A. Yeadon Slough College of Higher Education Slough. U.K.
Molecular Models Based on Petri Dishes
3 (Fig. 2). Assembly of the two complete Petri dishes is The stereochemistry of several chemical systems is such equivalent to salt formation. The salts are not enantiomorphs that the atoms which make up the structure lie either on and can only he diastereoisomers. The three-dimensional parallel planar surfaces or they can be projected on to parallel nature of the dish model is essential for this demonstration planar surfaces. A simple, convenient, and low cost method and a totally different result is ohtained if flat surfaces are of representingsuch systems involves projecting groups on to the or Petri dishes.' A stack of several Petri dishes used. ...-hnaes --.. .. .lids of .~~-~ is equivalent to several parallel planes. Such models are inR and S Structures exprnsivr, r d ~ u s tand , mas I I used ~ on the owrhead prujecwr. The interconversion of R or S ( I )structures and Fischer Thc circular lnaturt: uf the dish is nm\.t:n~enti i rotation a l r ~ u ~ projection formulas can he facilitated by Petri dish models. s tnmd is to he ihuwn and the depth of the dish assists in In general, the numbers 1,2, and 3, which represent functional mitint;tining a three-dmwnsiot~almodel. 'l'hc construction and groups are drawn on the lid of a dish so that they form the use a,isrvl rsl d these mudeli is described. Thc followin&??Icorners of an equilateral triangle. A figure 4 is drawn on the amples illustrate the scope of the idea which can clearcy he hase. The priority of groups is 1 > 2 > 3 > 4. developed further to include many other systems. The sequence of groups in Figure 3 represents an S arNewman Projection Formulas raneement. Fischer proiection formulas may be ohtained by . . placing any two groups nearer the observer. These groups are The arrangement of groups around the nearer carbon atom is drawn on the lid and that around the second carbon atom placed on the horizontal line and the two remaining groups are placed on the vertical line in the Fischer projection in the is drawn on the hase. Permanent figures are best drawn in order they are viewed on the Petri dish. naint from a ball-noint aunlicator. Fiaures on temuorary .. A complet~model models may be drawn with fiber-tip Double Quartet Theory is made by assembly of the individual, appropriately marked The double quartet theory due to Linnett ( 2 )successfully dishes. This model may be viewed directly or via an overhead accounts for hond orders, stabilities, and magnetic properties projector. Rotation of the lid relative to the hase enables of many diatomic and triatomic molecules and ions. The relvarious staggered and eclipsed structures to he viewed rapidly. ative distribution of the quartets of electrons is often initially The products of elimination reactions also can he predicted difficult to perceive. This problem may be solved by using quickly with the aid of such models. Petri dish models. For example, nitric oxide has the electronic Stereochemistry of a Diels-Alder Reaction atn~rtnre The nossibilitv of formation of exo and endo products from a reaction between maleic anhydride and furanmay he demonstrated hv drawing the structure of maleic anhydride on the hase of a dish so that the C=C group occurs across the center Siielectrons have spins antiparallel to the other five. Petri of the hase. The structure of the diene is drawn on the fid so dish models are assembled from a stock of lids and bases that the terminal carbon atoms of the diene unit appear over marked with one, two, or three markers to represent electrons the double hond on the base. The two distinct modes of approach are demonstrated by rotating the lid of the assembled dish. The greater overlap of the a electron systems in the formation of the endo isomer is clearly seen. ~~~~~~~~
Geometrical Isomerism
The reason for restricted rotation around a double bond may he demonstrated by drawing a p-orbital and groups R and S on the hase and lid of a Petri dish (Fig. 1). The assembled dish may he used to show that cis and trans isomers are interconvertible only if the bonding due to overlap of the porbitals becomes impossible.
Figure 1. A projection the arrangement of groups and a p-arbital around an So2 hvbriof
Resolution of Racemates
An understandine of the resolution of racemates via formation of diastere&omeric salts is often made easier by considerina the followina models. The letters A, B, and C are drawn on t i e bases of two dishes (Fig. 2) so as to give a mirror image pair. These dishes represent molecules of (say) a racemic acid. The neutralizing optically active base is represented by the lids of two dishes marked with the numbers 1,2, and 'Plastic disposable Petri dishes which have flat bases and lids are used. The diameter of the dishes used is 9.0 cm and depth 1.5 cm.
Fioure 3. An arranoemant of groups drawn on a lid to represent an S arrangement.
Figure 2. (A) An arrangement of groups drawn on the bases of dishes to represent a racemic mixture. (8)An arrangement of grwps which represent an optically active base drawn on the lids of dishes.
Volume 55, Number 1, January 1978 / 39
(Fig. 4A). Markers of different colors are used to distinguish between electrons of different spins. Bv choosinz a suitable cornhination of lids and bases, the electrons are placed in parallel planes. The relative distribution of quartets may be viewed through the stacked dishes. Dishes are chosen to give the distribution of electrons in three ulanes (Fie. 48). When the dishes are stacked vertically, the distrihution of the spin quartets in nitric oxide can he viewed. Orbital Symmetry Correlations As a further example of the use of Petri dish models, the thermal cyclization of trans, trans-hexa-2,4-diene will be described (3).The fragments A and D are drawn on the lids of two dishes while R and C are drawn on two bases (Fig. 5).
Assembly of two complete dishes side by side represents the
HOMO $r ground state for this molecule. Conrotatory motion and disrotatory motion correspond to both lids heing rotated clockwise or anticlockwise, respectively, while the bases remain stationary. The formation of trans- and cis-dimethylryrlc~l~nt~mt! hy conrotaric~n,ind diwutatim, rrspt.clively, may he d&mnnstratedand alsu the orhital symmetry at any srngtt in the cvclization can he easilv examined. If the two bases and one lid are rotated through 180" the dishes represent the HOMO h astate of cls. trans-hexadiene. In this case, disroillustrakd(Fig. 5) it is possible to demonstrate the products
of photochemical and thermal cyclization of the three isomeric hexadienes. Lanosterol Biosynthesis The stereochemistrv of the cvclization reaction involved in the hiosynthesis of lanosrrn,l from squalrne in+ Iw dewonatrated usinr! I'etri dish m~rlels.A model of swalene i< constructed by marking isoprene units on the inside base units of six dishes. The ~ o s i t i o nof the carhon atoms may he depicted using colored adhesive markers. One color represents a carhon atom originating from the methyl group of acetate and another color represents the carbon originating from the carboxyl group of acetate. Each dish is fitted with unmarked lids to act as supports. The honds indicated by the dotted lines need not he drawn on the dishes since these atoms are placed above each other in the stacked model (Fie. " 6). The formation of the A ring in lanosterol is shown by placing dish 11. face downwards on dish I. C-7 amears above C-2 and .. the c h k nature of the ring prdduced is evident from the Newman tvue formula which is apparent to the .. of uroiection . . .. \,iewer. The merhanism of this 1.yc11x;itIon I. thought to inwlw ~ : l ~ ! c t r o d di~ddition ic of 'OH to the dtmhle bond io that C 2 becomes a tertiary carhonium ion. The centers which become carhonium ions in the suhsequent addition reactions are marked The formation of rings B and C are depicted by placing dish I11 on dish I1 and dish IV over dish 111. When correctly placed the boat and chair forms of the B and C rings should he seen as Newman projections. Either the whole structure may be built up or problems concerning stereochemistry of ring formation may be discussed hv selectine two auurouriate dishes. An alternative to the kno& chair-bb;at-cha;;sequence (4) can be rapidly shown by simply turning the dishes over. The stereochemical prohlems involved in D ring formation can be illustrated by placing dish V over dish IV and the reasons whv further cvclization is impossible become evident with d i s h h over dish V. Several interesting points can be seen quickly from this model. Firstly, squalene can cyclize in either of two ways (5) and an identical model can be built up by starting with dish VI and the placing of dish V, then dish IV, etc. on top. Secondly, 'isoprene units' can be orientated in either of two ways. If required, allowance can be made for the inversion which occurs when two farnesyl units couple to form squalene (5) by placing the appropriate marker on the outside rather than the inside of the base of the Petri dish. Thirdly, the groups which are known to migrate are all seen to occupy axial positions on adjacent atoms (5). It is tempting to suggest that this model will also serve as a model for enzyme action in which a coiled squalene molecule is cvclized in a aroove on an enzvme surface. The methyl g r ~ ; ~would s he ield in specific sites and ring formation would occur hv electron movement from the double honds which are all heldclose to each other. Similar models could he used to account for the formation of triterpenes.
+.
Miscellaneous Examoles The use of Petri dish models to construct crystal structures has been oreviouslv described ( 6 ) .The idea is suitable even tor n,niplex .;tructures such as I)NA and a wale model of m r wm~,leteturn of the daul~lvhdix can he r a ~ i d l cunstructed s from Petri dishes (7).
C
D
Figure 5. The arrangement of orbitals representing the HOMO +b2 state of trans, trans-hem-2.4-diene. 40 1 Journal of Chemical Education
I
II
m
IP
P
Yt
Figure 6. A representation of squalene consisting of a linear arrangement of six isonrene units on Petri dishes.
Literature Cited 0 ,
