Chemistry and mechanism of organic reactions. Basic types and mechanisms of reactions in organic chemistry
Reactions of organic substances can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization). It is obvious that the entire variety of reactions of organic compounds cannot be reduced to the proposed classification (for example, combustion reactions). However, such a classification will help establish analogies with the reactions that occur between inorganic substances that are already familiar to you.
Typically, the main organic compound involved in the reaction is called substrate, and the other reaction component is conventionally considered as reagent.
Substitution reactions
Substitution reactions- these are reactions that result in the replacement of one atom or group of atoms in the original molecule (substrate) with other atoms or groups of atoms.
Substitution reactions involve saturated and aromatic compounds such as alkanes, cycloalkanes or arenes. Let us give examples of such reactions.
Under the influence of light, hydrogen atoms in a methane molecule can be replaced by halogen atoms, for example, by chlorine atoms:
Another example of replacing hydrogen with halogen is the conversion of benzene to bromobenzene:
The equation for this reaction can be written differently:
With this form of writing, the reagents, catalyst, and reaction conditions are written above the arrow, and the inorganic reaction products are written below it.
As a result of reactions substitutions in organic substances are formed not simple and complex substances, as in inorganic chemistry, and two complex substances.
Addition reactions
Addition reactions- these are reactions as a result of which two or more molecules of reacting substances combine into one.
Unsaturated compounds such as alkenes or alkynes undergo addition reactions. Depending on which molecule acts as a reagent, hydrogenation (or reduction), halogenation, hydrohalogenation, hydration and other addition reactions are distinguished. Each of them requires certain conditions.
1.Hydrogenation- reaction of addition of a hydrogen molecule through a multiple bond:
2. Hydrohalogenation- hydrogen halide addition reaction (hydrochlorination):
3. Halogenation- halogen addition reaction:
4.Polymerization- a special type of addition reaction in which molecules of a substance with a small molecular weight combine with each other to form molecules of a substance with a very high molecular weight - macromolecules.
Polymerization reactions are processes of combining many molecules of a low molecular weight substance (monomer) into large molecules (macromolecules) of a polymer.
An example of a polymerization reaction is the production of polyethylene from ethylene (ethene) under the action of ultraviolet radiation and a radical polymerization initiator R.
The covalent bond most characteristic of organic compounds is formed when atomic orbitals overlap and the formation of shared electron pairs. As a result of this, an orbital common to the two atoms is formed, in which a common electron pair is located. When a bond is broken, the fate of these shared electrons can be different.
Types of reactive particles
An orbital with an unpaired electron belonging to one atom can overlap with an orbital of another atom that also contains an unpaired electron. In this case, a covalent bond is formed according to the exchange mechanism:
The exchange mechanism for the formation of a covalent bond is realized if a common electron pair is formed from unpaired electrons belonging to different atoms.
The process opposite to the formation of a covalent bond by the exchange mechanism is the cleavage of the bond, in which one electron is lost to each atom (). As a result of this, two uncharged particles are formed, having unpaired electrons:
Such particles are called free radicals.
Free radicals- atoms or groups of atoms that have unpaired electrons.
Free radical reactions- these are reactions that occur under the influence and with the participation of free radicals.
In the course of inorganic chemistry, these are the reactions of hydrogen with oxygen, halogens, and combustion reactions. Reactions of this type are characterized by high speed and release of large amounts of heat.
A covalent bond can also be formed by a donor-acceptor mechanism. One of the orbitals of an atom (or anion) that has a lone pair of electrons overlaps with the unoccupied orbital of another atom (or cation) that has an unoccupied orbital, and a covalent bond is formed, for example:
The rupture of a covalent bond leads to the formation of positively and negatively charged particles (); since in this case both electrons from a common electron pair remain with one of the atoms, the other atom has an unfilled orbital:
Let's consider the electrolytic dissociation of acids:
It can be easily guessed that a particle having a lone pair of electrons R: -, i.e. a negatively charged ion, will be attracted to positively charged atoms or to atoms on which there is at least a partial or effective positive charge.
Particles with lone pairs of electrons are called nucleophilic agents (nucleus- “nucleus”, a positively charged part of an atom), i.e. “friends” of the nucleus, a positive charge.
Nucleophiles(Nu) - anions or molecules that have a lone pair of electrons that interact with parts of the molecules that have an effective positive charge.
Examples of nucleophiles: Cl - (chloride ion), OH - (hydroxide anion), CH 3 O - (methoxide anion), CH 3 COO - (acetate anion).
Particles that have an unfilled orbital, on the contrary, will tend to fill it and, therefore, will be attracted to parts of the molecules that have an increased electron density, a negative charge, and a lone electron pair. They are electrophiles, “friends” of the electron, negative charge, or particles with increased electron density.
Electrophiles- cations or molecules that have an unfilled electron orbital, tending to fill it with electrons, as this leads to a more favorable electronic configuration of the atom.
Not any particle is an electrophile with an unfilled orbital. For example, alkali metal cations have the configuration of inert gases and do not tend to acquire electrons, since they have a low electron affinity.
From this we can conclude that despite the presence of an unfilled orbital, such particles will not be electrophiles.
Basic reaction mechanisms
Three main types of reacting particles have been identified - free radicals, electrophiles, nucleophiles - and three corresponding types of reaction mechanisms:
- free radical;
- electrophilic;
- zeroophilic.
In addition to classifying reactions according to the type of reacting particles, in organic chemistry four types of reactions are distinguished according to the principle of changing the composition of molecules: addition, substitution, detachment, or elimination (from the English. to eliminate- remove, split off) and rearrangements. Since addition and substitution can occur under the influence of all three types of reactive species, several can be distinguished mainmechanisms of reactions.
In addition, we will consider elimination reactions that occur under the influence of nucleophilic particles - bases.
6. Elimination:
A distinctive feature of alkenes (unsaturated hydrocarbons) is their ability to undergo addition reactions. Most of these reactions proceed by the electrophilic addition mechanism.
Hydrohalogenation (addition of halogen hydrogen):
When a hydrogen halide is added to an alkene hydrogen adds to the more hydrogenated one to a carbon atom, i.e. an atom at which there are more atoms hydrogen, and halogen - to less hydrogenated.
Annex 1
REACTION MECHANISMS IN ORGANIC CHEMISTRY
N.V. Sviridenkova, NUST MISIS, Moscow
WHY STUDY THE MECHANISMS OF CHEMICAL REACTIONS?
What is the mechanism of a chemical reaction? To answer this question, consider the equation for the combustion reaction of butene:
C 4 H 8 + 6O 2 = 4CO 2 + 4H 2 O.
If the reaction actually proceeded as described in the equation, then one molecule of butene would have to collide with six molecules of oxygen at once. However, this is unlikely to happen: it is known that the simultaneous collision of more than three particles is almost impossible. The conclusion suggests itself that this reaction, like the vast majority of chemical reactions, occurs in several successive stages. The reaction equation shows only the starting materials and the final result of all transformations, and does not explain in any way how products are formed from initial substances. In order to find out exactly how a reaction proceeds, what stages it includes, what intermediate products are formed, it is necessary to consider the reaction mechanism.
So, reaction mechanism is a detailed description of the course of a reaction in stages, which shows in what order and how chemical bonds in the reacting molecules are broken and new bonds and molecules are formed.
Consideration of the mechanism makes it possible to explain why some reactions are accompanied by the formation of several products, while in other reactions only one substance is formed. Knowing the mechanism allows chemists to predict the products of chemical reactions before they are actually carried out. Finally, knowing the reaction mechanism, you can control the course of the reaction: create conditions to increase its speed and increase the yield of the desired product.
BASIC CONCEPTS: ELECTROPHILE, NUCLEOPHILE, CARBOCATION
In organic chemistry, reagents are traditionally divided into three types: nucleophilic, electrophilic And radical. You have already encountered radicals earlier when studying the halogenation reactions of alkanes. Let's take a closer look at other types of reagents.
Nucleophilic reagents or simply nucleophiles(translated from Greek as “nucleus lovers”) are particles with excess electron density, most often negatively charged or having a lone electron pair. Nucleophiles attack molecules with low electron density or positively charged reagents. Examples of nucleophiles are OH - , Br - ions, NH 3 molecules.
Electrophilic reagents or electrophiles(translated from Greek as “electron lovers”) are particles with a lack of electron density. Electrophiles often carry a positive charge. Electrophiles attack molecules with high electron density or negatively charged reagents. Examples of electrophiles are H +, NO 2 +.
An atom of a polar molecule that carries a partial positive charge can also act as an electrophile. An example is the hydrogen atom in the HBr molecule, on which a partial positive charge arises due to the displacement of the common electron bond pair to the bromine atom, which has a higher electronegativity value H δ + → Br δ - .
Reactions proceeding through the ionic mechanism are often accompanied by the formation of carbocations. Carbocation called a charged particle that has a free R-orbital on the carbon atom. One of the carbon atoms in the carbocation carries a positive charge. Examples of carbocations include particles CH 3 -CH 2 +, CH 3 -CH + -CH 3. Carbocations are formed at one of the stages in the reactions of addition of halogens to alkenes and hydrogen halides to alkenes, as well as in substitution reactions involving aromatic hydrocarbons.
MECHANISM OF ADDITION TO UNSATURED HYDROCARBONS
The addition of halogens, hydrogen halides, and water to unsaturated hydrocarbons (alkenes, alkynes, diene hydrocarbons) occurs through ionic mechanism, called electrophilic addition.
Let us consider this mechanism using the example of the reaction of addition of hydrogen bromide to an ethylene molecule.
Despite the fact that the hydrobromination reaction is described by a very simple equation, its mechanism includes several stages.
Stage 1. In the first stage, a hydrogen halide molecule forms with π -electron cloud of double bond unstable system – “ π -complex” due to partial transfer π -electron density per hydrogen atom carrying a partial positive charge.
Stage 2. The hydrogen-halogen bond is broken to form an electrophilic H + particle and a nucleophilic Br - particle. The released electrophile H+ adds to the alkene due to the electron pair of the double bond, forming σ
-complex – carbocation.
Stage 3. At this stage, a negatively charged nucleophile is added to the positively charged carbocation to form the final reaction product.
WHY DOES MARKOVNIKOV'S RULE FOLLOW?
The proposed mechanism explains well the formation of predominantly one of the products in the case of addition of hydrogen halides to unsymmetrical alkenes. Let us recall that the addition of hydrogen halides obeys Markovnikov’s rule, according to which hydrogen is added at the double bond to the most hydrogenated carbon atom (i.e., connected to the largest number of hydrogen atoms), and halogen to the least hydrogenated one. For example, when hydrogen bromide is added to propene, 2-bromopropane is predominantly formed:
In electrophilic addition reactions to unsymmetrical alkenes, two carbocations can be formed in the second stage of the reaction. Next, it reacts with a nucleophile, which means that the more stable of them will determine the reaction product.
Let us consider which carbocations are formed in the case of propene and compare their stability. The addition of an H+ proton at the site of a double bond can lead to the formation of two carbocations, secondary and primary:
The resulting particles are very unstable because the positively charged carbon atom in the carbocation has an unstable electronic configuration. Such particles are stabilized by distributing (delocalizing) the charge over as many atoms as possible. Electron donor alkyl groups, supplying electron density to the electron-deficient carbon atom, promote and stabilize carbocations. Let's look at how this happens.
Due to the difference in electronegativity of the carbon and hydrogen atoms, a certain excess of electron density appears on the carbon atom of the -CH 3 group, and some deficiency appears on the hydrogen atom, C δ- H 3 δ+. The presence of such a group next to a carbon atom bearing a positive charge inevitably causes a shift in the electron density towards the positive charge. Thus, the methyl group acts as a donor, giving away part of its electron density. Such a group is said to have positive inductive effect (+ I -effect). The more such electron donor (+ I ) - the substituents are surrounded by a carbon bearing a positive charge, the more stable the corresponding carbocation is. Thus, the stability of carbocations increases in the series:
In the case of propene, the most stable is the secondary carbocation, since in it the positively charged carbon atom of the carbocation is stabilized by two + I - effects of neighboring methyl groups. It is predominantly formed and reacts further. The unstable primary carbocation apparently exists for a very short time, so that during its “life” it does not have time to attach a nucleophile and form a reaction product.
When the bromide ion is added to the secondary carbocation at the last stage, 2-bromopropane is formed:
DOES MARKOVNIKOV'S RULE ALWAYS FOLLOW?
Consideration of the mechanism of the propylene hydrobromination reaction allows us to formulate a general rule for electrophilic addition: “when unsymmetrical alkenes interact with electrophilic reagents, the reaction proceeds through the formation of the most stable carbocation.” The same rule makes it possible to explain the formation in some cases of addition products contrary to Markovnikov’s rule. Thus, the addition of hydrogen halides to trifluoropropylene formally proceeds against Markovnikov’s rule:
How can such a product be obtained, since it was formed as a result of the addition of Br to the primary, and not to the secondary, carbocation? The contradiction is easily resolved by considering the reaction mechanism and comparing the stability of the intermediate particles formed:
The -CF 3 group contains three electron-withdrawing fluorine atoms, which draw electron density from the carbon atom. Therefore, a significant lack of electron density appears on the carbon atom. To compensate for the resulting partial positive charge, the carbon atom absorbs the electron density of neighboring carbon atoms. Thus, the -CF 3 group is electron-withdrawing and shows negative inductive effect (- I ) . In this case, the primary carbocation turns out to be more stable, since the destabilizing effect of the -CF 3 group through two σ bonds is weakened. And the secondary carbocation, destabilized by the neighboring electron-withdrawing group CF 3, is practically not formed.
The presence of electron-withdrawing groups –NO2, -COOH, -COH, etc. at the double bond has a similar effect on addition. In this case, the addition product is also formed formally against the Markovnikov rule. For example, when hydrogen chloride is added to propenoic (acrylic) acid, 3-chloropropanoic acid is predominantly formed:
Thus, the direction of addition to unsaturated hydrocarbons can be easily determined by analyzing the structure of the hydrocarbon. Briefly this can be reflected in the following diagram:
It should be noted that Markovnikov's rule is satisfied only if the reaction proceeds by the ionic mechanism. When carrying out radical reactions, Markovnikov's rule is not satisfied. Thus, the addition of hydrogen bromide HBr in the presence of peroxides (H 2 O 2 or organic peroxides) proceeds against Markovnikov’s rule:
The addition of peroxides changes the reaction mechanism; it becomes radical. This example shows how important it is to know the reaction mechanism and the conditions under which it occurs. Then, by choosing the appropriate conditions for the reaction, you can direct it according to the mechanism required in this particular case, and obtain exactly the products that are needed.
MECHANISM OF HYDROGEN ATOMS REPLACEMENT IN AROMATIC HYDROCARBONS
The presence in the benzene molecule of a stable conjugate π
-electronic system makes addition reactions almost impossible. For benzene and its derivatives, the most typical reactions are substitution of hydrogen atoms, which occur while maintaining aromaticity. In this case, the benzene ring containing π-
electrons interact with electrophilic particles. Such reactions are called electrophilic substitution reactions in the aromatic series. These include, for example, halogenation, nitration and alkylation of benzene and its derivatives.
All electrophilic substitution reactions in aromatic hydrocarbons follow the same path ionic mechanism regardless of the nature of the reagent. The mechanism of substitution reactions includes several stages: the formation of an electrophilic agent E +, the formation π -complex, then σ- complex and finally disintegration σ- complex to form a substitution product.
An electrophilic E+ particle is formed when a reagent interacts with a catalyst, for example, when a halogen molecule is exposed to aluminum chloride. The resulting E+ particle interacts with the aromatic ring, first forming π -, and then σ- complex:
During education σ- complex, the electrophilic particle E + attaches to one of the carbon atoms of the benzene ring through σ- communications. In the resulting carbocation, the positive charge is evenly distributed (delocalized) between the remaining five carbon atoms.
The reaction ends with the removal of a proton from σ- complex. In this case, two electrons σ -CH bonds return to the cycle, and a stable six-electron aromatic π - the system is regenerated.
In a benzene molecule, all six carbon atoms are equal. The replacement of a hydrogen atom can occur with equal probability for any of them. How will substitution occur in the case of benzene homologues? Let's take methylbenzene (toluene) as an example.
It is known from experimental data that electrophilic substitution in the case of toluene always occurs with the formation of two products. Thus, the nitration of toluene occurs with the formation P-nitrotoluene and O-nitrotoluene:
Other electrophilic substitution reactions (bromination, alkylation) proceed similarly. It was also found that in the case of toluene, substitution reactions proceed faster and under milder conditions than in the case of benzene.
It is very simple to explain these facts. The methyl group is electron-donating and, as a result, further increases the electron density of the benzene ring. A particularly strong increase in electron density occurs in O- And P- positions relative to the -CH 3 group, which facilitates the attachment of a positively charged electrophilic particle to these sites. Therefore, the rate of the substitution reaction generally increases, and the substituent is directed predominantly to ortho- And pair- provisions.
Guidelines for independent work of 1st year students in biological and bioorganic chemistry
(module 1)
Approved
Academic Council of the University
Kharkov KhNMU
Basic 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 characteristics of the topic
The study of this topic is the basis for understanding some of the biochemical reactions that take place during the metabolic process 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 drugs and analogues natural compounds.
Learning goal
Be able to predict the ability of the main classes of organic compounds to enter into homolytic and heterolytic reactions according to their electronic structure and the electronic effects of substituents.
1. FREE RADICAL AND ELECTROPHILIC REACTIONS (REACTIVITY OF HYDROCARBONS)
Educational-target questions
1. Be able to describe the mechanisms of the following reactions:
Radical substitution - R S
Electrophilic connection - A E
Electrophilic substitution - S E
2. Be able to explain the influence of substituents on the reactivity during electrophilic interactions based on electronic effects.
Baseline
1. 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. Intermediators - 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 reaction of ethylene hydrogenation.
2. Describe the mechanism of the hydration reaction of propenoic acid. Explain the role of acid catalysis.
3. Write the equation for the nitration reaction of toluene (methylbenzene). By what mechanism does this reaction occur?
4. Explain the deactivating and orienting effect of the nitro group in the nitrobenzene molecule using the example of the bromination reaction.
Educational tasks and algorithms for solving them
Task No. 1. Describe the reaction mechanism for the bromination of isobutane and cyclopentane upon irradiation with light.
Solution algorithm . Isobutane and cyclopentane molecules consist of sp 3 hybridized carbon atoms. The C - C bonds in their molecules are non-polar, and the C - H bonds are low-polar. These bonds are quite easily subject to homolytic cleavage with the formation of free radicals - particles that have unpaired electrons. Thus, in the molecules of these substances a radical substitution reaction must occur - an R S reaction or a chain reaction.
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 a cyclopentane molecule with the formation of a new cyclopentyl radical:
The cyclopentyl radical interacts with a new bromine molecule, causing homolytic cleavage of the bond in it and forming bromocyclopentane and a new bromine radical:
The bromine free radical attacks the new cyclopentane molecule. Thus, the chain growth stage is repeated many times, i.e., a chain reaction occurs. Chain termination completes the chain reaction by combining different radicals:
Since all the carbon atoms in the cyclopentane molecule are equal, only monocyclobromopentane is formed.
In isobutane, the C - H bonds are not equivalent. They differ in the energy of homolytic dissociation and the stability of the free radicals formed. It is known that the energy of C-H bond cleavage 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 observed. During chlorination, hydrogen atoms at any carbon atoms may be subject to substitution, but the content of the substitution product at tertiary carbon will be greatest.
Task No. 2. Using oleic acid as an example, describe the mechanism of the lipid peroxidation reaction that occurs during 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 radicals. During radioactive irradiation, water molecules may disintegrate into radicals. Hydroxyl radicals attack an unsaturated acid molecule at the methylene group adjacent to the double bond. In this case, a radical is formed, stabilized due to the participation of the unpaired electron in conjugation with the electrons of bonds. Next, 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 conjugated with the -electron cloud of the aromatic ring, which leads to its relative stability.
Task No. 3. Give the mechanism of the ethylene bromination reaction.
Solution algorithm. For compounds that consist of carbon atoms in the state of sp 2 - or sp-hybridization, typical reactions are those that occur with the rupture of bonds, i.e., addition reactions. These reactions can proceed by a radical or ionic mechanism depending on the nature of the reagent, the polarity of the solvent, temperature, etc. Ionic reactions occur under the action of either electrophilic reagents, which have an affinity for an electron, or nucleophilic reagents, which donate their electrons. Electrophilic reagents can be cations and compounds that have atoms with unfilled electron shells. The simplest electrophilic reagent is a proton. Nucleophilic reagents are anions, or compounds with atoms that have unshared electron pairs.
For alkenes - compounds that have an sp 2 - or sp-hybridized carbon atom, electrophilic addition reactions - A E reactions - are typical. In polar solvents in the absence of sunlight, the halogenation reaction proceeds by an ionic mechanism with the formation of carbocations:
Under the influence of the π bond in ethylene, the bromine molecule is polarized to form an unstable π complex, which turns into a carbocation. In it, bromine is bonded to carbon by a π bond. The process is completed by the interaction of the bromine anion with this carbocation to form the final reaction product, dibromoethane.
Task No. 4 . Using the example of the hydration reaction of propene, justify Markovnikov’s rule.
Solution algorithm. Since the water molecule is a nucleophilic reagent, its addition to the double bond without a catalyst is impossible. Acids act as catalysts in such reactions. The formation of carbocations occurs upon the addition of an acid proton when the π bond is broken:
A water molecule is attached to the carbocation that is formed due to the paired electrons of the oxygen atom. A stable alkyl derivative of oxonium is formed, which stabilizes with the release of a proton. The reaction product is sec-propanol (propan-2-ol).
In the hydration reaction, a proton is added according to Markovnikov’s 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 due to the addition of a proton is more stable than the primary one (the influence of two alkyl groups).
Task No. 5. Justify the possibility of the formation of 1,3-dibromopropane during the bromination of cyclopropane.
Solution algorithm. Molecules that are three- or four-membered rings (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 undergo addition reactions with ring rupture:
Task No. 6. Describe the reaction of hydrogen bromide with 1,3 butadiene. What is special about this reaction?
Solution algorithm. When hydrogen bromide reacts with 1,3 butadiene, the 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 No. 7. Describe the mechanism of the benzene bromination reaction.
Solution algorithm. For aromatic compounds that contain a closed conjugated electronic 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 halogen molecule to polarize, after which it attacks the π electrons of the benzene ring:
π-complex σ-complex
Initially, 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 at the expense of 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 release of a proton.
Electrophilic substitution reactions in aromatic compounds also include sulfonation and nitration. The role of a 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 SE reactions depends on the electron density in the aromatic nucleus (direct relationship). In this regard, the reactivity of substances should be considered in connection with the electronic effects of substituents and heteroatoms.
The amino group in aniline exhibits a +M effect, as a result of which the electron density in the benzene ring increases and its highest concentration is observed in the ortho and para positions. The reaction progresses easier.
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, meta-isomers are formed in this case. Thus, electron-donating substituents are ortho- and para-orientants (orientants of the first kind and activators of SE reactions; electron-withdrawing substituents are meta-orientants (orientants of the second kind) deactivators of SE reactions).
In five-membered heterocycles (pyrrole, furan, thiophene), which belong to π-excess systems, S E reactions occur more easily than in benzene; in this case, the α-position is more reactive.
Heterocyclic systems with a pyridine nitrogen atom are π-deficient, therefore they are more difficult to undergo electrophilic substitution reactions; in this case, the electrophile occupies the β-position relative to the nitrogen atom.
Classification of reactions
There are four main types of reactions in which organic compounds participate: substitution (displacement), addition, elimination (elimination), rearrangements.
3.1 Substitution reactions
In reactions of the first type, the substitution usually occurs at the carbon atom, but the substituted atom may be a hydrogen atom or some other atom or group of atoms. In electrophilic substitution, a hydrogen atom is most often replaced; An example is the classic aromatic substitution:
With nucleophilic substitution, it is not the hydrogen atom that is most often replaced, but other atoms, for example:
NC - + R−Br → NC−R +BR -
3.2 Addition reactions
Addition reactions can also be electrophilic, nucleophilic, or radical, depending on the type of species initiating the process. Attachment to ordinary carbon-carbon double bonds is usually induced by an electrophile or radical. For example, the addition of HBr
may begin with an attack of the double bond by the H+ proton or the Br· radical.
3.3 Elimination reactions
Elimination reactions are essentially the reverse of addition reactions; The most common type of such reaction is the elimination of a hydrogen atom and another atom or group from neighboring carbon atoms to form alkenes:
3.4 Rearrangement reactions
Rearrangements can also occur through intermediates that are cations, anions or radicals; most often these reactions occur with the formation of carbocations or other electron-deficient particles. Rearrangements may involve significant 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 final product.
A detailed description of a chemical reaction in stages is called a mechanism. From an electronic point of view, the mechanism of a chemical reaction is understood as the method of breaking covalent bonds in molecules and the sequence of states through which reacting substances pass before becoming reaction products.
4.1 Free radical reactions
Free radical reactions are chemical processes in which molecules that have unpaired electrons take part. Certain aspects of free radical reactions are unique compared to other types of reactions. The main difference is that many free radical reactions are chain reactions. This means that there is a mechanism by which many molecules are converted into a product through a repeating process initiated by the creation of a single reactive species. A typical example is illustrated using the following hypothetical mechanism:
The stage at which the reaction intermediate, in this case A·, is generated is called initiation. This stage occurs at high temperatures, under the influence of UV or peroxides, in non-polar solvents. The next four equations in this example repeat the sequence of two reactions; they represent the development phase of the chain. Chain reactions are characterized by the length of the chain, which corresponds to the number of development stages per initiation stage. The second stage occurs with simultaneous synthesis of the compound and the formation of a new radical, which continues the chain of transformations. The last step is the chain termination step, which involves any reaction in which one of the reaction intermediates necessary for chain progression is destroyed. The more stages of chain termination, the shorter the chain length becomes.
Free radical reactions occur: 1) in the light, at high temperatures or in the presence of radicals that are formed during the decomposition of other substances; 2) inhibited by substances that easily react with free radicals; 3) occur in non-polar solvents or in the vapor phase; 4) often have an autocatalytic and induction period before the start of the reaction; 5) kinetically they are chain.
Radical substitution reactions are characteristic of alkanes, and radical addition reactions are characteristic of alkenes and alkynes.
CH 4 + Cl 2 → CH 3 Cl + HCl
CH 3 -CH=CH 2 + HBr → CH 3 -CH 2 -CH 2 Br
CH 3 -C≡CH + HCl → CH 3 -CH=CHCl
The connection of free radicals with each other and chain termination occurs mainly on the walls of the reactor.
4.2 Ionic reactions
Reactions in which it occurs heterolytic the breaking of bonds and the formation of intermediate particles of the ionic type are called ionic reactions.
Ionic reactions occur: 1) in the presence of catalysts (acids or bases and are not affected by light or free radicals, in particular those arising from the decomposition of peroxides); 2) are not affected by free radical scavengers; 3) the nature of the solvent influences the course of the reaction; 4) rarely occur in the vapor phase; 5) kinetically, they are mainly first- or second-order reactions.
Based on the nature of the reagent acting on the molecule, ionic reactions are divided into electrophilic And nucleophilic. Nucleophilic substitution reactions are characteristic of alkyl and aryl halides,
CH 3 Cl + H 2 O → CH 3 OH + HCl
C 6 H 5 -Cl + H 2 O → C 6 H 5 -OH + HCl
C 2 H 5 OH + HCl → C 2 H 5 Cl + H 2 O
C 2 H 5 NH 2 + CH 3 Cl → CH 3 -NH-C 2 H 5 + HCl
electrophilic substitution – for alkanes in the presence of catalysts
CH 3 -CH 2 -CH 2 -CH 2 -CH 3 → CH 3 -CH(CH 3)-CH 2 -CH 3
and arenas.
C 6 H 6 + HNO 3 + H 2 SO 4 → C 6 H 5 -NO 2 + H 2 O
Electrophilic addition reactions are characteristic of alkenes
CH 3 -CH=CH 2 + Br 2 → CH 3 -CHBr-CH 2 Br
and alkynes,
CH≡CH + Cl 2 → CHCl=CHCl
nucleophilic addition – for alkynes.
CH 3 -C≡CH + C 2 H 5 OH + NaOH → CH 3 -C(OC 2 H 5) = CH 2
>> Chemistry: Types of chemical reactions in organic chemistry
Reactions of organic substances can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization). It is obvious that the entire variety of reactions of organic compounds cannot be reduced to the framework of the proposed classification (for example, combustion reactions). However, such a classification will help to establish analogies with the classifications of reactions occurring between inorganic substances that are already familiar to you from the course of inorganic chemistry.
Typically, the main organic compound involved in a reaction is called the substrate, and the other component of the reaction is conventionally considered the reactant.
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.
Division Dividing a number into classes
So did the USSR attack Poland?
Battle of Larga Excerpt from the Livonian Rhymed Chronicle