A.M. Theory Butlerova

1. Atoms in molecules are interconnected in a certain sequence of chemical bonds in accordance with their valence. The order of communication of atoms is called their chemical structure. Carbon in all organic compounds is four sheets.

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

3. Atoms or groups of atoms mutually affect each other, from 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 connections have near characteristic featureswhich distinguish them from inorganic. Almost all of them (with rare exceptions) combustion; most organic compounds does not dissociate on ions, which is due to the nature of a covalent bond in organic substancesoh. The ion type of communication is realized only in organic acid salts, for example, CH3Coona.

Homologic series - This is an infinite series of organic compounds having a similar structure and, therefore, similar chemical properties and differing from each other for any number of CH2 groups (homologous difference).

Even before the creation of the structure of the structure, the substance of the same elemental composition was known, but with different properties. Such substances were named with isomers, and this phenomenon itself is isomeria.

The basis of Isomerism, as A.M. Butlers lies a difference in the structure of molecules consisting of the same set of atoms.

Isomeria - This is the appearance of the existence of compounds having the same quality and quantitative composition, but different buildings and, therefore, different properties.

Distinguish 2 types of isomeria: structural Isomeria I. spatial Isomeria.

Structural isomeria

Structural isomers - compounds of the same qualitative and quantitative composition, characterized by the procedure for binding atoms, i.e. the chemical structure.

Spatial isomeria

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 isomeria

lies in the possibility of the location of the substituents on one or different directions of the double bond plane or non-aromatic cycles. cis-isomer The substituents are one side from the plane of the ring or double bond, in trans-Isomera - Different.

In the boothene-2 \u200b\u200bmolecule, CH3-CH \u003d CH-CH3 of the CH3 groups can be either one side of the double bond - in the cis-isomer, or on different sides - in the trans-isomer.

Optical isomeria

It appears when the carbon has four different deputies.
If you swap any two of them, another spatial isomer of the same composition is obtained. The physico-chemical properties of such isomers differ significantly. Compounds of this type are characterized by the ability to rotate the plane passed through the solution of such compounds of polarized light by a certain amount. In this case, one isomer rotates the plane of polarized light in one direction, and its isomer is in the opposite. Due to such optical effects, this kind of isomerism is called optical isomeria.

Lecture 15.

The theory of the structure of organic substances. Basic classes of organic compounds.

Organic chemistry -science engaged in the study of organic substances. Otherwise it can be defined as chemistry of carbon compounds. The latter occupies a special place in the Periodic system of D.I. REMEELEEVA by the diversity of compounds, which are known about 15 million, while the number of inorganic compounds is five hundred thousand. Organic substances are known to humanity for a long time, like sugar, vegetable and animal fats, coloring, fragrant and medicinal substances. Gradually, people learned by processing these substances to receive a variety of valuable organic products: wine, vinegar, soap, etc. Successes in organic chemistry rely on achievements in chemistry of protein substances, nucleic acids, vitamins, and others. Huge importance of organic chemistry has for the development of medicine, Since the overwhelming majority medicines It is organic compounds of not only natural origin, but also obtained mainly by synthesis. Exceptional significance high molecular weight Organic compounds (synthetic resins, plastics, fibers, synthetic rubbers, coloring substances, gerbecides, insecticides, fungicides, defolients ...). Great value of organic chemistry for the production of food and industrial goods.

Modern organic chemistry deeply penetrated into chemical processes occurring during storage and processing food products: Processes of drying, barbing and washing oils, fermentation, bread accumulations, boosters, receiving drinks, in the production of dairy products, etc. The discovery and study of enzymes, perfumery and cosmetic substances also played a big role.

One of the reasons for the greater diversity of organic compounds is the originality of their structure, which is manifested in the formation of carbon atoms of covalent bonds and chains, various types and length. At the same time, the number of linked carbon atoms in them can reach tens of thousands, and the configuration of carbon chains may be linear or cyclic. In the chain, in addition to carbon atoms, oxygen, nitrogen, sulfur, phosphorus, arsenic, silicon, tin, lead, titanium, iron, etc. can include oxygen, arsenic, silicon, tin, lead.

The manifestation of these properties is associated with several reasons. It was confirmed that the energy of the relationship with C - C and C - O comparable. Carbon has the ability to form three types of hybridization of orbitals: four SP 3 - hybrid orbitals, their orientation in the space tetrahedral and corresponds simple covalent bonds; Three hybrid SP 2 - orbitals located in the same plane in combination with a non-liberty orbital form double multiples communication (─c \u003d s─); Also, with the help of SP - hybrid orbitals of linear orientation and non-liberal orbitals between carbon atoms arise triple multiples Communications (─ C ≡ C ─). For this, such types of bonds of carbon atoms form not only with each other, but also with other elements. Thus, the modern theory of the structure of the substance explains not only a significant number of organic compounds, but also the influence of their chemical structure on the properties.

It also fully confirms the foundations theories of chemical structuredeveloped by the Great Russian scientist A.M. Butlerov. Its main provisions:

1) In organic molecules, atoms are connected to each other in a certain order according to their valence, which causes the structure of molecules;

2) the properties of organic compounds depend on nature and the number of atoms included in their composition, as well as on the chemical structure of molecules;

3) each chemical formula corresponds to a certain number of possible structures of isomers;

4) each organic compound has one formula and has certain properties;

5) In molecules, there is a mutual influence of atoms on each other.

Classes of organic compounds

According to the theory, organic compounds are divided into two rows - acyclic and cyclic compounds.

1. Aciclic compounds. (Alcans, alkenes) contain an open, unlocked carbon chain - direct or branched:

N n n n n n

│ │ │ │ │ │ │

N─ С─С─С─С н Н мас─С─С─Н

│ │ │ │ │ │ │

N n n n │ n

Normal Bhutan Isobutan (methylpropane)

2. a) alicyclic compounds - Compounds having closed (cyclic) carbon chains in molecules:

cyclobutane Cyclohexane

b) aromatic compoundsin the molecules of which there is a skeleton of benzene - a six-membered cycle with alternating simple and double connections (arena):

c) heterocyclic compounds - cyclic compounds containing nitrogen, sulfur, oxygen, phosphorus and some microelements, which are called heteroatoms.

furan Pyrrol Pyridin

In each row, organic substances are distributed by classes - hydrocarbons, alcohols, aldehydes, ketones, acids, ethers in accordance with the nature of the functional groups of their molecules.

There is also a classification according to the degree of saturation and by functional groups. According to the degree of saturation distinguish:

1. Male saturated - There are only single connections in the carbon skeleton.

─cms─

2. Unforeseen unsaturated - There are multiple (\u003d, ≡) communications in the carbon skeleton.

─c \u003d С─ ─С≡С

3. Aromatic - Disadvantaged cycles with ring pairing (4N + 2) π-electrons.

According to functional groups

1. Alcohols R-CH 2 OH

2. Phenols

3. Aldehydes R─COH ketones R─c─r

4. Carboxylic acids R─coh about

5. Essential esters R─coor 1

For cooking, dyes, clothes, medicines, a person has long learned to apply various substances. Over time, a sufficient amount of information about the properties of certain substances was accumulated, which made it possible to improve the methods for obtaining, processing, etc. And it turned out that many mineral (inorganic substances) can be obtained directly.

But some people used by a person were not synthesized them, because they were obtained from living organisms or plants. These substances called organic.Organic substances failed to synthesize in the laboratory. At the beginning of the nineteenth century, such a doctrine as Vitalyism (VITA - life) was actively developed, according to which organic substances arise only thanks to the "life force" and it is impossible to create their "artificial way".

But there was time and science developed, new facts about organic substances appeared, which were contrary to the existing theory of the Vitalists.

In 1824, the German scientist F. Vylerfor the first time in the history of chemical science synthesized oxalic acid – Organic substance from inorganic substances (dicyan and water):

(CN) 2 + 4H 2 O → COOH - COOH + 2NH 3

In 1828, Völler heated a cyanomonic sodium with sulfur ammonium and synthesized urea - Product of liveliness of animal organisms:

Naocn + (NH 4) 2 SO 4 → NH 4 OCN → NH 2 OCNH 2

These discoveries played an important role in the development of science in general, and chemistry in particular. Chemical scientists began to gradually move away from the vitalistic teaching, and the principle of dividing substances into organic and inorganic discovered their inconsistency.

Currently substances Still divide on organic and inorganic,but the criterion of separation is already a bit different.

Organic called substancescontaining carbon in its composition, they are also called carbon compounds. Such compounds are about 3 million, the rest of the same compounds about 300 thousand.

Substances that carbon is not included, called inorganicand. But there are exceptions to the general classification: there are a number of compounds that include carbon, but they relate to inorganic substances (oxide and carbon dioxide, servo carbon, coal acid and its salts). All of them in composition and properties they are similar to inorganic compounds.

During the study of organic substances, new difficulties appeared: on the basis of theories about inorganic substances, it is impossible to reveal the patterns of the structure of organic compounds, to explain the valence of carbon. Carbon in different compounds had various valence.

In 1861, Russian scientist A.M. Butlers for the first time the synthesis received a sugar substance.

When studying hydrocarbons, A.M. Butlersi realized that they constitute a completely special class of chemicals. Analyzing their structure and properties, the scientist revealed several patterns. They laid the basis created by him chemical building theory.

1. The molecule of any organic matter is not random, the atoms in molecules are connected to each other in a certain sequence according to their valenneys. Carbon in organic compounds are always quadricula.

2. The sequence of interatomic bonds in the molecule is called the ephythmic structure reflected by one structural formula (structure formula).

3. The chemical structure can be installed by chemical methods. (Currently used modern physical methods).

4. Properties of substances depend not only on the composition of the molecules of the substance, but from their chemical structure (sequence of the compound of the atoms of the elements).

5. According to the properties of this substance, it is possible to determine the structure of its molecule, and on the structure of the molecule – anticipate properties.

6. Atoms and groups of atoms in the molecule have a mutual influence on each other.

This theory has become a scientific foundation for organic chemistry and accelerated its development. Relying on the provisions of the theory, A.M. Butlers described and explained the phenomenon isomeria, predicted the existence of various isomers and first got some of them.

Consider the chemical structure of ethane – C 2 H 6. Denoticing the valence of the elements of the fuses, depicting the ethane molecule in the order of the compound of atoms, that is, we will write a structural formula. According to A.M. Theory Butlerova, it will have the following form:

Hydrogen and carbon atoms are associated with one particle, hydrogen valence is equal to one, and carbon – Four. Two carbon atoms are interconnected by carbon bond – Carbon (S. – FROM). Carbon ability to form with – C-bond is understandable, based on the chemical properties of carbon. On the outer electronic layer at the carbon atom, four electrons, the ability to give electrons is the same as to attach the missing. Therefore, carbon most often forms compounds with a covalent bond, that is, due to the formation of electronic pairs with other atoms, including carbon atoms with each other.

This is one of the reasons for the diversity of organic compounds.

The compounds that have the same composition, but different buildings are called isomers. Isomeriya phenomenon – One of the reasons for the variety of organic compounds

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

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

2. The chemical structure can be installed by chemical methods. (Currently used modern physical methods).

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

4. According to the properties of this substance, it is possible to determine the structure of its molecule, and on the structure of the molecule - to foresee properties.

5. Atoms and groups of atoms in the molecule have a mutual influence on each other.

The theory of Butlerov was a scientific foundation for organic chemistry and contributed to its rapid development. Relying on the provisions of the theory, A.M. Butlers gave an explanation to the phenomenon of Isomerism, predicted the existence of various isomers and first got some of them.

The development of the theory of structure contributed to the work of Kekule, Kolle, Cooper and Vant-Gooff. However, their theoretical provisions did not carry a general nature and served mainly the objectives of explaining the experimental material.

2. Build formulas

The formula of the structure (structural formula) describes the order of the compound of atoms in the molecule, i.e. Her chemical structure. Chemical bonds in the structural formula are depicted by dashes. The relationship between hydrogen and other atoms is usually not indicated (such formulas are called abbreviated structural formulas).

For example, complete (deployed) and abbreviated structural formulas of the N-Bhutan C4H10 are viewed:

Another example is formulas isobutane.

It is often used an even more brief record of the formula, when they do not depict not only connections with the hydrogen atom, but also the symbols of carbon and hydrogen atoms. For example, the structure of benzene C6H6 reflects formulas:

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

For example, n-butane and isobutane have one molecular formula C4H10, but a different sequence of connections.

Thus, the differences in substances are due not only to different qualitative and quantitative elemental composition, but also a different chemical structure, which can be reflected only by structural formulas.

3. The concept of isomerism

Even before the creation of the structure of the structure, the substance of the same elemental composition was known, but with different properties. Such substances were named with isomers, and this phenomenon itself is isomeria.

The basis of Isomerism, as A.M. Butlers lies a difference in the structure of molecules consisting of the same set of atoms. In this way,

isomerius is a phenomenon of the existence of compounds having the same quality and quantitative composition, but a different structure and, therefore, different properties.

For example, the content in the molecule of 4-atoms of carbon and 10 atoms of hydrogen is the existence of 2 isomeric connections:

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

4. Structural isomers

Structural isomers - compounds of the same qualitative and quantitative composition, characterized by the procedure for binding atoms, i.e. the 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 (different color balls denote different atoms or atomic groups):

Molecules of such isomers are incompatible in space.

Stereoisomeria plays an important role in organic chemistry. More information 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 an atom and chemical bond in organic chemistry was one of the most important stages of the development of the theory of the structure of organic compounds. The concept of the chemical structure as a sequence of links between atoms (A.M. Butlers) electronic theory supplemented with the views of the union and spatial structure and their influence on the properties of organic compounds. It is these ideas that make it possible to understand the methods of transmitting 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:

nature and electronic structure of atoms;

type of atomic orbitals and the nature of their interaction;

type of chemical bonds;

chemical, electronic and spatial structure of molecules.

7. Electron properties

Electron has a dual nature. In different experiments, it can show the properties of both particles and waves. The electron movement obeys the laws of quantum mechanics. The relationship between the wave and corpuscular properties of the electron reflects the de Broglyl ratio.

The energy and coordinates of the electron, as well as other elementary particles, cannot be measured simultaneously with the same accuracy (the principle of uncertainty-mesenberg). Therefore, the movement of the electron in the atom or in the molecule cannot be described using the trajectory. The electron can be at any point of space, but with a different probability.

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

For example:

8. Atomic orbitals

Atomic orbital (AO) - the area of \u200b\u200bthe most likely stay of the electron (electronic cloud) in electric field The nuclei of the atom.

The position of the element in the periodic system determines the type of orbitals of its atoms (S-, P-, D-, F-AO, etc.), differing energy, shape, sizes and spatial orientation.

For elements of the 1st period (H, HE), one AO \u200b\u200b- 1S is characterized.

In the elements of the 2nd period, electrons occupy five JSC in two energy levels: the first level 1S; Second level - 2S, 2PX, 2PY, 2PZ. (The numbers indicate the number of the energy level, the letter - the form of orbital).

The state of the electron in the atom is fully described by quantum numbers.

Organic chemistry - The section of chemistry, in which carbon compounds are studied, their structure, properties, mutual entertainment.

The very name of the discipline is "Organic Chemistry" - it has emerged for a long time. The reason for him lies in the fact that most carbon compounds faced by researchers on initial stage The formation of chemical science, had a vegetable or animal origin. However, in order of exception, separate carbon compounds refer to inorganic. For example, inorganic substances, carbon oxides, coalic acid, carbonates, hydrocarbonates, cyanorrod and some others are considered to be considered inorganic substances.

Currently, a little less than 30 million diverse organic substances is also known and this list is continuously replenished. Such a huge number of organic compounds is primarily due to the following specific carbon properties:

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

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

3) possible different types connections between carbon atoms, namely single, double and triple. At the same time, the valence of carbon in organic compounds is always equal to four.

In addition, the large variety of organic compounds also contributes to the fact that carbon atoms are able to form connections and with atoms of many others chemical elements, for example, hydrogen, oxygen, nitrogen, phosphorus, gray, halogens. At the same time, hydrogen, oxygen and nitrogen are most often found.

It should be noted that a rather long organic chemistry was represented for the "Dark Forest" scientists. For a while in science, the theory of vitalism was even popular, according to which organic substances cannot be obtained by an "artificial" method, i.e. Outside live matter. However, the theory of vitalyism existed not for a very long time, due to the fact that the substances of which are possible outside the living organisms are found.

The researchers caused bewilderment that many organic substances have the same qualitative and quantitative composition, but they often have completely unlike each other by physical and chemical properties. For example, dimethyl ether and ethyl alcohol have an absolutely identical elemental composition, however, dimethyl ether under normal conditions is gas, and ethyl alcohol - liquid. In addition, the sodium dimethyl ether does not react, and ethyl alcohol interacts with it, highlighting hydrogen gas.

Researchers of the XIX century, a lot of assumptions were put forward regarding how organic substances are arranged. Significantly important assumptions were nominated by German scientists F.A. Saekul, who first expressed the idea that atoms of different chemical elements have specific values \u200b\u200bof valences, and carbon atoms in organic compounds are tied and able to combine each other, forming a chain. Later, pushing away from the assumptions of Kekule, the Russian scientist Alexander Mikhailovich Butlerov developed the theory of the structure of organic compounds, which did not lose their relevance and in our time. Consider the main provisions of this theory:

1) All atoms in organic substances molecules are connected to each other in a certain sequence in accordance with their valence. Carbon atoms have permanent valenceequal to four and can form each other chain of various buildings;

2) the physical and chemical properties of any organic matter depend not only on the composition of its molecules, but also on the order of the compound of atoms in this molecule among themselves;

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

4) Exploring the physical and chemical properties of the organic compound can be installed its structure. The opposite structure of a molecule of a substance is also true, it is possible to predict its properties.

Similarly, the periodic law D.I. Ieldeva became a scientific foundation for inorganic chemistry, the theory of the structure of organic substances A.M. Butlerova actually became the starting point in the formation of organic chemistry as science. It should be noted that after creating the theory of Buchler's structure, organic chemistry began its development in very rapid pace.

Isomerius and Homology

According to the second position of the theory of butlerov, the properties of organic substances depend not only on the qualitative and quantitative composition of molecules, but also on the order of the compound of atoms in these molecules.

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

Isomeria - phenomenon, when different substances have an absolutely identical composition of molecules, i.e. The same molecular formula.

Very often, the isomers are very different in physical and chemical properties. For example:

Types of isomeria

Structural isomeria

a) isomeria carbon skeleton

b) isomeria position:

multiple touch

deputies:

functional groups:

c) Interclative isomeria:

Interclative isomerism occurs when the compounds that are fromomers belong to different classes of organic compounds.

Spatial isomeria

The spatial isomerism is a phenomenon, when different substances with the same order of attachment of atoms to each other differ from each other fixed-different positions of atoms or groups of atoms in space.

There are two types of spatial isomerism - geometric and optical. Tasks for optical isomerism on the exam are not found, so we consider only geometric.

If in the molecule of any compound there is a double C \u003d C Communication or cycle, sometimes in such cases there may be a geometric phenomenon or cis-trans.--omeria.

For example, this kind of isomerism is possible for Bootena-2. Its meaning is that the double bond between carbon atoms actually has a flat structure, and the substituents at these carbon atoms can be fixed to be fixed either above or under this plane:

When the same substituents are on one side of the plane say it is cis-Zeter, and when different - trance-Zeter.

On in the form of structural formulas cis and trance-Isomers (on the example of Bouthen-2) are depicted as follows:

It should be noted that the geometric isomerism is not possible if at least one carbon atom with a double bond will be two identical substituents. For example, cis-transisomerization is impossible for propene:

Propen has no cis-trans.-Someters, since with one of the carbon atoms with double bonds, two identical "substituent" (hydrogen atoms)

As can be seen from the illustration above, if you swap a methyl radical and a hydrogen atom located at a second carbon atom, along the different sides of the plane, we will get the same molecule to which they just looked from the other side.

The effect of atoms and groups of atoms on each other in organic compound molecules

The concept of O. chemical structure As the sequence of atoms connected with each other, it was significantly expanded with the advent of electronic theory. From the standpoint of this theory, one can explain how atoms and groups of atoms in the molecule have an impact on each other.

Distinguish two possible method The influence of some sections of the molecule to others:

1) inductive effect

2) mesomeric effect

Inductive effect

To demonstrate this phenomenon, take for an example 1-chloropropane molecule (CH 3 CH 2 CH 2 CL). The relationship between carbon and chlorine atoms is polar, since chlorine has much higher electronegility compared to 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, and on the chlorine atom - partial negative (Δ-):

The displacement of the electron density from one atom to another is often denoted by an arrow directed to a more electronegative atom:

However, interesting is such a moment that, in addition to offset of electron density from the first carbon atom to the chlorine atom, the displacement also takes place, but to a slightly lesser extent from the second carbon atom to the first, as well as from the third to the second:

Such a displacement of the electron density on the chain σ-links is called an inductive effect ( I.). This effect is fading as it removes from the influencing group and is already practically not manifested after 3 σ-links.

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

-F, -Cl, -Br, -i, -OH, -NH 2, -CN, -NO 2, -COH, -COOH

If the electronegability of the atom or group of atoms is less than the electric negatsis of the carbon atom, the electron density is actually transmitted from such substituents to carbon atoms. In this case, they say that the deputy has a positive inductive effect (+ I.) (is an electron donor).

So, deputies with + I.- Effect are limiting hydrocarbon radicals. In this case, severity + I.-Effect increases with the 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 electronegathy. Carbon atoms in the state of SP hybridization have greater electronegacity compared to carbon atoms in a state of SP 2-hybridization, which, in turn, are more electronegative than carbon atoms in the state SP 3-hybridization.

Mesomeric effect (m), or the conjugation effect, is the effect of the substituent transmitted according to the system of conjugate π-bonds.

The sign of the mesomeric effect is determined by the same principle as the sign of the inductive effect. If the substituent increases electron density in the conjugate system, it has a positive mesomeric effect (+ M.) And is an electron donor. Double carbon-carbon bonds, substituents containing a watered electronic pair: -NH 2, -n, halogen have a positive mesomeric effect.

Negative mesomeric effect (- M.) Posses have substituents, pulling electron density from the conjugate system, with the electronic density in the system decreases.

Negative mesomeric effects have groups:

-NO 2, -COOH, -SO 3 H, -COH,\u003e C \u003d O

Due to the redistribution of electron density due to 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.

A graphically mesomeric effect is shown by the curved arrow, which begins in the center of the electron density and is completed where the electron density shifts. For example, in the vinyl chloride molecule, the mesomeric effect occurs when conjugating a vapor electron pair of the chlorine atom, with electrones π-bond between carbon atoms. Thus, as a result of this, a partial positive charge appears on the chlorine atom, and having a mobility π-electron cloud under the influence of an electronic pair shifts towards the extreme carbon atom, which arises as a result of this partial negative charge:

If the molecule has alternating single and double bonds, they indicate that the molecule contains a conjugate π-electronic system. An interesting feature Such a system is that the mesomeric effect does not fade in it.