Unsaturated hydrocarbons include hydrocarbons containing multiple bonds between carbon atoms in molecules. Unlimited are alkenes, alkynes, alkadienes (polyenes). Cyclic hydrocarbons containing a double bond in the cycle also have an unsaturated character ( cycloalkenes), as well as cycloalkanes with a small number of 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.

The structure of alkenes

Acyclic hydrocarbons containing in the molecule, in addition to single bonds, one double bond between carbon atoms and corresponding to the general formula СnН2n. Its second name olefins- alkenes were obtained by analogy with unsaturated fatty acids (oleic, linoleic), the remains of which are part of liquid fats - oils.
Carbon atoms between which there is a double bond 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 a σ-bond, and due to unhybridized p-orbitals
neighboring carbon atoms, a second, π-bond is formed. Thus, a double bond consists of one σ- and one π-bond. The hybrid orbitals of the atoms that form a double bond are in the same plane, and the orbitals that form a π bond are located perpendicular to the plane of the molecule. A double bond (0.132 im) is shorter than a single bond, and its energy is greater, since it is more durable. However, the presence of a mobile, easily polarizable π-bond leads to the fact that alkenes are chemically more active than alkanes and are able to enter into addition reactions.

The structure of ethylene

Double bond formation in alkenes

Homologous series of ethene

Unbranched alkenes form the homologous series of ethene ( ethylene): C 2 H 4 - ethene, C 3 H 6 - propene, C 4 H 8 - butene, C 5 H 10 - pentene, C 6 H 12 - hexene, C 7 H 14 - heptene, etc.

Isomerism of alkenes

Alkenes are characterized by structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkene, which is characterized by structural isomers, is butene:

A special type of structural isomerism is the double bond position isomerism:

Alkenes are isomeric to cycloalkanes (interclass isomerism), for example:

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 and transisomerism.

Cis isomers differ from trans isomers the spatial arrangement of fragments of the molecule (in this case, methyl groups) relative to the plane of the π-bond, and, consequently, the properties.

Alkene nomenclature

1. Selecting the main circuit. 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:

If the position of the double bond cannot determine the beginning of the numbering of atoms in the chain, then it determines the position of the substituents in the same way as for saturated hydrocarbons.

3. Name formation. At the end of the name indicate the number of the carbon atom at which the double bond begins, and the suffix -en, denoting that the compound belongs to the class of alkenes. For example:

Physical properties of alkenes

The first three representatives of the homologous series of alkenes are gases; substances of the composition C5H10 - C16H32 - liquids; higher alkenes are solids.
The boiling and melting points naturally increase with an increase in the molecular weight of the compounds.

Chemical properties of alkenes

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 electrophilic addition.
1. Hydrogenation of alkenes. Alkenes are able to add hydrogen in the presence of hydrogenation catalysts, metals - platinum, palladium, nickel:

This reaction proceeds at atmospheric and elevated pressure and does not require high temperature, since 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 (CC14) 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.
3. Hydrohalogenation (addition of hydrogen halide).

This reaction is subject to
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.

Note that a primary alcohol (with a hydroxo group at the primary carbon) is formed only when ethene is hydrated. When propene or other alkenes are hydrated, secondary alcohols.

This reaction also proceeds in accordance with Markovnikov's rule - the hydrogen cation is added to the more hydrogenated carbon atom, and the hydroxo group 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.
1. Combustion. Like any organic compounds, alkenes burn in oxygen to form CO2 and H2O:

2. Oxidation in solutions. Unlike alkanes, alkenes are easily oxidized by the action of potassium permanganate solutions. In neutral or 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:

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 the pentene C 5 H 10 and up to the alkene with the formula C 17 H 34, they are liquids, and then there are 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

The first representative of the homologous series of 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 atoms of other chemical elements to the free valences of the carbon. 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 organic chemicals.

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 in the composition of their molecules can participate. 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 the school course of organic chemistry, the process of dehydration of ethyl alcohol is used with the help of water-removing 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.

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 of 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 on a large number of experimental data, a mechanism was proposed for this reaction, which 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): under normal conditions, the hydrogen of the hydrogen halide is attached at the double bond site 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 an 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 of the σ-bonds C-H is observed, associated with a shift in the 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 an aqueous solution of KMnO 4 acts on ethylene (under normal conditions, in a neutral or alkaline medium - the Wagner reaction), a dihydric alcohol - ethylene glycol is formed:

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. It plays an 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.

Lesson topic: Alkenes. Obtaining, chemical properties and application of alkenes.

Goals and objectives of the lesson:

• consider the specific chemical properties of ethylene and the general properties of alkenes;
• deepen and concretize the concepts of ?-bonds, the mechanisms of chemical reactions;
• give initial ideas about polymerization reactions and the structure of polymers;
• analyze laboratory and general industrial methods for obtaining alkenes;
• continue to develop the ability to work with a textbook.

Equipment: device for obtaining gases, KMnO 4 solution, ethyl alcohol, concentrated sulfuric acid, matches, spirit lamp, sand, tables "Structure of the molecule of ethylene", "Basic chemical properties of alkenes", demonstration samples "Polymers".

DURING THE CLASSES

I. Organizational moment

We continue to study the homologous series of alkenes. Today we have to consider the methods of obtaining, chemical properties and applications of alkenes. We must characterize the chemical properties due to the double bond, get an initial understanding of polymerization reactions, consider laboratory and industrial methods for obtaining alkenes.

II. Activation of students' knowledge

1. What hydrocarbons are called alkenes?

1. What are the features of their structure?

1. In what hybrid state are the carbon atoms that form a double bond in an alkene molecule?

Bottom line: alkenes differ from alkanes in the presence of one double bond in the molecules, which determines the features of the chemical properties of alkenes, methods for their preparation and use.

III. Learning new material

1. Methods for obtaining alkenes

Compose reaction equations confirming the methods for obtaining alkenes

– cracking of alkanes C 8 H 18 ––> C 4 H 8 + C 4 H 10 ; (thermal cracking at 400-700 o C)
octane butene butane
– dehydrogenation of alkanes C 4 H 10 ––> C 4 H 8 + H 2; (t, Ni)
butane butene hydrogen
– dehydrohalogenation of haloalkanes C 4 H 9 Cl + KOH ––> C 4 H 8 + KCl + H 2 O;
chlorobutane hydroxide butene chloride water
potassium potassium
– dehydrohalogenation of dihaloalkanes
- dehydration of alcohols C 2 H 5 OH -–> C 2 H 4 + H 2 O (when heated in the presence of concentrated sulfuric acid)
Remember! In the reactions of dehydrogenation, dehydration, dehydrohalogenation and dehalogenation, it must be remembered that hydrogen is predominantly detached from less hydrogenated carbon atoms (Zaitsev's rule, 1875)

2. Chemical properties of alkenes

The nature of the carbon - carbon bond determines the type of chemical reactions that organic substances enter into. The presence of a double carbon-carbon bond in the molecules of ethylene hydrocarbons determines the following features of these compounds:
- the presence of a double bond makes it possible to classify alkenes as unsaturated compounds. Their transformation into saturated ones is possible only as a result of addition reactions, which is the main feature of the chemical behavior of olefins;
- a double bond is a significant concentration of electron density, so the addition reactions are electrophilic in nature;
- a double bond consists of one - and one -bond, which is quite easily polarized.

Reaction equations characterizing the chemical properties of alkenes

a) Addition reactions

Remember! Substitution reactions are characteristic of alkanes and higher cycloalkanes having only single bonds, addition reactions are characteristic of alkenes, dienes and alkynes having double and triple bonds.

Remember! The following break-link mechanisms are possible:

a) if alkenes and the reagent are non-polar compounds, then the -bond breaks with the formation of a free radical:

H 2 C \u003d CH 2 + H: H -–> + +

b) if the alkene and the reagent are polar compounds, then breaking the bond leads to the formation of ions:

c) when connecting at the site of the break-bond of reagents containing hydrogen atoms in the molecule, hydrogen always attaches to a more hydrogenated carbon atom (Morkovnikov's rule, 1869).

- polymerization reaction nCH 2 = CH 2 ––> n – CH 2 – CH 2 ––> (– CH 2 – CH 2 –) n
ethene polyethylene

b) oxidation reaction

Laboratory experience. Obtain ethylene and study its properties (instruction on student desks)

Instructions for obtaining ethylene and experiments with it

1. Place 2 ml of concentrated sulfuric acid, 1 ml of alcohol and a small amount of sand into a test tube.
2. Close the test tube with a stopper with a gas outlet tube and heat it in the flame of an alcohol lamp.
3. Pass the escaping gas through a solution of potassium permanganate. Note the change in color of the solution.
4. Ignite the gas at the end of the gas tube. Pay attention to the color of the flame.

- Alkenes burn with a luminous flame. (Why?)

C 2 H 4 + 3O 2 -–> 2CO 2 + 2H 2 O (with complete oxidation, the reaction products are carbon dioxide and water)

Qualitative reaction: "mild oxidation (in aqueous solution)"

- alkenes decolorize a solution of potassium permanganate (Wagner reaction)

Under more severe conditions in an acidic environment, the reaction products can be carboxylic acids, for example (in the presence of acids):

CH 3 - CH \u003d CH 2 + 4 [O] -–> CH 3 COOH + HCOOH

– catalytic oxidation

Remember the main thing!

1. Unsaturated hydrocarbons actively enter into addition reactions.
2. The reactivity of alkenes is due to the fact that - the bond is easily broken under the action of reagents.
3. As a result of the addition, the transition of carbon atoms from sp 2 - to sp 3 - hybrid state occurs. The reaction product has a limiting character.
4. When ethylene, propylene and other alkenes are heated under pressure or in the presence of a catalyst, their individual molecules are combined into long chains - polymers. Polymers (polyethylene, polypropylene) are of great practical importance.

3. Use of alkenes(student's message according to the following plan).

1 - obtaining fuel with a high octane number;
2 - plastics;
3 - explosives;
4 - antifreeze;
5 - solvents;
6 - to accelerate the ripening of fruits;
7 - obtaining acetaldehyde;
8 - synthetic rubber.

III. Consolidation of the studied material

Homework:§§ 15, 16, ex. 1, 2, 3 p. 90, ex. 4, 5 p. 95.

For alkenes, the reactions occurring due to the opening of a less strong π-bond are most characteristic. In this case, the π-bond (in the starting alkene) is converted into a σ-bond in the reaction product. The initial unsaturated compound is converted into a saturated one without the formation of other products, i.e. going onaddition reaction.

What is the mechanism of addition reactions with alkenes?

1. Due to the electrons of the π-bond in the molecules of alkenes, there is a region of increased electron density (a cloud of π-electrons above and below the plane of the molecule):

Therefore, the double bond tends to be attacked by an electrophilic (electron-deficient) reagent. In this case, a heterolytic cleavage of the π-bond will occur and the reaction will go along ionic mechanism as an electrophilic addition.

The mechanism of electrophilic addition is indicated by the symbol Ad E

(according to the first letters of English terms: Ad - addition [attachment],

E - electrophile [electrophile]).

2. On the other hand, the carbon-carbon π-bond, being non-polar, can be broken homolytically, and then the reaction will go according to radical mechanism.

The mechanism of radical addition is denoted by the symbol Ad R

(R - radical - radical).

The addition mechanism depends on the reaction conditions.

In addition, alkenes are characterized by reactions isomerization And oxidation(including reaction burning characteristic of all hydrocarbons).

Addition reactions to alkenes

Alkenes undergo a variety of addition reactions.

1. Hydrogenation (hydrogen addition)

Alkenes interact with hydrogen when heated and at elevated pressure in the presence of catalysts (Pt, Pd, Ni, etc.) to form alkanes:

Hydrogenation of alkenes - reaction, reverse dehydrogenation of alkanes. According to principle of Le Chatelier, hydrogenation is favored by increased pressure, tk. this reaction is accompanied by a decrease in the volume of the system.

The addition of hydrogen to carbon atoms in alkenes leads to a decrease in the degree of their oxidation:

Therefore, the hydrogenation of alkenes is referred to as reduction reactions. This reaction is used in industry to produce high-octane fuel.

2. Halogenation (addition of halogens)

The addition of halogens to the C=C double bond occurs easily under normal conditions (at room temperature, without a catalyst). For example, the rapid discoloration of the red-brown color of a solution of bromine in water (bromine water) serves as a qualitative reaction to the presence of a double bond:

So, in the reaction of HCl with propylene, from two possible structural isomers of 1-chloropropane and 2-chloropropane, the latter is formed:

This pattern was initially established empirically. In modern organic chemistry, a theoretical substantiation of Markovnikov's rule is given on the basis of the position on the influence of the electronic structure of molecules on their reactivity.

It should be noted that Markovnikov's rule in its classical formulation is observed only for electrophilic reactions of the alkenes themselves. In the case of some alkene derivatives or when the mechanism of the reaction is changed, against the rule Markovnikov.

4. Hydration(water connection)

Hydration occurs in the presence of mineral acids by the mechanism of electrophilic addition:

In the reactions of unsymmetrical alkenes, Markovnikov's rule is observed.

1. Polymerization- the reaction of formation of a high molecular weight compound (polymer) by sequential addition of molecules of a low molecular weight substance (monomer) according to the scheme:

n M M n

Number n in the polymer formula ( M n ) is called the degree of polymerization. The polymerization reactions of alkenes are due to the addition of multiple bonds:

2. Dimerizationalkenes - the formation of a dimer (doubled molecule) as a result of an addition reaction. In the presence of a mineral acid (proton donor H + ) a proton is added to the double bond of the alkene molecule. This forms a carbocation:

The "dimeric carbocation" is stabilized by ejection of a proton, which leads to alkene dimerization products - a mixture of isomeric diisobutylenes (2,4,4-trimethypentene-2 ​​and 2,4,4-trimethylpentene-1):

This process takes place during the treatment of isobutylene (2-methypropene) with 60% sulfuric acid at a temperature of 70°C. The resulting mixture of diisobutylenes is hydrogenated to produce "isooctane" (2,2,4-trimethylpentane), which is used to improve the anti-knock capability of gasoline ("isooctane" is a 100 octane motor fuel standard).