Those containing a pi bond are unsaturated hydrocarbons. They are derivatives of alkanes, in the molecules of which two hydrogen atoms have been split off. The resulting free valences form a new type of bond, which is located perpendicular to the plane of the molecule. This is how a new group of compounds arises - alkenes. We will consider the physical properties, preparation and use of substances of this class in everyday life and industry in this article.
Homologous series of ethylene
The general formula for all compounds called alkenes, reflecting their qualitative and quantitative composition, is C n H 2 n. The names of hydrocarbons according to the systematic nomenclature are as follows: in the term of the corresponding alkane, the suffix changes from -an to -ene, for example: ethane - ethene, propane - propene, etc. In some sources, you can find another name for compounds of this class - olefins. Next, we will study the process of double bond formation and the physical properties of alkenes, and also determine their dependence on the structure of the molecule.
How is a double bond formed?
The electronic nature of the pi bond using the example of ethylene can be represented as follows: carbon atoms in its molecule are in the form of sp 2 hybridization. In this case, a sigma bond is formed. Two more hybrid orbitals, one each from carbon atoms, form simple sigma bonds with hydrogen atoms. The two remaining free hybrid clouds of carbon atoms overlap above and below the plane of the molecule - a pi bond is formed. It is she who determines the physical and chemical properties of alkenes, which will be discussed later.
Spatial isomerism
Compounds that have the same quantitative and qualitative composition of molecules, but a different spatial structure, are called isomers. Isomerism occurs in a group of substances called organic. The characterization of olefins is greatly influenced by the phenomenon of optical isomerism. It is expressed in the fact that ethylene homologues containing different radicals or substituents at each of the two carbon atoms in the double bond can occur in the form of two optical isomers. They differ from each other by the position of the substituents in space relative to the plane of the double bond. The physical properties of alkenes in this case will also be different. For example, this applies to the boiling and melting points of substances. Thus, straight chain olefins have higher boiling points than isomer compounds. Also, the boiling points of cis isomers of alkenes are higher than those of trans isomers. With regard to melting temperatures, the picture is opposite.
Comparative characteristics of the physical properties of ethylene and its homologues
The first three representatives of olefins are gaseous compounds, then, starting from pentene C 5 H 10 and up to alkene with the formula C 17 H 34, they are liquids, and then go solids. The ethene homologues show the following trend: the boiling points of the compounds decrease. For example, for ethylene this indicator is -169.1°C, and for propylene -187.6°C. But the boiling points increase with increasing molecular weight. So, for ethylene it is -103.7°C, and for propene -47.7°C. Summing up what has been said, we can conclude that the physical properties of alkenes depend on their molecular weight. With its increase, the aggregate state of the compounds changes in the direction: gas - liquid - solid, and the melting point also decreases, and the boiling points increase.
Characteristics of ethene
First Representative homologous series alkenes is ethylene. It is a colorless gas, slightly soluble in water, but highly soluble in organic solvents. Molecular weight - 28, ethene is slightly lighter than air, has a subtle sweet smell. It easily reacts with halogens, hydrogen and hydrogen halides. The physical properties of alkenes and paraffins, however, are quite close. For example, the state of aggregation, the ability of methane and ethylene to undergo severe oxidation, etc. How can alkenes be distinguished? How to reveal the unsaturated character of an olefin? For this, there are qualitative reactions, on which we will dwell in more detail. Recall what feature in the structure of the molecule alkenes have. The physical and chemical properties of these substances are determined by the presence of a double bond in their composition. To prove its presence, gaseous hydrocarbon is passed through a purple solution of potassium permanganate or bromine water. If they are discolored, then the compound contains pi bonds in the composition of the molecules. Ethylene enters into an oxidation reaction and decolorizes solutions of KMnO 4 and Br 2 .
Mechanism of addition reactions
The breaking of the double bond ends with the addition of other atoms to the free carbon valences. chemical elements. For example, the reaction of ethylene with hydrogen, called hydrogenation, produces ethane. A catalyst is needed, such as powdered nickel, palladium or platinum. The reaction with HCl ends with the formation of chloroethane. Alkenes containing more than two carbon atoms in their molecules undergo the addition reaction of hydrogen halides, taking into account V. Markovnikov's rule.
How ethene homologues interact with hydrogen halides
If we are faced with the task "Characterize the physical properties of alkenes and their preparation", we need to consider V. Markovnikov's rule in more detail. It has been established in practice that ethylene homologues react with hydrogen chloride and other compounds at the site of double bond rupture, obeying a certain pattern. It consists in the fact that the hydrogen atom is attached to the most hydrogenated carbon atom, and the chlorine, bromine or iodine ion is attached to the carbon atom containing the smallest number of hydrogen atoms. This feature of the course of addition reactions is called V. Markovnikov's rule.
Hydration and polymerization
Let us continue to consider the physical properties and application of alkenes using the example of the first representative of the homologous series - ethene. Its reaction with water is used in the organic synthesis industry and is of great practical importance. The process was first carried out in the 19th century by A.M. Butlerov. The reaction requires a number of conditions to be met. This is, first of all, the use of concentrated sulfuric acid or oleum as a catalyst and solvent for ethene, a pressure of about 10 atm and a temperature within 70 °. The hydration process occurs in two phases. At first, sulfate molecules are added to ethene at the point of rupture of the pi bond, and ethylsulfuric acid is formed. Then the resulting substance reacts with water, ethyl alcohol is obtained. Ethanol is an important product used in the food industry for the production of plastics, synthetic rubbers, varnishes and other products. organic chemistry.
Olefin based polymers
Continuing to study the issue of the use of substances belonging to the class of alkenes, we will study the process of their polymerization, in which compounds containing unsaturated chemical bonds within their molecules. Several types of polymerization reactions are known, according to which high-molecular products are formed - polymers, for example, such as polyethylene, polypropylene, polystyrene, etc. The free radical mechanism leads to the production of high-pressure polyethylene. It is one of the most widely used compounds in industry. The cationic-ionic type provides a polymer with a stereoregular structure, such as polystyrene. It is considered one of the safest and most convenient polymers to use. Products made of polystyrene are resistant to aggressive substances: acids and alkalis, non-flammable, easily painted. Another type of polymerization mechanism is dimerization, which leads to the production of isobutene, which is used as an antiknock additive for gasoline.
How to get
Alkenes, the physical properties of which we study, are obtained in the laboratory and industry by various methods. In experiments in school course organic chemistry uses the process of dehydration of ethyl alcohol with the help of dehydrating agents, such as phosphorus pentoxide or sulfate acid. The reaction is carried out when heated and is the reverse of the process of obtaining ethanol. Another common method for obtaining alkenes has found its application in industry, namely: heating halogen derivatives of saturated hydrocarbons, such as chloropropane with concentrated alcoholic solutions of alkalis - sodium or potassium hydroxide. In the reaction, a hydrogen chloride molecule is split off, a double bond is formed at the place where free valences of carbon atoms appear. The end product of the chemical process will be an olefin - propene. Continuing to consider the physical properties of alkenes, let us dwell on the main process for obtaining olefins - pyrolysis.
Industrial production of unsaturated hydrocarbons of the ethylene series
Cheap raw materials - gases formed in the process of oil cracking, serve as a source of olefins in the chemical industry. For this, a technological scheme of pyrolysis is used - the splitting of a gas mixture, which goes with the breaking of carbon bonds and the formation of ethylene, propene and other alkenes. Pyrolysis is carried out in special furnaces, consisting of individual pyro-coils. They create a temperature of the order of 750-1150°C and there is water vapor as a diluent. Reactions proceed by a chain mechanism that proceeds with the formation of intermediate radicals. The final product is ethylene or propene, and they are produced in large volumes.
We studied in detail the physical properties, as well as the application and methods for obtaining alkenes.
Knowledge Hypermarket >>Chemistry >>Chemistry Grade 10 >> Chemistry: Alkenes
Unsaturated hydrocarbons include hydrocarbons containing multiple bonds between carbon atoms in molecules. Unsaturated are alkenes, alkynes, alkadienes (polyenes). Cyclic hydrocarbons containing a double bond in the cycle (cycloalkenes), as well as cycloalkanes with non a large number carbon atoms in the cycle (three or four atoms). The property of "unsaturation" is associated with the ability of these substances to enter into addition reactions, primarily hydrogen, with the formation of saturated, or saturated, hydrocarbons - alkanes.
Structure
Alkenes are acyclic, containing in the molecule, in addition to single bonds, one double bond between carbon atoms and corresponding to the general formula C n H 2n.
Alkenes received their second name - "olefins" by analogy with unsaturated fatty acids (oleic, linoleic), the remains of which are part of liquid fats - oils (from the English oil - oil).
Carbon atoms between which there is a double bond, as you know, are in a state of sp 2 hybridization. This means that one s- and two p-orbitals participate in hybridization, while one p-orbital remains unhybridized. The overlap of hybrid orbitals leads to the formation of an α-bond, and due to the unhybridized α-orbitals of neighboring ethylene molecules, carbon atoms form a second, P-connection. Thus, a double bond consists of one z- and one p-bond.
The hybrid orbitals of the atoms that form the double bond are in the same plane, while the orbitals that form the n-bond are perpendicular to the plane of the molecule (see Fig. 5).
A double bond (0.132 nm) is shorter than a single bond, and its energy is greater, that is, it is more durable. Nevertheless, the presence of a mobile, easily polarizable 7r bond leads to the fact that alkenes are chemically more active than alkanes and are able to enter into addition reactions.
Homologous series of ethene
Unbranched alkenes make up the homologous series of ethene (ethylene).
C2H4 - ethene, C3H6 - propene, C4H8 - butene, C5H10 - pentene, C6H12 - hexene, etc.
Isomerism and nomenclature
For alkenes, as well as for alkanes, structural isomerism is characteristic. Structural isomers, as you remember, differ from each other in the structure of the carbon skeleton. The simplest alkene, which is characterized by structural isomers, is butene.
CH3-CH2-CH=CH2 CH3-C=CH2
l
CH3
butene-1 methylpropene
A special type of structural isomerism is the double bond position isomerism:
CH3-CH2-CH=CH2 CH3-CH=CH-CH3
butene-1 butene-2
Almost free rotation of carbon atoms is possible around a single carbon-carbon bond, so alkane molecules can take on a wide variety of shapes. Rotation around the double bond is impossible, which leads to the appearance of another type of isomerism in alkenes - geometric, or cis-trans isomerism.
Cis-isomers differ from thorax-isomers by the spatial arrangement of molecular fragments (in this case, methyl groups) relative to the plane P relationships, and hence properties.
Alkenes are isomeric to cycloalkanes (interclass isomerism), for example:
ch2=ch-ch2-ch2-ch2-ch3
hexene-1 cyclohexane
Nomenclature alkenes, developed by IUPAC, is similar to the nomenclature of alkanes.
1. Main circuit selection
The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in a molecule. In the case of alkenes, the main chain must contain a double bond.
2. Numbering of atoms of the main chain
The numbering of the atoms of the main chain starts from the end to which the double bond is closest. For example, the correct connection name is
ch3-chn-ch2-ch=ch-ch3 ch3
5-methylhexene-2, not 2-methylhexene-4 as might be expected.
If it is impossible to determine the beginning of the numbering of atoms in the chain by the location of the double bond, then it is determined by the position of the substituents in the same way as for saturated hydrocarbons.
CH3-CH2-CH=CH-CH-CH3
l
CH3
2-methylhexene-3
3. Name formation
The names of alkenes are formed in the same way as the names of alkanes. At the end of the name, the number of the carbon atom at which the double bond begins is indicated, and the suffix, indicating that the compound belongs to the class of alkenes, -ene.
Receipt
1. Cracking of petroleum products. In the process of thermal cracking of saturated hydrocarbons, along with the formation of alkanes, the formation of alkenes occurs.
2. Dehydrogenation of saturated hydrocarbons. When alkanes are passed over a catalyst at a high temperature (400-600 °C), a hydrogen molecule is split off and an alkene is formed: 
3. Dehydration of alcohols (cleavage of water). The effect of water-removing agents (H2804, Al203) on monohydric alcohols at high temperatures leads to the elimination of a water molecule and the formation of a double bond:
This reaction is called intramolecular dehydration (in contrast to intermolecular dehydration, which leads to the formation of ethers and will be studied in § 16 "Alcohols").
4. Dehydrohalogenation (elimination of hydrogen halide).
When a haloalkane reacts with an alkali in an alcoholic solution, a double bond is formed as a result of the elimination of a hydrogen halide molecule.
Note that this reaction produces predominantly butene-2 rather than butene-1, corresponding to Zaitsev's rule:
When a hydrogen halide is split off from secondary and tertiary haloalkanes, a hydrogen atom is split off from the least hydrogenated carbon atom.
5. Dehalogenation. Under the action of zinc on a dibromo derivative of an alkane, halogen atoms are split off from neighboring carbon atoms and a double bond is formed: 
Physical properties
The first three representatives of the homologous series of alkenes are gases, substances of the composition C5H10-C16H32 are liquids, higher alkenes are solids.
The boiling and melting points naturally increase with an increase in the molecular weight of the compounds.
Chemical properties
Addition reactions
Recall that a distinctive feature of the representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed by the mechanism of electrophilic addition.
1. Hydrogenation of alkenes. Alkenes are able to add hydrogen in the presence of hydrogenation catalysts - metals - platinum, palladium, nickel:
CH3-CH2-CH=CH2 + H2 -> CH3-CH2-CH2-CH3
This reaction proceeds both at atmospheric and elevated pressure and does not require high temperature, as it is exothermic. With an increase in temperature on the same catalysts, the reverse reaction, dehydrogenation, can occur.
2. Halogenation (addition of halogens). The interaction of an alkene with bromine water or a solution of bromine in an organic solvent (СCl4) leads to a rapid discoloration of these solutions as a result of the addition of a halogen molecule to the alkene and the formation of dihaloalkanes.
Markovnikov Vladimir Vasilievich
(1837-1904)
Russian organic chemist. Formulated (1869) rules on the direction of reactions of substitution, elimination, double bond addition and isomerization, depending on the chemical structure. Investigated (since 1880) the composition of oil, laid the foundations of petrochemistry as an independent science. Opened (1883) a new class of organic substances - cyclo-paraffins (naphthenes).
3. Hydrohalogenation (addition of hydrogen halide).
The hydrogen halide addition reaction will be discussed in more detail below. This reaction obeys Markovnikov's rule:
When a hydrogen halide is added to an alkene, hydrogen is attached to a more hydrogenated carbon atom, i.e., an atom at which there are more hydrogen atoms, and a halogen to a less hydrogenated one.
4. Hydration (water addition). Hydration of alkenes leads to the formation of alcohols. For example, the addition of water to ethene underlies one of the industrial methods for producing ethyl alcohol:
CH2=CH2 + H2O -> CH3-CH2OH
ethene ethanol
Note that a primary alcohol (with a hydroxyl group at the primary carbon) is formed only when ethene is hydrated. When propene or other alkenes are hydrated, secondary alcohols are formed. 
This reaction also proceeds in accordance with Markovnikov's rule - the hydrogen cation is added to the more hydrogenated carbon atom, and the hydroxy group is added to the less hydrogenated one.
5. Polymerization. A special case of addition is the polymerization reaction of alkenes:
This addition reaction proceeds by a free radical mechanism.
Oxidation reactions
Like any organic compounds, alkenes burn in oxygen to form CO2 and H20.
Unlike alkanes, which are resistant to oxidation in solutions, alkenes are easily oxidized by aqueous solutions of potassium permanganate. In neutral or slightly alkaline solutions, alkenes are oxidized to diols (dihydric alcohols), and hydroxyl groups are attached to those atoms between which a double bond existed before oxidation.
As you already know, unsaturated hydrocarbons - alkenes are able to enter into addition reactions. Most of these reactions proceed by the mechanism of electrophilic addition.
electrophilic addition
Electrophilic reactions are reactions that occur under the action of electrophiles - particles that have a lack of electron density, such as an unfilled orbital. The simplest electrophilic particle is the hydrogen cation. It is known that the hydrogen atom has one electron per 3-in-orbital. A hydrogen cation is formed when an atom loses that electron, so the hydrogen cation has no electrons at all:
H - 1e - -> H +
In this case, the cation has a rather high electron affinity. The combination of these factors makes the hydrogen cation a fairly strong electrophilic particle.
The formation of a hydrogen cation is possible during the electrolytic dissociation of acids:
HBr -> H + + Br -
It is for this reason that many electrophilic reactions occur in the presence and with the participation of acids.
Electrophilic particles, as mentioned earlier, act on systems containing regions of increased electron density. An example of such a system can be a multiple (double or triple) carbon-carbon bond.
You already know that the carbon atoms between which a double bond is formed are in a state of sp 2 hybridization. Unhybridized p-orbitals of neighboring carbon atoms, which are in the same plane, overlap, forming P-bond, which is less strong than the z-bond, and, most importantly, is easily polarized under the action of an external electric field. This means that when a positively charged particle approaches, the electrons of the TC bond are displaced in its direction and the so-called P- complex.
It turns out P-complex and upon addition of a hydrogen cation to P-connections. The hydrogen cation, as it were, stumbles upon an electron density protruding from the plane of the molecule P-links and joins it. 
At the next stage, the complete displacement of the electron pair occurs. P-bonds to one of the carbon atoms, which leads to the appearance of a lone pair of electrons on it. The orbital of the carbon atom on which this pair is located and the unfilled orbital of the hydrogen cation overlap, which leads to the formation covalent bond according to the donor-acceptor mechanism. At the same time, the second carbon atom remains an unfilled orbital, i.e., a positive charge.
The resulting particle is called a carbocation because it contains a positive charge on the carbon atom. This particle can combine with any anion, a particle that has an unshared electron pair, i.e., a nucleophile.
Consider the mechanism of the electrophilic addition reaction using the example of hydrobromination (addition of hydrogen bromide) of ethene:
CH2= CH2 + HBr --> CHBr-CH3
The reaction begins with the formation of an electrophilic particle - a hydrogen cation, which occurs as a result of the dissociation of a hydrogen bromide molecule.
Hydrogen cation attacks P-connection, forming P-a complex that quickly converts to a carbocation: 
Now consider a more complicated case.
The addition reaction of hydrogen bromide to ethene proceeds unambiguously, and the interaction of hydrogen bromide with propene can theoretically give two products: 1-bromopropane and 2-bromopropane. Experimental data show that mainly 2-bromopropane is obtained.
In order to explain this, we will have to consider an intermediate particle - a carbocation.
The addition of a hydrogen cation to propene can lead to the formation of two carbocations: if the hydrogen cation is attached to the first carbon atom, to the atom that is at the end of the chain, then the second one, i.e., in the center of the molecule (1), will have a positive charge; if it joins the second, then the first atom (2) will have a positive charge. 
The preferred direction of the reaction will depend on which carbocation will be more in the reaction medium, which, in turn, is determined by the stability of the carbocation. The experiment shows the predominant formation of 2-bromopropane. This means that the formation of carbocation (1) with a positive charge on the central atom occurs to a greater extent.
The greater stability of this carbocation is explained by the fact that the positive charge on the central carbon atom is compensated by the positive inductive effect of two methyl groups, the total effect of which is higher than the +/- effect of one ethyl group:
The patterns of reactions of hydrohalogenation of alkenes were studied by the famous Russian chemist V. V. Markovnikov, a student of A. M. Butlerov, who, as mentioned above, formulated the rule that bears his name.
This rule was established empirically, that is, empirically. At present, we can give a completely convincing explanation of it.
Interestingly, other electrophilic addition reactions also obey the Markovnikov rule, so it would be correct to formulate it in a more general view.
In electrophilic addition reactions, an electrophile (a particle with an unfilled orbital) is attached to a more hydrogenated carbon atom, and a nucleophile (a particle with a lone pair of electrons) is attached to a less hydrogenated one.
Polymerization
A special case of the addition reaction is the polymerization of alkenes and their derivatives. This reaction proceeds by the mechanism of free radical addition:
Polymerization is carried out in the presence of initiators - peroxide compounds, which are a source of free radicals. Peroxide compounds are called substances, the molecules of which include the -O-O- group. The simplest peroxide compound is hydrogen peroxide HOOH.
At a temperature of 100 °C and a pressure of 100 MPa, homolysis of an unstable oxygen-oxygen bond occurs and the formation of radicals - polymerization initiators. Under the action of KO- radicals, polymerization is initiated, which develops as a free radical addition reaction. Chain growth stops when the reaction mixture is recombined radicals - polymer chain and radicals or KOCH2CH2-.
Using the reaction of free radical polymerization of substances containing a double bond, a large number of macromolecular compounds are obtained: 
The use of alkenes with various substituents makes it possible to synthesize a wide range of polymeric materials with a wide range of properties. 
All these polymeric compounds are widely used in various fields of human activity - industry, medicine, are used to manufacture equipment for biochemical laboratories, some are intermediates for the synthesis of other macromolecular compounds.
Oxidation
You already know that in neutral or slightly alkaline solutions, alkenes are oxidized to diols (dihydric alcohols). In an acidic environment (a solution acidified with sulfuric acid), the double bond is completely destroyed and the carbon atoms between which the double bond existed are converted into carbon atoms of the carboxyl group: 
Destructive oxidation of alkenes can be used to determine their structure. So, for example, if acetic and propionic acids are obtained during the oxidation of some alkene, this means that pentene-2 has undergone oxidation, and if butyric (butanoic) acid and carbon dioxide, then the original hydrocarbon is pentene-1.
Application
Alkenes are widely used in the chemical industry as a raw material for the production of various organic substances and materials.
So, for example, ethene is the starting material for the production of ethanol, ethylene glycol, epoxides, dichloroethane.
A large amount of ethene is processed into polyethylene, which is used for the manufacture of packaging films, dishes, pipes, and electrical insulating materials.
Glycerin, acetone, isopropanol, solvents are obtained from propene. Polymerization of propene produces polypropylene, which is superior to polyethylene in many respects: it has a higher melting point and chemical resistance.
At present, fibers with unique properties are produced from polymers - analogues of polyethylene. For example, polypropylene fiber is stronger than all known synthetic fibers.
Materials made from these fibers are promising and are increasingly used in various fields of human activity.
1. What types of isomerism are characteristic of alkenes? Write the formulas for the possible isomers of pentene-1.
2. What compounds can be obtained from: a) isobutene (2-methylpropene); b) butene-2; c) butene-1? Write the equations for the corresponding reactions.
3. Decipher the following chain of transformations. Name compounds A, B, C. 4. Suggest a method for obtaining 2-chloropropane from 1-chloro-propane. Write the equations for the corresponding reactions.
5. Suggest a method for purifying ethane from ethylene impurities. Write the equations for the corresponding reactions.
6. Give examples of reactions that can be used to distinguish between saturated and unsaturated hydrocarbons.
7. Complete hydrogenation of 2.8 g of alkene consumed 0.896 l of hydrogen (n.a.). What is the molecular weight and structural formula of this compound, which has a normal chain of carbon atoms?
8. What gas is in the cylinder (ethene or propene), if it is known that it took 90 cm3 (n.a.) of oxygen to completely burn 20 cm3 of this gas?
9*. When an alkene reacts with chlorine in the dark, 25.4 g of dichloride is formed, and when this alkene of the same mass reacts with bromine in carbon tetrachloride, 43.2 g of dibromide is formed. Set all possible structural formulas of the starting alkene.
Discovery history
From the above material, we have already understood that ethylene is the ancestor of the homologous series of unsaturated hydrocarbons, which has one double bond. Their formula is C n H 2n and they are called alkenes.
The German physician and chemist Becher in 1669 was the first to obtain ethylene by the action of sulfuric acid on ethyl alcohol. Becher found that ethylene is more reactive than methane. But, unfortunately, at that time, the scientist could not identify the gas received, therefore he did not assign any name to it.
A little later, Dutch chemists also used the same method for obtaining ethylene. And since, when interacting with chlorine, it had the ability to form an oily liquid, it accordingly received the name "oxygen gas". Later it became known that this liquid is dichloroethane.
In French the term "oily" sounds like oléfiant. And after other hydrocarbons of this type were discovered, Antoine Fourcroix, a French chemist and scientist, introduced a new term that became common to the entire class of olefins or alkenes.
But already at the beginning of the nineteenth century, the French chemist J. Gay-Lussac showed that ethanol consists not only of "oily" gas, but also of water. In addition, the same gas was found in ethyl chloride.
And although chemists determined that ethylene consists of hydrogen and carbon, and already knew the composition of substances, they could not find its real formula for a long time. And only in 1862, E. Erlenmeyer managed to prove the presence of a double bond in the ethylene molecule. This was also recognized by the Russian scientist A. M. Butlerov and confirmed the correctness of this point of view experimentally.
Finding in nature and the physiological role of alkenes
Many are interested in the question of where alkenes can be found in nature. So, it turns out that they practically do not occur in nature, since its simplest representative, ethylene, is a hormone for plants and is synthesized in them only in small quantities.
True, in nature there is such an alkene as muscalur. This one of the natural alkenes is a sexual attractant of the female house fly.
It is worth paying attention to the fact that, having a high concentration, lower alkenes have a narcotic effect that can cause convulsions and irritation of the mucous membranes.
Application of alkenes
Life modern society today it is difficult to imagine without the use of polymeric materials. Since, unlike natural materials, polymers have different properties, they are easy to process, and if you look at the price, they are relatively cheap. Another important aspect in favor of polymers is that many of them can be recycled.
Alkenes have found their application in the production of plastics, rubbers, films, Teflon, ethyl alcohol, acetaldehyde and others. organic compounds.
IN agriculture it is used as a means that accelerates the process of fruit ripening. For getting various polymers and alcohols use propylene and butylenes. But in the production of synthetic rubber, isobutylene is used. Therefore, we can conclude that alkenes cannot be dispensed with, since they are the most important chemical raw materials.
Industrial use of ethylene
On an industrial scale, propylene is usually used for the synthesis of polypropylene and for the production of isopropanol, glycerol, butyric aldehydes, etc. Every year the need for propylene increases.
Continuation. For the beginning, see № 15, 16, 17, 18, 19/2004
Lesson 9
Chemical properties of alkenes
The chemical properties of alkenes (ethylene and its homologues) are largely determined by the presence of d ... bonds in their molecules. Alkenes enter into reactions of all three types, and the most characteristic of them are reactions p .... Consider them using propylene C 3 H 6 as an example.
All addition reactions proceed through a double bond and consist in the splitting of the α-bond of the alkene and the formation of two new α-bonds at the site of the break.
Addition of halogens:
Addition of hydrogen(hydrogenation reaction):
Water connection(hydration reaction):
Addition of hydrogen halides (HHal) and water to unsymmetrical alkenes according to the rule of V.V. Markovnikov (1869). Hydrogen acid Hhal attaches to the most hydrogenated carbon atom at the double bond. Accordingly, the Hal residue binds to the C atom, which has a smaller number of hydrogen atoms.
Combustion of alkenes in air.
When ignited, alkenes burn in air:
2CH 2 \u003d CHCH 3 + 9O 2 6CO 2 + 6H 2 O.
Gaseous alkenes form explosive mixtures with atmospheric oxygen.
Alkenes are oxidized by potassium permanganate in an aqueous medium, which is accompanied by discoloration of the KMnO 4 solution and the formation of glycols (compounds with two hydroxyl groups at adjacent C atoms). This process - hydroxylation of alkenes:
Alkenes are oxidized by atmospheric oxygen to epoxides. when heated in the presence of silver catalysts:
Polymerization of alkenes- the binding of many alkene molecules to each other. Reaction conditions: heating, presence of catalysts. The connection of molecules occurs by splitting intramolecular-bonds and the formation of new intermolecular-bonds:
In this reaction, the range of values n = 10 3 –10 4 .
Exercises.
1. Write the reaction equations for butene-1 with: a) Br2; b) HBr; V) H2O; G) H2. Name the reaction products.
2.
Conditions are known under which the addition of water and hydrogen halides to the double bond of alkenes proceeds against the Markovnikov rule. Write reaction equations
3-bromopropylene according to anti-Markovnikov with: a) water; b) hydrogen bromide.
3.
Write the equations for polymerization reactions: a) butene-1; b) vinyl chloride CH 2 =CHCl;
c) 1,2-difluoroethylene.
4. Write the equations for the reactions of ethylene with oxygen for following processes: a) combustion in air; b) hydroxylation with water KMnO 4 ; c) epoxidation (250 °C, Ag ).
5. Write the structural formula of an alkene, knowing that 0.21 g of this compound can add 0.8 g of bromine.
6. When burning 1 liter of gaseous hydrocarbon, which decolorizes the raspberry solution of potassium permanganate, 4.5 liters of oxygen are consumed, and 3 liters are obtained CO2. Write the structural formula for this hydrocarbon.
Lesson 10
Obtaining and using alkenes
Reactions for obtaining alkenes are reduced to reversing the reactions representing the chemical properties of alkenes (their flow from right to left, see lesson 9). You just need to find the right conditions.
Elimination of two halogen atoms from dihaloalkanes containing halogens at neighboring C atoms. The reaction proceeds under the action of metals (Zn, etc.):
Cracking of saturated hydrocarbons. So, during cracking (see lesson 7) of ethane, a mixture of ethylene and hydrogen is formed:
Dehydration of alcohols. When alcohols are treated with water-removing agents (concentrated sulfuric acid) or when heated to 350 ° C in the presence of catalysts, water is split off and alkenes are formed:
In this way, ethylene is obtained in the laboratory.
An industrial method for producing propylene, along with cracking, is the dehydration of propanol over alumina:
Dehydrochlorination of chloroalkanes is carried out under the action of an alkali solution in alcohol, because In water, the reaction products are not alkenes, but alcohols.
The use of ethylene and its homologues based on their chemical properties, i.e., the ability to turn into various useful substances.
Motor fuels, with high octane numbers, are obtained by hydrogenation of branched alkenes:
Discoloration of a yellow solution of bromine in an inert solvent (CCl 4) occurs when a drop of alkene is added or a gaseous alkene is passed through the solution. Interaction with bromine - characteristic qualitative reaction to the double bond:
The product of ethylene hydrochlorination, chloroethane, is used in chemical synthesis to introduce the C 2 H 5 group into the molecule:
Chloroethane also has a local anesthetic (pain relieving) effect, which is used in surgical operations.
Alcohols are obtained by hydration of alkenes, for example, ethanol:
Alcohol C 2 H 5 OH is used as a solvent, for disinfection, in the synthesis of new substances.
Hydration of ethylene in the presence of an oxidizing agent [O] leads to ethylene glycol - antifreeze and intermediate chemical synthesis :
Ethylene is oxidized to produce ethylene oxide and acetaldehyde. raw materials in the chemical industry:
Polymers and plastics- products of polymerization of alkenes, for example, polytetrafluoroethylene (Teflon):
Exercises.
1. Complete the equations for the reactions of elimination (cleavage), name the resulting alkenes:
2.
Make the equations for hydrogenation reactions: a) 3,3-dimethylbutene-1;
b) 2,3,3-trimethylbutene-1. These reactions produce alkanes used as motor fuels, give them names.
3. 100 g of ethyl alcohol was passed through a tube filled with heated alumina. C 2 H 5 OH. This resulted in 33.6 liters of hydrocarbon (n.o.s.). How much alcohol (in%) reacted?
4. How many grams of bromine will react with 2.8 liters (n.o.s.) of ethylene?
5. Write an equation for the polymerization of trifluorochloroethylene. (The resulting plastic is resistant to hot sulfuric acid, metallic sodium, etc.)
Answers to exercises for topic 1
Lesson 9
5. Reaction of alkene C n H2 n with bromine in general:
Molar mass of alkene M(WITH n H2 n) = 0.21 160/0.8 = 42 g/mol.
This is propylene.
Answer.
The alkene formula is CH 2 \u003d CHCH 3 (propylene).
6. Since all the substances involved in the reaction are gases, the stoichiometric coefficients in the reaction equation are proportional to their volume ratios. Let's write the reaction equation:
WITH a H V+ 4.5O 2 3CO 2 + 3H 2 O.
The number of water molecules is determined by the reaction equation: 4.5 2 = 9 O atoms reacted, 6 O atoms are bound in CO 2, the remaining 3 O atoms are part of three H 2 O molecules. Therefore, the indices are equal: A = 3, V\u003d 6. The desired hydrocarbon is propylene C 3 H 6.
Answer.
Structural formula propylene - CH 2 \u003d CHCH 3.
Lesson 10
1. Elimination (cleavage) reaction equations - synthesis of alkenes:
2. Hydrogenation reactions of alkenes when heated under pressure in the presence of a catalyst:
3. The reaction of dehydration of ethyl alcohol has the form:
Here through X the mass of alcohol converted to ethylene is indicated.
Let's find the value X: X\u003d 46 33.6 / 22.4 \u003d 69 g.
The proportion of reacted alcohol was: 69/100 = 0.69, or 69%.
Answer.
69% alcohol reacted.
4.
Since the stoichiometric coefficients in front of the formulas of the reactants (C 2 H 4 and Br 2) are equal to one, the relation is valid:
2,8/22,4 = X/160. From here X= 20 g Br 2 .
Answer.
20 g Br 2 .
The simplest alkene is ethene C 2 H 4. According to the IUPAC nomenclature, the names of alkenes are formed from the names of the corresponding alkanes by replacing the suffix "-an" with "-ene"; the position of the double bond is indicated by an Arabic numeral.
Spatial structure of ethylene
By the name of the first representative of this series - ethylene - such hydrocarbons are called ethylene.
Nomenclature and isomerism
Nomenclature
Alkenes of a simple structure are often called by replacing the suffix -an in alkanes with -ylene: ethane - ethylene, propane - propylene, etc.
According to the systematic nomenclature, the names of ethylene hydrocarbons are produced by replacing the suffix -an in the corresponding alkanes with the suffix -ene (alkane - alkene, ethane - ethene, propane - propene, etc.). The choice of the main chain and the order of name is the same as for alkanes. However, the chain must necessarily include a double bond. The numbering of the chain starts from the end to which this connection is closer. For example:
Rational names are sometimes used as well. In this case, all alkene hydrocarbons are considered as substituted ethylene:
Unsaturated (alkene) radicals are called trivial names or according to the systematic nomenclature:
H 2 C \u003d CH - - vinyl (ethenyl)
H 2 C \u003d CH - CH 2 - -allyl (propenyl-2)
isomerism
Alkenes are characterized by two types of structural isomerism. In addition to the isomerism associated with the structure of the carbon skeleton (as in alkanes), there is an isomerism that depends on the position of the double bond in the chain. This leads to an increase in the number of isomers in the alkene series.
The first two members of the homologous series of alkenes - (ethylene and propylene) - do not have isomers and their structure can be expressed as follows:
H 2 C \u003d CH 2 ethylene (ethene)
H 2 C \u003d CH - CH 3 propylene (propene)
Multiple bond position isomerism
H 2 C \u003d CH - CH 2 - CH 3 butene-1
H 3 C - CH \u003d CH - CH 3 butene-2
Geometric isomerism - cis-, trans-isomerism.
This isomerism is characteristic of compounds with a double bond.
If a simple σ-bond allows free rotation of individual links of the carbon chain around its axis, then such rotation does not occur around a double bond. This is the reason for the appearance of geometric ( cis-, trans-) isomers.
Geometric isomerism is one of the types of spatial isomerism.
Isomers in which the same substituents (at different carbon atoms) are located on one side of the double bond are called cis-isomers, and in different ways - trans-isomers:
cis- And trance- isomers differ not only in spatial structure, but also in many physical and chemical properties. Trance- isomers are more stable than cis- isomers.
Obtaining alkenes
Alkenes are rare in nature. Usually, gaseous alkenes (ethylene, propylene, butylenes) are isolated from refinery gases (during cracking) or associated gases, as well as from coal coking gases.
In industry, alkenes are obtained by dehydrogenation of alkanes in the presence of a catalyst (Cr 2 O 3).
Dehydrogenation of alkanes
H 3 C - CH 2 - CH 2 - CH 3 → H 2 C \u003d CH - CH 2 - CH 3 + H 2 (butene-1)
H 3 C - CH 2 - CH 2 - CH 3 → H 3 C - CH \u003d CH - CH 3 + H 2 (butene-2)
Of the laboratory methods of obtaining, the following can be noted:
1. Cleavage of hydrogen halide from halogenated alkyls under the action of an alcohol solution of alkali on them:
2. Hydrogenation of acetylene in the presence of a catalyst (Pd):
H-C ≡ C-H + H 2 → H 2 C \u003d CH 2
3. Dehydration of alcohols (cleavage of water).
Acids (sulphuric or phosphoric) or Al 2 O 3 are used as a catalyst:
In such reactions, hydrogen is split off from the least hydrogenated (with the smallest number hydrogen atoms) carbon atom (A.M. Zaitsev's rule):
Physical properties
The physical properties of some alkenes are shown in the table below. The first three representatives of the homologous series of alkenes (ethylene, propylene and butylene) are gases, starting with C 5 H 10 (amylene, or pentene-1) are liquids, and with C 18 H 36 are solids. As the molecular weight increases, the melting and boiling points increase. Normal alkenes boil at a higher temperature than their isomers. Boiling points cis-isomers higher than trance-isomers, and melting points - vice versa.
Alkenes are poorly soluble in water (however, better than the corresponding alkanes), but well - in organic solvents. Ethylene and propylene burn with a smoky flame.
Physical properties of some alkenes
Name |
t pl, °С |
t kip, ° С |
||
Ethylene (ethene) |
||||
propylene (propene) |
||||
Butylene (butene-1) |
||||
cis-butene-2 |
||||
Trans-butene-2 |
||||
Isobutylene (2-methylpropene) |
||||
Amilene (pentene-1) |
||||
Hexylene (hexene-1) |
||||
Heptylene (heptene-1) |
||||
Octene (octene-1) |
||||
Nonylene (nonene-1) |
||||
Decylen (decene-1) |
Alkenes have low polarity, but are easily polarized.
Chemical properties
Alkenes are highly reactive. Their chemical properties are determined mainly by the carbon-carbon double bond.
The π-bond, as the least strong and more accessible, breaks under the action of the reagent, and the released valences of carbon atoms are spent on attaching the atoms that make up the reagent molecule. This can be represented as a diagram:
Thus, in addition reactions, the double bond is broken, as it were, by half (with the preservation of the σ-bond).
For alkenes, in addition to addition, oxidation and polymerization reactions are also characteristic.
Addition reactions
More often, addition reactions proceed according to the heterolytic type, being electrophilic addition reactions.
1. Hydrogenation (addition of hydrogen). Alkenes, adding hydrogen in the presence of catalysts (Pt, Pd, Ni), pass into saturated hydrocarbons - alkanes:
H 2 C \u003d CH 2 + H 2 → H 3 C - CH 3 (ethane)
2. Halogenation (addition of halogens). Halogens easily add at the site of double bond rupture to form dihalogen derivatives:
H 2 C \u003d CH 2 + Cl 2 → ClH 2 C - CH 2 Cl (1,2-dichloroethane)
The addition of chlorine and bromine is easier, and iodine is more difficult. Fluorine with alkenes, as with alkanes, interacts with an explosion.
Compare: in alkenes, the halogenation reaction is a process of addition, not substitution (as in alkanes).
The halogenation reaction is usually carried out in a solvent at ordinary temperature.
The addition of bromine and chlorine to alkenes occurs by an ionic rather than a radical mechanism. This conclusion follows from the fact that the rate of halogen addition does not depend on irradiation, the presence of oxygen, and other reagents that initiate or inhibit radical processes. Based a large number experimental data for this reaction, a mechanism was proposed that includes several successive stages. At the first stage, the polarization of the halogen molecule occurs under the action of π-bond electrons. The halogen atom, which acquires some fractional positive charge, forms an unstable intermediate with the electrons of the π bond, called the π complex or charge transfer complex. It should be noted that in the π-complex, the halogen does not form a directed bond with any particular carbon atom; in this complex, the donor-acceptor interaction of the electron pair of the π-bond as a donor and the halogen as an acceptor is simply realized.
Further, the π-complex turns into a cyclic bromonium ion. In the process of formation of this cyclic cation, a heterolytic cleavage of the Br-Br bond occurs and an empty R-orbital sp 2 -hybridized carbon atom overlaps with R-orbital of the "lone pair" of electrons of the halogen atom, forming a cyclic bromonium ion.
At the last, third stage, the bromine anion, as a nucleophilic agent, attacks one of the carbon atoms of the bromonium ion. Nucleophilic attack by the bromide ion leads to the opening of the three-membered ring and the formation of a vicinal dibromide ( vic-near). This step can be formally considered as a nucleophilic substitution of S N 2 at the carbon atom, where the leaving group is Br + .
The result of this reaction is not difficult to predict: the bromine anion attacks the carbocation to form dibromoethane.
The rapid discoloration of a solution of bromine in CCl 4 is one of the simplest tests for unsaturation, since alkenes, alkynes, and dienes react rapidly with bromine.
The addition of bromine to alkenes (bromination reaction) is a qualitative reaction to saturated hydrocarbons. When unsaturated hydrocarbons are passed through bromine water (a solution of bromine in water), the yellow color disappears (in the case of limiting hydrocarbons, it remains).
3. Hydrohalogenation (addition of hydrogen halides). Alkenes easily add hydrogen halides:
H 2 C \u003d CH 2 + HBr → H 3 C - CH 2 Br
The addition of hydrogen halides to ethylene homologues follows the rule of V.V. Markovnikov (1837 - 1904): when normal conditions the hydrogen of the hydrogen halide is attached at the double bond to the most hydrogenated carbon atom, and the halogen to the less hydrogenated:
Markovnikov's rule can be explained by the fact that in unsymmetrical alkenes (for example, in propylene), the electron density is unevenly distributed. Under the influence of the methyl group bound directly to the double bond, the electron density shifts towards this bond (to the extreme carbon atom).
Due to this shift, the p-bond is polarized and partial charges appear on the carbon atoms. It is easy to imagine that a positively charged hydrogen ion (proton) will join a carbon atom (electrophilic addition), which has a partial negative charge, and a bromine anion, to carbon with a partial positive charge.
Such attachment is a consequence of the mutual influence of atoms in organic molecule. As you know, the electronegativity of the carbon atom is slightly higher than that of hydrogen.
Therefore, in the methyl group, some polarization σ- C-H connections associated with the shift of electron density from hydrogen atoms to carbon. In turn, this causes an increase in the electron density in the region of the double bond, and especially on its extreme, atom. Thus, the methyl group, like other alkyl groups, acts as an electron donor. However, in the presence of peroxide compounds or O 2 (when the reaction is radical), this reaction can also go against the Markovnikov rule.
For the same reasons, Markovnikov's rule is observed when not only hydrogen halides are added to unsymmetrical alkenes, but also other electrophilic reagents (H 2 O, H 2 SO 4 , HOCl, ICl, etc.).
4. Hydration (water addition). In the presence of catalysts, water is added to alkenes to form alcohols. For example:
H 3 C - CH \u003d CH 2 + H - OH → H 3 C - CHOH - CH 3 (isopropyl alcohol)
Oxidation reactions
Alkenes are more easily oxidized than alkanes. The products formed during the oxidation of alkenes and their structure depend on the structure of alkenes and on the conditions for this reaction.
1. Combustion
H 2 C \u003d CH 2 + 3O 2 → 2CO 2 + 2H 2 O
2. Incomplete catalytic oxidation
3. Oxidation at normal temperature. When acting on ethylene aqueous solution KMnO 4 (at normal conditions, in neutral or alkaline environment- Wagner reaction) the formation of dihydric alcohol - ethylene glycol occurs:
3H 2 C \u003d CH 2 + 2KMnO 4 + 4H 2 O → 3HOCH 2 - CH 2 OH (ethylene glycol) + 2MnO 2 + KOH
This reaction is qualitative: the violet color of a solution of potassium permanganate changes when an unsaturated compound is added to it.
Under more severe conditions (oxidation of KMnO 4 in the presence of sulfuric acid or a chromium mixture), the double bond breaks in the alkene to form oxygen-containing products:
H 3 C - CH \u003d CH - CH 3 + 2O 2 → 2H 3 C - COOH (acetic acid)
Isomerization reaction
When heated or in the presence of catalysts, alkenes are able to isomerize - a double bond moves or an isostructure is established.
polymerization reactions
Due to the breaking of π-bonds, alkene molecules can combine with each other, forming long chain molecules.
Finding in nature and the physiological role of alkenes
In nature, acyclic alkenes are practically not found. The simplest representative of this class of organic compounds - ethylene C 2 H 4 - is a hormone for plants and is synthesized in them in small quantities.
One of the few naturally occurring alkenes is muscalur ( cis- tricosen-9) is a sexual attractant of the female house fly (Musca domestica).
Lower alkenes in high concentrations have a narcotic effect. The higher members of the series also cause convulsions and irritation of the mucous membranes of the respiratory tract.
Individual representatives
Ethylene (ethene) is an organic chemical compound described by the formula C 2 H 4 . It is the simplest alkene. Contains a double bond and therefore refers to unsaturated or unsaturated hydrocarbons. Plays extremely important role in industry, and is also a phytohormone (low molecular weight organic substances produced by plants and having regulatory functions).
Ethylene - causes anesthesia, has an irritating and mutagenic effect.
Ethylene is the most produced organic compound in the world; the total world production of ethylene in 2008 amounted to 113 million tons and continues to grow by 2-3% per year.
Ethylene is the leading product of the main organic synthesis and is used to produce polyethylene (1st place, up to 60% of the total volume).
Polyethylene is a thermoplastic polymer of ethylene. The most common plastic in the world.
It is a waxy mass of white color (thin transparent sheets are colorless). It is chemically and frost-resistant, an insulator, not sensitive to shock (shock absorber), softens when heated (80-120 ° C), freezes when cooled, adhesion (adhesion of surfaces of dissimilar solid and / or liquid bodies) is extremely low. Sometimes in the popular mind it is identified with cellophane - a similar material of plant origin.
Propylene - causes anesthesia (stronger than ethylene), has a general toxic and mutagenic effect.
Resistant to water, does not react with alkalis of any concentration, with solutions of neutral, acidic and basic salts, organic and inorganic acids, even concentrated sulfuric acid, but decomposes under the action of 50% nitric acid at room temperature and under the influence of liquid and gaseous chlorine and fluorine. Over time, thermal aging occurs.
Polyethylene film (especially packaging, such as bubble wrap or tape).
Containers (bottles, jars, boxes, canisters, garden watering cans, pots for seedlings.
Polymer pipes for sewerage, drainage, water and gas supply.
electrical insulating material.
Polyethylene powder is used as a hot melt adhesive.
Butene-2 - causes anesthesia, has an irritating effect.
Chemical properties of alkanes
Alkanes (paraffins) are non-cyclic hydrocarbons, in the molecules of which all carbon atoms are connected only by single bonds. In other words, there are no multiple, double or triple bonds in the molecules of alkanes. In fact, alkanes are hydrocarbons containing the maximum possible number of hydrogen atoms, and therefore they are called limiting (saturated).
Due to saturation, alkanes cannot enter into addition reactions.
Since carbon and hydrogen atoms have fairly close electronegativity, this leads to the fact that the CH bonds in their molecules are extremely low polarity. In this regard, for alkanes, reactions proceeding according to the mechanism of radical substitution, denoted by the symbol S R, are more characteristic.
1. Substitution reactions
In reactions of this type, carbon-hydrogen bonds are broken.
RH + XY → RX + HY
Halogenation
Alkanes react with halogens (chlorine and bromine) under the action of ultraviolet light or with strong heat. In this case, a mixture of halogen derivatives with different degrees of substitution of hydrogen atoms is formed - mono-, di-tri-, etc. halogen-substituted alkanes.
On the example of methane, it looks like this:
By changing the halogen/methane ratio in the reaction mixture, it is possible to ensure that any particular methane halogen derivative predominates in the composition of the products.
reaction mechanism
Let us analyze the mechanism of the free radical substitution reaction using the example of the interaction of methane and chlorine. It consists of three stages:
- initiation (or chain initiation) - the process of formation of free radicals under the action of energy from the outside - irradiation with UV light or heating. At this stage, the chlorine molecule undergoes a homolytic cleavage of the Cl-Cl bond with the formation of free radicals:
Free radicals, as can be seen from the figure above, are called atoms or groups of atoms with one or more unpaired electrons(Cl, H, CH 3 , CH 2 etc.);
2. Chain development
This stage consists in the interaction of active free radicals with inactive molecules. In this case, new radicals are formed. In particular, when chlorine radicals act on alkane molecules, an alkyl radical and hydrogen chloride are formed. In turn, the alkyl radical, colliding with chlorine molecules, forms a chlorine derivative and a new chlorine radical:
3) Break (death) of the chain:
Occurs as a result of the recombination of two radicals with each other into inactive molecules:
2. Oxidation reactions
Under normal conditions, alkanes are inert with respect to such strong oxidizing agents as concentrated sulfuric and nitric acids, permanganate and potassium dichromate (KMnO 4, K 2 Cr 2 O 7).
Combustion in oxygen
A) complete combustion with an excess of oxygen. Leads to the formation of carbon dioxide and water:
CH 4 + 2O 2 \u003d CO 2 + 2H 2 O
B) incomplete combustion with a lack of oxygen:
2CH 4 + 3O 2 \u003d 2CO + 4H 2 O
CH 4 + O 2 \u003d C + 2H 2 O
Catalytic oxidation with oxygen
As a result of heating alkanes with oxygen (~200 o C) in the presence of catalysts, a wide variety of organic products can be obtained from them: aldehydes, ketones, alcohols, carboxylic acids.
For example, methane, depending on the nature of the catalyst, can be oxidized to methyl alcohol, formaldehyde or formic acid:
3. Thermal transformations of alkanes
Cracking
Cracking (from English to crack - to tear) is chemical process proceeding at high temperature, as a result of which the carbon skeleton of alkane molecules breaks with the formation of alkene and alkane molecules with lower molecular weights compared to the original alkanes. For example:
CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3 → CH 3 -CH 2 -CH 2 -CH 3 + CH 3 -CH \u003d CH 2
Cracking can be thermal or catalytic. For the implementation of catalytic cracking, due to the use of catalysts, significantly lower temperatures are used compared to thermal cracking.
Dehydrogenation
The elimination of hydrogen occurs as a result of breaking the C-H bonds; carried out in the presence of catalysts at elevated temperatures. Dehydrogenation of methane produces acetylene:
2CH 4 → C 2 H 2 + 3H 2
Heating methane to 1200 ° C leads to its decomposition into simple substances:
CH 4 → C + 2H 2
Dehydrogenation of other alkanes gives alkenes:
C 2 H 6 → C 2 H 4 + H 2
When dehydrogenating n-butane, butene-1 and butene-2 are formed (the latter in the form cis- And trance-isomers):
Dehydrocyclization
Isomerization
Chemical properties of cycloalkanes
The chemical properties of cycloalkanes with more than four carbon atoms in the cycles are generally almost identical to those of alkanes. For cyclopropane and cyclobutane, oddly enough, addition reactions are characteristic. This is due to the high tension within the cycle, which leads to the fact that these cycles tend to break. So cyclopropane and cyclobutane easily add bromine, hydrogen or hydrogen chloride:
Chemical properties of alkenes
1. Addition reactions
Since the double bond in alkene molecules consists of one strong sigma bond and one weak pi bond, they are quite active compounds that easily enter into addition reactions. Alkenes often enter into such reactions even under mild conditions - in the cold, in aqueous solutions and organic solvents.
Hydrogenation of alkenes
Alkenes are able to add hydrogen in the presence of catalysts (platinum, palladium, nickel):
CH 3 -CH \u003d CH 2 + H 2 → CH 3 -CH 2 -CH 3
Hydrogenation of alkenes proceeds easily even at normal pressure and slight heating. An interesting fact is that the same catalysts can be used for the dehydrogenation of alkanes to alkenes, only the dehydrogenation process proceeds at a higher temperature and lower pressure.
Halogenation
Alkenes easily enter into an addition reaction with bromine both in aqueous solution and in organic solvents. As a result of the interaction, initially yellow solutions of bromine lose their color, i.e. discolor.
CH 2 \u003d CH 2 + Br 2 → CH 2 Br-CH 2 Br
Hydrohalogenation
It is easy to see that the addition of a hydrogen halide to an unsymmetrical alkene molecule should theoretically lead to a mixture of two isomers. For example, when hydrogen bromide is added to propene, the following products should be obtained:
Nevertheless, in the absence of specific conditions (for example, the presence of peroxides in the reaction mixture), the addition of a hydrogen halide molecule will occur strictly selectively in accordance with the Markovnikov rule:
The addition of a hydrogen halide to an alkene occurs in such a way that hydrogen is attached to a carbon atom with a larger number of hydrogen atoms (more hydrogenated), and a halogen is attached to a carbon atom with a smaller number of hydrogen atoms (less hydrogenated).
Hydration
This reaction leads to the formation of alcohols, and also proceeds in accordance with the Markovnikov rule:
As you might guess, due to the fact that the addition of water to the alkene molecule occurs according to the Markovnikov rule, the formation of primary alcohol is possible only in the case of ethylene hydration:
CH 2 \u003d CH 2 + H 2 O → CH 3 -CH 2 -OH
It is by this reaction that the main amount of ethyl alcohol is carried out in the large-capacity industry.
Polymerization
A specific case of the addition reaction is the polymerization reaction, which, unlike halogenation, hydrohalogenation and hydration, proceeds through a free radical mechanism:
Oxidation reactions
Like all other hydrocarbons, alkenes burn easily in oxygen to form carbon dioxide and water. The equation for the combustion of alkenes in excess oxygen has the form:
C n H 2n + (3/2)nO 2 → nCO 2 + nH 2 O
Unlike alkanes, alkenes are easily oxidized. Under the action of an aqueous solution of KMnO 4 on alkenes, discoloration, which is a qualitative reaction to double and triple CC bonds in molecules of organic substances.
Oxidation of alkenes with potassium permanganate in a neutral or slightly alkaline solution leads to the formation of diols (dihydric alcohols):
C 2 H 4 + 2KMnO 4 + 2H 2 O → CH 2 OH–CH 2 OH + 2MnO 2 + 2KOH (cooling)
In an acidic environment, a complete rupture of the double bond occurs with the transformation of the carbon atoms that formed the double bond into carboxyl groups:
5CH 3 CH=CHCH 2 CH 3 + 8KMnO 4 + 12H 2 SO 4 → 5CH 3 COOH + 5C 2 H 5 COOH + 8MnSO 4 + 4K 2 SO 4 + 17H 2 O (heating)
If the double C=C bond is at the end of the alkene molecule, then carbon dioxide is formed as the oxidation product of the extreme carbon atom at the double bond. This is due to the fact that the intermediate oxidation product, formic acid, is easily oxidized by itself in an excess of an oxidizing agent:
5CH 3 CH=CH 2 + 10KMnO 4 + 15H 2 SO 4 → 5CH 3 COOH + 5CO 2 + 10MnSO 4 + 5K 2 SO 4 + 20H 2 O (heating)
In the oxidation of alkenes, in which the C atom at the double bond contains two hydrocarbon substituents, a ketone is formed. For example, the oxidation of 2-methylbutene-2 produces acetone and acetic acid.
The oxidation of alkenes, which breaks the carbon skeleton at the double bond, is used to establish their structure.
Chemical properties of alkadienes
Addition reactions
For example, the addition of halogens:
Bromine water becomes colorless.
Under normal conditions, the addition of halogen atoms occurs at the ends of the butadiene-1,3 molecule, while π bonds are broken, bromine atoms are attached to the extreme carbon atoms, and free valences form a new π bond. Thus, as if there is a "movement" of the double bond. With an excess of bromine, one more bromine molecule can be added at the site of the formed double bond.
polymerization reactions
Chemical properties of alkynes
Alkynes are unsaturated (unsaturated) hydrocarbons and therefore are capable of entering into addition reactions. Among the addition reactions for alkynes, electrophilic addition is the most common.
Halogenation
Since the triple bond of alkyne molecules consists of one stronger sigma bond and two weaker pi bonds, they are able to attach either one or two halogen molecules. The addition of two halogen molecules by one alkyne molecule proceeds by the electrophilic mechanism sequentially in two stages:
Hydrohalogenation
The addition of hydrogen halide molecules also proceeds by the electrophilic mechanism and in two stages. In both stages, the addition proceeds in accordance with the Markovnikov rule:
Hydration
The addition of water to alkynes occurs in the presence of ruthium salts in an acidic medium and is called the Kucherov reaction.
As a result of the hydration of the addition of water to acetylene, acetaldehyde (acetic aldehyde) is formed:
For acetylene homologues, the addition of water leads to the formation of ketones:
Alkyne hydrogenation
Alkynes react with hydrogen in two steps. Metals such as platinum, palladium, nickel are used as catalysts:
Alkyne trimerization
When acetylene is passed over activated carbon at high temperature, a mixture of various products is formed from it, the main of which is benzene, a product of acetylene trimerization:
Dimerization of alkynes
Acetylene also enters into a dimerization reaction. The process proceeds in the presence of copper salts as catalysts:
Alkyne oxidation
Alkynes burn in oxygen:
C n H 2n-2 + (3n-1) / 2 O 2 → nCO 2 + (n-1) H 2 O
The interaction of alkynes with bases
Alkynes with a triple C≡C at the end of the molecule, unlike other alkynes, are able to enter into reactions in which the hydrogen atom in the triple bond is replaced by a metal. For example, acetylene reacts with sodium amide in liquid ammonia:
HC≡CH + 2NaNH 2 → NaC≡CNa + 2NH 3,
and also with ammonia solution silver oxide, forming insoluble salt-like substances called acetylenides:
Thanks to this reaction, it is possible to recognize alkynes with a terminal triple bond, as well as to isolate such an alkyne from a mixture with other alkynes.
It should be noted that all silver and copper acetylenides are explosive substances.
Acetylides are able to react with halogen derivatives, which is used in the synthesis of more complex organic compounds with a triple bond:
CH 3 -C≡CH + NaNH 2 → CH 3 -C≡CNa + NH 3
CH 3 -C≡CNa + CH 3 Br → CH 3 -C≡C-CH 3 + NaBr
Chemical properties of aromatic hydrocarbons
The aromatic nature of the bond affects the chemical properties of benzenes and other aromatic hydrocarbons.
A single 6pi electron system is much more stable than conventional pi bonds. Therefore, for aromatic hydrocarbons, substitution reactions are more characteristic than addition reactions. Arenes enter into substitution reactions by an electrophilic mechanism.
Substitution reactions
Halogenation
Nitration
The nitration reaction proceeds best under the action of not pure nitric acid, but its mixture with concentrated sulfuric acid, the so-called nitrating mixture:
Alkylation
The reaction in which one of the hydrogen atoms at the aromatic nucleus is replaced by a hydrocarbon radical:
Alkenes can also be used instead of halogenated alkanes. Aluminum halides, ferric iron halides or inorganic acids can be used as catalysts.<
Addition reactions
hydrogenation
Accession of chlorine
It proceeds by a radical mechanism under intense irradiation with ultraviolet light:
Similarly, the reaction can proceed only with chlorine.
Oxidation reactions
Combustion
2C 6 H 6 + 15O 2 \u003d 12CO 2 + 6H 2 O + Q
incomplete oxidation
The benzene ring is resistant to oxidizing agents such as KMnO 4 and K 2 Cr 2 O 7 . The reaction does not go.
Division of substituents in the benzene ring into two types:
Consider the chemical properties of benzene homologues using toluene as an example.
Chemical properties of toluene
Halogenation
The toluene molecule can be considered as consisting of fragments of benzene and methane molecules. Therefore, it is logical to assume that the chemical properties of toluene should to some extent combine the chemical properties of these two substances taken separately. In particular, this is precisely what is observed during its halogenation. We already know that benzene enters into a substitution reaction with chlorine by an electrophilic mechanism, and catalysts (aluminum or ferric halides) must be used to carry out this reaction. At the same time, methane is also capable of reacting with chlorine, but by a free radical mechanism, which requires irradiation of the initial reaction mixture with UV light. Toluene, depending on the conditions under which it undergoes chlorination, is able to give either the products of substitution of hydrogen atoms in the benzene ring - for this you need to use the same conditions as in the chlorination of benzene, or the products of substitution of hydrogen atoms in the methyl radical, if on it, how to act on methane with chlorine when irradiated with ultraviolet light:
As you can see, the chlorination of toluene in the presence of aluminum chloride led to two different products - ortho- and para-chlorotoluene. This is due to the fact that the methyl radical is a substituent of the first kind.
If the chlorination of toluene in the presence of AlCl 3 is carried out in excess of chlorine, the formation of trichlorine-substituted toluene is possible:
Similarly, when toluene is chlorinated in the light at a higher chlorine / toluene ratio, dichloromethylbenzene or trichloromethylbenzene can be obtained:
Nitration
The substitution of hydrogen atoms for nitrogroup, during the nitration of toluene with a mixture of concentrated nitric and sulfuric acids, leads to substitution products in the aromatic nucleus, and not in the methyl radical:
Alkylation
As already mentioned, the methyl radical is an orientant of the first kind, therefore, its Friedel-Crafts alkylation leads to substitution products in the ortho and para positions:
Addition reactions
Toluene can be hydrogenated to methylcyclohexane using metal catalysts (Pt, Pd, Ni):
C 6 H 5 CH 3 + 9O 2 → 7CO 2 + 4H 2 O
incomplete oxidation
Under the action of such an oxidizing agent as an aqueous solution of potassium permanganate, the side chain undergoes oxidation. The aromatic nucleus cannot be oxidized under such conditions. In this case, depending on the pH of the solution, either a carboxylic acid or its salt will be formed.
