There are different classification systems for organic reactions, which are based on different characteristics. Among them are the following classifications:
- By the final result of the reaction, that is, a change in the structure of the substrate;
- By reaction mechanism, that is, by the type of bond breaking and the type of reagents.
Substances interacting in an organic reaction are divided into reagent And substrate. In this case, the reagent is considered to attack the substrate.
DEFINITION
Reagent- a substance that acts on an object - a substrate - and causes a change in the chemical bond in it. Reagents are divided into radical, electrophilic and nucleophilic.
DEFINITION
Substrate, is generally considered to be a molecule that provides a carbon atom for a new bond.
CLASSIFICATION OF REACTIONS ACCORDING TO THE FINAL RESULT (CHANGE IN THE STRUCTURE OF THE SUBSTRATE)
In organic chemistry, four types of reactions are distinguished according to the final result and change in the structure of the substrate: addition, substitution, detachment, or elimination(from English to eliminate- remove, split off), and rearrangements (isomerizations)). This classification is similar to the classification of reactions in inorganic chemistry according to the number of initial reagents and resulting substances, with or without a change in composition. Classification according to the final result is based on formal criteria, since the stoichiometric equation, as a rule, does not reflect the reaction mechanism. Let's compare the types of reactions in inorganic and organic chemistry.
Type of reaction in inorganic chemistry | Example | Type of reaction in organic chemistry | Variety and example Reactions |
|---|---|---|---|
1. Connection | C l2 + H2 = 2 H C l | Joining by multiple connections | Hydrogenation |
Hydrohalogenation
|
|||
Halogenation
|
|||
Hydration
|
|||
2. Decomposition | 2
H2
O=2 H2
+
O2
| Elimination | Dehydrogenation
|
Dehydrohalogenation
|
|||
Dehalogenation
|
|||
Dehydration
|
|||
3. Substitution | Z n + 2 H C l =ZnCl2+H2 | Substitution |
|
4. Exchange (special case - neutralization) | H2 S O4 + 2 N a O H=N a 2 S O 4 + 2 H 2 O | special case - esterification |
|
5. Allotropization | graphite ⇔ diamond Pred⇔ Pwhite P red ⇔ P white Srhombus.⇔ Splast. Srhomb.⇔Splastic | Isomerization | Isomerization alkanes
|
n) without replacing them with others.
Depending on which atoms are split off - neighboring ones C–C or isolated by two or three or more carbon atoms – C–C–C– C–, –C–C–C–C– C–, compounds can form with multiple bonds and or cyclic compounds. The elimination of hydrogen halides from alkyl halides or water from alcohols occurs according to Zaitsev’s rule.
DEFINITION
Zaitsev's rule: A hydrogen atom H is removed from the least hydrogenated carbon atom.
For example, the elimination of a hydrogen bromide molecule occurs from neighboring atoms in the presence of an alkali, resulting in the formation of sodium bromide and water.
DEFINITION
Regrouping- a chemical reaction that results in a change in the relative arrangement of atoms in a molecule, the movement of multiple bonds or a change in their multiplicity.
Rearrangement can be carried out while maintaining the atomic composition of the molecule (isomerization) or changing it.
DEFINITION
Isomerization- a special case of a rearrangement reaction leading to the transformation of a chemical compound into an isomer through a structural change in the carbon skeleton.
Rearrangement can also occur by a homolytic or heterolytic mechanism. Molecular rearrangements can be classified according to various criteria, for example, by the saturation of the systems, by the nature of the migrating group, by stereospecificity, etc. Many rearrangement reactions have specific names - Claisen rearrangement, Beckmann rearrangement, etc.
Isomerization reactions are widely used in industrial processes, such as petroleum refining to increase the octane number of gasoline. An example of isomerization is the transformation n-octane to isooctane:
CLASSIFICATION OF ORGANIC REACTIONS BY REAGENT TYPE
DISCONNECTION
Bond cleavage in organic compounds can be homolytic or heterolytic. 
DEFINITION
Homolytic bond cleavage- this is a gap as a result of which each atom receives an unpaired electron and two particles are formed that have a similar electronic structure - free radicals.
A homolytic break is characteristic of nonpolar or weakly polar bonds, such as C–C, Cl–Cl, C–H, and requires a large amount of energy.
The resulting radicals, which have an unpaired electron, are highly reactive, therefore the chemical processes occurring with the participation of such particles are often of a “chain” nature, they are difficult to control, and the reaction results in a set of substitution products. Thus, when methane is chlorinated, the substitution products are chloromethane C H3 C l CH3Cl, dichloromethane C H2 C l2 CH2Cl2, chloroform C H C l3 CHCl3 and carbon tetrachloride C C l4 CCl4. Reactions involving free radicals proceed through the exchange mechanism of the formation of chemical bonds.
The radicals formed during such bond cleavage cause radical mechanism the course of the reaction. Radical reactions usually occur at elevated temperatures or radiation (eg light).
Due to their high reactivity, free radicals can have a negative impact on the human body, destroying cell membranes, affecting DNA and causing premature aging. These processes are associated primarily with lipid peroxidation, that is, the destruction of the structure of polyunsaturated acids that form fat inside the cell membrane.
DEFINITION
Heterolytic bond cleavage- this is a gap in which an electron pair remains with a more electronegative atom and two charged particles are formed - ions: a cation (positive) and an anion (negative).
In chemical reactions, these particles perform the functions of " nucleophiles"("phil" - from gr. be in love) And " electrophiles", forming a chemical bond with the reaction partner according to the donor-acceptor mechanism. Nucleophilic particles provide an electron pair to form a new bond. In other words,
DEFINITION
Nucleophile- an electron-rich chemical reagent capable of interacting with electron-deficient compounds.
Examples of nucleophiles are any anions ( C l− , I− , N O− 3 Cl−,I−,NO3− etc.), as well as compounds having a lone electron pair ( N H3 , H2 O NH3,H2O).
Thus, when a bond is broken, radicals or nucleophiles and electrophiles can be formed. Based on this, three mechanisms of organic reactions occur.
MECHANISMS OF ORGANIC REACTIONS
Free radical mechanism: the reaction is started by free radicals formed when homolytic rupture bonds in a molecule.
The most typical option is the formation of chlorine or bromine radicals during UV irradiation.
1. Free radical substitution
methane bromomethane
Chain initiation
Chain growth
Open circuit
2. Free radical addition
ethene polyethylene
Electrophilic mechanism: the reaction begins with electrophilic particles that receive a positive charge as a result heterolytic rupture communications. All electrophiles are Lewis acids.
Such particles are actively formed under the influence of Lewis acids, which enhance the positive charge of the particle. Most often used A l C l3 , F e C l3 , F e B r3 ,ZnC l2 AlCl3,FeCl3,FeBr3,ZnCl2, performing the functions of a catalyst.
The site of attack of the electrophile particle is those parts of the molecule that have increased electron density, i.e., the multiple bond and the benzene ring.
The general form of electrophilic substitution reactions can be expressed by the equation:
1. Electrophilic substitution
benzene bromobenzene
2. Electrophilic connection
propene 2-bromopropane
propyne 1,2-dichloropropene
The addition to unsymmetrical unsaturated hydrocarbons occurs in accordance with Markovnikov’s rule.
DEFINITION
Markovnikov's rule: addition to unsymmetrical alkenes of molecules of complex substances with the conditional formula HX (where X is a halogen atom or hydroxyl group OH–), the hydrogen atom is added to the most hydrogenated (containing the most hydrogen atoms) carbon atom at the double bond, and X to the least hydrogenated.
For example, the addition of hydrogen chloride HCl to a propene molecule C H3 – C H = C H2 CH3–CH=CH2.
The reaction proceeds by the mechanism of electrophilic addition. Due to the electron-donating influence C H3 CH3-group, the electron density in the substrate molecule is shifted to the central carbon atom (inductive effect), and then along the system of double bonds - to the terminal carbon atom C H2 CH2-groups (mesomeric effect). Thus, the excess negative charge is localized precisely on this atom. Therefore, the attack begins with the hydrogen proton H+ H+, which is an electrophilic particle. A positively charged carbene ion is formed [ C H3 – C H − C H3 ] + + , to which the chlorine anion is added C l− Cl−.
DEFINITION
Exceptions to Markovnikov's rule: the addition reaction proceeds against Markovnikov’s rule if the reaction involves compounds in which the carbon atom adjacent to the carbon atom of the double bond partially absorbs the electron density, that is, in the presence of substituents that exhibit a significant electron-withdrawing effect (–C C l3 , – C N , – C O O H(–CCl3,–CN,–COOH and etc.).
Nucleophilic mechanism: the reaction begins with nucleophilic particles having a negative charge, formed as a result heterolytic rupture communications. All nucleophiles - Lewis's foundations.
In nucleophilic reactions, the reagent (nucleophile) has a free pair of electrons on one of the atoms and is a neutral molecule or anion ( H a l– , O H– , R O− , R S– , R C O O– , R– , C N – , H2 O, R O H, N H3 , R N H2 Hal–,OH–,RO−,RS–,RCOO–,R–,CN–,H2O,ROH,NH3,RNH2 and etc.).
The nucleophile attacks the atom in the substrate with the lowest electron density (i.e., with a partial or complete positive charge). The first step in the nucleophilic substitution reaction is the ionization of the substrate to form a carbocation. In this case, a new bond is formed due to the electron pair of the nucleophile, and the old one undergoes heterolytic cleavage followed by elimination of the cation. An example of a nucleophilic reaction is nucleophilic substitution (symbol SN SN) at a saturated carbon atom, for example alkaline hydrolysis of bromo derivatives.
1. Nucleophilic substitution
2. Nucleophilic addition
ethanal cyanohydrin
source http://foxford.ru/wiki/himiya
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.
It 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, which contains a common pair of electrons. When a bond is broken, the fate of these shared electrons can be different.
Exchange mechanism of covalent bond formation. Homolytic bond cleavage
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:
N· + ·N -> N: N, or N-N
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.
The mechanism of covalent bond cleavage, in which free radicals are formed, is called hemolytic or homolysis (homo - identical, i.e. this type of bond cleavage leads to the formation of identical particles).
Reactions that occur under the influence and with the participation of free radicals are called free radical reactions.
The hydroxyl anion is attracted to the carbon atom (attacks the carbon atom) on which the partial positive charge is concentrated, and replaces the bromine, or more precisely, the bromide anion.
In the 1-chloropropane molecule, the electron pair in the C-Cl bond is shifted towards the chlorine atom due to its greater electronegativity. In this case, the carbon atom, which has received a partial positive charge (§+), draws electrons from the associated carbon atom, which, in turn, from the following:
Thus, the inductive effect is transmitted through the circuit, but quickly fades: it is practically not observed after three st-connections.
Let's consider another reaction - the addition of hydrogen bromide to ethene:
CH2=CH2 + HBr -> CH3-CH2Br
At the initial stage of this reaction, a hydrogen cation is added to a molecule containing a multiple bond:
CH2=CH2 + H+ -> CH2-CH3
The electrons of the n-bond shifted to one carbon atom, and the neighboring one had a positive charge, an unfilled orbital.
The stability of such particles is determined by how well the positive charge on the carbon atom is compensated. This compensation occurs due to a shift in the electron density of the a-bond towards the positively charged carbon atom, i.e., a positive inductive effect (+1). 
The group of atoms, in this case the methyl group, from which the electron density is drawn, has a donor effect, which is denoted +1.
Mesomeric effect. There is another way that some atoms or groups influence others - the mesomeric effect, or the conjugation effect.
Consider the 1,3 butadiene molecule:
CH2=CH CH=CH2
It turns out that the double bonds in this molecule are not just two double bonds! Since they are nearby, there is overlap P-bonds included in neighboring double bonds, and a common bond is formed for all four carbon atoms P-electron cloud. In this case, the system (molecule) becomes more stable. This phenomenon is called conjugation (in this case P - P- pairing). 
Additional overlap, the conjugation of n-bonds separated by one o-bond, leads to their “averaging.” The central simple bond acquires a partial “double” character, becomes stronger and shorter, and the double bonds become somewhat weakened and lengthened.
Another example of conjugation is the effect of a double bond on an atom that has a lone pair of electrons.
So, for example, when a carboxylic acid dissociates, the lone electron pair remains on the oxygen atom:
This leads to an increase in the stability of the anion formed during dissociation and an increase in the strength of the acid.
The shift in electron density in conjugated systems involving n-bonds or lone electron pairs is called the mesomeric effect (M).
Basic reaction mechanisms
We have identified three main types of reacting particles - free radicals, electrophiles, nucleophiles and three corresponding types of reaction mechanisms:
Free radicals;
electrophilic;
nucleophilic.
In addition to classifying reactions according to the type of reacting particles, in organic chemistry there are four types of reactions based on the principle of changing the composition of molecules: addition, substitution, elimination, or elimination (from English to eliminate - remove, split off), and rearrangement. Since addition and substitution can occur under the influence of all three types of reactive particles, several main reaction mechanisms can be distinguished.
In addition, we will consider elimination reactions, which occur under the influence of nucleophilic particles - bases.
1. What are homolytic and heterolytic cleavages of a covalent bond? What mechanisms of covalent bond formation are they typical for?
2. What are called electrophiles and nucleophiles? Give examples of them.
3. What are the differences between mesomeric and inductive effects? How do these phenomena illustrate the position of A. M. Butlerov’s theory of the structure of organic compounds about the mutual influence of atoms in the molecules of organic substances?
4. In the light of the concepts of inductive and mesomeric effects, consider the mutual influence of atoms in molecules:
Support your conclusions with examples of chemical reaction equations.
Lesson content lesson notes supporting frame lesson presentation acceleration methods interactive technologies Practice tasks and exercises self-test workshops, trainings, cases, quests homework discussion questions rhetorical questions from students Illustrations audio, video clips and multimedia photographs, pictures, graphics, tables, diagrams, humor, anecdotes, jokes, comics, parables, sayings, crosswords, quotes Add-ons abstracts articles tricks for the curious cribs textbooks basic and additional dictionary of terms other Improving textbooks and lessonscorrecting errors in the textbook updating a fragment in a textbook, elements of innovation in the lesson, replacing outdated knowledge with new ones Only for teachers perfect lessons calendar plan for the year; methodological recommendations; discussion programs Integrated LessonsCH 3 -CH 3 + Cl 2 – (hv) ---- CH 3 -CH 2 Cl + HCl
C 6 H 5 CH 3 + Cl 2 --- 500 C --- C 6 H 5 CH 2 Cl + HCl
Addition reactions
Such reactions are typical for organic compounds containing multiple (double or triple) bonds. Reactions of this type include reactions of addition of halogens, hydrogen halides and water to alkenes and alkynes
CH 3 -CH=CH 2 + HCl ---- CH 3 -CH(Cl)-CH 3
Elimination reactions
These are reactions that lead to the formation of multiple bonds. When eliminating hydrogen halides and water, a certain selectivity of the reaction is observed, described by Zaitsev's rule, according to which a hydrogen atom is eliminated from the carbon atom at which there are fewer hydrogen atoms. Example reaction
CH3-CH(Cl)-CH 2 -CH 3 + KOH →CH 3 -CH=CH-CH 3 + HCl
Polymerization and polycondensation
n(CH 2 =CHCl) (-CH 2 -CHCl)n
Redox
The most intense of the oxidative reactions is combustion, a reaction characteristic of all classes of organic compounds. In this case, depending on the combustion conditions, carbon is oxidized to C (soot), CO or CO 2, and hydrogen is converted into water. However, for organic chemists, oxidation reactions carried out under much milder conditions than combustion are of great interest. Oxidizing agents used: solutions of Br2 in water or Cl2 in CCl 4 ; KMnO 4 in water or dilute acid; copper oxide; freshly precipitated silver(I) or copper(II) hydroxides.
3C 2 H 2 + 8KMnO 4 +4H 2 O→3HOOC-COOH + 8MnO 2 + 8KOH
Esterification (and its reverse hydrolysis reaction)
R 1 COOH + HOR 2 H+ R 1 COOR 2 + H 2 O
Cycloaddition
Y R Y-R
‖ + ‖ → ǀ ǀ
R Y R-Y
‖ + →
11. Classification of organic reactions by mechanism. Examples.
The reaction mechanism involves a detailed step-by-step description of chemical reactions. At the same time, it is established which covalent bonds are broken, in what order and in what way. The formation of new bonds during the reaction process is also carefully described. When considering the reaction mechanism, first of all, pay attention to the method of breaking the covalent bond in the reacting molecule. There are two such ways - homolytic and heterolytic.
Radical reactions proceed by homolytic (radical) cleavage of a covalent bond:
Non-polar or low-polar covalent bonds (C–C, N–N, C–H) undergo radical cleavage at high temperatures or under the influence of light. The carbon in the CH 3 radical has 7 outer electrons (instead of a stable octet shell in CH 4). Radicals are unstable; they tend to capture the missing electron (up to a pair or up to an octet). One of the ways to form stable products is dimerization (the combination of two radicals):
CH 3 + CH 3 CH 3 : CH 3,
N + N N : N.
Radical reactions - these are, for example, reactions of chlorination, bromination and nitration of alkanes:
Ionic reactions occur with heterolytic bond cleavage. In this case, short-lived organic ions - carbocations and carbanions - with a charge on the carbon atom are intermediately formed. In ionic reactions, the bonding electron pair is not separated, but passes entirely to one of the atoms, turning it into an anion:
Strongly polar (H–O, C–O) and easily polarizable (C–Br, C–I) bonds are prone to heterolytic cleavage.
Distinguish nucleophilic reactions (nucleophile– looking for the nucleus, a place with a lack of electrons) and electrophilic reactions (electrophile– looking for electrons). The statement that a particular reaction is nucleophilic or electrophilic always refers to the reagent. Reagent– a substance participating in the reaction with a simpler structure. Substrate– a starting substance with a more complex structure. Outgoing group is a replaceable ion that has been bonded to carbon. Reaction product– new carbon-containing substance (written on the right side of the reaction equation).
TO nucleophilic reagents(nucleophiles) include negatively charged ions, compounds with lone pairs of electrons, compounds with double carbon-carbon bonds. TO electrophilic reagents(electrophiles) include positively charged ions, compounds with unfilled electron shells (AlCl 3, BF 3, FeCl 3), compounds with carbonyl groups, halogens. Electrophiles are any atom, molecule or ion capable of adding a pair of electrons in the process of forming a new bond. The driving force of ionic reactions is the interaction of oppositely charged ions or fragments of different molecules with a partial charge (+ and –).
Examples of different types of ionic reactions.
Nucleophilic substitution :
Electrophilic substitution :
Nucleophilic addition (CN – is added first, then H +):
Electrophilic connection (H + is added first, then X –):
Elimination by the action of nucleophiles (bases) :
Elimination upon action electrophiles (acids) :
| Parameter name | Meaning |
| Article topic: | Mechanisms of organic reactions |
| Rubric (thematic category) | Education |
Classification of reactions
There are four main types of reactions in which organic compounds participate: substitution (displacement), addition, elimination (elimination), rearrangements.
3.1 Substitution reactions
In the first type of reaction, the substitution usually occurs at a carbon atom, but the substituted atom must be a hydrogen atom or some other atom or group of atoms. During electrophilic substitution, the hydrogen atom is most often replaced; An example is the classic aromatic substitution:
With nucleophilic substitution, it is not the hydrogen atom that is most often replaced, but other atoms, for example:
NC - + R−Br → NC−R +BR -
3.2 Addition reactions
Addition reactions can also be electrophilic, nucleophilic or radical based on the type of species initiating the process. Attachment to ordinary carbon-carbon double bonds is usually induced by an electrophile or radical. For example, the addition of HBr
may begin with an attack of the double bond by the H+ proton or the Br· radical.
3.3 Elimination reactions
Elimination reactions are essentially the reverse of addition reactions; The most common type of such reaction is the elimination of a hydrogen atom and another atom or group from neighboring carbon atoms to form alkenes:
3.4 Rearrangement reactions
Rearrangements can also occur through intermediate compounds that are cations, anions or radicals; Most often, these reactions occur with the formation of carbocations or other electron-deficient particles. Rearrangements may involve significant restructuring of the carbon skeleton. The rearrangement step itself in such reactions is often followed by substitution, addition or elimination steps, leading to the formation of a stable final product.
A detailed description of a chemical reaction by stages is usually called a mechanism. From an electronic point of view, the mechanism of a chemical reaction is understood as the method of breaking covalent bonds in molecules and the sequence of states through which reacting substances pass before becoming reaction products.
4.1 Free radical reactions
Free radical reactions are chemical processes in which molecules with unpaired electrons take part. Certain aspects of free radical reactions are unique compared to other types of reactions. The main difference is that many free radical reactions are chain reactions. This means that there is a mechanism by which many molecules are converted into a product through a repeating process initiated by the creation of a single reactive species. A typical example is illustrated using the following hypothetical mechanism:
The stage at which the reaction intermediate, in this case A·, is generated is usually called initiation. This stage occurs at high temperatures, under the influence of UV or peroxides, in non-polar solvents. The next four equations in this example repeat the sequence of two reactions; they represent the development phase of the chain. Chain reactions are characterized by the length of the chain, which corresponds to the number of development stages per initiation stage. The second stage occurs with simultaneous synthesis of the compound and the formation of a new radical, which continues the chain of transformations. The last step is the chain termination step, which involves any reaction in which one of the reaction intermediates necessary for chain progression is destroyed. The more stages of chain termination, the shorter the chain length becomes.
Free radical reactions occur: 1) in the light, at high temperatures or in the presence of radicals that are formed during the decomposition of other substances; 2) inhibited by substances that easily react with free radicals; 3) occur in non-polar solvents or in the vapor phase; 4) often have an autocatalytic and induction period before the start of the reaction; 5) kinetically they are chain.
Radical substitution reactions are characteristic of alkanes, and radical addition reactions are characteristic of alkenes and alkynes.
CH 4 + Cl 2 → CH 3 Cl + HCl
CH 3 -CH=CH 2 + HBr → CH 3 -CH 2 -CH 2 Br
CH 3 -C≡CH + HCl → CH 3 -CH=CHCl
The connection of free radicals with each other and chain termination occurs mainly on the walls of the reactor.
4.2 Ionic reactions
Reactions in which it occurs heterolytic the breaking of bonds and the formation of intermediate particles of the ionic type are called ionic reactions.
Ionic reactions occur: 1) in the presence of catalysts (acids or bases and are not affected by light or free radicals, in particular those arising from the decomposition of peroxides); 2) are not affected by free radical scavengers; 3) the nature of the solvent influences the course of the reaction; 4) rarely occur in the vapor phase; 5)kinetically they are mainly first- or second-order reactions.
Based on the nature of the reagent acting on the molecule, ionic reactions are divided into electrophilic And nucleophilic. Nucleophilic substitution reactions are characteristic of alkyl and aryl halides,
CH 3 Cl + H 2 O → CH 3 OH + HCl
C 6 H 5 -Cl + H 2 O → C 6 H 5 -OH + HCl
C 2 H 5 OH + HCl → C 2 H 5 Cl + H 2 O
C 2 H 5 NH 2 + CH 3 Cl → CH 3 -NH-C 2 H 5 + HCl
electrophilic substitution – for alkanes in the presence of catalysts
CH 3 -CH 2 -CH 2 -CH 2 -CH 3 → CH 3 -CH(CH 3)-CH 2 -CH 3
and arenas.
C 6 H 6 + HNO 3 + H 2 SO 4 → C 6 H 5 -NO 2 + H 2 O
Electrophilic addition reactions are characteristic of alkenes
CH 3 -CH=CH 2 + Br 2 → CH 3 -CHBr-CH 2 Br
and alkynes,
CH≡CH + Cl 2 → CHCl=CHCl
nucleophilic addition – for alkynes.
CH 3 -C≡CH + C 2 H 5 OH + NaOH → CH 3 -C(OC 2 H 5) = CH 2
Mechanisms of organic reactions - concept and types. Classification and features of the category "Mechanisms of organic reactions" 2017, 2018.
