Chemical properties of alkali and alkaline earth metals. Alkali and alkaline earth metals

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

The electronegativity of a carbon atom depends on the type of hybridization of the atomic orbitals. The contribution of the s-orbital is smaller at sp 3 - and more for sp 2 - and sp hybridization.

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

Atoms related polar link, 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 toward itself acquires a negative charge of D-. In a pair of atoms bound covalent bond, the more electronegative atom is called electron acceptor. His partner in the a-bond has a deficit in electron density - an equal in magnitude partial positive charge 6+; such an atom - electron donor.

The displacement 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 along the circuit with damping. The shift of the electron density of a-bonds is shown by a simple (straight) arrow (- "or *-).

Depending on whether the electron density of a 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 between the electronegativities of a carbon atom and another atom or functional group associated with them, i.e. influencing this carbon atom.

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

Electron donor substitutes, that is, an atom or a group of atoms that cause a shift of the electron density to the carbon atom (away from itself) exhibit positive inductive effect(+/- effect).

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

The inductive effect also manifests itself 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 bp 3 -hybrid state, and the gp 2 -hybrid atom at the double bond acts as an electron acceptor, since it has a higher electronegativity:

When the inductive effect of the methyl group is transferred to the double bond, first of all, its influence is experienced by the mobile

The influence of a substituent on the distribution of electron density transmitted along the 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 is shifted. For example, in a phenol molecule, the hydroxyl group has the + 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 off 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.

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

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

2.1. The electronic structure of the carbon atom

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

In chemistry, the concept of hybrid orbitals of the carbon atom and other elements is widely used. The concept of hybridization as a way of describing the rearrangement of orbitals is necessary when the number of unpaired electrons in the ground state of the 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 the orbitals at its outer electronic level in the ground state 1s 2 2s 2 2p 2 there are only two unpaired electrons (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 their greater overlap, form stronger bonds compared to unhybridized orbitals.

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

Rice. 2.1.Distribution of electrons over the orbitals of a carbon atom in the ground (a), excited (b) and hybridized states (c - sp 3, g- sp 2, d- sp)

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

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

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

Each hybrid orbital is filled with one electron. A carbon atom in the sp 3 -hybridization state has an 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 to the vertices 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 sp 3 -hybridized carbon atom and its two bonds are located in the plane of the drawing and are graphically denoted by the usual bar. A bold line or a bold wedge denotes a bond extending forward from the plane of the drawing and directed towards the observer; a dotted line or a shaded wedge (..........) - a connection leaving the observer for the plane of the drawing

Rice. 2.2.Types of hybridization of the carbon atom. Center point - atomic nucleus

za (Fig. 2.3, a). A carbon atom in a 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 sp 2 -hybrid orbitals and remains unhybridized 2p-AO. A carbon atom in a state sp 2-hybridization has an electronic configuration 1s 2 2 (sp 2) 3 2p 1 (see Fig. 2.1, d). This state of hybridization of a carbon atom is characteristic of unsaturated hydrocarbons (alkenes), as well as for some functional groups, for example, carbonyl and carboxyl.

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

sp-hybridization.When one 2s and one 2p orbitals of an excited carbon atom are mixed, two equivalent sp-hybrid AOs are formed, while two p-AOs remain unhybridized. The sp-hybridized carbon atom 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, e). This state of hybridization of a carbon atom occurs in compounds with a triple bond, for example, in alkynes, nitriles.

sp-hybrid orbitals are located at an angle of 180 °, and two unhybridized AOs are located in mutually perpendicular planes (see Fig. 2.2, c). The sp-hybridized carbon atom 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 can 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.

Covalent is a chemical bond formed as a result of the sharing of electrons of the bonded atoms.

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

2.2.1. σ- andπ -Communication

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

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

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

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

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

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 a π-bond in ethylene by lateral overlap r-AO

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

The 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 during the formation of a bond or required to separate two bound atoms. It serves as a measure of bond strength: the higher the energy, the stronger the bond (Table 2.1).

Link lengthis the distance between the centers of bound atoms. A double bond is shorter than a single bond, and a triple bond is shorter than a double one (see Table 2.1). The bonds between carbon atoms in different hybridization states 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 CH 3 CH 2 CH 3, propene CH 3 CH = CH 2, propyne CH 3 C = CH bond length CH 3 -C is respectively equal to 0.154; 0.150 and 0.146 nm.

Communication polarity due to the uneven distribution (polarization) of the electron density. The polarity of a molecule is quantified by the magnitude of its dipole moment. From the dipole moments of the molecule, one can calculate the dipole moments of individual bonds (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 values of bond energies, the American chemist L. Pauling (1901-1994) proposed a quantitative characterization of the relative electronegativity of atoms (Pauling's scale). In this scale (row), typical organogenic elements are arranged according to relative electronegativity (for comparison, two metals are shown) as follows:

Electronegativity is not an absolute constant of an element. It depends on the effective nuclear charge, the type of AO hybridization, and the effect of substituents. For example, the electronegativity of a carbon atom in the state of sp 2 - or sp-hybridization is higher than in the state of sp 3 -hybridization, which is associated with an increase in the fraction of the s-orbital in the hybrid orbital. At the transition of atoms from sp 3 - to sp 2 - and further to sp-hybridized state gradually decreases the length of the hybrid orbital (especially in the direction providing the greatest overlap during the formation of the σ-bond), which means that in the same sequence the maximum of the 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. With an increase in the difference in electronegativity, the polarity of the bond increases. With a difference of up to 0.4, one speaks of a weakly polar bond, more than 0.5 - a strongly polar covalent bond, and more than 2.0 - an ionic bond. Polar covalent bonds are prone to heterolytic cleavage

(see 3.1.1).

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

2.2.2. Donor-acceptor bonds

Overlapping of two one-electron AOs is not the only way formation of a covalent bond. A covalent bond can be formed when the two-electron orbital of one atom (donor) interacts with the vacant orbital of another atom (acceptor). The donors are compounds containing either orbitals with a lone pair of electrons or π-MO. Carriers of lone pairs of electrons (n-electrons, from the English. non-bonding) are the atoms of nitrogen, oxygen, halogens.

Lonely 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 of one of the bond partners is called donor-acceptor.

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

Donor-acceptor bonds are characteristic of complex compounds.

2.2.3. Hydrogen bonds

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

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

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

Intermolecular hydrogen bonds lead to 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. So, the boiling point of ethanol C 2 H 5 OH (78.3 ° C) is significantly higher than that having the same molecular weight dimethyl ether CH 3 OCH 3 (-24 ° C), not associated due to hydrogen bonds.

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

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

2.3. Coupled systems

The covalent bond can be localized and delocalized. Localized is called a bond, 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 are in most cases π-bonds. They are typical for coupled systems. In these systems, a special type of mutual influence of atoms is realized - conjugation.

Conjugation (mesomerism, from the Greek. mesos- medium) is the alignment of bonds and charges in a real molecule (particle) in comparison with an ideal, but not existing structure.

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

2.3.1. Open loop 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 atoms of carbon and hydrogen and, therefore, all σ-bonds in its molecule lie in the same plane, forming a flat σ-skeleton. Carbon atoms are in the sp 2 -hybridization state. The unhybridized р-AOs of each carbon atom are located perpendicular to the plane of the σ-skeleton and parallel to each other, which is necessary condition to overlap 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, resulting in a single π -system, that is, there is a delocalized covalent bond (see Fig. 2.6, b).

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

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

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

An open interface 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 member of the conjugation chain of three sp 2 -hybridized carbon atoms and an oxygen atom. Each of these atoms contributes one p-electron to the unified π-system.

pn-Conjugation.This type of conjugation is most often manifested 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 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. The conjugated systems with fully aligned bonds and charges include negatively charged particles, for example, the acetate ion.

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

There are other graphical ways to display pairing results. So, 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 the π-electron density (the double-sided arrow connecting these structures is a special symbol of the theory of resonance).

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

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

In the allyl anion CH 2 = CH-CH 2 sp 2 -hybridized carbon atom C-3 supplies the total conjugated

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

system two electrons, in the allyl radical CH 2 = CH-CH 2+ - one, and in the allylic carbocation CH 2 = CH-CH 2+ supplies none. 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, an unpaired electron in the allyl radical, and a negative charge in the allyl anion. In fact, in such conjugated systems, there is a delocalization (dispersal) of the electron density, which leads to the alignment of bonds and charges. Atoms C-1 and C-3 in these systems are equivalent. For example, in an allyl cation, each of them carries a positive charge+1/2 and is linked by a "one and a half" bond with the C-2 atom.

Thus, conjugation leads to a significant difference in the distribution of electron density in real structures compared to 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 in comparison with conjugated open systems. These compounds also have other special properties, the totality of which unite general concept aroma. These include the ability of such formally unsaturated compounds

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

Arenas and their derivatives are typical representatives of aromatic systems. The features 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 in equally can overlap with two adjacent R-AO. As a result of this 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 carbon atoms of the cycle (see Fig. 2.8, b). The π-electron density is evenly distributed throughout the cyclic system, which is indicated by a circle or a dotted line inside the cycle (see Fig. 2.8, c). All bonds between carbon atoms in the benzene ring have the same length (0.139 nm), intermediate between the lengths of single and double bonds.

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

A compound is aromatic if it has a flat cycle and a conjugateπ -electronic system, covering all the 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 that are common to more than

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

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

Stability of coupled systems. The formation of a conjugated and especially an aromatic system is an energetically favorable process, since in this case the degree of overlapping of orbitals increases and delocalization (dispersal) occurs. R-electrons. In this regard, conjugated and aromatic systems have increased thermodynamic stability. They contain a smaller store of internal energy and, in the ground state, occupy a lower energy level in comparison with 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 1,3-butadiene, it is small and amounts to about 15 kJ / mol. With an increase in the length of the conjugated chain, the conjugation energy and, accordingly, the thermodynamic stability of the compounds increase. The conjugation energy for benzene is much higher and amounts to 150 kJ / mol.

2.4. Electronic effects of substituents 2.4.1. Inductive effect

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

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

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

Due to the weak polarizability of the σ-bond, the inductive effect is attenuated through three to four bonds in the circuit. Its effect is most pronounced in relation to the carbon atom adjacent to the one with the substituent. The direction of the inductive effect of a substituent is qualitatively assessed by comparing it with a 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 dash and directed by the tip towards the more electronegative atom.

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

Such substituents generally lower the electron density of the system; they are called electron acceptor. Most of the functional groups belong to them: OH, NH 2, COOH, NO 2 and cationic groups such as -NH 3+.

A substituent that displaces the electron density in comparison with the hydrogen atomσ -bond towards the carbon atom of the chain, exhibitspositiveinductive effect (+/- effect).

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

2.4.2. Mesomeric effect

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

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

In this case, the deputy himself is a member of the coupled system. It can introduce into the conjugation system either a π-bond (carbonyl, carboxyl groups, etc.), or an unshared pair of electrons of a heteroatom (amino and hydroxy groups), or a vacant p-AO filled with one electron.

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

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

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

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

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

The displacement of the electron density is indicated by a curved arrow, the beginning of which shows which p- or π-electrons are displaced, and the end of which 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 assessing the effect of substituents on the distribution of electron density in a molecule, it is necessary to take into account the resulting effect of the inductive and mesomeric effects (Table 2.2).

Table 2.2.Electronic effects of some substituents

The electronic effects of substituents allow one to give a qualitative assessment of the distribution of electron density in a non-reactive molecule and predict its properties.

Atoms and atomic groups in the molecules of organic compounds affect each other, and not only atoms that are 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 electronic effects: inductive and mesomeric effect.

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

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

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

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

In the propene molecule, the carbon of the methyl group is sp 3 -hybridized, and the carbon atoms at the sp 2 double bond are 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 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.

An acrylic aldehyde molecule is
π.π-adjoint system. The oxygen atom gives up one electron to conjugation - the same as each carbon atom, but 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 on the p z -orbital, for example: -NH 2, -OH,
-F, -Cl, -Br-, -I, -OR (-OCH 3, -OC 2 H 5).

The substituents shifting the electron density along the conjugated system towards themselves exhibit a negative mesomeric effect. These include substituents in the structure of which there are double bonds, for example:

The substituent can exhibit both inductive and mesomeric effects at the same time. In some cases, the direction of these effects is the same (for example, -I and -M), in others - they act in opposite directions (for example, -I and + M). How, in these cases, can one determine the general effect of a substituent on the rest of the molecule (in other words, how to 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 donor, and substituents that lower the electron density in the rest of the molecule are called electron acceptor.

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

Consider specific examples:

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

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

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

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

These examples show that it is impossible to consider the influence of any substituent in general, but it is necessary to consider its influence in a specific molecule.

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

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

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

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

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

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

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

TO substitutes 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 have a positive inductive effect and / or a positive mesomeric effect. All of them, except for halogens, increase the electron density in the ring, especially in the ortho and para positions. Therefore, the electrophile is directed to these positions. Let's take a look at phenol as an example:

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

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 at once in three positions:

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 the meta-positions, it is not lowered so strong as in ortho and para. Let's consider this using benzoic acid as an example:

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

LECTURE 2

2.1. Mutual influence of atoms in molecules of bioorganic compounds

2.1.1. Electronic effects of substituents. Inductive and mesomeric effect. Donor and acceptor substituent groups.

2.1.2. Distribution of electron density in bioorganic molecules.

2.2. Acid-base properties of organic compounds.

2.2.1. Bronsted-Lowry theory. Definitions of "acid and base" according to the Bronsted-Lowry theory.

2.2.2. Bioorganic compounds - acids. Influence of the type of acid site and

substituents acidic properties.

2.2.3. Bioorganic compounds - bases. ... Influence of the type of main center and

substituents are basic properties. properties

2.3 Medical biological significance studying the topic "Acid-base properties of bioorganic compounds"

Initial level of knowledge for mastering the topic

Orbital hybridization and spatial orientation of orbitals of period 2 elements, types of chemical bonds, peculiarities of the formation of covalent σ- and π-bonds, polar and non-polar covalent bonds, change in the electronegativity of elements in a period and a group, functional groups, conjugated systems, delocalization.

2.1. Mutual influence of atoms in molecules of bioorganic compounds.

Electronic effects of substituents

Key words for section 2.1.

Substituent donor, acceptor, electronegativity, distribution of electron density in the molecule of bioorganic compounds, inductive effect, mesomeric

The shift in electron density in bioorganic compounds is associated with different electronegativity of atoms. The electron density is always shifted towards the more electronegative atom.

Electronegativity series:

F> O> N> C1> Br> I ~ S> C> H

Functional groups that shift the electron density in their direction are acceptors, and groups that "repulse" the electron density from themselves are donors.

To demonstrate these phenomena, compose electron density distribution diagrams that help you understand the direction organic reaction and explain why they proceed in this way, and not otherwise. Based on the distribution of electron density, it is possible to make an assumption about the reaction mechanism and the structure of the resulting substances.

The shift of the electron density along the σ-bonds is called inductive effect Atoms or functional groups, "repelling" the electron density from themselves, exhibit positive (+ I) effects, while shifting in their direction, negative (- I) effects.

The inductive effect is indicated by an arrow along the bond, which is directed from an atom with a partial positive charge (b +) towards the atom on which an excess negative charge arises (partial charge b-)

The inductive effect extends to neighboring 2-3 atoms in relation to the group causing this effect, and decreases with distance from the group.

Distribution of electron density in a butanoic acid molecule.

CH 3 -> CH 2 -> CH 2 -> CH 2 -> COOH

<------ charge b + decreases in the direction of the arrow

The displacement of the electron density in the system of conjugated bonds is called mesomeric effect (M-effect). The mesomeric effect encompasses the entire acyclic conjugated system, partial charges arise on the extreme atoms in the conjugated system, and in the benzene ring, the change in the electron density occurs at positions 2,4,6 (with respect to the group exhibiting the effect) .

Halogen atoms, hydroxy and amino groups contain lone pairs of electrons, which are displaced towards the π-bond, forming a common conjugated system. They exhibit the + M-effect. Carboxyl, carbonyl, nitro groups have the -M-effect, and they shift the π-electron density in their direction.

Examples: propenoic (acrylic) acid

CH2 == CH- C == O

Chlorvinyl (chloroethene)

C1-CH == CH2

If there is a substituent in the aromatic system of benzene - donor , then there is a partial (excess) charge δ- in positions 2,4,6 Donor groups. Hydroxy, amino groups, halogen atoms of fluorine and chlorine should be considered as exhibiting a positive mesomeric effect.

- if the deputy - acceptor , then the partial charge δ + at positions 2,4,6

Acceptor groups: carboxyl, aldehyde., Nitro, cyano.

The donor exhibits positive + I and + M - effects, and the acceptor - negative - I and - M - effects.

In a conjugated system, the main is the mesomeric M-effect.

The nitrogen atom in the six-membered aromatic heterocyclic compounds pyridine and pyrimidine has a negative mesomeric effect, therefore, the total electron density in the aromatic system decreases (remember the concept of π-insufficient cycles) and, in relation to the nitrogen atom in positions 2, 4, 6 of the cycle, there is a lack of electron density and a partial charge b + appears. In nicotinic acid, the introduction of a carboxyl group into a pyridine molecule increases the lack of electron density. The nitrogen atom and the carboxyl group act "in concert" and create a lack of electron density in the same 2,4,6 positions with respect to them.

An organic compound molecule is a collection of atoms linked in a specific order, usually by covalent bonds. In this case, the bound atoms can differ in magnitude electronegativity... The quantities electronegativities to a large extent determine such important characteristics of the bond 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. This is due to the share s - orbitals in a hybrid orbital: it is smaller for sp 3 - and more for sp 2 - and sp -hybrid atoms.

All atoms constituting a molecule are interconnected and experience mutual influence. This influence is transmitted mainly through a system of covalent bonds, using the so-called electronic effects.

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

Polarly bonded atoms carry partial charges, denoted by the Greek letter delta ( d ). The atom "pulling away" the 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 will accordingly have an equal electron density deficit, i.e. partial positive charge d + will be called electron donor.

Offset of electron density along the circuits -ties called inductive effect and denoted I.

The inductive effect is transmitted along the circuit with damping. The direction of the displacement of the electron density of alls -links 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 causes it.

Electron-withdrawing substituents, i.e. atom or group of atoms shifting electron densitys -bonds from a carbon atom to themselves, exhibit negative inductive effect (- I-effect).

Electro donorsubstituents, i.e. an atom or a group of atoms that shift the electron density to the carbon atom away from itself, exhibit positive inductive effect(+ I-effect).

The I-effect is manifested 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 is also manifested in the case when the bonded carbon atoms differ in the state of hybridization.

When the inductive effect of the metal group is transferred to the double bond, first of all, its influence is experienced by the mobilep - connection.

Effect of a substituent on the electron density distribution transmitted overp -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 is displaced.

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.