Alkene hydrocarbons (olefins) are one of the classes of organic substances that have their own. Types of isomerism of alkenes in representatives of this class do not repeat with the isomerism of other organic substances.
In contact with
Characteristic features of the class
Ethylene olefins are called one of the classes of unsaturated hydrocarbons containing one double bond.
By physical properties representatives of this category of unsaturated compounds are:
- gases,
- liquids,
- solid compounds.
In the composition of the molecules there is not only a "sigma" bond, but also a "pi" bond. The reason for this is the presence in the structural formula of hybridization " sp2”, which is characterized by the arrangement of atoms of the compound in the same plane.
At the same time, an angle of at least one hundred and twenty degrees is formed between them. unhybridized orbitals " R» is characteristic of the location both above the molecular plane and below it.
This feature of the structure leads to the formation of additional bonds - "pi" or " π ».
The described connection is less strong compared to the "sigma"-bonds, since the side overlap has a weak adhesion. The total distribution of the electron densities of the formed bonds is characterized by inhomogeneity. When rotating near the carbon-carbon bond, there is a violation of the overlap of "p" orbitals. For each alkene (olefin), such a pattern is a distinctive feature.
Almost all ethylene compounds have high boiling and melting points, which are not characteristic of all organic substances. Representatives of this class of unsaturated carbohydrates quickly dissolve in other organic solvents.
Attention! Acyclic unsaturated compounds ethylene hydrocarbons have the general formula - C n H 2n.
Homology
Based on the fact that the general formula of alkenes is C n H 2n, they have a certain homology. The homologous series of alkenes begins with the first representative ethylene or ethene. This substance in normal conditions is a gas and contains two carbon atoms and four hydrogen atoms -C 2 H 4. Behind ethene, the homologous series of alkenes continues with propene and butene. Their formulas are as follows: "C 3 H 6" and "C 4 H 8". Under normal conditions, they are also gases that are heavier, which means that they must be collected with a test tube turned upside down.
The general formula of alkenes allows you to calculate the next representative of this class, having at least five carbon atoms in the structural chain. This is a pentene with the formula "C 5 H 10".
By physical characteristics the specified substance belongs to liquids, as well as the twelve following compounds of the homologous line.
Among the alkenes with the indicated characteristics, there are also solids, which begin with the formula C 18 H 36 . Liquid and solid ethylene hydrocarbons do not tend to dissolve in water, but when they enter organic solvents, they react with them.
The described general formula for alkenes implies the replacement of the previously standing suffix "an" with "en". This is enshrined in IUPAC rules. Whichever representative of this category of compounds we take, they all have the described suffix.
In the name of ethylene compounds, there is always a certain number that indicates the location of the double bond in the formula. Examples of this are: "butene-1" or "pentene-2". Atomic numbering starts from the edge closest to the double configuration. This rule is "iron" in all cases.
isomerism
Depending on the existing type of hybridization of alkenes, they have certain types of isomerism, each of which has its own characteristics and structure. Consider the main types of isomerism of alkenes.
structural type
Structural isomerism is subdivided into isomers according to:
- carbon skeleton;
- location of the double bond.
Structural isomers of the carbon skeleton arise in the case of the appearance of radicals (branches from the main chain).
Isomers of alkenes of the indicated isomerism will be:
CH 2 \u003d CH — CH 2 — CH 3.
2-methylpropene-1:
CH2=C — CH 3
│
The presented compounds have a total number of carbon and hydrogen atoms (C 4 H 8), but a different structure of the hydrocarbon skeleton. These are structural isomers, although their properties are not the same. Butene-1 (butylene) has a characteristic odor and narcotic properties that irritate the respiratory tract. These features do not have 2-methylpropene-1.
In this case, ethylene (C 2 H 4) has no isomers, since it consists of only two carbon atoms, where radicals cannot be substituted.
Advice! The radical is allowed to be placed on the middle and penultimate carbon atoms, but it is not allowed to place them near the extreme substituents. This rule works for all unsaturated hydrocarbons.
Regarding the location of the double bond, isomers are distinguished:
CH 2 \u003d CH — CH 2 — CH 2 -CH 3.
CH 3 -CH = CH — CH 2 -CH 3.
The general formula for alkenes in the examples presented is:C 5 H 10,, but the location of one double bond is different. The properties of these compounds will vary. This is structural isomerism.
isomerism
Spatial type
Spatial isomerism of alkenes is associated with the nature of the arrangement of hydrocarbon substituents.
Based on this, isomers are distinguished:
- "cis";
- "Trance".
The general formula of alkenes allows the creation of "trans-isomers" and "cis-isomers" of the same compound. Take, for example, butylene (butene). For it, it is possible to create isomers of the spatial structure by arranging the substituents in different ways relative to the double bond. With examples, the isomerism of alkenes would look like this:
"cis-isomer" "trans-isomer"
Butene-2 Butene-2
From this example, it can be seen that the "cis-isomers" have two identical radicals on one side of the plane of the double bond. For "trans-isomers" this rule does not work, since they have two dissimilar substituents relative to the "C \u003d C" carbon chain. Given this regularity, it is possible to build "cis" and "trans" isomers for various acyclic ethylene hydrocarbons.
The presented "cis-isomer" and "trans-isomer" for butene-2 cannot be converted into one another, since this requires rotation around the existing carbon double chain (C=C). To carry out this rotation, a certain amount of energy is needed to break the existing “p-bond”.
Based on the foregoing, it can be concluded that the "trans" and "cis" isomers of the species are individual compounds with a certain set of chemical and physical properties.
Which alkene has no isomers. Ethylene has no spatial isomers due to the same arrangement of hydrogen substituents relative to the double chain.
Interclass
Interclass isomerism in alkene hydrocarbons is widespread. The reason for this is the similarity of the general formula of representatives of this class with the formula of cycloparaffins (cycloalkanes). These categories of substances have the same number of carbon and hydrogen atoms, a multiple of the composition (C n H 2n).
Interclass isomers would look like this:
CH 2 \u003d CH — CH 3.
Cyclopropane:
It turns out that the formulaC 3 H 6two compounds are responsible: propene-1 and cyclopropane. From the structural structure one can see the different arrangement of carbon relative to each other. The properties of these compounds are also different. Propene-1 (propylene) is a gaseous compound with a low boiling point. Cyclopropane is characterized by a gaseous state with a pungent odor and a pungent taste. The chemical properties of these substances also differ, but their composition is identical. In organic, this type of isomer is called interclass.
Alkenes. Isomerism of alkenes. USE. Organic chemistry.
Alkenes: Structure, nomenclature, isomerism
Conclusion
Alkene isomerism is their important characteristic, due to which new compounds with other properties appear in nature, which are used in industry and everyday life.
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 the reversal of reactions representing Chemical properties alkenes (flowing them 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 product of 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 view:
Molar mass 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.
The structural formula of propylene is CH 2 = 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 .
They are burning.
1. Combustion in air
2. Oxidation with an aqueous solution of permanganate (Wagner reaction)
In a neutral medium, brown manganese (IV) oxide is obtained, and two OH groups are attached to the double bond of the organic substance:
On the left is an alkene with potassium permanganate, on the right is an alkane. The organic layer (top) does not mix with the water layer (bottom). On the right, the color of the permanganate has not changed. Rice. 1.
Rice. 1. Wagner reaction
3. Oxidation with acidified permanganate solution
In an acidic environment, the solution becomes colorless: Mn +7 is reduced to Mn +2. Discoloration of an acidified solution of potassium permanganate - a qualitative reaction to unsaturated compounds.
5CH 2 \u003d CH 2 + 12KMnO 4 + 18H 2 SO 4 \u003d 12MnSO 4 + 10CO 2 + 6K 2 SO 4 + 28H 2 O.
Dependence of oxidation products on alkene structure:
Radical substitution in alkenes
Propene and chlorine at high temperature: 400-500 o C (conditions favorable radical reactions) give the product not of addition, but of substitution.
In industry alkenes are obtained by cracking or dehydrogenation of petroleum alkanes.
Laboratory methods obtaining alkenes based on cleavage reactions.
1. Dehalogenation
The reaction of dihaloalkanes, in the molecules of which halogen atoms are located at neighboring carbon atoms, with magnesium or zinc leads to the formation of a double bond:
CH 2 Cl-CH 2 Cl + Zn → CH 2 \u003d CH 2 + ZnCl 2
2. Dehydrohalogenation
When haloalkanes react with a hot alcoholic solution of alkali, a hydrogen halide molecule is split off and an alkene is formed:
CH 3 -CH 2 -CHCl-CH 3 + KOH alcohol. CH 3 -CH \u003d CH-CH 3 + KCl + H 2 O
3. Dehydration
Heating alcohols with concentrated sulfuric or phosphoric acid leads to the elimination of water and the formation of an alkene.
Elimination reactions of unsymmetrical haloalkanes and alcohols often proceed according to Zaitsev's rule: The hydrogen atom is predominantly split off from that of the C atoms, which is associated with the smallest number atoms H.
Zaitsev's rule, like Markovnikov's rule, can be explained by comparing the stability of the intermediate particles that are formed in the reaction.
Ethylene, propene and butenes are the starting materials for petrochemical synthesis, primarily for the production of plastics.
When chlorine is added to alkenes, chlorine derivatives are obtained.
CH 2 \u003d CH-CH 3 +Cl 2 → CH 2 Cl- CHCl- CH 3 (1,2-dichloropropane)
But back in 1884, the Russian scientist Lvov M.D. (Fig. 2) carried out the propene chlorination reaction under more severe conditions, at t = 400 0 C. As a result, the product was not the addition of chlorine, but a substitution.
CH 2 \u003d CH-CH 3 +Cl 2 CH 2 \u003d CH-CH 2Cl + HCl
Rice. 2. Russian scientist M.D. Lviv
The interaction of the same substances under different conditions lead to different results. This reaction is widely used to obtain glycerol. Sometimes ethylene is used in vegetable stores to speed up the ripening of fruits.
Summing up the lesson
In this lesson, you covered the topic “Alkenes. Chemical properties - 2. Preparation and use of alkenes. During the lesson, you were able to deepen your knowledge about alkenes, learned about the chemical properties of alkenes, as well as about the features of obtaining and using alkenes.
Bibliography
1. Rudzitis G.E. Chemistry. Basics general chemistry. Grade 10: textbook for educational institutions: a basic level of/ G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.
2. Chemistry. Grade 10. Profile level: studies. for general education institutions / V.V. Eremin, N.E. Kuzmenko, V.V. Lunin and others - M.: Drofa, 2008. - 463 p.
3. Chemistry. Grade 11. Profile level: textbook. for general education institutions / V.V. Eremin, N.E. Kuzmenko, V.V. Lunin and others - M.: Drofa, 2010. - 462 p.
4. Khomchenko G.P., Khomchenko I.G. Collection of problems in chemistry for those entering the universities. - 4th ed. - M.: RIA "New Wave": Publisher Umerenkov, 2012. - 278 p.
Homework
1. Nos. 12, 13 (p. 39) Rudzitis G.E., Feldman F.G. Chemistry: Organic Chemistry. Grade 10: textbook for educational institutions: basic level / G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.
2. What is a qualitative reaction for ethylene and its homologues?
3. Can not addition, but substitution occur during propene chlorination? What is it connected with?
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 a small number of carbon atoms in the cycle (three or four atoms) also have an unsaturated character. 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 homologous series alkenes - gases, substances of the composition C5H10-C16H32 - liquids, higher alkenes - 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., the atom at which it is located more atoms hydrogen, and halogen - to less hydrogenated.
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 form.
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. Install all possible structural formulas original 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 other 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.
1. Oxidation of alkenes.
1.1 Combustion.
In excess of air or oxygen, all alkenes burn to carbon dioxide and water:
CH 3 - CH \u003d CH 2 + 4.5 O 2 3 CO 2 + 3 H 2 O
The combustion of alkenes is not used in internal combustion engines, since during storage of gasoline they are resinous and the resins clog the fuel equipment (injector).
The possibility of combustion of alkenes should be taken into account during transportation and storage at chemical plants.
1.2 Oxidation of alkenes with a calculated amount of atmospheric oxygen in the presence of silver.
Epoxy compounds are used to create adhesives for various purposes.
1.3 Oxidation of alkenes with a 1% solution of potassium permanganate in water - a qualitative reaction for alkenes by E.E. Wagner.
The reaction was first described by E.E. Wagner in the Journal of the Russian Physical and Chemical Society in 1886. The oxidation of alkenes or other unsaturated compounds occurs at room temperature and is accompanied by the disappearance of the violet color of the permanganate ion and the precipitation of a brown precipitate of manganese dioxide .. Regardless of the structure of the alkene (but not alkadiene, for example), the coefficients in the Wagner reaction are always the same (324-322). Below are examples of the oxidation of specific alkenes and shows the half-reactions and the total OVR in ionic and molecular form:
:
As can be seen in the Wagner reaction, the final organic products are dihydric alcohols. They are also called glycols. For example, 1,2-ethanediol is called ethylene glycol.
1.4 Oxidation of alkenes by strong oxidizing agents in the liquid phase in an acidic medium.
Depending on the structure of alkenes, various products are obtained during oxidation under these conditions, namely CO 2 , carboxylic acids and ketones. The scheme for the oxidation of alkenes of various structures is shown below.
To illustrate the use of this scheme, an example of the oxidation of 2-methylpentene with potassium permanganate in a sulfuric acid medium is given. According to the oxidation scheme, the end organic products for a given alkene are a carboxylic acid and a ketone:
The half reactions for this process are:
Another example is the oxidation of 2-ethylbutene-1 with potassium dichromate in sulfuric acid. In accordance with the rules of the oxidation scheme, in this case, ketone and carbon dioxide are obtained:
Third example: oxidation cis- 3,4,5-trimethylheptene-3 sodium bismuthate in dilute nitric acid. In accordance with the rules of the oxidation scheme, two ketones are obtained in this case:
1.5 Ozonolysis
Ozonolysis is a two-stage process, at the first stage of which ozone is added to the alkene and ozonide is formed, and at the second stage this ozonide is either slowly destroyed by water with the formation of hydrogen peroxide, aldehydes and ketones, or is quickly reduced by zinc dust with the formation of zinc oxide and the same aldehydes and ketones.
Below is an example of ozonolysis of 3-methyl- cis-heptene-3.
Ozonolysis produces two different ketones:
Formaldehyde (methanal) can be obtained as one of the oxidation products if a terminal alkene is taken into the reaction:
2. Addition reactions at the double bond of alkenes.
Both non-polar and polar molecules can attach to the double bond of alkenes.
Non-polar: H 2, Cl 2, Br 2, J 2. Fluorine F 2 does not add to alkenes, but burns them to CF 4 and HF:
CH 3 - CH \u003d CH - CH 3 + 12 F 2 → 4 CF 4 + 8 HF
2.1 Addition of hydrogen.
Attachment occurs only in the presence of a catalyst. Most often in industry, palladium or platinum is used, which are easily regenerated by calcination. Nickel is practically not used, since under the conditions of ordinary calcination it turns into an oxide, which is not economically profitable to restore.
CH 3 - CH \u003d CH 2 + H 2 CH 3 - CH 2 - CH 3
2.2 Accession of chlorine.
Goes to two atoms with a double bond. Dichloro derivatives of alkanes are obtained. The reaction can proceed as aqueous solution at room or lower temperatures, and in organic solvents, for example, carbon tetrachloride CCl 4 or dichloroethane C 2 H 4 Cl 2:
2.3 Addition of bromine.
It passes similarly both with bromine water at temperatures up to 0 0 С, and in the same organic solvents. In the latter case, the reaction can also take place at temperatures up to -25 0 C, that is, in the cold.
The reaction with bromine is qualitative for the presence of alkenes in gaseous and liquid mixtures, as it is accompanied by the discoloration of orange bromine solutions:
2.4 Reaction with iodine.
The reaction is widely used to determine the total unsaturation of fats that are derived from unsaturated fatty acids containing double bonds, as in alkenes:
The mass of iodine in grams, which went to complete iodization of 100 g of fat, is called the iodine number. The higher it is, the more useful fat is for a person, since the body synthesizes hormones only from polyunsaturated fatty acids. Examples of iodine numbers: palm oil - 12, mutton fat - 35, olive oil - 80, soybean oil - 150, herring fat - 200, seal fat - 280
2.5 Reactions with polar molecules.
to polar molecules type H-A include the following: H-F, H-Cl, H-Br, H-J, H-OH,
H-O-R (alcohols) and carboxylic acids -
The addition of hydrogen chloride and other polar molecules occurs along, that is, the hydrogen atom from the polar molecule preferentially attaches to the more hydrogenated carbon atom at the double bond, and the remainder A to the other atom at the double bond.
Thus, the reaction is not selective.
With an increase in the difference in hydrogenation, the selectivity in the reaction increases. Indeed, the difference in hydrogenation at atoms 1 and 2 in propene is one hydrogen atom, and 85% of chlorine goes to the less hydrogenated carbon atom, while in
In 2-methylpropene, the difference in hydrogenation at atoms 1 and 2 is already two hydrogen atoms, and more than 98% of chlorine goes to atom 2:
The addition of HF, HBr, HJ proceeds similarly:
Otherwise, HBr is added (and only HBr, not HCl, HF and HI) in the presence of hydrogen peroxide H 2 O 2:
This reaction is called the Karasz HBr addition. The selectivity in it practically changes to the inverse in comparison with that for the addition of HBr in the absence of hydrogen peroxide (according to the Markovnikov rule).
The reaction of alkenes with chlorine at 500 ° C is very interesting. Under these conditions, the reaction of adding chlorine to the double bond is reversible, moreover, the equilibrium in it is strongly shifted towards the starting materials. On the contrary, it is much slower, but irreversible reaction radical substitution to the allyl position, that is, it goes to the end next to the double bond:
This reaction is of great practical importance. For example, one of the stages of the large-scale industrial synthesis of glycerol is the chlorination of propene to
3-chloropropene-1.
When water is added to alkenes in the presence of catalytic amounts of sulfuric or orthophosphoric acids, alcohols are obtained. The addition follows the Markovnikov rule:
When alcohols are added to alkenes, ethers are obtained:
These isomeric esters may be referred to as both alkoxy derivatives of alkanes and ethers. In the first case, the longest chain of carbon atoms is selected and numbered from the side that is closer to the alkoxy substituent. For example, for broadcast I chain numbered in brackets. And the corresponding name is also in brackets. For isomer II, on the contrary, the numbers in brackets number the chain starting from the carbon atom bonded to the oxygen atom. The name in this case is formed as follows: first, the simpler radical associated with the oxygen atom is called, then the more complex one, and finally, “new ether” is added.
When carboxylic acids are added to alkenes, esters are obtained:
The names of esters are formed as follows: first they name the hydrocarbon radical associated with oxygen. In this case, the carbon atom in contact with oxygen is taken as atom number 1. From this atom, the longest available chain is numbered. Groups of atoms that are not included in the main chain are considered substituents and are listed according to the usual rules. Then the "new ester of such and such an acid" is added.
