What are the reactions in organic chemistry? Mechanisms of organic reactions

Reactions of organic substances can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization). It is obvious that the entire variety of reactions of organic compounds cannot be reduced to the proposed classification (for example, combustion reactions). However, such a classification will help establish analogies with the reactions that occur between inorganic substances that are already familiar to you.

Typically, the main organic compound involved in the reaction is called substrate, and the other reaction component is conventionally considered as reagent.

Substitution reactions

Substitution reactions- these are reactions that result in the replacement of one atom or group of atoms in the original molecule (substrate) with other atoms or groups of atoms.

Substitution reactions involve saturated and aromatic compounds such as alkanes, cycloalkanes or arenes. Let us give examples of such reactions.

Under the influence of light, hydrogen atoms in a methane molecule can be replaced by halogen atoms, for example, by chlorine atoms:

Another example of replacing hydrogen with halogen is the conversion of benzene to bromobenzene:

The equation for this reaction can be written differently:

With this form of writing, the reagents, catalyst, and reaction conditions are written above the arrow, and the inorganic reaction products are written below it.

As a result of reactions substitutions in organic substances are formed not simple and complex substances, as in inorganic chemistry, and two complex substances.

Addition reactions

Addition reactions- these are reactions as a result of which two or more molecules of reacting substances combine into one.

Unsaturated compounds such as alkenes or alkynes undergo addition reactions. Depending on which molecule acts as a reagent, hydrogenation (or reduction), halogenation, hydrohalogenation, hydration and other addition reactions are distinguished. Each of them requires certain conditions.

1.Hydrogenation- reaction of addition of a hydrogen molecule through a multiple bond:

2. Hydrohalogenation- hydrogen halide addition reaction (hydrochlorination):

3. Halogenation- halogen addition reaction:

4.Polymerization- a special type of addition reaction in which molecules of a substance with a small molecular weight combine with each other to form molecules of a substance with a very high molecular weight - macromolecules.

Polymerization reactions are processes of combining many molecules of a low molecular weight substance (monomer) into large molecules (macromolecules) of a polymer.

An example of a polymerization reaction is the production of polyethylene from ethylene (ethene) under the action of ultraviolet radiation and a radical polymerization initiator R.

The covalent bond most characteristic of organic compounds is formed when atomic orbitals overlap and the formation of shared electron pairs. As a result of this, an orbital common to the two atoms is formed, in which a common electron pair is located. When a bond is broken, the fate of these shared electrons can be different.

Types of reactive particles

An orbital with an unpaired electron belonging to one atom can overlap with an orbital of another atom that also contains an unpaired electron. In this case, a covalent bond is formed according to the exchange mechanism:

The exchange mechanism for the formation of a covalent bond is realized if a common electron pair is formed from unpaired electrons belonging to different atoms.

The process opposite to the formation of a covalent bond by the exchange mechanism is the cleavage of the bond, in which one electron is lost to each atom (). As a result of this, two uncharged particles are formed, having unpaired electrons:

Such particles are called free radicals.

Free radicals- atoms or groups of atoms that have unpaired electrons.

Free radical reactions- these are reactions that occur under the influence and with the participation of free radicals.

In the course of inorganic chemistry, these are the reactions of hydrogen with oxygen, halogens, and combustion reactions. Reactions of this type are characterized by high speed and release of large amounts of heat.

A covalent bond can also be formed by a donor-acceptor mechanism. One of the orbitals of an atom (or anion) that has a lone pair of electrons overlaps with the unoccupied orbital of another atom (or cation) that has an unoccupied orbital, and a covalent bond is formed, for example:

The rupture of a covalent bond leads to the formation of positively and negatively charged particles (); since in this case both electrons from a common electron pair remain with one of the atoms, the other atom has an unfilled orbital:

Let's consider the electrolytic dissociation of acids:

It can be easily guessed that a particle having a lone pair of electrons R: -, i.e. a negatively charged ion, will be attracted to positively charged atoms or to atoms on which there is at least a partial or effective positive charge.
Particles with lone pairs of electrons are called nucleophilic agents (nucleus- “nucleus”, a positively charged part of an atom), i.e. “friends” of the nucleus, a positive charge.

Nucleophiles(Nu) - anions or molecules that have a lone pair of electrons that interact with parts of the molecules that have an effective positive charge.

Examples of nucleophiles: Cl - (chloride ion), OH - (hydroxide anion), CH 3 O - (methoxide anion), CH 3 COO - (acetate anion).

Particles that have an unfilled orbital, on the contrary, will tend to fill it and, therefore, will be attracted to parts of the molecules that have an increased electron density, a negative charge, and a lone electron pair. They are electrophiles, “friends” of the electron, negative charge, or particles with increased electron density.

Electrophiles- cations or molecules that have an unfilled electron orbital, tending to fill it with electrons, as this leads to a more favorable electronic configuration of the atom.

Not any particle is an electrophile with an unfilled orbital. For example, alkali metal cations have the configuration of inert gases and do not tend to acquire electrons, since they have a low electron affinity.
From this we can conclude that despite the presence of an unfilled orbital, such particles will not be electrophiles.

Basic reaction mechanisms

Three main types of reacting particles have been identified - free radicals, electrophiles, nucleophiles - and three corresponding types of reaction mechanisms:

• free radical;
• electrophilic;
• zeroophilic.

In addition to classifying reactions according to the type of reacting particles, in organic chemistry four types of reactions are distinguished according to the principle of changing the composition of molecules: addition, substitution, detachment, or elimination (from the English. to eliminate- remove, split off) and rearrangements. Since addition and substitution can occur under the influence of all three types of reactive species, several can be distinguished mainmechanisms of reactions.

In addition, we will consider elimination reactions that occur under the influence of nucleophilic particles - bases.
6. Elimination:

A distinctive feature of alkenes (unsaturated hydrocarbons) is their ability to undergo addition reactions. Most of these reactions proceed by the electrophilic addition mechanism.

Hydrohalogenation (addition of halogen hydrogen):

When a hydrogen halide is added to an alkene hydrogen adds to the more hydrogenated one carbon atom, i.e. the atom at which there are more atoms hydrogen, and halogen - to less hydrogenated.

Classification of reactions According to the number of starting and final substances: 1. Addition 2. Elimination (elimination) 3. Substitution

Classification of reactions According to the mechanism of bond breaking: 1. Homolytic (radical) radicals 2. Heterolytic (ionic) ions

Reaction mechanism Mechanism is a detailed description of a chemical reaction in stages, indicating intermediate products and particles. Reaction scheme: Reaction mechanism:

Classification of reactions by type of reagents 1. Radical A radical is a chemically active particle with an unpaired electron. 2. Electrophilic Electrophile is an electron-deficient particle or molecule with an electron-deficient atom. 3. Nucleophilic Nucleophile is an anion or neutral molecule having an atom with a lone electron pair.

Types of chemical bonds in organic substances The main type of bond is covalent (less common ionic) Sigma bond (σ-): Pi bond (-)

ALKANES - aliphatic (fatty) hydrocarbons “Alifatos” - oil, fat (Greek). Cn. H 2 n+2 Saturated hydrocarbons

Homologous series: CH 4 - methane C 2 H 6 - ethane C 3 H 8 - propane C 4 H 10 - butane C 5 H 12 - pentane, etc. C 6 H 14 - hexane C 7 H 16 - heptane C 8 H 18 - octane C 9 H 20 - nonane C 10 H 22 - decane and C 390 H 782 - nonocontatrictan (1985)

Atomic-orbital model of the methane molecule In the methane molecule, the carbon atom no longer has S- and P-orbitals! Its 4 hybrid SP 3 orbitals, equal in energy and shape, form 4 bonds with the S orbitals of the hydrogen atom. H H 4 bonds

Nitration reaction Konovalov Dmitry Petrovich (1856 -1928) 1880. The first successful attempt to revive the “chemical dead”, which were considered alkanes. I found the conditions for the nitration of alkanes. Rice. Source: http: //images. yandex. ru.

Chemical properties I. Reactions with the rupture of C-H bonds (substitution reactions): 1. halogenation 2. nitration 3. sulfochlorination II. Reactions with breaking of C-C bonds: 1. combustion 2. cracking 3. isomerization

How to find a chemist? If you want to find a chemist, ask what moth and non-ionized are. And if he starts talking about fur-bearing animals and the organization of labor, calmly leave. Science fiction writer, popularizer of science Isaac Asimov (1920–1992) Fig. Source: http: //images. yandex. ru.

1. Halogenation reaction Chlorination: RH + Cl 2 hv RCl + HCl Bromination: RH + Br 2 hv RBr + HBr For example, chlorination of methane: CH 4 + Cl 2 CH 3 Cl + HCl

Stages of the free radical mechanism Reaction scheme: CH 4 + Cl 2 CH 3 Cl + HCl Reaction mechanism: I. Chain initiation - the stage of generation of free radicals. Cl Cl 2 Cl Radical is an active particle, the initiator of a reaction. – – The stage requires energy in the form of heating or lighting. Subsequent stages can take place in the dark, without heating.

Stages of the free radical mechanism II. Chain growth is the main stage. CH 4 + Cl HCl + CH 3 + Cl 2 CH 3 Cl + Cl The stage may include several substages, at each of which a new radical is formed, but not H!!! At stage II, the main stage, the main product is necessarily formed!

Stages of the free radical mechanism III. Chain termination – recombination of radicals. Cl + Cl Cl 2 Cl + CH 3 CH 3 Cl CH 3 + CH 3 CH 3 -CH 3 Any two radicals combine.

Selectivity of substitution Selectivity – selectivity. Regioselectivity is selectivity in a certain area of ​​reactions. For example, halogenation selectivity: 45% 3% Conclusion? 55% 97%

The selectivity of halogenation depends on the following factors: Reaction conditions. At low temperatures it is more selective. Nature of halogen. The more active the halogen, the less selective the reaction. F 2 reacts very vigorously, with the destruction of C-C bonds. I 2 does not react with alkanes under these conditions. Alkane structure.

Influence of alkane structure on substitution selectivity. If the carbon atoms in an alkane are unequal, then the substitution for each of them occurs at a different rate. Relative rate of substitution reaction Primary. H atom Secondary atom H Tert. H atom chlorination 1 3, 9 5, 1 bromination 1 82 1600 Conclusion?

To remove a tertiary hydrogen atom requires less energy than to remove a secondary and primary one! Alkane formula Result of homolysis ED, kJ/mol CH 4 CH 3 + H 435 CH 3 - CH 3 C 2 H 5 + H 410 CH 3 CH 2 CH 3 (CH 3)2 CH + H 395 (CH 3)3 CH (CH 3)3 C + H 377

Direction of reactions Any reaction proceeds predominantly in the direction of the formation of a more stable intermediate particle!

The intermediate particle in radical reactions is a free radical. The most stable radical is formed most easily! Stability series of radicals: R 3 C > R 2 CH > RCH 2 > CH 3 Alkyl groups exhibit an electron-donating effect, due to which they stabilize the radical

Sulfochlorination reaction Reaction scheme: RH + Cl 2 + SO 2 RSO 2 Cl + HCl Reaction mechanism: 1. Cl Cl 2 Cl 2. RH + Cl R + HCl R + SO 2 RSO 2 + Cl 2 RSO 2 Cl + Cl, etc. .d. 3. 2 Cl Cl 2 etc.

Konovalov’s reaction D.P. Nitration according to Konovalov is carried out by the action of dilute nitric acid at a temperature of 140 o. C. Reaction scheme: RH + HNO 3 RNO 2 + H 2 O

Konovalov reaction mechanism HNO 3 N 2 O 4 1. N 2 O 4 2 NO 2 2. RH + NO 2 R + HNO 2 R + HNO 3 RNO 2 + OH RH + OH R + H 2 O, etc. 3 .Circuit break.

Alkenes are unsaturated hydrocarbons with one C=C bond, Cn. H 2 n С=С – functional group of alkenes

Chemical properties of alkenes General characteristics Alkenes are a reactive class of compounds. They undergo numerous reactions, most of which occur by breaking the weaker pi bond. E C-C (σ-) ~ 350 KJ/mol E C=C (-) ~ 260 KJ/mol

Characteristic reactions Addition is the most characteristic type of reaction. The double bond is an electron donor, so it tends to add: E - electrophiles, cations or radicals

Examples of electrophilic addition reactions 1. Addition of halogens – Not all halogens add, but only chlorine and bromine! – Polarization of a neutral halogen molecule can occur under the action of a polar solvent or under the action of the double bond of an alkene. Red-brown bromine solution becomes colorless

Electrophilic addition Reactions occur at room temperature and do not require lighting. The mechanism is ionic. Reaction scheme: XY = Cl 2, Br 2, HCl, HBr, HI, H 2 O

The sigma complex is a carbocation - a particle with a positive charge on the carbon atom. If other anions are present in the reaction medium, they can also join the carbocation.

For example, the addition of bromine dissolved in water. This qualitative reaction to the double C=C bond proceeds with the discoloration of the bromine solution and the formation of two products:

Addition to unsymmetrical alkenes Regioselectivity of addition! Markovnikov's rule (1869): acids and water add to unsymmetrical alkenes in such a way that hydrogen adds to the more hydrogenated carbon atom.

Markovnikov Vladimir Vasilievich (1837 - 1904) Graduate of Kazan University. Since 1869 - professor of the department of chemistry. Founder of a scientific school. Rice. Source: http: //images. yandex. ru.

Explanation of Markovnikov's rule The reaction proceeds through the formation of the most stable intermediate particle - a carbocation. primary secondary, more stable

Stability series of carbocations: tertiary secondary primary methyl Markovnikov's rule in the modern formulation: the addition of a proton to an alkene occurs with the formation of a more stable carbocation.

Anti-Markovnikov addition CF 3 -CH=CH 2 + HBr CF 3 -CH 2 Br Formally, the reaction goes against the Markovnikov rule. CF 3 – electron-withdrawing substituent Other electron-withdrawing agents: NO 2, SO 3 H, COOH, halogens, etc.

Anti-Markovnikov addition more stable unstable CF 3 – electron acceptor, destabilizes the carbocation. The reaction only formally goes against the Markovnikov rule. In fact, it obeys it, since it goes through a more stable carbocation.

Kharash peroxide effect X CH 3 -CH=CH 2 + HBr CH 3 -CH 2 Br X = O 2, H 2 O 2, ROOR Free radical mechanism: 1. H 2 O 2 2 OH + HBr H 2 O + Br 2. CH 3 -CH=CH 2 + Br CH 3 -CH -CH 2 Br more stable radical CH 3 -CH -CH 2 Br + HBr CH 3 -CH 2 Br + Br, etc. 3. Any two radicals combine between yourself.

Electrophilic addition 3. Hydration - addition of water - The reaction occurs in the presence of acid catalysts, most often sulfuric acid. – The reaction obeys Markovnikov’s rule. Cheap way to obtain alcohols

During the exam, Academician Ivan Alekseevich Kablukov asks the student to tell how hydrogen is produced in the laboratory. “From mercury,” he answers. “How do you mean “made of mercury”? ! They usually say “made of zinc,” but made of mercury is something original. Write a reaction." The student writes: Hg = H + g And says: “Mercury is heated; it decomposes into H and g. H is hydrogen, it is light and therefore flies away, but g is the acceleration of gravity, heavy, remains.” “For such an answer you should give an A,” says Kablukov. - Let's get a record book. I’ll just warm up the “five” first too. “Three” flies away, but “two” remains.”

Two chemists in the laboratory: - Vasya, put your hand in this glass. - I dropped it. - Do you feel anything? - No. - So there is sulfuric acid in another glass.

Aromatic hydrocarbons Aromatic – fragrant? ? Aromatic compounds are benzene and substances that resemble it in chemical behavior!

CH 3 -CH 3 + Cl 2 – (hv) ---- CH 3 -CH 2 Cl + HCl

C 6 H 5 CH 3 + Cl 2 --- 500 C --- C 6 H 5 CH 2 Cl + HCl

Addition reactions

Such reactions are typical for organic compounds containing multiple (double or triple) bonds. Reactions of this type include reactions of addition of halogens, hydrogen halides and water to alkenes and alkynes

CH 3 -CH=CH 2 + HCl ---- CH 3 -CH(Cl)-CH 3

Elimination reactions

These are reactions that lead to the formation of multiple bonds. When eliminating hydrogen halides and water, a certain selectivity of the reaction is observed, described by Zaitsev's rule, according to which a hydrogen atom is eliminated from the carbon atom at which there are fewer hydrogen atoms. Example reaction

CH3-CH(Cl)-CH 2 -CH 3 + KOH →CH 3 -CH=CH-CH 3 + HCl

Polymerization and polycondensation

n(CH 2 =CHCl)  (-CH 2 -CHCl)n

Redox

The most intense of the oxidative reactions is combustion, a reaction characteristic of all classes of organic compounds. In this case, depending on the combustion conditions, carbon is oxidized to C (soot), CO or CO 2, and hydrogen is converted into water. However, for organic chemists, oxidation reactions carried out under much milder conditions than combustion are of great interest. Oxidizing agents used: solutions of Br2 in water or Cl2 in CCl 4 ; KMnO 4 in water or dilute acid; copper oxide; freshly precipitated silver(I) or copper(II) hydroxides.

3C 2 H 2 + 8KMnO 4 +4H 2 O→3HOOC-COOH + 8MnO 2 + 8KOH

Esterification (and its reverse hydrolysis reaction)

R 1 COOH + HOR 2 H+  R 1 COOR 2 + H 2 O

Cycloaddition

Y R Y-R

‖ + ‖ → ǀ ǀ

R Y R-Y

‖ + →

11. Classification of organic reactions by mechanism. Examples.

The reaction mechanism involves a detailed step-by-step description of chemical reactions. At the same time, it is established which covalent bonds are broken, in what order and in what way. The formation of new bonds during the reaction process is also carefully described. When considering the reaction mechanism, first of all, pay attention to the method of breaking the covalent bond in the reacting molecule. There are two such ways - homolytic and heterolytic.

Radical reactions proceed by homolytic (radical) cleavage of a covalent bond:

Non-polar or low-polar covalent bonds (C–C, N–N, C–H) undergo radical cleavage at high temperatures or under the influence of light. The carbon in the CH 3 radical has 7 outer electrons (instead of a stable octet shell in CH 4). Radicals are unstable; they tend to capture the missing electron (up to a pair or up to an octet). One of the ways to form stable products is dimerization (the combination of two radicals):

CH 3 + CH 3 CH 3 : CH 3,

N + N N : N.

Radical reactions - these are, for example, reactions of chlorination, bromination and nitration of alkanes:

Ionic reactions occur with heterolytic bond cleavage. In this case, short-lived organic ions - carbocations and carbanions - with a charge on the carbon atom are intermediately formed. In ionic reactions, the bonding electron pair is not separated, but passes entirely to one of the atoms, turning it into an anion:

Strongly polar (H–O, C–O) and easily polarizable (C–Br, C–I) bonds are prone to heterolytic cleavage.

Distinguish nucleophilic reactions (nucleophile– looking for the nucleus, a place with a lack of electrons) and electrophilic reactions (electrophile– looking for electrons). The statement that a particular reaction is nucleophilic or electrophilic always refers to the reagent. Reagent– a substance participating in the reaction with a simpler structure. Substrate– a starting substance with a more complex structure. Outgoing group is a replaceable ion that has been bonded to carbon. Reaction product– new carbon-containing substance (written on the right side of the reaction equation).

TO nucleophilic reagents(nucleophiles) include negatively charged ions, compounds with lone pairs of electrons, compounds with double carbon-carbon bonds. TO electrophilic reagents(electrophiles) include positively charged ions, compounds with unfilled electron shells (AlCl 3, BF 3, FeCl 3), compounds with carbonyl groups, halogens. Electrophiles are any atom, molecule or ion capable of adding a pair of electrons in the process of forming a new bond. The driving force of ionic reactions is the interaction of oppositely charged ions or fragments of different molecules with a partial charge (+ and –).

Types of chemical reactions in inorganic and organic chemistry.

1. A chemical reaction is a process in which other substances are formed from one substance. Depending on the nature of the process, types of chemical reactions are distinguished.

1)According to the final result

2) Based on the release or absorption of heat

3) Based on the reversibility of the reaction

4) Based on changes in the oxidation state of the atoms that make up the reacting substances

According to the final result, reactions are of the following types:

A) Substitution: RH+Cl 2 →RCl+HCl

B) Accession: CH 2 =CH 2 +Cl 2 →CH 2 Cl-CH 2 Cl

B) Elimination: CH 3 -CH 2 OH → CH 2 =CH 2 +H 2 O

D) Decomposition: CH 4 →C+2H 2

D) Isomerization

E) Exchange

G) Connections

Decomposition reaction is a process in which two or more others are formed from one substance.

Exchange reaction is a process in which reacting substances exchange their constituent parts.

Substitution reactions occur with the participation of simple and complex substances, as a result of which new simple and complex substances are formed.

As a result compound reactions from two or more substances one new one is formed.

Based on the release or absorption of heat, reactions are of the following types:

A) Exothermic

B) Endothermic

Exothermic – These are reactions that occur with the release of heat.

Endothermic- These are reactions that occur with the absorption of heat from the environment.

Based on reversibility, reactions are of the following types:

A) Reversible

B) Irreversible

Reactions that proceed in only one direction and end with the complete conversion of the initial reactants into the final substances are called irreversible.

Reversible Reactions that simultaneously occur in two mutually opposite directions are called.

Based on changes in the oxidation state of the atoms that make up the reacting substances, reactions are of the following types:

A) Redox

Reactions that occur with a change in the oxidation state of atoms (in which electrons transfer from one atom, molecule or ion to another) are called redox.

2. According to the mechanism of reaction, reactions are divided into ionic and radical.

Ionic reactions– interaction between ions as a result of heterolytic rupture of a chemical bond (a pair of electrons goes entirely to one of the “fragments”).

Ionic reactions are of two types (based on the type of reagent):

A) electrophilic - during a reaction with an electrophile.

Electrophile– a group that has free orbitals or centers with reduced electron density in some atoms (for example: H +, Cl - or AlCl 3)

B) Nucleophilic - during interaction with a nucleophile

Nucleophile – a negatively charged ion or molecule with a lone electron pair (not currently involved in the formation of a chemical bond).

(Examples: F - , Cl - , RO - , I -).

Real chemical processes can only rarely be described by simple mechanisms. A detailed examination of chemical processes from a molecular kinetic point of view shows that most of them proceed along a radical chain mechanism; the peculiarity of chain reactions is the formation of free radicals at intermediate stages (unstable fragments of molecules or atoms with a short lifetime, all have free communications.

The processes of combustion, explosion, oxidation, photochemical reactions, and biochemical reactions in living organisms proceed through a chain mechanism.

Chain systems have several stages:

1) chain nucleation - the stage of chain reactions, as a result of which free radicals arise from valence-saturated molecules.

2) continuation of the chain - the stage of the circuit chain, proceeding while maintaining the total number of free stages.

3) chain break - the elementary stage of a chain of processes leading to the disappearance of free bonds.

There are branched and unbranched chain reactions.

One of the most important concepts of the chain is chain length- the average number of elementary stages of chain continuation after the appearance of a free radical until its disappearance.

Example: Hydrogen Chloride Synthesis

1) CL 2 absorbs a quantum of energy and the image of radical 2: CL 2 +hv=CL * +CL *

2) the active particle combines with the m-molecule H 2 to form hydrogen chloride and the active particle H 2: CL 1 + H 2 = HCL + H *

3)CL 1 +H 2 =HCL+CL * etc.

6)H * +CL * =HCL - open circuit.

Branched mechanism:

F * +H 2 =HF+H * etc.

F * +H 2 =HF+H * etc.

In water it is more complicated - OH*, O* radicals and the H* radical are formed.

Reactions that occur under the influence of ionizing radiation: X-rays, cathode rays, and so on - are called radiochemical.

As a result of the interaction of molecules with radiation, the disintegration of molecules is observed with the formation of the most reactive particles.

Such reactions promote the recombination of particles and the formation of substances with different combinations of them.

An example is hydrazine N 2 H 4 - a component of rocket fuel. Recently, attempts have been made to obtain hydrazine from ammonia as a result of exposure to γ-rays:

NH 3 → NH 2 * + H*

2NH 2 *→ N 2 H 4

Radiochemical reactions, for example radiolysis of water, are important for the life of organisms.

Literature:

1. Akhmetov, N.S. General and inorganic chemistry / N.S. Akhmetov. – 3rd ed. – M.: Higher School, 2000. – 743 p.

1. Korovin N.V. General chemistry / N.V. Korovin. – M.: Higher School, 2006. – 557 p.
2. Kuzmenko N.E. Short course in chemistry / N.E. Kuzmenko, V.V. Eremin, V.A. Popkov. – M.: Higher School, 2002. – 415 p.
3. Zaitsev, O.S. General chemistry. Structure of substances and chemical reactions / O.S. Zaitsev. – M.: Chemistry, 1990.
4. Karapetyants, M.Kh. Structure of matter / M.Kh. Karapetyants, S.I. Drakin. – M.: Higher School, 1981.
5. Cotton F. Fundamentals of inorganic chemistry / F. Cotton, J. Wilkinson. – M.: Mir, 1981.
6. Ugay, Ya.A. General and inorganic chemistry / Ya.A.Ugai. – M.: Higher School, 1997.

Annex 1
REACTION MECHANISMS IN ORGANIC CHEMISTRY
N.V.Sviridenkova, NUST MISIS, Moscow
WHY STUDY THE MECHANISMS OF CHEMICAL REACTIONS?
What is the mechanism of a chemical reaction? To answer this question, consider the equation for the combustion reaction of butene:

C 4 H 8 + 6O 2 = 4CO 2 + 4H 2 O.

If the reaction actually proceeded as described in the equation, then one molecule of butene would have to collide with six molecules of oxygen at once. However, this is unlikely to happen: it is known that the simultaneous collision of more than three particles is almost impossible. The conclusion suggests itself that this reaction, like the vast majority of chemical reactions, occurs in several successive stages. The reaction equation shows only the starting materials and the final result of all transformations, and does not explain in any way how products are formed from initial substances. In order to find out exactly how a reaction proceeds, what stages it includes, what intermediate products are formed, it is necessary to consider the reaction mechanism.

So, reaction mechanism is a detailed description of the course of a reaction in stages, which shows in what order and how chemical bonds in the reacting molecules are broken and new bonds and molecules are formed.

Consideration of the mechanism makes it possible to explain why some reactions are accompanied by the formation of several products, while in other reactions only one substance is formed. Knowing the mechanism allows chemists to predict the products of chemical reactions before they are actually carried out. Finally, knowing the reaction mechanism, you can control the course of the reaction: create conditions to increase its speed and increase the yield of the desired product.
BASIC CONCEPTS: ELECTROPHILE, NUCLEOPHILE, CARBOCATION
In organic chemistry, reagents are traditionally divided into three types: nucleophilic, electrophilic And radical. You have already encountered radicals earlier when studying the halogenation reactions of alkanes. Let's take a closer look at other types of reagents.

Nucleophilic reagents or simply nucleophiles(translated from Greek as “nucleus lovers”) are particles with excess electron density, most often negatively charged or having a lone electron pair. Nucleophiles attack molecules with low electron density or positively charged reagents. Examples of nucleophiles are OH - , Br - ions, NH 3 molecules.

Electrophilic reagents or electrophiles(translated from Greek as “electron lovers”) are particles with a lack of electron density. Electrophiles often carry a positive charge. Electrophiles attack molecules with high electron density or negatively charged reagents. Examples of electrophiles are H +, NO 2 +.

An atom of a polar molecule that carries a partial positive charge can also act as an electrophile. An example is the hydrogen atom in the HBr molecule, on which a partial positive charge arises due to the displacement of the common electron bond pair to the bromine atom, which has a higher electronegativity value H δ + → Br δ - .

Reactions proceeding through the ionic mechanism are often accompanied by the formation of carbocations. Carbocation called a charged particle that has a free R-orbital on the carbon atom. One of the carbon atoms in the carbocation carries a positive charge. Examples of carbocations include particles CH 3 -CH 2 +, CH 3 -CH + -CH 3. Carbocations are formed at one of the stages in the reactions of addition of halogens to alkenes and hydrogen halides to alkenes, as well as in substitution reactions involving aromatic hydrocarbons.
MECHANISM OF ADDITION TO UNSATURED HYDROCARBONS

The addition of halogens, hydrogen halides, and water to unsaturated hydrocarbons (alkenes, alkynes, diene hydrocarbons) occurs through ionic mechanism, called electrophilic addition.

Let us consider this mechanism using the example of the reaction of addition of hydrogen bromide to an ethylene molecule.

Despite the fact that the hydrobromination reaction is described by a very simple equation, its mechanism includes several stages.

Stage 1. In the first stage, a hydrogen halide molecule forms with π -electron cloud of double bond unstable system – “ π -complex” due to partial transfer π -electron density per hydrogen atom carrying a partial positive charge.

Stage 2. The hydrogen-halogen bond is broken to form an electrophilic H + particle and a nucleophilic Br - particle. The released electrophile H+ adds to the alkene due to the electron pair of the double bond, forming σ -complex – carbocation.

Stage 3. At this stage, a negatively charged nucleophile is added to the positively charged carbocation to form the final reaction product.

WHY DOES MARKOVNIKOV'S RULE FOLLOW?
The proposed mechanism explains well the formation of predominantly one of the products in the case of addition of hydrogen halides to unsymmetrical alkenes. Let us recall that the addition of hydrogen halides obeys Markovnikov’s rule, according to which hydrogen is added at the double bond to the most hydrogenated carbon atom (i.e., connected to the largest number of hydrogen atoms), and halogen to the least hydrogenated one. For example, when hydrogen bromide is added to propene, 2-bromopropane is predominantly formed:

In electrophilic addition reactions to unsymmetrical alkenes, two carbocations can be formed in the second stage of the reaction. Next, it reacts with a nucleophile, which means that the more stable of them will determine the reaction product.

Let us consider which carbocations are formed in the case of propene and compare their stability. The addition of an H+ proton at the site of a double bond can lead to the formation of two carbocations, secondary and primary:

The resulting particles are very unstable because the positively charged carbon atom in the carbocation has an unstable electronic configuration. Such particles are stabilized by distributing (delocalizing) the charge over as many atoms as possible. Electron donor alkyl groups, supplying electron density to the electron-deficient carbon atom, promote and stabilize carbocations. Let's look at how this happens.

Due to the difference in electronegativity of the carbon and hydrogen atoms, a certain excess of electron density appears on the carbon atom of the -CH 3 group, and some deficiency appears on the hydrogen atom, C δ- H 3 δ+. The presence of such a group next to a carbon atom bearing a positive charge inevitably causes a shift in the electron density towards the positive charge. Thus, the methyl group acts as a donor, giving away part of its electron density. Such a group is said to have positive inductive effect (+ I -effect). The more such electron donor (+ I ) - the substituents are surrounded by a carbon bearing a positive charge, the more stable the corresponding carbocation is. Thus, the stability of carbocations increases in the series:

In the case of propene, the most stable is the secondary carbocation, since in it the positively charged carbon atom of the carbocation is stabilized by two + I - effects of neighboring methyl groups. It is predominantly formed and reacts further. The unstable primary carbocation apparently exists for a very short time, so that during its “life” it does not have time to attach a nucleophile and form a reaction product.

When the bromide ion is added to the secondary carbocation at the last stage, 2-bromopropane is formed:

DOES MARKOVNIKOV'S RULE ALWAYS FOLLOW?

Consideration of the mechanism of the propylene hydrobromination reaction allows us to formulate a general rule for electrophilic addition: “when unsymmetrical alkenes interact with electrophilic reagents, the reaction proceeds through the formation of the most stable carbocation.” The same rule makes it possible to explain the formation in some cases of addition products contrary to Markovnikov’s rule. Thus, the addition of hydrogen halides to trifluoropropylene formally proceeds against Markovnikov’s rule:

How can such a product be obtained, since it was formed as a result of the addition of Br to the primary, and not to the secondary, carbocation? The contradiction is easily resolved by considering the reaction mechanism and comparing the stability of the intermediate particles formed:

The -CF 3 group contains three electron-withdrawing fluorine atoms, which draw electron density from the carbon atom. Therefore, a significant lack of electron density appears on the carbon atom. To compensate for the resulting partial positive charge, the carbon atom absorbs the electron density of neighboring carbon atoms. Thus, the -CF 3 group is electron-withdrawing and shows negative inductive effect (- I ) . In this case, the primary carbocation turns out to be more stable, since the destabilizing effect of the -CF 3 group through two σ bonds is weakened. And the secondary carbocation, destabilized by the neighboring electron-withdrawing group CF 3, is practically not formed.

The presence of electron-withdrawing groups –NO2, -COOH, -COH, etc. at the double bond has a similar effect on addition. In this case, the addition product is also formed formally against the Markovnikov rule. For example, when hydrogen chloride is added to propenoic (acrylic) acid, 3-chloropropanoic acid is predominantly formed:

Thus, the direction of addition to unsaturated hydrocarbons can be easily determined by analyzing the structure of the hydrocarbon. This can be briefly reflected in the following diagram:

It should be noted that Markovnikov's rule is satisfied only if the reaction proceeds by the ionic mechanism. When carrying out radical reactions, Markovnikov's rule is not satisfied. Thus, the addition of hydrogen bromide HBr in the presence of peroxides (H 2 O 2 or organic peroxides) proceeds against Markovnikov’s rule:

The addition of peroxides changes the reaction mechanism; it becomes radical. This example shows how important it is to know the reaction mechanism and the conditions under which it occurs. Then, by choosing the appropriate conditions for the reaction, you can direct it according to the mechanism required in this particular case, and obtain exactly the products that are needed.
MECHANISM OF HYDROGEN ATOMS REPLACEMENT IN AROMATIC HYDROCARBONS
The presence in the benzene molecule of a stable conjugate π -electronic system makes addition reactions almost impossible. For benzene and its derivatives, the most typical reactions are substitution of hydrogen atoms, which occur while maintaining aromaticity. In this case, the benzene ring containing π- electrons interact with electrophilic particles. Such reactions are called electrophilic substitution reactions in the aromatic series. These include, for example, halogenation, nitration and alkylation of benzene and its derivatives.

All electrophilic substitution reactions in aromatic hydrocarbons follow the same path ionic mechanism regardless of the nature of the reagent. The mechanism of substitution reactions includes several stages: the formation of an electrophilic agent E +, the formation π -complex, then σ- complex and, finally, disintegration σ- complex to form a substitution product.

An electrophilic E+ particle is formed when a reagent interacts with a catalyst, for example, when a halogen molecule is exposed to aluminum chloride. The resulting E+ particle interacts with the aromatic ring, first forming π -, and then σ- complex:

During education σ- complex, the electrophilic particle E + attaches to one of the carbon atoms of the benzene ring through σ- communications. In the resulting carbocation, the positive charge is evenly distributed (delocalized) between the remaining five carbon atoms.

The reaction ends with the removal of a proton from σ- complex. In this case, two electrons σ -CH bonds return to the cycle, and a stable six-electron aromatic π - the system is regenerated.

In a benzene molecule, all six carbon atoms are equal. The replacement of a hydrogen atom can occur with equal probability for any of them. How will substitution occur in the case of benzene homologues? Let's take methylbenzene (toluene) as an example.

It is known from experimental data that electrophilic substitution in the case of toluene always occurs with the formation of two products. Thus, the nitration of toluene occurs with the formation P-nitrotoluene and O-nitrotoluene:

Other electrophilic substitution reactions (bromination, alkylation) proceed similarly. It was also found that in the case of toluene, substitution reactions proceed faster and under milder conditions than in the case of benzene.

It is very simple to explain these facts. The methyl group is electron-donating and, as a result, further increases the electron density of the benzene ring. A particularly strong increase in electron density occurs in O- And P- positions relative to the -CH 3 group, which facilitates the attachment of a positively charged electrophilic particle to these sites. Therefore, the rate of the substitution reaction generally increases, and the substituent is directed predominantly to ortho- And pair- provisions.