Kenichi Fukui

Japanese chemist

• Born: October 4, 1918; Nara City, Japan
• Died: January 9, 1998; Kyoto, Japan

Kenichi Fukui was a pioneer in the field of theoretical chemistry. His theory of frontier orbitals, a term he coined in the early 1950s, explains why different types of molecules demonstrate varying levels of chemical reactivity.

Primary field: Chemistry

Specialties: Biochemistry; molecular biology

Early Life

Kenichi Fukui was born on October 4, 1918, in Nara City, Japan. His father, Ryokichi Fukui, worked as a factory manager and merchant. The Fukuis had three sons, of whom Kenichi was the eldest. After graduating from high school, Fukui had to make a decision about which course of study he would pursue in university, as college students in Japan were required to choose a concentration before beginning undergraduate studies. Fukui’s father consulted with his friend Professor Genitsu Kita, who suggested that the younger Fukui should apply to the Department of Industrial Chemistry at Kyoto Imperial University, a department that Kita himself had founded. Kita became a lifelong mentor of Fukui’s.

In 1938, Fukui began his undergraduate career at Kyoto Imperial University, where he remained until his graduation in 1941. Although he focused on industrial chemistry (the application of chemistry to problems in manufacturing), he also developed an interest in quantum mechanics, which is the realm of physics that focuses on the very smallest particles and seeks to explain the fundamental characteristics of the universe.

Life’s Work

When Fukui graduated from university, Japan was in the midst of World War II. He began working with the Japanese military, conducting experimental research on synthetic fuels. In 1943, Fukui took a position as a lecturer at Kyoto Imperial University, where he taught classes in the chemistry of fuel and also began doing work in experimental organic chemistry. Organic chemistry focuses on the structure and characteristics of chemical compounds that contain the element carbon. Fukui progressed through the ranks at Kyoto University, being promoted to assistant professor in 1945. In 1947, he married Tomoe Horie; the couple had two children, a daughter named Miyako and a son named Tetsyua.

After completing his doctorate in 1948, Fukui became a full professor of fuel chemistry in 1951. By this time, he had turned his focus toward the problem of molecular reactivity. A molecule is a structure made up of more than one atom. Every atom is composed of various smaller particles, including a central nucleus and electrons. Electrons are arranged around the nucleus of an atom, “orbiting” it like the Earth does the sun (hence the name orbitals). In a molecule, atoms are bonded to each other by attractive forces that exist between their respective electrons. A molecular reaction breaks the attractive force between one or more of the bonded electrons and forms one or more new bonds between different electrons.

Fukui and other scientists of his era wanted to determine the factors governing how bonds are broken and formed when two molecules react with each other. At the time, little was known about molecular reactions involving more than one bond. Fukui started to explore these problems with two graduate students, Haruo Shingu and Teijiro Yonezawa. Together, they developed the frontier molecular orbital theory of reactivity, which they outlined in a 1952 article for the Journal of Chemical Physics. The frontier orbital theory attempted to explain molecular reactivity in a particular type of organic compound called an aromatic hydrocarbon. The term frontier orbitals refers to the fact that some electrons in an atom are arranged in orbits that are especially far away from the nucleus, on the edges, or frontiers, of the molecule. This makes the electrons less strongly bonded to the molecule and more likely to be exchanged with another molecule in a chemical reaction.

Fukui proposed that there were two types of frontier orbitals in any given molecule, which he called LUMO, for lowest unoccupied molecular orbital, and HOMO, for highest occupied molecular orbital. He suggested that the LUMO exists in a state of comparatively low energy and is therefore likely to gain a new electron in a molecular reaction. The HOMO, on the other hand, tends to be in a state of high energy and is therefore likely to give away one of its own electrons in a molecular reaction.

Although Fukui’s theory is considered one of the century’s most important breakthroughs in chemistry, it remained obscure for several years after its introduction; Fukui had used advanced mathematical reasoning and calculations in order to arrive at his conclusions, and some criticized his work as confusing and difficult to follow. In addition, others saw the frontier orbital theory as too simple to be correct.

In 1965, American organic chemists Roald Hoffmann and Robert Woodward published a paper that derived conclusions similar to Fukui’s work. Hoffmann and Woodward’s work described how pairs of molecules react with each other and why some molecules are more reactive than others. The guidelines they set out, now known as the Woodward-Hoffmann stereoselection rules, helped to develop frontier orbital theory.

Impact

After Hoffman and Woodward’s paper was published, Fukui’s frontier orbital theory gained wide acceptance by practical chemists. In 1981, Fukui and Hoffmann both received the Nobel Prize in Chemistry, making Fukui the first Japanese scientist to be thus honored. Throughout the rest of his career, he continued to explore the question of molecular reactivity and worked in other areas of chemistry as well. Fukui was a prolific writer who published hundreds of scientific papers in both English and Japanese. He was also a great advocate of science education in Japan and a dedicated environmentalist.

In addition to the Nobel Prize, Fukui received the Japan Academy Medal and was named a foreign associate of the United States National Academy of Sciences. He retired from Kyoto University in 1982. Fukui died on January 9, 1998, at the age of seventy-nine.

Bibliography

Fukui, Kenichi, and Hiroshi Fujimoto, eds. Frontier Orbitals and Reaction Paths: Selected Papers of Kenichi Fukui. Hackensack: World Scientific, 1997. Print. A collection of Kenichi’s work on frontier orbitals, with introductory notes. Includes a review of how the theory was formulated and a basic conceptual overview of how it works.

Inagaki, Satoshi. Topics in Current Chemistry:Orbitals in Chemistry. Medford: Springer, 2009. Print. Presents a collection of reports on orbital research. Includes a discussion of research trends and research conducted in the years following the development of Fukui’s theory.

Miller, Bernard. Advanced Organic Chemistry: Reactions and Mechanisms. 2nd ed. Saddle River: Prentice, 2004. Print. Textbook covering a variety of chemical reactions, including reactions between heterocyclic compounds.