What are the provisions of the theory of chemical structure. The main provisions of the theory of the chemical structure of organic compounds by A.M. Butlerov

In the most general and systematic form, the theory of chemical structure (abbreviated as TCS) was first formulated by the Russian chemist A. M. Butlerov in 1861 and subsequently developed and supplemented by him and his students and followers (primarily V. V. Markovnikov, A. M. Zaitsev and others), as well as by many foreign chemists (Ya. G. van't Hoff, J. A. Le Bel and others).

Let us consider the main provisions of classical TCS and comment on them from the standpoint of modern structural chemistry.

1. Each atom in a molecule is capable of forming a certain number of chemical bonds with other atoms.

Already in the first half of the XIX century. in chemistry, ideas were formed about the ability of atoms to combine with each other in certain relationships. According to Butlerov, each atom “is born with a certain amount of force that produces chemical phenomena (affinities). In a chemical combination, ... a part of this force or all of its quantity is consumed. Thus, two features of the interatomic chemical interaction were emphasized: a) discreteness - all the affinity inherent in the atom was supposed to be composed of separate portions or, according to Butlerov, “separate units of chemical force”, which was clearly expressed by the symbolism of valence strokes (for example, H-O- H, H-C≡N, etc.), where each stroke characterized one chemical bond; b) saturation - the number of chemical bonds formed by an atom is limited, which is why there are, for example, such neutral molecular systems of various stability as CH, CH2, CH3, CH4, but there are no CH5, CH6 molecules, etc.

A quantitative measure of the ability of an atom to form chemical bonds is its valence. Formation in the 1850s the concepts of valence and chemical bond served as the most important prerequisite for the creation of TCS. However, before the beginning of the XX century. the physical meaning of the valence stroke, and hence the nature of the chemical bond and valency, remained unclear, which sometimes led to paradoxes. So, studying the properties of unsaturated hydrocarbons, Butlerov accepted in 1870 the idea of ​​the German chemist E. Erlenmeyer about the existence of multiple bonds in them. Meanwhile, it remained unclear why a multiple bond turned out to be less strong (prone to addition reactions) than a single bond (which does not enter into these reactions). There were other indications that some or all of the chemical bonds in the molecule were not of equal value.

With the creation of quantum chemistry, it became clear that, as a rule, a two-center two-electron bond corresponds to each valence stroke, and that chemical bonds can differ in energy, length, polarity, polarizability, orientation in space, multiplicity, etc. (see Chemical bond) .

The concept of a chemical bond entails the division of atoms of a molecule into chemically bound and chemically unbound (see Fig.), From which follows the second position of TCS.

H / O \ H Chemically bonded atoms

Chemically unbonded atoms

2. Atoms in a molecule are connected to each other in a certain order, according to their valency. It was the “order of chemical interaction”, or, in other words, the “method of mutual chemical bonding” of atoms in a molecule, that Butlerov called the chemical structure. As a result, the chemical structure, clearly expressed by the structural formula (sometimes also called graphic, and in recent years - topological), shows which pairs of atoms are chemically bonded and which are not, i.e., the chemical structure characterizes the topology of the molecule (see Molecule ). At the same time, Butlerov specifically emphasized that each compound corresponds to only one chemical structure and, consequently, only one structural (graphic) formula.

The considered provision of TCS is generally valid today. However, firstly, it is far from always possible to convey the molecular structure by one classical structural formula (see Benzene), secondly, in non-rigid molecules, the bond order of atoms can spontaneously change and rather quickly (see Molecule), and, thirdly , modern chemistry has discovered a wide range of molecules with "unusual" structures (for example, in some carboranes, the carbon atom is bonded to five neighboring atoms).

3. The physical and chemical properties of a compound are determined both by its qualitative and quantitative composition, and by its chemical structure, as well as by the nature of the bonds between atoms.

This provision is central to the TCS. It was his assertion in chemistry that constituted Butlerov's main scientific merit. A number of important consequences follow from this position: the explanation of isomerism by the difference in the chemical structure of isomers, the idea of ​​the mutual influence of atoms in a molecule, and also the meaning and significance of the structural formulas of molecules is revealed.

In 1874, TCS was enriched with stereochemical concepts (see Stereochemistry), within the framework of which it was possible to explain the phenomena of optical, geometric, and conformational isomerism (see Isomerism).

In modern chemistry, the term "structure of a molecule" is understood "in three ways: a) as a chemical structure (i.e., the topology of a molecule); b) as a spatial structure characterizing the arrangement and movement of nuclei in space; c) as an electronic structure (see Molecule, chemical bond).

Thus, the main position of TCS, from a modern point of view, can be represented as follows: the physical and chemical properties of compounds are determined by their quantitative and qualitative elemental composition, as well as the chemical (topological), spatial (nuclear) and electronic structure of their molecules.

4. The chemical structure can be studied by chemical methods, i.e., analysis and synthesis.

Developing this position, Butlerov formulated a number of rules for "recognizing the chemical structure" and widely applied them in his experimental work.

At present, the structure of molecules is studied both by chemical and physical methods (see Spectral analysis).

5. The atoms included in the molecule, both chemically bound and unbound, have a certain effect on each other, which is manifested in the reactivity of individual atoms and bonds of the molecule, as well as in its other properties.

TCS, like any scientific theory, is based on some model concepts that have a certain area of ​​applicability and reflect only certain aspects of reality. So, speaking of TCS, one should not forget that in reality a molecule is a single integral system of nuclei and electrons and the separation of individual atoms, functional groups, chemical bonds, lone electron pairs, etc. in it is an approximation. But as soon as this approximation turned out to be effective in solving various chemical problems, it became widespread. At the same time, the theoretical, mental dismemberment, structuring of an object (molecule) that is integral in nature forces us to introduce additional ideas into the theory, taking into account the fact that the selected molecular fragments (atoms, bonds, etc.) are actually connected and interact with each other . For this purpose, the concept of the mutual influence of atoms (VVA) was created.

The properties and state of each atom or functional group of a molecule are determined not only by their nature, but also by their environment. For example, introducing an OH group into a molecule can lead to different results:

Therefore, when studying the nature and intensity of the influence of various substituents on the properties of a molecule, they proceed as follows: they consider reaction series, i.e., a number of compounds of the same type that differ from each other either in the presence of a substituent or in the arrangement of multiple bonds, for example: CH2=CH-CH=CH- CH3, H2C=CH-CH2-CH=CH2, etc., or according to some other details of the structure. At the same time, the ability of substances of this series to participate in the same type of reactions is investigated, for example, they study the bromination of phenol and benzene. The observed differences are associated with the influence of various substituents on the rest of the molecule.

As for organic compounds, one of their characteristic features is the ability of a substituent to transfer its influence to chains of covalently bonded atoms (see Chemical bond). Of course, the substituents are also influenced by the rest of the molecule. The transfer of the influence of the substituent on a- and l-bonds leads to a change in these bonds. If the influence of substituents is transmitted with the participation of a-bonds, then the substituent is said to exhibit an inductive, or I-effect. If there are π-bonds in the chain, they are also polarized (π-effect). In addition, if the chain has a system of conjugated multiple bonds (-C=C-C=C-) or a substituent with an unshared electron pair with a multiple bond (CH3-O-CH=CH2) or with an aromatic nucleus, then the transfer of influence occurs along system of π-bonds (conjugation effect, or C-effect), while the electron cloud is partially shifted to the region of the neighboring σ-bond. For example, substituents such as -Br, -Cl, -OH, -NH2, having unshared electron pairs, are π-electron donors. Therefore, they are said to have a +C-effect. At the same time, they shift the electron density towards themselves along σ-bonds, i.e., they have the -I-effect. For -Br, -Cl, the I-effect prevails, for -OH and -NH2-, on the contrary, +C-effect. Therefore, say, in phenol, the π-electron density on the benzene nucleus is greater than in benzene, which facilitates the occurrence of electrophilic substitution reactions in phenol (compared to benzene).

The theory of chemical structure is also widely used in inorganic chemistry, especially after the creation by A. Werner in 1893 of the coordination theory (see Coordination compounds).

The largest event in the development of organic chemistry was the creation in 1961 by the great Russian scientist A.M. Butlerov's theory of the chemical structure of organic compounds.

Before A.M. Butlerov, it was considered impossible to know the structure of the molecule, that is, the order of the chemical bond between atoms. Many scientists even denied the reality of atoms and molecules.

A.M. Butlerov refuted this opinion. He proceeded from correct materialistic and philosophical ideas about the reality of the existence of atoms and molecules, about the possibility of knowing the chemical bond of atoms in a molecule. He showed that the structure of a molecule can be established empirically by studying the chemical transformations of a substance. Conversely, knowing the structure of the molecule, one can derive the chemical properties of the compound.

The theory of chemical structure explains the diversity of organic compounds. It is due to the ability of tetravalent carbon to form carbon chains and rings, combine with atoms of other elements and the presence of isomerism in the chemical structure of organic compounds. This theory laid the scientific foundations of organic chemistry and explained its most important regularities. The basic principles of his theory A.M. Butlerov stated in the report "On the theory of chemical structure".

The main provisions of the theory of structure are as follows:

1) in molecules, atoms are connected to each other in a certain sequence in accordance with their valency. The bonding order of atoms is called chemical structure;

2) the properties of a substance depend not only on which atoms and in what quantity are part of its molecule, but also on the order in which they are interconnected, that is, on the chemical structure of the molecule;

3) atoms or groups of atoms that formed a molecule mutually influence each other.

In the theory of chemical structure, much attention is paid to the mutual influence of atoms and groups of atoms in a molecule.

Chemical formulas, which depict the order of connection of atoms in molecules, are called structural formulas or structure formulas.

The value of the theory of chemical structure of A.M. Butlerov:

1) is an essential part of the theoretical foundation of organic chemistry;

2) in importance it can be compared with the Periodic system of elements of D.I. Mendeleev;

3) it made it possible to systematize a huge amount of practical material;

4) made it possible to predict in advance the existence of new substances, as well as indicate ways to obtain them.

The theory of chemical structure serves as the guiding basis in all research in organic chemistry.

5. Isomerism. The electronic structure of atoms of elements of small periods. Chemical bond

The properties of organic substances depend not only on their composition, but also on the order of connection of atoms in a molecule.

Isomers are substances that have the same composition and the same molar mass, but different molecular structure, and therefore have different properties.

Scientific significance of the theory of chemical structure:

1) deepens ideas about the substance;

2) indicates the way to the knowledge of the internal structure of molecules;

3) makes it possible to understand the facts accumulated in chemistry; predict the existence of new substances and find ways to synthesize them.

All this theory greatly contributed to the further development of organic chemistry and the chemical industry.

The German scientist A. Kekule expressed the idea of ​​connecting carbon atoms to each other in a chain.

The doctrine of the electronic structure of atoms.

Features of the doctrine of the electronic structure of atoms: 1) made it possible to understand the nature of the chemical bond of atoms; 2) find out the essence of the mutual influence of atoms.

The state of electrons in atoms and the structure of electron shells.

Electron clouds are areas of the greatest probability of an electron being present, which differ in their shape, size, and orientation in space.

In the atom hydrogen a single electron during its movement forms a negatively charged cloud of a spherical (spherical) shape.

S-electrons are electrons that form a spherical cloud.

The hydrogen atom has one s-electron.

In the atom helium are two s-electrons.

Features of the helium atom: 1) clouds of the same spherical shape; 2) the highest density is equally removed from the core; 3) electron clouds are combined; 4) form a common two-electron cloud.

Features of the lithium atom: 1) has two electronic layers; 2) has a cloud of spherical shape, but is much larger than the inner two-electron cloud; 3) the electron of the second layer is weaker attracted to the nucleus than the first two; 4) is easily captured by other atoms in redox reactions; 5) has an s-electron.

Features of the beryllium atom: 1) the fourth electron is an s-electron; 2) the spherical cloud coincides with the cloud of the third electron; 3) there are two paired s-electrons in the inner layer and two paired s-electrons in the outer.

The more electron clouds overlap when atoms connect, the more energy is released and the stronger chemical bond.

Chemical structure of a molecule represents its most characteristic and unique side, since it determines its general properties (mechanical, physical, chemical and biochemical). Any change in the chemical structure of a molecule entails a change in its properties. In the case of minor structural changes made to one molecule, small changes in its properties follow (usually affecting physical properties), but if the molecule has experienced deep structural changes, then its properties (especially chemical ones) will be profoundly changed.

For example, Alpha-aminopropionic acid (Alpha-alanine) has the following structure:

What we see:

1. The presence of certain atoms (C, H, O, N),
2. a certain number of atoms belonging to each class, which are connected in a certain order;

All these design features determine a number of properties of Alpha-alanine, such as: solid state of aggregation, boiling point 295 ° C, solubility in water, optical activity, chemical properties of amino acids, etc.

In the presence of a bond between the amino group and another carbon atom (i.e., there has been a slight structural change), which corresponds to beta-alanine:

The general chemical properties are still characteristic of amino acids, but the boiling point is already 200°C and there is no optical activity.

If, for example, two atoms in this molecule are connected by an N atom in the following order (deep structural change):

then the formed substance - 1-nitropropane in its physical and chemical properties is completely different from amino acids: 1-nitro-propane is a yellow liquid, with a boiling point of 131 ° C, insoluble in water.

In this way, structure-property relationship allows you to describe the general properties of a substance with a known structure and, conversely, allows you to find the chemical structure of a substance, knowing its general properties.

General principles of the theory of the structure of organic compounds

In the essence of determining the structure of an organic compound, the following principles lie, which follow from the relationship between their structure and properties:

a) organic substances, in an analytically pure state, have the same composition, regardless of the method of their preparation;

b) organic substances, in an analytically pure state, have constant physical and chemical properties;

c) organic substances with a constant composition and properties, has only one unique structure.

In 1861 the great Russian scientist A. M. Butlerov in his article “On the chemical structure of matter”, he revealed the main idea of ​​the theory of chemical structure, which consists in the influence of the method of bonding atoms in organic matter on its properties. He summarized all the knowledge and ideas about the structure of chemical compounds available by that time in the theory of the structure of organic compounds.

The main provisions of the theory of A. M. Butlerov

can be summarized as follows:

1. In the molecule of an organic compound, the atoms are connected in a certain sequence, which determines its structure.
2. The carbon atom in organic compounds has a valence of four.
3. With the same composition of a molecule, several options for connecting the atoms of this molecule to each other are possible. Such compounds having the same composition but different structures were called isomers, and a similar phenomenon was called isomerism.
4. Knowing the structure of an organic compound, one can predict its properties; Knowing the properties of an organic compound, one can predict its structure.
5. The atoms that form a molecule are subject to mutual influence, which determines their reactivity. Directly bonded atoms have a greater influence on each other, the influence of not directly bonded atoms is much weaker.

Pupil A.M. Butlerov - V. V. Markovnikov continued to study the issue of the mutual influence of atoms, which was reflected in 1869 in his dissertation work "Materials on the mutual influence of atoms in chemical compounds."

The merit of A.M. Butlerov and the importance of the theory of chemical structure is exceptionally great for chemical synthesis. The opportunity arose to predict the basic properties of organic compounds, to foresee the ways of their synthesis. Thanks to the theory of chemical structure, chemists first appreciated the molecule as an ordered system with a strict bond order between atoms. And at present, the main provisions of Butlerov's theory, despite changes and clarifications, underlie modern theoretical concepts of organic chemistry.

The first appeared at the beginning of the 19th century. the theory of radicals (J. Gay-Lussac, F. Wehler, J. Liebig). Radicals were called groups of atoms that pass unchanged during chemical reactions from one compound to another. This concept of radicals has been preserved, but most of the other provisions of the theory of radicals turned out to be incorrect.

According to the theory of types (C. Gerard), all organic substances can be divided into types corresponding to certain inorganic substances. For example, R-OH alcohols and R-O-R ethers were considered as representatives of the H-OH type of water, in which hydrogen atoms are replaced by radicals. The theory of types created a classification of organic substances, some of the principles of which are currently applied.

The modern theory of the structure of organic compounds was created by the outstanding Russian scientist A.M. Butlerov.

1. Atoms in a molecule are arranged in a certain sequence according to their valency. The valency of the carbon atom in organic compounds is four.

2. The properties of substances depend not only on which atoms and in what quantities are part of the molecule, but also on the order in which they are interconnected.

3. The atoms or groups of atoms that make up the molecule mutually influence each other, on which the chemical activity and reactivity of the molecules depend.

4. The study of the properties of substances allows you to determine their chemical structure.

The mutual influence of neighboring atoms in molecules is the most important property of organic compounds. This influence is transmitted either through a chain of single bonds or through a chain of conjugated (alternating) single and double bonds.

Classification of organic compounds is based on the analysis of two aspects of the structure of molecules - the structure of the carbon skeleton and the presence of functional groups.

Alexander Mikhailovich Butlerov was born on September 3 (15), 1828 in the city of Chistopol, Kazan province, into the family of a landowner, a retired officer. He received his first education in a private boarding school, then studied at the gymnasium and the Kazan Imperial University. From 1849 he taught, in 1857 he became an ordinary professor of chemistry at the same university. Twice he was its rector. In 1851 he defended his master's thesis "On the oxidation of organic compounds", and in 1854 at Moscow University - his doctoral dissertation "On essential oils". From 1868 he was an ordinary professor of chemistry at St. Petersburg University, from 1874 - an ordinary academician of the St. Petersburg Academy of Sciences. In addition to chemistry, Butlerov paid attention to the practical issues of agriculture, horticulture, beekeeping, and under his leadership tea cultivation began in the Caucasus. He died in the village of Butlerovka, Kazan province, on August 5 (17), 1886.

Before Butlerov, a considerable number of attempts were made to create a theory of the chemical structure of organic compounds. This issue was addressed more than once by the most eminent chemists of that time, whose work was partially used by the Russian scientist for his theory of structure. For example, the German chemist August Kekule concluded that carbon can form four bonds with other atoms. Moreover, he believed that for the same compound there may be several formulas, but he always added that, depending on the chemical transformation, this formula may be different. Kekule believed that formulas do not reflect the order in which atoms are connected in a molecule. Another prominent German scientist, Adolf Kolbe, generally considered it fundamentally impossible to elucidate the chemical structure of molecules.

Butlerov first expressed his main ideas about the structure of organic compounds in 1861 in the report “On the chemical structure of matter”, which he presented to the participants of the Congress of German Naturalists and Physicians in Speyer. In his theory, he incorporated the ideas of Kekule about valency (the number of bonds for a particular atom) and the Scottish chemist Archibald Cooper that carbon atoms could form chains. The fundamental difference between Butlerov's theory and others was the position on the chemical (and not mechanical) structure of molecules - the method by which atoms bonded to each other, forming a molecule. At the same time, each atom established a bond in accordance with the “chemical force” belonging specifically to it. In his theory, the scientist made a clear distinction between a free atom and an atom that has joined with another (it passes into a new form, and as a result of mutual influence, the connected atoms, depending on the structural environment, have different chemical functions). The Russian chemist was convinced that the formulas not only represent molecules schematically, but also reflect their real structure. Moreover, each molecule has a certain structure, which changes only in the course of chemical transformations. It followed from the provisions of the theory (subsequently it was confirmed experimentally) that the chemical properties of an organic compound are determined by its structure. This statement is especially important, since it made it possible to explain and predict the chemical transformations of substances. There is also an inverse relationship: the structural formula can be used to judge the chemical and physical properties of a substance. In addition, the scientist drew attention to the fact that the reactivity of compounds is explained by the energy with which atoms bind.

With the help of the created theory, Butlerov was able to explain isomerism. Isomers are compounds in which the number and "quality" of atoms are the same, but at the same time they have different chemical properties, and hence a different structure. The theory made it possible to explain well-known cases of isomerism in an accessible way. Butlerov believed that it was possible to determine the spatial arrangement of atoms in a molecule. His predictions were later confirmed, which gave impetus to the development of a new branch of organic chemistry - stereochemistry. It should be noted that the scientist was the first to discover and explain the phenomenon of dynamic isomerism. Its meaning lies in the fact that two or more isomers under certain conditions can easily pass into each other. Generally speaking, it was isomerism that became a serious test for the theory of chemical structure and was brilliantly explained by it.

The irrefutable propositions formulated by Butlerov very soon brought universal recognition to the theory. The correctness of the ideas put forward was confirmed by the experiments of the scientist and his followers. In their process, they proved the hypothesis of isomerism: Butlerov synthesized one of the four butyl alcohols predicted by the theory, deciphered its structure. In accordance with the rules of isomerism, which directly followed from the theory, the possibility of the existence of four valeric acids was also expressed. Later they were received.

These are just a few facts in a chain of discoveries: the chemical theory of the structure of organic compounds had an amazing predictive ability.

In a relatively short period, a large number of new organic substances and their isomers were discovered, synthesized and studied. As a result, Butlerov's theory gave impetus to the rapid development of chemical science, including synthetic organic chemistry. Thus, Butlerov's numerous syntheses are the main products of entire industries.

The theory of chemical structure continued to develop, which brought many revolutionary ideas to organic chemistry at that time. For example, Kekule put forward an assumption about the cyclic structure of benzene and the movement of its double bonds in a molecule, about the special properties of compounds with conjugated bonds, and much more. Moreover, the mentioned theory made organic chemistry more visual - it became possible to draw the formulas of molecules.

And this, in turn, marked the beginning of the classification of organic compounds. It was the use of structural formulas that helped to determine the ways of synthesis of new substances, to establish the structure of complex compounds, that is, it led to the active development of chemical science and its branches. For example, Butlerov began to conduct serious studies of the polymerization process. In Russia, this undertaking was continued by his students, which eventually made it possible to discover an industrial method for producing synthetic rubber.