A molecule of an organic compound is a collection of atoms linked in a certain order, usually by covalent bonds. In this case, bonded atoms can differ in size electronegativity. Quantities electronegativities largely determine such important bond characteristics as polarity and strength (energy of formation). In turn, the polarity and strength of bonds in a molecule, to a large extent, determine the ability of the molecule to enter into certain chemical reactions.

Electronegativityof a carbon atom depends on the state of its hybridization. This is due to the share s— orbitals in a hybrid orbital: it is smaller than y sp 3 - and more for sp 2 - and sp -hybrid atoms.

All the atoms that make up a molecule are interconnected and mutually influenced. This influence is transmitted mainly through a system of covalent bonds, using the so-called electronic effects.

Electronic effects called the shift in electron density in a molecule under the influence of substituents./>

Atoms connected by a polar bond carry partial charges, denoted by the Greek letter delta ( d ). Atom "pulling" electron densitys—connections in its favor, acquires negative charge d -. When considering a pair of atoms linked by a covalent bond, the more electronegative atom is called electron acceptor. His partner s -bond will accordingly have an equal-magnitude electron density deficit, i.e. partial positive charge d +, will be called electron donor.

Shift of electron density along the chains—connections are called inductive effect and is designated I.

The inductive effect is transmitted through the circuit with attenuation. The direction of shift of the electron density of alls—connections are indicated by straight arrows.

Depending on whether the electron density moves away from the carbon atom in question or approaches it, the inductive effect is called negative (- I ) or positive (+I). The sign and magnitude of the inductive effect are determined by differences in electronegativity between the carbon atom in question and the group causing it.

Electron-withdrawing substituents, i.e. an atom or group of atoms that shifts electron densitys—bonds from a carbon atom to itself exhibit negative inductive effect (- I-effect).

Electrodonorsubstituents, i.e. an atom or group of atoms that shifts electron density to a carbon atom away from itself exhibits positive inductive effect(+I-effect).

The I-effect is exhibited by aliphatic hydrocarbon radicals, i.e. alkyl radicals (methyl, ethyl, etc.). Most functional groups exhibit − I -effect: halogens, amino group, hydroxyl, carbonyl, carboxyl groups.

The inductive effect also manifests itself in the case when the bonded carbon atoms differ in their state of hybridization.

When the inductive effect of a methyl group is transferred to a double bond, its influence is first experienced by the mobilep— connection.

The influence of the substituent on the distribution of electron density transmitted throughp—connections are called mesomeric effect (M). The mesomeric effect can also be negative and positive. In structural formulas it is depicted as a curved arrow starting at the center of the electron density and ending at the place where the electron density shifts.

The presence of electronic effects leads to a redistribution of electron density in the molecule and the appearance of partial charges on individual atoms. This determines the reactivity of the molecule.

According to modern concepts, the nature and mechanism of mutual influence of atoms is determined by the nature of the distribution of electron density in the molecule and the polarizability of covalent bonds.

Electronic mixing in organic compounds are divided into two types: the inductive effect - mixing of electron density along a chain of π bonds and the mesomeric effect - displacement along a system of π bonds.

Inductive effect. Looking at Types chemical bonds, we noted that between atoms with the same electronegativity, a pair of bond electrons belongs equally to both bond participants (covalent Not polar connection). For example, the bonds in methane and butane molecules are non-polar, the electron density in them is distributed symmetrically and the molecule does not have a dipole moment. If in a butane molecule one hydrogen atom is replaced by a halogen - chlorine, then the electron density of the covalent C-Cl bond will mix with the more electronegative chlorine atom (polar covalent bond):

The pair of electrons of the a-bond belongs to both carbon and chlorine, but is somewhat mixed with chlorine, so chlorine acquires a partially negative charge (δ -), and the carbon atom of the C–Cl bond has an equal partially positive charge (δ +).

A decrease in electron density on C 1 leads to the fact that the latter, exhibiting acceptor properties, displaces S-bond electrons from the neighboring carbon atom. Polarization of the C 2 –C 1 bond occurs and a partial positive charge also appears on C 2, which in turn leads to polarization of the C 2 –C 3 bond and the appearance of a partial positive charge on C 3, etc. In this case, a fractional positive charge on carbon atoms in the chain from C 1 to C decreases: δ + > δ' + > δ'' + > δ''' +

Polarization of one carbon-halogen bond causes polarization of the molecule as a whole and, consequently, the appearance of a dipole moment.

Inductive (inductive) effect– transfer of the electronic influence of the substituent along the chain of σ-bonds, which arises due to the different electronegativity of the atoms.

The inductive effect is indicated by the letter I, and the electron density shift is depicted using an arrow along a simple σ bond, the tip of which indicates the direction of the shift.

Based on the direction of the electronic influence of the substituents, positive +I and negative –I inductive effect.

A negative inductive effect is exhibited by substituents that attract electrons from the o-bond, for example: –NO 3 –C≡N, –COOH, –Hal, –OH,

The negative inductive effect, as a rule, increases with increasing electronegativity of the atoms. It is more pronounced for a substituent with a triple bond, since it contains a more electronegative sp-hybridized carbon atom. In turn, the carbon atom in sp 3 hybridization, being less electronegative, exhibits + I in relation to carbon atoms in sp and sp 2 hybridization:

A positive inductive effect is exhibited by substituents that repel the electrons of the oσ bond, most often these are alkyl groups (Alk). The electron-donating properties of alkyl substituents increase with increasing length of the hydrocarbon chain (–C 4 H 9 > –CH 3) and increase in the series from primary to tertiary radicals ((CH 3) 3 C– > (CH 3) 2 CH– > CH 3 CH 2 – > CH 3 –). The latter is explained by the fact that the inductive effect decays along the circuit.

Summarizing the above, let us briefly dwell on the main properties of the inductive effect;

1. The inductive effect appears only when the molecule contains atoms with different electronegativity.

2. The inductive effect propagates only through o-bonds in one direction.

3. The inductive effect quickly decays along the circuit. Its maximum action is four σ bonds.

4. Inductive bias is determined by the presence of a dipole moment: μ≠0.

Mesomeric effect (conjugation effect). Before considering the transfer of the electronic influence of substituents through a system of π-bonds, let us define the concepts of conjugated system and conjugation.

A conjugated system is a system in which there is an alternation of simple and multiple bonds, or the proximity of an atom that has a vacant p-orbital or a lone pair of p-electrons. Coupled systems come in open and closed circuit types:

Each of the given chains of conjugated bonds is also called a conjugation chain (from Latin - overlapping, superimposition). They involve conjugation—an additional overlap of π- and p-orbitals that have parallel symmetry axes (coplanar). Due to conjugation, a redistribution (delocalization) of the π-electron density occurs and the formation of a single π-electron system.

Depending on the type of overlapping orbitals, several types of conjugation are distinguished: π,π-conjugation (overlapping two π-orbitals), p,π-conjugation (overlapping p- and π-orbitals):

Rice. 2.9. Conjugated systems of 1,3 butadiene, vinyl chloride and allylic cation

Conjugation is an energetically beneficial process that occurs with the release of energy. Conjugated systems are characterized by increased thermodynamic stability.

Having defined conjugation and conjugated systems, let us consider the electronic effects that are observed when various types of substituents are introduced into such systems.

Conjugation effect or mesomeric effect(M) – the process of transferring the electronic influence of a substituent through a conjugated system of π bonds. Mixing of electron density in conjugated systems is possible only when electron-donating or electron-withdrawing substituents are included in the system.

For example, a benzene molecule has conjugation but no substituents, so there is no mesomeric effect. The hydroxy group in the phenol molecule is part of the conjugated system and exhibits a mesomeric effect, while in the benzyl alcohol molecule the –OH group is isolated from the conjugated system by two σ bonds and does not exhibit a mesomeric effect.

The mesomeric effect is denoted by the letter M, and the shift in electron density in the conjugated system is denoted by a curved arrow. According to the directing effect of the substituent, the mesomeric effect is divided into positive (+M) and negative (–M).

A positive mesomeric effect is exhibited by substituents (electron-donating atoms or atomic groups) that provide electrons to the conjugated system, i.e., having lone pairs of electrons or a negative charge:

The maximum +M is for atoms with a negative charge. Substituents containing lone pairs of electrons have more +M, the less within the period the electronegativity of atoms containing lone pairs of electrons.

The negative mesomeric effect is manifested by substituents that shift the electron density of the conjugated system to themselves:

Maximum –M is exhibited by substituents carrying a positive charge. In unsaturated groups, the –M effect increases with increasing difference in the electronegativity of the multiple bond atoms.

Let's look at a few examples of the mesomeric effect:

The mesomeric effect, compared to the inductive effect, causes a stronger shift in the electron density and practically does not attenuate.

Combined manifestation of inductive and mesomeric effects of the substituent

The mesomeric and inductive effects of one substituent may or may not coincide in direction. For example, in the acrolein molecule the aldehyde group exhibits –I And –M, and the hydroxyl group in the phenol molecule has –I, But +M-effect,

As can be seen from the above example, in the phenol molecule the opposite electronic mixing leads to the fact that these two effects seem to “quench” each other. And in the acrolein molecule, the inductive and mesomeric effects reinforce each other. The mesomeric effect of the substituent is usually greater than the inductive one, since l-bonds are more easily polarized than σ-bonds.

The polarization caused by the mesomeric effect has an alternating character: under the influence of the substituent, not only π-electron clouds, but also clouds of σ bonds will mix. This phenomenon is observed in systems with open and closed circuit coupling:

Although the amino group exhibits –I-effect, causes a decrease in electron density on all carbon atoms of the aromatic cycle, but due to +M-the effect of a pair of electrons of the nitrogen atom, which is larger –I in general, an increase in electron density is observed on the carbon atoms of the benzene ring, especially in positions 2. 4. 6. Alternating polarization occurs.

In molecules with an open conjugation chain, the partial charges that are concentrated at the ends of the conjugated system are usually indicated:

Superconjugation effect (hyperconjugation). Along with π,π- and p,π-conjugation, there is a special type of conjugation - hyperconjugation (superconjugation) or σ,π-conjugation.

Superconjugation effect– interaction that occurs when the electron cloud of the o-orbitals of the C–H bond overlaps with the π-orbitals of the multiple bond. This type of electron cloud overlap is σ,π conjugation, which is present in both the aliphatic and aromatic series of compounds. The mixing of electrons is depicted using a curved arrow. Any of the σ bonds of the methyl group of propene can participate in σ,π conjugation.

Rice. 2.10. Diagram of the overlap of the σ-orbitals of the C–H bond with the π-orbital of the multiple bond in the propene molecule

The magnitude of the hyperconjugation effect is higher, the more hydrogen atoms there are at the carbon associated with the unsaturated system. The concept of superconjugation explains the increased reactivity and mobility of α-hydrogen atoms in the molecules of aldehydes, ketones, acids and their derivatives. Superconjugation is sometimes called the Nathan-Becker effect after the scientists who discovered it.

ISOMERITY OF ORGANIC COMPOUNDS. SPATIAL STRUCTURE OF MOLECULES

The term isomerism (from the Greek isos - identical, meros - part) was first introduced in 1830, when substances with the same qualitative and quantitative composition, but with different physical and chemical properties, became known.

Isomerism is a phenomenon consisting in the existence of compounds that have the same molecular formula, but differ in the order of bonding of atoms in the molecule or the arrangement of atoms in space, and as a result differ in physical and chemical properties

Such compounds are called isomers. There are two main types of isomerism - structural (structural isomerism) and spatial (stereoisomerism).

Atoms and atomic groups in the molecules of organic compounds influence each other, and not only the atoms directly connected to each other. This influence is somehow transmitted through the molecule. The transfer of the influence of atoms in molecules due to the polarization of bonds is called electronic effects . There are two types of electronic effects: inductive and mesomeric effects.

Inductive effect- this is the transfer of the influence of substituents along a chain of σ-bonds due to their polarization. The inductive effect is denoted by the symbol I. Let's consider it using 1-chlorobutane as an example:

The C-Cl bond is polar due to the higher electronegativity of chlorine. A partial positive charge (δ+) appears on the carbon atom. The electron pair of the next σ bond is shifted towards the electron-deficient carbon atom, i.e. polarized. Due to this, a partial positive charge (δ+’) also appears on the next carbon atom, etc. So chlorine induces polarization of not only the “own” σ bond, but also subsequent ones in the chain. Please note that each subsequent partial positive charge is smaller in magnitude than the previous one (δ+>δ+’>δ+’’>δ+’’’), i.e. the inductive effect is transmitted through the circuit with attenuation. This can be explained by the low polarizability of σ bonds. It is generally accepted that the inductive effect extends to 3-4 σ bonds. In the example given, the chlorine atom shifts electron density along a chain of bonds to myself. This effect is called the negative inductive effect and is denoted –I Cl.

Most substituents exhibit a negative inductive effect, because their structure contains atoms that are more electronegative than hydrogen (the inductive effect of hydrogen is assumed to be zero). For example: -F, -Cl, -Br, -I, -OH, -NH 2, -NO 2,
-COOH, >C=O.

If a substituent shifts the electron density along a chain of σ bonds Push, it exhibits a positive inductive effect (+I). For example:

Oxygen with a total negative charge exhibits a positive inductive effect.

In the propene molecule, the carbon of the methyl group is sp 3 -hybridized, and the carbon atoms at the double bond are sp 2 -hybridized, i.e. more electronegative. Therefore, the methyl group shifts the electron density away from itself, exhibiting a positive inductive effect (+I CH 3).

So, the inductive effect can manifest itself in any molecule in which there are atoms of different electronegativity.

Mesomeric effect– this is the transfer of the electronic influence of substituents in conjugated systems through the polarization of π bonds. The mesomeric effect is transmitted without attenuation, because π bonds are easily polarized. Please note: only those substituents that are themselves part of the conjugated system have a mesomeric effect. For example:

The mesomeric effect can be either positive (+M) or negative (-M).

In the vinyl chloride molecule, the lone electron pair of chlorine participates in p,π-conjugation, i.e. the contribution of chlorine to the conjugated system is greater than that of each of the carbon atoms. Therefore, chlorine exhibits a positive mesomeric effect.

The acrylic aldehyde molecule is
π.π-conjugate system. The oxygen atom gives up one electron to conjugation - the same as each carbon atom, but at the same time the electronegativity of oxygen is higher than that of carbon, therefore oxygen shifts the electron density of the conjugated system towards itself, the aldehyde group as a whole exhibits a negative mesomeric effect.

So, substituents that donate two electrons to conjugation have a positive mesomeric effect. These include:

a) substituents with a complete negative charge, for example, –O - ;

b) substituents, in the structure of which there are atoms with lone electron pairs in p z orbitals, for example: -NH 2, -OH,
-F, -Cl, -Br-, -I, -OR (-OCH 3, -OC 2 H 5).

Substituents that shift the electron density toward themselves along the conjugated system exhibit a negative mesomeric effect. These include substituents whose structure contains double bonds, for example:

A substituent can exhibit both inductive and mesomeric effects simultaneously. In some cases, the direction of these effects is the same (for example, -I and –M), in others they act in opposite directions (for example, -I and +M). In these cases, how can we determine the overall effect of the substituent on the rest of the molecule (in other words, how can we determine whether a given substituent is electron-donating or electron-withdrawing)? Substituents that increase the electron density in the rest of the molecule are called electron-donating, and substituents that lower the electron density in the rest of the molecule are called electron-withdrawing.

To determine the overall effect of a substituent, it is necessary to compare its electronic effects in magnitude. If the effect is positive in sign, the substituent is electron-donating. If an effect with a negative sign predominates, the substituent is electron-withdrawing. It should be noted that, as a rule, the mesomeric effect is more pronounced than the inductive effect (due to the greater ability of π bonds to polarize). However, there are exceptions to this rule: the inductive effect of halogens is stronger than the mesomeric effect.

Let's consider specific examples:

In this compound, the amino group is an electron-donating substituent, because its positive mesomeric effect is stronger than the negative inductive effect.

In this compound, the amino group is an electron-withdrawing site, because exhibits only a negative inductive effect.

In the phenol molecule, the hydroxyl group is an electron-donating substituent due to the predominance of the positive mesomeric effect over the negative inductive effect.

In the benzyl alcohol molecule, the hydroxyl group does not participate in conjugation and exhibits only a negative inductive effect. Therefore, it is an electron-withdrawing substituent.

These examples show that one cannot consider the influence of any substituent in general, but must consider its influence in a specific molecule.

Only halogens are always electron-withdrawing substituents, because their negative inductive effect is stronger than the positive mesomeric effect. For example:

Now let's return to electrophilic substitution reactions in benzene derivatives. So, we have found that the substituent already present in the ring affects the course of electrophilic substitution reactions. What is this influence expressed in?

The substituent affects the reaction rate S E and the position of the second substituent introduced into the ring. Let's look at both of these aspects of influence.

Effect on reaction speed. The higher the electron density in the ring, the easier electrophilic substitution reactions occur. It is clear that electron-donating substituents facilitate S E reactions (they are cycle activators), and electron-withdrawing substituents hinder them (they deactivate the cycle). Therefore, electrophilic substitution reactions in benzene derivatives containing electron-withdrawing substituents are carried out under more stringent conditions.

Let's compare the activity of phenol, toluene, benzene, chlorobenzene and nitrobenzene in the nitration reaction.

Since phenol and toluene contain electron-donating substituents, they are more active in SE reactions than benzene. On the contrary, chlorobenzene and nitrobenzene are less active in these reactions than benzene, because contain electron-withdrawing substituents. Phenol is more active than toluene due to the positive mesomeric effect of the OH group. Chlorine is not as strong an electron-withdrawing substituent as the nitro group, because the nitro group exhibits both negative inductive and negative mesomeric effects. So, in this series, activity in electrophilic substitution reactions decreases from phenol to nitrobenzene. It has been experimentally established that if the reaction rate of benzene nitration is taken to be 1, then this series will look like this:

The second aspect of the influence of a substituent on the aromatic ring on the course of electrophilic substitution reactions is the so-called orienting action of substituents. All substituents can be divided into two groups: ortho-, para-orientants (substituents of the 1st kind) and meta-orientants (substituents of the 2nd kind).

TO deputies of the 1st kind include: -OH, -O -, -NH 2, alkyl groups (-CH 3, -C 2 H 5, etc.) and halogens. You can see that all of these substituents exhibit a positive inductive effect and/or a positive mesomeric effect. All of them, except the halogens, increase the electron density in the ring, especially in the ortho and para positions. Therefore, the electrophile is directed to these positions. Let's look at this using phenol as an example:

Due to the positive mesomeric effect of the hydroxyl group, the electron density is redistributed throughout the conjugated system, and in the ortho- and para-positions it is especially increased.

When phenol is brominated, a mixture of ortho- and para-bromophenol is formed:

If bromination is carried out in a polar solvent ( bromine water) and use excess bromine, the reaction proceeds in three stages at once:

Substitutes of the 2nd kind are: -NH 3 + , -COOH, -CHO (aldehyde group), -NO 2 , -SO 3 H. All these substituents reduce the electron density in the aromatic ring, but due to its redistribution in meta positions, it is not so reduced strongly, as in ortho- and para-. Let's look at this using benzoic acid as an example:

The carboxyl group exhibits negative inductive and negative mesomeric effects. Due to redistribution throughout the conjugated system in the meta positions, the electron density remains higher than in the ortho and para positions, so the electrophile will attack the meta positions.

Mutual influence of atoms in molecules organic matter(Theory of electronic displacements by K. Ingold)

Atoms and groups of atoms within a molecule of an organic substance have a significant influence on each other. This influence is based on the redistribution of electron density under the influence of electrostatic forces acting inside the molecule.

The presence of mutual influence was also pointed out by A.M. Butlerov in the theory of the structure of organic substances. However, a rigorous theory of electronic displacements was developed only in 1926 - 1933 by the English chemist Christopher Ingold.

In molecules of organic substances there are two possibilities for redistribution of electron density:

• 1. A shift in electron density along the -bond caused by the difference in electronegativity of atoms (or groups of atoms) included in the molecule. The mutual influence transmitted through the chain of -bonds is called the induction effect (I-effect) (polar effect). The induction effect is always attributed to a specific atom or group of atoms, and depending on the direction of the electron density shift under the influence of the atom in question, two types of induction effects are distinguished:
  • A) positive induction effect (+I-effect) Push (electron-donating atoms and groups):

To determine the severity of the +I effect, there are a number of rules:

a) the +I-effect of a substituent is stronger, the lower its electronegativity:

b) Due to the albeit small polarity of the C - H bond, alkyl groups exhibit a +I effect:

b) negative induction effect (-I-effect): the atom or group in question shifts electron density along a chain of -bonds to yourself (electron-withdrawing atoms and groups):

The degree of severity of the -I-effect is determined by the following rules:

a) -I-effect is stronger, the greater the electronegativity of the element:

b) Unsaturated substituents cause an -I-effect, which increases with increasing degree of unsaturation:

This is due to a change in the electronegativity of carbon atoms when the degree of their hybridization changes.

Due to the rigidity of the -bonds, the induction effect quickly fades when moving along the chain. Its influence is most noticeable on the first and second atoms of the chain; its influence on subsequent atoms is negligible.

2. Shift of electron density along conjugated -bonds. Conjugation is a type of electronic interaction that occurs in molecules in the structure of which there is an alternation of simple and multiple bonds. Due to coupling, in such systems there is a single electronic cloud. This effect is called the conjugation effect (C-effect) or mesomeric effect (M-effect). Unlike the inductive effect, the mesomeric effect is transmitted through a chain of conjugated bonds without weakening, covering the entire molecule. Like the induction effect, the mesomeric effect can be positive and negative: +M-effect and -M-effect. Substituents containing a strongly electronegative element have a negative mesomeric effect. Substituents containing an atom with a free electron pair have a positive mesomeric effect. If the substituent contains a strongly electronegative atom with a lone pair, there is competition between the -M and +M effects (halogens).

A type of mesomeric effect is the superconjugation effect (hyperconjugation, Nathan-Becker effect, -conjugation). Superconjugation is caused by the overlap of the -orbitals -bonds of alkyl groups with the -electron system.

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

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

Currently, just under 30 million different organic substances are known, and this list is constantly growing. 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 a sequential (linear) connection of carbon atoms with each other is possible, but also a branched and even cyclic one;

3) possible different types bonds between carbon atoms, namely single, double and triple. Moreover, the valence of carbon in organic compounds is always four.

Besides, great variety organic compounds is also facilitated by the fact that carbon atoms are capable of forming bonds with atoms of many other chemical elements, for example, hydrogen, oxygen, nitrogen, phosphorus, sulfur, halogens. In this case, hydrogen, oxygen and nitrogen are 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 “artificially”, i.e. outside of living matter. However, the theory of vitalism did not last very long, due to the fact that one after another substances were discovered whose synthesis 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. For example, dimethyl ether and ethyl alcohol have exactly the same elemental composition, but 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 reacts with it, releasing hydrogen gas.

Researchers of the 19th century put forward many assumptions regarding how organic substances were structured. Significantly important assumptions were put forward by the German scientist F.A. Kekule, who was the first to express the idea that atoms of different chemical elements have specific valence values, and carbon atoms in organic compounds are tetravalent and are capable of combining with each other to form chains. Later, starting from Kekule’s assumptions, the Russian scientist Alexander Mikhailovich Butlerov developed a theory of the structure of organic compounds, which has not lost its relevance in our time. Let's consider the main provisions of this theory:

1) all atoms in 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 different 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 physical and chemical properties connections;

4) by studying 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 particular substance, you can predict its properties.

Similar to how periodic law D.I. Mendelev became the scientific foundation of inorganic chemistry, the theory of the structure of organic substances by A.M. Butlerov actually became the starting point in the development of organic chemistry as a science. It should be noted that after the creation of Butlerov’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 the molecules, but also on the order in which the atoms in these molecules are connected to each other.

In this regard, the phenomenon of isomerism is widespread among organic substances.

Isomerism is a phenomenon when different substances have absolutely the same molecular composition, i.e. 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) Positional isomerism:

multiple connection

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. Tasks on optical isomerism are not found on the Unified State Exam, so we will consider only geometric ones.

If the molecule of a compound contains a double C=C bond or a ring, 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 is that the double bond between carbon atoms actually has a planar structure, and the substituents on these carbon atoms can be fixedly located either above or below this plane:

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

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

Note that geometric isomerism is impossible if at least one carbon atom at the double bond has two identical substituents. For example, cis-trans- isomerism is not possible for propene:

Propen does not have cis-trans-isomers, since one of the carbon atoms at the double bond has two identical “substituents” (hydrogen atoms)

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

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

Concept of chemical structure as a sequence of atoms connected to each other was significantly expanded with the advent of 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 the influence of some parts of the molecule on others:

1) Inductive effect

2) Mesomeric effect

Inductive effect

For demonstration this phenomenon Let's take for example the 1-chloropropane molecule (CH 3 CH 2 CH 2 Cl). The bond between carbon and chlorine atoms is polar because chlorine has a much higher electronegativity compared to carbon. As a result of the shift of 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 in electron density from one atom to another is often indicated by an arrow pointing towards the more electronegative atom:

However, an interesting point is 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 slightly lesser extent, from the second carbon atom to the first, as well as from the third to the second:

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

In the case where an atom or group of atoms has 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 the carbon atoms. In this case, they say that the substituent has a positive inductive effect (+ I) (is electron donor).

So, substituents with + I-the effect is saturated hydrocarbon radicals. At the same time, the expression + I-effect increases with lengthening 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 located in different valence states also have different electronegativity. Carbon atoms in the sp 2 -hybridized state have greater electronegativity compared to carbon atoms in the sp 2 -hybridized state, which, in turn, are more electronegative than carbon atoms in the sp 3 -hybridized state.

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

Negative mesomeric effect (– M) have substituents that withdraw electron density 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 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.

Graphically, the mesomeric effect is shown by a curved arrow that begins at the center of the electron density and ends where the electron density shifts. For example, in a vinyl chloride molecule, the mesomeric effect occurs when the lone electron pair of the chlorine atom couples with the electrons of the π bond between the 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, is shifted towards the outermost carbon atom, on which a partial negative charge arises as a result:

If a molecule has 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 in it does not fade.