(b. Nara, Japan, 4 October 1918; d. Kyoto, Japan, 9 January 1998),
physical organic chemistry, chemical theory.
One of the great theoretical chemists of the twentieth century, Ken’ichi Fukui was Japan’s first recipient of the Nobel Prize in Chemistry for his theory of chemical reactions. Solidly based in quantam mechanics, Fukui’s work and that of Roald Hoffmann, with whom he shared the award, enhanced the ability of chemists to predict the course of chemical reactions. Significant applications followed in medicine and pharmacology.
Family Background . Ken’ichi Fukui was born in Nara, Japan on 4 October 1918. He was one of three brothers. His family, though not wealthy, was solidly middle class and well educated for the time. Both parents were formative influences on the chemist’s life. Fukui’s father, Ryokichi, was a graduate of the Tokyo institution that became Hitotsubashi University. Working in management for much of his career, Ryokichi Fukui at one point worked for a British export-import firm and spent a year in Europe during the mid-1920s. Ryokichi had a good command of English and was an ardent reader of National Geographic magazine. Ken’ichi Fukui often said that this publication was a significant influence on what ultimately became his deep interest in science because of its many articles on insects, plants, and other natural phenomena. The chemist’s mother, Chie Sugisawa Fukui, graduated from high school—the Nara High School for Women— at a time when few Japanese women did so. Never inclined to push her children in a particular direction, her approach was rather to introduce books and subjects to Fukui and his brothers and encourage them to cultivate their own interests. From his mother Fukui acquired a deep interest in the literary works of Natsume Soseki, Japan’s greatest writer of the early twentieth century and coincidentally an intellectual with a considerable interest in science.
Education . Fukui’s path into chemistry was by no means predictable. He was not a dedicated student but preferred hiking, fishing, the martial art of Kendo, and the board game of Go to classroom work. Chemistry in particular struck him as boring because it appeared to be based on endless memorization. But he did well in mathematics with very little studying, was active in the biology club, and as a middle school student discovered the writings of the French entomologist and chemist Jean Henri Fabre (1823–1915). It seems to have been the life and work of Fabre that ultimately led Fukui to chemistry. He was impressed as a youth with the natural world of plants and animals described by Fabre, in which he himself was actually living. And Fukui found Fabre’s development of alizarine dye, plus his courage to go ahead after commercial failure in its production, both intellectually stimulating and morally inspiring.
Given Fukui’s talent for mathematics and his general love of nature, the most likely course might have been for him to enroll in the Faculty of Science at Kyoto University, where he ultimately matriculated. Instead, Fukui enrolled in the university’s faculty of engineering on the advice of his father’s cousin, Gen’itsu Kita, an engineering professor who had studied at the Massachusetts Institute of Technology and the Pasteur Institute. Fukui’s enrollment in engineering proved a fateful decision. Overwhelmingly practical in its pedagogical and research orientation, the faculty of engineering seemed an inhospitable environment with its emphasis on ceramics, synthetic textiles, synthetic rubber, resins, petroleum chemistry and other applied subjects. However, Kita recommended it on the basis of Fukui’s mathematical talent and general orientation; and his emphasis on pursuing “basic studies”—which Kita never defined—and building on basic knowledge would in fact serve Fukui well.
A graduation thesis project in late 1940 under the direction of a new assistant professor, Haruo Shingu, proved to be the first step in arousing Fukui’s interest in a line of work that would sustain his entire career. Shingu assigned Fukui to study the Schaarschmidt reaction in hydrocarbon chemistry. Alfred Schaarschmidt had described the reaction patterns of hydrogen atoms attached to various carbon chains; and Fukui had to analyze the iso-paraffin in a hydrocarbon mixture by conducting various experiments. These tests were designed to ascertain slight differences in how the carbon atoms and hydrogen atoms in various paraffin hydrocarbons combine with each other and how the nature of these reactions differ from those involving antimony pentachloride. What he observed was that even when the chemical structures of two hydrocarbon compounds were very similar, they showed very different reaction patterns compared to the antimony pentachloride. He became interested in subtle differences in the reactivity of hydrocarbons like paraffin, benzene, or naphthalene. This work, in fact, elicited his long-term interest in the chemical reactions of pure hydrocarbon molecules. These investigations in the faculty of engineering prepared him well, in crucial respects, for the work in chemical reaction theory that led to his Nobel Prize.
But an engineering education was not sufficient by itself. Although he ably performed such chemical experiments, Fukui did not particularly enjoy this aspect of chemistry. He was interested, as always, in mathematics and even as an undergraduate spent considerable time attending physics lectures and consulting the library of the nearby faculty of science. Both were open to engineering students, and there Fukui found the Handbuch der Physik, works by Richard Courant and David Hilbert, and various texts on quantum mechanics that he considered essential to his education. On one occasion he wrote that he had agreed, following the advice of Gen’itsu Kita, to study applied chemistry on the presumption that it would gradually become a more theoretically and mathematically based field of study. In the spring of 1941, Fukui completed his undergraduate degree and began graduate study in the Department of Fuel Chemistry under the direction of Shinjiro Kodama.
Kodama was a protégé of Kita, who had studied in Germany during the late 1920s and who saw the future of chemistry much as Kita had done. At Kyoto Kodama was at first professor of hydrocarbon physical chemistry, then professor of high temperature chemistry. The combination of applied studies with the emphasis on theory under Kodama’s direction was ideal for Fukui. Often leaving Fukui to his own devices, Kodama lent him books on quantum mechanics, physics, and electromagnetism that were otherwise difficult to obtain in Japan. And in the late summer of 1941 when Fukui became subject to the military draft, Kodama arranged for him to work part time at the Imperial Army Fuels Research Laboratory at Fuchu near Tokyo.
Early Career . What might well have been a hardship assignment became a significant opportunity. The Fuchu laboratory held an extensive collection of books on subjects of interest to Fukui, such as Ralph Howard Fowler’s textbook on statistical mechanics and texts on quantum mechanics and hydrocarbon chemistry. Moreover, his assignment and that of his team played to his strengths and interests. Their task was to find a technique for raising the octane level of ordinary gasoline (or a substitute) so as to render it suitable for use in airplanes. Branched chain hydrocarbons such as 2,2,4-trimethylpentane were not available in Japan. Instead the Fukui group fermented sugar to obtain butyl alcohol, which was then used as a substitute for the trimethylpentane. Although their fuel additive actually worked in airplanes, it was never produced in volume; nonetheless, in 1944 Fukui and his team received an Order of Technical Merit from the army.
Although Fukui had formally launched his academic career in 1943 when the university made him a lecturer in the Department of Fuel Chemistry, it was his 1945 appointment as an assistant professor—together with the end of the war—that enabled him to pursue his academic interests full time. Interested in chemical reaction engineering, Kodama had been trying to produce polyethylene, a synthetic polymer, by high pressurization technology. When this senior colleague left the university for private industry, Fukui took over the project and produced one of his first publications. He also took up another of Kodama’s lines of inquiry—what temperature distributions or reaction conditions were most efficacious for producing chemical reactions in chemical factories— and made it the subject of his doctoral dissertation. Fukui received his doctoral degree, the DScEngr, in June 1948.
Chemical Reaction Theory . Fukui’s career took a major turn in 1951. In that year he was advanced to the rank of full professor at the age of thirty-three. Because of the power inherent in the senior occupant of a university chair in the Japanese system, he was able—while continuing to teach various applied subjects—to gradually shift the focus of his laboratory away from applied research and more toward chemical theory. Not coincidentally, 1951 was also the year in which he began to formulate his path-breaking theory of chemical reactions. When Fukui began this work, the reigning paradigm was the so-called electronic theory of organic chemistry associated with the English chemists Robert Robinson and Christopher Keld Ingold. First published in 1926, this theory held that a chemical reaction could easily occur in one of two situations: in response to a positively charged electrophilic reagent at a site where the electron density is high, or in response to a negatively charged nucleophilic reagent at a site of low electron density. Fukui accepted electrons as the crucial element in chemical reactions but considered the Robinson-Ingold formulation inadequate at worst and inelegant at best. Moreover, it was not based on quantum mechanics, an approach to theorizing that Robinson in particular had largely disdained. Because of his own extensive investigations of hydrocarbons both in Kyoto’s Department of Fuel Chemistry and at the army’s laboratory in Fuchu, Fukui realized that this formulation simply did not work for many hydrocarbon chemical reactions. On the contrary, his own experiments had shown that aromatic hydrocarbons such as naphthalene could undergo reactions at the same site, both in the presence of reagents which accept electrons and in the presence of reagents that contribute them.
Fukui was strongly influenced in his theoretical work by the investigations of Robert S. Mulliken, who had earlier adopted Erwin Schrödinger’s view of the electron as a wave that spread out in so-called orbitals over the atoms in a chemical bond. Both chemists defined an orbital not as a physical entity or path, but rather as a mathematical function referring to the movements of electrons within an atom or molecule. Mulliken’s analyses were not directed to the rearrangement of chemical bonds in chemical reactions, as were Fukui's. Nonetheless, the University of Chicago chemist’s focus on electric charge transference between molecules did involve the delocalization of electrons in such encounters. For this reason his work provided a valuable context for Fukui’s discussion.
This was the perspective from which Fukui developed his frontier orbitals theory of chemical reactions. Using quantum mechanics he began by investigating naphthalene, largely because of its more or less regular shape. He did not focus on all the orbitals of its electrons but only the orbital with the highest energy level. From this initial inquiry he then investigated anthracene, pyrene, perylene, and other aromatic hydrocarbons one by one, but found the inquiry tedious and time-consuming because of the irregular shapes of their molecules. Teijiro Yonezawa, a special research student at the time but later professor of molecular engineering at Kyoto, cooperated with the task. Haruo Shingu, who had been the first to interest Fukui in hydrocarbon chemistry back in 1940, was also involved in the project. Late in 1951 they submitted their first major paper from the new project to the Journal of Chemical Physics, published by the American Institute of Physics. After minor revisions it appeared in the journal’s April 1952 issue.
This 1952 paper did not attempt to offer a complete explanation of the new theory; in fact, even the theoretical nomenclature was evolving. The principal focus in 1952 was on the molecule that contributes electrons in a chemical reaction. The argument was that when two molecules exert a mutual influence on each other, the molecule contributing electrons gives a high-energy— so-called frontier—electron to the other molecule. In a subsequent 1954 article published in the same journal, the focus was on the molecule that receives the electron or electrons. In this case, reception took place in the unoccupied orbital of the lowest energy. With respect to the location of the reaction, Fukui and his colleagues argued that this occurs at a specific site within the expanse of the unoccupied orbital. This orbital became known as the lowest unoccupied molecular orbital (LUMO), while its occupied counterpart came to be known as the highest occupied molecular orbital (HOMO). Over the decade of the 1950s, Fukui and his colleagues demonstrated that the site and rate of a chemical reaction depends on the geometries and relative energies of the HOMO of one reactant and the LUMO of the other.
Reception of the Frontier Orbitals Theory . Presenting a theory was one thing, winning wide acceptance another. One early source of support was Mulliken himself. In 1953 Tokyo University hosted Japan’s first big international scientific meeting after World War II, the International Symposium on Molecular Structure and Spectroscopy. Mulliken attended the meeting and referred approvingly to Fukui’s 1952 paper. Fukui was also present and made other valuable contacts, including Per-Olov Löwdin of the University of Uppsala. In general, the meeting, together with Mulliken’s lecture and endorsement of Fukui’s approach, gave a significant boost to the frontier orbitals theory of chemical reactions. But there was still considerable opposition. Robinson was a Nobel laureate, and many chemists were reluctant to accept a theory of chemical reactions so thoroughly derived from physics and quantum mechanics. Did the new theory account for an experimentally verified, regularity of chemical reactions known as the Hammett rule? Fukui was able to show that it did so; moreover, he and his collaborators demonstrated through the course of the 1950s that the frontier orbitals approach explained all of the reactions explicable by the electronic theory as well as the hydrocarbon reactions that Robinson’s theory could not explain.
In the 1960s Fukui’s work slowly gained wider recognition both in Japan and the rest of the world. In 1962 he received the Japan Academy Prize for “research on chemical reactions and the electron states of conjugate compounds,” and in 1964 he was invited to a major meeting of chemists in Florida organized by Löwdin. There he met Roald Hoffmann, with whom he would later share the Nobel Prize. As a junior fellow at Harvard University, Hoffmann and the senior chemist and Nobel laureate Robert B. Woodward published, in 1965, what came to be called the Woodward-Hoffmann Rule. Their observations linked experimental results with the phenomenon of orbitals, highlighting the importance of orbital symmetry control in chemical reactions. Together with another 1965 paper by Hoffmann on stereo-selectivity, their work called additional attention to Fukui’s investigations on the importance of frontier orbital electrons in chemical reactions.
In 1970 Fukui spent six months as a visiting scholar at the Illinois Institute of Technology in Chicago with support from the U.S. National Science Foundation. During this period he wrote two major papers on chemical reaction theory. One dealt with the concept of orbital symmetry control developed by Hoffmann and Woodward as a way to demonstrate the greater breadth of the frontier orbitals theory as compared to the older electronic theory. His other paper applied perturbation theory to some original qualitative notions, showing that as perturbations distort the HOMO of one reactant and the LUMO of the other, they affect the energetics of various reaction pathways. By lecturing during this period at a number of American research universities while visiting corporate laboratories, Fukui gradually became better known in the United States. In 1981 he was named a member of the National Academy of Sciences.
Other awards followed, including the Order of Cultural Merit from Japan (1981), membership in the Pontifical Academy of Sciences in Rome (1985), and the 1981 Nobel Prize for Chemistry, together with Hoffmann. Following his retirement from Kyoto University, Fukui was named director of the Institute for Fundamental Chemistry in Kyoto, created with funds from the Kao Soap Corporation. As Japan’s first Nobel laureate in chemistry, Fukui attracted a great deal of public attention and was named chair of a committee to organize the twelve-hundredth anniversary celebration of Kyoto’s founding. On 9 January 1998 Fukui died in Kyoto of peritoneal cancer, survived by his wife Tomoe Horie Fukui, whom he married in 1947, and their two grown children.
WORKS BY FUKUI
With Teijiro Yonezawa and Haruo Shingu. “A Molecular Orbital Theory of Reactivity in Aromatic Hydrocarbons.” Journal of Chemical Physics 20 (1952): 722–725.
With Teijiro Yonezawa, Chikayoshi Nagata, et al. “Molecular Orbital Theory of Orientation in Aromatic, Heteroaromatic and Other Conjugated Molecules.” Journal of Chemical Physics 22 (1954): 1433–1442.
With Chikayoshi Nagata, Teijiro Yonezawa, et al. “Novel Perturbation Theory in Simple LCAO Treatment of Conjugated Molecules—Method of Perturbed Secular Determinant.” Journal of Chemical Physics 31 (1959): 287–293.
With T. Teijiro Yonezawa and Chikayoshi Nagata. “Reply to the Comments on the ‘Frontier Electron Theory.’” Journal of Chemical Physics 31 (1959): 550–551.
Hiroshi Fujimoto, Shigeki Kato, et al. “Orbital Symmetry Control in the Interaction of Three Systems.” Bulletin of the Chemical Society of Japan 46 (1973): 1071–1076.
Hiroshi Fujimoto, Morio Miyagi, et al. “On the MO Perturbation Theory of Molecular Rearrangements.” Bulletin of the Chemical Society of Japan 46 (1973): 1357–1361.
“A Simple Quantum-Theoretical Interpretation of the Chemical Reactivity of Organic Compounds.” In Molecular Orbitals in Chemistry, Physics, and Biology: A Tribute to Robert S. Mulliken, edited by Per-Olov Löwdin and Bernard Pullman, 513–537. New York: Academic Press, 1964.
Kagaku to Watakushi: Noberusho Kagakusha Fukui Ken’ichi. Edited by Yamabe Tokio. Kyoto, Japan: Kagaku Dojin, 1982.
Gakumon no Sozo. Tokyo: Kosei Shuppansha, 1984.
Davis, Scott. “Kenichi Fukui, 1981.” In The Nobel Prize Winners: Chemistry, vol. 3, edited by Frank N. Magill, 1061–1066. Pasadena, CA: Salem Press, 1990.
Hoffmann, Roald. “Obituary: Kenichi Fukui (1918–98).” Nature 391 (1998): 750.
James R. Bartholomew