Classification of organic reactions in organic chemistry. Reactions of organic compounds

During the reaction, some chemical bonds in the molecules of the reacting substances are broken and others are formed. 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 are processes that involve homolytic cleavage of a covalent bond. In homolytic 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 homolytic cleavage, free radicals are formed:

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

Ionic reactions are processes that involve heterolytic cleavage of covalent bonds, when both bond electrons remain with one of the previously bonded particles:

As a result of heterolytic bond cleavage, charged particles are obtained: nucleophilic and electrophilic.

A 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.

An electrophilic particle (electrophile) is a particle that has an unfilled outer electron level. An electrophile presents unfilled, vacant orbitals for the formation of a covalent bond due to the electrons of the particle with which it interacts.

IN organic chemistry all structural changes are considered relative to the carbon atom (or atoms) involved in the reaction.

In accordance with the above, the chlorination of methane under the influence of light is classified as radical substitution, the addition of halogens to alkenes as electrophilic addition, and the hydrolysis of alkyl halides as nucleophilic substitution.

Most common following types oeactions.

Main types chemical reactions

I. Substitution reactions(replacement of one or more hydrogen atoms with halogen atoms or a special group) RCH 2 X + Y → RCH 2 Y + X

II. Addition reactions RCH=CH 2 + XY → RCHX−CH 2 Y

III. Elimination reactions RCHX−CH 2 Y → RCH=CH 2 + XY

IV. Isomerization (rearrangement) reactions

V. Oxidation reactions(interaction with atmospheric oxygen or oxidizing agent)

In these above types of reactions, they also distinguish specialized And personalized reactions.

Specialized:

1) hydrogenation (interaction with hydrogen)

2) dehydrogenation (elimination from a hydrogen molecule)

3) halogenation (interaction with halogen: F 2, Cl 2, Br 2, I 2)

4) dehalogenation (elimination from a halogen molecule)

5) hydrohalogenation (interaction with hydrogen halide)

6) dehydrohalogenation (elimination from a hydrogen halide molecule)

7) hydration (interaction with water in irreversible reaction)

8) dehydration (cleavage from a water molecule)

9) hydrolysis (interaction with water in a reversible reaction)

10) polymerization (production of a multiple enlarged carbon skeleton from identical simple compounds)

11) polycondensation (obtaining a multiple enlarged carbon skeleton from two different compounds)

12) sulfonation (reaction with sulfuric acid)

13) nitration (interaction with nitric acid)

14) cracking (reduction of the carbon skeleton)

15) pyrolysis (decomposition of complex organic matter to simpler ones under the influence of high temperatures)

16) alkylation reaction (introduction of an alkane radical into the formula)

17) acylation reaction (introduction of the –C(CH 3)O group into the formula)

18) aromatization reaction (formation of hydrocarbons of a number of arenes)

19) decarboxylation reaction (elimination of the carboxyl group -COOH) from the molecule

20) esterification reaction (the interaction of an alcohol with an acid, or the production of an ester from an alcohol or carboxylic acid)

21) “silver mirror” reaction (interaction with an ammonia solution of silver (I) oxide)

Nominal reactions:

1) Wurtz reaction (elongation of the carbon skeleton during the interaction of a halogenated hydrocarbon with active metal)

2) Kucherov’s reaction (production of aldehyde by reacting acetylene with water)

3) Konovalov reaction (interaction of an alkane with dilute nitric acid)

4) Wagner reaction (oxidation of hydrocarbons with a double bond by oxygen of the oxidizing agent in a weakly alkaline or neutral environment at normal conditions)

5) Lebedev reaction (dehydrogenation and dehydration of alcohols to produce alkadienes)

6) Friedel-Crafts reaction (alkylation reaction of an arene with a chloroalkane to obtain benzene homologues)

7) Zelinsky reaction (production of benzene from cyclohexane by dehydrogenation)

8) Kirchhoff reaction (conversion of starch into glucose under the catalytic action of sulfuric acid)

Organic reactions can be classified into two general types.

Hemolytic reactions. These reactions proceed by a radical mechanism. We'll look at them in more detail in the next chapter. The kinetics and mechanism of reactions of this type were discussed in Chap. 9.

Heterolytic reactions. These reactions are essentially ionic reactions. They can, in turn, be divided into substitution, addition and elimination reactions.

Substitution reactions

In these reactions, an atom or group of atoms is replaced by another atom or group. As an example of reactions of this type Here is the hydrolysis of chloromethane with the formation of methanol:

The hydroxyl ion is a nucleophile. Therefore, the substitution in question is called nucleophilic substitution. It is designated by the symbol SN. The replaced particle (in this case, a chlorine ion) is called a leaving group.

If we denote the nucleophile by the symbol and the leaving group by the symbol, then we can write the generalized equation for the reaction of nucleophilic substitution at a saturated carbon atom in the alkyl group R as follows:

A study of the rate of reactions of this type shows that reactions can be divided into

Reactions of the type For some reactions of the SN type, the kinetic equation for the reaction rate (see Section 9.1) has the form

Thus, these reactions are first order in the substrate but zero order in the reactant. The kinetics characteristic of a first order reaction is a reliable indication that the rate-limiting step of the reaction is a unimolecular process. Therefore, reactions of this type are indicated by the symbol.

The reaction has zero order with respect to the reagent since its rate does not depend on the concentration of the reagent. Therefore, we can write:

Since the nucleophile does not participate in the rate-limiting step of the reaction, the mechanism of such a reaction must include at least two steps. The following mechanism has been proposed for such reactions:

The first stage is ionization with the formation of a carbocation. This stage is limiting (slow).

An example of this type of reaction is the alkaline hydrolysis of tertiary alkyl halides. For example

In the case under consideration, the reaction rate is determined by the equation

Reactions of the type For some reactions of nucleophilic substitution SN the rate equation has the form

In this case, the reaction is first order in the nucleophile and first order in . In general, it is a second order reaction. This is sufficient reason to believe that the rate-limiting stage of this reaction is a bimolecular process. Therefore, the reaction of the type under consideration is denoted by the symbol Since both the nucleophile and the substrate simultaneously participate in the rate-limiting stage of the reaction, we can think that this reaction proceeds in one stage through a transition state (see Section 9.2):

Hydrolysis of primary alkyl halides in an alkaline medium proceeds according to the mechanism

This reaction has the following kinetic equation:

So far we have considered nucleophilic substitution only at the saturated carbon atom. Nucleophilic substitution is also possible at an unsaturated carbon atom:

Reactions of this type are called nucleophilic acyl substitution.

Electrophilic substitution. Electrophilic substitution reactions can also occur on benzene rings. In this type of substitution, the benzene ring supplies the electrophile with two of its delocalized -electrons. In this case, an intermediate compound is formed - an unstable complex of an electrophile and a leaving group. For a schematic representation of such complexes, an open circle is used, indicating the loss of two -electrons:

An example of electrophilic substitution reactions is the nitration of benzene:

Nitration of benzene is carried out in an installation with a reflux condenser at a temperature of 55 to 60 ° C using a nitrating mixture. This mixture contains equal amounts of concentrated nitric and sulfuric acids. The reaction between these acids leads to the formation of a nitroyl cation

Addition reactions

In reactions of this type, an electrophile or nucleophile is added to an unsaturated carbon atom. We will consider here one example each of electrophilic addition and nucleophilic addition.

An example of electrophilic addition is the reaction between hydrogen bromide and an alkene. To obtain hydrogen bromide in laboratory conditions the reaction between concentrated sulfuric acid and sodium bromide can be used (see Section 16.2). Hydrogen bromide molecules are polar because the bromine atom has a negative inductive effect on hydrogen. Therefore, the hydrogen bromide molecule has the properties of a strong acid. According to modern views, the reaction of hydrogen bromide with alkenes occurs in two stages. In the first stage, a positively charged hydrogen atom attacks the double bond, which acts as a source of electrons. As a result, an activated complex and a bromide ion are formed:

The bromide ion then attacks this complex, resulting in the formation of an alkyl bromide:

An example of nucleophilic addition is the addition of hydrogen cyanide to any aldehyde or ketone. First the aldehyde or ketone is treated aqueous solution sodium cyanide Then add an excess amount of any mineral acid, which leads to the formation of hydrogen cyanide HCN. The cyanide ion is a nucleophile. It attacks the positively charged carbon atom on the carbonyl group of the aldehyde or ketone. The positive charge and polarity of the carbonyl group is due to the mesomeric effect, which was described above. The reaction can be represented by the following diagram:

Elimination reactions

These reactions are the reverse of addition reactions. They lead to the removal of any atoms or groups of atoms from two carbon atoms connected to each other by a simple covalent bond, resulting in the formation of a multiple bond between them.

An example of such a reaction is the elimination of hydrogen and halogen from alkyl halides:

To carry out this reaction, the alkyl halide is treated with potassium hydroxide in alcohol at a temperature of 60 °C.

It should be noted that treatment of an alkyl halide with hydroxide also leads to nucleophilic substitution (see above). As a result, two competing substitution and elimination reactions occur simultaneously, which leads to the formation of a mixture of substitution and elimination products. Which of these reactions will be predominant depends on a number of factors, including the environment in which the reaction is carried out. Nucleophilic substitution of alkyl halides is carried out in the presence of water. In contrast, elimination reactions are carried out in the absence of water and at higher temperatures.

So let's say it again!

1. During hemolytic cleavage of a bond, two shared electrons are distributed evenly between atoms.

2. During heterolytic bond cleavage, two shared electrons are distributed unevenly between atoms.

3. A carbanion is an ion containing a carbon atom with a negative charge.

4. A carbocation is an ion containing a carbon atom with a positive charge.

5. Solvent effects can have a significant impact on chemical processes and their equilibrium constants.

6. The influence of the chemical environment of the functional group inside the molecule on reactivity of this functional group is called a structural effect.

7. Electronic effects and steric effects are collectively called structural effects.

8. The two most important electronic effects are the inductive effect and the mesomeric (resonant) effect.

9. The inductive effect is the shift of electron density from one atom to another, which leads to polarization of the bond between the two atoms. This effect can be positive or negative.

10. Molecular particles with multiple bonds can exist in the form of resonant hybrids between two or more resonant structures.

11. The mesomeric (resonance) effect consists in the stabilization of resonant hybrids due to the delocalization of -electrons.

12. Steric hindrance can occur when bulky groups in a molecule mechanically impede the reaction.

13. Nucleophile is a particle that attacks a carbon atom, supplying it with its electron pair. The nucleophile is a Lewis base.

14. An electrophile is a particle that attacks a carbon atom, accepting its electron pair. The nucleophile is a Lewis acid.

15. Hemolytic reactions are radical reactions.

16. Heterolytic reactions are mainly ionic reactions.

17. The replacement of any group in a molecule with a nucleophilic reagent is called nucleophilic substitution. The group being replaced in this case is called the leaving group.

18. Electrophilic substitution on a benzene ring involves the donation of two delocalized electrons to some electrophile.

19. In electrophilic addition reactions, an electrophile is added to an unsaturated carbon atom.

20. The addition of hydrogen cyanide to aldehydes or ketones is an example of nucleophilic addition.

21. In elimination (elimination) reactions, some atoms or groups of atoms are separated from two carbon atoms connected to each other by a simple covalent bond. As a result, a multiple bond is formed between these carbon atoms.

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 chlorine chloroethane hydrogen chloride

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

chloroethane sodium hydroxide ethanol sodium chloride

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

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 of the reaction organic compounds can be very different: nucleophilic substitution S N, electrophilic substitution SE, 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 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.

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 degree of oxidation of carbon in organic atom, molecule or ion process due to the donation of electrons, as a result of which a new compound is formed:

Condensation and polycondensation reactions

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

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 empty electron shell and 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

Categories ,

Many substitution reactions open the way to the production of a variety of compounds that have economic applications. Huge role V chemical science and industry is devoted to electrophilic and nucleophilic substitution. In organic synthesis, these processes have a number of features that should be paid attention to.

Variety of chemical phenomena. Substitution reactions

Chemical changes associated with the transformation of substances are distinguished by a number of features. The final results and thermal effects may vary; Some processes go to completion, in others a change in substances occurs, often accompanied by an increase or decrease in the degree of oxidation. When classifying chemical phenomena according to their final result, attention is paid to the qualitative and quantitative differences between reagents and products. Based on these characteristics, 7 types of chemical transformations can be distinguished, including substitution, which follows the scheme: A-B + C A-C + B. A simplified notation of a whole class of chemical phenomena gives the idea that among the starting substances there is a so-called “attack "a particle that replaces an atom, ion, or functional group in a reagent. The substitution reaction is characteristic of limiting and

Substitution reactions can occur in the form of a double exchange: A-B + C-E A-C + B-E. One of the subspecies is the displacement, for example, of copper with iron from a solution of copper sulfate: CuSO 4 + Fe = FeSO 4 + Cu. The “attacking” particle can be atoms, ions or functional groups

Homolytic substitution (radical, SR)

With the radical mechanism of breaking covalent bonds, an electron pair common to different elements is proportionally distributed between the “fragments” of the molecule. Free radicals are formed. These are unstable particles, the stabilization of which occurs as a result of subsequent transformations. For example, when producing ethane from methane, free radicals appear that participate in the substitution reaction: CH 4 CH 3. + .N; CH 3. + .CH 3 → C2H5; N. + .N → N2. Homolytic bond cleavage according to the above substitution mechanism is of a chain nature. In methane, the H atoms can be successively replaced by chlorine. The reaction with bromine occurs similarly, but iodine is unable to directly replace hydrogen in alkanes; fluorine reacts with them too vigorously.

Heterolytic bond breaking method

With the ionic mechanism of substitution reactions, electrons are unevenly distributed between newly formed particles. The bonding pair of electrons goes entirely to one of the “fragments”, most often to the bond partner towards which the negative density in the polar molecule was shifted. Substitution reactions include the formation of methyl alcohol CH 3 OH. In bromomethane CH3Br, the cleavage of the molecule is heterolytic, and the charged particles are stable. Methyl acquires a positive charge, and bromine acquires a negative charge: CH 3 Br → CH 3 + + Br - ; NaOH → Na + + OH - ; CH 3 + + OH - → CH 3 OH; Na + + Br - ↔ NaBr.

Electrophiles and nucleophiles

Particles that lack electrons and can accept them are called “electrophiles.” These include carbon atoms connected to halogens in haloalkanes. Nucleophiles have increased electron density; they “donate” a pair of electrons when creating a covalent bond. In substitution reactions, nucleophiles rich in negative charges are attacked by electron-starved electrophiles. This phenomenon is associated with the movement of an atom or other particle - a leaving group. Another type of substitution reaction is the attack of an electrophile by a nucleophile. It is sometimes difficult to distinguish between two processes and to attribute substitution to one type or another, since it is difficult to accurately indicate which of the molecules is the substrate and which is the reagent. Typically in such cases the following factors are taken into account:

• the nature of the leaving group;
• nucleophile reactivity;
• nature of the solvent;
• structure of the alkyl part.

Nucleophilic substitution (SN)

During the interaction process in an organic molecule, an increase in polarization is observed. In equations, a partial positive or negative charge is indicated by a letter of the Greek alphabet. Bond polarization makes it possible to judge the nature of its rupture and the further behavior of the “fragments” of the molecule. For example, the carbon atom in iodomethane has a partial positive charge and is an electrophilic center. It attracts that part of the water dipole where oxygen, which has an excess of electrons, is located. When an electrophile interacts with a nucleophilic reagent, methanol is formed: CH 3 I + H 2 O → CH 3 OH + HI. Nucleophilic substitution reactions take place with the participation of a negatively charged ion or molecule with a free electron pair that is not involved in the creation chemical bond. The active participation of iodomethane in SN 2 reactions is explained by its openness to nucleophilic attack and the mobility of iodine.

Electrophilic substitution (SE)

An organic molecule may contain a nucleophilic center, which is characterized by an excess of electron density. It reacts with an electrophilic reagent lacking negative charges. Such particles include atoms with free orbitals and molecules with areas of low electron density. B carbon, which has a charge “-”, interacts with positive part water dipoles - with hydrogen: CH 3 Na + H 2 O → CH 4 + NaOH. The product of this electrophilic substitution reaction is methane. In heterolytic reactions, oppositely charged centers interact organic molecules, which makes them similar to ions in the chemistry of inorganic substances. It should not be overlooked that the transformation of organic compounds is rarely accompanied by the formation of true cations and anions.

Monomolecular and bimolecular reactions

Nucleophilic substitution is monomolecular (SN1). This mechanism is used to hydrolyze an important product of organic synthesis—tertiary butyl chloride. The first stage is slow; it is associated with gradual dissociation into carbonium cation and chloride anion. The second stage proceeds faster, the reaction of carbonium ion with water occurs. replacing the halogen in the alkane with an hydroxy group and obtaining a primary alcohol: (CH 3) 3 C—Cl → (CH 3) 3 C + + Cl - ; (CH 3) 3 C + + H 2 O → (CH 3) 3 C—OH + H + . The one-stage hydrolysis of primary and secondary alkyl halides is characterized by the simultaneous destruction of the carbon-halogen bond and the formation of a C-OH pair. This is a nucleophilic bimolecular substitution (SN2) mechanism.

Mechanism of heterolytic replacement

The substitution mechanism is associated with electron transfer and the creation of intermediate complexes. The faster the reaction occurs, the easier its characteristic intermediate products arise. Often the process goes in several directions simultaneously. The advantage usually goes to the path that uses particles that require the least amount of energy for their formation. For example, the presence of a double bond increases the probability of the appearance of an allylic cation CH2=CH—CH 2 + compared to the CH 3 + ion. The reason lies in the electron density of the multiple bond, which affects delocalization positive charge, dispersed throughout the molecule.

Benzene substitution reactions

The group characterized by electrophilic substitution is arenes. The benzene ring is a convenient target for electrophilic attack. The process begins with the polarization of the bond in the second reagent, resulting in the formation of an electrophile adjacent to the electron cloud benzene ring. As a result, a transition complex appears. There is not yet a full connection between the electrophilic particle and one of the carbon atoms; it is attracted to the entire negative charge of the “aromatic six” electrons. In the third stage of the process, the electrophile and one carbon atom of the ring are linked by a shared pair of electrons (covalent bond). But in this case, the “aromatic six” is destroyed, which is unfavorable from the point of view of achieving a stable, stable energy state. A phenomenon that can be called “proton ejection” is observed. H+ is eliminated and is restored sustainable system connections characteristic of arenas. The by-product contains a hydrogen cation from the benzene ring and an anion from the second reagent.

Examples of substitution reactions from organic chemistry

Alkanes are especially characterized by a substitution reaction. Examples of electrophilic and nucleophilic transformations can be given for cycloalkanes and arenes. Similar reactions in molecules of organic substances occur under normal conditions, but more often when heated and in the presence of catalysts. Common and well-studied processes include electrophilic substitution in the aromatic ring. The most important reactions of this type:

1. Nitration of benzene in the presence of H 2 SO 4 proceeds according to the scheme: C 6 H 6 → C 6 H 5 -NO 2.
2. Catalytic halogenation of benzene, in particular chlorination, according to the equation: C 6 H 6 + Cl 2 → C 6 H 5 Cl + HCl.
3. The aromatic process proceeds with “fuming” sulfuric acid, benzenesulfonic acids are formed.
4. Alkylation is the replacement of a hydrogen atom from the benzene ring with an alkyl.
5. Acylation—formation of ketones.
6. Formylation is the replacement of hydrogen with a CHO group and the formation of aldehydes.

Substitution reactions include reactions in alkanes and cycloalkanes in which halogens attack an accessible C-H bond. The formation of derivatives may involve the replacement of one, two or all hydrogen atoms in saturated hydrocarbons and cycloparaffins. Many of the haloalkanes with small molecular weight are used in the production of more complex substances belonging to different classes. The progress achieved in studying the mechanisms of substitution reactions has given a powerful impetus to the development of syntheses based on alkanes, cycloparaffins, arenes and halogenated hydrocarbons.