T H E 1 0 0 t h A N N I V E R S A R Y OF T H E T H E O R Y OF C H E M I C A L S T R U C T U R E

G. V. B y k o v

Translated from Izvestiya Akademii Nauk SSSR, Otdelenie Khimicheskikh Nauk, No. 9, pp. 1533-1538, September, 1961

In the seven-year plan for the development of the national economy, adopted by the XXIst Congress of the Communist Party of the Soviet Union, an important place was given to the development of chemical science and chemical industry. The Congress also pointed out the necessity for every possible extension of theoretical studies in the field of chemistry, without which no successful solution can be found for the many varied problems concerned with the production of new synthetic materials with desired properties, the development of new technological pro- cesses, and the improvement of existing processes, and without which the creative development of chemical science and industry in general is impossible. In the resolution of the Central Commit tee of the Communist Party of the So- viet Union and the Council of Ministers of the USSP,, dealing with the activities of the Academy of Sciences of the USSR, mention was also made of the need to concentrate the efforts of Soviet scientists towards finding a solution for the major theoretical problems which are of fundamental importance for the development of the main trends in sci- ence.

In the Draft Program of the Communist Part of the Soviet Union, the development of theoretical studies in the main branches of natural science is regarded as one of the most important problems of science in the Soviet Union. The leading branches of natural science referred to in the program include chemistry.

One of these most important theoretical problems in chemistry is the development of the modern theory of chemical s t ruc ture- tha t is the development of theories dealing with the structure of ehemieal compounds and the relationship existing between their properties and their structure, which research chemists could use with a success equal to that with which others in their time applied the classical theory of chemical structure. For the characteri- zation of the present state of the theory of structure in chemistry, A. M. Buflerov's statement of 100 years ago is quite appropriate: "Today, following the discovery of a mass of unexpected and important facts, almost everyone recognizes that the theoretical side of chemistry does not correspond to its factual development". And in another place: ~ This transitional state of our science gives rise to a mass of private theoretical opinions and different me- thods of expressing them". The transition state to which A. M. Butlerov referred ended with the creation of the the- ory of chemical structure.

The 19th of September, 1961 marks the 100th anniversary of A. M. Butierov's report to the Congress of German Naturalists and Physicians at Speier, in which the fundamental hypotheses of this theory were formulated for the first t ime. The 19th of September, 1861, can therefore be regarded as the date of the introduction of the chemical struc o rural theory itself. While noting this extremely important event in the history of chemistry, we shall find it appro- priate not only to remember it in its purely historical aspect, but also to examine, as far as possible, the significance of the ideas and methods of the classical theory of chemical structure at the present time,

The starting points for the creation of a theory of chemical structure were provided by the concept of valence (Hatomicity*) and in particular the tetravalence of carbon atoms (the main credit for the introduction of this concept belongs to A. Kekule -1857 and 1858), the concept of an interatomic bond and in particular the idea of the possible formation of carbon chains (these ideas were put forward almost simultaneously in 1858 by A. Kekule and A. S. Couper), and the formulation of an accurate picture of atoms and molecules with the simultaneous elimination, from formulae, of symbols corresponding to equivalent weights. This prolonged process, which began with the work of C. Gerhardt and A. Laurent, was completed at the International Congress of Chemists at Karlsruhe in 1860.

In the report cited, A. M. Butlerov formulated the concept of chemical structure on the basis of these hypothe- ses of chemical theory, which at that t ime were comparat ively new and unestablished: "Assuming that to each chem- ical atom there corresponds a characteristic, definite, and limited amount of chemical force (affinity), with which it takes part in the formation of a body, I would use the term "chemical structure" to denote this chemical bond or

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means by which atoms combine with one another in a complex body" *. In this definit ion A. M. Butlerov on the one hand summarized, as i t were, the whole of the previous development of the science and, on the other, placed before chemists a new t a s k - t h e study of the chemica l structure of chemica l compounds. The ex t remely important hypothe- sis of the classical theory of chemica l structure, which he la ter formulated - " The chemica l nature of a complex par t ic le is determined by the nature of the e lementary component pa r t s , their quantity, and by the chemica l struc- t u re" -opened the way to considerable development in this direction. The above hypothesis means that i t is possible to establish the chemica l structure of a substance from its chemica l properties and, having determined its structure, to obtain information on the other properties of the compound and substances of analogous structure.

The determinat ion of the chemica l structure of organic compounds was a prac t ica l task which could be achieved by chemica l methods, in contrast to the task, quite impossible at that t ime, of establishing the relationship between the spatial arrangement of atoms and the properties of substances. At the same t ime Butlerov never rejected the possi-

b i l i ty that a knowledge of the three-dimensional structure of molecules cottld be obtained and he even opposed those chemists who re jected such a possibili ty on principle . In his report Buflerov showed that in order to reach conclusions regarding chemica l structure i t is possible to use all types of chemica l reaction: synthesis, decomposition, and substi- tution. He also stated that in the future, by making a deta i led study of the chemica l structure of substances, chemists would be able to reach conclusions regarding the mutual influence of atoms in molecules and the rearrangements taking p lace during chemica l reactions.

The history of organic chemistry confirms the comple te accuracy, far-sightedness, and fer t i l i ty of the funda- menta l hypotheses which were put forward by Butlerov in his report and which, taken together, made up the classical theory of chemica l structure. The views of Butlerov, as is often the case with those who break with established t radi -

tions, were not however accepted at first. He had to defend and develop them further almost alone. In 1863, But- lerov provided a solution to the most difficult problem of that t i m e - h e explained the phenomenon of isomerism. He pointed out for the first t ime that, as a result of the difference in chemica l structure and the consequent difference in the mutual influence of the atoms in the molecules of isomers, the " c h e m i c a l value" of the e lementary atoms and groups in the la t ter were not ident ical , so that isomeric compounds exhibi t different properties. This work of Buflerov's a t t racted the at tention of chemists and the following is a typical result: in the 1st edi t ion of the 2rid volume of his textbook published in 1863, Kekule used complex types exclusively, but in the 2nd edi t ion of the same book (pub- lished in 1864) he abandoned them. Of part icular importance in the victory of the ideas of the theory of chemica l structure was the exper imenta l work of A. M. Butlerov. In 1864, he prepared t r imethylcarbinol and interpreted isomer- ism in the series of saturated alcohols. In the same year, in contradict ion of the opinions of Wurtz and Schorlemmer, he predicted the existence of isomerism in the saturated hydrocarbons. In 1865, Butlerov showed the existence of iso- butylene. In the same year he reported the preparation of the three higher homologs of t r imethyl carbinol, which he had predicted, in 1866 he reported the preparat ion of isobutane, and in 1867 the preparat ion of isobutylene. The the- ory of chemica l structure was thus br i l l ian t ly confirmed by exper iment and by the second half of the sixties i t had become the reigning theory of organic chemistry; chemists directed their efforts towards the study of the chemica l structure of the many substances then known and the preparat ion of new substances, with the theoret ical formula of chemica l structure as a guide. In 1865 special fruitful hypotheses regarding the structure of a romat ic compounds (by Kekule) and unsaturated aromat ic compounds (by Erlenmeyer) were put forward.

In the seventies the theory of chemica l structure underwent important further development . "Facts which can- not be explained by existing theories are most dear to s c i ence ' , wrote Butlerov; " i t is from the solution of these pro- blems that we can expect the greatest deve lopment of science in the near future ". In fact, as the Chemical structure of an ever- increas ing number of organic compounds was studied, there was a rapid accumula t ion of facts which con- t radicted the hypothesis that the molecules of isomers should always have a different distribution of the bonds between the atoms, i .e . , it was found that in some cases the theore t ica l ly -pred ic ted number of isomers for a given formula was less than the number of isomers ac tua l ly found. A great deal of work was done on the study of the isomerism of fu- marie and ma le i c acids, which contradicted this hypothesis, and also on the study of the many cases of opt ica l isomer- ism. The extensive genera l iza t ion of the facts known at that t ime regarding such isomerism, and the bold hypothesis put forward to expla in them, made it possible for L H. van ' t Hoff, in 1874, to lay the foundations of "chemistry in space ' , subsequently known as stereochemistry. The extent to which the t ime was ripe for the new theory can be judged from the fact that s l ightly la ter in the same year, the same ideas, but with less sweeping and bold conclusions,

* One year later , in a Russian version of the report, A. M. Butlerov gave an analogous definit ion of the concept of

chemica l structure (see A. M. Butlerov, Works, Vol. ,1, Acad. Sci. USSR Press, Moscow, 1953, p. 70).

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were put forward by J. Le Bel.

In the following 40 years, organic chemistry developed rapidly, based on the theory of chemical structure. Tens of thousands of new compounds were synthesized, and these found application far beyond the walls of scientific laboratories; a fundamentally new method for systematizing organic compounds was created, its foundations laid by Butlerov in his classical "Introduction to the Complete Study of Organic Chemistry" ; a successful attempt was made to develop a rational nomenclature for organic compounds; and phenomena relating to the topics of tautomerism and irreversible isomerisation were generalized and to some extent explained.

With the passage of time, moreover, many new problems arose, related primarily to the explanation of the mechanism of the mutual influence of atoms, and in this field the first generalizations were made in the sixties by A. M. Butlerov and his eminent successor V. V Markovnikov. The classical hypotheses of the theory of chemical structure, in spite of the development due to 1. Thiele, could not explain many features of the structure and proper- ties of aromatic and unsaturated aliphatie compounds with conjugated multiple bonds. The electronic theories which arose during the first decades of the present century answered, or at least attempted to answer, many of these problems, and a particularly successful achievement of theoretical chemistry in the XXth century was the development of quan- tum-mechanical theories of the chemical bond. It is quite evident that the significance which chemists attached to the concept of chemical structure was extending continuously. Stereochemistry introduced the idea of the directional properties of bonds in space and physical methods of study made it possible to determine valence angles and inter- atomic distances, expressed in absolute units. Quantum mechanics made it possible to discuss the distribution of elec- tron density in a molecule and even to calculate this distribution for the simplest molecules. When applied to more complex molecules, including all organic compounds, however, quantum-mechanical methods, due to their complex- ity, had and still have to be used with the introduction of additional assumptions (whose influence on the final results of the calculations cannot be estimated), while it is also necessary to use values of eertain parameters obtained from experimental data. This constitutes the main reason for the existence in modern chemistry, as in the period before the introduction of the theory of chemical structure and in the words of the founder of this theory, of "a mass structure of individual theoretieal opinions and different methods of expressing them' .

It is true that the present "transitional state" in. the theory of the structure of chemical compounds differs in sev- eral respects from that which existed 100 years ago. Firstly, at the present time all directions of development in the theory of structure are based on the same foundation-on the classical theory of chemical structure and stereochem- istry, which have both stood the test of time. Secondly, in order to solve structural problems at the present time it is possible to make use of the powerful methods of modern physics, both experimental, in the form of the many phy- sical methods of study, and theoretical, in the form of different quantum-mechanical models and methods of calcu- lation. Thirdly, since the modern theory of structure is acquiring, to an ever-increasing extent, all the more distinct features of a physical theory, it is undergoing a transition from a qualitative theory to a quantitative theory. 8utlerov predicted, as early as 1858, that theoretical chemistry would develop in this direction: "Experimental studies will provide us with the basis for a true chemical theory, which will be a mathematical theory of the molecular force which we call chemical affinity. Since, however, affinity is responsible not only for the changes but also for the de- finite grouping of the elementary atoms in a chemically eomplex molecule, it should be studied not only during the molecular movement which it produces, but also in the equilibrium state of matter. Up to the present time there has not been nearly enough of this type of study of those physical properties of substances which are not related to the breakdown of molecules and which are capable of providing us with the means for an intensive study of this grouping". In spite of this difference between the state of the theory of structure at the present time and that of a hundred years ago, it is undoubtedly worth while to examine the classical theory of chemical structure in order to find, in its history, examples of the solution of problems analogous to those of today.

A true materialist, A. M. Butlerov emphasized repeatedly that the ideas of chemists regarding atoms and mole- eules are not fiction, but have a definite relationship to that which actually exists. He opposed attempts to treat symbols in chemical formulae as merely "thinking tools', arising only as a result of "arithmetical operations'. Apropos of this subject, Butlerov stated: "What do "arithmetical operations" mean in realistic science, if they do not correspond to anything objective, existing in nature ? * He repeatedly had to argue with those chemists who, from positivistic standpoints, rejected or underestimated the part played in science by hypotheses and theoretical eonsiderations in gen- eral.

These pronouncements by A. M. Butlerov formed the basis on which Soviet chemists relied in the discussion of the theory of resonance, particularly at the All-Union Conference on the Theory of Chemical Structure in Moscow in

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June, 1951, We know now, even from the words of L. Pauling, the founder of the theory, that the initial idea which regarded the theory of resonance as a quantum-mechanical theory was erroneous. The method used in the theory of resonance to express the real structure of molecules, involving the superposition of ideal structures by means of more or less complex "artithmetical operations", could only lead to a vast number of speculative conclusions. The latter, as in the theories which A. M. Butlerov opposed in his lifetime, were somehow or other adjusted to suit the experi- mental data. The objectivizatlon of these ideal structures, which many chemists approached unwillingly, appeared in a philosophical plan as the purest water of idealism. The pragmatic assertion that the subjective and arbitrary na- ture of the theory of resonance is justified by its "convenience" and "usefulness" cannot be substantiated, if only for the reason that the theory of resonance has retarded the search for and development of the present promising directions of approach in theoretical organic chemistry, and this, in the case of this branch of chemistry, cannot be regarded as either "convenient" or "useful". The theory of resonance is now disappearing from the scene, and this can only be re- garded as a victory for the principles which guided A. M. Butlerov in theoretical chemistry.

Mention must also be made of certain other features of A. M. Butlerov's approach to theoretical problems. An example is the procedure which he adopted in formulating and solving theoretical problems. First of all the study of the fundamental relationship between the properties and the chemical structure, followed by the study of relationships of secondary character, between the properties of the molecules and the spatial arrangement of the atoms, the mani- festation of the mutual influence of atoms not directly joined, etc. From this point of view we cannot regard as com, pletely satisfactory the state of affairs in which most attention in modern theory of structure is paid to the study of "the chemistry of the lr-electrons", sometimes in very complex molecules, while the distribution of the o-electrons in the molectdes and its influence on the properties are insufficiently studied, and while quantum-mechanical calcu- lations, with a few isolated exceptions, are lacking even for the simplest molecules-that is for those molecules with which A. M. Butlerov considered it necessary to begin any study.

At the present time in the study of chemical compounds, research workers are divided not accordingto the meth- ods which they use-for the greater the number and variety of methods of approaching an object of study the bet ter- but according to their aims. Some are trying to determine the Vorder" of bonds and the "free valence number", others are trying to determine different "indices" for bonds and atoms, while others are trying to determine the effective or actual electronic charges of the atoms and bonds. In his report, as we have already mentioned above, A. M. Butlerov pointed out that the chief aim of the study of molecules should be the examination of the fundamental characteristic of chemical structure. Thus at the present time the most correct approach appears to be to determine not the various "indices" but to determine the actual electronic structure, which at this stage in the development of science, can be represented by a model oniy by allocating a certain fraction of the total electron cloud of the molecules to definite zones (for example the zones of atoms and bonds).

In this connection we are reminded again of the fundamental principle of the theory of chemical structure, according to which the structure can be determined by studying the properties of the molecules on the basis of a knowledge of the relationships existing between them. It is evident that this semiempirical method can find exten- sive application at the present time. If some sort of relationship exists between the electrordc characteristics of atoms and bonds on the one hand and the properties of their structural elements on the other, it is evident that data obtained by physical, physicochemical and chemical methods can be used, with practically the necessary accuracy, to deter- mine the electronic structure of molecules. In some cases at present the semiempirical method can give information which in the cognitive aspect is no less important than the incomparably more complex and laborious quantum-me- chanical methods of calculation.

The classical theory of chemical structure led in its time to formulae which were generally understood by all chemists. As A. M. Butlemv stated in his report of 1861, the formula of chemical structure, "when the general laws of the rela~onship between the chemical properties of a substance and "its chemical structure become generally known, will become the expression of aU these properties". In exactly the same way the formulae of the electronic structure can and should express the relationship between the structure and properties of molecules even more fuUy and deeply than the classical structural formulae. For this, however, it is necessary that the formulae themselves should express the results of the quantitative study of the electronic structure of the molecules. This aim can evidently best be achi- eved by means of the so-called electronic diagrams (but not molecular diagrams with different types of "indices" on the atoms and bonds), in which figures are used to indicate the fraction of the electronic cloud of the molecule in its different zones. Such diagrams, with the further development of this field of study, may also be constructed for molecules in an activated "reaction" state, and also for radicals and ions.

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This approach promises to lead to the solution of one of the fundamental problems of modem theory of struc-

ture- the elucidation of the internal mechanism of the mutual influence of atoms. Let us give one example. In 1869, V. V. Markovnikov formulated the following rule: " . . . in saturated hydrocarbons, the hydrogens belonging to those carbon shares (atoms-G. B.) which have spent most of their affinity on the bond with other carbons, undergo substitution more readily than the others". According to this rule, substitution in propane takes place most readily at the bond between hydrogen and the secondary carbon atom. In 1965 the electronic structure of propane was calcu- lated, albeit approximately, for the first time by the molecular orbital method. The results of the calculation are shown in the following electronic diagram:

H H H

1,9~l H

where each figure shows the magnitude of the electronic cloud (in units of electronic charge) belonging to each bond. The diagram shows that the substitution to which V. V. Markovnikov refers takes place predominatly at the bond with the lowest fraction of the o-electron cloud. This conclusion can be further extended to hydrocarbons in which there are not two forms of C-H bond, as in propane, but several forms differing in structural position ando-electron charge.

This example provides a good illustration of how modern electronic theories of structure and reactivity repre- sent the further development, extension, and modification of the classical theory of chemical structure, and also il- lustrates the great distance it has travelled during the 100 years of its existence.

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