The chemical properties of organic compounds are determined by the type of chemical bonds, the nature of the bound 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.

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 the 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 less than number formed connections. 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 1s22s22p2 (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

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.

sp3 hybridization. When mixing four external AOs of an excited carbon atom (see Fig. 2.1, b) - one 2s - and three 2p-orbitals - four equivalent sp3-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 sp3 hybridization has the electronic configuration 1s22(sp3)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, the sp3-hybrid AOs are directed in space to the vertices of the tetrahedron, and the angles between them are equal to 109.5? (the most advantageous location; Fig. 2.2, a).

The spatial structure is depicted using stereochemical formulas. In these formulas, the sp3-hybridized carbon atom and its two bonds are placed in the plane of the drawing and are 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)

sp2 hybridization. When mixing one 2s - and two 2p-AO of the excited carbon atom, three equivalent sp2-hybrid orbitals are formed and the 2p-AO remains unhybridized. The carbon atom in the state of sp2 hybridization has the electronic configuration 1s22(sp2)32p1 (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.

sp2-hybrid orbitals are located in the same plane at an angle of 120?, and the unhybridized AO is in the perpendicular plane (see Fig. 2.2, b). The carbon atom in the sp2 hybridization state has a trigonal configuration. 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 mixing one 2s- and one 2p-orbitals of the excited carbon atom, 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)

1s22(sp2)22p2 (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 state of sp-hybridization has a linear configuration, for example, in an acetylene molecule, all four atoms are on the same straight line (see Fig. 2.3, c).

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. y- and p-bonds

There are two types of covalent bonds: y (sigma)- and p (pi)-bonds.

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

The y-bond arises when any AO overlaps, including hybrid ones. Figure 2.4 shows the formation of a y-bond between carbon atoms as a result of the axial overlap of their hybrid sp3-AO and y-bonds C-H by overlap between the hybrid sp3-AO of carbon and the s-AO of hydrogen.

* For more details, see:, Puzakov chemistry. - M.: GEOTAR-Media, 2007. - Chapter 1.

Rice. 2.4. Formation of y-bonds in ethane by axial overlapping of AOs (small fractions of hybrid orbitals are omitted, sp3-AOs of carbon are shown in color, s-AOs of hydrogen are shown in black)

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 p-bond (Fig. 2.5).

p-atomic orbitals

Rice. 2.5. P-bond formation in ethylene by p-AO lateral overlap

A p-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 y - and p-bonds: double - one y - and one p-, triple - one y - and two p-bonds.

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

Bond energy is the energy released when a bond is formed 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).

The bond length is the distance between the centers of the 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 of covalent bonds

with an increase in the fraction of the s-orbital in the hybrid orbital, the bond length decreases. For example, in the series of compounds propane CH3CH2CH3, propene CH3CH=CH2, propyne CH3C=CH, the length of the CH3-C bond, respectively, is 0.154; 0.150 and 0.146 nm.

The polarity of the bond is 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 the bond electrons in its direction increases.

Based on the bond energies, the American chemist L. Pauling (1901-1994) proposed quantitative characteristic 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 sp2- or sp-hybridization state is higher than in the sp3-hybridization state, which is associated with an increase in the fraction of the s-orbital in the hybrid orbital. During the transition of atoms from sp3- to sp2- and further to the 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 y-bond), which means that the electron density maximum is located in the same sequence for all 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 - of a strongly polar covalent bond, and more than 2.0 - of an ionic bond. Polar covalent bonds are prone to heterolytic cleavage

The polarizability of a bond is expressed in the displacement of the 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 p-bond significantly exceeds the y-bond, since the maximum electron density of the p-bond is located farther from the bound 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 p-MO. The carriers of lone pairs of electrons (n-electrons, from the English non-bonding) are atoms of nitrogen, oxygen, halogens.

Lone pairs of electrons play important role in manifestation chemical properties connections. 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 bonded 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 physical (boiling and melting points, viscosity, spectral characteristics) and chemical (acid-base) properties of compounds. Thus, the boiling point of ethanol C2H5OH (78.3 ? C) is much higher than that of dimethyl ether CH3OCH3 (-24 ? C), which has the same molecular weight and is 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 spatial structure 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 p-bonds. They are characteristic of coupled systems. In these systems, a special kind of mutual influence of atoms is carried out - conjugation.

Conjugation (mesomerism, from the Greek mesos - middle) is the alignment of bonds and charges in a real molecule (particle) compared to an ideal, but non-existent structure.

The delocalized p-orbitals involved in conjugation can belong either to two or more p-bonds, or to a p-bond and one atom with a p-orbital. In accordance with this, p, p-conjugation and c, p-conjugation are distinguished. The conjugation system can be open or closed and contain not only carbon atoms, but also heteroatoms.

2.3.1. Open circuit systems

p, p-Conjugation. The simplest representative of p, p-conjugated systems with a carbon chain is butadiene-1,3 (Fig. 2.6, a). The carbon and hydrogen atoms and, consequently, all the y-bonds in its molecule lie in the same plane, forming a flat y-skeleton. The carbon atoms are in a state of sp2 hybridization. The unhybridized p-AOs of each carbon atom are located perpendicular to the y-skeleton plane and parallel to each other, which is necessary condition to cover them. 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, as a result of which a single p is formed covering 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 of conjugated double bonds are common in flora. These include, for example, carotenes, which determine the color of carrots, tomatoes, etc.

An open conjugation system can also include heteroatoms. Examples of open p, p-conjugated systems with a heteroatom in the chain are b, c-unsaturated carbonyl compounds. For example, the aldehyde group in acrolein CH2=CH-CH=O is a member of the conjugation chain of three sp2-hybridized carbon atoms and an oxygen atom. Each of these atoms contributes to unified p-system one p-electron.

pn-pairing. This type of conjugation is most often manifested in compounds containing a 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 the p-orbital of the oxygen atom. A delocalized three-center bond is formed by overlapping two p-AO of sp2-hybridized carbon atoms and one p-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 p-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 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 resonant hybrid is made by structures with different p-electron density distributions (the double-sided arrow connecting these structures is a special symbol of 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 c, p-conjugated systems with a carbon chain, conjugation can be carried out if there is a carbon atom with an unhybridized p-orbital next to the p-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 CH2=CH-CH2 sp2-hybridized carbon atom C-3 supplies the common conjugated

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

the system has two electrons, in the allyl radical CH2=CH-CH2+ - one, and in the allyl carbocation CH2=CH-CH2+ does not supply any. As a result, when the p-AO overlaps three sp2-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 the allyl cation, each of them carries a positive charge of +1/2 and is linked by a "one and a half" bond to 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 enhanced thermodynamic stability compared to conjugated open systems. These compounds also have other special properties, the totality of which is combined 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 sp2-hybridized carbon atoms. All y-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 p-AO can equally overlap with two neighboring p-AOs. As a result of this overlap, a single delocalized p-system arises, in which the highest electron density is located above and below the y-skeleton plane and covers all carbon atoms of the cycle (see Fig. 2.8, b). The p-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 found that for the formation of such stable molecules, a flat cyclic system must contain (4n + 2) p-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 conjugated p-electronic system, covering all atoms of the cycle and containing (4n + 2) p-electrons.

Hückel's rule applies to any planar 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) of p-electrons occurs. In this regard, conjugated and aromatic systems have increased thermodynamic stability. They contain less stock internal energy and in the ground state occupy a lower energy level compared to non-conjugated systems. The difference between these levels can be used to 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 y-bond in a molecule causes polarization of the nearest y-bonds and leads to the appearance of partial charges on neighboring atoms*.

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

The inductive effect is the transfer of the electronic influence of the substituents as a result of the displacement of the electrons of the y-bonds.

Due to the weak polarizability of the y-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.

/v\stronger than the hydrogen atom, exhibits a negative inductive effect (-/-effect).

Such substituents generally lower the electron density of the system; they are called electron-withdrawing substituents. These include most of the functional groups: OH, NH2, COOH, NO2 and cationic groups, for example - NH3+.

A substituent that shifts the electron density of the y-bond towards the carbon atom of the chain compared to the hydrogen atom exhibits a positive inductive effect (+/-effect).

Such substituents increase the electron density in the chain (or ring) and are called electron donor substituents. These include alkyl groups located at the sp2-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 p-electrons of delocalized covalent bonds. The effect that manifests itself as a shift in the electron density of a delocalized (conjugated) p-system is called the mesomeric (M-effect), or the conjugation effect.

Mesomeric effect - 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 p-bond (carbonyl, carboxyl group etc.), or an unshared pair of electrons of a heteroatom (amino - and hydroxy groups), or a vacant or filled with one electron p-AO.

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

The M-Effect is possessed by substituents, including atoms with a lone pair of electrons (for example, an amino group in an aniline molecule) or a whole negative charge. These substitutes are capable

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

A substituent that lowers the electron density in a conjugated system exhibits a negative mesomeric 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 groups are electron-withdrawing.

The displacement of the electron density is indicated by a curved arrow, the beginning of which shows which p- or p-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 effect, is transmitted over a system of conjugated bonds over a much greater distance.

When evaluating 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.

In an organic compound, the atoms are connected in a certain order, usually by covalent bonds. In this case, atoms of the same element in a compound can have different electronegativity. Important communication characteristics - polarity And strength (formation energy), and hence the reactivity of the molecule (the ability to enter into certain chemical reactions) is largely determined by electronegativity.

The electronegativity of the carbon atom depends on the type of hybridization of atomic orbitals. The contribution of the s-orbital is smaller at sp3- and more at sp2- and sp hybridization.

All atoms in a molecule exert mutual influence on each other mainly through a system of covalent bonds. The shift in electron density in a molecule under the influence of substituents is called the electronic effect.

Atoms connected by a polar bond carry partial charges (a partial charge is denoted by the Greek letter Y - “delta”). An atom that "pulls" the electron density of the a-bond onto itself acquires a negative charge R-. In a pair of atoms linked by a covalent bond, the more electronegative atom is called an electron acceptor. Its a-bond partner has a deficit of electron density - an equal partial positive charge of 6+; such an atom electron donor.

The shift of the electron density along the chain of a-bonds is called the inductive effect and is denoted by the letter I.

The inductive effect is transmitted through the circuit with damping. The shift in the electron density of a-bonds is shown by a simple (straight) arrow (-" or *-).

Depending on whether the electron density of the carbon atom decreases or increases, the inductive effect is called negative (-/) or positive (+/). The sign and magnitude of the inductive effect are determined by the difference in the electronegativity of the carbon atom and another atom or functional group associated with them, i.e. affecting that carbon atom.

electron-withdrawing substituents, i.e., an atom or a group of atoms that shifts the electron density of an a-bond from a carbon atom to itself, exhibit negative inductive effect(-/-Effect).

electron donating substituents, i.e., an atom or group of atoms that causes a shift in electron density towards a carbon atom (away from itself) exhibits positive inductive effect(+/- effect).

The N-Effect is exhibited by aliphatic hydrocarbon radicals, i.e., alkyls (methyl, ethyl, etc.). Many functional groups have -/-effect: halogens, amino group, hydroxyl, carbonyl, carboxyl groups.

The inductive effect also appears in the carbon-carbon bond if the carbon atoms differ in the type of hybridization. For example, in a propene molecule, the methyl group exhibits a +/- effect, since the carbon atom in it is in the vp 3 hybrid state, and the §p 2 hybrid atom acts as an electron acceptor with a double bond, since it has a higher electronegativity:

When the inductive effect of the methyl group is transferred to the double bond, it is primarily affected by the mobile

The influence of a substituent on the distribution of electron density, transmitted through n-bonds, is called the mesomeric effect ( M ). The mesomeric effect can also be negative and positive. IN structural formulas the mesomeric effect is shown by a curved arrow from the middle of the bond with excess electron density, directed to the place where the electron density shifts. For example, in a phenol molecule, the hydroxyl group has a +M effect: the lone pair of electrons of the oxygen atom interacts with the n-electrons of the benzene ring, increasing the electron density in it. In benzaldehyde, the carbonyl group with the -M effect pulls the electron density from the benzene ring towards itself.

Electronic effects lead to a redistribution of the electron density in the molecule and the appearance of partial charges on individual atoms. This determines the reactivity of the molecule.

Target: study of the electronic structure of organic compounds and ways of transferring the mutual influence of atoms in their molecules.

Plan:

Inductive effect

Pairing types.

Aromaticity of organic compounds

Mesomeric effect (conjugation effect)

1. Inductive effect

A molecule of an organic compound is a collection of atoms linked in a certain order by covalent bonds. In this case, the bound atoms can differ in the value of electronegativity (E.O.).

Electronegativity- the ability of an atom to attract the electron density of another atom for the implementation of a chemical bond.

The larger the value of E.O. given element, the stronger it attracts bond electrons. E.O. were established by the American chemist L. Pauling and this series is called the Pauling scale.

The E. O. of a carbon atom depends on the state of its hybridization, since carbon atoms in various types hybridizations differ from each other in E. O. and this depends on the proportion of the s-cloud in a given type of hybridization. For example, a C atom in the state of sp 3 hybridization has the lowest E.O. since the p-cloud accounts for the least s-cloud. Big E.O. possesses a C atom in sp hybridization.

All the atoms that make up the molecule are in mutual connection with each other and experience mutual influence. This influence is transmitted through covalent bonds with the help of electronic effects.

One of the properties of a covalent bond is a certain mobility of the electron density. It is capable of shifting towards the atom with a larger E, O.

Polarity covalent bond is an uneven distribution of electron density between bonded atoms.

Availability polar bond in a molecule affects the state of adjacent bonds. They are affected by the polar bond, and their electron density is also shifted towards more E.O. atom, i.e., there is a transfer of the electronic effect.

The shift of the electron density along the chain of ϭ bonds is called inductive effect and is denoted by I.

The inductive effect is transmitted along the circuit with damping, because when a ϭ bond is formed, a large amount of energy is released and it is poorly polarized, and therefore the inductive effect manifests itself to a greater extent on one or two bonds. The direction of displacement of the electron density of all ϭ-bonds is indicated by straight arrows.→

For example: CH 3 δ +< → CH 2 δ +< → CH 2 δ +< →Cl δ - Э.О. Сl >E.O. WITH

CH 3 δ +< → CH 2 δ +< → CH 2 δ +< →OH δ - Э.О. ОН >E.O. WITH

An atom or a group of atoms that shifts the electron density of a ϭ bond from a carbon atom to itself is called electron-withdrawing substituents and exhibit a negative inductive effect (- I-Effect).

They are halogens (Cl, Br, I), OH -, NH 2 -, COOH, COH, NO 2, SO 3 H, etc.

An atom or group of atoms that donate electron density is called electron donor substituents and exhibit a positive inductive effect (+ I-Effect).

I-effect exhibit aliphatic hydrocarbon radicals, CH 3 , C 2 H 5 , etc.

The inductive effect also manifests itself in the case when the bonded carbon atoms differ in the state of hybridization. For example, in a propene molecule, the CH 3 group exhibits a + I-effect, since the carbon atom in it is in the sp 3 hybrid state, and the carbon atoms with a double bond in the sp 2 hybrid state and show greater electronegativity, therefore, they exhibit -I- effect and are electron acceptors.

An organic compound molecule is a collection of atoms linked in a certain order, usually by covalent bonds. In this case, the bound atoms can differ in size electronegativity. Quantities electronegativity 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.

Electronegativitycarbon atom depends on the state of its hybridization. It is related to the share s- orbitals in a hybrid orbital: it is smaller than y sp 3 - and more at sp 2 - and sp -hybrid atoms.

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

electronic effects called the shift of the electron density in the 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 in s -bonds, respectively, will have an equal electron density deficit, i.e. partial positive charge d +, will be called electron donor.

Shift of electron density along the chains-ties is called inductive effect and denoted I.

The inductive effect is transmitted through the circuit with damping. The direction of displacement of the electron density of alls-connections are indicated by straight arrows.

Depending on whether the electron density moves away from the considered carbon atom 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 that calls 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 displaces electron density away from itself toward a carbon atom 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 the state of hybridization.

When transferring the inductive effect of a methyl group to a double bond, it is primarily affected by the mobilep- connection.

The influence of the substituent on the distribution of electron density, transmitted throughp-connections, called mesomeric effect (M). The mesomeric effect can also be negative and positive. In structural formulas, it is represented by 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 the electron density in the molecule and the appearance of partial charges on individual atoms. This determines the reactivity of the molecule.

According to the theory of structure organic matter(A. M. Butlerov, 1861) the properties of compounds are determined by the mutual influence of atoms, both connected to each other and not directly connected. Such mutual influence is carried out by successive displacement of electrons forming single and multiple bonds. The electronic effect that causes the displacement of the electrons of a-bonds is called the inductive or inductive effect (/). If the displacement of electrons is associated with multiple TC bonds, then this effect is called mesomeric (M).

Inductive effect

One of the properties of covalent bonds is the mobility of the electron pairs that form these bonds. Some of these bonds are non-polar (eg C-C bonds) or weakly polar (C-H bonds). Therefore, atoms connected by such bonds do not carry a charge. An example of such compounds can be alkanes and, in particular, ethane CH 3 -CH 3 . However, the atoms that form covalent bonds can differ significantly in electronegativity and therefore the electron pairs are shifted towards the more electronegative atom. Such a bond will be polar, and this leads to the formation of partial charges on the atoms. These charges are denoted by the Greek letter "8" (delta). An atom that attracts an electron pair to itself acquires a partial negative charge (-5), and an atom from which electrons are displaced receives a partial positive charge (+8). The displacement of electrons (electron density) of the o-bond is indicated by a straight arrow. For example:

The presence of a polar bond affects the polarity of neighboring bonds. The electrons of neighboring o-bonds are also shifted towards the more electronegative element (substituent).

The displacement of electrons along the a-bond system under the influence of a substituent is called the inductive effect.

The inductive effect is denoted by the letter "/" and tends to fade when transmitted along the chain of a-bonds (it is transmitted at a distance of only 3-4 o-bonds). Therefore, the charges on the atoms gradually decrease during transmission along the chain of bonds (SJ > 8^ > SJ > 8J). The inductive effect can have a "+" or "-" sign. Electron-withdrawing substituents (atoms or a group of atoms) shift the electron density towards themselves and exhibit a negative inductive effect -I(a negative charge appears on the substituent).

Electron-withdrawing substituents that cause a negative inductive effect include:

Electron-donor substituents that shift the electron density away from themselves exhibit a positive inductive effect (+/). These substituents include alkyl radicals, and the larger and more branched the alkyl radical, the more +1.

The inductive effect of the hydrogen atom is assumed to be zero.

The inductive effect of substituents affects the properties of substances and makes it possible to predict them. For example, it is necessary to compare the acidic properties of acetic, formic and chloroacetic acids.

In the chloroacetic acid molecule, there is a negative inductive effect caused by the high electronegativity of the chlorine atom. The presence of a chlorine atom leads to a shift of electron pairs along the a-bond system and, as a result, a positive charge (5+) is created on the oxygen atom of the hydroxyl group. This leads to the fact that oxygen attracts an electron pair from the hydrogen atom more strongly, while the bond becomes even more polar and the ability to dissociate, i.e., acidic properties, increases.

In the acetic acid molecule, the methyl radical (CH 3 -), which has a positive inductive effect, pumps electron density onto the oxygen of the hydroxyl group and creates a partial negative charge (5-) on it. At the same time, oxygen, saturated with electron density, does not attract the electron pair from the hydrogen atom so strongly, the polarity O-N connections decreases and therefore acetic acid splits off a proton (dissociates) worse than formic acid, in which instead of an alkyl radical there is a hydrogen atom, the inductive effect of which is zero. Thus, of the three acids, acetic acid is the weakest, and chloroacetic acid is the strongest.

mesomeric effect

The mesomeric effect is a shift in electron density, carried out with the participation of n-bonds under the influence of substituents.

The mesomeric effect is also called the conjugation effect and is denoted by the letter M. n-electrons of double or triple bonds have high mobility, since they are located farther from the nuclei of atoms than the electrons of o-bonds, and therefore experience less attraction. In this regard, atoms and atomic groups located at a distance of one o-bond from multiple bonds can shift their n-electrons towards their own side (if these atoms have electron-withdrawing properties) or away from themselves (if they have electron-donating properties).

Thus, several conditions must be met for the mesomeric effect to occur. The first, most important condition: a multiple bond must be located one a-bond from the orbital with which it will interact (conjugate) (Fig. 32).

The second important condition for the appearance of the mesomeric effect is the parallelism of the interacting orbitals. In the previous figure, all p-orbitals are parallel to each other, so a conjugation occurs between them. The orbitals are not parallel to each other in the figure.

Rice. 32. The conjugation between the n-bond and the p-orbital, therefore, there is either no interaction between them or it is significantly weakened.

And, finally, the third important condition is the size of the interacting orbitals (in other words, the radii of the atoms entering into conjugation must be the same or close to each other). If the interacting orbitals are very different in size, then there is no complete overlap, and hence no interaction.

The last two conditions are optional, but highly desirable for the appearance of a large mesomeric effect. Recall that the radii of atoms can be compared using the table of D. I. Mendeleev: atoms in the same period have close atomic radii, and those in different periods are very different from each other. Therefore, knowing the orbital of which atom takes part in conjugation, it is possible to determine the strength of the mesomeric effect and, in general, evaluate the electron density distribution in the molecule (Table 34).

Electron donor substituents exhibit a positive mesomeric effect (+M). These substituents contain an atom with an unshared electron pair (-NH 2, -OH

and etc.). The "+" or "-" sign of the mesomeric effect is determined by the charge that appears on the substituent during this effect. For example, in the scheme shown in table 34, the substituents are the groups: -OH, - NH 2, - N0 2, - COOH. As a result of the mesomeric effect, a partial positive (8+) or negative (8-) charge appears on these groups. This is due to the displacement of negatively charged electrons from the substituent in the case of the +M effect or to the substituent in the case of the -M effect. Graphically, the displacement of electrons is indicated by curved arrows. The beginning of the arrow indicates which electrons are displaced during the mesomeric effect, and the end of the arrow indicates to which of the atoms or to which bond. A partial positive charge (+M) appears on the electron-donating groups. For example, on groups -OH and - NH 2 in vinyl alcohol and aniline:

Electron-withdrawing substituents contain several very electronegative atoms that do not contain free electron pairs (-N0 2, -S0 3 H, -COOH, etc.) and therefore they shift electrons towards themselves and acquire a partial negative charge and exhibit a negative mesomeric effect ( -M). We see this in propenoic acid and nitrobenzene:

As noted above, multiple bonds take part in the mesomeric effect, but it is not at all necessary that they interact with some substituents. Multiple, most often double, bonds can conjugate with each other. Most simple example such an interaction is benzene (C 6 H 6). In its molecule, three double bonds alternate with single a-bonds. In this case, all six carbon atoms are in er 2 hybridization and non-hybrid p-orbitals are parallel to each other. Thus, non-hybrid p-orbitals are located next to each other and mutually parallel, all conditions are created for their overlap. For the sake of completeness, let us recall how the p-orbitals overlap in the ethylene molecule during the formation of a r-bond (Fig. 33).

As a result of the interaction of individual p-orbitals, they overlap and merge to form

Rice. 33. Conjugation (mesomeric effect) between parallel p-orbitals of a single mc-electron cloud. Such a merging of orbitals with the formation of a single molecular orbital and there is a mesomeric effect.

A similar picture is also observed in the 1,3-butadiene molecule, in which two n-bonds merge together (come into conjugation) to form a single n-electron cloud (Fig. 34).

The formation of a single electron cloud (mesomeric effect) is an energetically very favorable process. As you know, all molecules tend to the lowest energy, which makes such molecules very stable. When a single molecular cloud is formed, all n-electrons are in the same common orbital (there are four electrons in the butadiene-1,3 molecule in one orbital) and experience the attraction of several nuclei at once (four for butadiene), and this attraction acts on each electron in different directions , which greatly slows down the speed of their movement. Thus, the speed of movement of all electrons in a single molecular orbital decreases, which leads to a decrease in the kinetic, and in general, the total energy of the molecule.

Rice. 34.

In cases where atoms containing double bonds are connected to substituents, the p-orbitals of the double bonds merge with the parallel p-orbitals of the substituents to form a single molecular orbital. We see this in the example of nitrobenzene.

Mesomeric and inductive effects, as a rule, are present simultaneously in the same molecule. Sometimes they coincide in the direction of action, for example in nitrobenzene:

In some cases, these effects act in different directions, and then the electron density in the molecule is distributed taking into account the stronger effect. With a few exceptions, the mesomeric effect is greater than the inductive one:

Electronic effects make it possible to evaluate the distribution of electron density in the molecules of organic substances and make it possible to predict the properties of these compounds.

QUESTIONS AND EXERCISES

• 1. What is an inductive or inductive effect?
• 2. Which of the substituents have a positive and which negative inductive effect: - COOH, -OH, - 0 ", -CH 3, -C \u003d N, -N0 2, -Cl, -NH 2? How is the sign of the inductive effect determined?
• 3. Which of the substances has a large dipole moment: a) CHo-CHp-C1 or CHo-CH 9 -Br; b) CH 3 -CH? -C1 or CH 3 -CH 2 -CH 2 -C1?
• 4. Which of the substances has great acidic properties: CH 3 -COOH or F-CH 2 -COOH? Explain the answer.
• 5. Arrange the substances in ascending order of acidic properties: C1 2 CH - COOH, C1-CH 2 -COOH,

C1 3 C - COOH, CH 3 -COOH. Give explanations.

• 6. What is the mesomeric effect? How is the sign of the mesomeric effect determined?
• 7. Which of the groups have a positive (+M) and negative (-M) mesomeric effect? -S0 3 H, -N0 2, -CHO, -COOH, -NH 2, -N (CH 3) 2, -OH, -o-CH 3.
• 8. In which of the compounds is the mesomeric effect greater: C 6 H 5 -OH and C 6 H 5 -SH? How does this relate to the radius of the atom in the substituent? What is the sign of the mesomeric effect?

• 9. In which compound does the amino group conjugate with an aromatic ring: C 6 H 5 -CH 2 -NH 2 and C 6 H 5 -NH 2?
• 10. Determine the signs of the inductive and mesomeric effects in the phenol molecule (C 6 H 5 -OH). Directions of displacement of electrons are indicated by arrows.

• 1. Which of the substituents exhibits a positive inductive effect:
  • a) - CHO; c) CH 3 -CH 2 -
  • b) -COOH; d) -N0 2 .
• 2. Which of the substituents exhibits a negative inductive effect:
  • a) CH 3 -; c) -S0 3 H;
  • b) CH 3 -CH 2 -; d) -Na.
• 3. Which of the substances has the largest dipole moment:
  • a) CH 3 -C1; c) (CH 3) 3 C-C1;
  • b) CH 3 -CH 2 -CH 2 -C1; d) CH 3 -CH 2 -C1.
• 4. Which of the groups has a positive mesomeric effect:
  • a) -N0 2 ; c) -OH;
  • b) -C=N d) -COOH.
• 5. Which of the compounds has a mesomeric effect:
  • a) C fi H.-CH ? -NH? ; c) CH 3 -CH? -C1;
  • b) C 6 H 5 -OH; d) (CH 3) 3 C-C1.