Chemical Structure, Theory of

a theory that describes the structure of organic compounds—that is, the sequence (order) of the arrangement of atoms and bonds in a molecule—as well as the interaction of atoms and the relationship between a compound’s structure and its physical and chemical properties.

The theory’s principal propositions were first set forth by A. M. Butlerov in the paper “On the Chemical Structure of Matter,” presented at a congress of German naturalists in Speyer in 1861. Butlerov wrote, “Assuming that each chemical atom has only a definite and limited quantity of chemical strength (affinity), with which it participates in the formation of matter, I would use the term chemical structure to denote the distribution of the action of this strength, as a result of which chemical atoms, directly or indirectly influencing one another, combine to form a chemical particle” (Izbr. raboty po organicheskoi khimii, 1951, pp. 71–72). Butlerov subsequently developed the propositions in a series of articles and in the book Introduction to the Complete Study of Organic Chemistry (Kazan, 1864–66; German edition, Leipzig, 1867–68), the first book on organic chemistry that systematized all the material on the basis of the theory of chemical structure. The theory’s development was preceded by the clarification of such important concepts as the atom and molecule (first international congress of chemists, Karlsruhe, 1860) and by the postulation (1857–58) of the tetravalence of carbon by F. A. Kekulé and A. S. Couper. Graphic formulas of organic compounds, similar to the formulas that emerge from the theory of chemical structure, were proposed in 1858 by Couper (seeORGANIC CHEMISTRY).

The following are the main principles of the theory of chemical structure: (1) the atoms in organic molecules are bonded to one another in a definite order according to their valence, which determines the chemical structure of molecules; (2) the chemical and physical properties of organic compounds depend both on the nature and number of the atoms of the compounds and on the chemical structure of the molecules; (3) a specific number of theoretically possible structures (isomers) may be derived for each empirical formula; (4) each organic compound has one chemical structural formula, from which the compound’s properties can be deduced; and (5) there exists in molecules an interaction between atoms, both those that are bonded to one another and those that are not. The last principle was further developed by Butlerov’s student V. V. Markovnikov (seeMARKOVNIKOV RULE) and, subsequently, by many other scientists.

The theory of chemical structure helped explain known cases of isomerism (both spatial and structural), theretofore incomprehensible to chemists. Butlerov’s prediction (1863) of the possibility of determining the spatial arrangement of atoms in a molecule became a reality. In 1874, the Dutch chemist J. van’t Hoff and, independently, the French scientist J. A. Le Bel proposed the concept that the four valences of carbon have a clear spatial orientation and are directed toward the corners of a tetrahedron with the carbon atom in its center. This idea of the definite spatial orientation of chemical bonds served as the basis for a new branch of organic chemistry, namely, stereochemistry. It enabled scientists to explain a series of known cases of geometric and, primarily, optical isomerism, as well as to explain the effect that later came to be called tautomerism (Butlerov, 1862; the German chemist P. C. Laar, 1885).

Butlerov confirmed the validity of his theory by the synthesis of a number of organic compounds. Because of its tremendous predictive capacity with respect to the synthesis of organic compounds and the determination of structures of compounds already known, the theory facilitated the rapid development of both chemistry itself, especially synthetic organic chemistry, and the chemical industry.

The further development of the theory of chemical structure enriched organic chemistry with new concepts, such as the cyclic structure of benzene (Kekulé, 1865); the oscillation (shift) of double bonds in the benzene molecule (1872), a concept of major importance in the chemistry of aromatic and heterocyclic compounds; and the special properties of compounds with conjugated bonds (the theory of partial valence, F. K. J. Thiele, 1899). The development of stereochemistry led to the strain theory (A. von Baeyer, 1885), which explains the differing stability of rings according to their size, and subsequently to conformational analysis (the German chemists H. Sachse [1890] and E. W. M. Mohr [1918]). The main principles of the theory of chemical structure found confirmation in the study of organic compounds by chemical, physical, and computer methods.

Although concepts of the mutual interaction of atoms in organic molecules are of fundamental importance in the theory of chemical structure, the theory was unable to explain the nature of the interaction or its essential mechanism. This became possible only with advances in physics that paved the way for the elucidation of the concepts of valence and chemical bond. Beginning in the early 20th century, various electronic theories were advanced in organic chemistry, all based on an electronic interpretation of the nature of ions (J. J. Thomson), and the nature of the ionic bond (W. Kossel) and the covalent bond (the German physicist J. Stark and the American physical chemist G. N. Lewis). Such theories made it possible to explain the reason for the interaction of atoms (the steady-state and dynamic shift of electron density in a molecule) and to predict the direction of reactions from the chemical structure of the reagents. At the end of the 1920’s, the ’chemical bond began to be interpreted in terms of quantum-chemical ideas (seeQUANTUM CHEMISTRY).

Butlerov’s theory served as the basis for the nomenclature and systematization of organic compounds (seeNOMENCLATURE, CHEMICAL). The use of Butlerov’s structural formulas is helpful in determining ways of synthesizing new compounds and in establishing the structure of complex compounds, including natural ones.