Classification of chemical reactions

Abstract on chemistry by Alexey Nikolaev, 11th grade student of secondary school No. 653

The following classification criteria can be selected:

1. The number and composition of starting materials and reaction products.

2. Physical state of reagents and reaction products.

3. The number of phases in which the reaction participants are located.

4. The nature of the transferred particles.

5. Possibility of reaction occurring in forward and reverse directions.

6. Thermal effect.

7. The phenomenon of catalysis.

Classification according to the number and composition of starting substances and reaction products.

Compound reactions.

When a compound reacts from several reacting substances of relatively simple composition, one substance of a more complex composition is obtained:

A+B+C=D

As a rule, these reactions are accompanied by the release of heat, i.e. lead to the formation of more stable and less energy-rich compounds.

Inorganic chemistry.

Reactions of compounds of simple substances are always redox in nature. Compound reactions occurring between complex substances can occur without a change in valency:

CaCO 3 + CO 2 + H 2 O = Ca(HCO 3) 2,

and also be classified as redox:

2FeCl 2 + Cl 2 = 2FeCl 3.

Organic chemistry.

In organic chemistry, such reactions are often called addition reactions. They usually involve compounds containing a double or triple bond. Types of addition reactions: hydrogenation, hydration, hydrohalogenation, polymerization. Examples of these reactions:

T o

H 2 C = CH 2 + H 2 → CH 3 – CH 3

ethylene ethane

T o

HC=CH + HCl → H 2 C=CHCl

acetylene vinyl chloride

T o

n CH 2 =CH 2 → (-CH 2 -CH 2 -)n

Ethylene polyethylene

Decomposition reactions.

Decomposition reactions lead to the formation of several compounds from one complex substance:

A = B + C + D.

The decomposition products of a complex substance can be both simple and complex substances.

Inorganic chemistry.

Of the decomposition reactions that occur without changing the valence states, noteworthy is the decomposition of crystalline hydrates, bases, acids and salts of oxygen-containing acids:

2AgNO3 = 2Ag + 2NO2 + O2,

(NH 4) 2 Cr 2 O 7 = Cr 2 O 3 + N 2 + 4H 2 O.

Organic chemistry.

In organic chemistry, decomposition reactions include: dehydration, dehydrogenation, cracking, dehydrohalogenation, as well as depolymerization reactions, when the original monomer is formed from the polymer. The corresponding reaction equations are:

T o

C 2 H 5 OH → C 2 H 4 + H 2 O

T o

C 6 H 14 → C 6 H 6 + 4 H 2

hexane benzene

C 8 H 18 → C 4 H 10 + C 4 H 8

Octane butane butene

C 2 H5Br → C 2 H 4 + HBr

bromoethane ethylene

(-CH 2 – CH = C - CH 2 -)n → n CH 2 = CH – C = CH 2

\СНз \СНз

natural rubber 2-methylbutadiene-1,3

Substitution reactions.

In substitution reactions, usually a simple substance reacts with a complex one, forming another simple substance and another complex one:

A + BC = AB + C.

Inorganic chemistry.

These reactions overwhelmingly belong to redox reactions:

2Al + Fe 2 O 3 = 2Fe + Al 2 O 3

Zn + 2HCl = ZnСl 2 + H 2

2KBr + Cl 2 = 2KCl + Br 2

2 KS lO 3 + l 2 = 2KlO 3 + C l 2.

Examples of substitution reactions that are not accompanied by a change in the valence states of atoms are extremely few. It should be noted the reaction of silicon dioxide with salts of oxygen-containing acids, which correspond to gaseous or volatile anhydrides:

CaCO 3 + SiO 2 = CaSiO 3 + CO 2

Ca 3 (PO 4) 2 + 3SiO 2 = 3СаSiO 3 + P 2 O 5

Organic chemistry.

In organic chemistry, substitution reactions are understood more broadly, that is, not one atom, but a group of atoms can be replaced, or not an atom, but a group of atoms can be replaced. A type of substitution reaction includes nitration and halogenation of saturated hydrocarbons, aromatic compounds and alcohols:

C 6 H 6 + Br 2 → C 6 H 5 Br + HBr

benzene bromobenzene

C 2 H 5 OH + HCl → C 2 H 5 Cl + H 2 O

Ethanol chloroethane

Exchange reactions.

Exchange reactionsare reactions between two compounds that exchange their constituents with each other:

AB + CD = AD + CB.

Inorganic chemistry

If redox processes occur during substitution reactions, then exchange reactions always occur without changing the valence state of the atoms. This is the most common group of reactions between complex substances - oxides, bases, acids and salts:

ZnO + H 2 SO 4 = ZnSO 4 + H 2 O

AgNO 3 + KBr = AgBr + KNO 3

CrCl 3 + ZNaON = Cr(OH) 3 + ZNaCl.

A special case of these exchange reactions is the neutralization reaction:

HCl + KOH = KCl + H 2 O.

Typically, these reactions obey the laws of chemical equilibrium and proceed in the direction where at least one of the substances is removed from the reaction sphere in the form of a gaseous, volatile substance, precipitate or low-dissociating (for solutions) compound:

NaHCO 3 + HCl = NaCl + H 2 O + CO 2

Ca(HCO 3) 2 + Ca(OH) 2 = 2CaCO 3 ↓ + 2H 2 O

Organic chemistry

HCOOH + NaOH → HCOONa + H 2 O

formic acid sodium formate

hydrolysis reactions:

Na 2 CO3 + H 2 O
NaHCO3 + NaOH

sodium carbonate sodium bicarbonate

CO 3 + H 2 O
HCO 3 + OH

esterification reactions:

CH 3 COOH + C 2 H 5 OH
CH 3 COOC 2 H 5 + H 2 O

acetic ethanol ethyl acetic acid

Physical state of reagents and reaction products.

Gas reactions

Reactions in solutions

NaOH(pp) + HCl(p-p) = NaСl(p-p) + H 2 O(l)

Reactions between solids

The number of phases in which the reaction participants are located.

A phase is understood as a set of homogeneous parts of a system with the same physical and chemical properties and separated from each other by an interface.

Homogeneous (single-phase) reactions.

These include reactions occurring in the gas phase and a number of reactions occurring in solutions.

Heterogeneous (multiphase) reactions.

These include reactions in which the reactants and reaction products are in different phases. For example:

gas-liquid-phase reactions

CO 2 (g) + NaOH(p-p) = NaHCO 3 (p-p).

gas-solid-phase reactions

CO 2 (g) + CaO (tv) = CaCO 3 (tv).

liquid-solid-phase reactions

Na 2 SO 4 (pp) + BaCl 3 (pp) = BaSO 4 (tv)↓ + 2NaCl (p-p).

liquid-gas-solid-phase reactions

Ca(HCO 3) 2 (pp) + H 2 SO 4 (pp) = CO 2 (r) + H 2 O (l) + CaSO 4 (tv)↓.

The nature of the transferred particles.

Protolytic reactions.

Protolytic reactions include chemical processes, the essence of which is the transfer of a proton from one reacting substance to another.

This classification is based on the protolytic theory of acids and bases, according to which an acid is any substance that donates a proton, and a base is a substance that can accept a proton, for example:

Protolytic reactions include neutralization and hydrolysis reactions.

Redox reactions.

All chemical reactions are divided into those in which the oxidation states do not change (for example, an exchange reaction) and those in which the oxidation states change. They are called redox reactions. They can be decomposition reactions, compounds, substitutions and other more complex reactions. For example:

Zn + 2 H + → Zn 2 + + H 2

FeS 2 + 8HNO 3 (conc. ) = Fe(NO 3) 3 + 5NO + 2H 2 SO 4 + 2H 2 O

The vast majority of chemical reactions are redox reactions; they play an extremely important role.

Ligand exchange reactions.

These include reactions during which the transfer of an electron pair occurs with the formation of a covalent bond via a donor-acceptor mechanism. For example:

Cu(NO 3) 2 + 4NH 3 = (NO 3) 2

Fe + 5CO =

Al(OH) 3 + NaOH =

A characteristic feature of ligand exchange reactions is that the formation of new compounds, called complexes, occurs without changing the oxidation state.

Possibility of reaction occurring in forward and reverse directions.

Irreversible reactions.

Irreversible These are chemical processes whose products are not able to react with each other to form the starting substances. Examples of irreversible reactions include the decomposition of Berthollet salt when heated:

2КlО 3 → 2Кl + ЗО 2,

or oxidation of glucose by atmospheric oxygen:

C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O

Reversible reactions.

Reversible These are chemical processes whose products are capable of reacting with each other under the same conditions in which they were obtained to form the starting substances.

For reversible reactions, the equation is usually written as follows:

A + B
AB.

Two oppositely directed arrows indicate that, under the same conditions, both forward and reverse reactions occur simultaneously, for example:

CH 3 COOH + C 2 H 5 OH
CH 3 SOOS 2 H 5 + H 2 O.

2SO 2 +O 2
2SO 3 + Q

Consequently, these reactions do not go to completion, because two reactions occur simultaneously - direct (between the starting substances) and reverse (decomposition of the reaction product).

Classification by thermal effect.

The amount of heat that is released or absorbed as a result of a reaction is called the thermal effect of this reaction. According to the thermal effect, reactions are divided:

Exothermic.

Leaks with heat release

CH 4 + 2O 2 → CO 2 + 2H 2 O + Q

H 2 + Cl 2 → 2HC l + Q

Endothermic.

Occurs with heat absorption

N 2 + O 2 → 2NO-Q

2H 2 O → 2H 2 + O 2 - Q

Classification taking into account the phenomenon of catalysis.

Catalytic.

These include all processes involving catalysts.

Cat.

2SO2 + O2
2SO 3

Non-catalytic.

These include any instantaneous reactions in solutions

BaCl 2 + H 2 SO 4 = 2HCl + BaSO 4 ↓

Bibliography

Internet resources:

http://chem.km.ru – “World of Chemistry”

http://chemi. org. ru – “Handbook for applicants. Chemistry"

http://hemi. wallst. ru – “Alternative textbook on chemistry for grades 8-11”

"Guide to Chemistry. For those entering universities" - E.T. Oganesyan, M. 1991

Big Encyclopedic Dictionary. Chemistry" - M. 1998

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 –).

Most often, organic reactions are classified according to the type of breaking of chemical bonds in the reacting particles. From among them, two large groups of reactions can be distinguished - radical and ionic./>

Radical reactions- these are processes that occur with the hemolytic rupture of a covalent bond. During hemolytic cleavage, the pair of electrons forming the bond is divided in such a way that each of the resulting particles receives one electron. As a result of hemolytic rupture, free radicals are formed:

X :Y→ X . +.Y

A neutral atom or particle with an unpaired electron is called free radical.

Ionic reactions are processes that occur with heterolytic the breaking of covalent bonds, when both electrons of the bond remain with one of the previously bonded particles./>

X:Y → X + + :Y —

As a result heterolytic When a bond is broken, charged particles are obtained: nucleophilic and electrophilic.

Nucleophilic particle (nucleophile) is a particle that has a pair of electrons in the outer electron level. Due to a pair of electrons, a nucleophile is able to form a new covalent bond./>

Electrophilic particle (electrophile) is a particle that has a free orbital at the outer electronic level. An electrophile represents unfilled, vacant orbitals for the formation of a covalent bond due to the electrons of the particle with which it interacts./>

A particle with a positive charge on a carbon atom is called a carbocation.

According to another classification, organic reactions are divided into thermal, which are the result of collisions of molecules during their thermal motion, and photochemical, in which molecules, absorbing a light quantum Av, move to higher energy states and then undergo chemical transformations. For the same starting compounds, thermal and photochemical reactions usually lead to different products. A classic example here is the thermal and photochemical chlorination of benzene - in the first case, chlorobenzene is formed, in the second case, hexachlorocyclohexane.

In addition, in organic chemistry, reactions are often classified in the same way as in inorganic chemistry - by structural feature. In organic chemistry, all structural changes are considered relative to the carbon atom (or atoms) involved in the reaction. The most common types of transformations are:

1) addition R-CH=CH 2 + XY/>→ RCHX-CH 2 Y;

2) substitution R-CH 2 X + Y/>→ R-CH 2 Y + X;

3) elimination of R-CHX-CH 2 Y/>→ R-CH=CH 2 + XY;

(elimination)

4) polymerization n (CH 2 =CH 2) />→ (-CH 2 -CH 2 -) n

In most cases, the eliminated molecule is formed by the combination of two particles split off from neighboring carbon atoms. This process is called 1,2-elimination.

In addition to the above four types of simple mechanisms and reactions, in practice the following designations for some classes of reactions are used, given below.

Oxidation is a reaction in which, under the influence of an oxidizing reagent, a substance combines with oxygen (or another electronegative element, such as a halogen) or loses hydrogen (in the form of water or molecular hydrogen)./>

The action of an oxidizing reagent (oxidation) is indicated in the reaction scheme by the symbol [O], and the action of a reducing reagent (reduction) by the symbol [H].

Hydrogenation is a reaction that is a special case of reduction. Hydrogen is added to the multiple bond or aromatic ring in the presence of a catalyst. />

Condensation is a reaction in which chain growth occurs. Addition occurs first, usually followed by elimination.

Pyrolysis is a reaction in which a compound undergoes thermal decomposition without access to air (and usually under reduced pressure) to form one or more products. An example of pyrolysis is the thermal decomposition of coal. Sometimes, instead of pyrolysis, the term “dry distillation” is used (in the case of the decomposition of coal, the term “carbonization” is also used)./>

Some reactions get their names from the products they lead to. So, if a methyl group is introduced into a molecule, then we talk about methylation, if acetyl, then acetylation, if chlorine, then about chlorination, etc.

When chemical reactions occur, some bonds break and others form. Chemical reactions are conventionally divided into organic and inorganic. Organic reactions are considered to be reactions in which at least one of the reactants is an organic compound that changes its molecular structure during the reaction. The difference between organic reactions and inorganic ones is that, as a rule, molecules are involved in them. The rate of such reactions is low, and the product yield is usually only 50-80%. To increase the reaction rate, catalysts are used and the temperature or pressure is increased. Next, we will consider the types of chemical reactions in organic chemistry.

Classification by the nature of chemical transformations

• Substitution reactions
• Addition reactions
• Isomerization reaction and rearrangement
• Oxidation reactions
• Decomposition reactions

Substitution reactions

During substitution reactions, one atom or group of atoms in the initial molecule is replaced by other atoms or groups of atoms, forming a new molecule. As a rule, such reactions are characteristic of saturated and aromatic hydrocarbons, for example:

Addition reactions

When addition reactions occur, one molecule of a new compound is formed from two or more molecules of substances. Such reactions are typical for unsaturated compounds. There are reactions of hydrogenation (reduction), halogenation, hydrohalogenation, hydration, polymerization, etc.:

1. Hydrogenation– addition of a hydrogen molecule:

Elimination reaction

As a result of elimination reactions, organic molecules lose atoms or groups of atoms, and a new substance is formed containing one or more multiple bonds. Elimination reactions include reactions dehydrogenation, dehydration, dehydrohalogenation and so on.:

Isomerization reactions and rearrangement

During such reactions, intramolecular rearrangement occurs, i.e. the transition of atoms or groups of atoms from one part of the molecule to another without changing the molecular formula of the substance participating in the reaction, for example:

Oxidation reactions

As a result of exposure to an oxidizing reagent, the oxidation state of carbon in an organic atom, molecule or ion increases due to the loss of electrons, resulting in the formation of a new compound:

Condensation and polycondensation reactions

Consists in the interaction of several (two or more) organic compounds with the formation of new C-C bonds and a low molecular weight compound:

Polycondensation is the formation of a polymer molecule from monomers containing functional groups with the release of a low molecular weight compound. Unlike polymerization reactions, which result in the formation of a polymer having a composition similar to the monomer, as a result of polycondensation reactions, the composition of the resulting polymer differs from its monomer:

Decomposition reactions

This is the process of breaking down a complex organic compound into less complex or simple substances:

C 18 H 38 → C 9 H 18 + C 9 H 20

Classification of chemical reactions by mechanisms

Reactions involving the rupture of covalent bonds in organic compounds are possible by two mechanisms (i.e., a path leading to the rupture of an old bond and the formation of a new one) – heterolytic (ionic) and homolytic (radical).

Heterolytic (ionic) mechanism

In reactions proceeding according to the heterolytic mechanism, intermediate particles of the ionic type with a charged carbon atom are formed. Particles carrying a positive charge are called carbocations, and negative ones are called carbanions. In this case, it is not the breaking of the common electron pair that occurs, but its transition to one of the atoms, with the formation of an ion:

Strongly polar, for example H–O, C–O, and easily polarizable, for example C–Br, C–I bonds exhibit a tendency to heterolytic cleavage.

Reactions proceeding according to the heterolytic mechanism are divided into nucleophilic and electrophilic reactions. A reagent that has an electron pair to form a bond is called nucleophilic or electron-donating. For example, HO - , RO - , Cl - , RCOO - , CN - , R - , NH 2 , H 2 O , NH 3 , C 2 H 5 OH , alkenes, arenes.

A reagent that has an unfilled electron shell and is capable of attaching a pair of electrons in the process of forming a new bond. The following cations are called electrophilic reagents: H +, R 3 C +, AlCl 3, ZnCl 2, SO 3, BF 3, R-Cl, R 2 C=O

Nucleophilic substitution reactions

Characteristic for alkyl and aryl halides:

Nucleophilic addition reactions

Electrophilic substitution reactions

Electrophilic addition reactions

Homolytic (radical mechanism)

In reactions proceeding according to the homolytic (radical) mechanism, at the first stage the covalent bond is broken with the formation of radicals. The resulting free radical then acts as an attacking reagent. Bond cleavage by a radical mechanism is typical for non-polar or low-polar covalent bonds (C–C, N–N, C–H).

Distinguish between radical substitution and radical addition reactions

Radical displacement reactions

Characteristic of alkanes

Radical addition reactions

Characteristic of alkenes and alkynes

Thus, we examined the main types of chemical reactions in organic chemistry

The division of chemical reactions into organic and inorganic is rather arbitrary. Typical organic reactions are those that involve at least one organic compound that changes its molecular structure during the reaction. Therefore, reactions in which a molecule of an organic compound acts as a solvent or ligand are not typical organic reactions.

Organic reactions, like inorganic ones, can be classified according to general characteristics into transfer reactions:

– single electron (redox);

– electron pairs (complexation reactions);

– proton (acid-base reactions);

– atomic groups without changing the number of bonds (substitution and rearrangement reactions);

– atomic groups with a change in the number of bonds (reactions of addition, elimination, decomposition).

At the same time, the diversity and originality of organic reactions leads to the need to classify them according to other criteria:

– change in the number of particles during the reaction;

– the nature of the severance of ties;

– electronic nature of the reagents;

– the mechanism of elementary stages;

– activation type;

– private characteristics;

– molecularity of reactions.

1) Based on the change in the number of particles during the reaction (or according to the type of transformation of the substrate), reactions of substitution, addition, elimination (elimination), decomposition and rearrangement are distinguished.

In the case of substitution reactions, one atom (or group of atoms) in the substrate molecule is replaced by another atom (or group of atoms), resulting in the formation of a new compound:

CH 3 – CH 3 + C1 2  CH 3 – CH 2 C1 + HC1

ethane chloroethane chloride hydrogen chloride

CH 3 – CH 2 С1 + NaOH (aqueous solution)  CH 3 – CH 2 OH + NaC1

chloroethane sodium hydroxide ethanol sodium chloride

In the symbol of the mechanism, substitution reactions are designated by the Latin letter S (from the English “substitution” - substitution).

When addition reactions occur, one new substance is formed from two (or several) molecules. In this case, the reagent is added via a multiple bond (C = S, S  S, S = Oh, S  N) substrate molecules:

CH 2 = CH 2 + HBr → CH 2 Br – CH 3

ethylene hydrogen bromide bromoethane

Taking into account the symbolism of the mechanism of processes, addition reactions are designated by the letter A or the combination Ad (from the English “addition” - accession).

As a result of the elimination reaction (cleavage), a molecule (or particle) is split off from the substrate and a new organic substance containing a multiple bond is formed:

CH 3 – CH 2 OH CH 2 = CH 2 + H 2 O

ethanol ethylene water

In the symbol of the mechanism, substitution reactions are designated by the letter E (from the English “elimination” - elimination, splitting off).

Decomposition reactions proceed, as a rule, with the rupture of carbon-carbon bonds (C – C) and lead to the formation from one organic substance of two or more substances of a simpler structure:

CH 3 – CH(OH) – UNS
CH 3 – CHO + HCOOH

lactic acid acetaldehyde formic acid

Rearrangement is a reaction during which the structure of the substrate changes to form a product that is isomeric to the original, that is, without changing the molecular formula. This type of transformation is denoted by the Latin letter R (from the English “rearrangement” - rearrangement).

For example, 1-chloropropane rearranges into the isomeric compound 2-chloropropane in the presence of aluminum chloride as a catalyst.

CH 3 – CH 2 – CH 2 – С1  CH 3 – SNS1 – CH 3

1-chloropropane 2-chloropropane

2) Based on the nature of bond cleavage, homolytic (radical), heterolytic (ionic) and synchronous reactions are distinguished.

A covalent bond between atoms can be broken in such a way that the electron pair of the bond is divided between two atoms, the resulting particles gain one electron each and become free radicals - they say that homolytic cleavage occurs. A new bond is formed due to the electrons of the reagent and the substrate.

Radical reactions are especially common in the transformations of alkanes (chlorination, nitration, etc.).

With the heterolytic method of breaking a bond, a common electron pair is transferred to one of the atoms, the resulting particles become ions, have an integer electric charge and obey the laws of electrostatic attraction and repulsion.

Heterolytic reactions, based on the electronic nature of the reagents, are divided into electrophilic (for example, addition to multiple bonds in alkenes or hydrogen substitution in aromatic compounds) and nucleophilic (for example, hydrolysis of halogen derivatives or the interaction of alcohols with hydrogen halides).

Whether the reaction mechanism is radical or ionic can be determined by studying the experimental conditions that favor the reaction.

Thus, radical reactions accompanied by homolytic cleavage of the bond:

– accelerated by irradiation h, under conditions of high reaction temperatures in the presence of substances that easily decompose with the formation of free radicals (for example, peroxide);

– slow down in the presence of substances that easily react with free radicals (hydroquinone, diphenylamine);

– usually take place in non-polar solvents or the gas phase;

– are often autocatalytic and characterized by the presence of an induction period.

Ionic reactions accompanied by heterolytic bond cleavage:

– are accelerated in the presence of acids or bases and are not affected by light or free radicals;

– not affected by free radical scavengers;

– the speed and direction of the reaction is influenced by the nature of the solvent;

– rarely occur in the gas phase.

Synchronous reactions occur without the intermediate formation of ions and radicals: the breaking of old bonds and the formation of new bonds occur synchronously (simultaneously). An example of a synchronous reaction is yene synthesis – Diels-Alder reaction.

Please note that the special arrow used to indicate the homolytic cleavage of a covalent bond means the movement of one electron.

3) Depending on the electronic nature of the reagents, reactions are divided into nucleophilic, electrophilic and free radical.

Free radicals are electrically neutral particles with unpaired electrons, for example: Cl ,  NO 2,
.

In the reaction mechanism symbol, radical reactions are denoted by the subscript R.

Nucleophilic reagents are mono- or polyatomic anions or electrically neutral molecules having centers with an increased partial negative charge. These include anions and neutral molecules such as HO –, RO –, Cl –, Br –, RCOO –, CN –, R –, NH 3, C 2 H 5 OH, etc.

In the reaction mechanism symbol, radical reactions are denoted by the subscript N.

Electrophilic reagents are cations, simple or complex molecules that, by themselves or in the presence of a catalyst, have an increased affinity for electron pairs or negatively charged centers of molecules. These include cations H +, Cl +, + NO 2, + SO 3 H, R + and molecules with free orbitals: AlCl 3, ZnCl 2, etc.

In the mechanism symbol, electrophilic reactions are represented by the subscript E.

Nucleophiles are electron donors, and electrophiles are electron acceptors.

Electrophilic and nucleophilic reactions can be thought of as acid-base reactions; This approach is based on the theory of generalized acids and bases (Lewis acids are electron pair acceptors, Lewis bases are electron pair donors).

However, it is necessary to distinguish between the concepts of electrophilicity and acidity, as well as nucleophilicity and basicity, because they are not identical. For example, basicity reflects the affinity for a proton, and nucleophilicity is most often assessed as the affinity for a carbon atom:

OH – + H +  H 2 O hydroxide ion as a base

OH – + CH 3 +  CH 3 OH hydroxide ion as a nucleophile

4) Depending on the mechanism of the elementary stages, reactions of organic compounds can be very different: nucleophilic substitution S N, electrophilic substitution S E, free radical substitution S R, pairwise elimination, or elimination of E, nucleophilic or electrophilic addition of Ad E and Ad N, etc.

5) Based on the type of activation, reactions are divided into catalytic, non-catalytic and photochemical.

Reactions that require the presence of a catalyst are called catalytic reactions. If an acid acts as a catalyst, we are talking about acid catalysis. Acid-catalyzed reactions include, for example, esterification reactions with the formation of esters, dehydration of alcohols with the formation of unsaturated compounds, etc.

If the catalyst is a base, then we speak of basic catalysis (as shown below, this is typical for the methanolysis of triacylglycerols).

Non-catalytic reactions are reactions that do not require the presence of a catalyst. They only accelerate as the temperature increases, so they are sometimes called thermal, although this term is not widely used. The starting reagents in these reactions are highly polar or charged particles. These can be, for example, hydrolysis reactions, acid-base interactions.

Photochemical reactions are activated by irradiation (photons, h); these reactions do not occur in the dark, even with significant heating. The efficiency of the irradiation process is measured by the quantum yield, which is defined as the number of reacted reagent molecules per absorbed quantum of light. Some reactions are characterized by a quantum yield of less than unity; for others, for example, for chain reactions of the halogenation of alkanes, this yield can reach 10 6.

6) According to particular characteristics, the classification of reactions is extremely diverse: hydration and dehydration, hydrogenation and dehydrogenation, nitration, sulfonation, halogenation, acylation, alkylation, carboxylation and decarboxylation, enolization, cycle closure and opening, isomerization, oxidative destruction, pyrolysis, polymerization, condensation and etc.

7) The molecularity of an organic reaction is determined by the number of molecules in which a real change in covalent bonds occurs at the slowest stage of the reaction, which determines its speed. The following types of reactions are distinguished:

– monomolecular – one molecule participates in the limiting stage;

– bimolecular – there are two such molecules, etc.

As a rule, there is no molecularity higher than three. The exception is topochemical (solid-phase) reactions.

Molecularity is reflected in the symbol of the reaction mechanism by adding the corresponding number, for example: S N 2 - nucleophilic bimolecular substitution, S E 1 - electrophilic monomolecular substitution; E1 – monomolecular elimination, etc.

Let's look at a few examples.

Example 1. Hydrogen atoms in alkanes can be replaced by halogen atoms:

CH 4 + C1 2  CH 3 C1 + HC1

The reaction follows a chain radical mechanism (the attacking particle is the chlorine radical C1  ). This means that according to the electronic nature of the reagents, this reaction is free radical; by a change in the number of particles - a replacement reaction; by the nature of bond cleavage - homolytic reaction; activation type – photochemical or thermal; according to particular characteristics - halogenation; reaction mechanism – S R .

Example 2. Hydrogen atoms in alkanes can be replaced by a nitro group. This reaction is called the nitration reaction and follows the scheme:

R – H+HO – NO 2  R – NO 2 + H 2 O

The nitration reaction in alkanes also follows a chain radical mechanism. This means that according to the electronic nature of the reagents, this reaction is free radical; by a change in the number of particles - a replacement reaction; by the nature of the bond rupture - homolytic; activation type – thermal; according to particular characteristics - nitration; by mechanism – S R .

Example 3. Alkenes easily add a hydrogen halide to the double bond:

CH 3 – CH = CH 2 + HBr → CH 3 – CHBr – CH3.

The reaction can proceed according to the mechanism of electrophilic addition, which means that according to the electronic nature of the reagents - the reaction is electrophilic (attack particle - H +); by a change in the number of particles – an addition reaction; by the nature of the bond rupture - heterolytic; according to particular characteristics - hydrohalogenation; by mechanism – Ad E .

The same reaction in the presence of peroxides can proceed by a radical mechanism, then, due to the electronic nature of the reagents, the reaction will be radical (the attacking particle is Br  ); by a change in the number of particles – an addition reaction; by the nature of the bond rupture - homolytic; according to particular characteristics - hydrohalogenation; by mechanism – Ad R .

Example 4. The alkaline hydrolysis reaction of alkyl halides proceeds through the mechanism of bimolecular nucleophilic substitution.

CH 3 CH 2 I + NaOH  CH 3 CH 2 OH + NaI

This means that according to the electronic nature of the reagents, the reaction is nucleophilic (attack particle – OH –); by a change in the number of particles - a replacement reaction; according to the nature of bond cleavage - heterolytic, according to particular characteristics - hydrolysis; by mechanism – S N 2.

Example 5. When alkyl halides react with alcoholic solutions of alkalis, alkenes are formed.

CH 3 CH 2 CH 2 Br
[CH 3 CH 2 C + H 2 ]  CH 3 CH = CH 2 + H +

This is explained by the fact that the resulting carbocation is stabilized not by the addition of a hydroxyl ion, the concentration of which in alcohol is insignificant, but by the abstraction of a proton from the neighboring carbon atom. The reaction to change the number of particles is detachment; by the nature of the bond rupture - heterolytic; according to particular characteristics - dehydrohalogenation; according to the mechanism - elimination of E.

Control questions

1. List the characteristics by which organic reactions are classified.

2. How can the following reactions be classified:

– sulfonation of toluene;

– interaction of ethanol and sulfuric acid with the formation of ethylene;

– propene bromination;

– synthesis of margarine from vegetable oil.