Rediscovery in a Course for Nonscientists Gordon W. Wood University
of
Windsor
Use o f molecular models
Windsor, Ontario, C a n a d a
I
t o solve classical structural problems
The chemistry courses for non-science students which are now in vogue present new challenges to the teacher: Not least of these is the need for mature approaches to essential scientific content without help from the sequential development usual in courses for science students. This paper describes exercises using simple hall and stick models which students with no chemistry, background can solve in the context of the original discovery. Ideally, appreciation for the history of science, and the mixture of logic, perseverance, and creativity inherent in the act of discovery are all transmitted in the process. The examples given are both from organic chemistry, hut structural problems from other areas could undoubtedly be developed. Pasteur's resolution of the paradox inherent in the diastereomeric tartaric acids depends initially upon recognition of the existence of non-superimposahle mirror images for a single asymmetric carbon. The problem is presented to the student in these terms initially and the more exact model using two asymmetric carbons is developed later. The following written material is presented to each student along with sufficient hall, stick, and spring models1 Rediscovery in Chemistry -The
Figure 1 . Models representing mesa and racernlc tartarc acids. The meso form on the left corresponds to the paratartrate as descrbed by Mitscherlich. The molecule is arranged so that a horizontal mirror plane exists through its central C-C band. The tartrate of Mitscherlich corresponds to one of the enantiomers on the right. That molecule and its mirror image are non-superimposable.
Tartaric Acid Problem
Background2 1) In 1884, in a paper by E. Mitscherlich, read before the French Academy of Science, the following observations were recorded about the two known kinds of tartaric acid: "Paratartrate and tartrate (double) of soda and ammonia have the same chemical composition, the same crystalline forms showing the same angles, the same specific weight, the same double refraction, and consequently similar angles between the optical axes. If the two substances are dissolved in water, their refraction is the same. But the tartrate, when dissolved, deflects the plane of polarized light, whereas the paratartrate is indifferent. Nevertheless, the nature and the number of atoms, their arrangement, and their distances are the same in t h e two substances compared." 2) "Pasteur, in the library of the Ecole Normale, happened to come across the German scientist's Note, and he was much struck by the obvious contradiction it contained. He could not, in all logic, agree that if one of two bodies displayed a n action on polarized light and the other no such action, their molecular constitution could be the same." 3) Subsequent research by Pasteur salved this problem.
Definifion of Problem wifh Models 1) Let the composition and shape of tartaric acids be represented by ball and stick models as follows: 1 black ball (representing a carbon atom) is attached by sticks to 1 each of red, yellow, green, and purple balls (representing other groups). 2) Prepare several models of this kind and explore the paradox described above, trying to find isolution.
Once the majority of the class has constructed a pair of enantiomers, and their mirror image relationship is grasped, further analysis of the observations of Mitscherlich leads fairly readily to the idea of an inactive (meso) form and an active form.3 Figure 1 shows a model of meso tartaric acid (paratartrate) on the left and the enantiomeric pair of tartaric acids on the right. Mitscherlich's ac-
Figure 2. Modes representing, from ieft to right, the Dewar. KekulB, and Ladenburg proposals for the structure of benzene. These and a number of other valence isomers of benzene are constructed by a typical class in a few minutes. Note that the unsymmetrical hexagon ,mpiicit in Kekule's fir91 proposal was subsequently modified. The present resonance picture 01 benzene consists at two imaginary symmetrical hexagons differing only in the locationof thedoubleand single bonds.
tive material corresponds to a pure enantiomer from a natural product. ( A mirror is kept on hand during this exercise.) The structure of benzene lends itself readily to this approach, and the usual class answers include, in addition to the Kekule answer, the proposals of Dewar, Ladenhurg, and others (See Fig. 2). The historical background to these structural proposals is outlined in several standard Some of the philosophically inclined students may even appreciate the injustice of Kekule's answer being "right," in the sense that the springs used to make double bonds in these models are totally localized and thus not obviously different from any other site of unsaturation. In this vein, Ladenburg's prism is very attractive, hut the facts on isomers of the derivatives can be brought
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Sargent-Welch Scientific Company, 7300 North Linder Ave., Skokie, Ill. 60076. 2This material is adapted from Nicolle, J., "Louis Pasteur," Basic Books. Inc.. New York. 1961. Chaw 2. 3Terminalogy and background on asymmetric carbon can be found in anv modern oreanie chemistrv text. Royals, E. E., "Advanced Organic Chemistry." Prentiee-Hall. Englewood Clifis, N. J., 1954, p. 416. 5 Wheland, G.W., "Advanced Organic Chemistry." 3rd Ed., John Wiley and Sons, lnc., New Yark, 1960, p. 86. GAllinger, N. L., et al., "Organic Chemistry," Worth Puhlishers, Inc., New York, 1971, p. 346. Volume 52. Number 3. March 1975
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in t o eliminate this possibility, or alternatively one can skip directly to the X-ray results for the "correct" answer.
clude that there were no multiple linkages of that sort in benzene (i.e., no C=C or C=C). However, benzene clearly lacked eight hydrogen atoms in comparison with the saturated Cs molecule.
Rediscovery in Chemistry -The
Resolution of Anomaly
Benzene Problem
Background In the first half of the nineteenth century great advances were made in understanding of the structure of hydrocarbons. It was well known that carbon atoms could link with each other to form chains. In modern representation H
H
H
Rules for Construction of Models Black ball = carbon atom. All four holes must be used. Yellow ball = hydrogen atom. Sticks or springs may be used for bonds connecting atoms. Springs allow some bending of bond.
H
1 / 1 1 H--C-C--C--C-H I I I I R
H
H
H
It was also understood that the maximum number of hydrogen atoms in such B Structure was determined by rules of valence and could be expressed as a formula (C,H2,+d Certain compounds contained less than this maximum number of hydrogens and were said to be "unsaturated" (C,H2nnrd. Typically, these compounds were very reactive, undergoing various "addition reactions" in which they would take up additional atoms to become saturated. In modem representation H
H
H-!-c=c-r-H
I I I
H
H
H
I I
+ x.
-
H
H
S
X
H
H
H
H
H
I ! I H-c-I%-r-H I I I I
Benzene a s a Misfit At this time benzene was a relatively new compound and did not fit the scheme of things. Its formula, CeHe corresponding to C,H2,-s, suggested a very high degree of "unsaturation," but it was very inert to the "addition reactions" normal for the type. These addition reactions were specifically related to the presence of double bands or triple bands and logically one might then con-
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After 1850 this problem began to be resolved by Kekulh, Couper, and others without reference to any important experimental observations beyond those presented here. You are invited to use the models provided to test various CsHe structures in an attempt to "discover" the key to this anomaly.
T h e point is often made t h a t approaches t o non-science courses are subjective and therefore difficult to transfer from one teacher t o another. For t h a t reason, no attempt is made here to specify in detail a n appropriate context for these exercises, or the conclusions to be reached afterwards. It is my hope t h a t the outline presented here will be sufficient to encourage others t o prepare exercises compatible with the aims of their own specific courses. In this department these exercises have been used successfully several times in courses for non-science students t o help them become familiar with organic chemistry structures in preparation for discussions of drugs, pesticides, and other classes of organic molecules important t o the layman. Although it is difficult to get hard evidence t h a t the more fundamental aims of transmitting some notion of scientific logic and the nature of discovery were achieved, my impression is t h a t the exercises are a t least partially successful in this context a s well.
