G. Olof Larson'
Paper Stereomodels
Westminster College Salt Lake City, Utah
As
part of a continuing effort to provide students of r h ~ w i s t r ywit,h cheap and effect,ivemolecular models for personal use,= a series of paper stereomodcls has bccn developed. Some of the models demonstrat,e or emphasize stereochemical concepts, while others represent specific compounds. Two separate cut-and-fold combinations are used to make paper models of organic compounds. The first romhiuation results in a "t,et,rahedral fold," which eives t,etrahedral orientation to erouvs - . around a eentcr, i.e., toward t,he apexes of a tetrahedron. The second combination controls directed rot,ation of groups about. a sigma bond axis. This axial fold is also used to make pi bonds and to form separate platies in models of allenes, spirans, and hindered biphenyls. I'at,t,erns for the paper rnodels are pririt,ed in exact, register on both sidcs of heavy paper which can toleratc repeated flexing. After appropriate rutt,iug and creasing, the model is folded to completed for111.~ L.
relat,ionships, as exenlplified in the tetroses or tartaric acids, respectively. 111the model shown, ronfigurat,ioris have been "locked" by gluing an oppositrly folded piece a t each tetrahedral center. Note, however, {.hat rotation is still possible in the axial fold. Configuration locks (or braces) may also hc used t,o advantage iii other (especially larger) models, mherc t,hey help stiffen the paper. But such rigid models lark one feature of the foldable kind: They can't he stored flat between the pares of a book!
Tetrahedral Models
coded
Figure 2.
A simple paper st.ereomode1, with groups arranged tct,rahcdrally asnund a center atom, may vary from a slwletal type to one suggestsinga solid space ball-on-ball type (Fig. 1). 1"or demonstration of atomic asymmetry, e.g., as a hasis for opt,ical isomerism, t,he ball-onslick co~ivcl~tio~i appsars to he the best comproniise
of
E n o n t i o m e r i c poi.
models.
Figure 3.
C s d e d model ozymmetriccenfers.
wit"
two
A chain of six tetrahedral "carbons" (skeletal type) can be twisted to make a chair form ring (Fig. 4) which demouslratcs axial and equatorial bonds. -4ddit,ional pieces can he attached to the ring, as funrtional groups, or to huild hridged (e.g., norbornane) or lattiw (e.g., adamantaue) s t r u d u r ~ s(I:ig. 5 ) .
1.
Simple tetrahedra Upper left: boll-onboll type. Upper right: methane with van d e r Woalr' radii indicated. Lower left: ball-andstick coded model. Lower right: stick model. Figure
models.
form. 1\11 idrnlifiralio~i code on the balls can be recognized from any angle (Fig. 2). Enant,iomeric model pairs are made by folding ident,ical patterns in opposite directions. In an ext,ension of {,hisprinciple, models of glyceraldehyde and alanine are used t,o demonstrat,e configurational relationships by an easy ilit,crchange of group posit,ions a t C-2. A model with two asymmetric centers (Fig. 3) can he uscd lo demonstrate either erythreo-threo or d, 1, ,~izcso Presented hefore the Division of Chemical Education at the 147th Meeting oi the Ameriran Chemical Society, Philitdelphirt, April, 1964. I Aulhor'e address as N S F Science Faculty Fellow ior 196.5-66 will he: yoChemistry Dept., University of Colorado, Boulder. L.\aso~, G. OLIIP,J. CBEM.EDUC., 41, 219 (1!164). I t is expected t,hat in commercial kits, most ui the paper models will be precision m t and creased, ready for folding. J
274 / Journal o f Chemical Education
Figure 4.
Sir linked tetrahedra
con form o ring with oxiol
equatorial
bonds.
and
Figure 5 . Adimontane paper model cornpored with o vinylwire rtereozcolor m o d e l 2
"Bent Polygon" Models
Other paper models of cyclic compounds incorporate t,he tetrahedral fold a t thc corners of berrt polygon center pieces. For example, in a model of 8-D-glucose (Fig. G ) , axial and equatorial groups project from the comers of a folded hexagon." Rotation of groups is made possible by axial-fold cuts. A paper model of glucose is quite as effeclive as the more expensive types for demonstrating equivalence of older and newer representatious of aldohexoses. +4nd, For a similar appenring metal model, see Scwr:rs,
J. CHEM.EIIUC. 39, 641((1062).
H. I'.,
with cheap models available, every student will be able t,o examine his own conformationally correct "glucose wheel," to make sure, for example, that the C-5 oxygen really is "on the right," according to Fischer's convention. Also, wit,h a similar "fructose whecl" (Fig. 7) he will be able to make a leisurely exan~i~lat~ion of the C-2 stereochemistry of this familiar sugar-long enough, perhaps, to ponder the inconsistency in the " q p " designations of saccharides. Decalins (Fig. 8) and related systems, e.g., steroids, are also nodel led effectively by {,hepuckered hexagon palterns.
angle braces at. each sp2carbon (Fig. l l ) , it can be used to illust~ratethe importance of atom co-planarity to opt,iruum pi bond formation. Alternately, the pi electrons may be cut into strips and spread to suggest a cloud effect. Allenes, acetylenes, conjugated dienes and ene-ones are also easily represented, alone or as part of larger systems. The axial fold is also used to make simple spirans (Fig. 12). A spiran model wit,h identically constituted asymmetric atoms (LC., by tetrahedral folds) in each ring, is an excellent tool for illust~ratirigthe kind of d, 1, meso optical isomerism possible in a n~oleeularly asymmetric "propeller" system. Component Models
F i g u r e 6.
Glucose p a p e r model with o converted-ball
rlereorcaior
F i g u r e 8.
model.'
Decalin modelr
Figure 7. Fructaie. Oxygen is easily reen to be "on t h e right" at
Almost any molecule can be modeled in paper, either directly or by building with scalar components. A series of pieces which represent simple niolecules (e.g., CHI, HzO, NH3, C2H4, and CH20) can be asse~nhled (with transparent book mending tape) to make more complex models (e.g., ~net,hylnwthacrylate in Fig. 1 3 ) .
C-5 ( F i x h e r convention).
Figure 9. Three-fold model of cyclohexane.
hexagon
Other Ring Systems
.4n altelxale CGling model, with l.hree folds (Fig. 9, can oft,en be used to advantage inskad of the two-fold hexagon. For example, in the chair form (three folds up), this model suggests a dish or crown. A boat forin (t,wo folds down, one up) passes t,hrough a "twist" form as i,hc direction of either of t,he two "majority" folds is rhanged. That is, with one fold up, one down, and one straight, the three-fold hexagon represent,^ a relatively stable twist conformer of cyclohexane. Also, with this ring model, inore complex systems (Fig. 10) such as adamantane and "twist.anen5 can be niodcled. Adarnantane "blocks," (with compliment,ary "concave" and "convex" surfaces) can be assi:mhled to illustrate the diamond lattice and intermediate "adamantalogues" such as "congressa~c."~ Models Based on Simple Planes
I n models of unsaturated syslenls, the pi bonds are raised perpendicular to t,he plaues of adjacent atoms. Sonwt,imes it is advantageous to emphasize an atom plane by leaving t,he surrounding paper intact,. A simple group-coded alkene model can be used to denionstrate geonletrie isomerism. If t,he model is prepared wibh an axial fold in the pi bond and right WHITLOCK, H. W., J R . , J . Am. Chem. Soc., 84, 3412 (1062). 6 C u ~ C., ~ SCHLEYER, ~ , P. V O N It., AND TRECKER, D. J., J . Am. C h m . Soc., 87, 9 l i (1!165).
Figure 10. Model5 m o d e f r o m three-fold hexagons. Top centzr: "Iwi~tone."~U p p e r right: bicyclo12.2.21-oclone. Bottom: o d a m o n tone models with "convex" a n d "conc.ve" rurfocer.
Figure I I . Models of unraturoted ryatemr. Lower left: 3-cycloheptenone. Top left: coded ollene. Top right: acetylene. Lower right: coded olkene showing oxiol cut in "broken" pi bond.
Figure 12. Spiranr. Upper: simple t w o - p l o n e model. Lower: model showing d, I, mero irom-
models.
eri5m
13. Component p a p e r S i m d e c o m ~ o n e n t r ; CH:. NH:$, d [with unshoied electron$ showing on N ."d 0). Left: model of methyl methocrylote. Figure
k0
Inorganic Paper Models
Simple cut-and-fold methods are used to make hexaand penbacoordinate atom models. They are analogous to the tetrahedral fold, and produce pieces with apexcs ("valences") which project froin octahedral or t,rigonal bipyramidal centers. These pieces may be used dirertly (with color coded ends) for isomerisnl studies, or they can be used as scalar components in models of inorganic complexes (i.e., in connection mit,h the organic cornporreril~smentioned above). Another patt,ern gives a more conventional octahedral or trigorial bipyramidal model, with color coded balls projecting axially (Fig. 14). Recent experiments suggest that many other simple Volume 42, Number 5, May 1965
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inorganic systems (e.g., SP,P401r, boranes), and even polynlers (e.g., black phosphorus and silica) can be modeled in paper, by extensions of the methods described in this paper.
for specific purposes, and their limitations as well as advantages should be recognized. While many of the paper models are as good as (or even better than) conventional models, their consistent advantage over other types is lorn cost.
Conclusion
Paper stereomodels almost insistently draw attention to atom planes in molecules. This emphasis can possibly lead to second guessing in conformational analysis of familiar systems, as new and useful structural relationships are found. Moreover, because of the emphasis on planes, paper models of carbonyl compounds (e.g., keto-steroids) should find use in optical rotatory dispersion studies, particularly at the instructicnal level. Most chemical models, like other tools, are designed
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Figure 14. Octahedral mod& Model a t lower right rhowr hvo versions of ethylene diomine bridging connected or "ligondr".
I t is hoped that paper stereomodels \vill l~worne generally useful aids both in teaching and in research.
