Kekule von Stradonitz, (Friedrich) August

(b. Darmstandt, Germany, 7 September 1829; d. Bonn, Germany, 13 July 1896)

chemistry

Kekulé was descended from the Czech line of an old Bohemian noble family, Kekule ze Stradonič Stradonice being a village northeast of Prague. The family can be traced to the end of the fourteenth century; a branch emigrated to Germany during the Thirty Years’ War and in the eighteenth century became established in Darmstadt. Kekulé’s father, Oberkriegsrat Ludwig Carl Emil Kekule, added the accent to the family name following Napoleon’s inclusion of Hesse-Darmstadt in the Confederation of the Rhine. When Kekulé himself was ennobled by William II of Prussia, in March 1895, the terminal accent was dropped in the full style.

Kekulé attended the Gymnasium in Darmstadt, where he distinguished himself by his studiousness, aptitude for languages, and talent for drawing. His family intended him to be an architect, and he began the appropriate studies at the University of Giessen in the winter semester of 1847–1848. During the second semester, however, he so enjoyed Liebig’s chemistry course that he decided to become a chemist. Kekulé’s father had died, and the family council did not give its immediate consent to his new plan, although it was agreed that he might attend the Höhere Gewerbeschule in Darmstadt to study science and mathematics. He accordingly spent the winter to study chemistry at Giessen, beginning in the summer semester of 1849.

At Giessen, Kekulé first worked under the direction of Heinrich Will, undertaking a study on the ester of amylsulfuric acid and its salts. In the winter of 1850–1851 he began to work in Liebig’s laboratory. Liebig was at that time devoting his energies to enlarging his Chemische Briefe; he entrusted Kekulé with research on the composition of gluten and wheat bran, and cited Kekulé’s results in his twenty-seventh letter. He offered Kekulé an assistantship, but Kekulé found practical laboratory work unsympathetic to his speculative mind and decided to continue his studies abroad.

In 1851 Kekulé, upon Liebig’s advice, went to Paris, where he took courses in physics and chemistry and, in particular, became the student and friend of Charles Gerhardt. He thus came to know Gerhart’s unitary theory of chemistry, his theory of rad1icals, and his systematization of organic compounds into four types: water (H₂ O), hydrogen (H₂), hydrogen chloride (HCI), and ammonia (NH₃); Gerhardt further made the manuscript of his Traité de chimie organique available to Kekulé. It was at this time, too, that Kekulé became interested in the problems of the philosophy of chemistry that were to concern him for some time.

Kekulé returned to Germany when his mother died. At Giessen he defended a thesis on the ester of amylsulfuric acid and was awarded the doctorate on 25 June 1852. He then became assistant to Adolf von Planta at Reichenau, Switzerland, where he remained for a year and half before taking up a similar position, on Liebig’s recommendation, with John Stenhouse at St. Bartholomew’s Hospital in London. Kekulé stayed in London from the end of 1853 until the autumn of 1855. During this time he met several other of Liebig’s former students, including A. W. Williamson, who had shortly before synthesized simple and mixed ethers that corresponded exactly to Gerhardt’s water type. (Gerhardt himself had just discovered the anhydrides of organic acids, thereby confirming the significance of this same type.)

Williamson and Kekulé became friends, and Williamson was influential in the development of Kekulé’s theoretical views. It was at Williamson’s instigation, moreover, that Kekulé began his work on the reaction of phosphorus pentasulfide on acetic acid. From this reaction Kekulé was able to isolate thioacetic acid, which he classified as a new type, hydrogen sulfide, corresponding to Gerhardt’s water and hydrogen chloride types. This work, published in 1854, marks the beginning of Kekulé’s scientific maturity. At the same time Kukulé had begun to consider, in the Gerhardt types of organic molecules, not only the radicals, but more and more the atoms themselves; he himself gave an account of a vision that he had on top of a London omnibus, in which he saw the atoms “gambolling” before his eyes. This fantasy, which was perhaps influenced by his early training in architecture, was soon to result in his theory of valence and in his structure theory.

Kekulé was, however, eager to begin a university career, and at the suggestion of Liebig and Bunsen he enrolled in the University of Hedielberg in order to the admitted there as a privatdocent. Having passed the requisite examinations, in the summer semester of 1856 Kekulé began teaching organic chemistry. He further installed, at his own expense, a lecture room and a laboratory in the first two floors of a house on the main street of Heidelberg, and it was in this private laboratory that he carried out his experiments on the chemical constitution of fulminate of mercury. Here, too, Adolf von Baeyer studied compounds of arsenic trimethyl.

During these years, too, Kekulé arrived at the concept of polyvalent radicals and introduced multiple and mixed types in a single formula of a particular compound. He introduced also the marsh gas type and worked out the theory of the tetravalence of carbon, as may be seen from an article that he published in Justus Liebigs Annalen der Chemie in 1857; in a more extensive publication of the following year he was able to state not only that the carbon atom is tetravalen in such simple compounds as CH₄, CH₃ Cl, CCl₄, CHCl₃, and CO₂, but also that in compounds containing more than one carbon atom, the carbon atoms can link together in chains which can, in turn, form various polyvalent radicals. An ordered classification of organic compounds thus becomes possible. Indeed, by creating the new type CH₄ and by stating the ability of carbon atoms to join up with each other, Kekulé laid the foundation of structural chemistry. He based his courses at Heidelberg on these principles, illustrating his lectures with models of individual atoms and of molecular groupings. By projecting the shadows of these models on a blackboard or on paper Kekulé obtained the “graphic formulas” that were one of his favorite teaching aids. His innovative course was a great success, and Kekulé began to consider publishing a treatise on organic chemistry.

Before he could do so, however, a chair of chemistry became vacant at the University of Ghent. The Belgian chemist Jean Servais Stas, wishing to revivify the teaching of chemistry in Belgium, strongly urged Kekulé’s nomination as full professor. Kekulé accepted the position and, at the age of twenty-nine, moved to Ghent. Stas had obtained a promise that practical chemistry would be introduced into the curriculum at Ghent and Kekulé was promised a new laboratory for both teaching and research. He was also given permission to accept private students, of whom Baeyer, one of the first, became his personal research assistant.

Despite the difficulties of adjusting to a foreign environment and of teaching in French, Kekulé soon established himself in a scholoarly mode of life. He spent the entire day in the laboratory, dedicated the evening to composing the first sections of his Lehrbuch der organischen Chemie (of which the first fascicle was printed in June 1859), and, in the hours after midnight, prepared his courses for the next day. He also found time to take the initiative in organizing the first International Congress of Chemists, which met at Karlsruhe in September 1860. The purpose of the Congress was to reduce confusion in chemical nomenclature—Kekulé was as aware as anyone of discrepancies in defining such basic concepts as the atom, the molecule, and equivalence—and to promote greater uniformity of terminology in the world chemical literature. It served a further important end as well, since it was here that Cannizzaro reestablished the importance of the Avogadro-Ampére molecular hypothesis, which had lain neglected for nearly fifty years.

Kekulé achieved some significant experimental work even before his new laboratory was ready. In particular he was concerned with the chemical structure of the organic acids and carried out, in sealed tubes, the bromination of succinic acid; from the silver salt of dibromosuccinic acid he prepared optically inactive tartaric acid; and from the silver salt of the monobromosuccinic acid he obtained maleic acid. He further demonstrated that the same family relationship exists between salicylic acid and benzoic acid as between glycolic acid and acetic acid. These researches led Kekulé to recognize the isomerism of the phenolic aromatic acids, but he was unable to account for it.

The new laboratory, constructed according to plans drawn up by Stas and Kekulé, was inaugurated in 1861, and Kekulé began to study the unsaturated dibasic acids. He was aided in this undertaking by Théodore Swarts and Eduard Linnemann, his assistants, and by one of his students, Hermann Wichelhaus. His attention had been drawn to the subject by his discovery of fumaric acid and maleic acid, two unsaturated dibasic isomers, related to succinic acid, each of which contains four carbon atoms. These acids further readily fix bromine to form two different dibromide derivatives. Having identified these entities, Kekulé was unable to interpret their structure, and the problem became more complex when he discovered three other unsaturated isomeric dibasic acids with five carbon atoms each. Since Kekulé had long held the tetravalence of carbon to be as invariable as its atomic weight, it was necessary for him to create a new theory to acknowledge the presence, in unsaturated isomers, of lacunae or double bonds between two neighboring carbon atoms. This theory of unsaturates was published in 1862; by means of it Kekulé was able to account for both the two isomers with four carbon atoms and the three acids with five carbon atoms.

The problem of unsaturated substances almost immediately came again to Kekulé’s attention in the following year, since he was writing the second part of his Lehrbuch, in which he planned to deal with the chemical structure of the aromatic compounds. The solution in this instance came to Kekulé in a vision—half awake, he saw before his eyes the animated image of a chain of carbon atoms, closing upon itself like a snake biting its own tail. He was instantly aware of the significance of such a closure, and spent the rest of the night determining the consequences of his inspired hypothesis. He arrived at a closed chain of six carbon atoms, linked alternately by three single and three double bonds and constituting the common nucleus of all the aromatic substances. He then set himself the task of experimental confirmation, but his work toward this end was delayed by various events.

At the time of his arrival in Ghent, Kekulé had met George William Drory, inspector general of the Continental Gas Association. Like Kekulé, Drory was a Protestant; they soon became close friends and Kekulé became a frequent visitor to Drory’s house. There he met and fell in love with Drory’s youngest daughter, Stéphanie, whom he married on 24 June 1862. Kekulé was thirty-two, Stéphanie nineteen. Their son, Stephan, was born the following May, and two days later Stéphanie Kekulé died. Kekulé was unable to take up his creative work for several months following her death.

He returned to his research in 1864, again taking up the search for confirmation of his benzene theory, which he had already set down in manuscript form. He first tried to do the necessary work by himself, but soon recognized the actual extent of his project and hired two assistants, Karl Glaser and Wilhelm Körner, both trained at Giessen. All the activity of the laboratory was for some time thereafter concentrated upon the trained at Giessen. All the activity of the laboratory was for some time thereafter concentrated upon the derivatives of benzene and their isomers, but Kekulé still did not publish his theory. It was only after Tollens and Fitting brought out their excellent work on the synthesis of the hydrocarbons of the benzene series that he decided to make his own work known. Thus Wurtz presented Kekulé’s benzene theory to the Société chimique de Paris on 27 January 1865, in a session presided over by Pasteur. It was subsequently published in the Bulletin de la Société chimique de Paris under the title “Sur la constitution des substances aromatiques,” and concluded with a table of formulas for benzene and similar compounds.

On 11 May 1865, Kekulé presented to the Académie associate member, a “Note sur quelques produits de substitution de la benzine,” in which he considered the geometry of the benzene nucleus and used it to determine the number of its possible monosubstituted, disubstituted, and trisubstituted isomeric derivatives. He and his associates then set out to prove these figures experimentally, and succeeded after several years’ work. They found the most diverse substituents to be those fixed on the ring or onto the lateral chains—namely the halogens and the NO₂, NH₂, diazo, CO₂ H, SO₃ H, OH and SH groups—and attempted to localize these substituents in each of the benzene isomers. On 3 August 1867, Kekulé presented to the Academy a remarkable work on this subject by Körner, “Faits pour servir à la détermination du lieu chimique dans la série aromatique.” (Körner himself stated his “absolute” method, which provided an elegant means for establishing unambiguously the ortho, meta, and para positions of the disubstituted derivatives of benzene, some seven years later.)

In addition to his work on the structure of aromatic substances, beginning in 1865, Kekulé took up the study of their azo and diazo derivatives. He began this research with a view toward incorporating the results of it in the second volume of his Lehrbuch; in addition, the subject had assumed considerable industrial importance once the potential of the intermolecular transformation of diazobenzene into aminobenzene became known. In 1866 Kekulé provided a masterful interpretation of this transformation and of the catalytic role of the aniline salts, drawing upon his new theories of the constitution of the diazo group and its mode of fixation on the benzene ring. In his wonderful researches on diazocompounds Griess prepared a new compound, called phenylendisulfuric acid, formed by interaction of concentrated sulfuric acid and diazobenzensulfate. kekulé’s interest was aroused and he proved theoretically as well as experimentally that the product was in reality a disulfonic derivative of phenol. He turned then to the study of the sulfonic derivatives of phenol and was able to clarify the double mode of action of sulfuric acid on organic matter, showing that it produces both readily decomposable sulfuric esters and highly stable sulfonic derviatives; he further emphasized the striking analogy in this respect between the sulfonyl and carbonyl groups fixed on the benzene ring..

Kekulé also discovered that sulfonic derivatives of benzene fuse with potash to create their corresponding phenols. This discovery was to become important in the industrial production of phenols. In a variation of an earlier experiment, made in London, in which he used phosphorus pentasulfide, kekulé succeeded in transforming phenol into thiophenol by substituting sulfur for the oxygen of the former. he demonstrated thereby that the oxygen of phenol is more strongly bonded to the carbon of the benzene ring than to the OH group of the fatty alcohols.

Throughout this strenuous period of research Kekulé did not neglect his teaching duties. (In 1867, for example, he published and recommended as a teaching aid a new model of the carbon atom.) But he wished to be able to teach in German again, and when he was offered the chair of chemistry at the university of Bonn—vacant since A. W. Hofmann had gone to Berlin—he accepted it gladly. He was additionally assured the directorship of a new chemical institute, the construction of which was virtually complete. In September 1867, the Blegian government accepted Kekulé’s resignation and he left Ghent for Bonn.

The new chemical institute was officially opened in 1868; the inaugural ceremonies coincided with those in celebration of the fiftieth anniversary of the university itself. Kekulé was awarded an honorary M.D. on this occasion in recognition of his contributions to theoretical chemistry. Many students were drawn to Bonn to hear his lectures and observe his class experiments and laboratory work; one of them, in 1873, was J. H. van’t Hoff, to whom kekulé’s model of the carbon atom suggested the concept of the asymmetric carbon atom of his La chimie Bans l’espace of 1875. Other students became kekulé’s direct collaborators, among them Theodor Zincke (in work on condensation of aldehydes), Hermann Wichelhaus— who had followed him from Ghent—and Thomas Edward Thorpe (on aromatic compounds), Nicolas Franchimont (on triphenylmethane and anthraquinone), Otto Strecker (on the constitution of benzene), and Richard Anschütz (on oxyderivatives of fumaric and maleic acids).

At Bonn, Kekulé found it necessary to delegate some of his teaching responsibilities to others in order to concentrate on his own research, in which he had the aid of several private assistants. his first projects were continuations of work he had begun in Ghent; he resumed his study of the sulfonic derivatives of phenol and nitrophenol, and extended his earlier investigations of camphor and oil of turpentine to include cymol, thymol, and carvacrol (the latter work was completed in 1874). More important, however, was the resumption of his attempt to provide experimental evidence for his benzene theory, particularly for the presence in the ring of three alternating souble bonds. Having observed that trimethylbenzene is formed through the condensation of three acetone molecules, Kekulé hoped to synthesize the benzene ring through the condensation of aldehyde. he was unable to obtain such a synthesis; his attempts to do so, however, resulted in an elegant series of works (published between 1869 and 1872) on the condensation of acetaldehyde. These studies treat the formation of crotonaldehyde and some of its derivative products, as well as dealing with polymerization products of aldehyde.

Kekulé also wished to demonstrate the superiority of his own formula for benzene over those put forth by A. claus, H. Wichelhaus, and A. Ladenburg. By 1872 he had created the complementary “oscillation theory,” which took into account the existence of only one bisubstituted derivative in the ortho position, rather than two. He thus permitted the delocalization of single and double bonds, which he had considered to be fixed in his earlier theory.

In the same year Kekulé and Franchimont succeeded in synthesizing triphenylmethane, the fundamental hydrocarbon in rosaniline dyes, and also obtained anthraquinone in the course of preparing benzophenone. The elucidation of the structure of these compounds proved crucial to the development of synthetic dyes; the subsequent rapid growth of the German aniline dye industry, based on the triphenylmethane group and authraquinone, was its direct result.

The growing number of Kekulé’s students and co-workersc soon necessitated an expansion of the chemical institute, to which a number of new workrooms were added in 1874 and 1875. During this period Kekulé was offered the chair of chemistry at the University of Munich, which had become vacant with the death of Liebig; he declined the post, however, and recommended Adolph von Baeyer in his stead. At the same time, his health had begun to fail. Twenty years of overexertion had begun to take their toll, as had an unfortunate second marriage to his former housekeeper, a woman much younger than he, who was incapable of relieving him of his cares. A month after this marriage, too, Kekulé contracted measles from his son and suffered prolonged aftereffects. he nonetheless continued to serve the university, being elected rector in 1877, on which occasion he gave an address on the scientific goals and accomplishments of chemistry. upon completion of his office the following year, he spoke upon the principles of higher education and educational reform.

At about the same time Kekulé resumed work on the Lehrbuch, in collaboration with Gustav Schultz, Richard Anschütz, and, slightly later, Wilhelm la Coste; but the rapid growth of chemistry at that time did not allow them to maintain the original plan of composition, and the work was never completed. Although volume III appeared in 1882, volume IV, published in 1887, consisted of only one of the planned sections. From 1879 to 1885 Kekulé also engaged in research, primarily experiments designed to support his own benzene theory against the prismatic formula advocated by August Ladenburg. Ludwig Barth had become a partisan of the latter theory, arguing from the formation of carboxytartronic acid from pyrocatechol. In 1883 Kekulé was able to show that this acid is simply tetraoxysuccinic acid, the formation of which from pyrocatechol was better explanined by his own hexagonal theory. In a series of investigations on trichlorophenomalic acid made with Otto Strecker in the following year, Kekulé again corroborated his own thesis and confirmed the superiority of his own formula, which provided an atom-by-atom explanation of the formation of β-trichloracetylarcrylic acid through the oxydochlorination of quinone.

A high point in Kekulé’s career occurred in 1890, when he read his paper “Ueber die Konstitutionen des Pyridins” to the general assembly of the Deutsche Chemische Gesellschaft in Berlin on 10 March. The communication summed up the investigations on pyridine, of which the formula is comparable to that of benzene, that he had carried out since 1886. The day after this presentation, a great celebration was held to honor Kekulé on the occasion of the twenty-fifth anniversary of his benzene theory. kekulé in thanks, gave a remarkable speech in which he reviewed his life’s work and made public for the first time the details of his visionary solution of the benzene ring.

Although he had grown deaf by this time, Kekulé continued to teach and carry out administrative duties. In 1892 he also prepared formic aldehyde in the pure state, thus extending his earlier work on the condensation of the aldehydes. His health was again seriously impaired following an attack of influenza, and he died shortly thereafter. He was buried in the family vault in the cemetery of Poppelsdorf; a bronze statue of him, paid for largely by subscription from the German dyestuffs industry, was erected, facing his chemical institute, in 1903.

BIBLIOGRAPHY

Kekulé’s works were collected, with a biography, by Richard Anschütz to honor him at the centenary of his birth: August Kekulé, I, leben und Wirken, II, Abhandlungen, Berichte, kritiken, Artikel, Reden (Berlin, 1929).

Secondary literature includes G. V. Bykov, August Kekulé (Moscow, 1964), in Russian; j. Gillis, “Auguste Kekulé et son oeuvre, réalisée à Gand de 1858 à 1867,” in Mémoires de I’ Académie royale de Belgique. Classe des sciences,37 (1966), 1–40; Francis R. Japp, “Kekulé Memorial Lecture,” in Journal of the Chemical Society, 73 (1898), 97–138; and R. Wizinger-Aust et al., Kekulé und seine Benzolformel. . . (Weinheim, 1966).

Jean Gillis

KEKULé VON STRADONITZ (KEKULé), (FRIEDRICH) AUGUST

(b. Darmstadt, Germany, 7 September 1829; d. Bonn, 13 July 1896),

chemistry, benzene ring. For the original article on KekulØ see DSB, vol. 7.

KekulØ’s theoretical insights into the classification of organic compounds, valence and structural chemistry, and his suggestion of the hexagonal formula for benzene transformed the subject of organic chemistry. In his great paper of 1858, “On the Constitution and Metamorphoses of Chemical Compounds and on the Chemical Nature of Carbon,” in which he extended the quadrivalence of carbon in methane to all of its compounds, Kekulé stressed how indebted he was to the English and French schools of chemistry for his interpretation. With hindsight it can be seen that at one stroke organic chemistry had been unified: chemists no longer needed to separate “types” for paraffins, ethers, and amines; all organic compounds were now embraced within the idea of carbon chains (catenation) and the notion of carbon’s tetravalence.

Pathway to Insight . Historians have debated why this insight occurred to Kekulé. Alan J. Rocke has suggested that the concept of structure (application of valence rules to the supposed construction of molecules) probably owed much to the work of Adolphe Wurtz. In his influential Méthode de chimie (1854), Auguste Laurent had speculated that atoms might be divisible in order to explain why, for example, iron had both odd and even powers of combination to form ferrous and ferric salts. At the same time, Alexander Williamson and William Odling were developing the idea of double, triple, and mixed “types” in which one dibasic (diatomic) molecule linked together two monobasic radicals. In 1855, following Laurent’s hint, Wurtz wondered whether oxygen was dibasic and

nitrogen tribasic because these elements were formed from two or three juxtaposed subatoms. According to this concept, polyvalent atoms were really aggregates of monova-lent subatoms. This, as Rocke has pointed out, conforms to the later textbook rule that an element’s chemical equivalent is “atomic weight divided by valence” and is still reflected in the definition of atomicity as the number of atoms in a molecule (rather than of subatoms in an atom).

Kekulé’s reverie on a London omnibus (recounted in 1890 and occurring, if it was a real event, probably in 1855) therefore may have involved segmented wormlike entities made up from subatoms. The sausage-shaped graphic formulas that he first used in lectures in Heidelberg in 1857 followed from this model and seem confirmed by Kekulé’s statement in 1867 that “polyvalent atoms, with respect to their chemical value [valence], can be viewed in a sense as a conglomeration of several monovalent atoms” (Kekulé, 1867, p. 217). The graphic formulas that Kekulé used in his Lehrbuch der organischen Chemie (1859–1887) were visualizations of Wurtz’s speculation, though he continued to use type formulas as a means of classification. It was Alexander Crum Brown and Edward Frankland who slowly directed the chemical community toward graphic formulas in which the “carbon chain” property was made explicit. The heuristic nature of Kekulé’s formulas may have been responsible for his refusal to accept that valence could vary. Although he was proved wrong, nevertheless, it was his conviction that carbon’s valence was invariable that logically led him to posit the carbon chain, double and triple bonding, and the benzene ring (1865).

Benzene Ring and “Kekulé’s Dreams.” . No one can be certain exactly when Kekulé hit upon the structure of benzene as a closed chain of six carbon atoms, though it probably occurred to him around the time of his first marriage in 1862. Toward the end of his life, in typical self-deprecatory fashion, he said that, in a dream, he had imagined a chain of dancing carbon atoms forming a closed circle, like a snake eating its own tail. Since the 1970s, provoked principally by the Czech American chemist and historian John H. Wotiz (1919–2001), historians have debated the validity and nature of Kekulé’s two “dreams” on historical and chemical grounds and whether the stories they told were a cloak to disguise the fact that Kekulé was less original than he claimed. These reinterpretations of Kekulé’s character and reputation led to resentments between chemist-historians and historians of chemistry that culminated in 1995 in a complicated legal action that affected a national society and brought ad hominem restrictions on a few historians’ freedom of action (Wotiz, 1993, pp. 108–110).

Historians of chemistry have tended to stress the slow evolutionary continuity of Kekulé’s work in organic chemistry rather than the sudden emergence of particular insights. They have also stressed the importance of the careful examination of “context of use” when considering whether or not Kekulé was indebted to Laurent, Archibald Scott Couper, and Frankland for ideas concerning constitutional formulas, or Aleksandr Butlerov for the concept of chemical structure, or to Laurent, Albert Ladenburg, and others for the hexagonal formula of benzene. This aspect of historiography has not always been appreciated by chemist-historians. Another debated issue has been whether Kekulé’s benzene formula was indebted to the graphic circle (or ball) formula that Josef Loschmidt privately published in his Chemische Studien in 1861. Rocke and Schiemenz have argued plausibly that Loschmidt was portraying benzene as an indeterminate hexavalent superatom using Kekulé’s previously published concept of quadrivalent carbon catenation, rather than trying to represent six individual carbon atoms connected in a hexagon. It must also be recognized that the “Benzolfest” of 1890 at which Kekulé recounted his reveries was designed less to honor Kekulé than to impress participants and readers of the central economic significance of chemistry for the Reich. Consequently, accounts of the speeches have to be used with caution as historical texts.

Later Years . Despite his declining powers, lethargy, and fixation with family history and ennoblement, which Hermann Kolbe exploited in inexorable criticism of Kekulé’s structure theory, Kekulé did make some significant achievements at Bonn. This included, in 1872, the daring dynamic oscillation formula for benzene that explained the embarrassing lack of isomeric disubstituted derivatives in benzene that seemed otherwise possible. In the early 1880s, when several alternative possibilities had been touted, including the prism formula proposed by Laden-burg, Kekulé demonstrated a return of his old powers when he showed that a series of experimental transformations of pyrocatechol and quinone into dioxytartaric acid and trichloracetoacrylic acid were best and most simply explained if benzene was hexagonal.

While it is convenient to suppose that it was Kekulé’s architectural training that helped him conceive molecular structure and to play with molecular models, what is more striking is the view he acquired from Williamson of the dynamic nature of molecules. Architecture is essentially static, whereas Kekulé’s conception of structure was much more fluid and imprecise. To that end, the visionary giddy molecular dances of his Benzolfest address in 1890 ring true.

SUPPLEMENTARY BIBLIOGRAPHY

Kekulé’s principal archives are housed in the Kekulé Sammlung, Institut für Organische Chemie, Technische Hochschule Darmstadt, Petersenstrasse 22, and the University of Ghent, Belgium. The Museum of Science and Technology at Ghent, Korte Meer 9, displays Kekulé’s apparatus and models.

WORKS BY KEKULÉ

“Über die Constitution des Mesitylens.” Zeitschrift für Chemie 10 (1867): 214–218.

With Justus von Liebig. Liebigs Experimentalvorlesung: Vorlesungsbuch und Kekulés Mitschrift. Edited by Otto Paul Krätz and Claus Priesner. Weinheim, Germany: Verlag Chemie, 1983. Facsimiles and transcriptions of Liebig’s Giessen lectures on organic chemistry, together with Kekulé’s student notes of 1848 and valuable commentaries.

OTHER SOURCES

Brock, William H. “August Kekulé (1829–96): Theoretical Chemist.” Endeavour 20, no. 3 (1996): 121–125.

Brooke, John H. “Doing Down the Frenchies: How Much Credit Should Kekulé Have Given?” In The Kekulé Riddle: A Challenge for Chemists and Psychologists, edited by John H. Wotiz, 59–76. Clearwater, FL: Cache River Press, 1993.

Brush, Stephen G. “Dynamics of Theory Change in Chemistry, Part 1: The Benzene Problem, 1865–1945” and “Part 2: Benzene and Molecular Orbitals, 1945–1980.” Studies in History and Philosophy of Science 30, no. 1 (1999): 21–79; 30, no. 2 (1999): 263–302.

Fisher, Nicholas W. “Kekulé and Organic Classification.” Ambix 21 (1974): 29–52.

Göbel, Wolfgang. Friedrich August Kekulé. Leipzig, Germany: Teubner, 1984.

Kauffman, George B. “Werner, Kekulé, and the Demise of the Doctrine of Constant Valency.” Journal of Chemical Education 49 (1972): 813–817.

Meinel, Christoph. “Molecules and Croquet Balls.” In Models: The Third Dimension of Science, edited by Soraya de Chadarevian and Nick Hopwood, 242–275. Stanford, CA: Stanford University Press, 2004.

Noe, Christian R., and Alfred Bader. “Facts Are Better Than Dreams.” Chemistry in Britain 29 (February 1993): 126–128.

Paoloni, Leonello. “Stereochemical Models of Benzene, 1869–1875.” Bulletin for the History of Chemistry 12 (1992): 10–23.

Ramsay, O. Bertrand, and Alan J. Rocke. “Kekulé’s Dreams: Separating the Fiction from the Fact.” Chemistry in Britain 20 (December 1984): 1093–1094.

Rocke, Alan J. “Kekulé, Butlerov, and the Historiography of the Theory of Chemical Structure.” British Journal for the History of Science 14 (1981): 27–57.

———. “Subatomic Speculations and the Origin of Structure Theory.” Ambix 30 (1983): 1–18.

———. “Hypothesis and Experiment in the Early Development of Kekulé’s Benzene Theory.” Annals of Science 42 (1985): 355–381.

———. “Kekulé’s Benzene Theory and the Appraisal of Scientific Theories.” In Scrutinizing Science: Empirical Studies of Scientific Change, edited by Arthur Donovan, Larry Laudan, and Rachel Laudan, 145–161. Dordrecht, Netherlands, and Boston: Kluwer, 1988.

———. “Waking Up to the Facts?” Chemistry in Britain (May 1993): 401–402. A reply to Noe and Bader.

Rudofsky, Susanna F., and John H. Wotiz. “Psychologists and the Dream Accounts of August Kekulé.” Ambix 35 (1988): 31–38.

Russell, Colin A. “Kekulé and Frankland: A Psychological Puzzle?” In The Kekulé Riddle: A Challenge for Chemists and Psychologists, edited by John H. Wotiz, 78–101. Clearwater, FL: Cache River Press, 1993.

Schiemenz, Günter P. “Albert Ladenburg und die Kekuléformel des Benzols.” Mitteilungen der GDCh Fachgruppe Geschichte der Chemie 1 (1988): 51–69. A revised English version is in The Kekulé Riddle: A Challenge for Chemists and Psychologists, edited by John H. Wotiz, 104–122. Clearwater, FL: Cache River Press, 1993.

———. “Goodbye, Kekulé? Josef Loschmidt und die monocyclische Struktur des Benzols.” Naturwissenschaftliche Rundschau 46 (1993): 85–88.

———. “A Heretical Look at the Benzolfest.” British Journal for the History of Science 26 (1993): 195–205. Article suppressed from Wotiz (1993) because of views on Loschmidt.

Seltzer, Richard J. “Influence of Kekulé Dream on Benzene Structure Disputed.” Chemistry & Engineering News, 4 November 1985, pp. 22–23. A good account of historians’ dispute followed by spirited correspondence lasting three months.

Wotiz, John H. Interview by Herbert T. Pratt at Newcastle, Delaware, and Washington, DC, 7, 8, and 10 August 2000. Philadelphia: Chemical Heritage Foundation, Oral History Transcript #0197.

———, ed. The Kekulé Riddle: A Challenge for Chemists and Psychologists. Clearwater, FL: Cache River Press, 1993. Contains seventeen essays by chemists, historians, and psychologists.

Wotiz, John H., and Susanna Rudofsky. “Kekulé’s Dream: Fact or Fiction? Chemistry in Britain 20 (August 1984): 720–723.

———. “The Unknown Kekulé.” In Essays on the History of Organic Chemistry, edited by James G. Traynham, 21–34. Baton Rouge: Louisiana State University Press, 1987. Preliminary claim of Kekulé’s fraudulence.

———. “Louis Pasteur, August Kekulé, and the Franco-Prussian War.” Journal of Chemical Education 66 (1989): 34–36.

W. H. Brock