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Mutual influence of atoms in organic molecules. Electronic effects in molecules of organic compounds

CHAPTER 2. CHEMICAL BONDING 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. Electronic structure of the carbon atom

The part of the atomic space in which the probability of finding an electron is maximum is called an 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, its outer electronic level in the ground state 1s 2 2s 2 2p 2 contains only two unpaired electrons (Fig. 2.1, A and Appendix 2-1). In these cases, it is postulated that different atomic orbitals, similar in energy, can mix with each other, forming hybrid orbitals of the same shape and energy.

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

Depending on the number of orbitals that have entered into hybridization, a carbon atom can be in one of three states

Rice. 2.1.Distribution of electrons over orbitals of a carbon atom in the ground (a), excited (b) and hybridized states (c - sp3, g-sp2, d-sp)

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

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

sp 3-Hybridization.When four external AOs of an excited carbon atom (see Fig. 2.1, b) - one 2s and three 2p orbitals - are mixed, 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). This 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 towards 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 indicated by a regular line. A thick line or a thick wedge indicates a connection extending forward from the plane of the drawing and directed towards the observer; dotted line or shaded wedge (..........) - a connection extending from the observer beyond the plane of the drawing -

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

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

sp 2 -Hybridized orbitals are located in the same plane at an angle of 120?, and the non-hybridized AO is in a perpendicular plane (see Fig. 2.2, b). The carbon atom is in the state sp 2-hybridization has trigonal configuration. Carbon atoms connected 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, and two p-AOs remain unhybridized. The carbon atom in the sp-hybridized state has an 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. 2.1, d). This state of hybridization of the carbon atom occurs in compounds that have a triple bond, for example, in alkynes and nitriles.

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

Atoms of other organogenic elements may also be in a hybridized state.

2.2. Chemical bonds of a carbon atom

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

A covalent bond is a chemical bond formed as a result of the sharing of electrons between bonded atoms.

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

2.2.1. σ- Andπ -Connections

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 a maximum overlap on this straight line.

The σ-bond occurs when any AO, including hybrid ones, overlap. 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 the hybrid sp 3 -AO of carbon and s-AO of 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 overlap of AOs (small fractions of hybrid orbitals are omitted and shown in color sp 3 -AO carbon, black - s-AO hydrogen)

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

p-atomic orbitals

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

A π-bond is a bond formed by the lateral overlap of unhybridized p-AOs with a maximum 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 through characteristics such as energy, length, polarity and polarizability.

Communication energyis the energy released when a bond is formed or required to separate two bonded atoms. It serves as a measure of the strength of the bond: the higher 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). Bonds between carbon atoms in different states of hybridization have a common pattern -

Table 2.1.Basic characteristics of covalent bonds

As the fraction of the s orbital in the hybrid orbital increases, 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 bond length CH 3 -C is correspondingly equal to 0.154; 0.150 and 0.146 nm.

Communication polarity due to uneven distribution (polarization) of 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 bond polarity is the difference in electronegativity of the bonded atoms.

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

Based on the values of bond energy, the American chemist L. Pauling (1901-1994) proposed a quantitative characteristic of the relative electronegativity of atoms (Pauling scale). In this scale (series), typical organogen 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 influence of substituents. For example, the electronegativity of a carbon atom in the sp 2 or sp hybridization state is higher than in the sp 3 hybridization state, 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 extent of the hybrid orbital gradually decreases (especially in the direction that provides the greatest overlap during the formation of a σ bond), which means that in the same sequence the maximum electron density is located closer and 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. A difference of up to 0.4 is said to be weakly polar, more than 0.5 is a strongly polar covalent bond, and more than 2.0 is an ionic bond. Polar covalent bonds are prone to heterolytic cleavage

(see 3.1.1).

Bond polarizability is expressed in the displacement of bond electrons under the influence of an external electric field, including that of another reacting particle. Polarizability is determined by electron mobility. Electrons are more mobile the further they are from the nuclei of atoms. In terms of polarizability, the π bond is significantly superior to the σ bond, since the maximum electron density of the π bond is located further from the bonded nuclei. Polarizability largely determines the reactivity of molecules towards 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 English. non-bonding) are atoms of nitrogen, oxygen, halogens.

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 donor-acceptor interactions.

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

The resulting donor-acceptor bond differs only in the method of formation; its properties are identical to other covalent bonds. The donor atom thereby acquires a positive charge.

Donor-acceptor bonds are characteristic of complex compounds.

2.2.3. Hydrogen bonds

A hydrogen atom bonded to a strongly electronegative element (nitrogen, oxygen, fluorine, etc.) is capable of interacting 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 type of donor bond.

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 electrostatic interaction.

Intermolecular hydrogen bonds determine 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. Thus, the boiling point of ethanol is C 2 H 5 OH (78.3°C) is significantly higher than dimethyl ether CH 3 OCH 3 (-24°C), which has the same molecular weight and is not associated through hydrogen bonds.

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

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

2.3. Conjugate systems

A covalent bond can be localized or delocalized. A localized bond is one whose electrons are actually shared between the two nuclei of the bonded atoms. If the bonding electrons are shared between more than two nuclei, then they speak of a delocalized bond.

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

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

Conjugation (mesomerism, from Greek. mesos- average) 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 involved in conjugation can belong to either two or more π-bonds, or 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). The 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. The 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. Overlap occurs not only between p-AO of atoms C-1 and C-2, C-3 and C-4, but also between p-AO of atoms C-2 and C-3, resulting in the formation of a single π covering four carbon atoms -system, i.e., a delocalized covalent bond appears (see Fig. 2.6, b).

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

This is reflected in changes in bond lengths in the molecule. The length of the C-1-C-2 as well as C-3-C-4 bonds in 1,3-butadiene is slightly 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 equalization of bond lengths.

Hydrocarbons with a large number of conjugated double bonds are common in the plant world. 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 chainα,β-unsaturated carbonyl compounds can serve. For example, the aldehyde group in acrolein CH 2 =CH-CH=O is a participant in the conjugation chain of three sp 2 -hybridized carbon atoms and an oxygen atom. Each of these atoms contributes one p-electron to a single π-system.

pn-Pairing.This type of conjugation most often occurs in compounds containing the structural fragment -CH=CH-X, where X is a heteroatom having a lone 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-orbital of the 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 occurs 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 species, such as the acetate ion.

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

There are other graphical ways to 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 resonance structures, which differ from each other only in the distribution of electrons. In conjugated systems, the main contribution to the resonance hybrid is made by structures with different distributions of π-electron density (the double-sided arrow connecting these structures is a special symbol of resonance theory).

Limit (boundary) structures do not really exist. However, to one degree or another, they “contribute” to the real distribution of electron density in a molecule (particle), which is represented as a resonant hybrid obtained by superposition of limiting structures.

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

In the allyl anion CH 2 =CH-CH 2 sp 2 -hybridized carbon atom C-3 supplies to the common conjugate

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

system two electrons, in the allylic radical CH 2 =CH-CH 2+ - one, and in the allylic carbocation CH 2 =CH-CH 2+ does not supply any. As a result, when the p-AO of three sp 2 -hybridized carbon atoms overlaps, 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 carries an unpaired electron, and in the allyl anion it carries a negative charge. In fact, in such conjugated systems there is delocalization (dispersal) of the electron density, which leads to the alignment of bonds and charges. The C-1 and C-3 atoms in these systems are equivalent. 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 to the C-2 atom.

Thus, conjugation results in a significant difference in the electron density distribution in real structures compared to the structures depicted 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 by the general concept aromaticity. These include the ability of such formally unsaturated compounds

engage in substitution reactions rather than addition, resistance to oxidizing agents and temperature.

Typical representatives of aromatic systems are arenes and their derivatives. The peculiarities of the electronic structure of 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 ones R-AO. As a result of such overlap, a single delocalized π-system arises, the highest electron density in which is located above and below the plane of the σ-skeleton and covers all the 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 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 flat cyclic system must contain (4n + 2) π-electrons, where n= 1, 2, 3, etc. (Hückel’s rule, 1931). Taking these data into account, the concept of “aromaticity” can be specified.

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

Hückel's rule applies to any planar condensed systems in which there are no atoms shared by more than

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

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 this increases the degree of overlap of orbitals and delocalization (dispersal) occurs. R-electrons. In this regard, conjugated and aromatic systems have increased thermodynamic stability. They contain a smaller supply of internal energy and in the ground state occupy a lower energy level 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. As the length of the conjugated chain increases, 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 nearby 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 transfer of influence of atoms is called the inductive effect (/-effect).

The inductive effect is 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 fades after three or four bonds in the circuit. Its effect 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 assessed by comparing it with the hydrogen atom, the inductive effect of which is taken to be zero. Graphically, the result of the /-effect is represented by an arrow coinciding with the position of the valence line and pointing towards the more electronegative atom.

/V\stronger than the hydrogen atom, exhibitsnegativeinductive effect (-/- effect).

Such substituents generally reduce the electron density of the system; they are called electron-withdrawing. These include most functional groups: OH, NH 2, COOH, NO 2 and cationic groups, for example -NH 3+.

A substituent that shifts the electron density compared to the 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 π-electrons of delocalized covalent bonds play the main role in the transmission of electronic influence. The effect manifested in a shift in the electron density of a delocalized (conjugated) π-system is called the mesomeric (M-effect), or conjugation effect.

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

In this case, the deputy himself is a participant in the coupled system. It can introduce into the conjugation system either a π-bond (carbonyl, carboxyl groups, etc.), or a lone pair of heteroatom electrons (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 entire negative charge. These substituents are capable

to the transfer of a pair of electrons to a common conjugate system, i.e. they are electron donor.

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

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

An electron density shift is indicated by a curved arrow, the beginning of which shows which p or π electrons are displaced, and the end of which shows the bond or atom to which they are displaced. The mesomeric effect, in contrast to the inductive effect, 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 effect of inductive and mesomeric effects (Table 2.2).

Table 2.2.Electronic effects of some substituents

Electronic effects of substituents make it possible to qualitatively assess the distribution of electron density in a non-reacting molecule and predict its properties.

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 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 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 look at 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 an excess of bromine is used, 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.

Video tutorial 1: Inductive effect. The structure of molecules. Organic chemistry

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

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

Lecture: Theory of the structure of 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 structure, properties, and interconversions.

Organic substances include carbon oxides, carbonic acid, carbonates, and bicarbonates. 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 capable of connecting with each other in chains of arbitrary length. This connection can be not only serial, but also branched and cyclic. Different bonds occur 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 seeking 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 most often bonds with hydrogen, oxygen and nitrogen.

Theory of the structure of organic compounds

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

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 different 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 connections of atoms also leads to a change in the properties of the substance.

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

Thus, knowing the structure of the molecule of an organic substance, 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 position of Butlerov’s theory, it became clear to us that the properties of organic substances depend not only on the composition of the molecules, but also on the order of connection of the atoms of their molecules. 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, and arenes. We'll talk about them in subsequent lessons.

Let's consider the types of isomerism:

1. Structural isomerism

1.1. Carbon skeleton isomerism:

1.2. Position isomerism:

1.2.1. Multiple bond isomerism

1.2.2. Isomerism of substituents

1.2.3. Isomerism of functional groups

1.3. Interclass isomerism:

2. Spatial isomerism

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

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

In the case where identical substituents are located on the same side of the plane, we can say that this is a cis isomer. When the substituents are located on opposite sides, this is a trans isomer. This type of isomerism is impossible in the case when at least one carbon atom at the double bond has two identical substituents. For example, cis-trans isomerism is not possible for propene.

2.2. Optical isomerism. You know that it is possible for a carbon atom to combine with four atoms/groups of atoms. Eg:

In such cases, optical isomerism is formed, two compounds - antipodes, like the left and right hands of a person:

Mutual influence of atoms in molecules

The concept of chemical structure, as a sequence of atoms connected to each other, was expanded with the advent of electronic theory. There are two possible ways in which one part of a molecule influences another:

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 here is polar, since the latter is more electronegative. As a result of a shift in electron density 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. A shift in electron density is indicated by an arrow pointing toward the more electronegative atom.

In addition to a shift in the electron density, its displacement is also possible, but to a lesser extent. The displacement occurs from the second carbon atom to the first, from the third to the second. This density shift along a chain of σ-bonds is called the inductive effect (I). It fades away as it moves away from the influencing group. And after 3 σ-bonds it 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 a carbon atom, the transfer of electron density from these substituents to the carbon atoms begins. This means that the mixer contains a positive inductive effect (+I). Substituents with +I-effect are considered to be saturated hydrocarbon radicals. At the same time, the +I effect increases with lengthening 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 a state of sp-hybridization, contain a fairly high electronegativity compared to carbon atoms in a state of sp2-hybridization. These atoms, in turn, are more electronegative compared to carbon atoms in the state of sp3 hybridization.

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 a lone electron pair: -NH 2, -OH, halogens. Substituents that are capable of withdrawing electron density from the conjugated system have a negative mesomeric effect (–M). 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.

With the redistribution of electron density, as well as due to the occurrence of mesomeric and inductive effects, positive or negative charges are formed on the atoms. This formation is reflected in the chemical properties of the substance. Graphically, the mesomeric effect is often represented by a curved arrow. This arrow originates at the center of the electron density. 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 combines with the electrons of the π bond between the carbon atoms. As a result of this conjugation, a partial positive charge is formed on the chlorine atom.

The π-electron cloud, which has mobility, as a result of the influence of the 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 fade.

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—connection in its direction, acquires a 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.

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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 electron density in the molecule. This phenomenon is called electronic effect deputy

Inductive effect

Bond polarization:

Inductive effect (I-Effect) deputy called broadcast eleTothrone influence deputy By chains y-connections.

The inductive effect quickly fades (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 the sp 2 carbon:

Anions: --O -

Metals of 1st and 2nd groups:

Mesomeric effect

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

Mesomeric Effect or Effect pairing (M-Effect) - This laneedistribution electrons By conjugate system.

The mesomeric effect is exerted by those substituents whose atoms have an unhybridized p-orbital and can participate in conjugation with the rest of the molecule. In the direction of the mesomeric effect, substituents can act as 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 effect.

If a molecule has several substituents, then their electronic effects can be coordinated or inconsistent.

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

Spatial effects

The influence of a substituent, especially if it carries an electrical 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. This phenomenon is called spatial effect deputyestitela.

For example:

A substituent can prevent the active particle from approaching the reaction center and thereby reduce the reaction rate:

atom molecule electron deputy

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

Literature

1. Beloborodov V.L., Zurabyan S.E., Luzin A.P., Tyukavkina N.A. Organic chemistry (main 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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Mutual influence of atoms in organic molecules. Electronic effects in molecules of organic compounds

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

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