Theories of the structure of organic compounds. Types of bonds in molecules of organic substances

Theory of A.M. Butlerov

1. Atoms in molecules are interconnected in a certain sequence by chemical bonds in accordance with their valency. The bonding order of atoms is called their chemical structure. Carbon in all organic compounds is tetravalent.

2. The properties of substances are determined not only by the qualitative and quantitative composition of molecules, but also by their structure.

3. Atoms or groups of atoms mutually influence each other, on which the reactivity of the molecule depends.

4. The structure of molecules can be established on the basis of the study of their chemical properties.

Organic compounds have a number characteristic features that distinguish them from inorganic ones. Almost all of them (with rare exceptions) are combustible; majority organic compounds does not dissociate into ions, which is due to the nature of the covalent bond in organic substances. The ionic type of bond is realized only in salts of organic acids, for example, CH3COONa.

homologous series is an infinite series of organic compounds that have a similar structure and, therefore, similar Chemical properties and differing from each other by any number of CH2– groups (homologous difference).

Even before the creation of the theory of structure, substances of the same elemental composition, but with different properties, were known. Such substances were called isomers, and this phenomenon itself was called isomerism.

At the heart of isomerism, as shown by A.M. Butlerov, lies the difference in the structure of molecules consisting of the same set of atoms.

isomerism- this is the phenomenon of the existence of compounds that have the same qualitative and quantitative composition, but a different structure and, consequently, different properties.

There are 2 types of isomerism: structural isomerism and spatial isomerism.

Structural isomerism

Structural isomers- compounds of the same qualitative and quantitative composition, differing in the order of binding atoms, i.e. chemical structure.

Spatial isomerism

Spatial isomers(stereoisomers) with the same composition and the same chemical structure differ in the spatial arrangement of atoms in the molecule.
Spatial isomers are optical and cis-trans isomers (geometric).

Cis-trans isomerism

consists in the possibility of arranging substituents one by one or by different sides plane of a double bond or non-aromatic ring. In cis isomers substituents are on the same side of the plane of the ring or double bond, in trans isomers- in different ways.

In the butene-2 CH3–CH=CH–CH3 molecule, CH3 groups can be located either on one side of the double bond, in the cis isomer, or on opposite sides, in the trans isomer.

Optical isomerism

Appears when carbon has four different substituents.
If any two of them are interchanged, another spatial isomer of the same composition is obtained. The physicochemical properties of such isomers differ significantly. Compounds of this type are distinguished by their ability to rotate the plane of polarized light passed through the solution of such compounds by a certain amount. In this case, one isomer rotates the plane of polarized light in one direction, and its isomer in the opposite direction. Due to such optical effects, this kind of isomerism is called optical isomerism.

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

1. Atoms in molecules are connected to each other in a certain sequence according to their valencies. The sequence of interatomic bonds in a molecule is called its chemical structure and is reflected by one structural formula (structure formula).

2. The chemical structure can be established by chemical methods. (Currently modern physical methods are also used).

3. The properties of substances depend on their chemical structure.

4. By the properties of a given substance, you can determine the structure of its molecule, and by the structure of the molecule, you can predict the properties.

5. Atoms and groups of atoms in a molecule mutually influence each other.

Butlerov's theory was the scientific foundation of organic chemistry and contributed to its rapid development. Based on the provisions of the theory, A.M. Butlerov gave an explanation for the phenomenon of isomerism, predicted the existence of various isomers, and obtained some of them for the first time.

The development of the theory of structure was facilitated by the work of Kekule, Kolbe, Cooper and van't Hoff. However, their theoretical positions did not carry general and served mainly the purposes of explaining the experimental material.

2. Structure formulas

The structure formula (structural formula) describes the order of connection of atoms in a molecule, i.e. its chemical structure. Chemical bonds in the structural formula are represented by dashes. The bond between hydrogen and other atoms is usually not indicated (such formulas are called abbreviated structural formulas).

For example, the full (expanded) and abbreviated structural formulas of n-butane C4H10 are:

Another example is the isobutane formulas.

Often used even more short entry formulas when they do not represent not only bonds with the hydrogen atom, but also the symbols of carbon and hydrogen atoms. For example, the structure of benzene C6H6 is reflected by the formulas:

Structural formulas differ from molecular (gross) formulas, which show only what elements and in what ratio are included in the composition of the substance (i.e., the qualitative and quantitative elemental composition), but do not reflect the order of binding atoms.

For example, n-butane and isobutane have the same molecular formula C4H10 but a different bond sequence.

Thus, the difference in substances is due not only to different qualitative and quantitative elemental composition, but also to different chemical structures, which can only be reflected in structural formulas.

3. The concept of isomerism

At the heart of isomerism, as shown by A.M. Butlerov, lies the difference in the structure of molecules consisting of the same set of atoms. In this way,

isomerism is the phenomenon of the existence of compounds that have the same qualitative and quantitative composition, but a different structure and, consequently, different properties.

For example, when a molecule contains 4 carbon atoms and 10 hydrogen atoms, the existence of 2 isomeric compounds is possible:

Depending on the nature of the differences in the structure of isomers, structural and spatial isomerism are distinguished.

4. Structural isomers

Structural isomers are compounds of the same qualitative and quantitative composition, differing in the order of binding atoms, i.e., in chemical structure.

For example, the composition of C5H12 corresponds to 3 structural isomers:

Another example:

5. Stereoisomers

Spatial isomers (stereoisomers) with the same composition and the same chemical structure differ in the spatial arrangement of atoms in the molecule.

Spatial isomers are optical and cis-trans isomers (balls of different colors represent different atoms or atomic groups):

The molecules of such isomers are spatially incompatible.

Stereoisomerism plays an important role in organic chemistry. These issues will be considered in more detail when studying compounds of individual classes.

6. Electronic representations in organic chemistry

The application of the electronic theory of the structure of the atom and chemical bonding in organic chemistry was one of the most important stages in the development of the theory of the structure of organic compounds. The concept of chemical structure as a sequence of bonds between atoms (A.M. Butlerov) was supplemented by electronic theory with ideas about the electronic and spatial structure and their influence on the properties of organic compounds. It is these representations that make it possible to understand the ways of transferring the mutual influence of atoms in molecules (electronic and spatial effects) and the behavior of molecules in chemical reactions.

According to modern ideas, the properties of organic compounds are determined by:

the nature and electronic structure of atoms;

the type of atomic orbitals and the nature of their interaction;

type of chemical bonds;

chemical, electronic and spatial structure of molecules.

7. Electron properties

The electron has a dual nature. In different experiments, it can exhibit the properties of both particles and waves. The motion of an electron obeys the laws of quantum mechanics. The connection between the wave and corpuscular properties of an electron reflects the de Broglie relation.

The energy and coordinates of an electron, as well as other elementary particles, cannot be simultaneously measured with the same accuracy (Heisenberg's uncertainty principle). Therefore, the motion of an electron in an atom or molecule cannot be described using a trajectory. An electron can be at any point in space, but with different probabilities.

The part of space in which the probability of finding an electron is high is called an orbital or an electron cloud.

For example:

8. Atomic Orbitals

Atomic orbital (AO) - the region of the most probable stay of an electron (electron cloud) in the electric field of the atomic nucleus.

The position of an element in the Periodic system determines the type of orbitals of its atoms (s-, p-, d-, f-AO, etc.), which differ in energy, shape, size and spatial orientation.

The elements of the 1st period (H, He) are characterized by one AO - 1s.

In the elements of the 2nd period, electrons occupy five AOs at two energy levels: the first level is 1s; second level - 2s, 2px, 2py, 2pz. (the numbers indicate the number of the energy level, the letters indicate the shape of the orbital).

The state of an electron in an atom is completely described by quantum numbers.

Hydrogen type:

Such formulas are somewhat similar to modern ones. But supporters of the theory of types did not consider them to reflect the real structure of substances and wrote many different formulas for one compound, depending on the chemical reactions that they tried to write using these formulas. They considered the structure of molecules to be fundamentally unknowable, which harmed the development of science.

3. The introduction by J. Berzelius in 1830 of the term "isomerism" for the phenomenon of the existence of substances of the same composition with different properties.

4. Successes in the synthesis of organic compounds, as a result of which the doctrine of vitalism, that is, the "life force", under the influence of which organic substances are allegedly formed in the body of living beings, was dispelled:

In 1828, F. Wehler synthesized urea from an inorganic substance (ammonium cyanate);

In 1842, the Russian chemist N. N. Zinin received aniline;

In 1845, the German chemist A. Kolbe synthesized acetic acid;

In 1854, the French chemist M. Berthelot synthesized fats, and, finally,

In 1861, A. M. Butlerov himself synthesized a sugar-like substance.

5. In the middle of the XVIII century. chemistry becomes a more rigorous science. As a result of the work of E. Frankland and A. Kekule, the concept of the valence of atoms of chemical elements was established. Kekule developed the concept of tetravalence of carbon. Thanks to the works of Cannizzaro, the concepts of atomic and molecular masses became clearer, their meanings and methods of determination were refined.

In 1860 more than 140 leading chemists from different countries Europe gathered for an international congress in Karlsruhe. Congress became very important event in the history of chemistry: the successes of science were summarized and conditions were prepared for a new stage in the development of organic chemistry - the emergence of the theory of the chemical structure of organic substances by A. M. Butlerov (1861), as well as for the fundamental discovery of D. I. Mendeleev - the Periodic Law and systems of chemical elements (1869).

In 1861, A. M. Butlerov spoke at the congress of doctors and naturalists in the city of Speyer with a report "On the chemical structure of bodies." In it, he outlined the foundations of his theory of the chemical structure of organic compounds. Under the chemical structure, the scientist understood the order of connection of atoms in molecules.

Personal qualities of A. M. Butlerov

A. M. Butlerov was distinguished by the encyclopedic nature of chemical knowledge, the ability to analyze and generalize facts, and to predict. He predicted the existence of an isomer of butane, and then received it, as well as the isomer of butylene - isobutylene.

Butlerov Alexander Mikhailovich (1828-1886)

Russian chemist, academician of the St. Petersburg Academy of Sciences (since 1874). Graduated from Kazan University (1849). He worked there (since 1857 - professor, in 1860 and 1863 - rector). Creator of the theory of the chemical structure of organic compounds, which underlies modern chemistry. Substantiated the idea of the mutual influence of atoms in a molecule. He predicted and explained the isomerism of many organic compounds. Wrote "Introduction to the complete study of organic chemistry" (1864) - the first manual in the history of science based on the theory of chemical structure. Chairman of the Department of Chemistry of the Russian Physical and Chemical Society (1878-1882).

A. M. Butlerov created the first school of organic chemists in Russia, from which brilliant scientists emerged: V. V. Markovnikov, D. P. Konovalov, A. E. Favorsky and others.

No wonder D. I. Mendeleev wrote: “A. M. Butlerov is one of the greatest Russian scientists, he is Russian both in terms of his scientific education and the originality of his works.”

The main provisions of the theory of the structure of chemical compounds

The theory of the chemical structure of organic compounds, put forward by A. M. Butlerov in the second half of the last century (1861), was confirmed by the work of many scientists, including Butlerov's students and himself. It turned out to be possible on its basis to explain many phenomena that until then had no interpretation: isomerism, homology, the manifestation of tetravalence by carbon atoms in organic substances. The theory also fulfilled its prognostic function: on its basis, scientists predicted the existence of still unknown compounds, described properties and discovered them.

So, in 1862-1864. A. M. Butlerov considered the isomerism of propyl, butyl and amyl alcohols, determined the number of possible isomers and derived the formulas of these substances. Their existence was later experimentally proven, and some of the isomers were synthesized by Butlerov himself.

During the XX century. the provisions of the theory of the chemical structure of chemical compounds were developed on the basis of new views that have spread in science: the theory of the structure of the atom, the theory of chemical bonding, ideas about the mechanisms of chemical reactions. At present, this theory has a universal character, that is, it is valid not only for organic substances, but also for inorganic ones.

First position. Atoms in molecules are connected in a certain order in accordance with their valency. Carbon in all organic and most inorganic compounds is tetravalent.

It is obvious that the last part of the first provision of the theory can be easily explained by the fact that carbon atoms in compounds are in an excited state:

a) tetravalent carbon atoms can combine with each other, forming various chains:

open branched
- open unbranched
- closed

b) the order of connection of carbon atoms in molecules can be different and depends on the type of covalent chemical bond between carbon atoms - single or multiple (double and triple).

Second position. The properties of substances depend not only on their qualitative and quantitative composition, but also on the structure of their molecules.

This position explains the phenomenon of isomerism. Substances that have the same composition, but different chemical or spatial structure, and therefore different properties, are called isomers. The main types of isomerism:

Structural isomerism, in which substances differ in the order of bonding of atoms in molecules:

1) isomerism of the carbon skeleton

3) isomerism of homologous series (interclass)

Spatial isomerism, in which the molecules of substances differ not in the order of bonding of atoms, but in their position in space: cis-trans-isomerism (geometric).

This isomerism is typical for substances whose molecules have a planar structure: alkenes, cycloalkanes, etc.

Optical (mirror) isomerism also belongs to spatial isomerism.

The four single bonds around the carbon atom, as you already know, are arranged tetrahedrally. If a carbon atom is bonded to four different atoms or groups, then a different arrangement of these groups in space is possible, that is, two spatial isomeric forms.

Two mirror forms of the amino acid alanine (2-aminopropanoic acid) are shown in Figure 17.

Imagine that an alanine molecule is placed in front of a mirror. The -NH2 group is closer to the mirror, so it will be in front in the reflection, and the -COOH group will be in the background, etc. (see image on the right). Alanya exists in two spatial forms, which, when superimposed, do not combine with one another.

The universality of the second position of the theory of the structure of chemical compounds confirms the existence of inorganic isomers.

So, the first of the syntheses of organic substances - the synthesis of urea, carried out by Wehler (1828), showed that an inorganic substance - ammonium cyanate and an organic substance - urea are isomeric:

If you replace the oxygen atom in urea with a sulfur atom, you get thiourea, which is isomeric to ammonium thiocyanate, a well-known reagent for Fe 3+ ions. Obviously, thiourea does not give this qualitative reaction.

Third position. The properties of substances depend on the mutual influence of atoms in molecules.

For example, in acetic acid, only one of the four hydrogen atoms reacts with alkali. Based on this, it can be assumed that only one hydrogen atom is bonded to oxygen:

On the other hand, from the structural formula of acetic acid, one can conclude that it contains one mobile hydrogen atom, that is, that it is monobasic.

To verify the universality of the position of the theory of structure on the dependence of the properties of substances on the mutual influence of atoms in molecules, which exists not only in organic, but also in inorganic compounds, we compare the properties of hydrogen atoms in hydrogen compounds of non-metals. They have a molecular structure and under normal conditions are gases or volatile liquids. Depending on the position of the non-metal in the Periodic system of D. I. Mendeleev, a pattern can be identified in the change in the properties of such compounds:

Methane does not interact with water. The lack of basic properties of methane is explained by the saturation of the valence capabilities of the carbon atom.

Ammonia exhibits basic properties. Its molecule is capable of attaching a hydrogen ion to itself due to its attraction to the lone electron pair of the nitrogen atom (donor-acceptor bond formation mechanism).

In phosphine PH3, the basic properties are weakly expressed, which is associated with the radius of the phosphorus atom. It is much larger than the radius of the nitrogen atom, so the phosphorus atom attracts the hydrogen atom to itself more weakly.

In periods from left to right, the charges of the nuclei of atoms increase, the radii of atoms decrease, the repulsive force of the hydrogen atom with a partial positive charge g + increases, and therefore the acidic properties of hydrogen compounds of non-metals are enhanced.

In the main subgroups, the atomic radii of elements increase from top to bottom, non-metal atoms with 5- attract hydrogen atoms with 5+ weaker, the strength of hydrogen compounds decreases, they easily dissociate, and therefore their acidic properties are enhanced.

The different ability of hydrogen compounds of non-metals to remove or add hydrogen cations in solutions is explained by the unequal effect that a non-metal atom has on hydrogen atoms.

The different influence of atoms in the molecules of hydroxides formed by elements of the same period also explains the change in their acid-base properties.

The main properties of hydroxides decrease, while acid ones increase, as the degree of oxidation of the central atom increases, therefore, the energy of its bond with the oxygen atom (8-) and the repulsion of the hydrogen atom (8+) by it increase.

Sodium hydroxide NaOH. Since the radius of the hydrogen atom is very small, it attracts the oxygen atom to itself more strongly and the bond between hydrogen and oxygen atoms will be stronger than between sodium and oxygen atoms. Aluminum hydroxide Al(OH)3 exhibits amphoteric properties.

In perchloric acid HclO 4, the chlorine atom with a relatively large positive charge is more strongly bonded to the oxygen atom and repels the hydrogen atom with 6+ more strongly. Dissociation proceeds according to the acid type.

The main directions in the development of the theory of the structure of chemical compounds and its significance

At the time of A. M. Butlerov, empirical (molecular) and structural formulas were widely used in organic chemistry. The latter reflect the order of connection of atoms in a molecule according to their valency, which is indicated by dashes.

For ease of recording, abbreviated structural formulas are often used, in which only the bonds between carbon or carbon and oxygen atoms are indicated by dashes.

Abbreviated structural formulas

Then, with the development of knowledge about the nature of the chemical bond and the influence of the electronic structure of the molecules of organic substances on their properties, they began to use electronic formulas in which the covalent bond is conventionally denoted by two dots. In such formulas, the direction of displacement of electron pairs in a molecule is often shown.

It is the electronic structure of substances that explains the mesomeric and induction effects.

The inductive effect is the displacement of electron pairs of gamma bonds from one atom to another due to their different electronegativity. Denoted (->).

The induction effect of an atom (or a group of atoms) is negative (-/), if this atom has a high electronegativity (halogens, oxygen, nitrogen), attracts gamma bond electrons and acquires a partial negative charge. An atom (or group of atoms) has a positive inductive effect (+/) if it repels the electrons of the gamma bonds. This property is possessed by some limiting radicals C2H5). Remember Markovnikov's rule about how hydrogen and a halogen of a hydrogen halide are added to alkenes (propene) and you will understand that this rule is of a particular nature. Compare these two examples of reaction equations:

[[Theory_of_the_chemical_compounds_A._M._Butlerov| ]]

In the molecules of individual substances, both induction and mesomeric effects are manifested simultaneously. In this case, they either reinforce each other (in aldehydes, carboxylic acids), or mutually weaken (in vinyl chloride).

The result of the mutual influence of atoms in molecules is the redistribution of electron density.

The idea of the spatial direction of chemical bonds was first expressed by the French chemist J. A. Le Bel and the Dutch chemist J. X. Van't Hoff in 1874. The scientists' assumptions were fully confirmed by quantum chemistry. The properties of substances are significantly affected by the spatial structure of their molecules. For example, we have already given the formulas for cis- and trans-isomers of butene-2, which differ in their properties (see Fig. 16).

The average bond energy that must be broken during the transition from one form to another is approximately 270 kJ / mol; there is not so much energy at room temperature. For the mutual transition of butene-2 forms from one to another, it is necessary to break one covalent bond and form another instead. In other words, this process is an example of a chemical reaction, and both forms of butene-2 considered are different chemical compounds.

You obviously remember that the most important problem in the synthesis of rubber was getting stereoregular rubber. It was necessary to create a polymer in which the structural units would be arranged in a strict order (natural rubber, for example, consists only of cis-units), because such an important property of rubber as its elasticity depends on this.

Modern organic chemistry distinguishes two main types of isomerism: structural (chain isomerism, isomerism of the position of multiple bonds, isomerism of homologous series, isomerism of the position of functional groups) and stereoisomerism (geometric, or cis-trans-isomerism, optical, or mirror, isomerism).

So, you were able to make sure that the second position of the theory of chemical structure, clearly formulated by A. M. Butlerov, was incomplete. From a modern standpoint, this provision requires additions:
the properties of substances depend not only on their qualitative and quantitative composition, but also on their:

Chemical,

electronic,

Spatial structure.

The creation of the theory of the structure of substances played essential role in the development of organic chemistry. From a predominantly descriptive science, it turns into a creative, synthesizing science; it became possible to judge the mutual influence of atoms in the molecules of various substances (see Table 10). The theory of structure created the prerequisites for explaining and predicting various kinds isomerism organic molecules, as well as directions and mechanisms of chemical reactions.

On the basis of this theory, organic chemists create substances that not only replace natural ones, but significantly surpass them in their properties. So, synthetic dyes are much better and cheaper than many natural ones, for example, alizarin and indigo known in antiquity. Synthetic rubbers are produced in large quantities with a wide variety of properties. Plastics and fibers are widely used, products from which are used in engineering, everyday life, medicine, and agriculture.

The value of the theory of chemical structure of A. M. Butlerov for organic chemistry can be compared with the value of the Periodic law and the Periodic system of chemical elements of D. I. Mendeleev for inorganic chemistry. It is not for nothing that both theories have so much in common in the ways of their formation, directions of development and general scientific significance. However, in the history of any other leading scientific theory(Ch. Darwin's theory, genetics, quantum theory, etc.) one can find such general stages.

1. Establish parallels between the two leading theories of chemistry - the Periodic Law and the Periodic Table of Chemical Elements by D. I. Mendeleev and the theory of the chemical structure of organic compounds by A. M. Butlerov according to the following features: common in prerequisites, common in the directions of their development, common in the prognostic role.

2. What role did the theory of the structure of chemical compounds play in the formation of the Periodic Law?

3. What examples from inorganic chemistry confirm the universality of each of the provisions of the theory of the structure of chemical compounds?

4. Phosphorous acid H3PO3 refers to dibasic acids. Propose its structural formula and consider the mutual influence of atoms in the molecule of this acid.

5. Write the isomers having the composition С3Н8O. Name them according to the systematic nomenclature. Determine the types of isomerism.

6. Known the following formulas crystalline hydrates of chromium(III) chloride: [Cr(H20)6]Cl3; [Cr(H20)5Cl]Cl2 H20; [Cr(H20)4 * C12]Cl 2H2O. What would you call this phenomenon?

How science took shape at the beginning of the 19th century, when the Swedish scientist J. J. Berzelius first introduced the concept of organic substances and organic chemistry. The first theory in organic chemistry is the theory of radicals. Chemists have discovered that during chemical transformations, groups of several atoms pass unchanged from a molecule of one substance to a molecule of another substance, just as atoms of elements pass from molecule to molecule. Such "immutable" groups of atoms are called radicals.

However, not all scientists agreed with the theory of radicals. Many generally rejected the idea of atomism - the idea of the complex structure of the molecule and the existence of the atom as its constituent part. What is undeniably proven in our days and does not cause the slightest doubt, in the XIX century. was the subject of fierce controversy.

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Just as in inorganic chemistry the fundamental theoretical basis is the Periodic law and the Periodic system of chemical elements of D. I. Mendeleev, so in organic chemistry the leading scientific basis is the theory of the structure of organic compounds of Butlerov-Kekule-Cooper.

Like any other scientific theory, the theory of the structure of organic compounds was the result of a generalization of the richest factual material accumulated by organic chemistry, which took shape as a science at the beginning of the 19th century. More and more new carbon compounds were discovered, the number of which increased like an avalanche (Table 1).

Table 1
Number of organic compounds known in different years

Explain this variety of organic compounds scientists early XIX in. could not. Even more questions were raised by the phenomenon of isomerism.

For example, ethyl alcohol and dimethyl ether are isomers: these substances have the same composition C 2 H 6 O, but a different structure, that is, a different order of connection of atoms in molecules, and therefore different properties.

F. Wöhler, already known to you, in one of his letters to J. J. Berzelius, described organic chemistry as follows: “Organic chemistry can now drive anyone crazy. It seems to me a dense forest, full of amazing things, a boundless thicket from which you can’t get out, where you don’t dare to penetrate ... "

The development of chemistry was greatly influenced by the work of the English scientist E. Frankland, who, relying on the ideas of atomism, introduced the concept of valency (1853).

In the hydrogen molecule H 2, one covalent chemical is formed H-H connection, i.e., hydrogen is monovalent. The valence of a chemical element can be expressed by the number of hydrogen atoms that one atom of a chemical element attaches to itself or replaces. For example, sulfur in hydrogen sulfide and oxygen in water are divalent: H 2 S, or H-S-H, H 2 O, or H-O-H, and nitrogen in ammonia is trivalent:

In organic chemistry, the concept of "valence" is analogous to the concept of "oxidation state", which you are used to working with in the course of inorganic chemistry in elementary school. However, they are not the same. For example, in a nitrogen molecule N 2, the oxidation state of nitrogen is zero, and the valency is three:

In hydrogen peroxide H 2 O 2, the oxidation state of oxygen is -1, and the valency is two:

In the ammonium ion NH + 4, the oxidation state of nitrogen is -3, and the valency is four:

Usually, in relation to ionic compounds (sodium chloride NaCl and many other inorganic substances with an ionic bond), the term "valency" of atoms is not used, but their oxidation state is considered. Therefore, in inorganic chemistry, where most substances have a non-molecular structure, it is preferable to use the concept of "oxidation state", and in organic chemistry, where most compounds have a molecular structure, as a rule, use the concept of "valence".

The theory of chemical structure is the result of a generalization of the ideas of outstanding organic scientists from three European countries: the German F. Kekule, the Englishman A. Cooper and the Russian A. Butlerov.

In 1857, F. Kekule classified carbon as a tetravalent element, and in 1858, simultaneously with A. Cooper, he noted that carbon atoms can combine with each other in various chains: linear, branched and closed (cyclic).

The works of F. Kekule and A. Cooper served as the basis for the development of a scientific theory explaining the phenomenon of isomerism, the relationship between the composition, structure and properties of molecules of organic compounds. Such a theory was created by the Russian scientist A. M. Butlerov. It was his inquisitive mind that "dared to penetrate" the "dense forest" of organic chemistry and begin the transformation of this "boundless thicket" into a regular park filled with sunlight with a system of paths and alleys. The main ideas of this theory were first expressed by A. M. Butlerov in 1861 at the congress of German naturalists and doctors in Speyer.

Briefly formulate the main provisions and consequences of the Butlerov-Kekule-Cooper theory of the structure of organic compounds as follows.

1. The atoms in the molecules of substances are connected in a certain sequence according to their valency. Carbon in organic compounds is always tetravalent, and its atoms are able to combine with each other, forming various chains (linear, branched and cyclic).

Organic compounds can be arranged in series of substances similar in composition, structure and properties - homologous series.

Butlerov Alexander Mikhailovich (1828-1886)

For example, methane CH 4 is the ancestor of the homologous series of saturated hydrocarbons (alkanes). Its closest homologue is ethane C 2 H 6, or CH 3 -CH 3. The next two members of the homologous series of methane are propane C 3 H 8, or CH 3 -CH 2 -CH 3, and butane C 4 H 10, or CH 3 -CH 2 -CH 2 -CH 3, etc.

It is easy to see that for homologous series one can derive a general formula for the series. So, for alkanes, this general formula is C n H 2n + 2.

2. The properties of substances depend not only on their qualitative and quantitative composition, but also on the structure of their molecules.

This position of the theory of the structure of organic compounds explains the phenomenon of isomerism. Obviously, for butane C 4 H 10, in addition to the molecule linear structure CH 3 -CH 2 -CH 2 -CH 3, a branched structure is also possible:

This is a completely new substance with its own individual properties, different from those of linear butane.

Butane, in the molecule of which the atoms are arranged in the form of a linear chain, is called normal butane (n-butane), and butane, the chain of carbon atoms of which is branched, is called isobutane.

There are two main types of isomerism - structural and spatial.

In accordance with the accepted classification, three types of structural isomerism are distinguished.

Isomerism of the carbon skeleton. Compounds differ in the order of carbon-carbon bonds, for example, n-butane and isobutane considered. It is this type of isomerism that is characteristic of alkanes.

Isomerism of the position of a multiple bond (C=C, C=C) or a functional group (i.e., a group of atoms that determine whether a compound belongs to a particular class of organic compounds), for example:

Interclass isomerism. Isomers of this type of isomerism belong to different classes of organic compounds, for example, ethyl alcohol (the class of saturated monohydric alcohols) and dimethyl ether (the class of ethers) discussed above.

There are two types of spatial isomerism: geometric and optical.

Geometric isomerism is characteristic, first of all, for compounds with a double carbon-carbon bond, since the molecule has a planar structure at the site of such a bond (Fig. 6).

Rice. 6.
Model of the ethylene molecule

For example, for butene-2, if the same groups of atoms at carbon atoms in a double bond are on one side of the C=C bond plane, then the molecule is a cisisomer, if on opposite sides it is a transisomer.

Optical isomerism is possessed, for example, by substances whose molecules have an asymmetric, or chiral, carbon atom bonded to four various deputies. Optical isomers are mirror images of each other, like two palms, and are not compatible. (Now, obviously, the second name of this type of isomerism has become clear to you: Greek chiros - hand - a sample of an asymmetric figure.) For example, in the form of two optical isomers, there is 2-hydroxypropanoic (lactic) acid containing one asymmetric carbon atom.

Isomeric pairs arise in chiral molecules, in which the isomer molecules are related to each other in their spatial organization in the same way as an object and its mirror image are related to each other. A pair of such isomers always has the same chemical and physical properties, with the exception of optical activity: if one isomer rotates the plane of polarized light clockwise, then the other necessarily counterclockwise. The first isomer is called dextrorotatory, and the second is called levorotatory.

The importance of optical isomerism in the organization of life on our planet is very great, since optical isomers can differ significantly both in their biological activity and in compatibility with other natural compounds.

3. The atoms in the molecules of substances influence each other. You will consider the mutual influence of atoms in the molecules of organic compounds in the further study of the course.

The modern theory of the structure of organic compounds is based not only on the chemical, but also on the electronic and spatial structure of substances, which is considered in detail at the profile level of the study of chemistry.

Several types of chemical formulas are widely used in organic chemistry.

The molecular formula reflects the qualitative composition of the compound, that is, it shows the number of atoms of each of the chemical elements that form the molecule of the substance. For example, the molecular formula of propane is C 3 H 8 .

The structural formula reflects the order of connection of atoms in a molecule according to valence. The structural formula of propane is:

Often there is no need to depict in detail the chemical bonds between carbon and hydrogen atoms, therefore, in most cases, abbreviated structural formulas are used. For propane, such a formula is written as follows: CH 3 -CH 2 -CH 3.

The structure of molecules of organic compounds is reflected using various models. The best known are volumetric (scale) and ball-and-stick models (Fig. 7).

Rice. 7.
Models of the ethane molecule:
1 - ball-and-stick; 2 - scale

New words and concepts

Isomerism, isomers.
Valence.
Chemical structure.
Theory of the structure of organic compounds.
Homological series and homological difference.
Formulas molecular and structural.
Models of molecules: volumetric (scale) and spherical.

Questions and tasks

What is valence? How is it different from oxidation state? Give examples of substances in which the values of the oxidation state and valence of atoms are numerically the same and different,
Determine the valency and oxidation state of atoms in substances whose formulas are Cl 2, CO 2, C 2 H 6, C 2 H 4.
What is isomerism; isomers?
What is homology; homologues?
How, using knowledge of isomerism and homology, to explain the diversity of carbon compounds?
What is meant by the chemical structure of molecules of organic compounds? Formulate the position of the theory of structure, which explains the difference in the properties of isomers. Formulate the position of the theory of structure, which explains the diversity of organic compounds.
What contribution did each of the scientists - the founders of the theory of chemical structure - make to this theory? Why did the contribution of the Russian chemist play a leading role in the formation of this theory?
It is possible that there are three isomers of the composition C 5 H 12. Write down their full and abbreviated structural formulas,
According to the model of the substance molecule presented at the end of the paragraph (see Fig. 7), make up its molecular and abbreviated structural formulas.
Calculate the mass fraction of carbon in the molecules of the first four members of the homologous series of alkanes.

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