Bond
1: Clear I/~"TYP
sp2 Carbon Drill 0.149"D.to center 0.43" D.x 1/4"thick Light Blue Lucite Drill 0.098" D. to center 2 holes
Hydrogen
0-
Drill to center 0.078"D.
0.32"D. x l / 8 " thick Yellow or Oronge Lucite Figure 1. Construction parameters for double and biple bonds and for s$ carbon and hydrogen atoms.
516
Journal of Chemical Education
Figure 2. A blackand-whke sketch of a mlor photograph of the proiected images of the d e l s of 9-cisvitamin A and propyne. In the actual proiection the spZ carbons are in ligM blue, hydrogens at 10. 11. 12 and the hydroxyl group in yellow n orange, and lhw oxygen in green.
ed in a stack (using a shellac-based adhesive), turned to the desired diameter (8.2 mm) in a lathe, and separated by immersion in methanol. Holes of appropriate dimensions were drilled radially into the disk to permit the attachment of the desired bonds. For trigonal planar atoms (sp2-hybridized), three radial holes, spaced 120° apart, were drilled, two of 0.098 in. (drill #41) for single bonds, and one of 0.149 in. (drill #25) for a double bond. Double Bonds
These were constructed from a single lI4-in. diameter, transparent Lucite rod. The dimensions of the bond are shown in Figure 1. As a result of the cylindrical lens effect, this fragment, when placed on an overhead projector, projects an image of a double bond.
' Liu, R. S. H.; Asato, A. E. Proc. Nail. Acad Sci., U.S.A. 1985, 82, 959
Liu, R. S. H.; Mead. D.; Asato, A. E. J. 107,6609.
Amer. Chem. Soc. 1985,
Triple Bonds These fragments were constructed from two fused transnarent Lucite rods in a similar manner (Fie. . - 1). . The doublelens effect results in a triple-bond image. All other fraements were adanted from other commercial molecular moiel sets. From tke HGS student molecular model set for organic chemistry (Benjamin/Maruzen or Holden-Day/Maruzen), the shorter bond (20 mm) was used as connecting C-C single bonds. The longer honds (30 mm) were cut in half (and the tip rounded) as abbreviated C-H bonds. From the Theta Molecular model set (John Wilev & Sons), the tetrahedral atoms were converted to methyl-ene (CH,) or methyl ICHJ groups by cutting off an appropriate number of cunnectors followed bv drilline holes for connecting bonds.
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Application Fiaure 2 shows the imaaes on the screen of two molecules t h a t a e r e constructed based on the newly modified molecular fragments. I t is clear that even for a moderately large
molecule of vitamin A (retinal) there should be no difficulties in their being seen nor in handling them. This modified molecular model set has met the stated goal of maneuverability and audience visibility. I t has the added advantage of being able to demonstrate salient molecular features such as geometric isomerism without having to disassemble the molecule. In this regard. - . the model is narticularly convenient for demonstrating the concerted twist motions as recentlv sueeested for the nrimarv. nrocess of vi. sion.l,Vt is m a i l , theiefore ideal for ;ravelling lectures. The limitatiun is the two-dimensional nature of the imaee. It is. therefore, more suitable for the purpose of demon&ating planar or linear molecules.
Warshel, A. Nature (London)1976, 260,679.
Volume 63
Number 6
June 1986
517
