Theoretical foundations of the structure of organic compounds. The main provisions of the theory of the chemical structure of organic compounds by A.M. Butlerov

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.

Topic: The main provisions of the theory of the structure of organic compounds by A. M. Butlerova.

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: 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 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:

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

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):

This position explains the phenomenon.

Substances that have the same composition, but different chemical or spatial structure, and therefore different properties, are called isomers.

Main types:

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

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:

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, organic chemistry widely used

empirical (molecular) and structural formulas. 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.

And fibers, products from which are used in technology, 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.

Organic chemistry- a branch of chemistry in which carbon compounds are studied, their structure, properties, interconversions.

The very name of the discipline - "organic chemistry" - arose quite a long time ago. The reason for it lies in the fact that most of the carbon compounds encountered by researchers at the initial stage of the formation of chemical science were of plant or animal origin. However, as an exception, individual carbon compounds are classified as inorganic. So, for example, carbon oxides, carbonic acid, carbonates, hydrocarbonates, hydrogen cyanide and some others are considered to be inorganic substances.

Currently, a little less than 30 million various organic substances are known, and this list is constantly updated. Such a huge number of organic compounds is associated primarily with the following specific properties of carbon:

1) carbon atoms can be connected to each other in chains of arbitrary length;

2) not only sequential (linear) connection of carbon atoms among themselves is possible, but also branched and even cyclic;

3) different types of bonds between carbon atoms are possible, namely single, double and triple. In this case, the valence of carbon in organic compounds is always equal to four.

In addition, a wide variety of organic compounds is also facilitated by the fact that carbon atoms are able to form bonds with the atoms of many other chemical elements, for example, hydrogen, oxygen, nitrogen, phosphorus, sulfur, halogens. Hydrogen, oxygen and nitrogen are the most common.

It should be noted that for quite a long time organic chemistry represented a “dark forest” for scientists. For some time, the theory of vitalism was even popular in science, according to which organic substances cannot be obtained in an “artificial” way, i.e. outside of living matter. However, the theory of vitalism did not last very long, in view of the fact that one by one substances were discovered, the synthesis of which is possible outside living organisms.

Researchers were perplexed by the fact that many organic substances have the same qualitative and quantitative composition, but often have completely different physical and chemical properties. So, for example, dimethyl ether and ethyl alcohol have exactly the same elemental composition, however, under normal conditions, dimethyl ether is a gas, and ethyl alcohol is a liquid. In addition, dimethyl ether does not react with sodium, but ethyl alcohol interacts with it, releasing hydrogen gas.

Researchers of the 19th century put forward many assumptions about how organic substances are nevertheless arranged. Significantly important assumptions were put forward by the German scientist F.A. Kekule, who was the first to put forward the idea that atoms of different chemical elements have specific valence values, and carbon atoms in organic compounds are tetravalent and can combine with each other, forming chains. Later, starting from the assumptions of Kekule, the Russian scientist Alexander Mikhailovich Butlerov developed a theory of the structure of organic compounds, which has not lost its relevance in our time. Consider the main provisions of this theory:

1) all atoms in the molecules of organic substances are connected to each other in a certain sequence in accordance with their valency. Carbon atoms have a constant valency of four and can form chains of various structures with each other;

2) the physical and chemical properties of any organic substance depend not only on the composition of its molecules, but also on the order in which the atoms in this molecule are connected to each other;

3) individual atoms, as well as groups of atoms in a molecule, influence each other. This mutual influence is reflected in the physical and chemical properties of the compounds;

4) by examining the physical and chemical properties of an organic compound, its structure can be established. The opposite is also true - knowing the structure of the molecule of a substance, you can predict its properties.

Just as the periodic law of D.I. Mendelev became the scientific foundation of inorganic chemistry, the theory of the structure of organic substances A.M. Butlerova actually became the starting point in the development of organic chemistry as a science. It should be noted that after the creation of Butler's theory of structure, organic chemistry began its development at a very rapid pace.

Isomerism and homology

According to the second position of Butlerov's theory, the properties of organic substances depend not only on the qualitative and quantitative composition of molecules, but also on the order in which the atoms in these molecules are connected to each other.

In this regard, such a phenomenon as isomerism is widespread among organic substances.

Isomerism is a phenomenon when different substances have exactly the same molecular composition, i.e. the same molecular formula.

Very often, isomers differ greatly in physical and chemical properties. For example:

Types of isomerism

Structural isomerism

a) Isomerism of the carbon skeleton

b) Position isomerism:

multiple bond

deputies:

functional groups:

c) Interclass isomerism:

Interclass isomerism occurs when compounds that are isomers belong to different classes of organic compounds.

Spatial isomerism

Spatial isomerism is a phenomenon when different substances with the same order of attachment of atoms to each other differ from each other by a fixed-different position of atoms or groups of atoms in space.

There are two types of spatial isomerism - geometric and optical. There are no assignments for optical isomerism at the Unified State Examination, so we will consider only the geometric one.

If there is a double C=C bond or a cycle in the molecule of any compound, sometimes in such cases the phenomenon of geometric or cis-trans-isomerism.

For example, this type of isomerism is possible for butene-2. Its meaning lies in the fact that the double bond between carbon atoms actually has a planar structure, and substituents at these carbon atoms can be fixedly located either above or below this plane:

When the same substituents are on the same side of the plane, they say that this cis-isomer, and when different - trance-isomer.

On in the form of structural formulas cis- And trance-isomers (for example, butene-2) are depicted as follows:

Note that geometric isomerism is impossible if at least one carbon atom in the double bond has two identical substituents. For example, cis-trans- isomerism is impossible for propene:

As you can see from the illustration above, if we swap the methyl radical and the hydrogen atom located at the second carbon atom on opposite sides of the plane, we get the same molecule, which we just looked at from the other side.

The influence of atoms and groups of atoms on each other in the molecules of organic compounds

The concept of a chemical structure as a sequence of atoms connected to each other was significantly expanded with the advent of the electronic theory. From the standpoint of this theory, it is possible to explain how atoms and groups of atoms in a molecule influence each other.

There are two possible ways of influence of some parts of the molecule on others:

1) Inductive effect

2) Mesomeric effect

Inductive effect

To demonstrate this phenomenon, let us take, for example, a molecule of 1-chloropropane (CH 3 CH 2 CH 2 Cl). The bond between carbon and chlorine is polar because chlorine has a much higher electronegativity than carbon. As a result of the displacement of the electron density from the carbon atom to the chlorine atom, a partial positive charge (δ+) is formed on the carbon atom, and a partial negative charge (δ-) is formed on the chlorine atom:

The shift of electron density from one atom to another is often indicated by an arrow pointing towards the more electronegative atom:

However, it is interesting that, in addition to the shift in electron density from the first carbon atom to the chlorine atom, there is also a shift, but to a somewhat lesser extent, from the second carbon atom to the first, and also from the third to the second:

Such a shift of the electron density along the chain of σ-bonds is called the inductive effect ( I). This effect fades with distance from the influencing group and practically does not manifest itself after 3 σ-bonds.

In the case when an atom or group of atoms has a greater electronegativity compared to carbon atoms, such substituents are said to have a negative inductive effect (- I). Thus, in the example discussed above, the chlorine atom has a negative inductive effect. In addition to chlorine, the following substituents have a negative inductive effect:

–F, –Cl, –Br, –I, –OH, –NH 2 , –CN, –NO 2 , –COH, –COOH

If the electronegativity of an atom or group of atoms is less than the electronegativity of a carbon atom, there is actually a transfer of electron density from such substituents to carbon atoms. In this case, the substituent is said to have a positive inductive effect (+ I) (is electron-donating).

So, substituents with + I-effect are saturated hydrocarbon radicals. At the same time, the expression I-effect increases with elongation of the hydrocarbon radical:

–CH 3 , –C 2 H 5 , –C 3 H 7 , –C 4 H 9

It should be noted that carbon atoms in different valence states also have different electronegativity. Sp carbon atoms have a higher electronegativity than sp 2 carbon atoms, which in turn are more electronegative than sp 3 carbon atoms.

Mesomeric effect (M), or conjugation effect, is the influence of a substituent transmitted through a system of conjugated π-bonds.

The sign of the mesomeric effect is determined by the same principle as the sign of the inductive effect. If a substituent increases the electron density in the conjugated system, it has a positive mesomeric effect (+ M) and is electron-donating. Double carbon-carbon bonds, substituents containing an unshared electron pair: -NH 2, -OH, halogens have a positive mesomeric effect.

Negative mesomeric effect (– M) have substituents that pull the electron density away from the conjugated system, while the electron density in the system decreases.

The following groups have a negative mesomeric effect:

–NO 2 , –COOH, –SO 3 H, -COH, >C=O

Due to the redistribution of the electron density due to the mesomeric and inductive effects in the molecule, partial positive or negative charges appear on some atoms, which is reflected in the chemical properties of the substance.

Graphically, the mesomeric effect is shown by a curved arrow that starts at the center of the electron density and ends where the electron density shifts. So, for example, in the vinyl chloride molecule, the mesomeric effect occurs when the lone electron pair of the chlorine atom is conjugated with the electrons of the π-bond between carbon atoms. Thus, as a result of this, a partial positive charge appears on the chlorine atom, and the mobile π-electron cloud, under the influence of an electron pair, shifts towards the extreme carbon atom, on which a partial negative charge arises as a result:

If a molecule contains alternating single and double bonds, then the molecule is said to contain a conjugated π-electron system. An interesting property of such a system is that the mesomeric effect does not decay in it.

Lesson content: Theories of the structure of organic compounds: prerequisites for creation, basic provisions. Chemical structure as the order of connection and mutual influence of atoms in molecules. Homology, isomerism. The dependence of the properties of substances on the chemical structure. The main directions of development of the theory of chemical structure. The dependence of the appearance of toxicity in organic compounds on the composition and structure of their molecules (the length of the carbon chain and the degree of its branching, the presence of multiple bonds, the formation of cycles and peroxide bridges, the presence of halogen atoms), as well as on the solubility and volatility of the compound.

Lesson Objectives:

• Organize the activities of students to familiarize and consolidate the primary provisions of the theory of chemical structure.
• Show students the universal nature of the theory of chemical structure using the example of inorganic isomers and the mutual influence of atoms in inorganic substances.

During the classes:

1. Organizational moment.

2. Actualization of students' knowledge.

1) What does organic chemistry study?

2) What substances are called isomers?

3) What substances are called homologues?

4) Name the theories known to you that arose in organic chemistry at the beginning of the 19th century.

5) What were the disadvantages of the theory of radicals?

6) What were the shortcomings of type theory?

3. Setting goals and objectives of the lesson.

The concept of valency formed an important part of the theory of the chemical structure of A.M. Butlerov in 1861

The periodic law formulated by D.I. Mendeleev in 1869, revealed the dependence of the valency of an element on its position in the periodic system.

It remained unclear the wide variety of organic substances that have the same qualitative and quantitative composition, but different properties. For example, about 80 different substances were known that corresponded to the composition C 6 H 12 O 2 . Jens Jakob Berzelius suggested calling these substances isomers.

Scientists from many countries have paved the way for the creation of a theory explaining the structure and properties of organic substances.

At the congress of German naturalists and doctors in the city of Speyer, a report was read, called "Something in the chemical structure of bodies." The author of the report was Professor of Kazan University Alexander Mikhailovich Butlerov. It was this very “something” that constituted the theory of chemical structure, which formed the basis of our modern ideas about chemical compounds.

Organic chemistry received a solid scientific basis, which ensured its rapid development in the next century up to the present day. This theory made it possible to predict the existence of new compounds and their properties. The concept of the chemical structure made it possible to explain such a mysterious phenomenon as isomerism.

The main provisions of the theory of chemical structure are as follows:
1. Atoms in the molecules of organic substances are connected in a certain sequence according to their valency.

2. The properties of substances are determined by the qualitative, quantitative composition, the order of connection and the mutual influence of atoms and groups of atoms in a molecule.

3. The structure of molecules can be established on the basis of studying their properties.

Let's consider these provisions in more detail. Molecules of organic substances contain carbon atoms (valence IV), hydrogen (valence I), oxygen (valence II), nitrogen (valency III). Each carbon atom in the molecules of organic substances forms four chemical bonds with other atoms, while carbon atoms can be combined into chains and rings. Based on the first position of the theory of chemical structure, we will draw up the structural formulas of organic substances. For example, methane has been found to have the composition CH 4 . Given the valencies of carbon and hydrogen atoms, only one structural formula of methane can be proposed:

The chemical structure of other organic substances can be described by the following formulas:

ethanol

The second position of the theory of chemical structure describes the relationship known to us: composition - structure - properties. Let's look at the manifestation of this regularity on the example of organic substances.

Ethane and ethyl alcohol have different qualitative composition. An alcohol molecule, unlike ethane, contains an oxygen atom. How will this affect properties?

The introduction of an oxygen atom into a molecule dramatically changes the physical properties of the substance. This confirms the dependence of properties on the qualitative composition.

Let's compare the composition and structure of methane, ethane, propane and butane hydrocarbons.

Methane, ethane, propane and butane have the same qualitative composition, but different quantitative composition (the number of atoms of each element). According to the second position of the theory of chemical structure, they must have different properties.

As can be seen from the table, with an increase in the number of carbon atoms in a molecule, an increase in the boiling and melting points occurs, which confirms the dependence of the properties on the quantitative composition of the molecules.

The molecular formula C 4 H 10 corresponds not only to butane, but also to its isomer isobutane:

Isomers have the same qualitative (carbon and hydrogen atoms) and quantitative (4 carbon atoms and ten hydrogen atoms) composition, but differ from each other in the order of connection of atoms (chemical structure). Let's see how the difference in the structure of isomers will affect their properties.

A branched hydrocarbon (isobutane) has higher boiling and melting points than a normal hydrocarbon (butane). This can be explained by the closer arrangement of molecules to each other in butane, which increases the forces of intermolecular attraction and, therefore, requires more energy to separate them.

The third position of the theory of chemical structure shows the feedback of the composition, structure and properties of substances: composition - structure - properties. Consider this using the example of compounds of the composition C 2 H 6 O.

Imagine that we have samples of two substances with the same molecular formula C 2 H 6 O, which was determined in the course of a qualitative and quantitative analysis. But how to find out the chemical structure of these substances? To answer this question will help the study of their physical and chemical properties. When the first substance interacts with metallic sodium, the reaction does not proceed, and the second actively interacts with it with the release of hydrogen. Let us determine the quantitative ratio of substances in the reaction. To do this, we add a certain mass of sodium to the known mass of the second substance. Let's measure the volume of hydrogen. Let's calculate the amount of substances. In this case, it turns out that out of two moles of the substance under study, one mole of hydrogen is released. Therefore, each molecule of this substance is a source of one hydrogen atom. What conclusion can be drawn? Only one hydrogen atom differs in properties and, therefore, in structure (with which atoms it is associated) from all the others. Given the valency of carbon, hydrogen and oxygen atoms, only one formula can be proposed for a given substance:

For the first substance, a formula can be proposed in which all hydrogen atoms have the same structure and properties:

A similar result can be obtained by studying the physical properties of these substances.

Thus, based on the study of the properties of substances, one can draw a conclusion about its chemical structure.

The importance of the theory of chemical structure can hardly be overestimated. It provided chemists with a scientific basis for studying the structure and properties of organic substances. The Periodic Law, formulated by D.I. Mendeleev. The theory of structure generalized all the scientific views prevailing in chemistry of that time. Scientists were able to explain the behavior of organic substances during chemical reactions. Based on the theory of A.M. Butlerov predicted the existence of isomers of certain substances, which were later obtained. Like the Periodic Law, the theory of chemical structure was further developed after the formation of the theory of the structure of the atom, chemical bonding and stereochemistry.

The chemical nature of organic compounds, the properties that distinguish them from inorganic compounds, as well as their diversity, were explained in the theory of chemical structure formulated by Butlerov in 1861 (see § 38).

According to this theory, the properties of compounds are determined by their qualitative and quantitative composition, chemical structure, i.e., the sequential order of connection between the atoms that form the molecule, and their mutual influence. The theory of the structure of organic compounds, developed and supplemented by the latest views in the field of chemistry and physics of atoms and molecules, especially ideas about the spatial structure of molecules, the nature of chemical bonds and the nature of the mutual influence of atoms, forms the theoretical basis of organic chemistry.

In the modern theory of the structure of organic compounds, the main provisions are the following.

1. All features of organic compounds are determined primarily by the properties of the element carbon.

In accordance with the place that carbon occupies in the periodic system, there are four electrons in the outer electron layer of its atom (-shell). It does not show a pronounced tendency to donate or add electrons, in this respect it occupies an intermediate position between metals and non-metals and is characterized by a pronounced ability to form covalent bonds. The structure of the outer electron layer of the carbon atom can be represented by the following diagrams:

An excited carbon atom can participate in the formation of four covalent bonds. Therefore, in the vast majority of its compounds, carbon exhibits a covalence equal to four.

So, the simplest organic compound hydrocarbon methane has a composition. Its structure can be represented by the structure (a) or electronic structural (or electronic) (b) formulas:

The electronic formula shows that the carbon atom in the methane molecule has a stable eight-electron outer shell (electron octet), and hydrogen atoms have a stable two-electron shell (electron doublet).

All four covalent bonds of carbon in methane (and in other similar compounds) are equivalent and symmetrically directed in space. The carbon atom is located, as it were, in the center of the tetrahedron (regular tetrahedral pyramid), and the four atoms connected to it (in the case of methane, four atoms are at the vertices of the tetrahedron (Fig. 120). The angles between the directions of any pair of bonds (valence angles of carbon) are the same and amount to 109 ° 28".

This is explained by the fact that in the carbon atom, when it forms covalent bonds with four other atoms, from one s- and three p-orbitals, as a result of -hybridization, four hybrid -orbitals symmetrically located in space are formed, elongated towards the vertices of the tetrahedron.

Rice. 120. Tetrahedral model of the methane molecule.

Rice. 121. Scheme of the formation of -bonds in a methane molecule.

As a result of overlapping - hybrid electron clouds of carbon with electron clouds of other atoms (in methane with spherical clouds - electrons of hydrogen atoms), four tetrahedrally directed covalent bonds are formed (Fig. 121; see also p. 131).

The tetrahedral structure of the methane molecule is clearly expressed by its spatial models - spherical (Fig. 122) or segmental (Fig. 123). White balls (segments) represent hydrogen atoms, black - carbon. The ball model characterizes only the mutual spatial arrangement of atoms, the segment one also gives an idea of ​​the relative interatomic distances (distances between nuclei. As shown in Fig. 122, the structural formula of methane can be considered as a projection of its spatial model onto the plane of the drawing.

2. An exceptional property of carbon, which determines the variety of organic compounds, is the ability of its atoms to connect with each other by strong covalent bonds, forming carbon chains of almost unlimited length.

The valences of carbon atoms that have not gone into mutual connection are used to add other atoms or groups (in hydrocarbons, to add hydrogen).

Thus, the hydrocarbons ethane and propane contain chains of two and three carbon atoms, respectively.

Rice. 122. Ball model of the methane molecule.

Rice. 123. Segment model of the methane molecule.

Their structure is expressed by the following structural and electronic formulas:

Compounds containing hundreds or more carbon atoms are known.

The growth of the carbon chain by one carbon atom leads to an increase in the composition per group. Such a quantitative change in composition leads to a new compound with slightly different properties, i.e., already qualitatively different from the original compound; however, the general character of the compounds is retained. So, in addition to the hydrocarbons of methane, ethane, propane, there are butane, pentane, etc. Thus, in a huge variety of organic substances, series of compounds of the same type can be distinguished, in which each subsequent member differs from the previous one by a group. Such series are called homological series, their terms are homologues with respect to each other, and the existence of such series is called the phenomenon of homology.

Consequently, the hydrocarbons methane, stage, propane, butane, etc. are homologues of the same series, which is called the series of limiting, or saturated, hydrocarbons (alkanes) or, according to the first representative, the methane series.

Due to the tetrahedral orientation of carbon bonds, its atoms included in the chain are located not in a straight line, but in a zigzag manner, and, due to the possibility of rotation of atoms around the bond axis, the chain in space can take various forms (conformations):

This structure of the chains makes it possible to approach the terminal (b) or other non-adjacent carbon atoms (c); as a result of the appearance of a bond between these atoms, carbon chains can be closed into rings (cycles), for example:

Thus, the diversity of organic compounds is also determined by the fact that with the same number of carbon atoms in a molecule, compounds with an open, open chain of carbon atoms are possible, as well as substances whose molecules contain cycles (cyclic compounds).

3. Covalent bonds between carbon atoms formed by one pair of generalized electrons are called simple (or ordinary) bonds.

The bond between carbon atoms can be carried out not by one, but by two or three common pairs of electrons. Then chains are obtained with multiple - double or triple bonds; these relationships can be depicted as follows:

The simplest compounds containing multiple bonds are the hydrocarbons ethylene (with a double bond) and acetylene (with a triple bond):

Hydrocarbons with multiple bonds are called unsaturated or unsaturated. Ethylene and acetylene are the first representatives of two homologous series - ethylene and acetylene hydrocarbons.

Rice. 124. Scheme of the formation of -bonds in the ethane molecule.

A simple covalent bond (or C:C) formed by the overlap of two -hybrid electron clouds along a line connecting the centers of atoms (along the bond axis), as, for example, in ethane (Fig. 124), is a -bond (see § 42 ). Bonds are also -bonds - they are formed by overlapping along the bond axis of the -hybrid cloud of the C atom and the spherical cloud -electron of the H atom.

The nature of multiple carbon-carbon bonds is somewhat different. So, in the ethylene molecule, during the formation of a double covalent bond (or) in each of the carbon atoms, one -orbital and only two p-orbitals (-hybridization) participate in hybridization; one of the p-orbitals of each C atom does not hybridize. As a result, three -hybrid electron clouds are formed, which participate in the formation of three -bonds. In total, there are five bonds in the ethylene molecule (four and one); they are all located in the same plane at angles of about 120° to each other (Fig. 125).

Thus, one of the electron pairs in the bond carries out a -bond, and the second is formed by p-electrons that are not involved in hybridization; their clouds retain the shape of a volume eight, are oriented perpendicular to the plane in which the -bonds are located, and overlap above and below this plane (Fig. 126), forming a -bond (see § 42).

Rice. 125. Scheme of the formation of -bonds in the molecule of ethylene.

Rice. 126. Scheme of the formation of -bonds in the molecule of ethylene.

Therefore, the C=C double bond is a combination of one and one -bonds.

A triple bond (or ) is a combination of one -bond and two -bonds. For example, during the formation of an acetylene molecule in each of the carbon atoms, one -orbital and only one p-orbital (-hybridization) participate in hybridization; as a result, two -hybrid electron clouds are formed, participating in the formation of two -bonds. Clouds of two p-electrons of each C atom do not hybridize, retain their configuration and participate in the formation of two -bonds. Thus, in acetylene there are only three -bonds (one and two) directed along one straight line, and two -bonds oriented in two mutually perpendicular planes (Fig. 127).

Multiple (i.e., double and triple) bonds during reactions easily turn into simple ones; the triple first turns into a double, and the last one into a simple one. This is due to their high reactivity and takes place when any atoms are attached to a pair of carbon atoms linked by a multiple bond.

The transition of multiple bonds to simple ones is explained by the fact that, as a rule, -bonds have less strength and therefore greater lability compared to -bonds. When -bonds are formed, p-electron clouds with parallel axes overlap to a much lesser extent than electron clouds overlapping along the bond axis (i.e., hybrid, -electron or p-electron clouds oriented along the bond axis).

Rice. 127. Scheme of the formation of -bonds in the acetylene molecule.

Rice. 128. Models of the ethylene molecule: a - ball; b - segmented.

Multiple bonds are stronger than simple bonds. So, the bond breaking energy is , bonds , and bonds only .

From what has been said, it follows that in the formulas two lines out of three in a connection and one line out of two in a connection express connections that are less strong than a simple connection.

On fig. 128 and 129 are ball and segment spatial models of compounds with double (ethylene) and triple (acetylene) bonds.

4. The theory of structure has explained numerous cases of isomerism of organic compounds.

Chains of carbon atoms can be straight or branched:

So, the composition has three saturated hydrocarbons (pentane) with different chain structures - one with an unbranched chain (normal structure) and two with a branched one (isostructure):

The composition has three unsaturated hydrocarbons, two normal structures, but isomeric in the position of the double bond, and one isostructure:

Rice. 129. Models of the acetylene molecule: a ball; b - segmented.

Two cyclic hydrocarbons are isomeric to these unsaturated compounds, which also have a composition and are isomeric to each other in cycle size:

With the same composition, compounds can differ in structure due to different positions in the carbon chain and other non-carbon atoms, for example:

Isomerism can be due not only to the different order of connection of atoms. There are several types of spatial isomerism (stereoisometry), which consists in the fact that the corresponding isomers (stereoisomers) with the same composition and order of connection of atoms differ in a different arrangement of atoms (or groups of atoms) in space.

So, if a compound has a carbon atom bonded to four different atoms or groups of atoms (an asymmetric atom), then two spatial isomeric forms of such a compound are possible. On fig. 130 shows two tetrahedral models of lactic acid, in which the asymmetric carbon atom (it is marked with an asterisk in the formula) is in the center of the tetrahedron. It is easy to see that these models cannot be combined in space: they are mirrored and reflect the spatial configuration of the molecules of two different substances (in this example, lactic acids), which differ in some physical, and mainly biological properties. Such isomerism is called mirror stereoisomerism, and the corresponding isomers are called mirror isomers.

Rice. 130. Tetrahedral models of molecules of mirror isomers of lactic acid.

The difference in the spatial structure of the mirror isomers can also be represented using structural formulas, which show the different arrangement of atomic groups at an asymmetric atom; for example, for those shown in Fig. 130 mirror isomers of lactic acid:

As already stated, carbon atoms; connected by a double bond lie in the same plane with four bonds connecting them to other atoms; the angles between the directions of these bonds are approximately the same (Fig. 126). When different atoms or groups are connected to each of the carbon atoms in the double bond, the so-called geometric stereoisomerism, or cis-trans isomerism, is possible. An example is the spatial geometric isomers of dichloroethylene

In the molecules of one isomer, the chlorine atoms are located on one side of the double bond, and in the molecules of the other, on opposite sides. The first configuration is called cis-, the second - trans-configuration. Geometric isomers differ from each other in physical and chemical properties.

Their existence is due to the fact that the double bond excludes the possibility of free rotation of the connected atoms around the bond axis (such rotation requires breaking the bond; see Fig. 126).

5. Mutual influence in the molecules of organic substances is manifested primarily by atoms directly connected to each other. In this case, it is determined by the nature of the chemical bond between them, the degree of difference in their relative electronegativity and, consequently, the degree of polarity of the bond.

For example, judging by the summary formulas, then in a methane molecule and in a methyl alcohol molecule, all four hydrogen atoms must have the same properties. But, as will be shown later, in methyl alcohol one of the hydrogen atoms can be replaced by an alkali metal, while in methane the hydrogen atoms do not show such an ability. This is due to the fact that in alcohol the hydrogen atom is directly bonded not to carbon, but to oxygen.

In the above structural formulas, the arrows on the lines of the bonds conditionally show the displacement of pairs of electrons that form a covalent bond, due to the different electronegativity of the atoms. In methane, such a shift in the bond is small, since the electronegativity of carbon (2.5) only slightly exceeds the electronegativity of hydrogen in Table 1. 6, p. 118). In this case, the methane molecule is symmetrical. In the alcohol molecule, the bond is significantly polarized, since oxygen (electronegativity 3.5) draws an electron pair to itself much more; therefore, the hydrogen atom, combined with the oxygen atom, acquires greater mobility, i.e., it is more easily detached in the form of a proton.

In organic molecules, the mutual influence of atoms that are not directly connected to each other is also important. So, in methyl alcohol, under the influence of oxygen, the reactivity of not only the hydrogen atom associated with oxygen, but also the hydrogen atoms that are not directly associated with oxygen, but connected with carbon, increases. Due to this, methyl alcohol is rather easily oxidized, while methane is relatively resistant to the action of oxidizing agents. This is due to the fact that the oxygen of the hydroxyl group significantly draws a pair of electrons towards itself in the bond connecting it to carbon, the electronegativity of which is less.

As a result, the effective charge of the carbon atom becomes more positive, which causes an additional shift of electron pairs also in the bonds in methyl alcohol, compared with the same bonds in the methane molecule. Under the action of oxidizing agents, H atoms bonded to the same carbon atom to which the OH group is bonded are much easier than in hydrocarbons to break off and combine with oxygen to form water. In this case, the carbon atom associated with the OH group undergoes further oxidation (see § 171).

The mutual influence of atoms that are not directly connected to each other can be transmitted over a considerable distance along the chain of carbon atoms and is explained by a shift in the density of electron clouds in the entire molecule under the influence of atoms or groups of different electronegativity present in it. Mutual influence can also be transmitted through the space surrounding the molecule - as a result of overlapping electron clouds of approaching atoms.