H. Mark Brooklyn Polytechnic Institute Brooklyn, New York
The Early Days of
The expression "structure of a molecule" did not hecome significant and meaningful before the second half of the last century. Up to that date, organic chemists were satisfied to establish for a new molecule which they had synthesized, the chemical composition in terms of a stoichiometric formula and to describe its properties-color, specific gravity, refractive index, melting point, boiling point, etc.,-as completely as possible. When, about one hundred years ago, the establishment of a structure became an important part of a chemical publication, it was particularly Kekule who became the protagonist of the new approach when, in the 1850's, he had the vision of carbon atoms being bonded together and forming chains to which other atoms such as hydrogen, oxygen, or nitrogen could he attached. Later, Kekule added to the concept of an open chain that of a closed ring and explained in a global way the essential differences between aliphatic and aromatic chemistry. All formulas of these days referred to the structure and the hehavior of ordinary, small molecules, hut when Kekule in 1877 became Rektor of the University in Bonn and, as usual, delivered an inaugural address of general character and wider scope he advanced the hypothesis that the natural organic substances which are most directly connected with life-proteins, starch, and cellulose-may consist of very long chains and derive their special properties from this peculiar structure. The change in emphasis from composition to structure led to the demand that any chemist would have to present in his puhlication the structural formula of the material which he was investigating. Since publications are written and printed on paper it was unavoidable that the two dimensional character of this commodity created the inclination for simplifications and distortions of what, in the well known sense of van't Hoff and Le Bel, should have been three dimensional systems. One of the greatest promoters of structural organic chemistry around the turn of the century was Emil Fischer who ingeniously used two dimensional formulas to express without any inconsistencies the most complicated three dimensional structures in the chemistry of sugars and amino acids. As early as 1893 he had already, in general terms, the structure of cellulose as a polysaccharide in mind and expressed the opinion that it might he represented as a chain of glucose units ( I ) , and his later systematic work on polypeptides clearly indicated a long chain structure for natural proteins (2). For two decades, from the turn of the century to his death (July 1919), Emil Fischer's Institute in Berlin was the undisputed center of work on natural high polymers and their precursors. Many of the distinguished scientists who later figured prominently in the developments of the 1920's were connected with his school for a shorter or longer period: Ahderhalden, Bergmann, Delhrueck, Freudenherg, Gabriel, Harries, Leuchs, and Zemplen (compare also reference (2)).
In order to get a correct impression of the general ideas prevailing in the field of such important natural suhstances as cellulose, starch, proteins, and rubber, early in this century, let us treat each of them separately.
7' LOFCHH-CHTO-
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6
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12 H
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GOCH
I I HCCG I HC
H2COG
-
(G isa glucoryl
qmupl
m.Tollanr
V Lrthelamy
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Early structural proposals for cellulose.
Polysaccharides
French chemists-Braconnot, Payen, Fremy, and Pelouze (3)-studied the composition of various vegetable cell walls and arrived a t the conclusion that a certain suhstance, called "cellulose," was their most important component. Analysis showed that cellulose was a carhohydrate, isomeric with starch, and could he degraded hydrolytically into simple sugars. From these early observations it took a long time until H. Ost demonstrated in 1910 that many celluloses can he converted almost quantitatively into D-glucose by acid hydrolysis (4). In his "Handbook der Kohlenhydrate," [Leipzig, J.A. Barth, 19141, B. Tollens presented the concept that cellulose may he a long chain consisting of glucose units; no experimental evidence was offered for this idea. From 1899 to 1920 several structural proposals were offered for the "formula" of cellulose as shown in the figure; they all were based only on the fact that cellulose consists essentially of glucose hut did not have the benefit of any additional or more precise evidence. Some of the proposals favored the chain concept, others that of small cyclic building units (4). Under these conditions it was clear that the separation of celluhiose and other oligosaccharides as intermediate degradation products would he of great importance for the experimental support of any structural concept of cellulose, and considerable efforts were spent on the accumulation of new and reliable experimental data on this point (5). In 1920 and 1921 three important papers appeared which postulated long chain structure for several synthetic and natural compounds on .the basis of general considerations, and offered specifically for cellulose the long chain character as a preferred alternative in comparison with Volume 50, Number
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other structures. The first of these papers was published by Staudinger ( 6 ) and proposed for polystyrene, polyoxymethylene, and rubber, formulas which represented linear long chains such as H H H H H H
I I I I I I
polystyrene
-C-C-C-C-C-C-
I I H
I
-C-0-C-0-C-0-C-0-C-
I
H
I I
H
I I H
I I H
palyoxymethylene
Actually, they are still accepted today. The second paper was published by Freudenberg (71, offered new experimental data on the yield of cellubiose during cellulose degradation, and stated that the best available data were in conformity with a long chain structure and certainly did not contradict this concept. If Freudenherg on that occasion had taken a more aggressive attitude and had said that his data proued the chain structure of cellulose, much confusion would have been spared during the following years; but Freudenherg was much too conservative to go with his statements beyond what he could actually prove. As a consequence this early paper did not play a very important role in the scramble for a unified and convincina- formulation of macromolecular systems. The third article (8)refers to a lecture which M. Polanyi gave on March 7, 1921 commenting on a paper by Herzoa and Jancke ( 9 ) which presented X-ray data on various cellulosic samples. ~ o l a n i icame to the conclusion that the measured X-ray diffraction spots were in agreement either with long glucosidic chains or with rings consrsting of two glucose anhydride units. It was made quite clear on this occasion that, on the basis of X-ray data alone, one could not distinguish between these two possihilities. The cautious and guarded language of this article gave rise later to the false statement that the small basic unit of the lattice of crystalline cellulose was a proof for a low molecular weight of this material. Although additional attempts were made (10) to correct this wrong position one finds even now the erroneous opinion expressed that the molecular size of a compound cannot he larger than its crystallographic unit cell. A few years later Haworth, Hirst, and Iwine (11) demonstrated by a brilliant analytical technique that the hydroxyl groups 2, 3, and 6 in cellulose are still free and consequently, that the honding between the individual glucose units must he through the carbon atoms 1 and 4. Based on this new important experimental evidence Sponsler and Dore (12), in 1926, made another significant step in the interpretation of the existing X-ray data by correlating the Haworth glucose ring with the Bragg atomic radii and arrived a t a more detailed structure for the cellulose chain than Polanyi. Unfortunately they did not take into account the irrevocable chemical evidence of a 1-4 glucosidic bond in celluhiose and arrived, therefore, a t a wrong honding principle along the length of the chain molecules. If Polanyi, in 1921, had known Staudinger's article of 758
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Journal of Chemical Education
1920 in which chain structures are postulated for several natural and synthetic materials and Freudenbera's article of 1921, he would have referred to them and'prohahly placed more emphasis on the chain structure of cellulose, which he offered only as one of two possihilities compatible with the X-ray data alone with no additional information from chemical sources. If Sponsler and Dore, in 1926, had seen Polanvi's article of 192i in which he ruled out a non-polar sequence of glucose units in the chains, they would not have proposed the incorrect alternating 1-1 and 4-4 glucosidic bonds along the cellulose molecules hut would have preferred the correct continuous 1-4 enchainment. Even Staudinger in his book (13) makes no mention of Polanyi's article eleven years after its appearance although it was the first correct qualitative anticipation of the ultimately accepted macromolecular chain structure of cellulose. These and many similar examples show that at that time only insufficient and slow communications between chemists existed even if they worked on the same substances. K. Hess, for example, in his well known hook on the "Chemistry of Cellulose" (14) presents a detailed account of practically all formulations up to 1928, without arriving at a completely conclusive decision between those proposals which prefer the long chain concept and those which believe that unusuallv strona association forces between relatively small units- are responsible for the "colloidal" character of cellulose and its derivatives. On the hasis of all previously available data, with considerable additional evidence from X-ray diagrams of various cellulose derivatives and from the optical activity of cellulose and its degradation products, Meyer and Mark (15) in 1928 accumulated convincing material for the presently accepted chain structure of cellulose and for the &ystall
