Louis F. Fieser Harvard University. Cambridge, Massachusetts
Extensions in the Use of Plastic Tetrahedral Models
T h e models previously described1 and now available in improved form2 can be modified easily to provide additional pieces and used for construetion of organic and inorganic specialized structures. The models are now made of General Electric's Lexan, a polycarbonate plastic much stronger than the
It has a socket for support,ing a rod and another for supporting a tube, with corresponding grooves. It is useful for measuring nonbonded interaction distances, for example, for estimating the relative d~abilityof cisand trans-decalin. The 1: 3 diaxial distance in chaircyclohexann (2.52 A) served as the standard. Under development is a set of simple extension pieces with which the C, 0, and N models can be expanded to twice the present size for use in lecture demonstrations; an expanded double bond is included. Additional Models4
polystyrene previously used. An individual Lexan rod can be bent with the fingers to an angle of 2&30° without taking a permanent set; on release of the pressure the rod springs back to the original position. With excessive pressure the rod can be bent to a right angle, but it does not break. The two half-tetrahedrons are now joined, not by cementing, but by the new technique of ultrasonic ~ e l d i n g . The ~ pieces are assembled, with the pin of one piece fitting into the mating hole of the second, the assembled piece is placed in a holding fixture, and application of ultrasonic power produces vibrations which travel through the plastic to the joint line where the heat generated melts the plastic at the interface and fuses the joint in 1-2 seconds. Aluminum tubes turned out of preformed tubing are much stronger than the early ones stamped out of sheet metal. They are securcd to the short plastic rods with a crimping machine. An accessory now available is a rule of transparent plastic with a scale reading directly in angstrom units.
'FIE~EE, L. F.,J. CHEM.EDUC., 40, 457 (1963). Manufsctured by Morningstar Corporation, Cambridge, Mass., and distributed by Rinco Instrument Co., Greenville, Illinois. OBEDA,E., Modem Plastics,November (1964)
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Use of plastic in the models has the advantage that it can be bent and cut as well as painted for modificat,ion of the four basic pieces. For the permanent bondbending required for the construction of models of the type of cyclopropane and acetylene, the method of heating with a microburner suggested for the early polystyrene models1 is unsatisfactory as applied to Lexan because at the higher softening temperature the plastic easily catches fire. One hot-air blower is made by forming a part of a length of '/,-in. copper tubing into a flat coil which is mounted on a hot plate. A better heater (Fig. 1) is easily constructed from a commercial "Heat gun"5 and an a d a p t e r . V h e outer metal sleeve is removed and discarded. The inner metal sleeve is removed and the sheets of mica either left in place around the heating element or set aside. The circular screen is forced out by pressure of a screwdriver inserted in the sleeve, without dislodging the retaining spring. The adapter is then inserted, the circular screen is introduced next, the mica sheets are slipped into the sleeve, and this is screwed in place. The hot air stream (ca. 220") permits even heating of a model and easy bending. The piece is rotated in the stream until it suddenly softens and can be bent with hardly any pressure. Since in bending a plastic piece with an attached tube the metal becomes quite hot, it is advisable ~i~~~~ 1. not air blower. to first insert another plastic rod to sewe as a handle. The most satisfactory way to cut a plastic piece is to use a band saw and smooth the ends with a file. 'In collaboration with Roland A. E. Winter. Model HG201 heat gun (11Cb120 ac, dc, 5 amp) of Master Appliance Corporation, Raeine, Wisconsin. Ace Glaas 5005 adapter (24/40 to 14/20).
Figure 2.
101
Cyclobutane.
Quick-drying enamels7 that match the red and blue Lexan pieces extend the scope of the models in an attractive way. Application can be done by dipping the piece into a can of enamel, letting it drain into the can, and touching off excess drops onto a piece of filter paper. The nitrogen atom supplied has two rods and one tuhe; an atom with two tubes and one rod is made by cutting off one rod of a carbon atom, dipping the remaining rod in blue enamel, inserting No. 000 corks in the tubes, and dipping the tubes in enamel. Additional pieces are constructed as follows: Acetylene is made by bending three bonds of one carbon atom parallel to an unbent rod and three bonds of a second carbon parallel to a tuhe. For cyclopropane the three pairs of bonds to be joined are bent to an angle of 60"; for complete accuracy the bonds extending to hydrogen atoms are bent to an angle of 118O. An epoxide is made by bending an oxygen atom to an angle of 60' and joining it to two cyclopropane carbons. Cyclopropene can be made from a double bond and a tetrahedron. Cyclobutanegcan be made if the four pairs of bonds to he joined are bent to an angle slightly less than 90'. The result is the planar conformation shown in Figure 2(b) (the bent bonds are identified by encircling washers). Trigonometric calculation shows that outof-plane folding of the ring to an estimated angle of 2530' requires a surprisingly small change in the carhonbond angle, namely, from 90' to 88' 20'. Structure 2(b) is severely destabilized by eight eclipsed 1,2 H:H interactions, but strain is relieved by a flip to either of the puckered forms 2(a) and 2(c), in which all the 1,2 H : H interactions are close-to-skew. I n 2(a) the quasi axial la,3a- and 2P,4P-hydrogen pairs are too distant for interaction.
' Larcolaid enamels recommended for oxygen, nitrogen, sulfur, and cobalt are: 9048 Chinese Red, 9132 Light Blue, 9066 Medium Yellow, and 9075 Colonial Ivory, manufactured by Carpenter-Morton Co., Everett, mass. 'The topic of cyclobutane conformation is reviewed by WILSON, A,, AND GOLDMAN, D., J. CHEM.EDUC., 40, 504 (1963).
Allene can be made in two ways: (a) the 1,l-bonds of ethylene are bent until parallel to the double bond and joined to a carbon atom in which a pair of corresponding bonds have been bent in the same way; (b) C1 of each of two double bonds is split or sawed along the parting line and the two pieces are cemented together with methylene chloride. A model of bivalent sulfur for a &membered ring or for attachment to alkyl groups should have a ho%d angle of 105" and the C-S bond a length of 1.82 A. These requirements are met by converting an oxygen atom to sulfur by changing the bond angle from 109" to 105', extending each bond by 7 mm (0.14 A), and applying yellow paint (Fig. 3). After the two bonds have been bent, the a section of rod is protected with masking tape and the section b-ed is dipped into yellow enamel. When dry, the masking tape is removed and the 7-mm section b is wrapped with transparent tape to build up a shoulder that will stop the tube of a carbon atom. This is done conveniently by wrapping with '/%-in. transparent tape, cutting through the tape with a copper-pipe cutter (3/le to 11/*in.) and removing excess tape with a razor blade. The tube, with the original indenture at e, is provided with new indentures at f and g so that the button of an inserted rod will stop between f and g, with extension of the bond length by 7 mm. The grooves are made with a pipe cutter; groove f should be the deeper of the two, but no deeper than needed to stop the inserted rod. Bivalent sulfur for a 5-membered ring (tetramethylene sulfide) is made in the same way except that the bond angle is reduced to 94". I n tetramethylene sulfide the ring is puckered, with the sulfur atom out of the plane of the methylene groups. For construction of a disulfide group, as in CHaSSCH8or cystine, two oxygen atoms are first modified by bending the bonds to an angle of 105'. As shown in Figure 4 on the left, the rod of one atom is extended by 7 mm (paint and tape) and the tube is extended by 12.5 mm [new indentations at f and g]; the section b-c-d is dipped in yellow enamel.
4 mm.
Figure 3. Sulfur atom for RSR and for 5 in o 6-membered ring.
Figure 4.
Volume 42, Number
8, August 1965
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Figure 5.
Hydrogen bonding.
Figure
The second atom is modified as shown on the right and the two long bonds are joined. Two carbonyl groups are made by cutting a double bond in two, smoothing the ends, and dipping each half into red enamel. Hydrogen bonding, for example in o-hydroxyacetophenone or cystine, can be represented with a section of 3/lrin. rubber tubing (Fig. 5). Use of yellow paint gives a thiocarbonyl group. A nitrile group is made by forming half of an acetylene molecule and dipping it part-way into blue enamel (Fig. 6). An imino group (Fig. 7), made by cutting off one bond of a double bond and painting the remainder blue, serves for use in acyclic structures and 6-membered rings to make imines, Schiff bases, oximes, and pyridine. An azo group, cis or trans, is made by cutting off appropriate bonds from ethylene and painting the entire piece blue. An immonium group for the construction of an N-alkylpyridinium cation is made by painting half of an ethylene molecule blue. To make a C=C double bond, endo-6-ring (isolated or conjugated), the two bonds involved in forming the ring are compressed to an angle of 108' as shown in Figure 8(a) and 8(b). The same applies to the C=N double bond of an azole, as in Figure 8(c). A carbonium ion results when one rod and the two tubes of a tetrahedron are bent into a plane and the remaining rod is cut offwith a band saw. To show the SN2-transition state, two rods and one tube of a tetrahedron are bent into a plane and the remaining tube is left in place (tube on the right in Fig. 9).9 A fifth bond opposite this tube is made by sawing another carbon atom through the tetrahedral center a t the base of a rod to expose a triangular surface which can be cemented (CH2C12)to the first carbon. Note that this piece, without the balls, can be used to represent pentavalent sulfur or phosphorus. An octahedral piece with which one can make models of virtually every known transition metal'o complex is 9
Figure 7.
6. Model of -C=N.
Model of
> C=N-,
constructed $0 the scale of the cobalt-nitrogen bond of length 1.99 A. The central piece (Fig. 10) is a square block of pine wood cut from a length of 15/a X 15/8 in. kiln-dried baluster stock, available at any lumber yard. In the center of each face a hole '/% in. deep is cut with a '/lein. drill for insertion of pegs cut to a length of 2'/g in. from a 7/16-in.dowel after each has been provided with a rod or a tube cut from three carbon atoms. Tubes cut off flush with the metal are 4.4 cm long; rods cut at the point of enlargement have an average length of 4.5 cm. The circular protrusion on each rod, which is formed by operation of a pin that knocks the molded piece out of the mold, is removed by filing. A hole in. deep is cut with a No. 4 drill in the center of each wooden peg, and a rod or tube is tested for fit and then glued and inserted in place. The holes cut in the block are made slightly deeper than required in order to permit adjustment in the final operation of applying glue and inserting the wooden pegs so that the distance from the end of each tube to the inside of the button a t the end of the corresponding rod measures exactly 19.9 cm. Figure 11 shows use of the octahedron for construction of a model of the complex cation from cobalt chloride and three molecules of 2,2'-diamin0diphenyI.~~ The bond distances of other transition metals are so close to those of cobalt that the model described can be used to represent complexes of other metals with a close approximation. If a pair of vertical bonds is left off, or ignored, the piece accommodates square-planar complexes. For example, the cupric ion can be represented by attaching to each of four coplanar bonds a No. 4 ball to represent a molecule of water. 'oInclusioo of such a piece was suggested by Professor Brian W. Moores, University of Illinois. "For a discussion of the stereoohemistry of such complexes E. J., AND BAILAR, J. C.,JR.,J . Am. Chem. 80%81, See COREY, 2620 (1959).
When a ball is reauired larger than the No. 4 hall in-
are usually rwaila.ble at dime stores, florists, etc
Figure 8.
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Figure 9.
Tronr~tionstate
Figure 10.
Cobalt.
Figure
1 1.
Co(2,2'-~iaminodiphenyl)s]~~.
2,4,&Octatriene is too slender to serve as an acceptable guest for the 14.3-cm channel. Hydrocarbons which form complexes with thiourea but not with nrea are isooctaue, adamantane, and twistane; the models are all accommodated by a 26.4-cm guest cylinder. The prediction, based on models, that 1,4-di-thutylbenzene wonld form a thiourea complex whereas its 2,5dimethoxy derivative wonld not, was verified by experimentation.la Are any of the macrocycloalkanes capable of forming urea complexes? Until an experimental answer becomes available, it is interesting to speculate on the possibility that specific hydrocarbons will he accommodated by one or by two urea channels. The phenomena of molecular overcrowding and of molecular asymmetry can he demonstrated dramatically with a model of one of the enantiomeric forms of hexahelicene (Fig. 12). Other apt examples of molecular asymmetry can be illustrated with models of the (+)- and (-)-forms of twistane and of trans-cyclooctene. Models show in a striking way why tetracyanoethylene forms a stable rr-complex with hexamethylbenzene but not with hexaethylbenzene. The
Use in Teaching
I n the author's course for some 300 beginners, each student this year was required to purchase or obtain on loan and bring to the second lecture the following set contained in a plastic hag: 10 carbons, 1 double bond, 1 oxygen, 1 nitrogen, 1 sheet of labels, and two sets of balls of four sizes (angstrom rules were not then available). With the students constructing each structure discussed, the lecture introduced the following concepts: Bond distances: C-E, C - C , C - 4 1 , C=C. Bond angles: tetrahedral, trigonal, oxygen, nitrogen. Ethane and 1,Z-diehloroethane: free rotation about a single bond; nonbonded interactions; relative eonformational stability; Newman projections of the staggered, eclipsed, and skew oonformations. The hutan-; stable conformation of n-octane: uncoiled ~igeag. 3-Methylhexme: Do you see anything interesting? Try a model made with one carbon and balls Nos. 1 4 . Check with neighbors and discover asymmetry and optical activity. 2-Butene: i s isomerism possible? Cyclopropane and cyclobutane: Show models with bent bonds, but advise students to use rubber-tube connectors. Cyclopentane: Show how to assemble and dissssemble the model without damage. To take the model apart, grasp two adjacent atoms with thumb and forefingers and brace the thumbs against the breast to permit pulling the atoms apart about 3-4 mm and then stopping. After a second and a third pair have been treated in the same way, complete separation is easy. Cyelohexane: As a take-home problem, the students were told to construct models and see what predictions they couldmake about number and relative stability of conformstions, interconversion of forms, and types of bonds.
With this beginning, models were used freely in further development of the subject. For example, nrea inclusion compounds can be demonstrated nicely with models in combination with cellulose acetate cylinders of appropriate size.I2 n-Octane fits snugly into a cylinder 14.3 cm in diameter and 52 cm long. 3-Nonyne is not accommodated by the same cylinder but fits into a 16.2-cm cylinder, also representing urea. Seeillustration in FIEBER, L. F., "Chemistry in Three Dimensions," pp. 96-100, Rineo Instrument Ca., 1963. lP
Figure Ir.
nexonemene.
interesting experiment by Wittig, in which benzyne was generated in the absence of a trapping agent and the resulting mixture of diphenylene and triphenylene was separated by precipitating triphenylene as the trinitrobenzene =-complex, can be elucidated with models showing that the rr-acid fits nicely over the planar surface of triphenylene but not over that of diphenylene. Advanced students are finding the models useful in considering the effect of stereochemistry and conformation on physical properties and chemical reactivity; in a course given by Dr. Roy A. Olofson students are required to bring models to the examinations. An interesting innovation introduced by Dr. Olofson is representation of partial bonds (dotted) by rubber bands, as in a model for the nonclassical carhonium ion from exo-2-norbornyl bromide [Fig. 13(a)1. Figure 13(b) shows the result of expulsion of the bromide ion, breaking of the C1-C6 bond, and development of partial bonds a t CI-C2, CICs, and C6-CI. It is not easily apparent from Figure 13(b) that the ion possesses a plane of symmetry, but symmetry is obvious when the
" FIESER?L. F., "Organic
Experiments,"D. C. EeathandCo.,
1964, p p 1E5, 188. One mole of the hydrocarbon combines with approxm~str:ly4.4 moles of thiourea. Volume 42, Number 8, August 1965
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Use of Sheet Aluminum for Conformation Models Dr. N. Entwistle, Bradford Institute of Technology, Bradford, U. K., suggests that the conformational aspects of the decalins e m be shown clearly by taking advantage of the rigidity of sheet aluminum cut and bent in the appropriate size and form. This practice is a modification of paper models [see THIS JOURNAL, 42, 274 (1965)l and gillvclniaed iron examples [see THIS JOURNAL, 39, 648 (1962)l.
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