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AUGUST KEKULE AND THE BIRTH OF THE STRUCTURAL THEORY OF ORGANIC CHEMISTRY IN 1858' 0. THEODOR BENPEY (Translator) Earlham College, Richmond, Indiana August Kokul6 and Archibald Scott Couper almost simultaneously enunciated in 1858 the two principles underlying the structural theory of organic chemistry: the tetravalence and selflinking of carbon. In 1890, twenty five years after the benzene structure was elucidated by Kekul6, he made a major speech on the origins and birth of the structural theory st a festivd.given in his honor in the Berlin City Hall. That speech was published in the Berichte d m Deutschen Chemiseha Ge:esellschajt,23, 1302 (1890). Parts of the speech have been translated by various authors, notably by F. R. Japp in his Kekul6 Memorial Leeture.2 The speech is of such general and oontempormy interest, especially in view of the concern for the finding and training of oreative scientists, that an essentially complete translation is here attempted. Japp's translation has been used in unchanged or slightly modified form where suitable and represents approximately one third of the present version.
Y o u are celebrating the jubilee of the benzene theory. I must first of all tell you that for me the benzene theory was only a consequence, and a very obvious consequence of the views that I had formed about the valences of the atoms and of the nature of their binding, the views, therefore, which we now call valence and structural theory. What else could I have done with the unused valences? . . . My colleagues! We all stand on the shoulders of our predecessors; is it then surprising that we can see further than they? If we follow the roads built by our predecessors or the footpaths trodden by them, and effortlessly reach the places which they had finally attained after overcoming countless obstacles, what special merit is it then if we have some energy left to penetrate further into the unknown? Everyone of my colleagues has contributed to these developments, each in his own way. Certainly yon cannot blame my colleagues of twenty-five years ago for not developing and publishing the benzene theory; but, on the other hand, it would be going too far t o give me special credit for that development. Certain ideas a t certain times are in the air; if one man does not enunciate them, another will do so soon afterwards. It was said that the benzene theory appeared like a meteor in the sky, absolutely new and unheralded. Gentlemen! The human intellect does not operate in that way. Something absolutely new has never been thought, certainly not in chemistry. Anyone who has studied the historical development of his science from his student days on, as I have, first as a hobby and then later, as befits old age, by immersing himself anew in thorough studies of the pioneers, can certify that no science has developed as steadily as has chemistry. Admittedly, during one period of its development, which fell partly within my own experience, the very 'Supported in part by Research Grant NSG-G4207 of the National Science Foundation. 'JAPP, F. R., J . Chem. Soe., 73,97 (1898).
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opposite seemed for a time to be the case. Fifty years ago, the stream of chemical progress had divided into two branches. The one flowed, chiefly on French soil, through luxuriant flowerdecked plains, and those who followed it, with Laurent and Dumas a t their head, could reap, during the whole voyage, almost without effort, an abundant harvest. The other followed the course indicated by an old and approved guide-post set up by the great Swedish chemist, Berzelius; it led for the most part through broken boulders, and only later did it again reach fertile country. At length, as the two branches had again approached much nearer to one another, they were separated by a thick growth of misunderstandings, so that those who were sailing along on the one side neither saw those on the other, nor understood their speech. Suddenly a loud shout of triumph resounded from the host of the adherents of the type theory. The others also had arrived-Frankland a t their head. Both sides saw that they had heen striving toward the same goal, although by d i e r e n t routes. They exchanged experiences; each side profited by the conquests of the other; and with united force they sailed onward on the reunited stream. One or two held themselves apart and sulked; they thought that they alone had held the true conrse-the right fair-way-but they too followed the stream. Our present opinions do not, as has frequently been asserted, stand on the ruins of earlier theories. None of the earlier theories has been recognized by later generations as entirely false; each one, when stripped of certain ill-proportioned, meaningless excrescences, could be utilized in the later structure, and form with i t one harmonious whole. Here and there a seed may have lain in the ground without germinating; but everything that grew came from seed that had been previously sown. My views also have grown out of those of my predecessors and are based on them. There is no such thing as absolute novelty in the matter.
It has been remarked that the benzene theory emerged, armed like Pallas Athene, from the head of a chemical Zeus. It may have looked that way, but even if it did, that isn't the way it happened. I am in a position t o clarify the question under discussion. My conceptions as to chemical valence and the mode of linking of atoms-what we now call structural theory-had arisen during my early stay in London. As a young instructor in Heidelberg, I put these views on paper and shared them with two of my closer friends. They shook their heads, full of misgivings. I decided that one of two things was not quite ripe, either the theory or the t i m e s o I let my manuscript stay in the desk: Nonumque prematur in annum. More than a year later an article by Limpricht provided the occasion for publishing it, though of course in a modified form. The article did not gain materially from this alteration. It would have suited the purpose better had the polemical part not been printed. To my mind the earlier form was preferable. The benzene theory fared similarly. For almost a year it lay completely finished among my papers, till the beautiful syntheses of aromatic hydrocarbons by Fittig and Tollens led me to publish it. We should always leave fruit on trees till it is ripe. Unripe fruit brings little gain even to the farmer; it hurts the health of those who enjoy it: it particularly endangers the young who don't yet know how to distinguish ripe from unripe. Someone has spoken of genius and the benzene theory has been called a work of genius. I have often asked myself: What is a genius and what a work of genius? It is said that a genius recognizes truth without knowing the evidence for it. I do not doubt that even in ancient times this kind of thinking occurred. Would Pythagoras have offered up a hecatomb if he had recognized his famous theorem only after finding a way to prove it? It is also said that genius thinks in leaps. Gentlemen, the growing intellect does not think in leaps. That is not given to it. Perhaps it will interest yon, if I let you know through highly indiscreet disclosures from my inner life, how I arrived a t some of my ideas. During my stay in London I resided for a considerable time in Clapham Road in the neighborhood of Clapham Common. I frequently, however, spent my evenings with my friend Hugo Miiller a t Islington a t the opposite end of the metropolis. We talked of many things but most often of our beloved chemistry. One fine summer evening I was returning by the last bus, "outside," as usual, through the deserted streets of the city, which are a t other times so full of life. I fell into a reverie (Traumerei),and lo, the atoms were gamboling before my eyes! Whenever, hitherto, these diminutive beings had appeared to me, they had always been in motion; but up to that time I had never been able to discern the nature of their motion. Now, however, I saw how, frequently, two smaller atoms united t o form a pair; how a larger one embraced the two smaller ones; how still larger ones kept hold of three or even four of the smaller; whilst the whole kept whirling in a giddy dance. I saw how the larger ones formed a chain, dragging the smaller ones after them but only a t the
ends of the chain. I saw what our past master, Kopp, my highly honored teacher and friend, has depicted with such charm in his L'Molekular-welt"; but I saw it long before him. The cry of the conductor: "Clapham Road," awakened me from by dreaming; but I spent a part of the night in putting on paper a t least sketches of these dream forms. This was the origin of the "Structural Theory." Something similar happened with the benzene theory. During my stay in Ghent I resided in elegant bachelor quarters in the main thoroughfare. My study, however, faced a narrow side-alley and no daylight penetrated it. For the chemist who spends his day in the laboratory this mattered little. I was sitting writing a t my textbook but the work did not progress; my thoughts were elsewhere. I turned my chair to the fire and dozed. Again the atoms were gamboling before my eyes. This time the smaller groups kept modestly in the background. My mental eye, rendered more acute by repeated visions of the kind, could now distinguish larger structures of manifold conformation: long rows, sometimes more closely fitted together all twining and twisting in snake-like motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lightning I awoke; and this time also I spent the rest of the night in working out the consequences of the hypothesis. Let us learn to dream, gentlemen, then perhaps we shall find the truth. And to those who don't think The truth will be given. They'll have it without effort.
But let us beware of publishing our dreams till they have been tested by the making understanding. Countless spores of the inner life fill the universe, hut only in a few rare beings do they find the soil for their development; in them the ides, whose origin is known to no men, comes to life in creative action. (J. YON LIEBIG)
I said earlier that in a given period certain ideas are in the air. Now we hear Liebig's view that the germs of ideas, like spores, fill the atmosphere. Why, then, did the spores of the structure and benzene theory that were swarming about twenty-five years ago find the necessary food stuffs for their development in my head? Again, I must burden you with details from my life. I n high school I showed particular aptitude in mathematics and drawing. My father, who was a close friend of a number of famous architects, decided that I should study architecture. It seems to be the fact that parents normally decide the vocations of their sons. Accordingly, I entered the university as studiosus architecturae and followed with commendable diligence under Ritgen's guidance descriptive geometry, perspective, shadowtheory, stonecutting, and other interesting subjects. But Liebig's lectures tempted me to change subjects and this I decided to do. My relatives insisted that I take some time to reflect on my decision, and thus I spent a semester a t the Darmstadt Polytechnic.. . . Only then could I devote myself to my beloved chemistry, under Will's and Liebig's guidance. JOURNAL OF CHEMICAL EDUCATION
My years of apprenticeship took me to Paris, where I was able to listen t o what I believe were the last lectures of the famous Dumas. I spent much time with Wurtz with whom a close friendship later developed. By accident I met and became friends with Gerhardt, who a t that time was discovering the acid anhydrides and was preparing the completed manuscript of his famous textbook for the printer. A stay of a year and a half at a lonely Swiss castle gave me ample leisure to assimilate independently the insights I had gained from the still unpublished manuscript. My travel years took me farther, to London. While Paris gave me the opportunity to become acquainted with the unpublished views of Gerhardt, so now I had the good fortune to become good friends with Williamson and to familiarize myself with the mode of thought of this philosophical intellect. Originally a student of Liebig, I had become a student of Dumas, Gerhardt, and Williamson. No longer did I belong to any one school. This circumstance and the direction which my earlier architectural studies gave my intellect-an irresistible urge to visualize everything-these seem to be the reasons why the chemical ideas which were in the air twenty-five years ago found suitable soil inside my head. The human being, it seems clear, is the product of the conditions in which he grew up; no merit accrues to him for what he becomes. If I may be permitted to add a lesson for my younger colleagues, it is this: Free yourselves from the spirit of the "Schools," then you will be capable of doing original work. Remember that it was Mephistopheles who advised the scholar: Here also it is best If only one you bear And on the master's words you swesr
For one thing only, I believe, do I deserve credit. I have loyally followed the advice which that past master Liebig gave to the young beginner: "If you want to become a chemist," so said Liebig as I started work in his laboratory, "yon will have to ruin your health. If you don't ruin your health studying, you won't accomplish anything these days in chemistry." That was forty years ago; is it still valid today? I loyally followed that advice. For many years four or even three hours of sleep were enough for me. One night spent with my books did not count; only if two or three followed each other did I feel I had earned some merit. I n those days I had acquired such a fund of knowledge that my friends considered me more reliable than the Jahresbericht (Berzelius' Annual Review of Chemical Progress). Those good days are long past. Of the various mental powers, the imagination is the first to go; memory follows-fortunately slowly; the longest to remain intact is the critical faculty, but this may still do good service provided that i t rests on the broad foundation of solid knowledge acquired by thorough
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industry. May I draw a moral? I would advise my young fellow-chemists t o be diligent during their youth. One cannot explore new countries in express trains, nor will the study of even the best textbooks qualify a man to become a discoverer. Whoever is content to follow well-laid promenades until he reaches some pleasant eminence frequented by tourists, may, by striking into the thickets, gather some forgotten flower; or if cryptograms, mosses and lichens satisfy him he may even bring home a well-filled vasculum; but anything essentially new he will not find. Whoever wishes to train himself as an investigator must study the traveler's original works; and that, too, so thoroughly that he is able to read between the lines-to divine the author's unexpressed thought. He must follow the paths of the Pathfinders; he must note every footprint, every bent twig, every fallen leaf. Then, standing a t the extreme point reached by his predecessors, it will be easy for him to perceive where the foot of a further pioneer may find solid ground. Surveying everything, I find no reason that could have motivated you to arrange this celebration. And yet you did arrange this celebration. I believe I can tell you why. The ambitious little community of chemists, proud of its past and full of hope for the future, felt the need, in our jubilee-addicted century, to hold a jubilee on its part also. I n this time of feverish living the period could of course not be longer than twenty-five years and most theories can't be preserved for more than twenty-five years anyway. Now i t is to my regret incontestable that as the theory of substitution and of types put its stamp on the previous quarter of a century, so, for the immediately concluded quarter, next to the structural theory it was that theory's keystone, the benzene theory, which was its most characteristic signature. Much of importance has been achieved in other areas of chemistry; other branches too have grown mightily; some t o whom the future no doubt will belong have newly arisen; but in general the parole was: aromatic. . . . I had been of the opinion that the gentlemen from industry, among whom I count many valued friends and former students, consider only the bee meritorious that brings in the honey but not the flower that produces the nectar. The flower acts, I thought, out of an inner urge, in order to bring pleasure to itself and friends. The realization that my opinion was in error has provided me the greatest pleasure. I cannot deny that some of my studies, including the benzene theory, have been of use in the coal tar dye industry; but I can assure you, I have never worked for technical ends, only for pure science. I have always had the greatest interest in industry but I have never derived my interests from such sources. Precisely for that reason I am doubly happy and doubly thankful that the representatives of industry desire to recognize my slight contributions to the practical arts. . . .
