Organic chemistry types of chemical reactions. Types of organic reactions
When chemical reactions occur, some bonds break and others form. 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 are involved in them. The rate of such reactions is low, and the product yield is usually only 50-80%. To increase the reaction rate, catalysts are used and the temperature or pressure is increased. Next, we will consider the types of chemical reactions in organic chemistry.
Classification by 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
When addition reactions occur, one molecule of a new compound is formed from two or more molecules of substances. Such reactions are typical for unsaturated compounds. There are reactions of hydrogenation (reduction), halogenation, hydrohalogenation, hydration, polymerization, etc.:
- Hydrogenation– addition of a hydrogen molecule:
Elimination reaction
As a result of elimination 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 and so on.:
Isomerization reactions and rearrangement
During 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 oxidation state of carbon in an organic atom, molecule or ion increases due to the loss of electrons, resulting in the formation of a new compound: 
Condensation and polycondensation reactions
Consists 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 polymerization reactions, which result in the formation of a polymer having a composition similar to the monomer, as a result of polycondensation reactions, the composition of the resulting polymer differs from its monomer:
Decomposition reactions
This is the process of breaking down 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
Reactions involving the rupture of covalent bonds in organic compounds are possible by two mechanisms (i.e., a path leading to the rupture of an 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 carrying a positive charge are called carbocations, and negative ones are called carbanions. In this case, it is not the breaking of the common electron pair that occurs, 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 exhibit 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 nucleophilic or electron-donating. 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 capable of attaching 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. The resulting free radical then acts as an attacking reagent. Bond cleavage by a radical mechanism is typical for non-polar or low-polar covalent bonds (C–C, N–N, C–H).
Distinguish between radical substitution and radical addition reactions
Radical displacement reactions
Characteristic of alkanes 
Radical addition reactions
Characteristic of alkenes and alkynes 
Thus, we examined the main types of chemical reactions in organic chemistry
Categories , Abstract: “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.
Under the influence of light, hydrogen atoms in a methane molecule can be replaced by halogen atoms, for example, chlorine atoms:
CH4 + Cl2→ CH3Cl + HCl
Another example of replacing hydrogen with halogen is the conversion of benzene to bromobenzene:
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.
Addition reactions
Reactions in which two or more molecules of reacting substances combine into one are called addition reactions.
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:
CH3-CH = CH2 + H2 → CH3-CH2-CH3
propene propane
2 . Hydrohalogenation - hydrogen halide addition reaction (for example, hydrochlorination):
CH2=CH2 + HCl → CH3-CH2-Cl
ethene chloroethane
3 . Halogenation - halogen addition reaction (for example, chlorination):
CH2=CH2 + Cl2 → CH2Cl-CH2Cl
ethene 1,2-dichloroethane
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 - these are the processes of combining many molecules of a low-molecular 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.
Types of chemical reactions in organic chemistry
Elimination reactions
Reactions that result in the formation of molecules of several new substances from a molecule of the original compound are called elimination or elimination reactions.
Examples of such reactions include the production of ethylene from various organic substances.
Types of chemical reactions in organic chemistry
Of particular importance among the elimination reactions is the reaction of thermal splitting of hydrocarbons, on which cracking of alkanes is based - the most important technological process:
In most cases, the cleavage of a small molecule from a molecule of the parent substance leads to the formation of an additional n-bond between the atoms. Elimination reactions occur under certain conditions and with certain reagents. The given equations reflect only the final result of these transformations.
Isomerization reactions
Reactions as a result of which molecules of one substance are formed from molecules of other substances of the same qualitative and quantitative composition, i.e., with the same molecular formula, are called isomerization reactions.
An example of such a reaction is the isomerization of the carbon skeleton of linear alkanes into branched ones, which occurs on aluminum chloride at high temperature:
Types of chemical reactions in organic chemistry
1 . What type of reaction is this:
a) obtaining chloromethane from methane;
b) obtaining bromobenzene from benzene;
c) producing chloroethane from ethylene;
d) producing ethylene from ethanol;
e) conversion of butane to isobutane;
f) ethane dehydrogenation;
g) conversion of bromoethane to ethanol?
2 . What reactions are typical for: a) alkanes; b) alkenes? Give examples of reactions.
3 . What are the features of isomerization reactions? What do they have in common with reactions producing allotropic modifications of one chemical element? Give examples.
4. In which reactions (addition, substitution, elimination, isomerization) is the molecular weight of the starting compound:
a) increases;
b) decreases;
c) does not change;
d) does it increase or decrease depending on the reagent?
Organic compounds can react both with each other and with inorganic substances - nonmetals, metals, acids, bases, salts, water, etc. Therefore, their reactions turn out to be very diverse both in the nature of the reacting substances and in the type of transformations that occur. There are many registered reactions named after the scientists who discovered them.
The organic compound molecule involved in the reaction is called a substrate.
A particle of an inorganic substance (molecule, ion) in an organic reaction is called a reagent.
For example:
A chemical transformation can involve the entire molecule of an organic compound. Of these reactions, the most widely known is combustion, which leads to the transformation of a substance into a mixture of oxides. They are of great importance in the energy sector, as well as in the destruction of waste and toxic substances. From the point of view of both chemical science and practice, reactions leading to the transformation of some organic substances into others are especially interesting. A molecule always has one or more reactive sites where one or another transformation occurs.
The atom or group of atoms in a molecule where a chemical transformation directly occurs is called a reaction center.
In multielement substances, the reaction centers are functional groups and the carbon atoms to which they are bonded. In unsaturated hydrocarbons, the reaction center is carbon atoms connected by a multiple bond. In saturated hydrocarbons, the reaction center is predominantly secondary and tertiary carbon atoms.
Molecules of organic compounds often have several reaction centers exhibiting different activities. Therefore, as a rule, several parallel reactions occur, giving different products. The reaction that occurs at the fastest rate is called main Other reactions - side effects. The resulting mixture contains the largest amount of the product of the main reaction, and the products of side reactions are impurities. After the reaction, it is almost always necessary to purify the main product from impurities of organic substances. Note that in inorganic chemistry, substances usually have to be purified from impurities of compounds of other chemical elements.
It has already been noted that organic reactions are characterized by relatively low rates. Therefore, it is necessary to widely use various means of accelerating reactions - heating, irradiation, catalysis. Catalysts are of utmost importance in organic chemistry. Their role is not limited to huge time savings when carrying out chemical processes. By choosing catalysts that accelerate certain types of reactions, one can purposefully carry out one or another of the parallel reactions and obtain the desired products. During the existence of the organic compounds industry, the discovery of new catalysts radically changed the technology. For example, ethanol was produced for a long time only by fermentation of starch, and then switched to its production
adding water to ethylene. To do this, it was necessary to find a well-functioning catalyst.
Reactions in organic chemistry are classified according to the nature of the transformation of the substrate:
a) addition reactions (symbol A)- a small molecule (water, halogen, etc.) is attached to an organic molecule;
b) substitution reactions (symbol S) - in an organic molecule an atom (group of atoms) is mixed with another atom or group of atoms;
c) detachment or elimination reactions (symbol E)- an organic molecule loses some fragments, which, as a rule, form inorganic substances;
d) cracking - splitting a molecule into two or more parts, also representing organic compounds;
e) decomposition - the transformation of an organic compound into simple substances and inorganic compounds;
f) isomerization - transformation of a molecule into another isomer;
g) polymerization - the formation of a high-molecular compound from one or more low-molecular compounds;
h) polycondensation - the formation of a high-molecular compound with the simultaneous release of a substance consisting of small molecules (water, alcohol).
In the processes of transformation of organic compounds, two types of breaking of chemical bonds are considered.
Homolytic bond cleavage. From the electron pair of a chemical bond, each atom retains one electron. The resulting particles having unpaired electrons are called free radicals. In composition, such a particle can be a molecule or an individual atom. The reaction is called radical (symbol R):
Heterolytic bond cleavage. In this case, one atom retains an electron pair and becomes a base. The particle containing this atom is called nucleophile. The other atom, deprived of an electron pair, has a vacant orbital and becomes an acid. The particle containing this atom is called electrophile:
This type of l-bond is especially easy to break while maintaining
For example, a certain particle A, attracting an n-electron pair, itself forms a bond with a carbon atom:
The same interaction is depicted by the following diagram: 
If a carbon atom in a molecule of an organic compound accepts an electron pair, which it then transfers to a reagent, then the reaction is called electrophilic, and the reagent is called an electrophile.
Types of electrophilic reactions - addition A E and replacement S E .
The next stage of the reaction is the formation of a bond between the C + atom (it has a free orbital) and another atom that has an electron pair.
If a carbon atom in a molecule of an organic compound loses an electron pair and then accepts it from a reagent, then the reaction is called nucleophilic, and the reagent is called a nucleophile.
Types of nucleophilic reactions - addition of Ad, and substitution S N .
Heterolytic rupture and the formation of chemical bonds actually represent a single coordinated process: the gradual rupture of an existing bond is accompanied by the formation of a new bond. In a coordinated process, the activation energy is lower.
QUESTIONS AND EXERCISES
1. When 0.105 g of organic matter was burned, 0.154 g of carbon dioxide, 0.126 g of water and 43.29 ml of nitrogen were formed (21 ° C, 742 mm Hg). Suggest one of the possible structural formulas of the substance.
2. In the C 3 H 7 X molecule, the total number of electrons is 60. Identify the element X and write the formulas for possible isomers.
3. There are 10 moles of electrons per 19.8 g of compound C 2 H 4 X 2. Identify element X and write formulas for possible isomers.
4. Gas volume 20 l at 22 "C and 101.7 kPa contains 2.5 10 i atoms and has a density of 1.41 g/l. Draw conclusions about the nature of this gas.
5. Indicate a radical that has two isomers: -C 2 H 5, -C 3 H 7, -CH 3.
6. Indicate the substance that has the highest boiling point: CH 3 OH, C 3 H 7 OH, C 5 H 11 OH.
7. Write the structural formulas of C 3 H 4 isomers.
8. Write the formula for 2,3,4-trimethyl-4-ethylheptene. Give the structural formulas of two isomers of this substance containing one and two quaternary carbon atoms.
9. Write the formula for 3,3-dimethylpentane. Give the formula of a cyclic hydrocarbon without multiple bonds with the same number of carbon atoms. Are they isomers?
10. Write the formula of a four-element organic compound with the structure C10, in which atoms of additional elements are located at the 2nd and 7th carbon atoms, and the name contains the root “hepta”.
11. Name a hydrocarbon that has a carbon structure
12.Write the structural formula of the compound C 2 H X F X Cl X with different substituents on each carbon atom.
Hydrocarbons
Hydrocarbons are among the most important substances that determine the way of life of modern civilization. They serve as a source of energy (energy carriers) for land, air and water transport, for heating homes. It is also the raw material for the production of hundreds of household chemical products, packaging materials, etc. The initial source of all of the above is oil and natural gas. The welfare of states depends on the availability of their reserves. International crises have arisen over oil.
Among the most well-known hydrocarbons are methane and propane, used in household stoves. Methane is transported through pipes, and propane is transported and stored in red cylinders. Another hydrocarbon, silt-butane, gaseous under normal conditions, can be seen in a liquid state in transparent lighters. Oil refining products - gasoline, kerosene, diesel fuel - are mixtures of hydrocarbons of different compositions. Mixtures of heavier hydrocarbons are semi-liquid petroleum jelly and solid paraffin. Hydrocarbons also include a well-known substance used to protect wool and fur from moths - naphthalene. The main types of hydrocarbons from the point of view of the composition and structure of molecules are saturated hydrocarbons - alkanes, cyclic saturated hydrocarbons - cycloalkanes, unsaturated hydrocarbons, i.e. containing multiple bonds - alkenes And
alkynes, cyclic conjugate aromatic hydrocarbons - arenas. Some homologous series of hydrocarbons are characterized in table. 15.1.
Table 15.1. Homologous series of hydrocarbons
Alkanes
Chapter 14 already provides data on the structure, composition, isomerism, names and some properties of alkanes. Recall that in alkane molecules, carbon atoms form tetrahedrally oriented bonds with hydrogen atoms and neighboring carbon atoms. In the first compound of this series, methane, carbon is bonded only to hydrogen. In the molecules of saturated hydrocarbons there is a continuous internal rotation of the terminal CH 3 groups and individual sections of the chain, as a result of which different conformations arise (p. 429). Alkanes are characterized by isomerism of the carbon skeleton. Compounds with unbranched molecules are called
normal, n-alkanes, and with branched ones - iso alkanes. Data on the names and some physical properties of alkanes are given in table. 15.2.
The first four members of the alkanes series - methane, ethane, propane and butane - are used in large quantities as individual substances. Other individual alkanes are used in scientific research. Mixtures of alkanes, usually containing hydrocarbons and other homologous series, are of great practical importance. Gasoline is one of these mixtures. It is characterized boiling temperature range 30-205 °C. Other types of hydrocarbon fuels are also characterized by boiling ranges, since as light hydrocarbons volatilize from them, the boiling point increases. All alkanes are practically insoluble in water.
Table 15.2. Names and boiling and melting points of normal alkanes
task 15.1. Group alkanes based on their state of aggregation at 20 °C and normal atmospheric pressure (according to Table 15.2).
task 15.2. Pentane has three isomers with the following boiling points (°C):
Explain the decrease in boiling points in the series of these isomers.
Receipt. Oil is an almost unlimited source of any alkanes, but isolating individual substances from it is a rather difficult task. Conventional petroleum products are fractions obtained during rectification (fractional distillation) of oil and consisting of a large number of hydrocarbons.
A mixture of alkanes is obtained by hydrogenating coal at a temperature of -450 0 C and a pressure of 300 atm. Gasoline can be produced using this method, but it is still more expensive than gasoline from oil. Methane is formed in a mixture of carbon monoxide (II) and hydrogen on a nickel catalyst:
In the same mixture on catalysts containing cobalt, both a mixture of hydrocarbons and individual hydrocarbons are obtained. These can be not only alkanes, but also cycloalkanes.
There are laboratory methods for obtaining individual alkanes. Carbides of some metals produce methane upon hydrolysis:
Haloalkanes react with an alkali metal to form hydrocarbons with twice the number of carbon atoms. This is Wurtz's reaction. It goes through the hemolytic cleavage of the bond between carbon and halogen with the formation of free radicals:
task 15.3. Write the overall equation for this reaction.
Example 15.1. Potassium was added to the mixture of 2-bromopropane and 1-bromopropane. Write equations for possible reactions.
SOLUTION. Radicals formed during the reactions of bromoalkanes with potassium can combine with each other in different combinations, resulting in three hydrocarbons in the mixture. Summary reaction equations:
When heated with alkali, sodium salts of organic acids lose the carboxyl group (decarboxylate) to form an alkane:
During the electrolysis of these same salts, decarboxylation occurs and the remaining radicals combine into one molecule:
Alkanes are formed during the hydrogenation of unsaturated hydrocarbons and the reduction of compounds containing functional groups:
Chemical properties. Saturated hydrocarbons are the least active organic substances. Their original name paraffins reflects weak affinity (reactivity) for other substances. They react, as a rule, not with ordinary molecules, but only with free radicals. Therefore, reactions of alkanes occur under conditions of the formation of free radicals: at high temperature or irradiation. Alkanes burn when mixed with oxygen or air and play a vital role as fuel.
task 15.4. The heat of combustion of octane is determined with particular accuracy:
How much heat will be released during the combustion of 1 liter of a mixture consisting equally of n-octane and silt-octane (р = = 0.6972 Alkanes react with halogens by a radical mechanism (S R). The reaction begins with the breakdown of a halogen molecule into two atoms, or, as is often said, into two free radicals:
A radical removes a hydrogen atom from an alkane, such as methane:
The new molecular radical methyl H 3 C- reacts with a chlorine molecule, forming a substitution product and at the same time a new chlorine radical:
Then the same stages of this chain reaction are repeated. Each radical can generate a chain of transformations of hundreds of thousands of links. Collisions between radicals are also possible, leading to chain termination: 
The overall chain reaction equation is:
task 15.5. As the volume of the vessel in which the chain reaction occurs decreases, the number of transformations per radical (chain length) decreases. Give an explanation for this.
The reaction product chloromethane belongs to the class of halogenated hydrocarbons. In the mixture, as chloromethane is formed, a reaction begins to replace the second hydrogen atom with chlorine, then the third, etc. At the third stage, the well-known substance chloroform CHClg, used in medicine for anesthesia, is formed. The product of complete replacement of hydrogen with chlorine in methane - carbon tetrachloride CC1 4 - is classified as both organic and inorganic substances. But, if you strictly adhere to the definition, it is an inorganic compound. In practice, carbon tetrachloride is obtained not from methane, but from carbon disulfide.
When methane homologues are chlorinated, secondary and tertiary carbon atoms become more reactive. From propane, a mixture of 1-chloropropane and 2-chloropropane is obtained, with a larger proportion of the latter. The replacement of the second hydrogen atom with a halogen occurs predominantly at the same carbon atom:
Alkanes react when heated with dilute nitric acid and nitrogen(IV) oxide to form nitroalkanes. Nitration also follows a radical mechanism, and therefore it does not require concentrated nitric acid:
Alkanes undergo various transformations when heated in the presence of special catalysts. Normal alkanes isomerize into zo-alkanes:
Industrial isomerization of alkanes to improve the quality of motor fuel is called reforming. The catalyst is metal platinum deposited on aluminum oxide. Cracking is also important for oil refining, i.e. the splitting of an alkane molecule into two parts - an alkane and an alkene. The splitting occurs predominantly in the middle of the molecule: 
Aluminosilicates serve as cracking catalysts.
Alkanes with six or more carbon atoms in the chain cyclize on oxide catalysts (Cr 2 0 3 / /A1 2 0 3), forming cycloalkanes with a six-membered ring and arenes:
This reaction is called dehydrocyclization.
It is gaining increasing practical importance functionalization alkanes, i.e., converting them into compounds containing functional groups (usually oxygen). Butane is oxidized by acid
oxygen with the participation of a special catalyst, forming acetic acid:
Cycloalkanes C n H 2n with five or more carbon atoms in the ring are very similar in chemical properties to non-cyclic alkanes. They are characterized by substitution reactions S R . Cyclopropane C 3 H 6 and cyclobutane C 4 H 8 have less stable molecules, since the angles between the C-C-C bonds in them differ significantly from the normal tetrahedral angle of 109.5°, characteristic of sp 3 carbon. This leads to a decrease in binding energy. When exposed to halogens, rings are broken and joined at the ends of the chain:
When hydrogen reacts with cyclobutane, normal butane is formed:
TASK 15.6. Is it possible to obtain cyclopentane from 1,5-dibromopentane? If you think it is possible, then select the appropriate reagent and write the reaction equation.
Alkenes
Hydrocarbons containing less hydrogen than alkanes due to the presence of multiple bonds in their molecules are called unlimited, and unsaturated. The simplest homologous series of unsaturated hydrocarbons are alkenes C n H 2n, having one double bond:
The other two valences of carbon atoms are used to add hydrogen and saturated hydrocarbon radicals.
The first member of the series of alkenes is ethene (ethylene) C 2 H 4. It is followed by propene (propylene) C 3 H 6, butene (butylene) C 4 H 8, pentene C 5 H 10, etc. Some radicals with a double bond have special names: vinyl CH 2 = CH-, allyl CH 2 =CH-CH 2 -.
Carbon atoms connected by a double bond are in a state of sp 2 hybridization. Hybrid orbitals form σ bond between them, and the non-hybrid p-orbital is π bond(Fig. 15.1). The total energy of the double bond is 606 kJ/mol, with the a-bond accounting for about 347 kJ/mol, and the π bond- 259 kJ/mol. The increased strength of the double bond is manifested by a decrease in the distance between carbon atoms to 133 pm compared to 154 pm for a C-C single bond.
Despite the formal strength, it is the double bond in alkenes that turns out to be the main reaction center. Electron pair π -bonds form a fairly diffuse cloud, relatively distant from atomic nuclei, as a result of which it is mobile and sensitive to the influence of other atoms (p. 442). π -The cloud moves towards one of the two carbon atoms, which
Rice. 15.1. Formation of a multiple bond between carbon atoms sp 2
it belongs, under the influence of substituents in the alkene molecule or under the influence of an attacking molecule. This results in the high reactivity of alkenes compared to alkanes. A mixture of gaseous alkanes does not react with bromine water, but in the presence of alkene impurities, it becomes discolored. This sample is used to detect alkenes.
Alkenes have additional types of isomerism that are absent in alkanes: isomerism of the position of the double bond and spatial cis-trans isomerism. The last type of isomerism is due to special symmetry π - connections. It prevents internal rotation in the molecule and stabilizes the arrangement of four substituents on the C=C atoms in the same plane. If there are two pairs of different substituents, then with a diagonal arrangement of the substituents of each pair, a trans isomer is obtained, and with an adjacent arrangement, a cis isomer is obtained. Ethene and propene do not have isomers, but butene has both types of isomers:
task 15.7. All alkenes have the same elemental composition both by mass (85.71% carbon and 14.29% hydrogen) and by the ratio of the number of atoms n(C): n(H) = 1:2. Can we assume that each alkene is an isomer with respect to other alkenes?
task 15.8. Are spatial isomers possible in the presence of three or four different substituents on sp 2 carbon atoms?
task 15.9. Draw the structural formulas of pentene isomers.
Receipt. We already know that alkanes can be converted into unsaturated compounds. This happened
occurs as a result of hydrogen removal (dehydrogenation) and cracking. Dehydrogenation of butane produces predominantly butene-2:
task 15.10. Write the cracking reaction of malka-
Dehydrogenation and cracking require fairly high temperatures. Under normal conditions or gentle heating, alkenes are formed from halogen derivatives. Chloro- and bromoalkanes react with an alkali in an alcohol solution, eliminating halogen and hydrogen from two adjacent carbon atoms:
This is an elimination reaction (p. 441). If two neighboring carbon atoms have a different number of hydrogen atoms attached to them, then elimination follows Zaitsev’s rule.
In the elimination reaction, hydrogen is preferentially eliminated from the less hydrogenated carbon atom.
Example 15.2. Write the elimination reaction of 2-chlorobutane.
solution. According to Zaitsev’s rule, hydrogen is split off from the 3 C atom:
When the metals zinc and magnesium act on dihaloalkanes with adjacent halogen positions, alkenes are also formed:
Chemical properties. Alkenes can either decompose at high temperatures to simple substances or polymerize, turning into high-molecular substances. Ethylene polymerizes at very high pressure (-1500 atm) with the addition of a small amount of oxygen as an initiator that produces free radicals. From liquid ethylene under these conditions, a white flexible mass is obtained, transparent in a thin layer - polyethylene. This is material that is well known to everyone. The polymer is made up of very long molecules
Molecular weight 20 LLC-40 LLC. In structure it is a saturated hydrocarbon, but there may be oxygen atoms at the ends of the molecules. At a high molecular weight, the proportion of terminal groups is very small and it is difficult to determine their nature.
task 15.11. How many molecules of ethylene are included in one molecule of polyethylene with a molecular weight of 28000?
Polymerization of ethylene also occurs at low pressure in the presence of special Ziegler-Natta catalysts. These are mixtures of TiCl and organoaluminum compounds AlR x Cl 3-x, where R is alkyl. Polyethylene obtained by catalytic polymerization has better mechanical properties, but ages faster, i.e., it is destroyed under the influence of light and other factors. The production of polyethylene began around 1955. This material significantly influenced everyday life, as packaging bags began to be made from it. Of the other alkene polymers, polypropylene is the most important. It produces a more rigid and less transparent film than polyethylene. Polymerization of propylene is carried out with
Ziegler-Natta talizer. The resulting polymer has the correct isotactic structure
When polymerized under high pressure it turns out Atlantic polypropylene with a random arrangement of CH 3 radicals. This is a substance with completely different properties: a liquid with a solidification temperature of -35 °C.
Oxidation reactions. Alkenes under normal conditions are oxidized at the double bond upon contact with solutions of potassium permanganate and other oxidizing agents. In a slightly alkaline environment they form glycols, i.e. diatomic alcohols:
In an acidic environment, when heated, alkenes are oxidized with complete cleavage of the molecule at the double bond:
task 15.12. Write the equation for this reaction.
task 15.13. Write the reaction equations for the oxidation of butene-1 and butene-2 with potassium permanganate in an acidic medium.
Ethylene is oxidized by oxygen on an Ag/Al 2 O 3 catalyst to form a cyclic oxygen-containing substance called ethylene oxide:
This is a very important product of the chemical industry, produced annually in the amount of millions of tons. It is used to produce polymers and detergents.
Electrophilic addition reactions. Molecules of halogens, hydrogen halides, water and many others are attached to alkenes via a double bond. Let us consider the mechanism of addition using bromine as an example. When a Br 2 molecule attacks one of the carbon atoms of the unsaturated center, an electron pair π -bond shifts to the latter and further to bromine. Thus, bromine acts as an electrophilic reagent:
A bond between bromine and carbon is formed, and at the same time the bond between the bromine atoms is broken:
A carbon atom that has lost an electron pair is left with an empty orbital. A bromine ion is added to it via a donor-acceptor mechanism:
The addition of hydrogen halides occurs through the stage of proton attack on the unsaturated carbon. Next, as in the reaction with bromine, a halogen ion is added:
If water is added, there are few protons (water is a weak electrolyte), and the reaction occurs in the presence of an acid as a catalyst. The addition to ethylene homologues follows Markovnikov's rule.
In the reactions of electrophilic addition of hydrogen halides and water to unsaturated hydrocarbons, hydrogen preferentially forms a bond with the most hydrogenated carbon atom.
Example 15.3. Write the reaction for the addition of hydrogen bromide to propene.
The essence of Markovnikov's rule is that hydrocarbon radicals are less electronegative (more electron-donating) substituents than the hydrogen atom. Therefore, mobile π electrons shift to sp 2 -carbon not associated with a radical or associated with a smaller number of radicals:
Naturally, hydrogen H+ attacks a carbon atom with a negative charge. It is more hydrogenated.
In functional derivatives of alkenes, substitution may go against Markovnikov's rule, but when considering the shift in electron density in specific molecules, it always turns out that hydrogen is added to the carbon atom on which there is an increased electron density. Let us consider the distribution of charges in 3-fluoropropene-1. The electronegative fluorine atom acts as an electron density acceptor. In a chain of o-bonds, electron pairs are displaced towards the fluorine atom, and mobile π electrons shift from the outermost to the middle carbon atom:
As a result, the accession goes against the Markovnikov rule:
One of the main mechanisms of mutual influence of atoms in molecules operates here - inductive effect:
The inductive effect (±/) is the displacement of electron pairs in a chain of o-bonds under the influence of an atom (group of atoms) with increased (-/) or decreased (+/) electronegativity relative to hydrogen:
The halogen atom has a different effect if it is located at the carbon atom sp2. Here the addition follows Markovnikov's rule. In this case it applies mesomeric Effect. The lone electron pair of the chlorine atom is displaced to the carbon atom, as if increasing the multiplicity of the Cl-C bond. As a result, the electrons of the n-bond are displaced to the next carbon atom, creating an excess of electron density on it. During the reaction, a proton is added to it: 
Then, as can be seen from the diagram, the chlorine ion goes to the carbon atom to which chlorine was already bonded. The mesomeric effect occurs only if the lone pair of electrons coupled With π bond, i.e. they are separated by only one single bond. When the halogen is removed from the double bond (as in 3-fluoropropene-1), the mesomeric effect disappears. The inductive effect operates in all halogen derivatives, but in the case of 2-chloropropene the mesomeric effect is stronger than the inductive effect.
Mesomeric (±M) the effect is called displacement I-electrons in the chain of sp 2 -carbon atoms with the possible participation of a lone electron pair of a functional group.
The mesomeric effect can be either positive (+M) or negative (-M). Halogen atoms have a positive mesomeric effect and at the same time a negative inductive effect. Functional groups with double bonds at oxygen atoms have a negative mesomeric effect (see below).
task 15.14. Write the structural formula of the reaction product of the addition of hydrogen chloride to 1-chlorobutene-1.
Oxosynthesis. The reaction of alkenes with carbon monoxide (II) and hydrogen is of industrial importance. It is carried out at elevated temperatures under pressure of more than 100 atm. The catalyst is metal cobalt, which forms intermediate compounds with CO. The reaction product is an oxo compound - an aldehyde containing one more carbon atom than the original alkene:
Alcadienes
Hydrocarbons with two double bonds are called alkadienes, and also more briefly dienes. The general formula of dienes is C n H 2n-2. There are three main homologous series of diene hydrocarbons:
task 15.15. Indicate in what hybrid states the carbon atoms are found in the diene hydrocarbons given above.
Conjugated diene hydrocarbons are of greatest practical importance, as they serve as raw materials for the production of various types of rubber. Non-conjugated dienes have the usual properties of alkenes. Conjugated dienes have four consecutive sp 2 carbon atoms. They are in the same plane, and their non-hybrid p-orbitals are oriented in parallel (Fig. 15.2). Therefore, overlap occurs between all neighboring p-orbitals, and π bonds not only between 1 -
2 and 3 -
4, but also between 2-3 carbon atoms. At the same time, the electrons should form two two-electron clouds. There is an overlap (resonance) of different states of n-electrons with an intermediate multiplicity of coupling between single and double: 
These connections are called conjugated. The bond between 2-3 carbon atoms turns out to be shortened compared to a regular single bond, which confirms its increased multiplicity. At low temperatures, conjugated dienes behave predominantly as compounds with two double bonds, and at elevated temperatures, as compounds with conjugated bonds.
The two most important dienes - butadiene-1,3 (divinyl) and 2-methylbutadiene-1,3 (isoprene) - are obtained from buta-
Rice. 15.2. Overlapping p-orbitals in a diene molecule
new And pentane fractions that are products of natural gas processing:
Butadiene is also obtained using the method of S.V. Lebedev from alcohol:
Electrophilic addition reactions in conjugated dienes proceed in a unique way. Butadiene, when cooled to -80 °C, attaches the first bromine molecule to position 1,2:
This product is obtained with a yield of 80%. The remaining 20% comes from the 1,4-addition product:
The remaining double bond is located between the second and third carbon atoms. First, bromine attaches to the terminal carbon atom, forming a carbonate (a particle with a positive charge on the carbon):
During the movement, the π electrons find themselves either in positions 2, 3, or in positions 3, 4. At low temperatures, they more often occupy positions 3, 4, and therefore the 1,2-addition product predominates. If bromination is carried out at a temperature of 40 °C, then the 1,4-addition product becomes the main one, its yield rises to 80%, and the rest is the 1,2-addition product.
task 15.16. Write the products of the sequential addition of bromine and chlorine to isoprene at elevated temperatures.
Butadiene and isoprene readily polymerize to form various rubbers. Polymerization catalysts can be alkali metals, organic compounds of alkali metals, and Ziegler-Natta catalysts. Polymerization occurs according to the 1,4-addition type. By their structure, rubber molecules belong to non-conjugated polyenes, that is, hydrocarbons with a large number of double bonds. These are flexible molecules that can both stretch and curl into balls. On double bonds in rubbers it appears as cis-, and the trans arrangement of hydrogen atoms and radicals. The best properties are found in cis-butadiene and cis-isoprene (natural) rubbers. Their structure is shown in Fig. 15.3. Trans-polyisoprene (gutta-percha) is also found in nature. On the given formulas
Rice. 15.3. The molecular structure of some rubbers
chuk around the connections shown by the dotted line, internal rotation is possible. Rubbers, in the molecules of which there are both double bonds cis-, and the thorax configuration are called irregular. Their properties are inferior to regular rubbers.
task 15.17. Draw the structure trans polybu Tadiene.
task 15.18. A chloro derivative of butadiene, chloroprene (2-chlorobutadiene-1,3), is known, from which chloroprene rubber is obtained. Write the structural formula of cis-chloroprene rubber.
Rubber is produced from rubber, the practical application of which is extremely wide. The largest amount of it is used to make wheel tires. To obtain rubber, rubber is mixed with sulfur and heated. Sulfur atoms join through double bonds, creating many bridges between rubber molecules. A spatial network of bonds is formed, uniting almost all existing rubber molecules into one molecule. While rubber dissolves in hydrocarbons, rubber can only swell, absorbing solvent into the empty cells between sections of hydrocarbon chains and sulfur bridges.
Alkynes
Another homologous series consists of alkynes- hydrocarbons with a triple bond between carbon atoms: 
The general formula of this series C n H 2n _ 2 is the same as for the homologous series of dienes. The first member of the series is acetylene C 2 H 2, or, according to systematic nomenclature, ethyn. The following members of the series are propyne C 3 H 4, butine C 4 H 6, pentine C 5 H 8, etc. Like alkenes and dienes, these are also unsaturated hydrocarbons, but in this series the carbon atoms are triple-linked
bond, are in a state of sp-hybridization. Their hybrid orbitals are directed in opposite directions at an angle of 180° and create a linear grouping that includes hydrogen or carbon atoms of the radicals:
task 15.19. Write the structural formulas of propyne and butine. Do they have isomers?
task 15.20. Consider the pattern of overlapping orbitals in the acetylene molecule (p. 188). What orbitals form n-bonds between carbon atoms?
The triple bond in alkenes is characterized by energy E St = 828 kJ/mol. This is 222 kJ/mol more than the double bond energy in alkenes. The C=C distance is reduced to 120 pm. Despite the presence of such a strong bond, acetylene is unstable and can decompose explosively into methane and coal:
This property is explained by the fact that the number of less durable substances in decomposition products decreases. π bonds, instead of which are created σ bonds in methane and graphite. The instability of acetylene is associated with a large release of energy during its combustion. The flame temperature reaches 3150 °C. This is sufficient for cutting and welding steel. Acetylene is stored and transported in white cylinders, in which it is in an acetone solution under a pressure of -10 atm.
Alkynes exhibit isomerism in the carbon skeleton and multiple bond positions. Spatial cistrans there is no isomerism.
task 15.21. Write the structural formulas of all possible isomers of C 5 H 8 that have a triple bond.
Receipt. Acetylene is formed by the hydrolysis of calcium carbide:
Another practically important method for producing acetylene is based on rapid heating of methane to 1500-1600 °C. In this case, methane decomposes and at the same time up to 15% acetylene is formed. The mixture of gases is quickly cooled. Acetylene is separated by dissolving it in water under pressure. The volumetric solubility coefficient of acetylene is higher than that of other hydrocarbons: K V = 1.15 (15 ° C).
Alkynes are formed when double elimination of dihalogen derivatives:
Example 15.4. How to obtain butine-2 from butene-1 in four steps?
solution. Let's write the reaction equations.
Chemical properties. Acetylene explodes at a temperature of -500 °C or under a pressure of more than 20 atm, decomposing into coal and hydrogen with an admixture of methane. Acetylene molecules can also connect with each other. In the presence of CuCl, dimerization occurs to form vinyl acetylene:
task 15.22. Name vinyl acetylene using systematic nomenclature.
When passed over heated coal, acetylene trimerizes to form benzene:
Potassium permanganate in a weakly alkaline medium oxidizes alkynes while maintaining σ bonds between carbon atoms:
In this example, the reaction product is potassium oxalate, a salt of oxalic acid. Oxidation with potassium permanganate in an acidic environment leads to complete cleavage of the triple bond:
TASK 15.23. Write an equation for the oxidation of butine-2 with potassium permanganate in a slightly alkaline medium.
Despite the greater unsaturation of the molecules, electrophilic addition reactions in alkynes are more difficult (slower) than in alkenes. Alkynes add two halogen molecules in series. The addition of hydrogen halides and water follows Markovnikov’s rule. To add water, a catalyst is required - mercury sulfate in an acidic medium (Kucherov reaction):
Hydroxyl group OH bonded to sp 2 -yvnepo house, unstable. An electron pair moves from oxygen to the nearest carbon atom, and a proton moves to the next carbon atom:
Thus, the final product of the reaction of propyne with water is the oxo compound acetone.
Hydrogen substitution reaction. Carbon in the sp-hybridization state is characterized by a slightly higher electronegativity than in the states sp 2 And sp3. Therefore, in alkynes the polarity of the C-H bond is increased, and hydrogen becomes relatively mobile. Alkynes react with solutions of heavy metal salts, forming substitution products. In the case of acetylene, these products are called acetylenides:
Calcium carbide also belongs to acetylenides (p. 364). It should be noted that acetylenides of alkali and alkaline earth metals are completely hydrolyzed. Acetylenides react with halogen derivatives of hydrocarbons to form various homologues of acetylene.
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 carbon atom, i.e. the atom at which there are more atoms hydrogen, and halogen - to less hydrogenated.
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