LECTURE 4
Classification and
mechanisms
organic reactions

Plan
4.1. Organic classification
reactions
4.2. Classification of reagents
4.3 Reactions
(SR)
radical
replacement
4.4 Electrophilic addition reactions (AE)

4.1 Classification of organic reactions

4.1 Classification
organic reactions
towards
by molecularity
S substitution reactions
Addition reactions A
Elimination reactions
E
Molecular
rearrangements
Monomolecular
Bimolecular
Trimolecular

According to the method of breaking and forming bonds

Heterolytic
(ionic)
* electrophilic
* nucleophilic
Homolytic
(radical)
Molecular

Scheme of breaking chemical bonds

A:B
+
AT:
.
.
BUT
A:B
heterolytic
A: B
g ohm lytic
A + B
glad ikala
+
+ V:
BUT
e associated ions

Scheme of the formation of chemical bonds

+
BUT
.
+ V:
A + B
.
BUT
AT
heterolytic
BUT
AT
homolytic.

heterolytic reactions
called ionic because
they are accompanied
the formation of organic
ions flow into
organic solvents
Homolytic reactions
flow predominantly in
gas phase

Heterolytic reactions in
dependence on electronic
the nature of the attacking particle
divided into nucleophiles (symbol
N) and electrophilic (symbol E).
At the same time, it is conventionally assumed
one of the interacting particles
reagent and the other substrate
on which the reagent acts

A substrate is a molecule that
provides a carbon atom
formation of a new connection
type of reaction (nucleophilic
or electrophilic) is determined by the nature of the reagent

Reagent with lone
electron pair,
interacting with
substrate that has
lack of electrons
called "nucleophilic"
(loving, looking for the core), and
nucleophilic reactions

Reagent with electronic deficit,
interacting with
a substrate with an excess of electrons
called
"electrophilic" and
electrophilic reaction

Nucleophilic and
electrophilic reactions are always
interconnected
reactions accompanied by
simultaneous
(consensual) gap and
bonding is called
molecular (synchronous,
agreed)

diene synthesis

CH 2
HC
CH 2
+
HC
CH 2
CH 2
Cyclog exen

4.2. Classification of reagents

4.2. Classification of reagents
To nucleophilic reagents
include molecules that contain
one or more unshared
pairs of electrons; ions that carry
negative charge (anions);
molecules with centers
increased density

Nucleophilic reagents

neutral molecules,
having lone pairs
electrons:
..
..
..
..
NH3; R - NH2; R2 - NH; R3N;
..
H2O;
..
..
R-OH;
..
..
;
R-O
R
..
anions:
OH-; CN-; NH2-; RCOO-; RS-; Cl-;
Br-; I-; HSO3-;

Nucleophilic reagents

connections,
containing centers with
increased electron density:
C
C
;
C
C
;

Electrophilic reagents

neutral molecules,
having a vacant orbital:
SO3, Lewis acids (AlCl3,
SnCl4, FeBr3, BF3)
cations: proton (H+), ions
metals (Men+), SO3H+, NO2+, NO+

molecules,
having
centers
With
reduced electron density:
halogen derivatives of hydrocarbons Rδ+-
Halδ-, halogens (Cl2, Br2, I2), compounds with
carbonyl group:
R
C
O
;
H
R
C
O
;
R1
R
C
O
; R
Oh
C
O
;
OR

In organic chemistry reactions,
usually take place in
several stages, i.e. With
the formation of intermediate
short-lived particles
(intermediates): carbocations,
carbanions, radicals

Carbocations - positive
charged particles, atom
carbon bearing positive
the charge is in sp2 -
hybridization.
Carbon atom with acquisition
positive charge changes
its valence state from sp3 to
sp2, which is energetically more
profitable.

An important characteristic
carbocations is their
sustainability, which
determined by the degree
delocalization
positive charge

Carbocation stability
falls in the line:
tertiary
atom C
>
secondary
atom C
>
primary
atom C

Carbocation stability

+
CH3 CH3
m ethylium
cation
+
CH2
ethylium
cation
CH3
CH3
+
CH
isopropylium
cation
CH3
CH3
INCREASED STABILITY
+
C
CH3
tertbutylium
cation

Carbanions - negative
charged particles, charge
which is due to the presence in them
structure of the C atom with a lone
electronic pair. At the same time, the atom
carbon bearing negative
charge, can be both in sp2 and
in sp3 hybridization

The stability of carbanions depends on
degree of delocalization of the negative
charge on the carbon atom. Than she
higher, the higher their stability and the
lower their reactivity.
The most stable cyclic
carbanions, in the structure of which
there is a common π-electron
density, including
4n+2 π-electrons

cyclopentadienyl anion

Free radicals - any
electrically neutral active
particle containing
one-electron orbital.
Free radicals can
be assigned particles,
containing an unpaired electron
not only on the carbon atom (C ), but
and on other atoms: R2N· ; RO

4.3. Radical substitution reactions (SR)

4.3. Reactions of the radical
substitution (SR)
SR reactions are characteristic of
compounds of aliphatic and
alicyclic series. How
as a rule, they flow
chain mechanism, the main
the stages of which are:
initiation, development (growth
chain) and open circuit.

At the initiation stage
free radicals are formed
starting a chain
process
Free radicals can
occur due to thermal
or photochemical
initiation, as well as
as a result of OB reactions

Radical substitution reactions (SR)

R-H+A-A
substrate
reagent
h
R-A+HA
product
reactions

reaction mechanism
radical substitution (SR)
1. Initiation
A-A
h
.
2A

2. Chain development

.
A
.
+R-H
R+A-A
.
R
+AH
R-A+
.
A

3. Open circuit
.
R
.
A
.
A
+
.
R
R-R
+
.
R
R-A
+
.
A
A-A

The ease of detachment of the H atom from the carbon atom falls in the series of hydrocarbons

CH3
CH3
H3C
C
CH3
H>H3C
C
H
H
H
H>H3C
C
H
H > H
C
H
H

Bromine radicals (Br˙) have
high selectivity: if
molecule has a secondary, and
especially the tertiary carbon atom,
then bromination is predominantly
goes to the tertiary (secondary)
carbon atom. Such reactions
called regioselective
(selective by place
actions) reactions

Bromination of alkanes (regioselective reactions)

H3C
CH
H
CH3 + Br2
h
H3C
CH
CH3 + HBr
Br
2-bromopropane

reaction mechanism
bromination of alkanes
1. Initiation
Br2
h
.
2Br

2. Chain development
.
Br + H3C
CH
CH3
H3C
.
CH
CH3 + HBr
H
Br2 + H3C
.
CH
CH3
H3C
CH
Br
.
CH3 + Br

3. Open circuit
.
.
H3C
CH3 + Br
CH
H3C
CH
CH3
Br
.
Br
H3C
.
Br2
+Br
.
.
CH+H3C
CH
CH3
CH3
H3C
CH
CH
CH3
CH3
2,3-dim ethylbutane
CH3

4.4. Electrophilic addition reactions

Electrophilic addition (AE)
characteristic of unsaturated systems,
containing double or triple bonds.
The nucleophilic nature of these
compounds due to the presence of a π-bond,
which is an area with
increased electron density,
is polarizable and easily
breaks down under
electrophilic reagents

AE reaction mechanism

+ X
C=C
substrate
Y
reagent
X
C
+
C
-complex
+Y
C=C
X
Y
-complex
X
C
C
Y

Halogenation

H
H
C=C
H
+Br
Br
H
H
C=C
H
H
Br
Br
CH2
H2C
+
Br
onium bromine
cation
+Br
H2C
CH2
Br
1,2-d ibromo ethane
H
Br

hydrogenation
H
C=C
+ H2
t, Kt
C
C
H
Hydrohalogenation
Cl
C=C
+ HCl
C
H
C

Hydration
Oh
C=C
+HOH
H
+
C
H
C

Markovnikov's rule:
when interacting
HX-type reagents with
asymmetrical
alkenes, hydrogen
joins
most
hydrogenated Vladimir
Markovnikov
carbon atom
(1837 – 1904)

Hydrohalogenation of alkenes
Morkovnikov's rule
CH3 CH = CH2 + HCl
CH3
CH
Cl
2-chloropropane
CH3

reaction mechanism
hydrohalogenation
CH3
CH3
+
+
CH
CH3
CH2
+
CH2
CH = CH2 + H
CH3
CH3
CH
Cl
CH3
+Cl
-

Alkene hydration reaction scheme

Scheme of the hydration reaction
alkenes
+
H2C = CH2 + H2O
H
H3C
CH2
Oh
ethanol

Hydration Reaction Mechanism
alkenes
..
+
+HOH
..
+
H C = CH + H
H C CH
2
2
H3C
3
CH2
+
O
H
+
-H
return
catalyst
H
Oxonium cation
2
H3C
CH2
Oh

classic rule
Markovnikova is perfect
applicable only to
alkenes, in the case of their
derivatives needed
take into account the mechanism
reactions and stability
formed intermediates

Hydration reaction mechanism of unsaturated carboxylic acids against Morkovnikov's rule

R
R
CH=CH
+
CH
O
CH2
C
Oh
+
+ H
C
O
Oh
R
CH2
+
CH
C
O
Oh

..
HOH
..
O
R
CH
+
O
H
H
CH2
C
O
R
-H+
CH
CH2
C
Oh return
catalyst
Oh
Oh
-hydroxy acid

This type of hydration in
vivo is part of the process
β-oxidation of unsaturated
fatty acids in the body

Related systems
(alkadienes)
thermodynamically the most
stable, so often
are found in nature.
Reactions of AE with such dienes
proceed with the formation of two
products
1,4- and 1,2-attachments

AE reactions in the alkadiene series

1, 4
H2C=CH
CH = CH2 + HCl
H3C
CH=CH
CH2Cl
1-chlorobutene-2
1, 2
H3C
CH
Cl
3-chlorobutene-1
CH=CH2

AE reactions in the alkadiene series Reaction mechanism

+
H3C
H2C=CH
CH = CH2 + H+
H3C Hydration reaction mechanism
acetylene derivatives
H3C
C
+
CH+H
H3C
+
C=CH2
..
+HOH
..

Hydration Reaction Mechanism
acetylene derivatives
H3C
C=CH2
+
O
H
-H+
H3C
C=CH2
Oh
H

The reactions of organic substances can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization).

Obviously, the whole variety of reactions of organic compounds cannot be reduced to the proposed classification (for example, combustion reactions). However, such a classification will help to establish analogies with the reactions already familiar to you that occur between inorganic substances.

As a rule, the main organic compound involved in the reaction is called substrate, and the other component of the reaction is conditionally 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 action 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 reagents, catalyst, reaction conditions write above the arrow, and inorganic reaction products- under it.

Addition reactions

Addition reactions are reactions in which two or more molecules of reactants combine into one.

Unsaturated compounds, such as alkenes or alkynes, enter into 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- the reaction of adding a hydrogen molecule to a multiple bond:

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

3. Halogenation- halogen addition reaction:

4. Polymerization- a special type of addition reactions, during which molecules of a substance with a small molecular weight are combined 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 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 by overlapping atomic orbitals and the formation of common electron pairs. As a result of this, an orbital common to two atoms is formed, on which a common electron pair is located. When the bond is broken, the fate of these common electrons can be different.

Types of reactive particles in organic chemistry

An orbital with an unpaired electron belonging to one atom can overlap with an orbital of another atom that also contains an unpaired electron. This is where education takes place covalent bond by 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 disconnection at which one electron goes to each atom. As a result, two uncharged particles with unpaired electrons are formed:

Such particles are called free radicals.

free radicals- atoms or groups of atoms having unpaired electrons.

Free radical reactions are reactions that occur under the action and with the participation of free radicals.

I know inorganic chemistry these are reactions of interaction of hydrogen with oxygen, halogens, combustion reactions. Reactions of this type are characterized by high speed, release of a large amount of heat.

A covalent bond can also form donor-acceptor mechanism. One of the orbitals of an atom (or anion), which contains a lone electron pair, overlaps with an unfilled orbital of another atom (or cation), which has an unfilled orbital, while forming covalent bond, for example:

Breaking 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:

Consider electrolytic dissociation of acids:

One can easily guess that a particle having lone electron pair 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 electron pairs are called nucleophilic agents(nucleus - "nucleus", the positively charged part of the atom), that is, the "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 on which an effective positive charge is concentrated.

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

Particles that have unfilled orbital, on the contrary, will tend to fill it and, therefore, will be attracted to the regions of the molecules where there is an increased electron density, a negative charge, an unshared 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 every particle is an electrophile with an empty orbital. For example, cations alkali metals have the configuration of inert gases and do not tend to acquire electrons, as 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.

Main reaction mechanisms

There are three main types of reacting particles - free radicals, electrophiles, nucleophiles- and three corresponding types of reaction mechanism:

Free radical;

Electrophilic;

Nuleophilic.

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

1. Free radical substitution:

2. Free radical addition:

3. Electrophilic substitution:

4. Electrophilic addition:

5. Nucleophilic addition:

In addition, consider the cleavage or elimination reactions that take place under the influence of nucleophilic particles - bases.

6. Elimination:

Rule of V. V. Markovnikov

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

Hydrohalogenation (addition of hydrogen halide):

This reaction obeys the rule of V. V. Markovnikov.

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

Reference material for passing the test:

periodic table

Solubility table

It is formed when atomic orbitals overlap and the formation of common electron pairs. As a result of this, an orbital common to two atoms is formed, on which a common pair of electrons is located. When the bond is broken, the fate of these common electrons can be different.

Exchange mechanism for the formation of a covalent bond. Homolytic bond breaking

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, the formation of a covalent bond occurs according to the exchange mechanism:

H + H -> H: H, or H-H

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 bond breaking, in which one electron goes to each atom. As a result, two uncharged particles with unpaired electrons are formed:

Such particles are called free radicals.

free radicals- atoms or groups of atoms having unpaired electrons.

The mechanism of breaking a covalent bond, in which free radicals are formed, is called hemolytic or homolysis (homo is the same, that is, this type of bond breaking leads to the formation of identical particles).

Reactions that take place under the action 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, 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 carbon atom associated with it, which, in turn, from the following:

Thus, the inductive effect is transmitted along the chain, but quickly decays: it is practically not observed already after three st-couplings.

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 have shifted to one carbon atom, the neighboring one has 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 the shift in the electron density of the a-bond towards the positively charged carbon atom, i.e., the 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 by +1.

mesomeric effect. There is another way of influence of some atoms or groups on others - the mesomeric effect, or the conjugation effect.

Consider a 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 close, there is an overlap P-bonds that make up neighboring doubles, and a common for all four carbon atoms is formed P- electron cloud. In this case, the system (molecule) becomes more stable. This phenomenon is called conjugation (in this case P - P- conjugation).

Additional overlap, 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 somewhat weaken and lengthen.

Another example of conjugation is the effect of a double bond on an atom that has an unshared electron pair.

So, for example, during the dissociation of a carboxylic acid, the unshared 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 unshared electron pairs is called the mesomeric effect (M).

Main reaction mechanisms

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

Free radical;
electrophilic;
nucleophilic.

In addition to classifying reactions according to the type of reacting particles, organic chemistry distinguishes four types of reactions according to 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 action of all three types of reactive particles, several main reaction mechanisms can be distinguished.

In addition, we will consider the cleavage or elimination reactions that take place under the influence of nucleophilic particles - bases.

1. What are homolytic and heterolytic breaks of a covalent bond? What mechanisms of covalent bond formation are they characteristic of?

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 on 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.

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

As a rule, the main organic compound involved in the reaction is called substrate, and the other component of the reaction is conditionally 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 action 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 recording, the reagents, catalyst, reaction conditions are written above the arrow, and the inorganic reaction products 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 are reactions in which two or more molecules of reactants combine into one.

Unsaturated compounds, such as alkenes or alkynes, enter into 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- the reaction of adding a hydrogen molecule to a multiple bond:

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

3. Halogenation- halogen addition reaction:

4.Polymerization- a special type of addition reactions, during which molecules of a substance with a small molecular weight are combined with each other to form molecules of a substance with a very high molecular weight - macromolecules.

Polymerization reactions are the 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 common electron pairs. As a result of this, an orbital common to two atoms is formed, on which a common electron pair is located. When the bond is broken, the fate of these common 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, the formation of a covalent bond occurs 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 bond breaking, in which one electron () goes to each atom. As a result, two uncharged particles with unpaired electrons are formed:

Such particles are called free radicals.

free radicals- atoms or groups of atoms having unpaired electrons.

Free radical reactions are reactions that occur under the action and with the participation of free radicals.

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

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

Breaking 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:

Consider the electrolytic dissociation of acids:

It can be easily guessed that a particle having an unshared electron pair 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 unshared electron pairs are called nucleophilic agents (nucleus- "nucleus", the positively charged part of the atom), that is, the "friends" of the nucleus, a positive charge.

Nucleophiles(Nu) - anions or molecules that have a lone pair of electrons, interacting with the regions of the molecules, on which the effective positive charge is concentrated.

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 the regions of the molecules that have an increased electron density, a negative charge, and an unshared electron pair. They are electrophiles, "friends" of an electron, a negative charge, or particles with an 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 every particle is an electrophile with an empty orbital. So, 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.

Main reaction mechanisms

There are three main types of reacting particles - free radicals, electrophiles, nucleophiles - and three corresponding types of reaction mechanism:

• free radical;
• electrophilic;
• nullophilic.

In addition to classifying reactions according to the type of reacting particles, organic chemistry distinguishes four types of reactions according to the principle of changing the composition of molecules: addition, substitution, elimination, or elimination (from the English. to eliminate- delete, split off) and regroup. Since addition and substitution can occur under the action of all three types of reactive species, several majorreaction mechanisms.

In addition, consider the cleavage or elimination reactions that take place under the influence of nucleophilic particles - bases.
6. Elimination:

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

Hydrohalogenation (addition of halogen hydrogen):

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

Reaction classification

There are four main types of reactions in which organic compounds participate: substitution (displacement), addition, elimination (cleavage), rearrangement.

3.1 Substitution reactions

In reactions of the first type, substitution usually occurs at the carbon atom, but the substituted atom may be a hydrogen atom or some other atom or group of atoms. In electrophilic substitution, a hydrogen atom is most often replaced; an example is classical aromatic substitution:

In nucleophilic substitution, it is more often not the hydrogen atom that is 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, depending on the type of species initiating the process. Attachment to conventional carbon-carbon double bonds is usually induced by an electrophile or a radical. For example, adding HBr

may begin with an attack on 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 intermediates that are cations, anions, or radicals; most often these reactions go with the formation of carbocations or other electron-deficient particles. The rearrangements may involve a significant rearrangement of the carbon skeleton. The actual rearrangement step in such reactions is often followed by substitution, addition, or elimination steps leading to the formation of a stable end product.

Detailed description chemical reaction by stages is called a mechanism. From an electronic point of view, the mechanism of a chemical reaction is understood as a method of breaking covalent bonds in molecules and the sequence of states through which reactants pass before being converted into 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 repetitive process initiated by the creation of a single reactive species. A typical example is illustrated with the following hypothetical mechanism:

The stage at which the reaction intermediate is generated, in this case A·, is called initiation. This stage takes place during high temperature, under the action of UV or peroxides, in non-polar solvents. In the next four equations this example the sequence of two reactions is repeated; they represent the development phase of the chain. chain reactions characterized by a chain length that corresponds to the number of developmental stages per initiation stage. The second stage proceeds with the 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 includes any reaction that destroys one of the reaction intermediates necessary for chain propagation. The more stages of chain termination, the shorter the chain length becomes.

Free radical reactions proceed: 1) in the light, at high temperature or in the presence of radicals, which are formed during the decomposition of other substances; 2) inhibited by substances that easily react with free radicals; 3) proceed 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 \u003d 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

The reactions in which heterolytic rupture of bonds and the formation of intermediate particles of the ionic type are called ionic reactions.

Ionic reactions proceed: 1) in the presence of catalysts (acids or bases and are not affected by light or free radicals, in particular, arising from the decomposition of peroxides); 2) are not affected by free radical scavengers; 3) the nature of the solvent affects the course of the reaction; 4) rarely occur in the vapor phase; 5) kinetically, they are mainly reactions of the first or second order.

According to 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 \u003d 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