John C. Godfrey' Bristol Laboratories Syracuse, N e w York
Stereo
The past five years have seen the intro. duction of a number of model designs which show accurately the positions of atomic nuclei in molecules. Perhaps the most prominent of these have been the Cenco-Peterson models (8) and the Dreiding stereomodels (9). While they are certainly excellently designed models and easy to use, they are nevertheless too expensive for many individuals to own (10). In addition, they have limitations with respect to the representation of strained systems by the use of standard units. I n the interest of providing a less expensive set of models, and with the hope of rendering some functional improvements upon existing molecular models, the author has designed and executed an entirely new and less expensive set of model^.^ The components of the new set of models are shown in Figure 1 and in the table. All parts (with the exception of the white tubes, which are extruded and precision-cut) are injectioc-molded of low-density polyethylene on a scale of 1 A = 2.50 cm corresponding to a magnification factor of 250,000,000. All components are basically flat pieces which are 2 mm thick. The
' Inquiries and reprint requests shordd be addressed to the author, Bronwill Scientific 1)ivision of Will Scientific, Ine., Box 277, Rochester, New York 14601. These models are awilahle as Bronwill Kit No. 90 from Bronwill Scieut,ific Division (above address); the price is $29.511 per kit, which includes 50 hydrogen atoms and atom model? or groups totaling 119 other atoms. A student kit is also avsilahle at a much lower price.
three-dimensional characteristics of tetrahedral carbon and tetrahedral (amino and quaternary) nitrogen are achieved by assembling each individual atom from two identical halves (see Fig. 2). Each half-atom has a bond angle of 109'28', so that when two half-atoms are snapped together via their interlocking slots, a perfect tetrahedron results (as seen in Fig. 1). Because of locking teeth which are an integral part of each halfatom, the complete tetrahedral atoms me permanently assembled and cannot be separated into their halves except by a special manipulation. A second snap-lock mechanism is built into the connectors, as show11 in Figure 2. This interlocking feature is so powerful Co-
Internuc.
valont
dig-
Bond angle
Model and Color Tetrahedral carbon. -, blsck Trigonsl carbon, blsck Olefinic double bond, hlaek (embossed "olef.") Conjugated and aromatic double bond, black (embossed "AIZ") Benzene ring, blsck Acetylenie triple band,
".*"..
109' 28' ( 4 , 6 ) 120" 120e 120" 120' 180"
hlonG
Cyclopropane, black
0.76
Tetrahedral nitrogen,
0.70
hh.~
Ref.
1.52
109' 28' (4)
Trigonsl nitrogen, blue Amide group, blue
0.656 0.696C 0.6KiN Digonsl oxygen, red 0.66 Digonal 8ulfur, yellow 1.04 Hydrogen, white tube 0.30 Carhonyl oxygen, red tip 0.51 Thionyl sulfur, yellow tip 0.94 1.85 Nitrile group, blue tip Fluorine, colorless tip 0.60 0.99 Chlorine, green t,ip Bromine, brown t,ip 1.14 Iodine, purple tip 1 36
Figure 2. F'grre I 1, oefnir d0.o e oond. 2. tevrohedral nnvrogen; 3, acetylenic trnp c bone. 4, lrigonai "itrogen. 5, telrohcdrol corbon; 6. omide group
dorble bone;. 8.. lnolr ioen1:n.r carbon end.. 7 . oromok Ironiuaotedl . . digond oxygen; 9 , triganol carbon; 10, unirerrol connector; 11, benzene ring; 12, digonol rvlfur; 13, nitrile group; 14, thionyl sulfur; 15, carbonyl oxygen; 16, hydrogen; 17, fluorine; 18, chlorine; 19. bromine; 20, iodine; 21, cyclopropane ring.
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Journol of Chemicol Educdion
By interlocking two hdf-carbon atoms (blackl, one complete
tetrahedral corbon atom may b e prepared. This is easily accomplished b y (11 pushing the two large dots together so thot the atom ir half-asemhied 01 shown in drawing o; 121 twisting the two holves so thot they lie nearly in a single plane agoinst each other (drowing b); (3) then pushing them together until the teeth snap into the small slots opposite the large dots. Drowing c is a sectional view of the universal connector. showing the map-lacking rings in each end of the tube.
Figure 3. The two extreme confarmotianr of cycloheione are easily interconverted by twkting neighboring pairs of carbon otomr. Left to right, chair, twirl, and boat conformations.
that the author has constructed a polymethylene chain 50 feet long which can be swung like a skipping rope without breaking, yet is readily assembled and disassembled without the use of tools. A further advantage of the snap-lock connectors is that they have been designed to possess a considerable degree of torsional friction so that, for example, the currently interesting stereoregular polymers such as isotactic and syndiotactic polypropylene (11) may be constructed and set in any desired conformational sequence. The model may be handled roughly without losing the set conformation, and yet a simple twist of any bond produces a new conformation which will again remain as set until it is purposely altered. The "twist" conformation of cyclohexane provides another example of the value of torsional friction, since it will remain as set up for an indefinite period. Nevertheless, the extreme boat and chair conformations fall together naturally and are easily interconverted by simply twisting two pairs of atoms with respect t,o each other. The torsional friction allows easy and accurate measurement of internuclear distances on any desired conformer. I t has also proved valuable in the interpretation of reaction mechanisms involving freely-rot,ating systems. Since the molecules will usually he expected to approach a specific conformation as the transition state is approached, it is most convenient to have models which will hold a given con for ma ti or^ corresponding to what is t,hought to represent the transition state.
Figure 4. Top left, cornphor; top rght. cyclopropane; bottom left, C ~ C I O bvtene; bottom right, tropolone.
Because of the knowr~ lack of free rotation in unsaturated systems, this kit has been simplified by molding mukiple-bond units and planar rings (benzene and cyclopropane) as single units. Since conjugated and aromatic double bonds are nearly the same length (with the length of conjugated double bonds approaching very closely the length of aromatic double bonds as the length of the conjugated system increases), both are represent.ed by the unit which is embossed "AR." The olefinic double bond unit is embossed "OLEF." Because the units are flexible, only these two types of C=C are required to accurately model compounds having double bonds in any conceivable situation, such as exocyclic and endocyclic double bonds in 4-, 5- and 6-membered rings, as well as indenes and indoles. The amide group is known to be planar and to exhibit, restricted rotation about the carbonyl C-N bond (7). It is therefore represented by a single unit which resembles the olefinic double bond but which is blue and slightly tapered (reflecting the difference between carbon and nitrogen covalent radii); it is further identified by a 6 mm hole at the carbon end (see Fig. 1). This same unit also accurately represents the C=S of imine, amidine, oxime, iminium, semicarbazone and hydmzone, as well as the C-N of oxidized nitrogen groups such as nitroso and nitro. In all of these cases (except nitro) the second bond receptacle (peg) on the nitrogen atom is left vacant and thereby represents the unshared pair of electrons on nitrogen. Atoms and groups which have valence in only one direction (hydrogen, the halogens, carbonyl oxygen, thionyl sulfur, and the nitrile group) are represented by white polyethylene tubes which terminate in colorcoded tips (with the exception of the ubiquitous hydrogen atom); see Figure 1. In a11 cases the correct internuclear distance is represented by measurement from the end of the identifying tip to the center of the atom to which t,he white tube is joined. It is important to remember that this set, without molecular orbital attachments, shows internuclear distances correctly, without regard to van der Waals' radii. Figure 4 shows some typical examples of highlystrained systems which are very satisfactorily represented with combinations of the standard units. The strain in cyclobutene is readily apparent from thc visible bending of the bonds. The opposite outwardbending type of strain is seen in the model of tropolone, in which all of the carbon atoms lie in a plane. I t is interesting t,o note that the model of cyclooctatetraene, with one more trigonal carbon in the ring, cannot be forced to lie flat, but naturally assumes the wcllestablished "t,ub" conformation. The greatest challenge which these new models have faced, with regard to strained systems, is Professor Jerrold Meinwald's 5-(N-t-butyl) carboxamido tricyclo (2, 1, 1, 0".6)hexane (12) (Fig. 3 ,which is seen to be a bicyclobutane having a 1,3-ethylene bridge. The preformed cyclopropane unit was not used in this model, since the distortion in t.he bicyclobutane portion would then have been unevenly distributed. Instead, six tetrahedral carbon atoms were used in constructing the tricyclohexane system, which resulted in a realistically symmetrical model. The ability to construct nonplanar 3-member rings from tetrahedral atoms is thus seen to be a great advantage in certain instances. Volume 42, Number 8, August 1965
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Although four of the tetrahedral carbons were severely distorted in this model, they quickly returned to their perfect tetrahedral shape when the model was disassembled after several weeks. Models also can be very satisfactorily coustructed for cyclopropene (using one tetrahedral and two trigonal carbons), cyclopropanone, ethyleneimine, ethylene sulfide, ethylene oxide, and even the cyclopropene cation and cyclopropenone. All are constructed entirely with standard units (Fig. 6). The ultimate is strained systems is of course tricyclobutane ("tetrahedrane"), which is a valence-tautomer of cyclobutadiene. It too can be represented with tetrahedral carbon atoms, although the strain defies belief.
together like the other tetrahedral atoms, as shown in Figure 7. The result is not a perfect tetrahedron, since the bond angles between adjacent pairs of groups are 104' and 112', but this situation is in agreement with the well-established skewed-tetrahedron geometry of sulfur bearing two pairs of non-equivalent substituents ( 1 ) . Models of isocyanate and isothiocyanate groups are easily prepared from standard components. The isocyanate group, -N=C=O, islinear and has an N to 0 internuclear distance of 2.38A (1). It is accurately represented by removing the green identifying tip from a chlorine atom and replacing it with a carbonyl oxyger~ (red tip in hydrogen tube). A piece of wood or plastic approximately 2 X 5 X 20 mm is used to join the two tubes. The assembly is completed by attaching it to a trigonal nitrogen atom as shown in Figure 7. The third (unoccupied) connector peg of the nitrogen atom represents the nnshared pair of electrons on nitrogen. The isothiocxanate group [ - N = W , internuclear distance 2.78A ( I ) ] is correctly shown in an analogous manner using a trigonal nitrogen atom, an iodine tube, a hydrogen tube, and a yellow thionyl sulfur identifying tip (Fig. 7). Ordinary cyanates and thiocyanates may be shown by attaching a &rile group to digonal oxygen or digonal sulfur. The hydrogen bond, which is an important feature of many organic molecules, is represented by an appropriate length of L/rin. insidediameter "Tygon" or rubber tubing. It can be shown with complete accuracy in the following manner, using a 2.93A hydrogen bond of a polypeptide (7) as an example. The total C- to Ninternuclear distance for the system C 4 - - - H - N is the sum of 2.93A ( 0 to N internuclear distance) plus 0.51A (covalent radius of carbonyl oxygen) plus 0.69A (covalent radius of trigonal carbon) = 4.14i. Multiplying this value by the conversion factor 2.50 gives a total internuclear distance of 10.35 cm. Since the tube does not go to the nuclei of the C and N atom models but stops at the circumference of the circnlar portion of these models, their combined radii, 0.53 cm 0.64 cm, must be subtracted from the total length of 10.35 cm, leaving an actual tube length of 9.18 cm. For hydrogen bond lengths other than 2.93A, snbst.itnte
+
Figure 6. I, Ethylem oxide; 2, ethylene rulfide; 3. ethyieneimine; 4. cyclopropene; C cycloproponone; 6, cyclopropenone; 7 , cyclopmpene cotion.
Special-purpose Applications
The allenic linkage, which has an internuclea~;distance between terminal carhon atoms of 2.59A, is accurately represent,ed by joining two trigonal carbon atoms by one of the nitrile-group white h b e s from which the blue identifying tip has been removed. The trigonal carbon atoms must of course be inserted in the tube so that t,heir planes are mutually perpendicular. The friction fit is very tight, and the trigonal carbons will maintain t,heir perpendicular relationship indefinitely (Fig. 7). A tetrahedral sulfu~' atonr (for snlfones, sulfoxides, sulfonium ions, and some halogenated sulfur compounds) is readily prepared by cutting 2-mm slots in earh of two digonal sulfur atoms and fitt,ing them 406
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Journal of Chemical Education
Figure 7. I. d e n e ; 2. tetrahedral sulfur; 3, half-letrahedrol ruifur .torn; 4, irocy.n.+e group; 5, irothiocyonote group.
the appropriate value (7) in the above calculation. The resulting hydrogen bond representation is accurate and realistic, since t,he bond is both elastic and flexible. Even very large hydrogen-bonded structures are exceptionally durable. The author has constructed alpha-helices up to 15 feet long which show no tendency to come apart under their own weight, even when handled roughly (see Fig. 8).3 Literature Cited (1) GORDY, W., SMITE,W. V., AND TEOMBARUM, R. L., "Micrw wave Spectroscopy," John Wiley & Sons, New York, 1953, pp. 371-3. (2) MIZUSHIMA, S., A N D SHIMANOUCHI, T., Ann. Rev. Phys. Chem., 7 , 445 (1956). ( 3 ) HUGHES, E. W., Ann. Rev. Phys. Chem., 6 , 261 (1955). (4) . . PAWL IN^-. L.. "Nature of the Chemical Bond." Come11 Universit; press, Ibhaca, N. Y., 1939, pp. 164 189. (5) STUART, N.A,, "Die Phrjik der Hochpolymeren," SpringerVerlag, Berlin, 1952, pp. 99, 162, 163. (6) C o n r s o ~ , C. A., in "Theoretical Organic Chemistry," Butterworth Scientific Publications, London, 1959, p. 49. (7) PIMENTEL G. C., AND MCCLELLAN, A. L., "The Hydrogen Bond," W. H. Freeman & Co., SanFrancisoo, 1960,pp. 299. 303. ( 8 ) ~ h e m : ~Nn m ~ ., 38, Oct. 12, 1959, p. 108. A. S., Helv. Chim. Ada, 42, 1339 (1959). (9) DREIDING,
The author is grateful to Mr. Len Rosenberg and Mr. Gary Andrews far the photographs.
Figure 8. Alpha-helices. Left: poly-L-phenylmlonene, right-hand helix. Right: poly-L-alanene, left-hand helix. Each helix ir suspended by a single thread.
(10) FIESER,L. F., J. CHEM.EDUC.,40, 457 (1963). (11) Chem. Eng. N m , 41, Nov. 18, 1963, p. 42. (12) MEINWALD, J., SWITHENBANH, C., A N D LEWIS,A,, J . Am. Chem. Soe., 85, 1880 (1963). W., SMITH,W. V., AND TROMBARULO, R. L.,o p .
