Aaron J. lhde
University of Wisconsin Madison
I I
The Karbruhe Congress:
A Centennial Retrospect
O n September 3,1860, there gathered in the resort city of Karlsmhe about 140 European chemists, assembled to seek a meeting of minds regarding some of the problems responsible for the chaotic state of mid-nineteenth century chemistry. Friedrich August Kekul6, then a professor a t the University of Ghent and one of the promising figures among the younger chemists, was the leading stimulus in bringing about the conference. He had gained the backing, a year earlier, of Adolf Wurtz and Carl Weltzein. The latter, professor a t the Technische Hochschule in Karlsruhe, shouldered the burden connected with arrangements for the meeting (1). Forty-five prominent chemists' permit,ted their names to be used on the invitations sent out on July 10, 1860. The Chaos That was Chemistry
The meaning of such terms as "atom," "molecule," and "equivalent" had been in a state of chaos during the previous 50 years. Failure to agree on a uniform system of atomic weights meant that formulas varied from laboratory to laboratory. As an example, the atomic weight of 8 was used by some chemists for oxygen, 16 by others. Atomic weights of 6 and 12 were both in use for carbon. Consequently, formulas for the same substance could not be in agreement. Even when there was agreement on the empirical formula there was still a variation in ideas on the nature of combination in compounds. When Kekul6 prepared the first volume of his textbook (8) in 1861 he listed 19 different formulas for acetic acid, all of them to be found in the literature (Fig. 1). I n an era such as this, chemistry was not only complicated but without sound foundations. The whole problem began a half century earlier with the introduction of Dalton's atomic theory. Dalton, in his early work, had introduced a list of atomic weights which proved highly inaccurate and quite inadequate. In 1808 Gay-Lussac (S), in his paper on
combining volumes, published information which suggested that two atoms of hydrogen combine with one of oxygen. This led to use of the formula H20 by Berzelius although Dalton preferred the formula HO. While the law of combining volumes actually bolstered Dalton's atomic theory, Dalton refused to consider it of significance and questioned the reliability of Gay-Lussac's work. The Italian physicist Avogadro (4) brought about a reconciliation of Gay-Lussac's law and Dalton's atomic theory in 1811 when he suggested that equal volumes of gases a t the same temperature and pressure contain the same number of molecules. He then went on to show that the apparent discrepancies between the law of combining volumes and the atomic theory might be reconciled if elemental gas molecules were considered to be polyatomic. Nowhere did Avogadro argue that all elemental gases were diatomic, but since each of CaH,O, . . . C,H,O, + HO . CaH,04. H . . C.H, + 0. . C,H,II,+ HO, C,H + R,O. C,H,O,.O + HO C,H, . 0, + no
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Presented before the Division d History of Chemistry a t the 138th Meeting of the American Chemical Society, New York City, September 13, 1060. The following names were appended t o the invitations sent out by Weltzein (asterisks show those who did attend the meeting). Baho,* Balard, Beketoff, Boussingault,' Brodie, Bunsen,' Bussy, Cahours, Cmnisaaro,* H. Deville, Dumas,* Engelhardt, 0. I,. Erdmann,' Fehling,* Franldand, Fremy, Fritzsche, Hlasiwets,* Hofmann, KekulB,* Kopp,* Liebig, Malaguti, Marignac,* Mitsoherlich, Odling,* Pastcur, Payen, Pebal,' Pelignt,, Pelouse, Pirirt, Regnault, Romoe,' Schbtter, Socoloff, Staerlclcr, %as,* Strecker,* Wellzein,* Will,* Williamson, WF'ler, Wurtq* Zinin.*
Figure 1. Tabulation of formutoe for acetic acid in use before 1861. From F. A. Kekule, "Lehrbuch der orgonixhen Chemie;' 1861.
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his examples was a diatomic molecule, the few chemists who attempted to deal with the hypothesis supposed that molecules of all elemental gases or vapors were diatomic. Nearly all chemists, however, found Avogadro incomprehensible or unconvincing; the hypothesis, t,ogether with AmpBre's independent reiteration of it in 1814, lay in the discard for a half century. Berzelius. leading student of electrochemistry, was of the opinion that atoms were held together incompounds by electrical forces; hence, two atoms of the same element would repel each other and diatomic molecules would not he possible. Although Berzelius utilized the principle that differential elemental gases contained the same number of atoms per volume, he could not accept the not,ion that compound gases also contained the same number of molecules. When Dumas (5) developed his method for vapor density determinations in 1826, he found disagreement between atomic weights calculated from vapor densities and the atomic weights for the same elements report,ed from purely chemical investigations. Although Gandin (6) suggested that elemental gas molecules might contain ot,her than two atoms, Dumas and everyone else remained nnconvinced and vapor densities played no further part in the atomic weight problem. The next 30 years saw the dramatic rise of organic chemistry, with an attendant intensification of the problem of inconsistent formulas. The various radical and type theories of the period confused rather than clarified the problem. Despite the excellent analytical work of Berzelius, and his shrewd deductions, there was a hesitancy to accept atomic weights as having value in the calculation of formulas. Following Leopold Gmelin, many chemists felt mom comfortable with equivalent weights and used t,hem in dealing with proportionate relationships in chemical compounds. Gerhardt, early in the forties, reiterated the greater significance of atomic over equivalent weights, but recognized certain inconsistencies in the Berzelius table and made corrections. However, in correctly halving the Berzelius' values for silver, sodium, and potassium, he also halved the correct values for zinc, calcium, and other divalent metals. The work of Wurtz and Hofmaun on amines led to the recognition around 1850 of the ammonia type of compound. About the same time, Williamson's work led to recognition of the water type of compound in the alcohols and ethers. Gerhardt added the hydrogen and hydrogen chloride types shortly thereafter and sought to develop a new type theory for the classification of organic compounds. Kolbe's electrochemical studies on fatty acids led him to grope toward a primitive sort of structural formulation. Frankland, as a consequence of his work with zinc alkyls, was arriving a t the valence concept for organic groups and metallic atoms. Kekul6 and Couper independently extended the valence concept when they recognized the quadrivalency of the carbon atom. Both further recognized that carbon atoms might satisfy a part of their valency by carbon-to-carhon bonding. KekulB and Couper each sought to deal with structure in organic compounds on the basis of valence, Couper's structures bearing a close resemblance to those which later came into common use. 84
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journal o f Chemical Education
I t becomes readily apparent that the discipline of chemistry was in a state of extreme chaos a t midcentury and was, in fact., ripe for reform. The older chemists like Berzelius, Liebig, Dumas, Gmelin, and Mitscherlich were giving ground to a new generation of chemists. Some, particularly Laurent and Gerhardt, were openly contemptuous of authority. Others like KekulB were respectful of the past, but not prevented by it from examining unique new concepts. It was in this atmosphere that t,he Karlsruhe Congress was called. Following the customary preliminaries, the Congress settled down to business with the appointment of a steering committee, with Kopp as chairman, BBchamp, Canizzaro (Fig. 3, Erdmann, Fresenius, KekulB, Schischkoff, Strecker, and Wurtz, to draft a set of questions which would form the basis for the second day's discussion. This session was spent in an examination of the various ways in which t,he terms "atom," ',molecule," "radical," and "equiwlent" were used. There was no unanimity of opinion and no decisions were reached
Figure 2.
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Cannizzaro's Influence
At the last meeting on September 5 the steering committee proposed three questions for discu~sion.~ One of them raised the advisability of returning to the principles of Berzelius. Cannizzaro spoke against the proposal, pointing out that Gerhardt had set chemistry on the right track by basing molecular weights on the concept introduced by Avogadro and AmpBre. Hence, it was important to utilize Gerhardt's contributions rather than retreat to the position of Berzelius. Cannizzaro, then a t the University of Genoa, had published two years earlier a sketch of his method of tearhing chemist,ry (7). In this paper he revealed 'For details see C. DE MILT,C h ~ m i a ,1, 165 (1948), or better, KARLENOLER in the "Festgabe zum Jubililrtum der vierzigjhhrigen Regierung Seiner Koniglichen Hoheit des Grossherzogs Friedrich von Baden," ICarlsruhe, 1802. pp. 346-55. Tho latter paper carries the official report of t,he meeting which was p r e pared hy Wurtz and filed in the Archives of the Technical School in Karlsruhr.
how, by an examination of historical theories and experiments, it was possible "to lead my students t o the conviction which I have reached myself" (8). He described the significance of certain contributions of Gay-Lussac, Avogadro, AmpBre, Berselius, Dumas, Regnault, Gerhardt, and Laurent, and showed in his lectures, how t,hese rontributions were useful in the clarification of the subject matter. Cannizzaro's speech before the Congress emphasized the significance of Gerhardt's work, in which t,he latter based his molecular weights on t,he hypothesis of Avogadro and AmpBre. Attention was called t o the work of Dumas on vapor densities and the reasoning of Gaudin on the size of molecules of elemental gases. The application of Avogadro's hypothesis to the determinat,ion of at,omir weights was explained and a plea was made for the adoption of atomic and molecular weight,^ based on the application of these principles. During the discussion which followed Strecker announced his intention of adopting the proposed atomic weights and Kekul6 agreed, but with certain reservat,ions. Kopp and Erdmann argued, however, that votes must not be taken on scientific questions and t,he meet,ing closed on a note of uncertainty. At that t,ime Angelo Pavesi, professor of chemistry a t Pavia and follower of Cannizzaro, distributed reprints of Cannizzaro's paper of 1858. While many copies were doubt,less consigned t o the t,rash can, a few were read with understanding. Lothar Meyer was profoundly influenced (8) and utilized Cannizzaro's suggestions as the basis for his own text,book, "Die Modernen Theorien der Chemie" (1864). Mendeleev was anothe~. who made ext~ensiveuse of the proposals as had Newlands before him. I t is impossible t o overemphasize the role of Cannizzaro in bringing about the acceptance of Avogadro's hypothesis, and t,hereby pointing chemist,ry along a fruit,ful pat,h. By comparing t,he density of gases and vapors wit,h t,he density of hydrogen, and by accepting t,he diatomic character of the hydrogen molecule, he was able t,o show how correct molecular weights might be determined. By accepting the comparative densities observed by use of the balance, and by refusing t,o entert,ain preconceived notions regarding molecular composition, he was able t o avoid the uncertainties which plagued Dumas with respect t o the vapors of sulfur, mercury, phosphorus, and arsenic. It became increasingly obvious that molecular size might be dealt wit,h with certainty. As a consequence of the certainty of molecular weight,s, it now became possible t o distinguish between empirical and molecular formulas. Hydrocarbons, alcohols, organic acids, aromatic compounds, and virtually all of the simpler organic molecules could be formulated correctly. Structural Chemistry Possible
With the advent of reliable molecular formulas it was possible t o make the step t o structural chemistry. Although KekuM and Couper had grappled with the problem as early as 1858, it could not be convincingly handled until molecular formulas were no longer open to question. During the eighteen sixties a vigorous period of chemira1,mrhitecture developed through the work of w r h c h h i s t s as Keknl6 and Butlerov. This
t,ype of t,hought paved the way for the successful solution of the molecular basis for optical activity which had plagued chemists since the forties. Van't Hoff and 1.e Be1 successfully dealt mith the problem of the asymmetric carbon atom in 1874. Their work was successfully extended during the next decades, particularly in the hands of J. Wislicenus and, in the case of the sugars, of Emil Fischer. The last four decades of the 19th century were extremely fruit,ful ones for the development of organic chemistry. This is evidenced by t,he successful development of t,he synt,hetic dye industry, the branching out into the field of synthetic drugs, and the development of a significant indicator theory before t,he end of the century. The successful elucidation of the structure of such nat,ural dyes as alizarin and indigo, and of the sugars, purines, and terpenes, reveals the power held by the organic chemists of the day. It is true that, the successes in structural chemistry did not extend into the inorganic field until the end of t,he century. The attempt of Blomstrand and of J$rgensen to deal with structure of inorganic compounds was a miserable failure. There was an inability t o realize that whereas organic structures involve a very limited number of elements where carbon atoms serve as a central skelet,on, inorganic compounds are made up of a greater varict,y of elements with no single element serving as a central core. The problem only found resolution in the 1890's when Alfred Werner arrived a t the concept of the coordination compound and began dealing mith roordinat,ion complexes from the standpoint of geomet,ric st,ructure. However, it was necessary for Werner to int,roduce the concept of primary and secondary valences. Since a sound theory of chemical bonding only began t o develop after the rise of the struct,nral atom around 1914, it was difficult for inorganic chemists t o reconcile themselves t,o the demands of primary and secondary valences. Prototype of International Meetings
Besides its impact on the development of chemical theory, the Xarlsruhe Congress represents the first int,ernational chemical meeting. Although it failed t o mark the introduction of a regular series of international meetings it represented a preliminary step among chemists to seek, through a large gathering representative of many countries, solutions to problems which are difficult t o resolve on t,he local level. As chemistry advanced during subsequent decades more of these problems began t o be apparent, leading to new international meetings. By 1889 the problem of naming organic compounds was sufficiently acute t,o demand the attention of the International Congress of chemist,^ which assembled in Paris in that year. A commission was set up a t t'hat time t o make an interim study. A report was submitted a t the Geneva meeting of the Congress three years later. This was approved by about fort,y chemists in attendance, forming the basis of the Geneva system of organic nomenclat,ure. The International Congress of Chemists continued t o meet a t regular intervals until World War I. Following the termination of the war, the need for an international organizat,ion of chemists was recognized Volume 38, Number 2, February 1961
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in the formation of the International Union of Pure and A ~ ~ l i eChemistrv d a t meetines of an international group held in London &nd ~rnsselHduringJuly in 1919. This organization has continued active in dealing with nomenclature and other problems of world-wide interest. International Congresses and Conferences have been held, except during World w a r 11,a t intervals of several years. The Karlsruhe Congress, in retrospect, had signifieven though it appeared to end cant that with little accomplished (10). There is the problems facing the conferees would have been solved without a Karlsrnhe Congress, but they would have given ground more slowly. The Congress dramatized the importance of Avogadro's hmothesis in the minds of a few of the leading younger chemists, resnlting in the application of a sound principle which
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would make possible the dramatic progress in chemistry during the next four decades of the centurv.
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Literature Cited DEMILTz CLARA,Chmia, (lg48). (2) KEKULB,F. A., "Lehrbuch der organisehe Chemie," Erlangen, 1861, vol. 1, p. 58. (3) GAT-LUSSAC, J. L., Mem. SOC. Areueil, 2,207 (1809); or see Alembic Club Reprints, No. 4. (4) AVOQADRO, A., Journal de physique, 73, 58 (1811); or see Alembic Club Reprints, No. 4. (5) D D ~ sJ., B., Ann. chim. phys., [2] 33,341 (1826). ( 6 ) GAUDIN, M. A,, Ann. chim. phys., [21 52, 113 (1833). (7) CANNIZZARO, S., I1 NUOWCimento, 7, 321 (1858). This is avdilsble in English translation in Alembic Club Reprints, No. 18, Edinburgh, 1910. (8) Alembic Club Rep~ints,No. 18, Edinburgh, 1910, p. 1. remarks in OsTwALDrs Klassikel.der ezakten (9) see L. Wissenschaften, No. 30, LEIPZIG,1891, p. 59. (10) METER,ERNSTYON, J . praet. Chem., [21, 83, 182 (1911).
