Ionic mechanism of the reaction. Types of chemical reactions in organic chemistry - Knowledge Hypermarket

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.

Literature:

1. Akhmetov, N.S. General and inorganic chemistry / N.S. Akhmetov. - 3rd ed. - M .: Higher school, 2000. - 743 p.

1. Korovin N.V. General chemistry / N.V. Korovin. - M.: Higher school, 2006. - 557 p.
2. Kuzmenko N.E. A short course in chemistry / N.E. Kuzmenko, V.V. Eremin, V.A. Popkov. - M.: Higher School, 2002. - 415 p.
3. Zaitsev, O.S. General chemistry. Structure of substances and chemical reactions / O.S. Zaytsev. – M.: Chemistry, 1990.
4. Karapetyants, M.Kh. The structure of matter / M.Kh. Karapetyants, S.I. Drakin. - M .: Higher School, 1981.
5. Cotton F. Fundamentals of inorganic chemistry / F. Cotton, J. Wilkinson. – M.: Mir, 1981.
6. Ugay, Ya.A. General and inorganic chemistry / Ya.A.Ugai. - M .: Higher School, 1997.

Guidelines for independent work of 1st year students in biological and bioorganic chemistry

(module 1)

Approved

Academic Council of the University

Kharkiv KhNMU

Main types and mechanisms of reactions in organic chemistry: Method. decree. for 1st year students / comp. A.O. Syrovaya, L.G. Shapoval, V.N. Petyunina, E.R. Grabovetskaya, V.A. Makarov, S.V. Andreeva, S.A. Nakonechnaya, L.V. Lukyanova, R.O. Bachinsky, S.N. Kozub, T.S. Tishakova, O.L. Levashova, N.V. Kopoteva, N.N. Chalenko. - Kharkov: KhNMU, 2014. - P. 32.

Compiled by: A.O. Syrovaya, L.G. Shapoval, V.N. Petyunina, E.R. Grabovetskaya, V.A. Makarov, S.V. Andreeva, L.V. Lukyanova, S.A. Nakonechnaya, R.O. Bachinsky, S.N. Kozub, T.S. Tishakova, O.L. Levashova, N.V. Kopoteva, N.N. Chalenko

Topic I: classification of chemical reactions.

Reactivity of Alkanes, Alkenes, Arenes, Alcohols, Phenols, Amines, Aldehydes, Ketones, and Carboxylic Acids

Motivational characteristic of the topic

The study of this topic is the basis for understanding some of the biochemical reactions that take place in the process of metabolism in the body (lipid peroxidation, the formation of hydroxy acids from unsaturated ones in the Krebs cycle, etc.), as well as for understanding the mechanism of such reactions in the synthesis of medical preparations and analogues natural compounds.

learning goal

To be able to predict the ability of the main classes of organic compounds to enter into reactions of homolytic and heterolytic interactions according to their electronic structure and electronic effects of substituents.

1. FREE RADICAL AND ELECTROPHILIC REACTIONS (REACTIVITY OF HYDROCARBONS)

Learning-targeted questions

1. Be able to describe the mechanisms of the following reactions:

Radical substitution - R S

Electrophilic addition - A E

Electrophilic substitution - S E

2. Be able to explain the effect of substituents on reactivity in electrophilic interactions based on electronic effects.

Baseline

1. The structure of the carbon atom. Types of hybridization of its electronic orbitals.

2. Structure, length and energy of - and -bonds.

3. Conformations of cyclohexane.

4. Pairing. Open and closed (aromatic) conjugated systems.

5. Electronic effects of substituents.

6. Transition state. Electronic structure of the carbocation. Intermediaries - and  - complexes.

Practical navski

1. Learn to determine the possibility of breaking a covalent bond, the type and mechanism of the reaction.

2. Be able to experimentally perform bromination reactions of compounds with double bonds and aromatic compounds.

Control questions

1. Give the mechanism of the ethylene hydrogenation reaction.

2. Describe the mechanism of propenoic acid hydration reaction. Explain the role of acid catalysis.

3. Write the reaction equation for the nitration of toluene (methylbenzene). What is the mechanism of this reaction?

4. Explain the deactivating and orienting effect of the nitro group in the nitrobenzene molecule using the bromination reaction as an example.

Learning tasks and algorithms for their solution

Task number 1. Describe the reaction mechanism of bromination of isobutane and cyclopentane under light irradiation.

Solution algorithm . Molecules of isobutane and cyclopentane consist of sp 3 hybridized carbon atoms. C - C bonds in their molecules are non-polar, and C - H bonds are of low polarity. These bonds are quite easily subjected to homolytic rupture with the formation of free radicals - particles that have unpaired electrons. Thus, in the molecules of these substances, a radical substitution reaction must occur - R S -reaction or chain.

The stages of any R S -reaction are: initiation, growth and chain termination.

Initiation is the process of formation of free radicals at high temperature or ultraviolet irradiation:

Chain growth occurs due to the interaction of a highly reactive free radical  Br with a low-polar C - H bond in the cyclopentane molecule with the formation of a new cyclopentyl radical:

The cyclopentyl radical interacts with a new bromine molecule, causing a homolytic bond cleavage in it and forming bromocyclopentane and a new bromine radical:

The free bromine radical attacks the new cyclopentane molecule. Thus, the stage of chain growth is repeated many times, i.e., a chain reaction occurs. Chain termination completes the chain reaction by combining different radicals:

Since all carbon atoms in a cyclopentane molecule are equal, only monocyclobromopentane is formed.

In isobutane, C - H bonds are not equivalent. They differ in the energy of homolytic dissociation and the stability of the formed free radicals. It is known that the breaking energy of the C-H bond increases from the tertiary to the primary carbon atom. The stability of free radicals decreases in the same order. That is why in the isobutane molecule the bromination reaction proceeds regioselectively - at the tertiary carbon atom:

It should be pointed out that for the more active chlorine radical, regioselectivity is not fully adhered to. During chlorination, hydrogen atoms at any carbon atoms can be replaced, but the content of the substitution product at tertiary carbon will be the largest.

Task number 2. Using oleic acid as an example, describe the mechanism of the lipid peroxidation reaction that occurs in radiation sickness as a result of damage to cell membranes. What substances act as antioxidants in our body?

Solution algorithm. An example of a radical reaction is lipid peroxidation, in which unsaturated fatty acids, which are part of cell membranes, are exposed to the action of radicals. With radioactive irradiation, the possible decay of water molecules into radicals. Hydroxyl radicals attack the unsaturated acid molecule at the methylene group adjacent to the double bond. In this case, a radical stabilized due to the participation of an unpaired electron in conjugation with electrons of  bonds is formed. Further, the organic radical interacts with a diradical oxygen molecule to form unstable hydroperoxides, which decompose to form aldehydes, which are oxidized to acids - the final products of the reaction. The consequence of peroxide oxidation is the destruction of cell membranes:

The inhibitory effect of vitamin E (tocopherol) in the body is due to its ability to bind free radicals that are formed in cells:

In the phenoxide radical that is formed, the unpaired electron is in conjugation with the -electron cloud of the aromatic ring, which leads to its relative stability.

Task number 3. Give the mechanism of ethylene bromination reaction.

Solution algorithm. For compounds that consist of carbon atoms in the state of sp 2 - or sp-hybridization, there are typical reactions that proceed with the breaking of -bonds, i.e., addition reactions. These reactions can proceed by a radical or ionic mechanism, depending on the nature of the reactant, the polarity of the solvent, temperature, etc. Ionic reactions proceed under the action of either electrophilic reagents, which have an electron affinity, or nucleophilic ones, which donate their electrons. Electrophilic reagents can be cations and compounds that have atoms with unfilled electron shells. The simplest electrophilic reagent is the proton. Nucleophilic reagents are anions, or compounds with atoms that have unshared electron pairs.

For alkenes - compounds that have sp 2 - or sp-hybridized carbon atom, electrophilic addition reactions are typical - A E reactions. In polar solvents, in the absence of sunlight, the halogenation reaction proceeds according to the ionic mechanism with the formation of carbocations:

Under the action of the π-bond in ethylene, the bromine molecule is polarized with the formation of an unstable π-complex, which turns into a carbocation. In it, bromine is bonded to carbon by a π bond. The process ends with the interaction of the bromine anion with this carbocation to the final reaction product, dibromoethane.

Task #4 . On the example of propene hydration reaction justify Markovnikov's rule.

Solution algorithm. Since the water molecule is a nucleophilic reagent, its addition via a double bond without a catalyst is impossible. The role of catalysts in such reactions is played by acids. The formation of carbocations occurs when a proton of an acid is added when a π-bond is broken:

A water molecule is attached to the carbocation that has been formed due to the paired electrons of the oxygen atom. A stable alkyl derivative of oxonium is formed, which is stabilized with the release of a proton. The reaction product is sec-propanol (propan-2-ol).

In the hydration reaction, the proton joins according to the Markovnikov rule - to a more hydrogenated carbon atom, since, due to the positive inductive effect of the CH 3 group, the electron density is shifted to this atom. In addition, the tertiary carbocation formed as a result of the addition of a proton is more stable than the primary one (the influence of two alkyl groups).

Task number 5. Substantiate the possibility of formation of 1,3-dibromopropane during bromination of cyclopropane.

Solution algorithm. Molecules that are three- or four-membered cycles (cyclopropane and cyclobutane) exhibit the properties of unsaturated compounds, since the electronic state of their "banana" bonds resembles a π-bond. Therefore, like unsaturated compounds, they enter into addition reactions with a ring break:

Task number 6. Describe the reaction of interaction of hydrogen bromide with butadiene-1,3. What is the nature of this reaction?

Solution algorithm. In the interaction of hydrogen bromide with butadiene-1,3, products 1,2 addition (1) and 1,4 addition (2) are formed:

The formation of product (2) is due to the presence in the conjugated system of a π-electron cloud common to the entire molecule, as a result of which it enters into an electrophilic addition reaction (A E - reaction) in the form of a whole block:

Task number 7. Describe the mechanism of the benzene bromination reaction.

Solution algorithm. For aromatic compounds that contain a closed conjugated electron system and which therefore have significant strength, electrophilic substitution reactions are characteristic. The presence of increased electron density on both sides of the ring protects it from attack by nucleophilic reagents and, vice versa, facilitates the possibility of attack by cations and other electrophilic reagents.

The interaction of benzene with halogens occurs in the presence of catalysts - AlCl 3 , FeCl 3 (the so-called Lewis acids). They cause the polarization of the halogen molecule, after which it attacks the π-electrons of the benzene ring:

π-complex σ-complex

At the beginning, a π-complex is formed, which slowly transforms into a σ-complex, in which bromine forms a covalent bond with one of the carbon atoms due to two of the six electrons of the aromatic ring. The four π electrons that remain are evenly distributed among the five atoms of the carbon ring; The σ-complex is a less favorable structure due to the loss of aromaticity, which is restored by the emission of a proton.

Electrophilic substitution reactions in aromatic compounds also include sulfonation and nitration. The role of the nitrating agent is performed by the nitroyl cation - NO 2+, which is formed by the interaction of concentrated sulfuric and nitric acids (nitrating mixture); and the role of the sulfonating agent is the SO 3 H + cation, or sulfur oxide (IV), if sulfonation is carried out with oleum.

Solution algorithm. The activity of compounds in S E reactions depends on the value of the electron density in the aromatic nucleus (direct dependence). In this regard, the reactivity of substances should be considered in conjunction with the electronic effects of substituents and heteroatoms.

The amino group in aniline exhibits the +M effect, as a result of which the electron density in the benzene nucleus increases and its highest concentration is observed in the ortho and para positions. The reaction is facilitated.

The nitro group in nitrobenzene has -I and -M effects, therefore, it deactivates the benzene ring in the ortho and para positions. Since the interaction of the electrophile occurs at the site of the highest electron density, in this case meta-isomers are formed. Thus, electron-donating substituents are ortho- and para-orientants (orientants of the first kind and activators of S E reactions; electron-withdrawing substituents are meta-orientants (orientants of the second kind) deactivators of S E reactions).

In five-membered heterocycles (pyrrole, furan, thiophene), which belong to π-excess systems, S E reactions proceed more easily than in benzene; while the α-position is more reactive.

Heterocyclic systems with a pyridine nitrogen atom are π-insufficient, therefore they are more difficult to enter into electrophilic substitution reactions; while the electrophile occupies the β-position with respect to the nitrogen atom.

CH 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 characteristic of organic compounds containing multiple (double or triple) bonds. Reactions of this type include addition reactions of halogens, hydrogen halides and water to alkenes and alkynes

CH 3 -CH \u003d CH 2 + HCl ---- CH 3 -CH (Cl) -CH 3

Cleavage (elimination) reactions

These are reactions that lead to the formation of multiple bonds. When splitting off hydrogen halides and water, a certain selectivity of the reaction is observed, described by the Zaitsev rule, according to which a hydrogen atom is split off from the carbon atom at which there are fewer hydrogen atoms. Reaction Example

CH3-CH(Cl)-CH 2 -CH 3 + KOH →CH 3 -CH=CH-CH 3 + HCl

Polymerization and polycondensation

n(CH 2 \u003d 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, of great interest to organic chemists are oxidation reactions carried out under much milder conditions than combustion. Used oxidizing agents: solutions of Br2 in water or Cl2 in CCl 4 ; KMnO 4 in water or dilute acid; copper oxide; freshly precipitated hydroxides of silver (I) or copper (II).

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

YR 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. Equally carefully describe the formation of new bonds in the course of the reaction. Considering the reaction mechanism, first of all, attention is paid 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) breaking of the covalent bond:

Non-polar or low-polarity covalent bonds (C–C, N–N, C–H) undergo radical rupture at high temperature or under the action of light. The carbon in the CH 3 radical has 7 outer electrons (instead of the 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 (combination of two radicals):

CH 3 + CH 3 CH 3 : CH 3,

H + H H : N.

Radical reactions - these are, for example, the reactions of chlorination, bromination and nitration of alkanes:

Ionic reactions occur with heterolytic bond cleavage. In this case, short-lived organic ions are intermediately formed - carbocations and carbanions - with a charge on the carbon atom. In ionic reactions, the binding electron pair does not separate, 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 this or that reaction is nucleophilic or electrophilic, conditionally always refers to the reagent. Reagent- a substance participating in the reaction with a simpler structure. substrate is the starting material with a more complex structure. Leaving group is a displaceable 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. An electrophile is any atom, molecule, or ion that can accept 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 -).

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 added 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? - No. - 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.