Mutual influence of atoms in the molecules of organic substances (Theory of electronic displacements of K. Ingold)

Atoms and groups of atoms in a molecule of organic matter have a significant effect on each other. This effect is based on the redistribution of electron density under the action 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 the redistribution of electron density:

• 1. The shift of the electron density along the -bond, caused by the difference in the electronegativity of the 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 inductive effect is always attributed to a specific atom or group of atoms, and depending on the direction of the shift of the electron density under the action of the considered atom, two types of induction effects are distinguished:
  • a) positive induction effect (+I-effect) Push (electron donor atoms and groups):

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

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

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

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

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

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

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

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

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

2. Shift of the 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 single and multiple bonds. Due to conjugation, in such systems there is a single electron cloud. This effect is called the conjugation effect (C-effect) or mesomeric effect (M-effect). In contrast to the inductive effect, the mesomeric effect is transmitted along the chain of conjugated bonds without weakening, covering the entire molecule. Like induction, the mesomeric effect can be positive and negative: +M-effect and -M-effect. Substituents having a strongly electronegative element in their composition have a negative mesomeric effect. Substituents having an atom with a free electron pair have a positive mesomeric effect. In the event that the substituent contains a strongly electronegative atom with a lone pair, there is competition between the -M and +M effects (halogens).

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

Video lesson 1: inductive effect. The structure of molecules. Organic chemistry

Video lesson 2: Mesomeric effect (conjugation effect). Part 1

Video lesson 3: Mesomeric effect (conjugation effect). Part 2

Lecture: Theory of structure organic compounds: homology and isomerism (structural and spatial). Mutual influence of atoms in molecules

Organic chemistry

Organic chemistry- a branch of chemistry that studies carbon compounds, as well as their structures, properties, interconversions.

Organic compounds include oxides of carbon, carbonic acid, carbonates, hydrocarbons. At the moment, about 30 million organic substances are known and this number continues to grow. A huge number of compounds are associated with the specific properties of carbon. Firstly, the atoms of a given element are able to connect with each other in a chain of arbitrary length. This connection can be not only sequential, but also branched, cyclic. There are different bonds between carbon atoms: single, double and triple. Secondly, the valency of carbon in organic compounds is IV. This means that in all organic compounds, carbon atoms are in an excited state, having 4 unpaired electrons actively looking for their pair. Therefore, carbon atoms have the ability to form 4 bonds with atoms of other elements. These elements include: hydrogen, oxygen, nitrogen, phosphorus, sulfur, halogen. Of these, carbon bonds most frequently to hydrogen, oxygen, and nitrogen.

Theory of the structure of organic compounds

The Russian scientist A.M. Butlerov developed the theory of the structure of organic compounds, which became the basis of organic chemistry and is currently relevant.

The main provisions of this theory:

The atoms of molecules of organic substances are intertwined with each other in a sequence corresponding to their valency. Since the carbon atom is tetravalent, it forms chains of various chemical structures.

The sequence of connection of atoms of molecules of organic substances determines the nature of their physical and chemical properties.

A change in the sequence of connection of atoms leads to a change in the properties of matter.

The atoms of molecules of organic substances influence each other, which affects the change in their chemical behavior.

Thus, knowing the structure of an organic substance molecule, one can predict its properties, and vice versa, knowledge of the properties of a substance will help to establish its structure.

Homology and isomerism

From the second proposition of Butlerov's theory, it became clear to us that the properties of organic substances depend not only on the composition of molecules, but also on the order in which the atoms of their molecules are combined. Therefore, homologues and isomers are common among organic substances.

homologues- these are substances that are similar in structure and chemical properties, but different in composition.

Isomers- These are substances that are similar in quantitative and qualitative composition, but different in structure and chemical properties.

Homologues differ in composition by one or more CH 2 groups ​.​​​ This difference is called homologous. There are homologous series of alkanes, alkenes, alkynes, arenes. We will talk about them in the next lesson.

Consider the types of isomerism:

1. Structural isomerism

1.1. Isomerism of the carbon skeleton:

1.2. Position isomerism:

1.2.1. Multiple bond isomerism

1.2.2. Substituent isomerism

1.2.3. isomerism functional groups

1.3. Interclass isomerism:

2. Spatial isomerism

This is such a chemical phenomenon in which different substances that have the same order of attachment of atoms to each other differ in a fixed-different position of atoms or groups of atoms in space. This type isomerism is geometric and optical.

2.1. Geometric isomerism. If a C=C double bond or cycle is present in the molecule of any chemical compound, then in these cases geometric or cis-trans isomerism is possible.

In the case when the same substituents are located on one side of the plane, we can say that this is a cis isomer. When the mixers are located on opposite sides, then this is a trans isomer. This type of isomerism is impossible when at least one carbon atom in the double bond has two identical substituents. For example, cis-trans isomerism is impossible for propene.

2.2. Optical isomerism. You know that it is possible for a carbon atom to bond to four atoms/groups of atoms. For example:

In such cases, optical isomerism is formed, two compounds are antipodes, like left and right hand person:

Mutual influence of atoms in molecules

The concept of a chemical structure, as a sequence of atoms connected to each other, was supplemented with the advent of the electronic theory. There are two possible ways of influence of some parts of the molecule on others:

inductive effect.

mesomeric effect.

Inductive effect (I). As an example, we can take the 1-chloropropane molecule (CH 3 CH 2 CH 2 Cl). The bond between the carbon and chlorine atoms is polar here, since the latter is more electronegative. As a result of the electron density shift from the carbon atom to the chlorine atom, a partial positive charge (δ+) begins to form on the carbon atom, and a partial negative charge (δ-) begins to form on the chlorine atom. The electron density shift is indicated by an arrow pointing towards the more electronegative atom.

In addition to the electron density shift, its shift is also possible, but to a lesser extent. The shift occurs from the second carbon atom to the first, from the third to the second. Such a shift in density along the chain of σ-bonds is called the inductive effect (I). It fades away from the influencing group. And after 3 σ-bonds practically does not appear. The most negative inductive effect (-I) contains the following substituents: -F, -Cl, -Br, -I, -OH, -NH 2, -CN, -NO 2, -COH, -COOH. Negative because they are more electronegative than carbon.

When the electronegativity of an atom is less than the electronegativity of the carbon atom, the transfer of electron density from these substituents to carbon atoms begins. This means that the mixer contains a positive inductive effect (+I). Substituents with +I-effect are saturated hydrocarbon radicals. At the same time, the +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 is important to remember that carbon atoms that are in different valence states have different electronegativity. Carbon atoms, being in the state of sp hybridization, contain a sufficiently large electronegativity compared to carbon atoms in the state of sp2 hybridization. These atoms, in turn, are more electronegative than carbon atoms in the sp3 hybridization state.

mesomeric effect(M) , the conjugation effect is a certain influence of the substituent, which is transmitted through the system of conjugated π-bonds. The sign of this effect is determined by the same principle as the sign of the inductive effect. In the case when the substituent begins to increase the electron density in the conjugated system, it will contain a positive mesomeric effect (+M). It will also be an electron donor. Only double carbon-carbon bonds, substituents, can have a positive mesomeric effect. They, in turn, must contain an unshared electron pair: -NH 2, -OH, halogens. The negative mesomeric effect (–M) is possessed by substituents that are able to withdraw the electron density from the conjugated system. It should also be noted that the electron density in the system will decrease. The following groups have a negative mesomeric effect: –NO 2 , –COOH, –SO 3 H, -COH, >C=O.

When the electron density is redistributed, as well as due to the occurrence of mesomeric and inductive effects, positive or negative charges are formed on the atoms. This education is reflected in chemical properties substances. Graphically, the mesomeric effect is often represented by a curved arrow. This arrow originates at the center of the electron density. At the same time, it ends where the electron density shifts.

Example: in a vinyl chloride molecule, the mesomeric effect is formed when the lone electron pair of the chlorine atom is conjugated with the electrons of the π-bond between carbon atoms. As a result of this conjugation, a partial positive charge is formed on the chlorine atom.

The mobile π-electron cloud, as a result of the action of an electron pair, begins to shift towards the outermost carbon atom.

If a molecule contains alternating single and double bonds, then the molecule contains a conjugated π-electron system.

The mesomeric effect in this molecule does not decay.

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Mutual influence of atoms in a molecule and methods of its transmission

The atoms that make up the molecule influence each other, this influence is transmitted along the chain of covalently bonded atoms and leads to a redistribution of the electron density in the molecule. Such a phenomenon is called electronic effect deputy.

Inductive effect

Bond polarization:

inductive effect (I-Effect) deputy called broadcast eletothrone influence deputy on chains y-connections.

The inductive effect quickly decays (after 2-3 connections)

Effect H accepted = 0

Electron acceptors (- I-Effect):

Hal, OH, NH 2 , NO 2 , COOH, CN

strong acceptors - cations: NH 3 +, etc.

Electron donors (+ I-Effect):

Alkyl groups next to sp 2 -carbon:

Anions: --O -

Metals of the 1st and 2nd groups:

mesomeric effect

The main role in the redistribution of the electron density of the molecule is played by delocalized p- and p-electrons.

Mesomeric Effect or Effect conjugation (M-Effect) - this is laneedistribution electrons on conjugate system.

The substituents whose atoms have an unhybridized p-orbital and can participate in conjugation with the rest of the molecule have a mesomeric effect. In the direction of the mesomeric effect, substituents can be both electron acceptors:

and electron donors:

Many substituents have both inductive and mesomeric effects (see table). For all substituents, with the exception of halogens, the mesomeric effect in absolute value significantly exceeds the inductive one.

If there are several substituents in the molecule, then their electronic effects may be consistent or inconsistent.

If all substituents increase (or decrease) the electron density in the same places, then their electronic effects are said to be consistent. Otherwise, their electronic effects are called inconsistent.

Spatial effects

The influence of the deputy, especially if he carries electric charge, can be transmitted not only through chemical bonds, but also through space. In this case, the spatial position of the substituent is of decisive importance. Such a phenomenon is called spatial effect deputyestetel.

For example:

The substituent can prevent the approach of the active particle to the reaction center and thereby reduce the reaction rate:

atom molecule electron substituent

The interaction of a drug substance with a receptor also requires a certain geometric correspondence of the contours of the molecules, and a change in the molecular geometric configuration significantly affects the biological activity.

Literature

1. Beloborodov V.L., Zurabyan S.E., Luzin A.P., Tyukavkina N.A. Organic chemistry (basic course). Bustard, M., 2003, p. 67 - 72.

2. N.A. Tyukavkina, Yu.I. Baukov. Bioorganic chemistry. DROFA, M., 2007, p. 36-45.

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The atoms and atomic groups in the molecules of organic compounds influence each other, and not only the atoms directly bonded 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 the chain of σ-bonds due to their polarization. The inductive effect is denoted by the symbol I. Consider it using the example of 1-chlorobutane:

The C-Cl bond is polar due to the higher electronegativity of chlorine. A partial positive charge (δ+) arises 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 arises on the next carbon atom, etc. So chlorine induces polarization not only of the "own" σ-bond, but also of subsequent ones in the chain. Please note that each subsequent partial positive charge is less than the previous one (δ+>δ+’>δ+’’>δ+’’’), i.e. the inductive effect is transmitted through the circuit with damping. 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 given example, the chlorine atom shifts the electron density along the bond chain to myself. Such an effect is called a negative inductive effect and is denoted by -I Cl.

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

If the substituent shifts the electron density along the 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, showing 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 both positive (+M) and negative (-M).

In the vinyl chloride molecule, the unshared 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 donates one electron to conjugation - the same number as each carbon atom, but the electronegativity of oxygen is higher than that of carbon, so oxygen shifts the electron density of the conjugated system towards itself, the aldehyde group as a whole exhibits a negative mesomeric effect.

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

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

b) substituents, in the structure of which there are atoms with unshared 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 in the conjugated system onto themselves exhibit a negative mesomeric effect. These include substituents in the structure of which there are 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). How in these cases to determine the overall effect of the substituent on the rest of the molecule (in other words, how to determine whether a given substituent is an electron donor or an electron acceptor)? Substituents that increase the electron density in the rest of the molecule are called electron-donating substituents, and substituents that decrease the electron density in the rest of the molecule are called electron-withdrawing substituents.

To determine the overall influence of a substituent, it is necessary to compare its electronic effects in magnitude. If the positive sign effect prevails, the substituent is an electron donor. If the negative effect prevails, the substituent is an electron-withdrawing substituent. It should be noted that, as a rule, the mesomeric effect is stronger than the inductive one (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 one.

Consider specific examples:

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

In this compound, the amino group is an electron-withdrawing substituent, 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 one.

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 one must consider its influence in a particular molecule.

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

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

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

Effect on reaction rate. The higher the electron density in the ring, the easier the electrophilic substitution reactions proceed. It is clear that electron-donating substituents facilitate the 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 severe 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 S E 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 row activity in electrophilic substitution reactions decreases from phenol to nitrobenzene. It has been experimentally established that if the rate of the benzene nitration reaction is taken as 1, then this series will look like this:

The second aspect of the influence of a substituent in 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 substituents 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 for 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 take phenol as an example:

Due to the positive mesomeric effect of the hydroxyl group, the electron density is redistributed along 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 an excess of bromine, the reaction proceeds immediately in three positions:

Substituents of the 2nd kind are: -NH 3 + , -COOH, -CHO (aldehyde group), -NO 2 , -SO 3 H. All these substituents lower the electron density in the aromatic ring, but due to its redistribution in meta positions, it is lowered not so strongly, as in ortho- and para-. Consider this using the example of benzoic acid:

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

CHAPTER 2. CHEMICAL BOND AND MUTUAL INFLUENCE OF ATOMS IN ORGANIC COMPOUNDS

CHAPTER 2. CHEMICAL BOND AND MUTUAL INFLUENCE OF ATOMS IN ORGANIC COMPOUNDS

The chemical properties of organic compounds are determined by the type of chemical bonds, the nature of the bonded atoms, and their mutual influence in the molecule. These factors, in turn, are determined by the electronic structure of atoms and the interaction of their atomic orbitals.

2.1. The electronic structure of the carbon atom

The part of the atomic space in which the probability of finding an electron is maximum is called atomic orbital(AO).

In chemistry, the concept of hybrid orbitals of the carbon atom and other elements is widely used. The concept of hybridization as a way of describing the rearrangement of orbitals is necessary when the number of unpaired electrons in the ground state of an atom is less than the number of bonds formed. An example is the carbon atom, which in all compounds manifests itself as a tetravalent element, but in accordance with the rules for filling orbitals on its outer electronic level, only two unpaired electrons are in the ground state 1s 2 2s 2 2p 2 (Fig. 2.1, a and Appendix 2-1). In these cases, it is postulated that different atomic orbitals, close in energy, can mix with each other, forming hybrid orbitals of the same shape and energy.

Hybrid orbitals, due to the greater overlap, form stronger bonds compared to non-hybridized orbitals.

Depending on the number of hybridized orbitals, a carbon atom can be in one of three states

Rice. 2.1.The distribution of electrons in orbitals at the carbon atom in the ground (a), excited (b) and hybridized states (c - sp 3 , g-sp2, d- sp)

hybridization (see Fig. 2.1, c-e). The type of hybridization determines the orientation of hybrid AOs in space and, consequently, the geometry of molecules, i.e., their spatial structure.

The spatial structure of molecules is the mutual arrangement of atoms and atomic groups in space.

sp 3-Hybridization.When mixing four external AOs of an excited carbon atom (see Fig. 2.1, b) - one 2s- and three 2p-orbitals - four equivalent sp 3 -hybrid orbitals arise. They have the shape of a three-dimensional "eight", one of the blades of which is much larger than the other.

Each hybrid orbital is filled with one electron. The carbon atom in the state of sp 3 hybridization has the electronic configuration 1s 2 2(sp 3) 4 (see Fig. 2.1, c). Such a state of hybridization is characteristic of carbon atoms in saturated hydrocarbons (alkanes) and, accordingly, in alkyl radicals.

Due to mutual repulsion, sp 3 -hybrid AOs are directed in space to the vertices tetrahedron, and the angles between them are 109.5? (the most advantageous location; Fig. 2.2, a).

The spatial structure is depicted using stereochemical formulas. In these formulas, the sp 3 hybridized carbon atom and its two bonds are placed in the plane of the drawing and graphically denoted by a regular line. A bold line or a bold wedge denotes a connection that extends forward from the plane of the drawing and is directed towards the observer; a dotted line or a hatched wedge (..........) - a connection that goes away from the observer beyond the plane of the drawing

Rice. 2.2.Types of hybridization of the carbon atom. The dot in the center is the nucleus of the atom (small fractions of hybrid orbitals are omitted to simplify the figure; unhybridized p-AOs are shown in color)

zha (Fig. 2.3, a). The carbon atom is in the state sp 3-hybridization has a tetrahedral configuration.

sp 2-Hybridization.When mixing one 2s- and two 2p-AO of the excited carbon atom, three equivalent sp 2-hybrid orbitals and remains unhybridized 2p-AO. The carbon atom is in the state sp 2-hybridization has an electronic configuration 1s 2 2(sp 2) 3 2p 1 (see Fig. 2.1, d). This state of hybridization of the carbon atom is typical for unsaturated hydrocarbons (alkenes), as well as for some functional groups, such as carbonyl and carboxyl.

sp 2 - Hybrid orbitals are located in the same plane at an angle of 120?, and unhybridized AO is in perpendicular to the plane(see Fig. 2.2, b). The carbon atom is in the state sp 2-hybridization has triangular configuration. The carbon atoms bound by a double bond are in the plane of the drawing, and their single bonds directed towards and away from the observer are designated as described above (see Fig. 2.3, b).

sp hybridization.When one 2s and one 2p orbitals of an excited carbon atom are mixed, two equivalent sp hybrid AOs are formed, while two p AOs remain unhybridized. The carbon atom in the sp hybridization state has the electronic configuration

Rice. 2.3.Stereochemical formulas of methane (a), ethane (b) and acetylene (c)

1s 2 2(sp 2) 2 2p 2 (see Fig. 2.1e). This state of hybridization of the carbon atom occurs in compounds having a triple bond, for example, in alkynes, nitriles.

sp-hybrid orbitals are located at an angle of 180?, and two unhybridized AOs are in mutually perpendicular planes (see Fig. 2.2, c). The carbon atom in the sp hybridization state has line configuration, for example, in an acetylene molecule, all four atoms are on the same straight line (see Fig. 2.3, in).

Atoms of other organogen elements can also be in a hybridized state.

2.2. Chemical bonds of carbon atom

Chemical bonds in organic compounds are mainly represented by covalent bonds.

Covalent is called chemical bond, formed as a result of the socialization of the electrons of the bonded atoms.

These shared electrons occupy molecular orbitals (MOs). As a rule, MO is a multicenter orbital and the electrons filling it are delocalized (dispersed). Thus, MO, like AO, can be vacant, filled with one electron or two electrons with opposite spins*.

2.2.1. σ- andπ -Communications

There are two types of covalent bonds: σ (sigma)- and π (pi)-bonds.

A σ-bond is a covalent bond formed when an AO overlaps along a straight line (axis) connecting the nuclei of two bonded atoms with the overlap maximum on this straight line.

The σ-bond arises when any AO overlaps, including hybrid ones. Figure 2.4 shows the formation of a σ-bond between carbon atoms as a result of the axial overlap of their hybrid sp 3 -AO and σ -C-H bonds by overlapping hybrid sp 3 -AO carbon and s-AO hydrogen.

* For more details see: Popkov V.A., Puzakov S.A. General chemistry. - M.: GEOTAR-Media, 2007. - Chapter 1.

Rice. 2.4.Formation of σ-bonds in ethane by axial overlapping of AO (small fractions of hybrid orbitals are omitted, color shows sp 3 -AO carbon, black - s-AO hydrogen)

In addition to the axial overlap, another type of overlap is possible - the lateral overlap of the p-AO, leading to the formation of a π bond (Fig. 2.5).

p-atomic orbitals

Rice. 2.5.π-bond formation in ethylene by lateral overlap r-AO

A π-bond is a bond formed by lateral overlap of unhybridized p-AOs with a maximum of overlap on both sides of the straight line connecting the nuclei of atoms.

Multiple bonds found in organic compounds are a combination of σ- and π-bonds: double - one σ- and one π-, triple - one σ- and two π-bonds.

The properties of a covalent bond are expressed in terms of characteristics such as energy, length, polarity, and polarizability.

Bond energyis the energy released during the formation of a bond or required to separate two bonded atoms. It serves as a measure of bond strength: the greater the energy, the stronger the bond (Table 2.1).

Link lengthis the distance between the centers of bonded atoms. A double bond is shorter than a single bond, and a triple bond is shorter than a double bond (see Table 2.1). The bonds between carbon atoms in different states of hybridization have a common pattern -

Table 2.1.Main characteristics covalent bonds

with an increase in the fraction of the s-orbital in the hybrid orbital, the bond length decreases. For example, in a series of compounds, propane CH 3 CH 2 CH 3, propene CH 3 CH=CH 2, propyne CH 3 C=CH CH 3 bond length -C, respectively, is equal to 0.154; 0.150 and 0.146 nm.

Communication polarity due to the uneven distribution (polarization) of the electron density. The polarity of a molecule is quantified by the value of its dipole moment. From the dipole moments of a molecule, the dipole moments of individual bonds can be calculated (see Table 2.1). The larger the dipole moment, the more polar the bond. The reason for the polarity of the bond is the difference in the electronegativity of the bonded atoms.

Electronegativity characterizes the ability of an atom in a molecule to hold valence electrons. With an increase in the electronegativity of an atom, the degree of displacement of bond electrons in its direction increases.

Based on the bond energies, the American chemist L. Pauling (1901-1994) proposed a quantitative characteristic of the relative electronegativity of atoms (Pauling scale). In this scale (row), typical organogenic elements are arranged according to relative electronegativity (two metals are given for comparison) as follows:

Electronegativity is not an absolute constant of an element. It depends on the effective charge of the nucleus, the type of AO hybridization, and the effect of substituents. For example, the electronegativity of a carbon atom in the state of sp 2 - or sp-hybridization is higher than in the state of sp 3 -hybridization, which is associated with an increase in the proportion of the s-orbital in the hybrid orbital. During the transition of atoms from sp 3 - to sp 2 - and further to sp-hybridized state, the length of the hybrid orbital gradually decreases (especially in the direction that provides the greatest overlap during the formation of the σ-bond), which means that in the same sequence, the electron density maximum is located closer to the nucleus of the corresponding atom.

In the case of a non-polar or practically non-polar covalent bond, the difference in the electronegativity of the bonded atoms is zero or close to zero. As the difference in electronegativity increases, the polarity of the bond increases. With a difference of up to 0.4, they speak of a weakly polar, more than 0.5, a strongly polar covalent bond, and more than 2.0, an ionic bond. Polar covalent bonds are prone to heterolytic cleavage

(see 3.1.1).

Communication polarizability is expressed in the displacement of bond electrons under the influence of an external electric field, including another reacting particle. Polarizability is determined by the electron mobility. Electrons are more mobile the farther they are from the nuclei of atoms. In terms of polarizability, the π-bond significantly exceeds the σ-bond, since the maximum electron density of the π-bond is located farther from the bonded nuclei. Polarizability largely determines the reactivity of molecules with respect to polar reagents.

2.2.2. Donor-acceptor bonds

The overlap of two one-electron AOs is not the only way to form a covalent bond. A covalent bond can be formed by the interaction of a two-electron orbital of one atom (donor) with a vacant orbital of another atom (acceptor). Donors are compounds containing either orbitals with a lone pair of electrons or π-MO. Carriers of lone pairs of electrons (n-electrons, from the English. non-bonding) are nitrogen, oxygen, halogen atoms.

Lone pairs of electrons play an important role in the manifestation of the chemical properties of compounds. In particular, they are responsible for the ability of compounds to enter into a donor-acceptor interaction.

A covalent bond formed by a pair of electrons from one of the bond partners is called a donor-acceptor bond.

The formed donor-acceptor bond differs only in the way of formation; its properties are the same as other covalent bonds. The donor atom acquires a positive charge.

Donor-acceptor bonds are characteristic of complex compounds.

2.2.3. Hydrogen bonds

A hydrogen atom bound to a strongly electronegative element (nitrogen, oxygen, fluorine, etc.) is able to interact with the lone pair of electrons of another sufficiently electronegative atom of the same or another molecule. As a result, a hydrogen bond arises, which is a kind of donor-

acceptor bond. Graphically, a hydrogen bond is usually represented by three dots.

The hydrogen bond energy is low (10–40 kJ/mol) and is mainly determined by the electrostatic interaction.

Intermolecular hydrogen bonds cause the association of organic compounds, such as alcohols.

Hydrogen bonds affect the physical (boiling and melting points, viscosity, spectral characteristics) and chemical (acid-base) properties of compounds. For example, the boiling point of ethanol C 2H5 OH (78.3 ° C) is significantly higher than that of having the same molecular weight dimethyl ether CH 3 OCH 3 (-24 ? C), not associated due to hydrogen bonds.

Hydrogen bonds can also be intramolecular. Such a bond in the anion of salicylic acid leads to an increase in its acidity.

Hydrogen bonds play an important role in the formation of the spatial structure of macromolecular compounds - proteins, polysaccharides, nucleic acids.

2.3. Related systems

A covalent bond can be localized or delocalized. A bond is called localized, the electrons of which are actually divided between the two nuclei of the bonded atoms. If the bond electrons are shared by more than two nuclei, then one speaks of a delocalized bond.

A delocalized bond is a covalent bond whose molecular orbital spans more than two atoms.

Delocalized bonds in most cases are π-bonds. They are characteristic of coupled systems. In these systems, a special kind of mutual influence of atoms occurs - conjugation.

Conjugation (mesomeria, from the Greek. mesos- medium) is the alignment of bonds and charges in a real molecule (particle) in comparison with an ideal, but non-existent structure.

The delocalized p-orbitals participating in conjugation can belong either to two or more π-bonds, or to a π-bond and one atom with a p-orbital. In accordance with this, a distinction is made between π,π-conjugation and ρ,π-conjugation. The conjugation system can be open or closed and contain not only carbon atoms, but also heteroatoms.

2.3.1. Open circuit systems

π,π -Pairing. The simplest representative of π, π-conjugated systems with a carbon chain is butadiene-1,3 (Fig. 2.6, a). Carbon and hydrogen atoms and, consequently, all σ-bonds in its molecule lie in the same plane, forming a flat σ-skeleton. Carbon atoms are in a state of sp 2 hybridization. Unhybridized p-AOs of each carbon atom are located perpendicular to the plane of the σ-skeleton and parallel to each other, which is a necessary condition for their overlap. Overlapping occurs not only between the p-AO of the C-1 and C-2, C-3 and C-4 atoms, but also between the p-AO of the C-2 and C-3 atoms, resulting in the formation of a single π spanning four carbon atoms -system, i.e., a delocalized covalent bond arises (see Fig. 2.6, b).

Rice. 2.6.Atomic orbital model of the 1,3-butadiene molecule

This is reflected in the change in bond lengths in the molecule. The bond length C-1-C-2, as well as C-3-C-4 in butadiene-1,3 is somewhat increased, and the distance between C-2 and C-3 is shortened compared to conventional double and single bonds. In other words, the process of electron delocalization leads to the alignment of bond lengths.

Hydrocarbons with a large number conjugated double bonds are common in the plant kingdom. These include, for example, carotenes, which determine the color of carrots, tomatoes, etc.

An open conjugation system can also include heteroatoms. An example of open π,π-conjugated systems with a heteroatom in the chain can serve as α,β-unsaturated carbonyl compounds. For example, the aldehyde group in acrolein CH 2 =CH-CH=O is a member of the chain of conjugation of three sp 2 -hybridized carbon atoms and an oxygen atom. Each of these atoms contributes one p-electron to the single π-system.

pn-pairing.This type of conjugation is most often manifested in compounds containing the structural fragment -CH=CH-X, where X is a heteroatom having an unshared pair of electrons (primarily O or N). These include, for example, vinyl ethers, in the molecules of which the double bond is conjugated with R the orbital of an oxygen atom. A delocalized three-center bond is formed by overlapping two p-AO sp 2 -hybridized carbon atoms and one R-AO of a heteroatom with a pair of n-electrons.

The formation of a similar delocalized three-center bond exists in the carboxyl group. Here, the π-electrons of the C=O bond and the n-electrons of the oxygen atom of the OH group participate in conjugation. Conjugated systems with fully aligned bonds and charges include negatively charged particles, such as the acetate ion.

The direction of electron density shift is indicated by a curved arrow.

There are others graphic ways display pairing results. Thus, the structure of the acetate ion (I) assumes that the charge is evenly distributed over both oxygen atoms (as shown in Fig. 2.7, which is true).

Structures (II) and (III) are used in resonance theory. According to this theory, a real molecule or particle is described by a set of certain so-called resonant structures, which differ from each other only in the distribution of electrons. In conjugated systems, the main contribution to the resonant hybrid is made by structures with different π-electron density distributions (the two-sided arrow connecting these structures is a special symbol of the resonance theory).

Limit (boundary) structures do not really exist. However, they "contribute" to some extent to the real distribution of electron density in a molecule (particle), which is represented as a resonant hybrid obtained by superimposition (superposition) of limiting structures.

In ρ,π-conjugated systems with a carbon chain, conjugation can occur if there is a carbon atom with an unhybridized p-orbital next to the π-bond. Such systems can be intermediate particles - carbanions, carbocations, free radicals, for example, allyl structures. Free radical allyl fragments play an important role in the processes of lipid peroxidation.

In the allyl anion CH 2 \u003d CH-CH 2 sp 2 -hybridized carbon atom C-3 delivers to the common conjugated

Rice. 2.7.Electron density map of the COONa group in penicillin

two electron system, in the allyl radical CH 2=CH-CH 2+ - one, and in the allyl carbocation CH 2=CH-CH 2+ does not supply any. As a result, when the p-AO overlaps three sp 2 -hybridized carbon atoms, a delocalized three-center bond is formed, containing four (in the carbanion), three (in the free radical), and two (in the carbocation) electrons, respectively.

Formally, the C-3 atom in the allyl cation carries a positive charge, in the allyl radical it has an unpaired electron, and in the allyl anion it has a negative charge. In fact, in such conjugated systems, there is a delocalization (dispersal) of the electron density, which leads to the alignment of bonds and charges. The C-1 and C-3 atoms are equivalent in these systems. For example, in an allyl cation, each of them carries a positive charge+1/2 and is connected by a "one and a half" bond with the C-2 atom.

Thus, conjugation leads to a significant difference in the electron density distribution in real structures compared to structures represented by conventional structure formulas.

2.3.2. Closed loop systems

Cyclic conjugated systems are of great interest as a group of compounds with increased thermodynamic stability compared to conjugated open systems. These compounds also have other special properties, the totality of which is united general concept aromaticity. These include the ability of such formally unsaturated compounds

enter into substitution reactions, not addition, resistance to oxidizing agents and temperature.

Typical representatives of aromatic systems are arenes and their derivatives. Peculiarities electronic structure aromatic hydrocarbons are clearly manifested in the atomic orbital model of the benzene molecule. The benzene framework is formed by six sp 2 hybridized carbon atoms. All σ-bonds (C-C and C-H) lie in the same plane. Six unhybridized p-AOs are located perpendicular to the plane of the molecule and parallel to each other (Fig. 2.8, a). Each R-AO can equally overlap with two neighboring R-AO. As a result of this overlap, a single delocalized π-system arises, in which the highest electron density is located above and below the σ-skeleton plane and covers all carbon atoms of the cycle (see Fig. 2.8, b). The π-electron density is evenly distributed throughout the cyclic system, which is indicated by a circle or a dotted line inside the cycle (see Fig. 2.8, c). All bonds between carbon atoms in the benzene ring have the same length (0.139 nm), intermediate between the lengths of single and double bonds.

Based on quantum mechanical calculations, it was established that for the formation of such stable molecules, a planar cyclic system must contain (4n + 2) π-electrons, where n= 1, 2, 3, etc. (Hückel's rule, 1931). Taking into account these data, it is possible to concretize the concept of "aromaticity".

A compound is aromatic if it has a planar ring and a conjugatedπ -electronic system covering all atoms of the cycle and containing(4n+ 2) π-electrons.

Hückel's rule is applicable to any flat condensed systems in which there are no atoms that are common to more than

Rice. 2.8.Atomic orbital model of the benzene molecule (hydrogen atoms omitted; see text for explanation)

two cycles. Compounds with condensed benzene rings, such as naphthalene and others, meet the criteria for aromaticity.

Stability of coupled systems. The formation of a conjugated and especially aromatic system is an energetically favorable process, since the degree of overlapping of the orbitals increases and delocalization (dispersal) occurs. R-electrons. In this regard, conjugated and aromatic systems have increased thermodynamic stability. They contain a smaller amount of internal energy and occupy a lower energy level in the ground state compared to non-conjugated systems. From the difference between these levels, one can quantify the thermodynamic stability of the conjugated compound, i.e., its conjugation energy(delocalization energy). For butadiene-1,3, it is small and amounts to about 15 kJ/mol. With an increase in the length of the conjugated chain, the conjugation energy and, accordingly, the thermodynamic stability of the compounds increase. The conjugation energy for benzene is much higher and amounts to 150 kJ/mol.

2.4. Electronic effects of substituents 2.4.1. Inductive effect

A polar σ-bond in a molecule causes polarization of the nearest σ-bonds and leads to the appearance of partial charges on neighboring atoms*.

Substituents cause polarization not only of their own, but also of neighboring σ-bonds. This type of transmission of the influence of atoms is called the inductive effect (/-effect).

Inductive effect - the transfer of the electronic influence of substituents as a result of the displacement of electrons of σ-bonds.

Due to the weak polarizability of the σ-bond, the inductive effect is attenuated after three or four bonds in the circuit. Its action is most pronounced in relation to the carbon atom adjacent to the one that has a substituent. The direction of the inductive effect of the substituent is qualitatively estimated by comparing it with the hydrogen atom, the inductive effect of which is taken as zero. Graphically, the result of the /-effect is depicted by an arrow coinciding with the position of the valence line and pointing towards the more electronegative atom.

/in\stronger than a hydrogen atom, exhibitsnegativeinductive effect (-/-effect).

Such substituents generally lower the electron density of the system, they are called electron-withdrawing. These include most of the functional groups: OH, NH 2, COOH, NO2 and cationic groups, such as -NH 3+.

A substituent that shifts electron density compared to a hydrogen atomσ -bonds towards the carbon atom of the chain, exhibitspositiveinductive effect (+/- effect).

Such substituents increase the electron density in the chain (or ring) and are called electron donor. These include alkyl groups located at the sp 2 -hybridized carbon atom, and anionic centers in charged particles, for example -O - .

2.4.2. mesomeric effect

In conjugated systems, the main role in the transfer of electronic influence is played by π-electrons of delocalized covalent bonds. The effect that manifests itself as a shift in the electron density of a delocalized (conjugated) π-system is called the mesomeric (M-effect), or the conjugation effect.

The mesomeric effect is the transfer of the electronic influence of substituents along the conjugated system.

In this case, the substitute is itself a member of the conjugated system. It can introduce into the conjugation system either a π-bond (carbonyl, carboxyl groups, etc.), or a lone pair of electrons of a heteroatom (amino and hydroxy groups), or a vacant or one-electron-filled p-AO.

A substituent that increases the electron density in a conjugated system exhibitspositivemesomeric effect (+M- effect).

The M-effect is exhibited by substituents that include atoms with a lone pair of electrons (for example, an amino group in an aniline molecule) or an integer negative charge. These substitutes are capable

to the transfer of a pair of electrons to a common conjugated system, that is, they are electron donor.

A substituent that lowers the electron density in a conjugated system exhibitsnegativemesomeric effect (-M- effect).

The M-effect in the conjugated system is possessed by oxygen or nitrogen atoms bound by a double bond to a carbon atom, as shown in the example of acrylic acid and benzaldehyde. Such groupings are electron-withdrawing.

The displacement of the electron density is indicated by a curved arrow, the beginning of which shows which p- or π-electrons are being displaced, and the end is the bond or atom to which they are displaced. The mesomeric effect, in contrast to the inductive one, is transmitted through a system of conjugated bonds over a much greater distance.

When assessing the influence of substituents on the distribution of electron density in a molecule, it is necessary to take into account the resulting action of the inductive and mesomeric effects (Table 2.2).

Table 2.2.Electronic effects of some substituents

The electronic effects of substituents make it possible to give a qualitative estimate of the electron density distribution in a nonreacting molecule and to predict its properties.