All chemical reactions are accompanied by the breaking of some bonds and the formation of others. In principle, organic reactions obey the same laws as inorganic ones, but have qualitative originality.
Thus, while inorganic reactions usually involve ions, organic reactions involve molecules.
Reactions proceed much more slowly, in many cases requiring a catalyst, or selection external conditions(temperature, pressure).
Unlike inorganic reactions, proceeding quite unambiguously, most organic reactions are accompanied by one or another number of side reactions. In this case, the yield of the main product often does not exceed 50%, but it happens that the yield is even less. But in some cases the reaction can proceed quantitatively, i.e. with 100% yield. Due to the fact that the composition of products is ambiguous, equations are rarely used in organic chemistry chemical reactions. Most often, a reaction scheme is written down, which reflects starting materials and the main product of the reaction, and instead of the “=” sign between the right and left parts of the diagram, use “” or the reversibility sign.
There are two approaches to the classification of organic reactions: according to the nature of chemical transformations and according to the mechanisms of their occurrence.
Based on the nature of chemical transformations, they are distinguished:
|
Substitution reactions (S - from the English Substitution - substitution) |
One atom or group of atoms is replaced by another atom or group of atoms: |
|
Addition reactions (Ad - from the English Addition - accession) |
From two or more molecules one new substance is formed. The addition occurs, as a rule, through multiple bonds (double, triple): |
|
Elimination reactions (E - from English Elimination - elimination, removal) |
Reactions of hydrocarbon derivatives in which a functional group is eliminated together with hydrogen atoms to form an -bond (double, triple): |
|
Regroupings (Rg - from the English Re-grouping - regrouping) |
Intramolecular reactions of redistribution of electron density and atoms: (Favorsky regrouping). |
Classification of organic reactions according to the mechanism of their occurrence.
The mechanism of a chemical reaction is the path that leads to the breaking of an old bond and the formation of a new one.
There are two rupture mechanisms covalent bond:
1. Heterolytic (ionic). In this case, the bonding electron pair is completely transferred to one of the bonded atoms:
2. Homolytic (radical). The shared electron pair is broken in half to form two particles with free valencies - radicals:
The nature of the decay mechanism is determined by the type of attacking particle (reagent). There are three types of reagents in organic chemistry.
1. Nucleophilic reagents (N - from the Latin Nucleophilic - having an affinity for the nucleus).
Particles (atoms, groups, neutral molecules) containing excess electron density. They are divided into strong, medium strength and weak. The strength of a nucleophile is a relative concept, depending on the reaction conditions (solvent polarity). In polar solvents strong nucleophiles: , as well as neutral molecules with lone electron pairs (in non-bonding orbitals). Medium strength nucleophiles: . Weak nucleophiles: anions of strong acids - as well as phenols and aromatic amines.
2. Electrophilic reagents (E - from the Latin Electrophilic - having an affinity for electrons).
Particles (atoms, groups, neutral molecules) that carry a positive charge or a vacant orbital, as a result of which they have an affinity for negatively charged particles or an electron pair. To the number strong electrophiles include the proton, metal cations (especially multiply charged ones), molecules with a vacant orbital on one of the atoms (Lewis acids), molecules of oxygen-containing acids with high charges on the oxidized atom ().
It often happens that a molecule contains several reaction centers of different natures - both nucleophilic and electrophilic.
3. Radicals (R).
Depending on the type of reagent and the route of heterolytic bond cleavage in the substrate molecule, various products are formed. This can be represented in general form:
Reactions occurring according to such schemes are called electrophilic substitution reactions (SE), because the reaction is essentially a displacement, and the attacking agent is an electrophilic species.
Reactions proceeding according to such schemes are called nucleophilic substitution reactions (S N), because the reaction is essentially a displacement, and the attacking agent is a nucleophilic species.
If the attacking agent is a radical, then the reaction proceeds by a radical mechanism.
>> Chemistry: Types of chemical reactions in organic chemistry
Reactions organic matter can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization). It is obvious that the whole variety of reactions 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.
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.:
- Hydrogenation– addition of a hydrogen molecule:
Elimination reaction
As a result of elimination reactions organic molecules lose atoms or groups of atoms, and a new substance is formed containing one or more multiple bonds. Elimination reactions include reactions dehydrogenation, dehydration, dehydrohalogenation and so on.:
Isomerization reactions and rearrangement
During such reactions, intramolecular rearrangement occurs, i.e. the transition of atoms or groups of atoms from one part of the molecule to another without changing the molecular formula of the substance participating in the reaction, for example: 
Oxidation reactions
As a result of exposure to an oxidizing reagent, the oxidation degree of carbon in an organic atom, molecule or ion increases due to the loss of electrons, resulting in the formation of a new compound: 
Condensation and polycondensation reactions
Consists in the interaction of several (two or more) organic compounds with the formation new S-S bonds 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 unfilled electron shell and is capable of attaching a pair of electrons in the process of forming a new bond. The following cations are called electrophilic reagents: H +, R 3 C +, AlCl 3, ZnCl 2, SO 3, BF 3, R-Cl, R 2 C=O
Nucleophilic substitution reactions
Characteristic for alkyl and aryl halides: 
Nucleophilic addition reactions 
Electrophilic substitution reactions
Electrophilic addition reactions 
Homolytic (radical mechanism)
In reactions proceeding according to the homolytic (radical) mechanism, at the first stage the covalent bond is broken with the formation of radicals. The resulting free radical then acts as an attacking reagent. Bond cleavage by a radical mechanism is typical for non-polar or low-polar covalent bonds (C–C, N–N, C–H).
Distinguish between radical substitution and radical addition reactions
Radical displacement reactions
Characteristic of alkanes 
Radical addition reactions
Characteristic of alkenes and alkynes 
Thus, we examined the main types of chemical reactions in organic chemistry
Categories ,Types of chemical reactions in inorganic and organic chemistry.
1. A chemical reaction is a process in which other substances are formed from one substance. Depending on the nature of the process, types of chemical reactions are distinguished.
1)According to the final result
2) Based on the release or absorption of heat
3) Based on the reversibility of the reaction
4) Based on changes in the oxidation state of the atoms that make up the reacting substances
According to the final result, reactions are following types:
A) Substitution: RH+Cl 2 →RCl+HCl
B) Accession: CH 2 =CH 2 +Cl 2 →CH 2 Cl-CH 2 Cl
B) Elimination: CH 3 -CH 2 OH → CH 2 =CH 2 +H 2 O
D) Decomposition: CH 4 →C+2H 2
D) Isomerization
E) Exchange
G) Connections
Decomposition reaction is a process in which two or more others are formed from one substance.
Exchange reaction is a process in which reacting substances exchange their constituent parts.
Substitution reactions proceed with the participation of simple and complex substance, as a result, new simple and complex substances are formed.
As a result compound reactions from two or more substances one new one is formed.
Based on the release or absorption of heat, reactions are of the following types:
A) Exothermic
B) Endothermic
Exothermic – These are reactions that occur with the release of heat.
Endothermic- These are reactions that occur with the absorption of heat from the environment.
Based on reversibility, reactions are of the following types:
A) Reversible
B) Irreversible
Reactions that proceed in only one direction and end with the complete conversion of the original reactants into final substances, are called irreversible.
Reversible Reactions that simultaneously occur in two mutually opposite directions are called.
Based on changes in the oxidation state of the atoms that make up the reacting substances, reactions are of the following types:
A) Redox
Reactions that occur with a change in the oxidation state of atoms (in which electrons transfer from one atom, molecule or ion to another) are called redox.
2. According to the mechanism of reaction, reactions are divided into ionic and radical.
Ionic reactions – interaction between ions due to heterolytic rupture chemical bond(a pair of electrons goes entirely to one of the “fragments”).
Ionic reactions are of two types (based on the type of reagent):
A) electrophilic - during a reaction with an electrophile.
Electrophile– a group that has free orbitals or centers with reduced electron density in some atoms (for example: H +, Cl - or AlCl 3)
B) Nucleophilic - during interaction with a nucleophile
Nucleophile – a negatively charged ion or molecule with a lone electron pair (not currently involved in the formation of a chemical bond).
(Examples: F - , Cl - , RO - , I -).
Real chemical processes, only in rare cases can be described simple mechanisms. A detailed examination of chemical processes from a molecular kinetic point of view shows that most of them proceed along a radical chain mechanism; the peculiarity of chain reactions is the formation of free radicals at intermediate stages (unstable fragments of molecules or atoms with a short lifetime, all have free communications.
The processes of combustion, explosion, oxidation, photochemical reactions, and biochemical reactions in living organisms proceed through a chain mechanism.
Chain systems have several stages:
1) chain nucleation - the stage of chain reactions, as a result of which free radicals arise from valence-saturated molecules.
2) continuation of the chain - the stage of the circuit chain, proceeding while maintaining the total number of free stages.
3) chain break - the elementary stage of a chain of processes leading to the disappearance of free bonds.
There are branched and unbranched chain reactions.
One of the most important concepts chains of phenomena chain length- the average number of elementary stages of chain continuation after the appearance of a free radical until its disappearance.
Example: Hydrogen Chloride Synthesis
1) CL 2 absorbs a quantum of energy and the image of radical 2: CL 2 +hv=CL * +CL *
2) the active particle combines with the m-molecule H 2 to form hydrogen chloride and the active particle H 2: CL 1 + H 2 = HCL + H *
3)CL 1 +H 2 =HCL+CL * etc.
6)H * +CL * =HCL - open circuit.
Branched mechanism:
F * +H 2 =HF+H * etc.
F * +H 2 =HF+H * etc.
In water it is more complicated - OH*, O* radicals and the H* radical are formed.
Reactions occurring under the influence ionizing radiation: X-rays, cathode rays and so on – are called radiochemical.
As a result of the interaction of molecules with radiation, the disintegration of molecules is observed with the formation of the most reactive particles.
Such reactions promote the recombination of particles and the formation of substances with different combinations of them.
An example is hydrazine N 2 H 4 - a component of rocket fuel. IN Lately Attempts are being made to obtain hydrazine from ammonia as a result of exposure to γ-rays:
NH 3 → NH 2 * + H*
2NH 2 *→ N 2 H 4
Radiochemical reactions, for example radiolysis of water, are important for the life of organisms.
Literature:
1. Akhmetov, N.S. General and inorganic chemistry / N.S. Akhmetov. – 3rd ed. – M.: graduate School, 2000. – 743 p.
- Korovin N.V. general chemistry/ N.V.Korovin. – M.: Higher School, 2006. – 557 p.
- Kuzmenko N.E. Short course chemistry / N.E. Kuzmenko, V.V. Eremin, V.A. Popkov. – M.: Higher School, 2002. – 415 p.
- Zaitsev, O.S. General chemistry. Structure of substances and chemical reactions / O.S. Zaitsev. – M.: Chemistry, 1990.
- Karapetyants, M.Kh. Structure of matter / M.Kh. Karapetyants, S.I. Drakin. – M.: Higher School, 1981.
- Cotton F. Fundamentals of inorganic chemistry / F. Cotton, J. Wilkinson. – M.: Mir, 1981.
- Ugay, Ya.A. General and inorganic chemistry / Ya.A.Ugai. – M.: Higher School, 1997.
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, during 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. This type of reaction is different high speed, highlighting large quantity 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 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 the 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.
