Addition and substitution reactions in organic matter. Types of organic reactions

Abstract: “Types of chemical reactions in organic chemistry”

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

Typically, the main organic compound involved in a reaction is called the substrate, and the other component of the reaction is conventionally considered the reactant.

Substitution reactions

Reactions that result in the replacement of one atom or group of atoms in the original molecule (substrate) with other atoms or groups of atoms are called substitution reactions.

Substitution reactions involve saturated and aromatic compounds, such as, for example, alkanes, cycloalkanes or arenes.

Let us give examples of such reactions.

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

CH4 + Cl2→ CH3Cl + HCl

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

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

Addition reactions

Reactions in which two or more molecules of reacting substances combine into one are called addition reactions.

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

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

CH3-CH = CH2 + H2 → CH3-CH2-CH3

propene propane

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

CH2=CH2 + HCl → CH3-CH2-Cl

ethene chloroethane

3 . Halogenation - halogen addition reaction (for example, chlorination):

CH2=CH2 + Cl2 → CH2Cl-CH2Cl

ethene 1,2-dichloroethane

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

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

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

Types of chemical reactions in organic chemistry

Elimination reactions

Reactions that result in the formation of molecules of several new substances from a molecule of the original compound are called elimination or elimination reactions.

Examples of such reactions include the production of ethylene from various organic substances.

Types of chemical reactions in organic chemistry

Of particular importance among the elimination reactions is the reaction of thermal splitting of hydrocarbons, on which cracking of alkanes is based - the most important technological process:

In most cases, the cleavage of a small molecule from a molecule of the parent substance leads to the formation of an additional n-bond between the atoms. Elimination reactions occur under certain conditions and with certain reagents. The given equations reflect only the final result of these transformations.

Isomerization reactions

Reactions as a result of which molecules of one substance are formed from molecules of other substances of the same qualitative and quantitative composition, i.e., with the same molecular formula, are called isomerization reactions.

An example of such a reaction is the isomerization of the carbon skeleton of linear alkanes into branched ones, which occurs on aluminum chloride at high temperature:

Types of chemical reactions in organic chemistry

1 . What type of reaction is this:

a) obtaining chloromethane from methane;

b) obtaining bromobenzene from benzene;

c) producing chloroethane from ethylene;

d) producing ethylene from ethanol;

e) conversion of butane to isobutane;

f) ethane dehydrogenation;

g) conversion of bromoethane to ethanol?

2 . What reactions are typical for: a) alkanes; b) alkenes? Give examples of reactions.

3 . What are the features of isomerization reactions? What do they have in common with reactions producing allotropic modifications of one chemical element? Give examples.

4. In which reactions (addition, substitution, elimination, isomerization) is the molecular weight of the starting compound:

a) increases;

b) decreases;

c) does not change;

d) does it increase or decrease depending on the reagent?

Nucleophilicis a reaction in which a reagent attacks the substrate with its nucleophile; it is denoted by an index N (nucleophlle).

In electrophilic reactions, the reagent is usually called an electrophile. In organic chemistry, the electrophilicity of a reagent characterizes its ability to interact with a carbon atom of the substrate that carries a full or partial negative charge.

In fact, the mechanism and result of any electrophilic-nucleophilic reaction is determined not only by the properties of the reagent, but also by the properties of the substrate, the resulting reaction products, the solvent and the conditions for its implementation. Therefore, the division of electrophilic-nucleophilic reactions into nucleophilic and electrophilic only based on the properties of the reagent is conditional. In addition, as can be seen from the above diagrams, in these reactions the electrophiles and nucleophiles contained in the substrate and reagent always interact with each other. In many reactions, only conditionally one component can be considered a substrate and the other a reagent.

Free radical reactions. Homolytic decay is characteristic of non-polar or low-polar bonds. It is accompanied by the formation of free radicals - particles with an unpaired electron.

Homolysis of a covalent bond can be considered as the cleavage of this bond by an exchange mechanism. To carry out homolysis of a bond, energy (heat, light) is required sufficient to break this bond. The presence of an unpaired electron is the reason for the low stability of free radicals (the lifetime in most cases is a fraction of a second) and high reactivity in free radical reactions. The presence of a free radical (R۰) in the system can lead to the formation of new radicals due to its interaction with existing molecules: R۰ + A – B → R – A + ۰B

Free radical reactionsare accompanied by the interaction of free radicals with molecules or with each other with the formation of new free radicals (nucleation or development of a chain) or only molecules (chain termination).

Free radical reactions are characterized by a chain mechanism, which includes three stages: initiation, development and chain termination. These reactions stop when free radicals disappear from the system. Free radical reactions are designated by the index R (radical).

Radical particles, depending on their electron affinity, can both accept electrons (i.e., be oxidizing agents) and donate electrons (i.e., be reducing agents). In this case, the affinity of a radical for an electron is determined not only by its properties, but also by the properties of its reaction partner. The features of free radical oxidation-reduction processes occurring in the body are considered separately when describing the properties of certain classes of organic compounds.

In complexation reactions, radicals can be both complexing agents and ligands. In the case of charge transfer complexes, radical formation can occur within the complex due to intramolecular oxidation-reduction between the complexing agent and the ligand.

The formation of radicals most easily occurs during the homolysis of nonpolar simple bonds between atoms of the same element:

C1 2 → C1۰ + ۰С1 HO-OH → СО۰ + ۰ОН

R-O-O-R" → RO۰ + ۰OR" R-S-S-R" →RS۰ + ۰SR"

Homolysis of a low-polarity CH bond produces alkyl radicals in which the unpaired electron is located at the carbon atom. The relative stability of these radicals depends on the type of substitution of the carbon atom bearing the unpaired electron, and increases in the series: CH 3< CH 2 R < CHR 2 < CR 3 . Это объясняется положительным индуктивным эффектом алкильных групп, который, повышая электронную плотность на атоме углерода, способствует стабилизации радикала.

The stability of free radicals increases significantly when it is possible to delocalize the unpaired electron due to the π-electrons of neighboring multiple bonds. This is especially clearly observed in the allylic and benzyl radicals:

allylic radical benzyl radical

When familiarizing yourself with possible reaction mechanisms in substrate and reagent molecules, reaction centers should be distinguished by their nature: nucleophilic, electrophilic And radical.

According to the final result of the chemical transformation, the simplest organic reactions are classified into reactions: substitution, addition, elimination (elimination) And regrouping.

Substitution reactions. Substitution refers to the replacement of an atom or group with another atom or group. In a substitution reaction, two different products are always formed. This type of reaction is designated by the symbol S (substitution).

Substitution reactions include: halogenation and nitration of alkanes, esterification and alkylation of carboxylic acids, as well as numerous reactions of simple polar molecules (H 2 O, NH 3, NGal) with ethers, alcohols and halogen derivatives.

Addition reactions. By addition we mean the introduction of atoms or groups into the molecule of an unsaturated compound, accompanied by the breaking of π bonds. In this case, double bonds turn into single bonds, and triple bonds into double or single bonds. This type of reaction is indicated by the symbol A (addition).

Elimination reactions. Elimination refers to the removal of atoms or groups from an organic molecule to form a multiple bond. Therefore, elimination reactions are the opposite of addition reactions. This type of reaction is designated by the symbol E (elimination).

Each of the organic reactions of substitution (S), addition (A) or elimination (E) can be electrophilic (E), nucleophilic (N) or radical (R). Thus, in organic chemistry there are nine typical reactions, denoted by the symbols S, A or E with the subscripts R, N or E:

The given types of organic reactions should be considered model ones, since they are not always realized in their pure form. For example, substitution and elimination can occur simultaneously:

With further acquaintance with specific classes of organic compounds, we will consider their following chemical properties: acid-base, complexing, redox, electrophilic-nucleophilic, as well as the ability for free radical interaction. Particular attention will be paid to the peculiarities of the occurrence of the reactions under consideration in biological systems.

The types of reactions characteristic of various classes of hydrocarbons, the mechanism of their occurrence and the biological significance of the processes are presented in Table 10.

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, we give 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 the laboratory, a 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 with an aqueous solution of sodium cyanide. Then an excess amount of any mineral acid is added, 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 effect of the chemical environment of a functional group within a molecule on the reactivity of that functional group is called the 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 (resonance) 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.

Lesson topic: Types of chemical reactions in organic chemistry.

Lesson type: a lesson in studying and initially consolidating new material.

Lesson objectives: create conditions for the formation of knowledge about the peculiarities of the occurrence of chemical reactions involving organic substances when becoming familiar with their classification, and consolidate the ability to write reaction equations.

Lesson objectives:

Educational: study the types of reactions in organic chemistry, based on the students’ knowledge of the types of reactions in inorganic chemistry and their comparison with the types of reactions in organic chemistry.

Developmental: promote the development of logical thinking and intellectual skills (analyze, compare, establish cause-and-effect relationships).

Educational: continue to create a culture of mental work; communication skills: listen to other people’s opinions, prove your point of view, find compromises.

Teaching methods:verbal (story, explanation, problem presentation); visual (multimedia visual aid); heuristic (written and oral exercises, problem solving, test tasks).

Means of education:implementation of intra- and interdisciplinary connections, multimedia visual aid (presentation), symbolic and graphic table.

Technologies: elements of cooperation pedagogy, student-oriented learning (competency-oriented learning, humane-personal technology, individual and differentiated approach), information and communication technology, health-saving educational technologies (organizational and pedagogical technology).

Brief description of the lesson progress.

I. Organizational stage: mutual greetings between the teacher and students; checking students' preparedness for the lesson; organization of attention and mood for the lesson.

Checking homework completion.Questions for verification: 1. Complete the sentences: a) Isomers are... b) A functional group is... 2. Distribute the indicated formulas of substances into classes (the formulas are offered on cards) and name the classes of compounds to which they belong. 3. Make possible abbreviated structural formulas of isomers corresponding to molecular formulas (for example: C 6 H 14, C 3 H 6 O)

Communication of the topic and objectives of studying new material; showing its practical significance.

II. Learning new material:

Updating knowledge.(The teacher’s story is based on slide diagrams that students transfer to their notebooks as a reference note)

Chemical reactions are the main object of the science of chemistry. (Slide 2)

In the process of chemical reactions, the transformation of some substances into others occurs.

Reagent 1 + Reagent 2 = Products (inorganic chemistry)

Substrate + Attack Reagent = Products (organic chemistry)

In many organic reactions, not all molecules undergo changes, but their reaction parts (functional groups, their individual atoms, etc.), which are called reaction centers. The substrate is the substance in which the old bond is broken at the carbon atom and a new bond is formed, and the compound acting on it or its reaction particle is called a reagent.

Inorganic reactions are classified according to several criteria: by the number and composition of starting substances and products (compounds, decomposition, substitution, exchange), by thermal effect (exo- and endothermic), by changes in the oxidation state of atoms, by the reversibility of the process, by phase (homo- and heterogeneous), according to the use of catalyst (catalytic and non-catalytic). (Slides 3,4)

The result of the lesson stage is that students complete a task (slide 5), which allows them to test their skills in writing equations of chemical reactions, arranging stoichiometric coefficients, and classifying inorganic reactions. (Tasks are offered at different levels)

(A “brain” gymnastics exercise for the development of cognitive and mental processes – “Owl”: improves visual memory, attention and relieves tension that develops during prolonged sitting.)Grab your left shoulder with your right hand and squeeze it, turn to the left so you are looking behind you, breathe deeply and roll your shoulders back. Now look over your other shoulder, drop your chin to your chest and breathe deeply, allowing your muscles to relax.

Presentation of new material.(During the presentation of the material, students make notes in notebooks, which the teacher focuses on - information from the slides)

Reactions involving organic compounds obey the same laws (the law of conservation of mass and energy, the law of mass action, Hess’s law, etc.) and exhibit the same patterns (stoichiometric, energetic, kinetic) as the reactions of inorganic substances. (Slide 6)

Organic reactions are usually classified according to the mechanisms of their occurrence, the direction and final products of the reaction. (Slide 7)

The method of breaking covalent bonds determines the type of reaction mechanism. The reaction mechanism is understood as the sequence of stages of the reaction, indicating the intermediate particles formed at each of these stages. (The reaction mechanism describes its path, i.e. the sequence of elementary acts of interaction of the reagents through which it proceeds.)

In organic chemistry, there are two main types of reaction mechanisms: radical (homolytic) and ionic (heterolytic). (Slide 8)

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:

X:Y → X . + . Y

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

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

X:Y → X + + :Y -

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 a free orbital at the outer electronic 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.

Radical reactions have a characteristic chain mechanism, which includes three stages: nucleation (initiation), development (growth) and chain termination. (Slide 9)

Ionic reactions occur without breaking the electron pairs that form chemical bonds: both electrons move to the orbital of one of the atoms of the reaction product to form an anion. (Slide 10) Heterolytic decomposition of a covalent polar bond leads to the formation of nucleophiles (anions) and electrophiles (cations). Depending on the nature of the attacking reagent, reactions can be nucleophilic or electrophilic.

According to the direction and final result of chemical transformation, organic reactions are divided into the following types: substitution, addition, elimination (elimination), rearrangement (isomerization), oxidation and reduction. (Slide 11)

Substitution refers to the replacement of an atom or group of atoms with another atom or group of atoms. The substitution reaction produces two different products.

R-CH 2 X + Y→ R-CH 2 Y + X

An addition reaction is understood as the introduction of an atom or group of atoms into the molecule of an unsaturated compound, which is accompanied by the breaking of π bonds in this compound. During the interaction, double bonds are converted into single bonds, and triple bonds into double or single bonds.

R-CH=CH 2 + XY→ RCHX-CH 2 Y

Problem: What type of reaction can we classify a polymerization reaction as? Prove that it belongs to a certain type of reaction and give an example.

Addition reactions also include polymerization reactions (for example: producing polyethylene from ethylene).

n(CH 2 =CH 2 ) → (-CH 2 -CH 2 -) n

Elimination reactions, or elimination, are reactions during which atoms or their groups are eliminated from an organic molecule to form a multiple bond.

R-CHX-CH 2 Y→ R-CH=CH 2 + XY

Rearrangement (isomerization) reactions. In this type of reaction, a rearrangement of atoms and their groups in the molecule takes place.

Polycondensation reactions belong to substitution reactions, but they are often distinguished as a special type of organic reactions that have specificity and great practical importance.

Oxidation-reduction reactions are accompanied by a change in the oxidation state of the carbon atom in compounds where the carbon atom is the reaction center.

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].

[O]

CH 3 CHO → CH 3 COOH

Reduction is the reverse reaction of oxidation. Under the action of a reducing reagent, a compound gains hydrogen atoms or loses oxygen atoms: the action of a reducing reagent (reduction) is indicated by the symbol [H].

[H]

CH 3 COCH 3 → CH 3 CH(OH)CH 3

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.

To consolidate the studied material, students perform a test task: slides 12,13.

III. Homework: § 8 (exercise 2), 9

IV. Summarizing

Conclusions: (Slide 14)

Organic reactions obey general laws (the law of conservation of mass and energy) and the general laws of their occurrence (energetic, kinetic - revealing the influence of various factors on the reaction rate).

They have common characteristics for all reactions, but also have their own characteristic features.

According to the mechanism of reaction, reactions are divided into homolytic (free radical) and heterolytic (electrophilic-nucleophilic).

According to the direction and final result of the chemical transformation, reactions are distinguished: substitution, addition, elimination (elimination), rearrangement (isomerization), polycondensation, oxidation and reduction.

Used Books:UMK: O.S. Gabrielyan et al. Chemistry 10 M. Bustard 2013

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Slide captions:

Types of chemical reactions in organic chemistry.

A chemical reaction is the transformation of one substance into another. The substances obtained as a result of the reaction differ from the starting substances in composition, structure and properties. Reagent 1 + Reagent 2 = Substrate Products + Attacker = Reagent Products

Signs of classification of chemical reactions in inorganic chemistry by the number and composition of starting substances and products by thermal effect by change in the oxidation state of atoms by reversibility of the process by phase by use of a catalyst

Classification according to the number and composition of the starting and resulting substances: Compound reactions: A + B = AB Zn + Cl 2 = ZnCl 2 CaO + CO 2 = CaCO 3 Decomposition reactions: AB = A + B 2H 2 O = 2H 2 + O 2 Cu (OH) 2 = CuO + H 2 O Substitution reactions: AB + C = A + CB CuSO 4 + Fe = Cu + FeSO 4 Cr 2 O 3 + 2Al = 2Cr + Al 2 O 3 Exchange reactions: AB + CD = AD + CB CuO + H2SO4 = CuSO4 + H2O NaOH + HCl = NaCl + H 2 O

Reaction schemes are given: 1. Copper(II) hydroxide → copper(II) oxide + water 2. Barium chloride + sodium sulfate → … 3. Hydrochloric acid + zinc → zinc chloride + hydrogen 4. Phosphorus(V) oxide + water → … Level I: Indicate the types of reactions, write down one of the equations (optional). Level II: Indicate the types of reactions, write down one of the equations in which the products are not indicated (optional). Level III: Indicate the types of reactions and write down all the equations.

Organic reactions are usually classified according to their mechanisms. A reaction mechanism is understood as the sequence of individual stages of a reaction, indicating the intermediate particles formed at each of these stages. according to the direction and final products of the reaction - addition; - cleavage (elimination); - substitutions; - rearrangement (isomerization); - oxidation; - recovery.

The method of breaking the covalent bond determines the type of reaction mechanism: Radical (homolytic) X:Y → X. + . Y R . (X . , . Y) – radicals (free atoms or particles with unpaired electrons, unstable and capable of undergoing chemical transformations) Ionic (heterolytic) X:Y → X + + :Y - X + - electrophilic reagent (electrophile: electron loving ):Y - - nucleophilic reagent (nucleophile: proton loving)

Radical reactions have a chain mechanism, including stages: initiation, development and chain termination. Chain nucleation (initiation) Cl 2 → Cl. +Cl. Growth (development) of the CH 4 + Cl chain. → CH 3. + H Cl CH 3 . + Cl 2 → CH 3 -Cl + Cl. Open circuit CH 3. +Cl. → CH 3 Cl CH 3 . + CH 3 . → CH 3 -CH 3 Cl. +Cl. →Cl2

Ionic reactions occur without breaking the electron pairs that form chemical bonds: both electrons move to the orbital of one of the atoms of the reaction product to form an anion. Heterolytic decomposition of a covalent polar bond leads to the formation of nucleophiles (anions) and electrophiles (cations). CH 3 -Br + Na + OH - → CH 3 -OH + Na + Br - substrate reagent reaction products (nucleophile) C 6 H 5 -H + HO: NO 2 → C 6 H 5 -NO 2 + H-OH substrate reagent reaction products (electrophile)

Classification by direction and final result Substitution reactions A-B + C → A-C + B Addition reactions C=C + A-B → A-C-C-B Elimination reactions A-C-C-B → C =C + A-B Rearrangement (isomerization) reactions X-A-B → A-B-X Oxidation and reduction reactions are accompanied by a change in the oxidation state of the carbon atom in compounds where the carbon atom is the reaction center. Problem: What type of reaction is a polymerization reaction? Prove that it belongs to a certain type of reaction and give an example.

Test. 1. Match: Section of chemistry Type of reaction Inorganic a) substitution b) exchange Organic c) compounds d) decomposition e) elimination f) isomerization g) addition 2. Match: Reaction scheme Type of reaction AB + C → AB + C a) substitution ABC → AB + C b) addition of ABC → ACB c) elimination of AB + C → AC + B d) isomerization

3. Butane reacts with a substance whose formula is: 1) H 2 O 2) C 3 H 8 3) Cl 2 4) HCl 4. The substrate in the proposed reaction schemes is the substance CH 3 -COOH (A) + C 2 H 5 -OH (B) → CH 3 COOC 2 H 5 + H 2 O CH 3 -CH 2 -OH (A) + H-Br ( B) → CH 3 -CH 2 -Br + H 2 O CH 3 -CH 2 -Cl (A) + Na-OH (B) → CH 2 =CH 2 + NaCl + H 2 O 5. The left side of the equation C 3 H 4 + 5O 2 → ... corresponds to the right side: → C 3 H 6 + H 2 O → C 2 H 4 + H 2 O → 3CO 2 + 4H 2 O → 3CO 2 + 2H 2 O 6. The volume of oxygen that will be required for complete combustion of 5 l of methane, equal to 1) 1 l 2) 5 l 3) 10 l 4) 15 l

Conclusions Organic reactions obey general laws and general patterns of their occurrence. They have common characteristics for all reactions, but also have their own characteristic features. According to the mechanism of reaction, reactions are divided into free radical and ionic. According to the direction and final result of the chemical transformation: substitution, addition, oxidation and reduction, isomerization, elimination, polycondensation, etc.

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

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

Substitution reactions

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

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

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

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

The equation for this reaction can be written differently:

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

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

Addition reactions

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

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

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

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

3. Halogenation- halogen addition reaction:

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

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

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

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

Types of reactive particles

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

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

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

Such particles are called free radicals.

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

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

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

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

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

Let's consider the electrolytic dissociation of acids:

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

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

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

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

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

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

Basic reaction mechanisms

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

free radical;
electrophilic;
zeroophilic.

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

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

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

Hydrohalogenation (addition of halogen hydrogen):

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