Basic types and mechanisms of reactions in organic chemistry. Radical and Ionic Reaction Mechanisms Reaction Mechanisms in Organic Chemistry with Examples
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 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 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.
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 form 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), which has an unfilled orbital, while forming covalent bond, eg:
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. 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;
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
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? - Not. - So sulfuric acid in another glass.
Aromatic hydrocarbons Aromatic - fragrant? ? Aromatic compounds are benzene and substances that resemble it in chemical behavior!
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-polar 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 -).
Examples of ionic reactions of various types.
Nucleophilic substitution :
Electrophilic substitution :
Nucleophilic addition (first CN - joins, then H +):
electrophilic addition (first H + joins, then X -):
Elimination under the action of nucleophiles (bases) :
Elimination on action electrophiles (acids) :
During the course of chemical reactions, some bonds are broken and other bonds are formed. Chemical reactions are conventionally divided into organic and inorganic. Organic reactions are considered to be reactions in which at least one of the reactants is an organic compound that changes its molecular structure during the reaction. The difference between organic reactions and inorganic ones is that, as a rule, molecules participate in them. The rate of such reactions is low, and the yield of the product is usually only 50-80%. To increase the reaction rate, catalysts are used, the temperature or pressure is increased. Next, consider the types of chemical reactions in organic chemistry.
Classification according to the nature of chemical transformations
- Substitution reactions
- Addition reactions
- Isomerization reaction and rearrangement
- Oxidation reactions
- Decomposition reactions
Substitution reactions
During substitution reactions, one atom or group of atoms in the initial molecule is replaced by other atoms or groups of atoms, forming a new molecule. As a rule, such reactions are characteristic of saturated and aromatic hydrocarbons, for example:
Addition reactions
In the course of addition reactions, one molecule of a new compound is formed from two or more molecules of substances. Such reactions are characteristic of unsaturated compounds. There are reactions of hydrogenation (reduction), halogenation, hydrohalogenation, hydration, polymerization, etc.:
- hydrogenation– addition of a hydrogen molecule:
Elimination reaction (cleavage)
As a result of cleavage reactions, organic molecules lose atoms or groups of atoms, and a new substance is formed containing one or more multiple bonds. Elimination reactions include reactions dehydrogenation, dehydration, dehydrohalogenation etc.:
Isomerization reactions and rearrangement
In the course of such reactions, intramolecular rearrangement occurs, i.e. the transition of atoms or groups of atoms from one part of the molecule to another without changing the molecular formula of the substance participating in the reaction, for example: 
Oxidation reactions
As a result of exposure to an oxidizing reagent, the degree of oxidation of carbon in an organic atom, molecule or ion increases due to the donation of electrons, as a result of which a new compound is formed: 
Condensation and polycondensation reactions
They consist in the interaction of several (two or more) organic compounds with the formation of new C-C bonds and a low molecular weight compound:
Polycondensation is the formation of a polymer molecule from monomers containing functional groups with the release of a low molecular weight compound. Unlike the polymerization reaction, which results in the formation of a polymer having a composition similar to the monomer, as a result of polycondensation reactions, the composition of the formed polymer differs from its monomer:
Decomposition reactions
This is the process of splitting a complex organic compound into less complex or simple substances:
C 18 H 38 → C 9 H 18 + C 9 H 20
Classification of chemical reactions by mechanisms
The occurrence of reactions with the breaking of covalent bonds in organic compounds is possible by two mechanisms (i.e., the path leading to the breaking of the old bond and the formation of a new one) - heterolytic (ionic) and homolytic (radical).
Heterolytic (ionic) mechanism
In reactions proceeding according to the heterolytic mechanism, intermediate particles of the ionic type with a charged carbon atom are formed. Particles that carry a positive charge are called carbocations, and a negative charge is called carbanions. In this case, there is not a break in the common electron pair, but its transition to one of the atoms, with the formation of an ion: 
Strongly polar, for example, H–O, C–O, and easily polarizable, for example, C–Br, C–I bonds show a tendency to heterolytic cleavage.
Reactions proceeding according to the heterolytic mechanism are divided into nucleophilic and electrophilic reactions. A reagent that has an electron pair to form a bond is called a nucleophilic or electron donor. For example, HO -, RO -, Cl -, RCOO -, CN -, R -, NH 2, H 2 O, NH 3, C 2 H 5 OH, alkenes, arenes.
A reagent that has an unfilled electron shell and is able to attach a pair of electrons in the process of forming a new bond. The following cations are called electrophilic reagents: H +, R 3 C +, AlCl 3, ZnCl 2, SO 3, BF 3, R-Cl, R 2 C=O
Nucleophilic substitution reactions
Characteristic for alkyl and aryl halides: 
Nucleophilic addition reactions 
Electrophilic substitution reactions
Electrophilic addition reactions 
Homolytic (radical mechanism)
In reactions proceeding according to the homolytic (radical) mechanism, at the first stage, the covalent bond is broken with the formation of radicals. Further, the formed free radical acts as an attacking reagent. Bond cleavage by a radical mechanism is characteristic of non-polar or low-polarity covalent bonds (C–C, N–N, C–H).
Distinguish between radical substitution and radical addition reactions
Radical substitution reactions
characteristic of alkanes 
Radical addition reactions
characteristic of alkenes and alkynes 
Thus, we have considered the main types of chemical reactions in organic chemistry
Categories ,Mechanisms of organic reactions
| Parameter name | Meaning |
| Article subject: | Mechanisms of organic reactions |
| Rubric (thematic category) | Education |
Reaction classification
There are four main types of reactions in which organic compounds participate: substitution (displacement), addition, elimination (cleavage), rearrangements.
3.1 Substitution reactions
In reactions of the first type, substitution usually occurs at the carbon atom, but the substituted atom must be a hydrogen atom or some other atom or group of atoms. In electrophilic substitution, most often a hydrogen atom is 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 are also electrophilic, nucleophilic, or radical, depending on the type of particles that initiate the process. Attachment to ordinary 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 proceed 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.
A detailed description of a chemical reaction by stages is usually called a mechanism. From an electronic point of view, the mechanism of a chemical reaction is understood as a method of breaking covalent bonds in molecules and a sequence of states through which the reacting substances pass before being converted into reaction products.
4.1 Free radical reactions
Free radical reactions - ϶ᴛᴏ 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 commonly called initiation. This stage takes place at high temperature, under the action of UV or peroxides, in non-polar solvents. The next four equations in this example repeat the sequence of two reactions; they represent the development phase of the chain. Chain reactions are characterized by the chain length, which 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 chain termination, 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 combination 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
Mechanisms of organic reactions - concept and types. Classification and features of the category "Mechanisms of organic reactions" 2017, 2018.
