Nobel lecture, 8 December, 1981
Kyoto 606, Japan
Since the 3rd century for more than a thousand years chemistry has been
thought of as a complicated, hard-to-predict science. Efforts to improve even a
part of its unpredictable character are said to have born fruit first of all in the
success of the “electronic theory”. This was founded mainly by organic chem-
ists, such as Fry, Stieglitz, Lucas, Lapworth and Sidgwick, brought to a
completed form by Robinson and Ingold, and developed later by many other
In the electronic theory, the mode of migration of electrons in
purpose, a criterion is necessary with respect to the number of electrons which
should originally exist in an atom or a bond in a molecule. Therefore, it can be
said to be the concept by Lewis of the sharing of electrons that has given a firm
basis to the electronic theory.
In the organic electronic theory, the chemical concepts such as acid and
a long time ago. Furthermore, there are terms centring closer around the
electron concept, such as electrophilicity and nucleophilicity, and electron
donor and acceptor both being pairs of relative concepts.
One may be aware that these concepts can be connected qualitatively to the
scale of electron density or electric charge. In the electronic theory, the static
and dynamic behaviours of molecules are explained by the electronic effects
which are based on nothing but the distribution of electrons in a molecule.
The mode of charge distribution in a molecule can be sketched to some
extent by the use of the electronegativity concept of atoms through organic
chemical experience. At the same time, it is given foundation, made quantita-
tive, and supported by physical measurements of electron distribution and
theoretical calculations based on quantum theory.
The distribution of electrons or electric charge - with either use the result is
unchanged - in a molecule is usually represented by the total numbers (gener-
ally not integer) of electrons in each atom and each bond, and it was a concept
easily acceptable even to empirical chemists as having a tolerably realistic
meaning. Therefore, chemists employed the electron density as a fundamental
concept to explain or to comprehend various phenomena. In particular, for the
purpose of promoting chemical investigations, researchers usually rely upon
the analogy through experience, and the electron density was very effectively
and widely used as the basic concept in that analogy.
When the magnitude of electron density is adopted as the criterion the
electrostatic attraction and repulsion caused by the electron density are taken
into account. Therefore, it is reasonable to infer that an electrophilic reagent
will attack the position of large electron density in a molecule while a nucleo-
philic reaction will occur at the site of small electron density. In fact, Wheland
explained the orientation of aromatic substitutions in substituted
other chemical reactions followed in the same fashion.
However, the question why one of the simple reactions known from long
before, the electrophilic substitution in naphthalene, for instance, such as
α-substituted derivatives predominantly was not so easy to
answer. That was because, in many of such unsubstituted aromatic hydrocar-
bons, both the electrophile and the nucleophile react at the same location. This
point threw some doubt on the theory of organic reactivity, where the electron
density was thought to do everything.
THE CONCEPT OF FRONTIER ORBITAIL INTERACTIONS
The interpretation of this problem was attempted by many people from various
different angles. Above all, Coulson and Longuet-Higgins
took up the
reagent. The explanation by Wheland
was based on the calculation of the
tried to attack this problem in a way which was at that time slightly unusual,
Taking notice of the principal role played by the valence electrons in the case of
the molecule formation from atoms, only the distribution of the electrons
occupying the highest energy
π orbital of aromatic hydrocarbons was calculat-
ed. The attempt resulted in a better success than expected, obtaining an almost
perfect agreement between the actual position of electrophilic attack and the
site of large density of these specified electrons as exemplified in Fig. 1.
Fig. 1. Nitration of naphthalene.