Mechanisms of chemical reactions in organic chemistry. Types of chemical reactions in organic chemistry - Knowledge Hypermarket

>> Chemistry: Types of chemical reactions in organic chemistry

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

As a rule, the main organic compound participating in the reaction is called the substrate, and the other component of the reaction is conditionally considered as a reagent.

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.

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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
+
IN:
.
.
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
IN
heterolytic
BUT
IN
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
from
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. from
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

Classification of reactions According to the number of initial and final substances: 1. Accession 2. Elimination (elimination) 3. Substitution

Classification of reactions According to the mechanism of bond breaking: 1. Homolytic (radical) radicals 2. Heterolytic (ionic) ions

Reaction mechanism Mechanism - a detailed description of a chemical reaction by stages, indicating intermediate products and particles. Reaction scheme: Reaction mechanism:

Classification of reactions according to the type of reagents 1. Radical A radical is a chemically active particle with an unpaired electron. 2. Electrophilic An electrophile is an electron-deficient particle or molecule with an electron-deficient atom. 3. Nucleophilic A nucleophile is an anion or a neutral molecule that has an atom with an unshared electron pair.

Types of chemical bonds in organic substances The main type of bond is covalent (ionic is less common) Sigma bond (σ-): Pi bond (-)

ALKANE - aliphatic (fatty) hydrocarbons "Alifatos" - oil, fat (Greek). Cn. H 2 n+2 Limit, saturated hydrocarbons

Homologous series: CH 4 - methane C 2 H 6 - ethane C 3 H 8 - propane C 4 H 10 - butane C 5 H 12 - pentane, etc. C 6 H 14 - hexane C 7 H 16 - heptane C 8 H 18 - octane C 9 H 20 - nonane C 10 H 22 - decane and C 390 H 782 - nonocontactican (1985)

Atomic Orbital Model of the Methane Molecule In the methane molecule, the carbon atom no longer has S- and P-orbitals! Its 4 hybrid SP 3 orbitals, which are equivalent in energy and shape, form 4 bonds with the S orbitals of the hydrogen atom. H H 4 -bonds

Nitration reaction Konovalov Dmitry Petrovich (1856 -1928) 1880. The first successful attempt to revive the "chemical dead", which were considered alkanes. Found the conditions for the nitration of alkanes. Rice. Source: http: //images. yandex. ru.

Chemical properties I. Reactions with cleavage of C-H bonds (substitution reactions): 1. halogenation 2. nitration 3. sulfochlorination II. Reactions with rupture of C-C bonds: 1. combustion 2. cracking 3. isomerization

How to find a chemist? If you want to find a chemist, ask what a mole and non-ionized are. And if he starts talking about fur animals and the organization of labor, calmly leave. Fiction writer, popularizer of science Isaac Asimov (1920–1992) Fig. Source: http: //images. yandex. ru.

1. Halogenation reaction Chlorination: RH + Cl 2 hv RCl + HCl Bromination: RH + Br 2 hv RBr + HBr For example, methane chlorination: CH 4 + Cl 2 CH 3 Cl + HCl

Stages of the free-radical mechanism Reaction scheme: CH 4 + Cl 2 CH 3 Cl + HCl Reaction mechanism: I. Chain initiation - the stage of generation of free radicals. Cl Cl 2 Cl The radical is an active particle, the initiator of the reaction. – – The stage requires energy in the form of heating or lighting. The subsequent steps can proceed in the dark, without heating.

Stages of the free-radical mechanism II. Chain growth is the main stage. CH 4 + Cl HCl + CH 3 + Cl 2 CH 3 Cl + Cl The stage may include several substages, each of which forms a new radical, but not H !!! At II, the main stage, the main product is necessarily formed!

Stages of the free-radical mechanism III. Chain termination is the recombination of radicals. Cl + Cl Cl 2 Cl + CH 3 CH 3 Cl CH 3 + CH 3 CH 3 -CH 3 Any two radicals combine.

Selectivity of substitution Selectivity - selectivity. Regioselectivity - selectivity in a certain area of ​​​​reactions. For example, halogenation selectivity: 45% 3% Conclusion? 55% 97%

The selectivity of halogenation depends on the following factors: Reaction conditions. At low temperatures it is more selective. nature of the halogen. The more active the halogen, the less selective the reaction. F 2 reacts very vigorously, with the destruction of C-C bonds. I 2 does not react with alkanes under these conditions. The structure of an alkane.

Influence of alkane structure on substitution selectivity. If the carbon atoms in the alkane are unequal, then the substitution for each of them proceeds at a different rate. Relatively. substitution reaction rate atom H Secondary atom H tert. H atom chlorination 1 3, 9 5, 1 bromination 1 82 1600 Conclusion?

The detachment of a tertiary hydrogen atom requires less energy than the detachment of a secondary and primary! Alkane formula Result of homolysis ED, k. J / mol CH 4 CH 3 + H 435 CH 3 - CH 3 C 2 H 5 + H 410 CH 3 CH 2 CH 3 (CH 3) 2 CH + H 395 (CH 3) 3 CH (CH 3) 3 C + H 377

Direction of reactions Any reaction proceeds predominantly in the direction of formation of a more stable intermediate particle!

An intermediate particle in radical reactions is a free radical. The most stable radical is formed most easily! Radical stability series: R 3 C > R 2 CH > RCH 2 > CH 3 Alkyl groups exhibit an electron-donor effect, due to which they stabilize the radical

Sulfochlorination reaction Reaction scheme: RH + Cl 2 + SO 2 RSO 2 Cl + HCl Reaction mechanism: 1. Cl Cl 2 Cl 2. RH + Cl R + HCl R + SO 2 RSO 2 + Cl 2 RSO 2 Cl + Cl etc 3. 2 Cl Cl 2 etc.

D. P. Konovalov's reaction. Nitration according to Konovalov is carried out by the action of dilute nitric acid at a temperature of 140 o. C. Reaction scheme: RH + HNO 3 RNO 2 + H 2 O

The mechanism of the Konovalov reaction HNO 3 N 2 O 4 1. N 2 O 4 2 NO 2 2. RH + NO 2 R + HNO 2 R + HNO 3 RNO 2 + OH RH + OH R + H 2 O, etc. 3 .Open circuit.

Alkenes are unsaturated hydrocarbons with one C=C Cn bond. H 2 n C \u003d C - functional group of alkenes

Chemical properties of alkenes General characteristics Alkenes are a reactive class of compounds. They enter into numerous reactions, most of which are due to the breaking of a less strong pi bond. Е С-С (σ-) ~ 350 KJ/mol Е С=С (-) ~ 260 KJ/mol

Characteristic reactions Addition is the most characteristic type of reactions. The double bond is an electron donor, so it tends to add: E - electrophiles, cations or radicals

Examples of electrophilic addition reactions 1. Addition of halogens - Not all halogens are added, but only chlorine and bromine! – Polarization of a neutral halogen molecule can occur under the action of a polar solvent or under the action of the double bond of an alkene. The red-brown solution of bromine becomes colorless

Electrophilic addition Reactions proceed at room temperature and do not require illumination. Ionic mechanism. Reaction scheme: XY \u003d Cl 2, Br 2, HCl, HBr, HI, H 2 O

The sigma complex is a carbocation - a particle with a positive charge on the carbon atom. If other anions are present in the reaction medium, they can also attach to the carbocation.

For example, the addition of bromine dissolved in water. This qualitative reaction for a double C=C bond proceeds with the decolorization of the bromine solution and the formation of two products:

Addition to unsymmetrical alkenes Regioselectivity of addition! Markovnikov's rule (1869): acids and water are added to unsymmetrical alkenes in such a way that hydrogen is attached to the more hydrogenated carbon atom.

Markovnikov Vladimir Vasilievich (1837 - 1904) Graduate of Kazan University. Since 1869 - Professor of the Department of Chemistry. Founder of the scientific school. Rice. Source: http: //images. yandex. ru.

Explanation of Markovnikov's rule The reaction proceeds through the formation of the most stable intermediate particle - carbocation. primary secondary, more stable

Carbocation stability series: tertiary secondary primary methyl Markovnikov's rule in the modern formulation: the addition of a proton to an alkene occurs with the formation of a more stable carbocation.

Anti-Markovnikov addition CF 3 -CH=CH 2 + HBr CF 3 -CH 2 Br Formally, the reaction goes against Markovnikov's rule. CF 3 - electron-withdrawing substituent Other electron-withdrawing agents: NO 2, SO 3 H, COOH, halogens, etc.

Anti-Markovnikov addition more stable unstable CF 3 - electron acceptor, destabilizes carbocation The reaction only formally goes against Markovnikov's rule. In fact, it obeys, as it goes through a more stable carbocation.

Harash peroxide effect X CH 3 -CH \u003d CH 2 + HBr CH 3 -CH 2 Br X \u003d O 2, H 2 O 2, ROOR Free radical mechanism: 1. H 2 O 2 2 OH + HBr H 2 O + Br 2. CH 3 -CH \u003d CH 2 + Br CH 3 -CH -CH 2 Br is a more stable radical CH 3 -CH -CH 2 Br + HBr CH 3 -CH 2 Br + Br, etc. 3. Any two radicals are connected between yourself.

Electrophilic addition 3. Hydration - addition of water - The reaction proceeds in the presence of acid catalysts, most often it is sulfuric acid. The reaction obeys Markovnikov's rule. Cheap way to get alcohols

At the exam, Academician Ivan Alekseevich Kablukov asks the student to tell how hydrogen is obtained in the laboratory. "Mercury," he replies. “How is it “from mercury”? ! Usually they say "from zinc", but from mercury - this is something original. Write a reaction. The student writes: Hg \u003d H + g And says: “The mercury is heated; it decomposes into H and g. H is hydrogen, it is light and therefore flies away, and g is the acceleration of gravity, heavy, remains. “For such an answer, you need to put the“ five, ”says Kablukov. - Let's take a note. Only the "five" I will also warm up first. "Three" flies away, and "two" remains.

Two chemists in the laboratory: - Vasya, put your hand in this glass. - I dropped it. - Do you feel anything? - Not. - So sulfuric acid in another glass.

Aromatic hydrocarbons Aromatic - fragrant? ? Aromatic compounds are benzene and substances that resemble it in chemical behavior!

Many substitution reactions open the way to obtaining a variety of compounds that have economic applications. A huge role in chemical science and industry is given to electrophilic and nucleophilic substitution. In organic synthesis, these processes have a number of features that should be taken into account.

variety of chemical phenomena. Substitution reactions

Chemical changes associated with the transformations of substances are distinguished by a number of features. The final results, thermal effects may be different; some processes go to the end, in others a change in substances is often accompanied by an increase or decrease in the degree of oxidation. When classifying chemical phenomena according to their end result, attention is paid to the qualitative and quantitative differences between the reactants and the products. According to these features, 7 types of chemical transformations can be distinguished, including substitution, following the scheme: A-B + C A-C + B. A simplified record of a whole class of chemical phenomena gives an idea that among the starting substances there is a so-called "a particle that replaces an atom, ion, or functional group in a reagent. The substitution reaction is typical for limiting and

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

Substitution homolytic (radical, SR)

With a radical mechanism for breaking covalent bonds, an electron pair common to different elements is proportionally distributed among the "fragments" of the molecule. Free radicals are formed. These are unstable particles, the stabilization of which occurs as a result of subsequent transformations. For example, when ethane is obtained from methane, free radicals appear that participate in the substitution reaction: CH 4 CH 3. + .H; CH 3 . + .CH 3 → C2H5; H. + .H → H2. Homolytic bond breaking according to the given substitution mechanism is of a chain nature. In methane, H atoms can be successively replaced by chlorine. The reaction with bromine proceeds similarly, but iodine is unable to directly replace hydrogen in alkanes, fluorine reacts too vigorously with them.

Heterolytic cleavage method

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

Electrophiles and nucleophiles

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

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

Substitution nucleophilic (SN)

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

Electrophilic substitution (SE)

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

Monomolecular and bimolecular reactions

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

Heterolytic substitution mechanism

The substitution mechanism is associated with electron transfer, the creation of intermediate complexes. The reaction proceeds the faster, the easier it is to form the intermediate products characteristic of it. Often the process goes in several directions at the same time. The advantage is usually obtained by the way in which the particles that require the least energy costs for their formation are used. For example, the presence of a double bond increases the probability of the appearance of the allyl cation CH2=CH—CH 2 + , compared to the ion CH 3 + . The reason lies in the electron density of the multiple bond, which affects the delocalization of the positive charge dispersed throughout the molecule.

Benzene substitution reactions

The group for which electrophilic substitution is characteristic is arenas. The benzene ring is a convenient target for electrophilic attack. The process begins with the polarization of the bond in the second reactant, resulting in the formation of an electrophile adjacent to the electron cloud of the benzene ring. The result is a transitional complex. There is still no full-fledged connection of an electrophilic particle with one of the carbon atoms, it is attracted to the entire negative charge of the “aromatic six” of electrons. At the third stage of the process, the electrophile and one carbon atom of the ring are connected by a common pair of electrons (covalent bond). But in this case, the “aromatic six” is destroyed, which is unfavorable from the point of view of achieving a stable sustainable energy state. There is a phenomenon that can be called "proton ejection". There is a splitting of H + , a stable bond system, characteristic of arenes, is restored. The by-product contains a hydrogen cation from the benzene ring and an anion from the composition of the second reagent.

Examples of substitution reactions from organic chemistry

For alkanes, the substitution reaction is especially characteristic. Examples of electrophilic and nucleophilic transformations can be given for cycloalkanes and arenes. Similar reactions in the molecules of organic substances occur under normal conditions, but more often when heated and in the presence of catalysts. Electrophilic substitution in the aromatic nucleus is one of the widespread and well-studied processes. The most important reactions of this type are:

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

Substitution reactions include reactions in alkanes and cycloalkanes, in which halogens attack the available C-H bond. The preparation of derivatives may be associated with the substitution of one, two or all hydrogen atoms in saturated hydrocarbons and cycloparaffins. Many of the low molecular weight haloalkanes find use in the production of more complex substances belonging to different classes. The progress made in studying the mechanisms of substitution reactions gave a powerful impetus to the development of syntheses based on alkanes, cycloparaffins, arenes, and halogen derivatives of hydrocarbons.

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) On the basis of the release or absorption of heat

3) Based on the reversibility of the reaction

4) On the basis of a change in the degree of oxidation of the atoms that make up the reactants

According to the final result, the reactions are of the following types:

A) Substitution: RH+Cl 2 → RCl+HCl

B) Accession: CH 2 \u003d CH 2 + Cl 2 → CH 2 Cl-CH 2 Cl

C) Cleavage: CH 3 -CH 2 OH → CH 2 \u003d CH 2 + H 2 O

D) Decomposition: CH 4 → C + 2H 2

D) Isomerization

E) Exchange

G) Connections

Decomposition reaction A process in which two or more other substances are formed from one substance.

Exchange reaction called the process in which reactants exchange constituents.

Substitution reactions occur with the participation of simple and complex substances, as a result, new simple and complex substances are formed.

As a result compound reactions one new substance is formed from two or more substances.

On the basis of the release or absorption of heat of reaction, there are the following types:

A) exothermic

B) Endothermic

Exothermic - These are reactions that release heat.

Endothermic are reactions that absorb heat from the environment.

On the basis of 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 initial reactants into final substances are called irreversible.

reversible Reactions are called those that simultaneously proceed in two mutually opposite directions.

Based on the change in the oxidation state of the atoms that make up the reactants, the 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 atoms, molecules or ions to others) are called redox.

2. According to the mechanism of the reaction, they are divided into ionic and radical.

Ionic reactions- interaction between ions as a result of heterolytic rupture of a chemical bond (a pair of electrons passes entirely to one of the "fragments").

Ionic reactions are of two types (according to the type of reagent):

A) electrophilic - during the reaction with an electrophile.

electrophile- a grouping that has free orbitals for some atoms, or centers with a reduced electron density (for example: H +, Cl - or AlCl 3)

B) Nucleophilic - in the course of interaction with a nucleophile

Nucleophile - a negatively charged ion or molecule with an unshared electron pair (not currently participating in the formation of a chemical bond).

(Examples: F - , Cl - , RO - , I -).

Real chemical processes, only in rare cases, can be described by simple mechanisms. A detailed examination of chemical processes from a molecular kinetic point of view shows that most of them proceed through a radical chain mechanism, a feature of chain p-tions is the formation of free radicals at intermediate stages (unstable fragments of molecules or atoms with a short lifetime, all have free connections.

The processes of combustion, explosion, oxidation, photochemical reactions, biochemical reactions in living organisms proceed according to the chain mechanism.

Chain districts have several stages:

1) chain nucleation - the stage of chain p-tion, as a result of which free radicals arise from valence-saturated molecules.

2) the continuation of the chain - the stage of the chain of the p-tion, proceeding with the preservation of the total number of free stages.

3) chain breakage - an elementary stage of the chains of the p-tion leading to the disappearance of free bonds.

There are branched and unbranched chain reactions.

One of the most important concepts of the chain is 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) m-la CL 2 absorbs a quantum of energy and an image of 2 radicals: CL 2 + hv \u003d CL * + CL *

2) the active particle combines with the m-molecule H 2 forming hydrochloric acid and the active particle H 2: CL 1 + H 2 \u003d HCL + H *

3)CL 1 +H 2 =HCL+CL * etc.

6) H * + CL * \u003d HCL - open circuit.

Branched mechanism:

F * + H 2 \u003d HF + H *, etc.

F * + H 2 \u003d HF + H *, etc.

In water, it is more difficult - OH*, O* radicals and H* radicals are formed.

Reactions that occur under the influence of ionizing radiation: X-rays, cathode rays, and so on - called radiochemical.

As a result of the interaction of molecules with radiation, the decay of molecules is observed with the formation of the most reactive particles.

Such reactions contribute to the recombination of particles, and the formation of substances with their various combinations.

An example is hydrazine N 2 H 4 - a component of rocket fuel. Recently, attempts have been 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, such as radiolysis of water, are important for the vital activity of organisms.

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