Types of chemical reactions in organic matter. Organic chemistry. By the nature of the severance of ties

Lecture: Classification of chemical reactions in inorganic and organic chemistry

Types of chemical reactions in inorganic chemistry

A) Classification according to the amount of initial substances:

Decomposition – as a result of this reaction, from one existing complex substance, two or more simple and also complex substances are formed.

Example: 2H 2 O 2 → 2H 2 O + O 2

Compound - this is a reaction in which two or more simple, as well as complex substances, form one, but more complex one.

Example: 4Al+3O 2 → 2Al 2 O 3

Substitution - this is a certain chemical reaction that takes place between some simple and also complex substances. Atoms of a simple substance, in this reaction, are replaced by atoms of one of the elements found in the complex substance.

Example: 2КI + Cl2 → 2КCl + I 2

Exchange - This is a reaction in which two substances of complex structure exchange their parts.

Example: HCl + KNO 2 → KCl + HNO 2

B) Classification by thermal effect:

Exothermic reactions - These are certain chemical reactions in which heat is released.
Examples:

S + O 2 → SO 2 + Q

2C 2 H 6 + 7O 2 → 4CO 2 +6H 2 O + Q

Endothermic reactions - These are certain chemical reactions in which heat is absorbed. As a rule, these are decomposition reactions.

Examples:

CaCO 3 → CaO + CO 2 – Q
2KClO 3 → 2KCl + 3O 2 – Q

The heat that is released or absorbed as a result of a chemical reaction is called thermal effect.

Chemical equations that indicate the thermal effect of a reaction are called thermochemical.

B) Classification by reversibility:

Reversible reactions - these are reactions that occur under the same conditions in mutually opposite directions.

Example: 3H 2 + N 2 ⇌ 2NH 3

Irreversible reactions - these are reactions that proceed in only one direction, and also end with the complete consumption of all starting substances. In these reactions, release there is gas, sediment, water.
Example: 2KClO 3 → 2KCl + 3O 2

D) Classification by change in oxidation state:

Redox reactions – during these reactions, a change in the oxidation state occurs.

Example: Cu + 4HNO 3 → Cu(NO 3) 2 + 2NO 2 + 2H 2 O.

Not redox – reactions without changing the oxidation state.

Example: HNO 3 + KOH → KNO 3 + H 2 O.

D) Classification by phase:

Homogeneous reactions – reactions occurring in one phase, when the starting substances and reaction products have the same state of aggregation.

Example: H 2 (gas) + Cl 2 (gas) → 2HCL

Heterogeneous reactions – reactions occurring at the interface, in which the reaction products and starting substances have different states of aggregation.
Example: CuO+ H 2 → Cu+H 2 O

Classification by catalyst use:

A catalyst is a substance that speeds up a reaction. A catalytic reaction occurs in the presence of a catalyst, a non-catalytic reaction occurs without a catalyst.
Example: 2H 2 0 2 MnO2 → 2H 2 O + O 2 catalyst MnO 2

The interaction of alkali with acid occurs without a catalyst.
Example: KOH + HCl → KCl + H 2 O

Inhibitors are substances that slow down a reaction.
Catalysts and inhibitors themselves are not consumed during the reaction.

Types of chemical reactions in organic chemistry

Substitution is a reaction during which one atom/group of atoms in the original molecule is replaced by other atoms/groups of atoms.
Example: CH 4 + Cl 2 → CH 3 Cl + HCl

Accession - These are reactions in which several molecules of a substance combine into one. Addition reactions include:

• Hydrogenation is a reaction during which hydrogen is added to a multiple bond.

Example: CH 3 -CH = CH 2 (propene) + H 2 → CH 3 -CH 2 -CH 3 (propane)

Hydrohalogenation– reaction that adds hydrogen halide.

Example: CH 2 = CH 2 (ethene) + HCl → CH 3 -CH 2 -Cl (chloroethane)

Alkynes react with hydrogen halides (hydrogen chloride, hydrogen bromide) in the same way as alkenes. Addition in a chemical reaction takes place in 2 stages, and is determined by Markovnikov’s rule:

When protic acids and water add to unsymmetrical alkenes and alkynes, a hydrogen atom is added to the most hydrogenated carbon atom.

The mechanism of this chemical reaction. Formed in the 1st, fast stage, the p-complex in the 2nd slow stage gradually turns into an s-complex - a carbocation. In the 3rd stage, stabilization of the carbocation occurs - that is, interaction with the bromine anion:

I1, I2 are carbocations. P1, P2 - bromides.

Halogenation - a reaction in which a halogen is added. Halogenation also refers to all processes as a result of which halogen atoms are introduced into organic compounds. This concept is used in a “broad sense”. In accordance with this concept, the following chemical reactions based on halogenation are distinguished: fluorination, chlorination, bromination, iodination.

Halogen-containing organic derivatives are considered the most important compounds that are used both in organic synthesis and as target products. Halogen derivatives of hydrocarbons are considered starting products in a large number of nucleophilic substitution reactions. As for the practical use of halogen-containing compounds, they are used in the form of solvents, for example chlorine-containing compounds, refrigerants - chlorofluoro derivatives, freons, pesticides, pharmaceuticals, plasticizers, monomers for the production of plastics.

Hydration– reactions of addition of a water molecule through a multiple bond.

Polymerization is a special type of reaction in which molecules of a substance with a relatively low molecular weight attach to each other, subsequently forming molecules of a substance with a high molecular weight.

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.

Due to the huge variety of organic substances (more than 27 million), reactions between them also occur in a variety of ways. If you add reactions with representatives of inorganic chemistry (oxides, salts, acids, etc.), then your head can generally go spinning and it is really impossible to keep track of all the diversity. A huge number of reactions are named in honor of the scientists who discovered them. Therefore, this publication will present those basic principles of the reactivity of organic substances that are currently known to science.

The chemical properties of atoms and molecules, expressed in their ability to interact with each other, are determined by the state of the electrons they contain. In simpler cases, the main role in chemical interaction is played by valence electrons of the outer shells of reacting atoms.

Chemical reactions, to a first approximation, can be characterized as processes in which the redistribution of outer shell electrons occurs. The direction of the reaction depends significantly on the distribution of electrons in the reacting molecules. The set of factors that control the distribution of electron density and the possibility of the formation of a new, more stable system with minimal potential energy ultimately determines the occurrence of a chemical reaction and is its driving force.

From a production point of view, one of the most significant reactions will be the combustion reaction. It is used in energy, toxic waste disposal, etc. The view of the scientific world is attracted by the reactions of transformation of some organic compounds into others. In a molecule of an organic substance there is always a section of the organic chain capable of reaction. Such atoms are called reaction centers. Since organic substances often have several reaction centers that have different activities, several reactions occur, with different rates and different end products. The fastest reaction is called the main reaction, the rest are secondary. Due to this, it is even possible to regulate the reactions that occur to obtain the desired end products. This is done with the help of catalysts, which have long been not only process accelerators, but also their regulators.

A huge number of different transformations of organic compounds are known, with the help of which chemists can obtain almost any substance of a given structure. Their classification helps to navigate the variety of organic reactions. This section sets out the basis for the classification of transformations of organic substances.

All reactions in organic chemistry are classified as follows:

According to the nature of the substrate transformation

The starting compounds in organic reactions are called reactants, and the resulting compounds are called products. In this equation, R-X and Y are reactants, and R-Y and X are products. For convenience, one of the reagents is usually called a substrate, and the other - an attack reagent.

The X group in the R-X substrate is usually called the leaving group, and the Y group is the entering group. Typically, the substrate has more complex structure, the attacking reagent often has inorganic nature. For example, in the reaction of methane with chlorine


methane is the substrate and chlorine is the attacking agent.

The symbols above the arrow (below the arrow) indicate conditions required to carry out the reaction; in this reaction, such conditions are UV irradiation and heating.

Substitution reactions are designated by a Latin letter S(from English, “substitution” - replacement).

Hydrogen substitution reactions are often named by the functional group involved. This reaction is called, for example, the chlorination reaction (H → Cl, i.e., a hydrogen atom is replaced by a chlorine atom).

Another example of a hydrogen substitution reaction is the reaction nitration benzene(H → NO 2, i.e. the hydrogen atom is replaced by a nitro group).

Not only hydrogen atoms can be subject to substitution, but also various functional groups previously introduced into hydrocarbon molecules. For example, substitution Cl → OH:

Here, Cl is the leaving group, OH is the entering group.

For example, the isomerization of 1-chloropropane to 2-chloropropane is observed in the presence of aluminum chloride:

By activation type

Non-catalytic are reactions that do not require the presence of a catalyst. These reactions only speed up as the temperature increases and are sometimes called thermal reactions. This activation method is indicated by the icon t°.

Non-catalytic reactions include some of the reactions already discussed, such as the nitration of benzene and the hydrobromination of ethylene. The starting reagents in these reactions are highly polar or charged particles.

Catalytic Reactions that require the presence of a catalyst are called reactions. If the catalyst is an acid, we are talking about acid catalysis. Acid-catalyzed reactions include, for example, the dehydration of isopropanol in the presence of sulfuric acid and the isomerization of chlorpropane. If a base acts as a catalyst, we are talking about mostly catalysis.

Photochemical reactions are reactions that are activated by radiation; This method of activation is designated hν . Photochemically activated reactions include the reaction of methane chlorination. The ethylene dimerization reaction is also photochemically activated:

It is important to note that this reaction does not leak in the dark even with significant heating.

By the nature of the severance of ties

• Radical reactions

Radical reactions are accompanied homolytic rupture bonds and the formation of radicals - neutral particles containing one or more unpaired electrons.

Radical reactions are especially common in the transformations of alkanes. For example, in the reaction of methane chlorination

the chlorine atom acts as radical reagent, and the reaction as a whole proceeds as a reaction radical substitution and is designated S R.

• Ionic reactions

Ionic reactions occur with the participation of ions and, as a rule, are accompanied heterolytic rupture connections in the substrate.

A charged particle having a vacant p-orbital on a carbon atom is called a carbocation.

A charged particle containing a lone electron pair (LEP) on a carbon atom is called a carbanion.

Ionic reactions are most common among transformations of organic compounds. The simplest example of heterolytic cleavage of a covalent bond is the reaction carboxylic acid dissociation.

The above reaction is an ionic reaction hydrolysis of chloromethane:

In this reaction, the reactant, which has a negative charge, donates its pair of electrons to form a bond with the substrate. A similar reaction occurs when ammonia reacts with ethyl bromide.

In this reaction, the attacking reagent is a neutral ammonia molecule, which donates its electron pair to form a bond with the substrate. Reagents that donate their electron pair during a reaction to form a bond with the substrate are called nucleophilic reagents, or nucleophiles.

Nucleophiles can usually be negative charged ions: hydroxide ion OH - , alkoxide ion OR - , alkylthio ion RS - , alkylcarboxy ion RCOO - , halogen ion Hal - , cyanide ion CN - , hydride ion H -

The reaction of ethylene hydrobromination also begins with the addition of a positively charged particle - a proton - due to a pair of π-electrons of the substrate.

Positively charged reagents that accept an electron pair during a reaction to form a covalent bond with the substrate are called electrophilic reagents, or electrophiles.

Electrophiles can be:a) positive ions: H +, Br +, NO 2 +, R +, etc.

b) neutral molecules that have polar bonds, and, therefore, atoms that carry a partial positive charge and are capable of forming a bond due to a pair of electrons of the substrate.

Determining the nature of the reagent - radical, nucleophilic and electrophilic - allows you to clarify the classification of organic reactions according to type of substrate transformation.

Substitution reactions in which a leaving group in a substrate is replaced by nucleophilic reagents are called reactions nucleophilic substitution. The hydrolysis reactions of chloromethane and ammonia with ethyl bromide are such and are designated as S N -type reactions.

Substitution reactions that occur with the participation of electrophilic reagents are called reactions electrophilic substitution and denote S E. Such a reaction may include benzene nitration.

Thus, the classification of organic reactions according to the type of transformation of the substrate, as well as the designations of these reactions, can be presented in the form of a table.

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Reagent	Transformation type	Designation
Radical

Classification of chemical reactions in inorganic and organic chemistry

Chemical reactions, or chemical phenomena, are processes as a result of which from some substances others are formed that differ from them in composition and (or) structure.

During chemical reactions, a change in substances necessarily occurs, in which old bonds are broken and new bonds are formed between atoms.

Chemical reactions must be distinguished from nuclear reactions. As a result of a chemical reaction, the total number of atoms of each chemical element and its isotopic composition do not change. Nuclear reactions are a different matter - processes of transformation of atomic nuclei as a result of their interaction with other nuclei or elementary particles, for example, the transformation of aluminum into magnesium:

$↙(13)↖(27)(Al)+ ()↙(1)↖(1)(H)=()↙(12)↖(24)(Mg)+()↙(2)↖(4 )(He)$

The classification of chemical reactions is multifaceted, i.e. it can be based on various features. But any of these characteristics can include reactions between both inorganic and organic substances.

Let's consider the classification of chemical reactions according to various criteria.

Classification of chemical reactions according to the number and composition of reactants. Reactions that occur without changing the composition of the substance

In inorganic chemistry, such reactions include the processes of obtaining allotropic modifications of one chemical element, for example:

$С_((graphite))⇄С_((diamond))$

$S_((rhombic))⇄S_((monoclinic))$

$Р_((white))⇄Р_((red))$

$Sn_((white tin))⇄Sn_((gray tin))$

$3О_(2(oxygen))⇄2О_(3(ozone))$.

In organic chemistry, this type of reaction can include isomerization reactions, which occur without changing not only the qualitative, but also the quantitative composition of the molecules of substances, for example:

1. Isomerization of alkanes.

The isomerization reaction of alkanes is of great practical importance, because hydrocarbons of isostructure have a lower ability to detonate.

2. Isomerization of alkenes.

3. Alkyne isomerization(reaction of A.E. Favorsky).

4. Isomerization of haloalkanes(A.E. Favorsky).

5. Isomerization of ammonium cyanate by heating.

Urea was first synthesized by F. Wöhler in 1882 by isomerizing ammonium cyanate when heated.

Reactions that occur with a change in the composition of a substance

Four types of such reactions can be distinguished: combination, decomposition, substitution and exchange.

1. Compound reactions- These are reactions in which one complex substance is formed from two or more substances.

In inorganic chemistry, the whole variety of compound reactions can be considered using the example of reactions for the production of sulfuric acid from sulfur:

1) obtaining sulfur oxide (IV):

$S+O_2=SO_2$ - one complex substance is formed from two simple substances;

2) obtaining sulfur oxide (VI):

$2SO_2+O_2(⇄)↖(t,p,cat.)2SO_3$ - one complex substance is formed from simple and complex substances;

3) obtaining sulfuric acid:

$SO_3+H_2O=H_2SO_4$ - two complex substances form one complex substance.

An example of a compound reaction in which one complex substance is formed from more than two initial substances is the final stage of producing nitric acid:

$4NO_2+O_2+2H_2O=4HNO_3$.

In organic chemistry, joining reactions are commonly called addition reactions. The whole variety of such reactions can be considered using the example of a block of reactions characterizing the properties of unsaturated substances, for example ethylene:

1) hydrogenation reaction - addition of hydrogen:

$CH_2(=)↙(ethene)CH_2+H_2(→)↖(Ni,t°)CH_3(-)↙(ethane)CH_3;$

2) hydration reaction - addition of water:

$CH_2(=)↙(ethene)CH_2+H_2O(→)↖(H_3PO_4,t°)(C_2H_5OH)↙(ethanol);$

3) polymerization reaction:

$(nCH_2=CH_2)↙(ethylene)(→)↖(p,cat.,t°)((-CH_2-CH_2-)_n)↙(polyethylene)$

2. Decomposition reactions- These are reactions in which several new substances are formed from one complex substance.

In inorganic chemistry, the whole variety of such reactions can be considered using the example of a block of reactions for producing oxygen by laboratory methods:

1) decomposition of mercury (II) oxide:

$2HgO(→)↖(t°)2Hg+O_2$ - two simple ones are formed from one complex substance;

2) decomposition of potassium nitrate:

$2KNO_3(→)↖(t°)2KNO_2+O_2$ - from one complex substance one simple and one complex are formed;

3) decomposition of potassium permanganate:

$2KMnO_4(→)↖(t°)K_2MnO_4+MnO_2+O_2$ - from one complex substance two complex and one simple are formed, i.e. three new substances.

In organic chemistry, decomposition reactions can be considered using the example of a block of reactions for the production of ethylene in the laboratory and industry:

1) dehydration reaction (elimination of water) of ethanol:

$C_2H_5OH(→)↖(H_2SO_4,t°)CH_2=CH_2+H_2O;$

2) dehydrogenation reaction (elimination of hydrogen) of ethane:

$CH_3—CH_3(→)↖(Cr_2O_3,500°C)CH_2=CH_2+H_2;$

3) propane cracking reaction:

$CH_3-CH_2CH_3(→)↖(t°)CH_2=CH_2+CH_4.$

3. Substitution reactions- these are reactions as a result of which atoms of a simple substance replace atoms of an element in a complex substance.

In inorganic chemistry, an example of such processes is a block of reactions characterizing the properties, for example, of metals:

1) interaction of alkali and alkaline earth metals with water:

$2Na+2H_2O=2NaOH+H_2$

2) interaction of metals with acids in solution:

$Zn+2HCl=ZnCl_2+H_2$;

3) interaction of metals with salts in solution:

$Fe+CuSO_4=FeSO_4+Cu;$

4) metallothermy:

$2Al+Cr_2O_3(→)↖(t°)Al_2O_3+2Cr$.

The subject of the study of organic chemistry is not simple substances, but only compounds. Therefore, as an example of a substitution reaction, we present the most characteristic property of saturated compounds, in particular methane, the ability of its hydrogen atoms to be replaced by halogen atoms:

$CH_4+Cl_2(→)↖(hν)(CH_3Cl)↙(chloromethane)+HCl$,

$CH_3Cl+Cl_2→(CH_2Cl_2)↙(dichloromethane)+HCl$,

$CH_2Cl_2+Cl_2→(CHCl_3)↙(trichloromethane)+HCl$,

$CHCl_3+Cl_2→(CCl_4)↙(carbon tetrachloride)+HCl$.

Another example is the bromination of an aromatic compound (benzene, toluene, aniline):

Let us pay attention to the peculiarity of substitution reactions in organic substances: as a result of such reactions, not a simple and a complex substance is formed, as in inorganic chemistry, but two complex substances.

In organic chemistry, substitution reactions also include some reactions between two complex substances, for example, the nitration of benzene:

$C_6H_6+(HNO_3)↙(benzene)(→)↖(H_2SO_4(conc.),t°)(C_6H_5NO_2)↙(nitrobenzene)+H_2O$

It is formally an exchange reaction. The fact that this is a substitution reaction becomes clear only when considering its mechanism.

4. Exchange reactions- These are reactions in which two complex substances exchange their constituent parts.

These reactions characterize the properties of electrolytes and in solutions proceed according to Berthollet’s rule, i.e. only if the result is the formation of a precipitate, gas or slightly dissociating substance (for example, $H_2O$).

In inorganic chemistry, this can be a block of reactions that characterize, for example, the properties of alkalis:

1) neutralization reaction that occurs with the formation of salt and water:

$NaOH+HNO_3=NaNO_3+H_2O$

or in ionic form:

$OH^(-)+H^(+)=H_2O$;

2) the reaction between alkali and salt, which occurs with the formation of gas:

$2NH_4Cl+Ca(OH)_2=CaCl_2+2NH_3+2H_2O$

or in ionic form:

$NH_4^(+)+OH^(-)=NH_3+H_2O$;

3) the reaction between alkali and salt, which occurs with the formation of a precipitate:

$CuSO_4+2KOH=Cu(OH)_2↓+K_2SO_4$

or in ionic form:

$Cu^(2+)+2OH^(-)=Cu(OH)_2↓$

In organic chemistry, we can consider a block of reactions that characterize, for example, the properties of acetic acid:

1) reaction that occurs with the formation of a weak electrolyte - $H_2O$:

$CH_3COOH+NaOH⇄NaCH_3COO+H_2O$

$CH_3COOH+OH^(-)⇄CH_3COO^(-)+H_2O$;

2) reaction that occurs with the formation of gas:

$2CH_3COOH+CaCO_3=2CH_3COO^(-)+Ca^(2+)+CO_2+H_2O$;

3) reaction that occurs with the formation of a precipitate:

$2CH_3COOH+K_2SiO_3=2KCH_3COO+H_2SiO_3↓$

$2CH_3COOH+SiO_3^(−)=2CH_3COO^(−)+H_2SiO_3↓$.

Classification of chemical reactions according to changes in oxidation states of chemical elements forming substances

Reactions that occur with a change in the oxidation states of elements, or redox reactions.

These include many reactions, including all substitution reactions, as well as those reactions of combination and decomposition in which at least one simple substance is involved, for example:

1.$(Mg)↖(0)+(2H)↖(+1)+SO_4^(-2)=(Mg)↖(+2)SO_4+(H_2)↖(0)$

$((Mg)↖(0)-2(e)↖(-))↙(reducing agent)(→)↖(oxidation)(Mg)↖(+2)$

$((2H)↖(+1)+2(e)↖(-))↙(oxidizer)(→)↖(reduction)(H_2)↖(0)$

2.$(2Mg)↖(0)+(O_2)↖(0)=(2Mg)↖(+2)(O)↖(-2)$

$((Mg)↖(0)-2(e)↖(-))↙(reducing agent)(→)↖(oxidation)(Mg)↖(+2)|4|2$

$((O_2)↖(0)+4(e)↖(-))↙(oxidizer)(→)↖(reduction)(2O)↖(-2)|2|1$

As you remember, complex redox reactions are compiled using the electron balance method:

$(2Fe)↖(0)+6H_2(S)↖(+6)O_(4(k))=(Fe_2)↖(+3)(SO_4)_3+3(S)↖(+4)O_2+ 6H_2O$

$((Fe)↖(0)-3(e)↖(-))↙(reducing agent)(→)↖(oxidation)(Fe)↖(+3)|2$

$((S)↖(+6)+2(e)↖(-))↙(oxidizer)(→)↖(reduction)(S)↖(+4)|3$

In organic chemistry, a striking example of redox reactions is the properties of aldehydes:

1. Aldehydes are reduced to the corresponding alcohols:

$(CH_3-(C)↖(+1) ()↖(O↖(-2))↙(H↖(+1))+(H_2)↖(0))↙(\text"aceticaldehyde") (→)↖(Ni,t°)(CH_3-(C)↖(-1)(H_2)↖(+1)(O)↖(-2)(H)↖(+1))↙(\text "ethyl alcohol")$

$((C)↖(+1)+2(e)↖(-))↙(oxidizer)(→)↖(reduction)(C)↖(-1)|1$

$((H_2)↖(0)-2(e)↖(-))↙(reducing agent)(→)↖(oxidation)2(H)↖(+1)|1$

2. Aldehydes are oxidized into the corresponding acids:

$(CH_3-(C)↖(+1) ()↖(O↖(-2))↙(H↖(+1))+(Ag_2)↖(+1)(O)↖(-2)) ↙(\text"aceticaldehyde"))(→)↖(t°)(CH_3-(Ag)↖(0)(C)↖(+3)(O)↖(-2)(OH)↖(-2 +1)+2(Ag)↖(0)↓)↙(\text"ethyl alcohol")$

$((C)↖(+1)-2(e)↖(-))↙(reducing agent)(→)↖(oxidation)(C)↖(+3)|1$

$(2(Ag)↖(+1)+2(e)↖(-))↙(oxidizer)(→)↖(reduction)2(Ag)↖(0)|1$

Reactions that occur without changing the oxidation states of chemical elements.

These include, for example, all ion exchange reactions, as well as:

• many compound reactions:

$Li_2O+H_2O=2LiOH;$

• many decomposition reactions:

$2Fe(OH)_3(→)↖(t°)Fe_2O_3+3H_2O;$

• esterification reactions:

$HCOOH+CH_3OH⇄HCOOCH_3+H_2O$.

Classification of chemical reactions by thermal effect

Based on the thermal effect, reactions are divided into exothermic and endothermic.

Exothermic reactions.

These reactions occur with the release of energy.

These include almost all compound reactions. A rare exception is the endothermic reaction of the synthesis of nitric oxide (II) from nitrogen and oxygen and the reaction of hydrogen gas with solid iodine:

$N_2+O_2=2NO - Q$,

$H_(2(g))+I(2(t))=2HI - Q$.

Exothermic reactions that occur with the release of light are classified as combustion reactions, for example:

$4P+5O_2=2P_2O_5+Q,$

$CH_4+2O_2=CO_2+2H_2O+Q$.

Hydrogenation of ethylene is an example of an exothermic reaction:

$CH_2=CH_2+H_2(→)↖(Pt)CH_3-CH_3+Q$

It runs at room temperature.

Endothermic reactions

These reactions occur with the absorption of energy.

Obviously, these include almost all decomposition reactions, for example:

a) calcination of limestone:

$CaCO_3(→)↖(t°)CaO+CO_2-Q;$

b) butane cracking:

The amount of energy released or absorbed as a result of a reaction is called thermal effect of reaction, and the equation of a chemical reaction indicating this effect is called thermochemical equation, For example:

$H_(2(g))+Cl_(2(g))=2HCl_((g))+92.3 kJ,$

$N_(2(g))+O_(2(g))=2NO_((g)) - 90.4 kJ$.

Classification of chemical reactions according to the state of aggregation of the reacting substances (phase composition)

Heterogeneous reactions.

These are reactions in which the reactants and reaction products are in different states of aggregation (in different phases):

$2Al_((t))+3CuCl_(2(sol))=3Cu_((t))+2AlCl_(3(sol))$,

$CaC_(2(t))+2H_2O_((l))=C_2H_2+Ca(OH)_(2(solution))$.

Homogeneous reactions.

These are reactions in which the reactants and reaction products are in the same state of aggregation (in the same phase):

Classification of chemical reactions according to the participation of a catalyst

Non-catalytic reactions.

Non-catalytic reactions occur without the participation of a catalyst:

$2HgO(→)↖(t°)2Hg+O_2$,

$C_2H_4+3O_2(→)↖(t°)2CO_2+2H_2O$.

Catalytic reactions.

Catalytic reactions are in progress with the participation of a catalyst:

$2KClO_3(→)↖(MnO_2,t°)2KCl+3O_2,$

$(C_2H_5OH)↙(ethanol)(→)↖(H_2SO-4,t°)(CH_2=CH_2)↙(ethene)+H_2O$

Since all biological reactions occurring in the cells of living organisms occur with the participation of special biological catalysts of a protein nature - enzymes, they are all catalytic or, more precisely, enzymatic.

It should be noted that more than $70%$ of chemical industries use catalysts.

Classification of chemical reactions by direction

Irreversible reactions.

Irreversible reactions flow under these conditions only in one direction.

These include all exchange reactions accompanied by the formation of a precipitate, gas or slightly dissociating substance (water), and all combustion reactions.

Reversible reactions.

Reversible reactions under these conditions proceed simultaneously in two opposite directions.

The overwhelming majority of such reactions are.

In organic chemistry, the sign of reversibility is reflected by the antonyms of the processes:

• hydrogenation - dehydrogenation;
• hydration - dehydration;
• polymerization - depolymerization.

All reactions of esterification (the opposite process, as you know, is called hydrolysis) and hydrolysis of proteins, esters, carbohydrates, and polynucleotides are reversible. Reversibility underlies the most important process in a living organism - metabolism.