There are different classification systems for organic reactions, which are based on different characteristics. Among them are the following classifications:
- By the final result of the reaction, that is, a change in the structure of the substrate;
- By reaction mechanism, that is, by 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, the reagent is considered to attack 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 (CHANGE IN THE STRUCTURE OF THE SUBSTRATE)
In organic chemistry, four types of reactions are distinguished according to the final result and change in the structure of the substrate: addition, substitution, detachment, or elimination(from English to eliminate- remove, split off), and rearrangements (isomerizations)). This classification is similar to the classification of reactions in inorganic chemistry according to the number of initial reagents and resulting substances, with or without a change in composition. Classification according to the final result is based on formal criteria, 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 | Joining by multiple connections | Hydrogenation |
Hydrohalogenation
|
|||
Halogenation
|
|||
Hydration
|
|||
2. Decomposition | 2
H2
O=2 H2
+
O2
| Elimination | Dehydrogenation
|
Dehydrohalogenation
|
|||
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=N a 2 S O 4 + 2 H 2 O | special case - esterification |
|
5. Allotropization | graphite ⇔ diamond Pred⇔ Pwhite P red ⇔ P white Srhombus.⇔ Splast. Srhomb.⇔Splastic | Isomerization | Isomerization alkanes
|
n) without replacing them with others.
Depending on which atoms are split off - neighboring ones C–C or isolated by two or three or more carbon atoms – C–C–C– C–, –C–C–C–C– C–, compounds can form with multiple bonds and or cyclic compounds. The elimination of hydrogen halides from alkyl halides or water from alcohols occurs according to Zaitsev’s rule.
DEFINITION
Zaitsev's rule: A hydrogen atom H is removed from the least hydrogenated carbon atom.
For example, the elimination of a hydrogen bromide molecule occurs from neighboring atoms in the presence of an alkali, resulting in the formation of sodium bromide and water.
DEFINITION
Regrouping- a chemical reaction that results in a change in the relative arrangement of atoms in a molecule, the movement of multiple bonds or a change in their multiplicity.
Rearrangement can be carried out while maintaining the atomic composition of the molecule (isomerization) or changing it.
DEFINITION
Isomerization- a special case of a rearrangement reaction leading to the transformation of a chemical compound into an isomer through a structural change in the carbon skeleton.
Rearrangement can also occur by a homolytic or heterolytic mechanism. Molecular rearrangements can be classified according to various 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, Beckmann rearrangement, etc.
Isomerization reactions are widely used in industrial processes, such as petroleum refining to increase the octane number of gasoline. An example of isomerization is the transformation n-octane to isooctane:
CLASSIFICATION OF ORGANIC REACTIONS BY REAGENT TYPE
DISCONNECTION
Bond cleavage in organic compounds can be homolytic or heterolytic. 
DEFINITION
Homolytic bond cleavage- this is 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.
A homolytic break is characteristic of nonpolar or weakly polar bonds, such as C–C, Cl–Cl, C–H, and requires a large amount of energy.
The resulting radicals, which have an unpaired electron, are highly reactive, therefore the chemical processes occurring with the participation of such particles are often of a “chain” nature, they are difficult to control, and the reaction results in a set of substitution products. Thus, when methane is chlorinated, the substitution products are chloromethane C H3 C l 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 through the exchange mechanism of the formation of chemical bonds.
The radicals formed during such bond cleavage cause radical mechanism the course of the reaction. Radical reactions usually occur at elevated temperatures or radiation (eg light).
Due to their high reactivity, free radicals can have a negative impact 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 cleavage- this is a gap in which an electron pair remains with 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 according to the donor-acceptor mechanism. Nucleophilic particles provide an electron pair to form 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 a lone 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 of organic reactions occur.
MECHANISMS OF ORGANIC REACTIONS
Free radical mechanism: the reaction is started by free radicals formed when homolytic rupture bonds in a molecule.
The most typical option is the formation of chlorine or bromine radicals during UV irradiation.
1. Free radical substitution
methane bromomethane
Chain initiation
Chain growth
Open circuit
2. Free radical addition
ethene polyethylene
Electrophilic mechanism: the reaction begins with electrophilic particles that receive a positive charge as a result heterolytic rupture communications. All electrophiles are Lewis acids.
Such particles are actively formed under the influence of Lewis acids, which enhance the positive charge of the particle. Most often used A l C l3 , F e C l3 , F e B r3 ,ZnC l2 AlCl3,FeCl3,FeBr3,ZnCl2, performing the functions of a catalyst.
The site of attack of the electrophile particle is those parts of the molecule that have increased electron density, i.e., the multiple bond and the benzene ring.
The general form of electrophilic substitution reactions can be expressed by the equation:
1. Electrophilic substitution
benzene bromobenzene
2. Electrophilic connection
propene 2-bromopropane
propyne 1,2-dichloropropene
The addition to unsymmetrical unsaturated hydrocarbons occurs in accordance with Markovnikov’s rule.
DEFINITION
Markovnikov's rule: addition to unsymmetrical alkenes of molecules of complex substances with the conditional formula HX (where X is a halogen atom or hydroxyl group OH–), the hydrogen atom is added to the most hydrogenated (containing the most hydrogen atoms) carbon atom at the 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-donating influence C H3 CH3-group, 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 attack begins with the hydrogen proton 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 added C l− Cl−.
DEFINITION
Exceptions to Markovnikov's rule: the addition reaction proceeds against Markovnikov’s rule if the reaction involves compounds in which the carbon atom adjacent to the carbon atom of the double bond partially absorbs 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 begins with nucleophilic particles having a negative charge, formed as a result heterolytic rupture communications. All nucleophiles - Lewis's foundations.
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 complete 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 heterolytic cleavage followed by elimination of the cation. An example of a nucleophilic reaction is 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
Nucleophilicis a reaction in which a reagent attacks the substrate with its nucleophile; it is denoted by an index N (nucleophlle).
In electrophilic reactions, the reagent is usually called an electrophile. In organic chemistry, the electrophilicity of a reagent characterizes its ability to interact with a carbon atom of the substrate that carries a full or partial negative charge.
In fact, the mechanism and result of any electrophilic-nucleophilic reaction is determined not only by the properties of the reagent, but also by the properties of the substrate, the resulting reaction products, the solvent and the conditions for its implementation. Therefore, the division of electrophilic-nucleophilic reactions into nucleophilic and electrophilic only based on the properties of the reagent is conditional. In addition, as can be seen from the above diagrams, in these reactions the electrophiles and nucleophiles contained in the substrate and reagent always interact with each other. In many reactions, only conditionally one component can be considered a substrate and the other a reagent.
Free radical reactions. Homolytic decay is characteristic of non-polar or low-polar bonds. It is accompanied by the formation of free radicals - particles with an unpaired electron.
Homolysis of a covalent bond can be considered as the cleavage of this bond by an exchange mechanism. To carry out homolysis of a bond, energy (heat, light) is required sufficient to break this bond. The presence of an unpaired electron is the reason for the low stability of free radicals (the lifetime in most cases is a fraction of a second) and high reactivity in free radical reactions. The presence of a free radical (R۰) in the system can lead to the formation of new radicals due to its interaction with existing molecules: R۰ + A – B → R – A + ۰B
Free radical reactionsare accompanied by the interaction of free radicals with molecules or with each other with the formation of new free radicals (nucleation or development of a chain) or only molecules (chain termination).
Free radical reactions are characterized by a chain mechanism, which includes three stages: initiation, development and chain termination. These reactions stop when free radicals disappear from the system. Free radical reactions are designated by the index R (radical).
Radical particles, depending on their electron affinity, can both accept electrons (i.e., be oxidizing agents) and donate electrons (i.e., be reducing agents). In this case, the affinity of a radical for an electron is determined not only by its properties, but also by the properties of its reaction partner. The features of free radical oxidation-reduction processes occurring in the body are considered separately when describing the properties of certain classes of organic compounds.
In complexation reactions, radicals can be both complexing agents and ligands. In the case of charge transfer complexes, radical formation can occur within the complex due to intramolecular oxidation-reduction between the complexing agent and the ligand.
The formation of radicals most easily occurs during the homolysis of nonpolar simple bonds between atoms of the same element:
C1 2 → C1۰ + ۰С1 HO-OH → СО۰ + ۰ОН
R-O-O-R" → RO۰ + ۰OR" R-S-S-R" →RS۰ + ۰SR"
Homolysis of a low-polarity CH bond produces alkyl radicals in which the unpaired electron is located at the carbon atom. The relative stability of these radicals depends on the type of substitution of the carbon atom bearing the unpaired electron, and increases in the series: CH 3< CH 2 R < CHR 2 < CR 3 . Это объясняется положительным индуктивным эффектом алкильных групп, который, повышая электронную плотность на атоме углерода, способствует стабилизации радикала.
The stability of free radicals increases significantly when it is possible to delocalize the unpaired electron due to the π-electrons of neighboring multiple bonds. This is especially clearly observed in the allylic and benzyl radicals:
 |
allylic radical benzyl radical
When familiarizing yourself with possible reaction mechanisms in substrate and reagent molecules, reaction centers should be distinguished by their nature: nucleophilic, electrophilic And radical.
According to the final result of the chemical transformation, the simplest organic reactions are classified into reactions: substitution, addition, elimination (elimination) And regrouping.
Substitution reactions. Substitution refers to the replacement of an atom or group with another atom or group. In a substitution reaction, two different products are always formed. This type of reaction is designated by the symbol S (substitution).
Substitution reactions include: halogenation and nitration of alkanes, esterification and alkylation of carboxylic acids, as well as numerous reactions of simple polar molecules (H 2 O, NH 3, NGal) with ethers, alcohols and halogen derivatives.
Addition reactions. By addition we mean the introduction of atoms or groups into the molecule of an unsaturated compound, accompanied by the breaking of π bonds. In this case, double bonds turn into single bonds, and triple bonds into double or single bonds. This type of reaction is indicated by the symbol A (addition).
Elimination reactions. Elimination refers to the removal of atoms or groups from an organic molecule to form a multiple bond. Therefore, elimination reactions are the opposite of addition reactions. This type of reaction is designated by the symbol E (elimination).
Each of the organic reactions of substitution (S), addition (A) or elimination (E) can be electrophilic (E), nucleophilic (N) or radical (R). Thus, in organic chemistry there are nine typical reactions, denoted by the symbols S, A or E with the subscripts R, N or E:
The given types of organic reactions should be considered model ones, since they are not always realized in their pure form. For example, substitution and elimination can occur simultaneously:
With further acquaintance with specific classes of organic compounds, we will consider their following chemical properties: acid-base, complexing, redox, electrophilic-nucleophilic, as well as the ability for free radical interaction. Particular attention will be paid to the peculiarities of the occurrence of the reactions under consideration in biological systems.
The types of reactions characteristic of various classes of hydrocarbons, the mechanism of their occurrence and the biological significance of the processes are presented in Table 10.
 |
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 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.
CH 3 -CH 3 + Cl 2 – (hv) ---- CH 3 -CH 2 Cl + HCl
C 6 H 5 CH 3 + Cl 2 --- 500 C --- C 6 H 5 CH 2 Cl + HCl
Addition reactions
Such reactions are typical for organic compounds containing multiple (double or triple) bonds. Reactions of this type include reactions of addition of halogens, hydrogen halides and water to alkenes and alkynes
CH 3 -CH=CH 2 + HCl ---- CH 3 -CH(Cl)-CH 3
Elimination reactions
These are reactions that lead to the formation of multiple bonds. When eliminating hydrogen halides and water, a certain selectivity of the reaction is observed, described by Zaitsev's rule, according to which a hydrogen atom is eliminated from the carbon atom at which there are fewer hydrogen atoms. Example reaction
CH3-CH(Cl)-CH 2 -CH 3 + KOH →CH 3 -CH=CH-CH 3 + HCl
Polymerization and polycondensation
n(CH 2 =CHCl) (-CH 2 -CHCl)n
Redox
The most intense of the oxidative reactions is combustion, a reaction characteristic of all classes of organic compounds. In this case, depending on the combustion conditions, carbon is oxidized to C (soot), CO or CO 2, and hydrogen is converted into water. However, for organic chemists, oxidation reactions carried out under much milder conditions than combustion are of great interest. Oxidizing agents used: solutions of Br2 in water or Cl2 in CCl 4 ; KMnO 4 in water or dilute acid; copper oxide; freshly precipitated silver(I) or copper(II) hydroxides.
3C 2 H 2 + 8KMnO 4 +4H 2 O→3HOOC-COOH + 8MnO 2 + 8KOH
Esterification (and its reverse hydrolysis reaction)
R 1 COOH + HOR 2 H+ R 1 COOR 2 + H 2 O
Cycloaddition
Y R Y-R
‖ + ‖ → ǀ ǀ
R Y R-Y
‖ + →
11. Classification of organic reactions by mechanism. Examples.
The reaction mechanism involves a detailed step-by-step description of chemical reactions. At the same time, it is established which covalent bonds are broken, in what order and in what way. The formation of new bonds during the reaction process is also carefully described. When considering the reaction mechanism, first of all, pay attention to the method of breaking the covalent bond in the reacting molecule. There are two such ways - homolytic and heterolytic.
Radical reactions proceed by homolytic (radical) cleavage of a covalent bond:
Non-polar or low-polar covalent bonds (C–C, N–N, C–H) undergo radical cleavage at high temperatures or under the influence of light. The carbon in the CH 3 radical has 7 outer electrons (instead of a stable octet shell in CH 4). Radicals are unstable; they tend to capture the missing electron (up to a pair or up to an octet). One of the ways to form stable products is dimerization (the combination of two radicals):
CH 3 + CH 3 CH 3 : CH 3,
N + N N : N.
Radical reactions - these are, for example, reactions of chlorination, bromination and nitration of alkanes:
Ionic reactions occur with heterolytic bond cleavage. In this case, short-lived organic ions - carbocations and carbanions - with a charge on the carbon atom are intermediately formed. In ionic reactions, the bonding electron pair is not separated, but passes entirely to one of the atoms, turning it into an anion:
Strongly polar (H–O, C–O) and easily polarizable (C–Br, C–I) bonds are prone to heterolytic cleavage.
Distinguish nucleophilic reactions (nucleophile– looking for the nucleus, a place with a lack of electrons) and electrophilic reactions (electrophile– looking for electrons). The statement that a particular reaction is nucleophilic or electrophilic always refers to the reagent. Reagent– a substance participating in the reaction with a simpler structure. Substrate– a starting substance with a more complex structure. Outgoing group is a replaceable ion that has been bonded to carbon. Reaction product– new carbon-containing substance (written on the right side of the reaction equation).
TO nucleophilic reagents(nucleophiles) include negatively charged ions, compounds with lone pairs of electrons, compounds with double carbon-carbon bonds. TO electrophilic reagents(electrophiles) include positively charged ions, compounds with unfilled electron shells (AlCl 3, BF 3, FeCl 3), compounds with carbonyl groups, halogens. Electrophiles are any atom, molecule or ion capable of adding a pair of electrons in the process of forming a new bond. The driving force of ionic reactions is the interaction of oppositely charged ions or fragments of different molecules with a partial charge (+ and –).
Examples of different types of ionic reactions.
Nucleophilic substitution :
Electrophilic substitution :
Nucleophilic addition (CN – is added first, then H +):
Electrophilic connection (H + is added first, then X –):
Elimination by the action of nucleophiles (bases) :
Elimination upon action electrophiles (acids) :
Organic reactions can be classified into two general types.
Hemolytic reactions. These reactions proceed by a radical mechanism. We'll look at them in more detail in the next chapter. The kinetics and mechanism of reactions of this type were discussed in Chap. 9.
Heterolytic reactions. These reactions are essentially ionic reactions. They can, in turn, be divided into substitution, addition and elimination reactions.
Substitution reactions
In these reactions, an atom or group of atoms is replaced by another atom or group. As an example of reactions of this type, we give the hydrolysis of chloromethane with the formation of methanol:
The hydroxyl ion is a nucleophile. Therefore, the substitution in question is called nucleophilic substitution. It is designated by the symbol SN. The replaced particle (in this case, a chlorine ion) is called a leaving group.
If we denote the nucleophile by the symbol and the leaving group by the symbol, then we can write the generalized equation for the reaction of nucleophilic substitution at a saturated carbon atom in the alkyl group R as follows:
A study of the rate of reactions of this type shows that reactions can be divided into
Reactions of the type For some reactions of the SN type, the kinetic equation for the reaction rate (see Section 9.1) has the form
Thus, these reactions are first order in the substrate but zero order in the reactant. The kinetics characteristic of a first order reaction is a reliable indication that the rate-limiting step of the reaction is a unimolecular process. Therefore, reactions of this type are indicated by the symbol.
The reaction has zero order with respect to the reagent since its rate does not depend on the concentration of the reagent. Therefore, we can write:
Since the nucleophile does not participate in the rate-limiting step of the reaction, the mechanism of such a reaction must include at least two steps. The following mechanism has been proposed for such reactions:
The first stage is ionization with the formation of a carbocation. This stage is limiting (slow).
An example of this type of reaction is the alkaline hydrolysis of tertiary alkyl halides. For example
In the case under consideration, the reaction rate is determined by the equation
Reactions of the type For some reactions of nucleophilic substitution SN the rate equation has the form
In this case, the reaction is first order in the nucleophile and first order in . In general, it is a second order reaction. This is sufficient reason to believe that the rate-limiting stage of this reaction is a bimolecular process. Therefore, the reaction of the type under consideration is denoted by the symbol Since both the nucleophile and the substrate simultaneously participate in the rate-limiting stage of the reaction, we can think that this reaction proceeds in one stage through a transition state (see Section 9.2):
Hydrolysis of primary alkyl halides in an alkaline medium proceeds according to the mechanism
This reaction has the following kinetic equation:
So far we have considered nucleophilic substitution only at the saturated carbon atom. Nucleophilic substitution is also possible at an unsaturated carbon atom:
Reactions of this type are called nucleophilic acyl substitution.
Electrophilic substitution. Electrophilic substitution reactions can also occur on benzene rings. In this type of substitution, the benzene ring supplies the electrophile with two of its delocalized -electrons. In this case, an intermediate compound is formed - an unstable complex of an electrophile and a leaving group. For a schematic representation of such complexes, an open circle is used, indicating the loss of two -electrons:
An example of electrophilic substitution reactions is the nitration of benzene:
Nitration of benzene is carried out in an installation with a reflux condenser at a temperature of 55 to 60 ° C using a nitrating mixture. This mixture contains equal amounts of concentrated nitric and sulfuric acids. The reaction between these acids leads to the formation of a nitroyl cation
Addition reactions
In reactions of this type, an electrophile or nucleophile is added to an unsaturated carbon atom. We will consider here one example each of electrophilic addition and nucleophilic addition.
An example of electrophilic addition is the reaction between hydrogen bromide and an alkene. To obtain hydrogen bromide in the laboratory, a reaction between concentrated sulfuric acid and sodium bromide can be used (see Section 16.2). Hydrogen bromide molecules are polar because the bromine atom has a negative inductive effect on hydrogen. Therefore, the hydrogen bromide molecule has the properties of a strong acid. According to modern views, the reaction of hydrogen bromide with alkenes occurs in two stages. In the first stage, a positively charged hydrogen atom attacks the double bond, which acts as a source of electrons. As a result, an activated complex and a bromide ion are formed:
The bromide ion then attacks this complex, resulting in the formation of an alkyl bromide:
An example of nucleophilic addition is the addition of hydrogen cyanide to any aldehyde or ketone. First, the aldehyde or ketone is treated with an aqueous solution of sodium cyanide. Then an excess amount of any mineral acid is added, which leads to the formation of hydrogen cyanide HCN. The cyanide ion is a nucleophile. It attacks the positively charged carbon atom on the carbonyl group of the aldehyde or ketone. The positive charge and polarity of the carbonyl group is due to the mesomeric effect, which was described above. The reaction can be represented by the following diagram:
Elimination reactions
These reactions are the reverse of addition reactions. They lead to the removal of any atoms or groups of atoms from two carbon atoms connected to each other by a simple covalent bond, resulting in the formation of a multiple bond between them.
An example of such a reaction is the elimination of hydrogen and halogen from alkyl halides:
To carry out this reaction, the alkyl halide is treated with potassium hydroxide in alcohol at a temperature of 60 °C.
It should be noted that treatment of an alkyl halide with hydroxide also leads to nucleophilic substitution (see above). As a result, two competing substitution and elimination reactions occur simultaneously, which leads to the formation of a mixture of substitution and elimination products. Which of these reactions will be predominant depends on a number of factors, including the environment in which the reaction is carried out. Nucleophilic substitution of alkyl halides is carried out in the presence of water. In contrast, elimination reactions are carried out in the absence of water and at higher temperatures.
So let's say it again!
1. During hemolytic cleavage of a bond, two shared electrons are distributed evenly between atoms.
2. During heterolytic bond cleavage, two shared electrons are distributed unevenly between atoms.
3. A carbanion is an ion containing a carbon atom with a negative charge.
4. A carbocation is an ion containing a carbon atom with a positive charge.
5. Solvent effects can have a significant impact on chemical processes and their equilibrium constants.
6. The effect of the chemical environment of a functional group within a molecule on the reactivity of that functional group is called the structural effect.
7. Electronic effects and steric effects are collectively called structural effects.
8. The two most important electronic effects are the inductive effect and the mesomeric (resonant) effect.
9. The inductive effect is the shift of electron density from one atom to another, which leads to polarization of the bond between the two atoms. This effect can be positive or negative.
10. Molecular particles with multiple bonds can exist in the form of resonant hybrids between two or more resonant structures.
11. The mesomeric (resonance) effect consists in the stabilization of resonant hybrids due to the delocalization of -electrons.
12. Steric hindrance can occur when bulky groups in a molecule mechanically impede the reaction.
13. Nucleophile is a particle that attacks a carbon atom, supplying it with its electron pair. The nucleophile is a Lewis base.
14. An electrophile is a particle that attacks a carbon atom, accepting its electron pair. The nucleophile is a Lewis acid.
15. Hemolytic reactions are radical reactions.
16. Heterolytic reactions are mainly ionic reactions.
17. The replacement of any group in a molecule with a nucleophilic reagent is called nucleophilic substitution. The group being replaced in this case is called the leaving group.
18. Electrophilic substitution on a benzene ring involves the donation of two delocalized electrons to some electrophile.
19. In electrophilic addition reactions, an electrophile is added to an unsaturated carbon atom.
20. The addition of hydrogen cyanide to aldehydes or ketones is an example of nucleophilic addition.
21. In elimination (elimination) reactions, some atoms or groups of atoms are separated from two carbon atoms connected to each other by a simple covalent bond. As a result, a multiple bond is formed between these carbon atoms.
