Types of chemical reactions in organic chemistry - Knowledge hypermarket. Mechanisms of organic reactions - substitution, addition, elimination Main types of chemical reactions in organic chemistry
>> Chemistry: Types of chemical reactions in organic chemistry
The reactions of organic substances can be formally divided into four main types: substitution, addition, elimination (elimination) and rearrangement (isomerization). It is obvious that the whole variety of reactions of organic compounds cannot be reduced to the framework of the proposed classification (for example, combustion reactions). However, such a classification will help to establish analogies with the classifications of reactions that take place between inorganic substances already familiar to you from the course of inorganic chemistry.
As a rule, the main organic compound participating in the reaction is called the substrate, and the other component of the reaction is conditionally considered as a reagent.
Substitution reactions
Reactions that result in the replacement of one atom or group of atoms in the original molecule (substrate) with other atoms or groups of atoms are called substitution reactions.
Substitution reactions involve saturated and aromatic compounds, such as, for example, alkanes, cycloalkanes or arenes.
Let us give examples of such reactions.
There are different classification systems for organic reactions that are based on different features. Among them are the following classifications:
- on end result of the reaction, that is, a change in the structure of the substrate;
- on reaction mechanism, that is, according to the type of bond breaking and the type of reagents.
Substances interacting in an organic reaction are divided into reagent and substrate. In this case, it is considered that the reagent attacks the substrate.
DEFINITION
Reagent- a substance that acts on an object - a substrate - and causes a change in the chemical bond in it. Reagents are divided into radical, electrophilic and nucleophilic.
DEFINITION
Substrate is generally considered to be a molecule that provides a carbon atom for a new bond.
CLASSIFICATION OF REACTIONS ACCORDING TO THE FINAL RESULT (CHANGES IN THE STRUCTURE OF THE SUBSTRATE)
In organic chemistry, four types of reactions are distinguished according to the final result and the change in the structure of the substrate: addition, substitution, splitting off, or elimination(from English. to eliminate- remove, split off), and rearrangements (isomerizations)). Such a classification is similar to the classification of reactions in inorganic chemistry according to the number of initial reagents and formed substances, with or without a change in composition. Classification according to the final result is based on formal features, since the stoichiometric equation, as a rule, does not reflect the reaction mechanism. Let's compare the types of reactions in inorganic and organic chemistry.
Type of reaction in inorganic chemistry | Example | Type of reaction in organic chemistry | Variety and example Reactions |
|---|---|---|---|
1. Connection | C l2 + H2 = 2 H C l | Attachment by multiple bonds | hydrogenation |
Hydrohalogenation
|
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Halogenation
|
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Hydration
|
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2. Decomposition | 2
H2
O = 2 H2
+
O2
| elimination | Dehydrogenation
|
Dehydrohalogenation
|
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Dehalogenation
|
|||
Dehydration
|
|||
3. Substitution | Z n + 2 H C l =ZnCl2+H2 | substitution |
|
4. Exchange (special case - neutralization) | H2 S O4 + 2 N a O H\u003d N a 2 S O 4 + 2 H 2 O | special case - esterification |
|
5. Allotropization | graphite ⇔ diamond Pred⇔ Pwhite Pred⇔P white Srhombus.⇔ Sreservoir Srhombus⇔Splast. | Isomerization | Isomerization alkanes
|
n) without replacing them with others.
Depending on which atoms are split off - neighboring C–C or isolated by two or three or more carbon atoms - C-C-C- C–, –C-C-C-C- C- may form compounds with multiple bonds and or cyclic compounds. The elimination of hydrogen halides from alkyl halides or water from alcohols occurs according to the Zaitsev rule.
DEFINITION
Zaitsev's rule: the hydrogen atom H is split off from the least hydrogenated carbon atom.
For example, the splitting off of a hydrogen bromide molecule occurs from neighboring atoms in the presence of alkali, with the formation of sodium bromide and water.
DEFINITION
regrouping- a chemical reaction, as a result of which there is a change in the mutual arrangement of atoms in a molecule, the movement of multiple bonds or a change in their multiplicity.
The rearrangement can be carried out with the preservation of the atomic composition of the molecule (isomerization) or with its change.
DEFINITION
Isomerization- a special case of a rearrangement reaction, leading to the transformation of a chemical compound into an isomer by structural changes in the carbon skeleton.
The rearrangement can also be carried out by a homolytic or heterolytic mechanism. Molecular rearrangements can be classified according to different criteria, for example, by the saturation of the systems, by the nature of the migrating group, by stereospecificity, etc. Many rearrangement reactions have specific names - Claisen rearrangement, Beckman rearrangement, etc.
Isomerization reactions are widely used in industrial processes, such as oil refining to increase the octane number of gasoline. An example of isomerization is the transformation n-octane to isooctane:
CLASSIFICATION OF ORGANIC REACTIONS BY TYPE OF REAGENT
DISCONNECTION
Bond cleavage in organic compounds can be homolytic or heterolytic. 
DEFINITION
Homolytic bond breaking- this is such a gap, as a result of which each atom receives an unpaired electron and two particles are formed that have a similar electronic structure - free radicals.
Homolytic gap is characteristic of non-polar or weakly polar bonds, for example C–C, Cl–Cl, C–H, and requires a large amount of energy.
The resulting radicals with an unpaired electron are highly reactive, so the chemical processes that occur with the participation of such particles are often of a “chain” nature, they are difficult to control, and as a result of the reaction a set of substitution products is obtained. So, in the chlorination of methane, the substitution products are chloromethane C H3 Cl CH3Cl, dichloromethane C H2 C l2 CH2Cl2, chloroform C H C l3 CHCl3 and carbon tetrachloride C C l4 CCl4. Reactions involving free radicals proceed according to the exchange mechanism of the formation of chemical bonds.
The radicals formed during this bond rupture cause radical mechanism the course of the reaction. Radical reactions usually take place at elevated temperatures or with radiation (such as light).
Due to their high reactivity, free radicals can have a negative effect on the human body, destroying cell membranes, affecting DNA and causing premature aging. These processes are associated primarily with lipid peroxidation, that is, the destruction of the structure of polyunsaturated acids that form fat inside the cell membrane.
DEFINITION
Heterolytic bond breaking- this is such a gap in which an electron pair remains at a more electronegative atom and two charged particles are formed - ions: a cation (positive) and an anion (negative).
In chemical reactions, these particles perform the functions of " nucleophiles"(" phil "- from gr. be in love) and " electrophiles”, forming a chemical bond with the reaction partner by the donor-acceptor mechanism. Nucleophilic particles provide an electron pair for the formation of a new bond. In other words,
DEFINITION
Nucleophile- an electron-rich chemical reagent capable of interacting with electron-deficient compounds.
Examples of nucleophiles are any anions ( C l− , I− , N O− 3 Cl−,I−,NO3− etc.), as well as compounds having an unshared electron pair ( N H3 , H2 O NH3,H2O).
Thus, when a bond is broken, radicals or nucleophiles and electrophiles can be formed. Based on this, three mechanisms for the occurrence of organic reactions are distinguished.
MECHANISMS OF ORGANIC REACTIONS
Free radical mechanism: the reaction is initiated by free radicals formed during homolytic rupture bonds in a molecule.
The most typical variant is the formation of chlorine or bromine radicals during UV irradiation.
1. Free radical substitution
methane bromine
Chain initiation
chain growth
chain break
2. Free radical addition
ethene polyethylene
Electrophilic mechanism: the reaction begins with electrophilic particles, which receive a positive charge as a result heterolytic gap connections. All electrophiles are Lewis acids.
Such particles are actively formed under the influence of Lewis acids, which increase the positive charge of the particle. The most commonly used A l C l3 , F e C l3 , F e B r3 , Z n C l2 AlCl3,FeCl3,FeBr3,ZnCl2 acting as a catalyst.
The place of attack of the particle-electrophile are those parts of the molecule that have an increased electron density, i.e., a multiple bond and a benzene ring.
The general form of electrophilic substitution reactions can be expressed by the equation:
1. Electrophilic substitution
benzene bromobenzene
2. electrophilic addition
propene 2-bromopropane
propyne 1,2-dichloropropene
Attachment to asymmetric unsaturated hydrocarbons occurs in accordance with Markovnikov's rule.
DEFINITION
Markovnikov's rule: the addition of molecules of complex substances to unsymmetrical alkenes with the conditional formula HX (where X is a halogen atom or a hydroxyl group OH–), a hydrogen atom is attached to the most hydrogenated (containing the most hydrogen atoms) carbon atom with a double bond, and X to the least hydrogenated.
For example, the addition of hydrogen chloride HCl to a propene molecule C H3 – C H = C H2 CH3–CH=CH2.
The reaction proceeds by the mechanism of electrophilic addition. Due to the electron donor influence C H3 CH3-groups, the electron density in the substrate molecule is shifted to the central carbon atom (inductive effect), and then, along the system of double bonds, to the terminal carbon atom C H2 CH2-groups (mesomeric effect). Thus, the excess negative charge is localized precisely on this atom. Therefore, the hydrogen proton starts the attack H+ H+, which is an electrophilic particle. A positively charged carbene ion is formed [ C H3 – C H − C H3 ] + + , to which the chlorine anion is attached C l− Cl−.
DEFINITION
Exceptions to Markovnikov's rule: the addition reaction proceeds against the Markovnikov rule, if compounds enter into the reaction in which the carbon atom adjacent to the carbon atom of the double bond partially withdraws the electron density, that is, in the presence of substituents that exhibit a significant electron-withdrawing effect (– C C l3 , – C N , – C O O H(–CCl3,–CN,–COOH and etc.).
Nucleophilic mechanism: the reaction is started by nucleophilic particles having a negative charge, formed as a result of heterolytic gap connections. All nucleophiles are Lewis founding.
In nucleophilic reactions, the reagent (nucleophile) has a free pair of electrons on one of the atoms and is a neutral molecule or anion ( H a l– ,O H– , R O− , R S– , R C O O– , R– , C N – , H2 O , R O H , N H3 , R N H2 Hal–,OH–,RO–,RS–,RCOO–,R–,CN–,H2O,ROH,NH3,RNH2 and etc.).
The nucleophile attacks the atom in the substrate with the lowest electron density (i.e., with a partial or full positive charge). The first step in the nucleophilic substitution reaction is the ionization of the substrate to form a carbocation. In this case, a new bond is formed due to the electron pair of the nucleophile, and the old one undergoes a heterolytic cleavage with subsequent elimination of the cation. An example of a nucleophilic reaction is a nucleophilic substitution (symbol SN SN) at a saturated carbon atom, for example, alkaline hydrolysis of bromo derivatives.
1. Nucleophilic substitution
2. Nucleophilic addition
ethanal cyanohydrin
source http://foxford.ru/wiki/himiya
Organic compounds can react both with each other and with inorganic substances - non-metals, 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 taking place. There are many registered reactions named after the scientists who discovered them.
The molecule of an organic compound participating in a 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 cover the entire molecule of an organic compound. Of these reactions, combustion is most widely known, leading 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 of particular interest. 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 the chemical transformation takes place is called the reaction center.
In multielement substances, the reaction centers are the functional groups and the carbon atoms with which they are associated. In unsaturated hydrocarbons, the reaction center is carbon atoms bound by a multiple bond. In saturated hydrocarbons, the reaction center is predominantly secondary and tertiary carbon atoms.
Molecules of organic compounds often contain several reaction centers exhibiting different activities. Therefore, as a rule, there are several parallel reactions that give different products. The fastest reaction is called main. The rest of the reactions side effects. The resulting mixture contains the product of the main reaction in the largest amount, 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 great importance in organic chemistry. Their role is not limited to huge time savings in chemical processes. By choosing catalysts that accelerate certain types of reactions, one or another of the parallel reactions can be purposefully carried out and the desired products can be obtained. During the existence of the organic compound industry, the discovery of new catalysts has fundamentally changed technology. For example, ethanol was obtained for a long time only by starch fermentation, and then they switched to its production.
addition of 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 BUT)- 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) cleavage or elimination reactions (symbol E)- an organic molecule loses some fragments, forming, as a rule, inorganic substances;
d) cracking - the splitting of 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 - the transformation of a molecule into another isomer;
g) polymerization - the formation of a high molecular weight compound from one or more low molecular weight 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 break. From the electron pair of a chemical bond, each atom has one electron left. The resulting particles having unpaired electrons are called free radicals. The composition of such a particle can be a molecule or a single atom. The reaction is called radical (symbol R):
Heterolytic cleavage. In this case, one atom retains an electron pair and becomes a base. The particle containing this atom is called nucleophile. Another atom, deprived of an electron pair, has a free orbital and becomes an acid. The particle containing this atom is called electrophile:
According to this type, the n-bond is especially easily broken while maintaining
For example, some particle A, attracting an n-electron pair, itself forms a bond with a carbon atom:
This interaction is represented 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 reactant, then the reaction is called electrophilic, and the reactant is called an electrophile.
Varieties of electrophilic reactions - addition A E and substitution 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 reactant, then the reaction is called nucleophilic, and the reactant is called a nucleophile.
Varieties of nucleophilic reactions - addition Ad, and substitution S N .
The heterolytic rupture and the formation of chemical bonds are actually a single coordinated process: the gradual rupture of the existing bond is accompanied by the formation of a new bond. In a coordinated process, the activation energy is less.
QUESTIONS AND EXERCISES
1. When burning 0.105 g of organic matter, 0.154 g of carbon dioxide, 0.126 g of water and 43.29 ml of nitrogen (21 ° C, 742 mm Hg) were formed. 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. Determine the element X and write the formulas for possible isomers.
3. For 19.8 g of the compound C 2 H 4 X 2, there are 10 mol of electrons. Identify the element X and write the formulas for the 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 the radical having two isomers: -C 2 H 5 , -C 3 H 7 , -CH 3 .
6. Indicate the substance with 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 of 2,3,4-trimethite-4-ethylheptene. Give the structural formulas of two isomers of this substance containing one and two quaternary carbon atoms.
9. Write the formula of 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 C 10 structure, in which the atoms of additional elements are located at 2 and 7 carbon atoms, and the name contains the root "hepta".
11. Name a hydrocarbon having 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 chemicals, packaging materials, etc. The initial source of all of these is oil and natural gas. The well-being of states depends on the availability of their reserves. Oil has caused international crises.
Among the most well-known hydrocarbons are methane and propane, which are used in household stoves. Methane is transported through pipes, while propane is transported and stored in red cylinders. Another hydrocarbon, ilo-butane, which is 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 composition. Mixtures of heavier hydrocarbons are semi-liquid vaseline and solid paraffin. Hydrocarbons also include a well-known substance used to protect wool and fur from moths - naphthalene. The main types of hydrocarbons in terms 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 contains data on the structure, composition, isomerism, names and some properties of alkanes. Recall that in alkane molecules, carbon atoms form tetrahedrally directed 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 end groups of CH 3 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 - iso alkanes. Data on the names and some physical properties of alkanes are given in Table. 15.2.
In the form of individual substances, the first four members of the alkane series - methane, ethane, propane and butane - are used in large quantities. 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 such mixture. It is characterized boiling point 30-205 °С. Other types of hydrocarbon fuels are also characterized by boiling intervals, since as light hydrocarbons evaporate from them, the boiling point rises. All alkanes are practically insoluble in water.
Table 15.2. Names and boiling and melting points of normal alkanes
assignment 15.1. Group the alkanes according to the state of aggregation at 20 ° C and normal atmospheric pressure (according to Table 15.2).
assignment 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 the isolation of individual substances from it is a rather difficult task. Ordinary petroleum products are fractions obtained during the 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. This method can produce gasoline, but it is still more expensive than gasoline from oil. Methane is formed in a mixture of carbon monoxide (P) and hydrogen on a nickel catalyst:
In the same mixture on catalysts containing cobalt, both a mixture of hydrocarbons and individual hydrocarbons are obtained. It can be not only alkanes, but also cycloalkanes.
There are laboratory methods for obtaining individual alkanes. Carbides of some metals during hydrolysis give methane:
Halogenalkanes react with an alkali metal to form hydrocarbons with twice the number of carbon atoms. This is the Wurtz reaction. It goes through a hemolytic rupture 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 a mixture of 2-bromopropane and 1-bromopropane. Write the equations of possible reactions.
SOLUTION. The radicals formed during the reactions of bromoalkanes with potassium can combine with each other in various combinations, resulting in three hydrocarbons in the mixture. Summary reaction equations:
Sodium salts of organic acids, when heated with alkali, lose the carboxyl group (decarboxylate) with the formation of an alkane:
During the electrolysis of the same salts, decarboxylation and the combination of the remaining radicals into one molecule occur:
Alkanes are formed during the hydrogenation of unsaturated hydrocarbons and the reduction of compounds containing functional groups:
Chemical properties. Limit hydrocarbons are the least active organic substances. Their original name paraffins reflects a weak affinity (reactivity) in relation to other substances. They react, as a rule, not with ordinary molecules, but only with free radicals. Therefore, the reactions of alkanes proceed under the conditions of the formation of free radicals: at high temperature or irradiation. Alkanes burn when mixed with oxygen or air and play an important role as a 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 yl-octane (p = = 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:
The radical takes away a hydrogen atom from an alkane, for example, from 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 give rise to 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:
task 15.5. With a decrease in the volume of the vessel in which the chain reaction takes place, the number of transformations per radical (chain length) decreases. Give this an explanation.
The reaction product chloromethane belongs to the class of halogenated hydrocarbons. In the mixture, as chloromethane is formed, the reaction of substitution of the second hydrogen atom for chlorine, then the third, etc., begins. At the third stage, the well-known substance chloroform CHClg, used in medicine for anesthesia, is formed. The product of the complete replacement of hydrogen by 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 are 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 by a halogen occurs predominantly on the same carbon atom:
Alkanes react when heated with dilute nitric acid and nitric oxide (IV), forming 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 to zo-alkanes:
The industrial isomerization of alkanes to improve the quality of motor fuel is called reforming. The catalyst is platinum metal deposited on alumina. Cracking is also important for oil refining, i.e., the splitting of an alkane molecule into two parts - alkane and alkene. Cleavage occurs predominantly in the middle of the molecule: 
Cracking catalysts are aluminosilicates.
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.
Of increasing practical importance functionalization alkanes, i.e., their transformation into compounds containing functional groups (usually oxygen). Butane is oxidized with acid
lorode 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 in them the angles between C-C-C bonds differ significantly from the normal tetrahedral angle of 109.5 ° characteristic of sp 3 -carbon. This leads to a decrease in the binding energy. Under the action of halogens, the cycles are broken and attached 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 choose 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, as well as unsaturated. The simplest homologous series of unsaturated hydrocarbons is C n H 2n alkenes having one double bond:
The other two valencies of carbon atoms are used to add hydrogen and saturated hydrocarbon radicals.
The first member of the alkene series 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 \u003d CH-, allyl CH 2 \u003d CH-CH 2 -.
Carbon atoms linked 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 - π bond(Fig. 15.1). The total energy of the double bond is 606 kJ / mol, and the a-bond accounts for about 347 kJ / mol, and π bond- 259 kJ/mol. The increased strength of the double bond is manifested in a decrease in the distance between carbon atoms to 133 pm compared to 154 pm for a single C-C bond.
Despite the formal strength, it is the double bond in alkenes that turns out to be the main reaction center. Electronic pair π -bonds form a fairly scattered cloud, relatively remote from atomic nuclei, as a result of which it is mobile and sensitive to the influence of other atoms (p. 442). π -The cloud is shifted to 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 action of an attacking molecule. This leads to the high reactivity of alkenes compared to alkanes. A mixture of gaseous alkanes does not react with bromine water, but in the presence of an admixture of alkenes, it becomes discolored. This sample is used to detect alkenes.
Alkenes have additional types of isomerism that are absent in alkanes: double bond position isomerism and spatial cis-trans isomerism. The last type of isomerism is due to a special symmetry π -connections. It prevents internal rotation in the molecule and stabilizes the arrangement of four substituents at the C=C atoms in one plane. If there are two pairs of different substituents, then with the diagonal arrangement of the substituents of each pair, a trans isomer is obtained, and with an adjacent arrangement, a cis isomer. Ethene and propene have no isomers, but butene has both types of isomers:
assignment 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 each alkene be considered an isomer with respect to other alkenes?
assignment 15.8. Are spatial isomers possible in the presence of three and four different substituents at 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 is
walks as a result of the removal of hydrogen (dehydrogenation) and cracking. Dehydrogenation of butane gives predominantly butene-2:
task 15.10. Write the reaction of cracking malk-
Dehydrogenation and cracking require relatively high temperatures. Under normal conditions or mild heating, alkenes are formed from halogen derivatives. Chloro- and bromoalkanes react with alkali in an alcohol solution, splitting off a halogen and hydrogen from two adjacent carbon atoms:
This is an elimination reaction (p. 441). If a different number of hydrogen atoms are attached to two neighboring carbon atoms, then elimination proceeds according to the Zaitsev rule.
In an elimination reaction, hydrogen is predominantly split off from a less hydrogenated carbon atom.
example 15.2. Write the reaction for the elimination of 2-chlorobutane.
solution. According to the Zaitsev rule, hydrogen is split off from the 3 C atom:
Under the action of zinc and magnesium metals on dihaloalkanes with a neighboring position of halogens, alkenes are also formed:
Chemical properties. Alkenes can both decompose at high temperature to simple substances, and polymerize, turning into high-molecular substances. Ethylene is polymerized at very high pressure (-1500 atm) with the addition of a small amount of oxygen as a free radical initiator. From liquid ethylene under these conditions, a white flexible mass is obtained, transparent in a thin layer, - polyethylene. This is a well-known material. The polymer is made up of very long molecules.
Molecular weight 20 OOO-40 OOO. In structure, this is a saturated hydrocarbon, but oxygen atoms can be located at the ends of the molecules. With a large molecular weight, the proportion of end groups is very small and it is difficult to establish their nature.
task 15.11. How many ethylene molecules are included in one molecule of polyethylene with a molecular weight of 28,000?
Ethylene polymerization 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., is destroyed by light and other factors. The production of polyethylene began around 1955. This material had a significant impact on everyday life, as packaging bags began to be made from it. Of the other alkene polymers, polypropylene is the most important. It produces a tougher and less transparent film than polyethylene. The polymerization of propylene is carried out with
Ziegler-Natta talizator. The resulting polymer has the correct isotactic structure
High pressure polymerization results in 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. Under normal conditions, alkenes are oxidized at the double bond upon contact with solutions of potassium permanganate and other oxidizing agents. In a weakly alkaline environment, glycols, i.e. diatomic alcohols:
In an acidic environment, when heated, alkenes are oxidized with a complete rupture of the molecule along the double bond:
task 15.12. Write an equation for this reaction.
task 15.13. Write the equations for the oxidation of butene-1 and butene-2 with potassium permanganate in an acidic medium.
Ethylene is oxidized with 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 for the production of polymers and detergents.
Reactions of electrophilic addition. Molecules of halogens, hydrogen halides, water, and many others are attached to alkenes at the double bond. Let us consider the addition mechanism using bromine as an example. When the Br 2 molecule attacks one of the carbon atoms of the unsaturated center, the electron pair π -bond is shifted 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 bromine atoms is broken:
The carbon atom that has lost an electron pair has a free orbital left. The bromine ion is added to it by the donor-acceptor mechanism:
The addition of hydrogen halides goes through the stage of proton attack on unsaturated carbon. Further, as in the reaction with bromine, a halogen ion is added:
In the case of addition of water, there are few protons (water is a weak electrolyte), and the reaction proceeds in the presence of an acid as a catalyst. The addition to ethylene homologues follows the Markovnikov rule.
In reactions of electrophilic addition of hydrogen halides and water to unsaturated hydrocarbons, hydrogen predominantly forms a bond with the most hydrogenated carbon atom.
example 15.3. Write the addition reaction of hydrogen bromide to propene.
The essence of Markovnikov's rule is that hydrocarbon radicals are less electronegative (more electron-donating) substituents than a hydrogen atom. Therefore mobile π electrons are shifted 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 also more hydrogenated.
In functional derivatives of alkenes, substitution can go against Markovnikov's rule, but when considering the shift in electron density in specific molecules, it always turns out that hydrogen is attached to the carbon atom, which has an increased electron density. Let us consider the charge distribution in 3-fluoropropene-1. The electronegative fluorine atom acts as an electron density acceptor. In the chain of o-bonds, electron pairs are shifted to the fluorine atom, and mobile π electrons shift from the outermost to the middle carbon atom:
As a result, the addition goes against Markovnikov's rule:
Here one of the main mechanisms of the mutual influence of atoms in molecules operates - inductive effect:
The inductive effect (±/) is the displacement of electron pairs in the chain of o-bonds under the action of an atom (group of atoms) with increased (-/) or decreased (+/) electronegativity relative to hydrogen:
A halogen atom has a different effect if it is located at a carbon atom sp2. Here, the adjunction follows the Markovnikov rule. In this case, the mesomeric Effect. The unshared electron pair of the chlorine atom is shifted to the carbon atom, as if increasing the multiplicity of the Cl-C bond. As a result, the electrons of the n-bond are shifted 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 with which chlorine has already been bound. The mesomeric effect occurs only if the lone pair of electrons conjugated 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 is active in all halogen derivatives, but in the case of 2-chloropropene the mesomeric effect is stronger than the inductive one.
Mesomeric (±M) effect is displacement I-electrons in the chain of sp 2 -carbon atoms with the possible participation of the unshared electron pair of the functional group.
The mesomeric effect can be both positive (+M) and negative (-M). Halogen atoms have a positive mesomeric effect and at the same time a negative inductive effect. The negative mesomeric effect has functional groups with double bonds at oxygen atoms (see below).
task 15.14. Write the structural formula of the reaction product of the addition of hydrogen chloride to 1-chlorobutene-1.
Oxosynthesis. Of great industrial importance is the reaction of alkenes with carbon monoxide (II) and hydrogen. It is carried out at an elevated temperature under a pressure of more than 100 atm. The catalyst is metallic cobalt, which forms intermediate compounds with CO. The reaction product is an oxo compound - an aldehyde containing one carbon atom more than the original alkene:
Alkadienes
Hydrocarbons with two double bonds are called alkadienes, and also shorter dienes. The general formula of dienes C n H 2n-2 There are three main homologous series of diene hydrocarbons:
task 15.15. Indicate in what hybrid states are the carbon atoms in the diene hydrocarbons given above.
Conjugated diene hydrocarbons are of the greatest practical importance, since they serve as raw materials for the production of various types of rubber and 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, there is an overlap between all neighboring p-orbitals, and are formed π bonds not only between 1 -
2 and 3 -
4, but also between 2-3 carbon atoms. At the same time, the electrons must form two two-electron clouds. There is a superposition (resonance) of different states of n-electrons with an intermediate bond multiplicity between single and double: 
These connections are called conjugated. The bond between 2-3 carbon atoms is shortened compared to the usual 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 by the method of S. V. Lebedev from alcohol:
Electrophilic addition reactions in conjugated dienes proceed in a peculiar way. Butadiene, when cooled to -80 ° C, adds 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 carbonatone (a particle with a positive charge on carbon):
In the process of movement, the n-electrons turn out to be in the 2, 3 position, then in the 3, 4 position. At low temperatures, they more often occupy the 3, 4 position, 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 sequential addition of bromine and chlorine to isoprene at elevated temperature.
Butadiene and isoprene readily polymerize to form various rubbers. Alkali metals, organic compounds of alkali metals, Ziegler-Natta catalysts can serve as polymerization catalysts. Polymerization proceeds according to the 1,4-addition type. Rubber molecules in their structure belong to non-conjugated polyenes, i.e., to 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 arises as cis-, and the trans arrangement of hydrogen atoms and radicals. The best properties are 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 above formulas, kau-
Rice. 15.3. Molecule structure of some rubbers
Chooks around the links shown by a dotted line, internal rotation is possible. Rubbers, in the molecules of which, with double bonds, there is both cis-, and thorax configuration are called irregular. In terms of properties, they are inferior to regular rubbers.
task 15.17. draw the structure trans polybu tadiene.
task 15.18. A chloroprene butadiene derivative 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 unusually wide. The largest amount of it goes to the manufacture of wheel tires. To make rubber, rubber is mixed with sulfur and heated. Sulfur atoms are attached via double bonds, creating many bridges between rubber molecules. A spatial network of bonds is formed, which combines almost all available rubber molecules into one molecule. While rubber dissolves in hydrocarbons, rubber can only swell by absorbing the solvent into the empty spaces between the hydrocarbon chains and the sulfur bridges.
Alkynes
Another homologous series is alkynes- hydrocarbons with a triple bond between carbon atoms: 
The general formula for this series C n H 2n _ 2 is the same as for the homologous series of dienes. The first member of the series is C 2 H 2 acetylene, or, according to the systematic nomenclature, ethyne. The following members of the series 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 bound by the triple
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, including hydrogen or carbon atoms of the radicals:
task 15.19. Write the structural formulas of propyne and butyne. Do they have isomers?
task 15.20. Consider the scheme 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 the energy E St = 828 kJ/mol. This is 222 kJ/mol more than the energy of the double bond 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 compounds decreases in the decomposition products. 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. It is enough for cutting and welding steel. Acetylene is stored and transported in white cylinders, in which it is in acetone solution at a pressure of -10 atm.
Alkynes exhibit carbon skeletal isomerism and multiple bond positions. Spatial cistrans isomerism is absent.
task 15.21. Write the structural formulas of all possible C 5 H 8 isomers with a triple bond.
Receipt. Acetylene is formed by the hydrolysis of calcium carbide:
Another practically important method for producing acetylene is based on the 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 rapidly cooled. Acetylene is separated by dissolving in pressurized water. The volumetric solubility coefficient of acetylene is greater 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 get butyn-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 combine with each other. In the presence of CuCl, dimerization occurs with the formation of vinylacetylene:
task 15.22. Name vinylacetylene according to the systematic nomenclature.
When passed over heated charcoal, acetylene trimerizes to form benzene:
Potassium permanganate in a weakly alkaline medium oxidizes alkynes with preservation 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 a complete cleavage of the triple bond:
ASSIGNMENT 15.23. Write an equation for the oxidation of butyne-2 with potassium permanganate in a slightly alkaline medium.
Despite the high 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 the Markovnikov rule. To add water, a catalyst is needed - mercury sulfate in an acidic environment (Kucherov's reaction):
Hydroxyl group OH associated with sp 2 -yvnepo house, unstable. The electron pair moves from oxygen to the nearest carbon atom, and the 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 salts of heavy metals, forming substitution products. In the case of acetylene, these products are called acetylides:
Calcium carbide also belongs to acetylenides (p. 364). It should be noted that acetylenides of alkali and alkaline earth metals are completely hydrolyzed. Acetylides react with halogen derivatives of hydrocarbons to form various acetylene homologues.
In the course of a reaction, some chemical bonds are broken in the molecules of the reacting substances and others are formed. Organic reactions are classified according to the type of breaking of chemical bonds in reacting particles. Of these, two large groups of reactions can be distinguished - radical and ionic.
Radical reactions are processes that go with a homolytic rupture of a covalent bond. In a homolytic rupture, a pair of electrons forming a bond is divided in such a way that each of the formed particles receives one electron. As a result of homolytic rupture, free radicals are formed:
A neutral atom or particle with an unpaired electron is called a free radical.
Ionic reactions are processes that occur with heterolytic breaking of covalent bonds, when both bond electrons remain with one of the previously bound particles:
As a result of heterolytic bond cleavage, charged particles are obtained: nucleophilic and electrophilic.
A nucleophilic particle (nucleophile) is a particle that has a pair of electrons in the outer electronic level. Due to the pair of electrons, the nucleophile is able to form a new covalent bond.
An electrophilic particle (electrophile) is a particle that has an unfilled outer electronic level. The electrophile represents unfilled, vacant orbitals for the formation of a covalent bond due to the electrons of the particle with which it interacts.
In organic chemistry, all structural changes are considered relative to the carbon atom (or atoms) involved in the reaction.
In accordance with the above, the chlorination of methane by the action of light is classified as a radical substitution, the addition of halogens to alkenes as an electrophilic addition, and the hydrolysis of alkyl halides as a nucleophilic substitution.
The following types of actions are the most common.
Main types of chemical reactions
I. Substitution reactions(replacement of one or more hydrogen atoms with halogen atoms or a special group) RCH 2 X + Y → RCH 2 Y + X
II. Addition reactions RCH=CH 2 + XY → RCHX−CH 2 Y
III. Cleavage (elimination) reactions RCHX−CH 2 Y → RCH=CH 2 + XY
IV. Isomerization reactions (rearrangements)
v. Oxidation reactions(interaction with air oxygen or an oxidizing agent)
In these types of reactions listed above, there are also specialized and registered reactions.
Specialized:
1) hydrogenation (interaction with hydrogen)
2) dehydrogenation (cleavage from a hydrogen molecule)
3) halogenation (interaction with halogen: F 2, Cl 2, Br 2, I 2)
4) dehalogenation (cleavage from a halogen molecule)
5) hydrohalogenation (interaction with hydrogen halide)
6) dehydrohalogenation (cleavage from a hydrogen halide molecule)
7) hydration (interaction with water in an irreversible reaction)
8) dehydration (cleavage from a water molecule)
9) hydrolysis (interaction with water in a reversible reaction)
10) polymerization (obtaining a multiple enlarged carbon skeleton from identical simple compounds)
11) polycondensation (obtaining a multiple enlarged carbon skeleton from two different compounds)
12) sulfonation (interaction with sulfuric acid)
13) nitration (interaction with nitric acid)
14) cracking (reduction of the carbon skeleton)
15) pyrolysis (decomposition of complex organic substances into simpler ones under the influence of high temperatures)
16) alkylation reaction (introduction of an alkane radical into the formula)
17) acylation reaction (introduction of the -C (CH 3) O group into the formula)
18) aromatization reaction (formation of a hydrocarbon of a series of arenes)
19) decarboxylation reaction (cleavage from the molecule of the carboxyl group -COOH)
20) esterification reaction (reaction of an alcohol with an acid, or obtaining an ester from an alcohol or a carboxylic acid)
21) the reaction of the "silver mirror" (interaction with an ammonia solution of silver oxide (I))
Nominal reactions:
1) Wurtz reaction (elongation of the carbon skeleton during the interaction of a halogenated hydrocarbon with an active metal)
2) Kucherov reaction (obtaining aldehyde by reacting acetylene with water)
3) Konovalov reaction (reaction of an alkane with dilute nitric acid)
4) Wagner reaction (oxidation of hydrocarbons with a double bond by the oxygen of an oxidizing agent in a weakly alkaline or neutral medium under normal conditions)
5) Lebedev reaction (dehydrogenation and dehydration of alcohols in the production of alkadienes)
6) Friedel-Crafts reaction (alkylation reaction of arene with chloroalkane to obtain benzene homologues)
7) Zelinsky reaction (obtaining benzene from cyclohexane by dehydrogenation)
8) Kirchhoff reaction (conversion of starch to glucose under the catalytic action of sulfuric acid)
Lesson topic: Types of chemical reactions in organic chemistry.
Lesson type: lesson of studying and primary consolidation of new material.
Lesson Objectives: create conditions for the formation of knowledge about the features of the flow of chemical reactions involving organic substances when getting acquainted with their classification, consolidate the ability to write reaction equations.
Lesson objectives:
Teaching: to study the types of reactions in organic chemistry, based on the knowledge of students about the types of reactions in inorganic chemistry and their comparison with the types of reactions in organic chemistry.
Developing: to promote the development of logical thinking and intellectual skills (analyze, compare, establish cause-and-effect relationships).
Educational: to continue the formation of a culture of mental work; communication skills: listen to other people's opinions, prove their point of view, find compromises.
Teaching methods:verbal (story, explanation, problem presentation); visual (multimedia visual aid); heuristic (written and oral exercises, problem solving, test tasks).
Means of education:implementation of intra- and interdisciplinary connections, multimedia visual aid (presentation), symbolic-graphic table.
Technology: elements of cooperation pedagogy, student-centered learning (competence-based learning, humane-personal technology, individual and differentiated approach), information and communication technology, health-saving educational technologies (organizational and pedagogical technology).
Brief description of the course of the lesson.
I. Organizational stage: mutual greetings of the teacher and students; checking the preparedness of students for the lesson; organization of attention and mood for the lesson.
Checking homework.Questions for verification: 1. Finish the sentences: a) Isomers are ... b) The functional group is ... 2. Classify the indicated formulas of substances (the formulas are offered on the cards) and name the classes of compounds to which they belong. 3. Make possible abbreviated structural formulas of isomers corresponding to molecular formulas (for example: C 6 H 14, C 3 H 6 O)
Reporting the topic and tasks of studying new material; showing its practical significance.
II. Learning new material:
Knowledge update.(The teacher's story is based on slide schemes that students transfer to notebooks as a reference note)
Chemical reactions are the main object of the science of chemistry. (Slide 2)
In the process of chemical reactions, one substance is transformed into another.
Reagent 1 + Reagent 2 = Products (inorganic chemistry)
Substrate + Attack Reagent = Products (Organic Chemistry)
In many organic reactions, not all molecules undergo a change, but their reaction parts (functional groups, their individual atoms, etc.), which are called reaction centers. The substrate is the substance in which the old carbon atom is broken and a new bond is formed, and the compound acting on it or its reactive particle is called a reagent.
Inorganic reactions are classified according to several criteria: according to the number and composition of the starting materials and products (compounds, decomposition, substitution, exchange), according to the thermal effect (exo- and endothermic), according to the change in the oxidation state of atoms, according to the reversibility of the process, according to the phase (homo- and heterogeneous), by the use of a catalyst (catalytic and non-catalytic). (Slides 3,4)
The result of the stage of the lesson is the fulfillment by the students of the task (slide 5), which allows you to test your skills in writing equations of chemical reactions, arranging stoichiometric coefficients, and classifying inorganic reactions. (Assignments are offered at different levels)
(The exercise of "brain" gymnastics for the development of cognitive and mental processes - "Owl": improves visual memory, attention and relieves stress that develops with prolonged sitting.)Grab your left shoulder with your right hand and squeeze it, turn to the left so that you look back, breathe deeply and push your shoulders back. Now looking over your other shoulder, drop your chin on your chest and breathe deeply, letting your muscles relax..
Presentation of new material.(During the presentation of the material, students make notes in notebooks on which the teacher focuses attention - slide information)
Reactions involving organic compounds obey the same laws (the law of conservation of mass and energy, the law of mass action, Hess's law, etc.) and exhibit the same patterns (stoichiometric, energy, kinetic) as the reactions of inorganic substances. (Slide 6)
Organic reactions are usually classified according to the mechanisms of occurrence, direction and final products of the reaction. (Slide 7)
The way in which covalent bonds are broken determines the type of reaction mechanism. Under the reaction mechanism understand the sequence of stages of the reaction with an indication of the intermediate particles formed at each of these stages. (The reaction mechanism describes its path, i.e. the sequence of elementary acts of interaction of the reagents through which it flows.)
In organic chemistry, two main types of reaction mechanism are distinguished: radical (homolytic) and ionic (heterolytic). (Slide 8)
In a homolytic rupture, a pair of electrons forming a bond is divided in such a way that each of the formed particles receives one electron. As a result of homolytic rupture, free radicals are formed:
X:Y → X . + . Y
A neutral atom or particle with an unpaired electron is called a free radical.
As a result of heterolytic bond cleavage, charged particles are obtained: nucleophilic and electrophilic.
X:Y → X + + :Y -
A nucleophilic particle (nucleophile) is a particle that has a pair of electrons in the outer electronic level. Due to the pair of electrons, the nucleophile is able to form a new covalent bond.
An electrophilic particle (electrophile) is a particle that has a free orbital on the outer electronic level. The electrophile represents unfilled, vacant orbitals for the formation of a covalent bond due to the electrons of the particle with which it interacts.
Radical reactions have a characteristic chain mechanism that includes three stages: nucleation (initiation), development (growth), and chain termination. (Slide 9)
Ionic reactions occur without breaking the electron pairs that form chemical bonds: both electrons go to the orbital of one of the atoms of the reaction product with the formation of an anion. (Slide 10) The heterolytic decay of a covalent polar bond leads to the formation of nucleophiles (anions) and electrophiles (cations). Depending on the nature of the attacking reagent, reactions can be nucleophilic or electrophilic.
According to the direction and final result of the chemical transformation, organic reactions are divided into the following types: substitution, addition, elimination (elimination), rearrangement (isomerization), oxidation and reduction. (Slide 11)
Substitution is understood as the replacement of an atom or group of atoms with another atom or group of atoms. As a result of the substitution reaction, two different products are formed.
R-CH 2 X + Y → R-CH 2 Y + X
The addition reaction is understood as the introduction of an atom or a group of atoms into the molecule of an unsaturated compound, which is accompanied by a break in this compound of π bonds. During the interaction, double bonds are converted into single bonds, and triple bonds are converted into double or single bonds.
R-CH=CH 2 + XY → RCHX-CH 2 Y
Problem: What type of reaction can we classify as a polymerization reaction? Prove that it belongs to a certain type of reactions and give an example.
Addition reactions also include polymerization reactions (for example: obtaining polyethylene from ethylene).
n(CH 2 \u003d CH 2) → (-CH 2 -CH 2 -) n
Elimination reactions, or cleavage, are reactions during which atoms or their groups are cleaved from an organic molecule to form a multiple bond.
R-CHX-CH 2 Y → R-CH=CH 2 + XY
Rearrangement reactions (isomerization). In this type of reaction, the rearrangement of atoms and their groups in the molecule takes place.
Polycondensation reactions are substitution reactions, but they are often distinguished as a special type of organic reactions that have specific features and great practical importance.
Oxidation-reduction reactions are accompanied by a change in the degree of oxidation of the carbon atom in compounds, where the carbon atom is the reaction center.
Oxidation is a reaction in which, under the action of an oxidizing reagent, a substance combines with oxygen (or another electronegative element, such as halogen) or loses hydrogen (in the form of water or molecular hydrogen). The action of an oxidizing reagent (oxidation) is indicated in the reaction scheme by the symbol [O].
[o]
CH 3 CHO → CH 3 COOH
Recovery is the reverse reaction of oxidation. Under the action of a reducing reagent, the compound accepts hydrogen atoms or loses oxygen atoms: the action of a reducing reagent (reduction) is indicated by the symbol [H].
[H]
CH 3 COCH 3 → CH 3 CH(OH)CH 3
Hydrogenation is a reaction that is a special case of reduction. Hydrogen is added to a multiple bond or aromatic nucleus in the presence of a catalyst.
To consolidate the studied material, students perform a test task: slides 12.13.
III. Homework: § 8 (exercise 2), 9
IV. Summarizing
Conclusions: (Slide 14)
Organic reactions obey general laws (the law of conservation of mass and energy) and general laws of their course (energy, kinetic - revealing the influence of various factors on the reaction rate).
They have common characteristics for all reactions, but they also have their own characteristic features.
According to the mechanism of the reaction, they are divided into homolytic (free radical) and heterolytic (electrophilic-nucleophilic).
According to the direction and final result of a chemical transformation, reactions are distinguished: substitution, addition, elimination (elimination), rearrangement (isomerization), polycondensation, oxidation and reduction.
Used Books:UMK: O.S. Gabrielyan et al. Chemistry 10 M. Bustard 2013
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Slides captions:
Types of chemical reactions in organic chemistry.
A chemical reaction is the transformation of one substance into another. The substances obtained as a result of the reaction differ from the starting substances in composition, structure and properties. Reagent 1 + Reagent 2 = Products Substrate + Attacker = Products Reagent
Signs of the classification of chemical reactions in inorganic chemistry according to the number and composition of the starting substances and products according to the thermal effect according to the change in the degree of oxidation of atoms according to the reversibility of the process according to the phase according to the use of a catalyst
Classification according to the number and composition of the starting and resulting substances: Connection reactions: A + B = AB Zn + Cl 2 = ZnCl 2 CaO + CO 2 = CaCO 3 Decomposition reactions: AB = A + B 2H 2 O = 2H 2 + O 2 Cu (OH) 2 \u003d CuO + H 2 O Substitution reactions: AB + C \u003d A + CB CuSO 4 + Fe \u003d Cu + FeSO 4 Cr 2 O 3 + 2Al \u003d 2Cr + Al 2 O 3 Exchange reactions: AB + CD \u003d AD + CB CuO + H2SO4 = CuSO4 + H2O NaOH + HCl = NaCl + H 2 O
Reaction schemes are given: 1. Copper(II) hydroxide → copper(II) oxide + water 2. Barium chloride + sodium sulfate → ... 3. Hydrochloric acid + zinc → zinc chloride + hydrogen 4. Phosphorus(V) oxide + water → ... Level I: Indicate the types of reactions, write down one of the equations (optional). Level II: Indicate the types of reactions, write down one of the equations in which the products are not indicated (optional). Level III: Indicate the types of reactions and write down all the equations.
Reactions involving organic compounds obey the same laws (the law of conservation of mass and energy, the law of mass action, Hess's law, etc.) and exhibit the same patterns (stoichiometric, energy, kinematic) as inorganic reactions.
Organic reactions are usually classified according to the mechanisms of the course. The reaction mechanism is understood as the sequence of individual stages of the reaction, indicating the intermediate particles formed at each of these stages. in the direction and end products of the reaction - addition; - splitting off (elimination); - substitutions; - rearrangement (isomerization); - oxidation; - recovery.
The method of breaking the covalent bond determines the type of reaction mechanism: Radical (homolytic) X:Y → X. + . Y R . (X . , . Y) - radicals (free atoms or particles with unpaired electrons, unstable and capable of entering into chemical transformations) Ionic (heterolytic) X: Y → X + +: Y - X + - electrophilic reagent (electrophile: loving electron ) :Y - - nucleophilic reagent (nucleophile: proton loving)
Radical reactions have a chain mechanism, including the stages: nucleation, development, and chain termination. Chain nucleation (initiation) Cl 2 → Cl. +Cl. Growth (development) of the CH 4 + Cl chain. → CH 3 . + H Cl CH 3 . + Cl 2 → CH 3 -Cl + Cl. CH 3 chain break. +Cl. → CH 3 ClCH 3 . +CH3. → CH 3 -CH 3 Cl. +Cl. →Cl2
Ionic reactions occur without breaking the electron pairs that form chemical bonds: both electrons go to the orbital of one of the atoms of the reaction product with the formation of an anion. Heterolytic decay of a covalent polar bond leads to the formation of nucleophiles (anions) and electrophiles (cations). CH 3 -Br + Na + OH - → CH 3 -OH + Na + Br - substrate reagent reaction products (nucleophile) C 6 H 5 -H + HO: NO 2 → C 6 H 5 -NO 2 + H-OH substrate reagent reaction products (electrophile)
Classification by direction and final result Substitution reactions A-B + C → A-C + B Addition reactions C \u003d C + A-B → A-C-C-B Elimination reactions A-C-C-B → C \u003d C + A-B Rearrangement (isomerization) reactions X-A-B → A-B-X Oxidation and reduction reactions are accompanied by a change in the oxidation state of the carbon atom in compounds where the carbon atom is the reaction center. Problem: What type of reactions can be attributed to the polymerization reaction? Prove that it belongs to a certain type of reactions and give an example.
Test. 1. Correlate: Section of chemistry Reaction type Inorganic a) substitution b) exchange Organic c) compounds d) decomposition e) elimination f) isomerization g) addition 2. Correlate: Reaction scheme Reaction type AB + C → AB + C a) substitution ABC → AB + C b) addition of ABC → DIA c) elimination of AB + C → AC + B d) isomerization
3. Butane reacts with a substance whose formula is: 1) H 2 O 2) C 3 H 8 3) Cl 2 4) HCl 4. The substrate in the proposed reaction schemes is the substance CH 3 -COOH (A) + C 2 H 5 -OH (B) → CH 3 COOS 2 H 5 + H 2 O CH 3 -CH 2 -OH (A) + H-Br ( B) → CH 3 -CH 2 -Br + H 2 O CH 3 -CH 2 -Cl (A) + Na-OH (B) → CH 2 \u003d CH 2 + NaCl + H 2 O 5. The left side of the equation C 3 H 4 + 5O 2 → ... corresponds to the right side: → C 3 H 6 + H 2 O → C 2 H 4 + H 2 O → 3CO 2 + 4H 2 O → 3CO 2 + 2H 2 O 6. The amount of oxygen that will be required for complete combustion of 5 l of methane, is equal to 1) 1 l 2) 5 l 3) 10 l 4) 15 l
Conclusions Organic reactions obey the general laws and general laws of their course. They have common characteristics for all reactions, but they also have their own characteristic features. According to the mechanism of the reaction, they are divided into free radical and ionic. According to the direction and final result of a chemical transformation: substitution, addition, oxidation and reduction, isomerization, elimination, polycondensation, etc.
