Electronic Charges of Bonds in Organic Compounds

195 Pages·1964·5.049 MB·English

by  G. V. Bykov (Auth.)

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ELECTRONIC CHARGES
OF BONDS IN ORGANIC
COMPOUNDS
G. V. BYKOV
Translated from the Russian by
J. T. GREAVES
Translation edited by
R. W. CLARKE
A.E.R.E., HARWELL
PERGAMON PRESS
OXFORD · LONDON · NEW YORK · PARIS
1964
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Copyright © 1964
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Library of Congress Catalog Card Number 63-10116
This is a translation of the original Russian Elektronnyye zaryady
svyazei v organicheskikh soyedineniyakh, published in 1960 by the
Publishing House of the Academy of Sciences of the U.S.S.R.,
Moscow
MADE IN GREAT BRITAIN
PREFACE
"For any scientific viewpoint, in order that it may
claim preference over others, certain qualities are
required. It must explain and predict what is not explained
and predicted from other points of view; it must at
least describe more simply and clearly problems which
are not satisfactorily explained by previous viewpoints".
A. M. BUTLEROV
CONTEMPORARY organic chemistry involves a constant search for
new theories concerning structure. This is due to the fact that existing
theoretical schemes are inadequate to form a basis for the formula-
tion of a fundamental electronic theory of organic chemistry which,
like the classical theory of chemical structure and stereochemistry,
could give real help to organic chemists in their everyday research
work. Such a theory must be characterized by a pattern of ideas,
universality and mathematical approach practicable for the ordinary
chemist.
Although a variety of treatments in theoretical organic chemistry
is extant, hardly any of them answer all requirements to the full
extent and at the same time. The present monograph is devoted to
a relatively new trend in the present-day electronic theory of structure
and reactivity of organic compounds, which perhaps satisfies to a
greater extent than others the combination of conditions outlined.
At the basis of the work in this direction there already lies a suffic-
iently well-founded hypothesis that the electronic charges of bonds
represent a real and substantial feature of the molecules, on which
their physical and chemical properties largely depend. By electronic
charge is implied the portion of the envelope of a- and π-electrons,
concentrated on a given bond, and generally not equal to the total
number of electrons.
In the present author's work, begun in 1951, it was shown that
simple and diverse methods existed for calculating the electronic
charges of bonds from the results of physical methods of investi-
vii
viii Preface
gation, and that the concept of electronic charges of bonds opened up
new possibilities for the study of the chemical and physical properties
of organic compounds. The results obtained by the author during
the past 9 years are summarized in the monograph. In Chapter I
brief information is given about the work of other research workers
in this direction. In Chapters II-VI the results of a study of the
relationship of the electronic charges of bonds to the physical and
physicochemical characteristics of molecules and their structural
elements are described. In Chapters VII-X, and particularly in the
last two chapters the relationship between the electronic charges of
bonds and chemical properties is discussed.
We are still at the very beginning of the systematic use of the
theoretical ideas to which this book is devoted for solving present
problems in organic chemistry. One may hope, therefore, that the
reader will himself try to apply the new theoretical aspects to the
field in which he is interested and thus take part in their development.
The contents of this monograph have been repeatedly discussed
in a number of scientific associations in Moscow and Leningrad.
The author thanks the organizers and participators in these dis-
cussions, and particularly V. N. Kondrat'yev for valuable comments
on the manuscript of the first eight chapters. The author would
also like to take the opportunity of expressing his sincere gratitude
to M. M. Dubinin, K. T. Poroshin and N. A. Figurovskii for their
kind interest in his work and for their help in overcoming difficulties.
Moscow, 1960
G. V. BYKOV
CHAPTER I
HISTORICAL OUTLINE AND
INTRODUCTION
1. HISTORY OF THE CLASSICAL AND ELECTRONIC THEORY
OF THE STRUCTURE OF ORGANIC COMPOUNDS
IN THIS section it is intended very sketchily to touch on the history
of the structural theory of organic compounds, in order to demonstrate
the development of the idea of electronic bond charges, and to show
the close analogy between the methods of studying them described
in this book and the methods which were widely used by chemists
before the discovery of the electron.
In 1861 Butlerov formulated the basis (or, as he put it, the "rule")
of the classical theory of chemical structure, according to which the
chemical properties of molecules depend on their composition and
chemical structure. By chemical structure was understood the
distribution of interatomic or chemical bonds in the mole-
cules. Many advances in organic chemistry, and especially in
organic synthesis, followed from application of this principle.
Butlerov not only indicated the correct basis of the theory of
chemical structure, but also showed that its development was quite
possible by the means which chemists had at their disposal at that
time.
Subsequently it was found that the properties of organic compounds
also depend to a certain extent on the spatial distribution of their
bonds, which may be different for molecules of the same chemical
structure. This is the basic idea of stereochemistry, first expressed in
1874 by van 't Hoff. In their studies of stereochemistry van 't Hoff
and Le Bel considered not only the facts obtained regarding the
chemical properties of organic molecules, but also the results of an
investigation of optical activity. Ten years before this, Butlerov had
1
2 Electronic Charges of Bonds in Organic Compounds
indicated that the chemical and physical properties of chemical
compounds "had a mutual causative connection", and that a study
of physical properties was of great significance for the elucidation
of "the mutual relationships existing between the component parts
of these substances" [1 ]. In the 1860's and 1870's numerous more
or less successful attempts were made to establish the relationship
between the physical properties and chemical structure of organic
compounds, but systematic studies were begun only in the 1880's
with the work of Thomsen on thermochemistry and of Briihl on
refractometry. It was Briihl who pointed out that ideas on the structure
of molecules and the physical methods of investigation had reached
a stage of development where "the time had come to make use of
the instruments proposed by physics for practical use in chemistry"
[2, p. 141].
Thomsen, Briihl and many of their followers gave their main atten-
tion to a study of the additive properties of organic compounds.
They obtained valuable results, but they frequently came to contra-
dictory conclusions, mainly because the perceptional possibilities
of the semi-empirical methods of which they made extensive use were
not visualized clearly enough.
At the end of the 1880's it was clearly evident that within the
framework of the classical theory of chemical structure and stereo-
chemistry it was not possible to provide a satisfactory answer to
many questions, for example about the structure of benzene, in spite
of the help of physical methods of investigation, and that it was not
possible to answer these questions until the physical idea of inter-
atomic reaction was discovered, or in other words, until the nature
of the chemical bond was elucidated [3 ].
In 1897 the electron was discovered (Wiehert, Thomson), although
theoretically its existence had been predicted 20 years earlier (Helm-
holtz, Stoni). Two years after the discovery of the electron, Abegg
and Bodlender expressed the idea of the electronic nature of the
chemical bond. Thus, physics gave the answer to the most profound
question which chemistry had ever provided it with. A chemical
compound is a system of electrical charges—such was stated to be
the physical model of the molecule, which chemists have used ever
since.
It is possible to note five periods [4 ] in the history of the electronic
theories which subsequently developed concerning the structure
Historical Outline and Introduction 3
and properties of organic compounds. To the first period, lasting
from the beginning of the 1900's to the beginning of the J920's,
belong the theories on which the hypothesis of the existence of an
ionic bond in organic molecules is based. Since this hypothesis did
not, however, correspond to the known facts, theories based on it
could not be used without stretching matters to explain the properties
of organic compounds. The second period, which lasted from the
beginning of the 1920's to approximately the middle of the 1930's,is
characterized by the formulation of the theory of electron displacements
and other qualitative ideas based on an understanding of the covalent
bond. An increased infiltration of quantum-mechanical ideas into
theoretical organic chemistry is characteristic of the third period,
which lasted from the beginning of the 1930's to the middle of the
1940's. In this period the concept of valency bonds (electron pairs)
underwent the greatest development, including a distorted and
unsuccessful modification known as the "theory of resonance." The
idea of molecular orbits played a secondary part, although several
important papers were devoted to it in principle. In the fourth period,
from the middle of the 1940's to the beginning of the 1950's, the cal-
culation of so-called "molecular diagrams" by the method of molecular
orbits was widespread (the English school with Coulson at the head)
and by the method of valency bonds (French school with Daudel
at the head). From the beginning of the 1950's the fifth period set
in, which is characterized by the recognition of the inadequacy and
limitations of the models presented previously for the theoretical
study of the electronic structure of organic compounds,* by attempts
to improve the existing models and introduce new models."'' One of
these concepts is the idea of localization of the cloud of valency
electrons in the molecule on individual bonds and atoms—the idea
of electronic charges of bonds and atoms.
* See, for example, the recent statements in connection with this by Pauling
[5] and Erich Hiickel [6].
t See a paper by the present author on this question, devoted to the history
of quantitative theories of the electronic structure of organic compounds [7].
4 Electronic Charges of Bonds in Organic Compounds
2. CONCEPTION OF ELECTRONIC CHARGES OF BONDS
Intercomplementary Conceptions of the ^-Electron Charge
of Bonds and the jr-Electron Charge of Atoms
The term "electronic charge of a bond" was introduced in 1945,
together with the term "electronic charge of an atom", by Daudel
and Pullman, [8 ] to denote that part of the π-electron cloud of a
molecule associated with the given bond and atom. In order to cal-
culate the electronic charges they worked out a quantum-mechanical
method called the "mesomer method", which was one of the simplest
modifications of the valency bonds concept. The calculated values
of the electronic charges, almost exclusively for polynuclear aromatic
hydrocarbons, were correlated with chemical and other properties
and in particular with carcinogenic properties. Daudel and his co-
workers [9 ] soon found however, that when the electronic charges
were calculated by a different method other values were found, and
therefore it was expedient to call the values found by the mesomer
method "indices" of bonds and free valencies, by analogy with the
arrangements of bonds and numbers of free valencies in the mole-
cular orbits concept. In recent years, only a paper by Bak [10] has
appeared in which the ττ-electron charges of the bonds and atoms
were determined by the superposition of valency structures of different
weight. The "mesomer method" was not used to calculate the weights
of these structures but they were chosen in such a way that the final
picture corresponded to the experimental results for the interatomic
distances and dipole moments. Bak calculated the π-electron charges
of the bonds and atoms in pyridine, pyrrole and pyran. As an example
the electron diagram obtained by him for pyridine is shown here.
00408
/ Νβ 04Ι0
I I 0-0410.
%/
0-5446
Attempts were also made to calculate the ττ-electron charges of
bonds and atoms by the molecular orbits method. The first paper
Historical Outline and Introduction 5
was by McWeeny [11], dated 1951, and 6 years later the calculations
of Maslen and Coulson [12], and still more recently of Payette and
Sandorfy [13], were published. In these papers it was considered
expedient to determine local electronic charges in organic molecules
instead of the so-called orders of bonds. McWeeny calculated the
π-electron charges of bonds and atoms only in naphthalene, Maslen
and Coulson in ethylene and in several conjugate aliphatic and aro-
matic ketones. As an example electron diagrams obtained in this way
for ethylene and butadiene [12] and acrolein and benzophenone
[13] are
0-82 0-82 0-75 1*442 0*408 0*791 0*590
c=c c = c -c=c o = c - c = c
0*4 0*35 0*14 0*269 0*166 0*335
As seen from Fig. 1 there are some very large discrepancies between
the results of the calculations by the molecular orbit method and
the mesomer method. The reason for such discrepancies lies in
the fact that the distribution of the ττ-electron cloud of the molecule
0-122 0-767
FIG. 1. π-Electron charges of bonds and atoms in benzene and naphthalene,
calculated by the method of molecular orbits [11, 12] (figures to the right) and
by the mesomer method [14] (figures to the left).
into ττ-electron charges of bonds and π-electron charges of atoms is
essentially arbitrary, and it must not be imagined that any real
boundary line could exist between them. Thus, failure of the attempts
to obtain identical results by different methods could have been
forecast, because the method of modelling the electronic structure of
the molecules was defective.