The Nobel Prize in Chemistry 1981
Kenichi Fukui, Roald Hoffmann
Presentation Speech by Professor Inga
Fischer-Hjalmars of the Royal
Academy of Sciences
Translation from the Swedish text
Your Majesties, Your Royal Highnesses,
Ladies and Gentlemen,
The laureates in chemistry of this year have studied the theory of chemical reactions. Chemical reactions is something that fills our daily life. All of us are constantly starting chemical reactions, by turning the key of the car or by cooking at the stove, to say nothing about the endless number of reactions in our own body that follows every breath.
In chemical reactions new compounds are created. It is possible to make designs for their preparation. But these designs are not realiable until the events at the micro level have been understood and the laws have been found that are governing the transformations of the molecules.
A molecule is composed of atoms that are tied together by aid of the electrons. Atomic nuclei and electrons are not at rest but are constantly moving. The paths of the electrons are usually called orbitals. The forms of these orbitals are determining the bonds between the atoms.
In a reaction molecules are impinging against each other. During the collision the electrons are influenced by new atomic nuclei and the orbitals are changed. Some of the bonds are broken and others are created. Afterwards, new molecules have been formed.
What is it then that decides the sequence of events during the collision? One governing factor is the energy. The new molecules are found at a lower energy level than the original ones. Sometimes, the change will take place without difficulty. The reaction complex is simply sliding down an energy slope. But in general some hindrance must be overcome. It is necessary first to go upwards before starting the downhill ride. Then, the problem is to find the lowest passage over the height barrier. Frequently, rather much is known about the starting material and the final product, about the energy valley of the starting point and the valley of the final destination. But about the character of the ridge in between very little has been known. This year's laureates in chemistry have helped us to foresee the obstacles and to, find the best way to the final goal. The barriers depend on the fact that the electronic orbitals must be transformed. Now, there is a large number of electrons in every molecule, each with its orbital. A drastic simplification of the barrier problem was achieved in the beginning of the 1950's when Kenichi Fukui discovered that only a few orbitals, those with highest energy, are dominating the frontier of the reaction. He therefore called them frontier orbitals. By use of his theory Fukui found the laws for many groups of organic-chemical reactions. An example: Naphthalene is an important initial material in dye-stuff industry, among others. For a long time the puzzling fact had been known that hydrogen atoms at different positions in the naphthalene molecule are reacting most unequally. The explanation came first through Fukui's theory.
Many molecules have no stereo-symmetry. Then, the molecule and its mirror image have very different effects, as the right-hand and left-hand are functioning differently. The finger-tips of a right-hand can easily match those of a left-hand, but not those of another right-hand. Usually, chemical reactions will give rise to a mixture of right-and left-molecules, but our bodies are only producing one kind. For the preparation of vitamins and drugs that are to react in the body it is therefore often necessary to use methods giving only right- or only left-molecules. Such methods have been found through an exquisite combination of theory and experiment. The problem to prepare vitamin B12 was attacked among others by the brilliant molecule builder Woodward at Harvard. There was also the theoretician Roald Hoffmann. Together, Hoffmann and Woodward discovered that not only the energy of the orbital but also its symmetry is decisive for the reactivity. Thus, the Woodward-Eschenmoser synthesis of vitamin B12 could be accomplished.
Hoffmann continued to develop the theory of orbital symmetry to an exceedingly practical instrument for synthetic work of widely different character. At the same time, Fukui showed that the frontier orbital theory leads to another powerful method of solving the intricate problems of stereochemistry. In this way, the theoreticians Fukui and Hoffmann have radically changed the conditions for the design of chemical experiments.
Professor Fukui, Professor Hoffmann. Each of you have independently developed important theories of chemical reactivity. The concepts of frontier orbitals and conservation of orbital symmetry have revealed completely new aspects of the interaction between molecules in collision. Through drastic simplifications you have been able to make beautiful generalizations. From your theoretical work new tools have emerged of the greatest importance for the design of chemical experiments. In recognition of your outstanding work the Royal Academy of Sciences has decided to award this year's Nobel Prize for Chemistry to you.
On behalf of the Royal Academy of Sciences I wish to convey to you our warmest congratulations, and now I ask you to receive your Prizes from the hands of His Majesty the King.
From Nobel Lectures, Chemistry 1981-1990, Editor-in-Charge Tore Frängsmyr, Editor Bo G. Malmström, World Scientific Publishing Co., Singapore, 1992
Copyright © The Nobel Foundation 1981