Mutual influence of atoms in molecules of bioorganic compounds. electronic effects of substituents

The material "Electronic effects in molecules of organic compounds" is intended to help teachers working in grades 10-11. The material contains a theoretical and practical part on the topic “The theory of the structure of organic compounds by N.M. Butlerov, the mutual influence of atoms in molecules.” You can use the presentation on this topic.

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Electronic effects in molecules of organic compounds

The replacement of hydrogen atoms in alkane molecules with any heteroatom (halogen, nitrogen, sulfur, oxygen, etc.) or group causes a redistribution of electron density. The nature of this phenomenon is different. It depends on the properties of the heteroatom (its electronegativity) and on the type of bonds along which this influence spreads.

Inductive effect

If the influence of the substituent is transmitted with the participation of -bonds, then a gradual change in the electronic state of the bonds occurs. This polarization is calledinductive effect (I), is depicted by an arrow in the direction of the electron density shift:

CH 3 -CH 2 Cl,

HOCH 2 -CH 2 Cl,

CH 3 -CH 2 COOH,

CH 3 -CH 2 NO 2, etc.

The inductive effect is due to the desire of an atom or group of atoms to supply or withdraw electron density, and therefore it can be positive or negative. A negative inductive effect is exhibited by elements that are more electronegative than carbon, i.e. halogens, oxygen, nitrogen and others, as well as groups with a positive charge on the element associated with carbon. The negative inductive effect decreases from right to left in a period and from top to bottom in a group of the periodic system:

F > O > N,

F > Cl > Br > J.

In the case of fully charged substituents, the negative inductive effect increases with increasing electronegativity of the atom bonded to the carbon:

>O + - >> N +

In the case of complex substituents, the negative inductive effect is determined by the nature of the atoms that make up the substituent. In addition, the inductive effect depends on the nature of the hybridization of atoms. Thus, the electronegativity of carbon atoms depends on the hybridization of electron orbitals and changes in the following direction:

Elements that are less electronegative than carbon exhibit a positive inductive effect; groups with a complete negative charge; alkyl groups. The +I-effect decreases in the series:

(CH 3 ) 3 C- > (CH 3 ) 2 CH- > CH 3 -CH 2 - > CH 3 - > H-.

The inductive effect of the substituent quickly decays as the chain length increases.

Table 1. Summary table of substituents and their electronic effects

Mesomeric effect

The presence of a substituent with a free pair of electrons or a vacant p-orbital attached to a system containing p-electrons leads to the possibility of mixing the p-orbitals of the substituent (occupied or vacant) with p-orbitals and a redistribution of electron density in compounds. This effect is called mesomeric.

The shift in electron density is usually insignificant and bond lengths remain virtually unchanged. A slight shift in the electron density is judged by the dipole moments, which are small even in the case of large conjugation effects on the outer atoms of the conjugated system.

The mesomeric effect is depicted by a curved arrow directed towards the shift in electron density:

Depending on the direction of displacement of the electron cloud, the mesomeric effect can be positive (+M):

and negative (-M):

The positive mesomeric effect (+M) decreases with an increase in the electronegativity of the atom carrying a lone pair of electrons, due to a decrease in the tendency to donate it, as well as with an increase in the volume of the atom. The positive mesomeric effect of halogens changes in the following direction:

F > Cl > Br > J (+M effect).

Groups with lone pairs of electrons on the atom attached to the conjugate have a positive mesomeric effect. pi system:

NH 2 (NHR, NR 2 ) > OH (OR) > X (halogen)(+M-effect).

The positive mesomeric effect decreases if the atom is bonded to an electron acceptor group:

NH 2 > -NH-CO-CH 3 .

The negative mesomeric effect increases with increasing electronegativity of the atom and reaches maximum values ​​if the acceptor atom carries a charge:

>C=O + H >> >C=O.

A decrease in the negative mesomeric effect is observed if the acceptor group is conjugated with a donor group:

CO-O- 2 (–M-effect).

Table 2. Summary table of substituents and their electronic effects

Hyperconjugation or superconjugation

An effect similar to positive mesomeric occurs when hydrogen at a multiple bond is replaced by an alkyl group. This effect is directed towards the multiple bond and is called hyperconjugation (superconjugation):

The effect resembles a positive mesomeric one, since it donates electrons to the conjugated -system:

Superconjugation decreases in the sequence:

CH 3 > CH 3 -CH 2 > (CH 3 ) 2 CH > (CH 3 ) 3 C.

For the effect of hyperconjugation to manifest itself, it is necessary to have at least one hydrogen atom at the carbon atom adjacent to the - system. The tert-butyl group does not exhibit this effect, and therefore its mesomeric effect is zero.

Table 3. Summary table of substituents and their electronic effects

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Solving problems on the reactivity of organic substances.

Exercise 1 . Arrange the substances in order of increasing acid activity: water, ethyl alcohol, phenol.

Solution

Acidity is the ability of a substance to produce an H ion upon dissociation.+ .

C 2 H 5 OH C 2 H 5 O – + H + , H 2 O H + + OH – (or 2H 2 O H 3 O + + OH – ),

C 6 H 5 OH C 6 H 5 O – + H + .

The stronger acidic character of phenols compared to water is explained by the influence of the benzene ring. The lone pair of electrons of the oxygen atom enters into conjugation with-electrons of the benzene ring. As a result, the electron density of the oxygen atom moves partially to the oxygen–carbon bond (while increasing the electron density in the ortho and para positions in the benzene ring). The electron pair of the oxygen–hydrogen bond is more strongly attracted to the oxygen atom.

This creates a greater positive charge on the hydrogen atom of the hydroxyl group, which promotes the removal of this hydrogen in the form of a proton.

When alcohol dissociates, the situation is different. The oxygen–hydrogen bond is affected by a positive mesomeric effect (injection of electron density) from CH 3 -groups. Therefore, it is more difficult to break the O–H bond in alcohol than in a water molecule, and therefore phenol.

These substances are ranked in order of acidity:

C 2 H 5 OH 2 O 6 H 5 OH.

Task 2. Arrange the following substances in order of increasing rate of reaction with bromine: ethylene, chloroethylene, propylene, butene-1, butene-2.

Solution

All of these substances have a double bond and will react with bromine. But depending on where the double bond is located and which substituents affect the electron density shift, the reaction rate will be different. Let's consider all these substances as derivatives of ethylene:

Chlorine has a negative inductive effect - it draws electron density from the double bond and therefore reduces its reactivity.

Three substances have alkyl substituents that have a positive inductive effect, and therefore have greater reactivity than ethylene. The positive effect of ethyl and two methyl groups is greater than one methyl group, therefore, the reactivity of butene-2 ​​and butene-1 is greater than propene.

Butene-2 ​​is a symmetrical molecule, and the C–C double bond is nonpolar. In 1-butene the bond is polarized, so overall the compound is more reactive.

These substances, in order of increasing reaction rate with bromine, are arranged in the following row:

chloroethene

Task 3. Which acid will be stronger: chloroacetic acid, trichloroacetic acid or trifluoroacetic acid?

Solution

The strength of the acid is stronger, the easier the separation of H occurs.+ :

CH 2 ClCOOH CF 3 COO – + H + .

All three acids differ in that they have different numbers of substituents. Chlorine is a substituent that exhibits a fairly strong negative inductive effect (it pulls electron density towards itself), which helps to weaken the O–H bond. Three chlorine atoms further demonstrate this effect. This means that trichloroacetic acid is stronger than chloroacetic acid. In the series of electronegativity, fluorine occupies the most extreme place; it is an even greater electron acceptor, and the O–H bond is further weakened compared to trichloroacetic acid. Therefore, trifluoroacetic acid is stronger than trichloroacetic acid.

These substances are arranged in the following order in order of increasing acid strength:

CH2ClCOOH 3 COOH 3 COOH.

Task 4. Arrange the following substances in order of increasing basicity: aniline, methylamine, dimethylamine, ammonia, diphenylamine.

Solution

The main properties of these compounds are associated with the lone electron pair on the nitrogen atom. If in a substance the electron density is pumped onto this electron pair, then this substance will be a stronger base than ammonia (let’s take its activity as one); if the electron density in the substance is pulled away, then the substance will be a weaker base than ammonia.

The methyl radical has a positive inductive effect (increases electron density), which means that methylamine is a stronger base than ammonia, and the substance dimethylamine is an even stronger base than methylamine.

The benzene ring, through the conjugation effect, pulls electron density onto itself (negative induction effect), therefore aniline is a weaker base than ammonia, diphenylamine is an even weaker base than aniline.

These substances are arranged in order of basicity:

Task 5. Write dehydration schemes n-butyl, sec-butyl and tert -butyl alcohols in the presence of sulfuric acid. Arrange these alcohols in order of increasing rate of dehydration. Give an explanation.

The rate of many reactions is affected by the stability of intermediate compounds. In these reactions, the intermediate substances are carbocations, and the more stable they are, the faster the reaction proceeds.

The tertiary carbocation is the most stable. These alcohols can be classified according to the rate of dehydration reaction into the following series:

The mutual influence of atoms in molecules is the most important property of organic compounds, distinguishing them from simple inorganic compounds. Mutual influence, as a result of the interaction of neighboring atoms, in organic molecules is transmitted along the chain of a-C-C bonds and especially successfully along the chain of conjugated C-C bonds and determines the selectivity of the reaction centers in the molecule to certain reagents. It was already mentioned earlier that reagents are divided into electron-donating (nucleophilic) and electron-accepting (electrophilic). It should also be added that they can be electron neutral when the electron donor and electron acceptor properties are compensated or absent altogether. In addition, one should distinguish between free radical (R* or R*), molecular and ionic reagents. The classification and properties of the reagents will be discussed in detail later.

The mutual influence of atoms follows from the classical theory of the structure of A.M. Butlerov and was first formulated by his student from the Kazan School of Chemists V.V. Markovnikov in his doctoral dissertation “Materials on the question of the mutual influence of atoms in chemical compounds” (1869).

For the first time, the mutual influence of atoms was discovered in the molecules of alkenes and halogen derivatives of alkanes. Markovnikov found that alkenes with an asymmetric electron shell, for example CH-CH=CH,

^C=CH2, /C=CH-CH3, add hydrogen bromide like this

in such a way that HBr donates its hydrogen atom to the most hydrogenated (with the maximum number of H-atoms) carbon:

The reaction practically does not proceed along the second route

because in 2-methylpropene it appears induction electron effect methyl (alkyl) groups in relation to 4 / l - orbitals. The induction effect (/-effect) is caused by the polarization of the α-electron cloud in the chain of chemical bonds of atoms having different electronegativity.

Under polarization of the molecule understand the redistribution of electron density under the influence of electrostatic forces, during which a partial separation of the “centers of gravity” of positive and negative charges occurs. At the same time, low-polar particles (having low electric dipole moments) are more prone to polarization. Moreover, not only atoms, but also atomic groups have electronegativity. Alkyl groups have relative to a,t-orbitals, i.e. atoms

C located at 5/? 2-hybrid state, electron-donating ability. As a result of this electronic influence of alkyl groups, the bonding 4^-orbital is polarized towards the second carbon atom.

As a result, fractional charges C arise, =^-=- C 2 Such an effect

going with the transfer of a-electron density to other atoms is called the +/- effect, since a small fractional charge 6+ appears on the CH 3 groups. Therefore, the polar covalent H-Br molecule is oriented in such a way with respect to the r-bond that in the elementary event a short-lived transition state first appears

which first turns into the carbocation (CH 3) 2 C-CH 3 and Br~, and then they quickly and easily recombine into (CH 3) 2 C(Br)-CH 3.

One can also consider the issue from the point of view of the stability of the carbocation formed in the elementary act (the most energetically favorable delocalization of the positive charge in it). This is a more simplified approach, although the essence again comes down to the influence of the ±/-electronic effect as the reason for the stability of the tertiary carbocation compared to the primary one.

In addition to methyl and other alkyl groups, the +/- effect, i.e. the ability to lose a small part of their electron density, is possessed by metal atoms of groups I - III of the periodic system (Li, Ca, A1, etc.), as well as hydride groups (-SiH 3, -PH 2, -BH 2, -A1H 2), their alkyl derivatives (-SiR 3, -PR 2, -BR 2, -A1R 2, etc.).

Derivatives of nitrogen, oxygen and halogens, which have a higher electronegativity than C atoms, cause polarization, i.e., a shift in the electron density of a-electrons towards their orbitals and have a negative inductive (-/) effect. For example, chloroform СНС1 3, in contrast to methane, has a fairly high proton-donor ability, since the c-orbital of the Н-СС1 3 bond is strongly polarized towards chlorine atoms. Similarly, the polarizing effect of three atoms

chlorine in CC1 3 COOH is transferred to the O-H bond of trichloroacetic acid, as a result the O-H bond is so strongly polarized that CC1 3 COOH, unlike weak acetic acid CH 3 COOH, becomes a relatively strong acid. In these examples, both short-range (SHC1 3) and long-range mutual influence of more electropositive or electronegative atoms and their groups on other atoms connected to them using a- or p-orbitals occurs. It has been established that the ±/-effect is transmitted more weakly along the chain of c-bonds, practically dying out at the fifth or sixth carbon atom, while the conjugated l-bond freely transmits the ±/-effect to the final atom of the conjugated chain.

The mechanism of easy transmission in l-systems ±/-effect has not been studied. It can be assumed that it is due to the delocalization of n-electrons, their high mobility throughout the conjugated chain of n-electrons, resulting in a (5+)- or (b-)-charge on the carbon atom associated with the contact atom of the functional group, which induces this fractional charge is “quenched” due to the displacement of the generalized π-electron cloud onto this atom, and the maximum (5±) charge appears on the final atom of the π-system. This maximum separation of (8+)- and (8-)-charges is energetically beneficial, since it allows electrons to use maximum space.

The mutual influence of atoms in elimination reactions (from English, elimination) of simple stable molecules is peculiar, as the addition of elements of functional groups located at neighboring carbon atoms:

The elimination of HBr (HC1, HI) according to the rule established by A. M. Zaitsev, a student of A. M. Butlerov, occurs opposite to the Markovnikov addition rule, i.e., the H atom is removed together with Br from the least hydrogenated carbon atom. H20 is similarly split off from alcohols and hydroxides of quaternary amines. This is due to the fact that the bonding a-orbitals of secondary C-H bonds are much less stable (located higher in energy) than the c-orbitals of primary C-H bonds. Even less

The tertiary - “C”H orbitals are stable, which donate hydrogen to the leaving partner (Br, OH) with maximum ease. The energies of C-H bonds are: primary 414, secondary 393 and tertiary 376 kJ/mol. As can be seen, the stability of the binding 4^“n MOs varies very significantly.

This is due to the greater electronegativity of the carbon atom (according to Pauling 2.5) than that of hydrogen (2.1). The tertiary carbon atom spends three of its bonds on more electronegative neighbors, the secondary carbon atom spends on two, while the primary C-atom spends a minimum of chemical affinity on the carbon bond and a maximum on the Z-atom.

Another important type of mutual influence of atoms is displacement P- electron density to a more electronegative atom or group of atoms. If in a chain of bonded atoms, in addition to a-electrons, there are also d-electrons, then the introduction of a more electron-donor or electron-acceptor atom into the conjugated π-system of such atoms will cause polarization of the d-system. For example, 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene) and 2-chloro-1,3-butadiene (chloroprene):

In butadiene, there is a mutual influence of a-bonded carbon atoms due to the presence of p g electrons on them (l symmetry). As follows from the above, such mutual influence of unsaturated atoms is called l, l-conjugation, or l-conjugation. The introduction of an a-electron-donor CH 3 group into such a conjugated system polarizes the a-system and leads to an increase in the bond orders in the conjugated chain, while the polarization of the a,l-system of butadiene with the introduction of a strong electron acceptor (C1 atom), which simultaneously has the ability to enter into tg-conjugation, partially transferring its electron pair to the orbital, leads to

ultimately to a decrease in bond orders in the conjugated n-system. It follows that the -/- effect of the chlorine atom in relation to the diene conjugated l-system is much greater than its +C-effect (positive conjugation effect). Polarization of conjugated n-systems by electron-donating (ED) or electron-withdrawing (EA) substituents also changes the ISV and leads to the appearance of small fractional charges on the atoms. The largest negative (-CH 3) or positive (-C1) charge will be located on the C 4 butadiene chain.

From the examples given, we can conclude that in organic molecules there is mutual influence of chemically bonded atoms of several types. This influence is expressed in the form of various electronic effects (electronic influences, actions, displacements), the action of electric fields of closely located atoms on each other (field effect, ±F- effect) and spatial obstacles (steric effects, ± ?-effect), which arise in chemical reactions as a result of shielding of the reaction centers of molecules by atoms.

It is customary to distinguish between the induction effect (±/-effect) and the coupling effect (±C-effect). Induction effects, denoted by signs (+) or (-) depending on the a-electron-donor or acceptor ability, are clearly manifested in the acid-base properties of molecules. Thus, all haloacetic acids are stronger than acetic acid, since halogens polarize

all st-bonds along the G-CH^- chain, causing a decrease in bond order

O-H and increasing its polarity, and consequently, the ability to remove H +. Polarization of the O-H, N-H, S-H bond is called its protonization and leads to an increase in acid dissociation (increase K L- acid dissociation constants). All substituents - electronic acceptors - act similarly. All electron donors that replace the H-atom in the methyl group of acetic acid act oppositely, reducing K L. The action of electronactive substituents (donors and acceptors, i.e. functional

F groups, on the basic centers (-B) of molecules of a simple type (B-R-F) is the opposite of the effect on acidic centers (-O-H, etc.). The effect of ED and EA substituents on nucleophilic substitution reactions is similar, when one electron-donating functional group (F") is replaced by another

In this case, the cleavage of the bond of the hydrocarbon residue (R) with the contact atom of the functional group (Ф") occurs due to a decrease in the order of the Ф^-R bond and its polarization as a result of the attack of Ф" by its electron pair:

The leaving group F" can be any groups of a basic nature, such as SG, Br", G, NCS", NOj, HSO, etc., and the entering group can be those already listed (F"), as well as such as NH 2, SH", SR", RCOO", HSOj, HPO^ and the like, i.e. stronger nucleophiles.

Any substituents can act as an accompanying group (F), covalently bonded to the hydrocarbon residue R of a non-aromatic nature. Usually these are H-atoms, CH 3 and other alkyls, C 6 Hs- and residues of any other arenes. More complex cases of substitution have been little studied and not developed theoretically.

In molecules and other molecular particles (ions, free radicals) containing more than three p-orbitals in a chain, conjugation effects occur. They are caused, as was indicated in the example of substituted butadiene, by the interaction of two or more similar or different n-systems (CH 2 =CH-, CH=C- groups) or r-systems (for example, CH 2 =CH-, QH5-, etc.) with chemically active atoms or groups

atoms that have orbitals: 1) p z - or d K -; 2) filled with one electron or a pair of electrons; 3) free from electrons. Examples of such

atoms or their groups can be -CH 2, -CH 2, -CH 2, -CH-, -CH, ?

CH -, numerous heteroatoms (non-metals and metals) and their groups, for example, =0, -0-, “OH, -NH 2, -NH-, -N=, -SH, -S-, etc.

Examples PC- and n-conjugate systems have already been given. Among the most common and important, we will name the following:

In the allyl radical, as a result of the conjugation of three n-electrons, the spin density created by the unpaired electron is distributed over all three carbon atoms, predominant on the outermost atoms (-0.622) and amounting to +0.231 on the middle carbon atom. These figures, reflecting the current, far from perfect state of the theory of quantum chemical calculations of complex molecules, show that calculations cannot give absolute values ​​of quantities. The given figures in themselves are meaningless, if not absurd, since it is natural that the spin density created in a conjugated system by a single unpaired electron should not be interrupted by any atom and in total should be equal to unity. From the given figures it follows that the electron is split into two parts that do not interact, since there is no electron density from the unpaired electron on the average carbon atom. But, despite the imperfection of the modern calculation apparatus of complex molecules, the given figures show that the sum of spin densities (2-0.622) - 0.231 is close to 1 and that it is energetically more favorable for an electron to occupy the maximum space with the maximum distribution of electron spin density on the outermost atoms of the conjugated system.

It is even more difficult to comprehend the results of calculating the spin density distribution in the benzyl radical

From the above diagram it can be seen that the electron density deficit in the calculation reached 0.2 units. electron charge.

Due to this, the radical is significantly stabilized. In the benzyl cation, the vacant p-orbital interacts with the HOMO, i.e. H*, - orbital with the pumping of a- and p-electron densities from the benzene nucleus, especially from ortho- and other provisions. In this case, 5" + 38 = 1. As a result of this mutual influence of a-, p-orbitals, the reactivity of the benzene ring with electrophilic reagents decreases greatly, and with nucleophilic reagents, on the contrary, it increases greatly. Delocalization of the (+) charge r g

orbitals noticeably stabilizes the carbocation C 6 H 5 CH 2 .

In the triphenylmethyl anion (C 6 H 5) 3 C', which has a flat structure and a bright (cherry-red) color as a result of the a-conjugation of the filled r - orbital of the methyl carbon atom with LUMO (i.e. Ch** e) benzene nuclei,

the negative charge is delocalized along ortho- and lard positions of benzene rings. Due to this mutual influence of atoms, the carbanion is greatly stabilized, and benzene nuclei acquire the ability to interact very easily even with weak electron acceptors (1 2 , pyridine, C 6 H 5 N0 2, etc.).

Vinyl chloride is an example of mt-conjugation between a nonbonding orbital (p C|, carrying a pair of p electrons, and a 71 bond. As a result, the C-C 1 bond becomes noticeably stronger (by 12 kJ/mol) and slightly shorter. The halogen atom becomes low-reactive in substitution reactions with alkali. Below is the energy diagram of the binding MOs of vinyl chloride (Fig. 2.21). The energy gain of mt-conjugation occurs in

Rice. 2.21. Vinyl chloride MO couplings (ll-conjugation)

as a result of an increase in the so-called region of residence of mt electrons due to their delocalization. For n-electrons CH 2 =CH-, the weak reservoir of delocalization can be the 3^-orbital of chlorine. A mt-conjugation situation similar to vinyl chloride occurs in the molecule of chlorobenzene QHs-Cl, aniline SbH5-HH2, phenol QHs-OH:

where X is a heteroatom.

The third type of mutual influence of atoms in molecules, which is called the field effect (F-effect), deserves serious attention. The field effect concept is less developed than the I- and C-effects.

Currently, the field effect is understood as the influence of the static electric charge of ionized atoms that make up a molecule, or large fractional charges of dipoles of polarized ad bonds on neighboring or nearby atoms of the same molecular particle.

An example would be molecules with a semipolar bond, for example + +

CH 3 - S- CH 3 (dimethyl sulfoxide), (C 2 H 5) 3 N-O (triethylamine oxide),

molecules with complete separation of charges due to strong intramolecular

acid-base interactions, such as NH-CH 2 - COO (glycine) and all other amino acids and complexons, highly polar molecules n -l/2

Cools, for example, C 6 H-_ 1/2 (nitrobenzene) or ions (C 2 H 5) 4 N (tetraethyl-

ammonium), C 2 H 5 -N=N (benzenediazonium), (C 2 H 5) 2 OH (diethylhydroxonium),

ions of complex compounds - cations and anions, including [Co(UN 2 CH 3) 6 ] 3+ (cobalt hexamethyl laminate), *~ (ferricia-

nide), Fe(C 5 H 5) 2 (ferricinium cation), (nickel dimethylglyoximine).

The number of known compounds of this structure is infinitely large. Under the influence of the electric field of point charge carriers, neighboring chemical bonds are polarized, as a result of which the chemical properties of bonded atoms and atomic groups located in close proximity to the charge source change. In this case, the F-effect has a much greater long-range action than the covalent /-effect. The influence of electric charge extends over a distance of up to 3 nm.

In a number of complex molecules containing functional groups in a conjugated system that are prone to ionization (acid-base dissociation) in solutions of polar solvents capable of donor-acceptor interactions, the charge is delocalized throughout the entire molecule or its individual parts.

An example of a simple particle of this kind is the dimethylglyoxymine anion

In these cases, a field effect occurs, but the mechanism of its action is complex and is not considered here.

Mutual influence is similarly manifested in both organic and inorganic complex compounds. In the latter, the mutual influence of atomic and molecular orbitals of the valence shell is called electronic coordination effects. These effects are due to the influence on coordinated ligands of such types of coordination chemical bonds as o-connection, dative k-connection, reverse dative p-connection, as well as action cation fields. They received the name in Russian literature c-coordination effect, % coordination effect, inverse p-coordination effect And charge effect.

As a result of these effects, during the formation of a complex compound (M + L ML) from a metal ion (M - charge omitted) and a ligand (L - charge omitted), the properties of the ligands L in the coordinated state can differ greatly from free L:

This fact is widely used in the catalysis of industrial and laboratory reactions. In these cases, due to the introduction of β-metal salts into the reaction sphere, complex compounds are formed and such catalysis is called “complex catalysis”.

One of the fundamental concepts of organic chemistry is the mutual influence of atoms in molecules. Without knowledge of electronic effects (inductive and mesomeric), organic chemistry appears to be a set of factual material, often unrelated to each other. It has to be learned and memorized. Mastery of the elements of the theory of mutual influence of atoms allows you to:

Systematize knowledge;

Connect the structure of a substance with its properties;

Predict the reactivity of molecules;

Correctly determine the main directions of chemical reactions;

Consciously perceive the interaction of substances with each other.

In addition, the application of the concepts of mutual influence of atoms in the process of studying the properties of organic substances creates great opportunities for enhancing the cognitive activity of students and developing intellectual skills.

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In an organic compound, the atoms are joined in a specific 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 (energy of formation), which means that the reactivity of a 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 atomic orbitals. The contribution of the s-orbital is less at sp 3 - and more at sp 2 - and sp hybridization.

All atoms in a molecule mutually influence 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 α-bond toward itself acquires a negative charge of J-. In a pair of atoms connected by a covalent bond, the more electronegative atom is called electron acceptor. Its a-bond partner has a deficiency of electron density - an equal partial positive charge of 6+; such an atom - electron donor.

The shift of electron density along a 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 attenuation. 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 electronegativity 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 group of atoms that shifts the electron density of the a-bond from the carbon atom to itself exhibits negative inductive effect(-/-Effect).

Electron-donating substituents i.e., an atom or group of atoms that causes a shift in electron density towards the 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 a -/- effect: halogens, amino group, hydroxyl, carbonyl, carboxyl groups.

The inductive effect also appears in carbon-carbon bonds 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 p 3 -hybrid state, and the §p 2 -hybrid atom at the double bond acts as an electron acceptor, since it has a higher electronegativity:

When the inductive effect of a methyl group is transferred to a double bond, its influence is primarily experienced 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 electron density from the benzene ring towards itself.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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