Lesson topic: Alkenes. Preparation, chemical properties and applications of alkenes.

Goals and objectives of the lesson:

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

Equipment: device for producing gases, KMnO 4 solution, ethyl alcohol, concentrated sulfuric acid, matches, alcohol lamp, sand, tables “Structure of the ethylene molecule”, “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 look at the methods of preparation, chemical properties and applications of alkenes. We must characterize the chemical properties caused by the double bond, gain an initial understanding of polymerization reactions, and consider laboratory and industrial methods for producing alkenes.

II. Activating 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 their molecules, which determines the peculiarities of the chemical properties of alkenes, methods of their preparation and use.

III. Learning new material

1. Methods for producing alkenes

Draw up reaction equations confirming methods for producing 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 preferentially abstracted 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 in which organic matter. 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 allows alkenes to be classified 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;
– the double bond represents a significant concentration of electron density, so 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, which have only single bonds; addition reactions are characteristic of alkenes, dienes and alkynes, which have double and triple bonds.

Remember! The following mechanisms for breaking the -bond are possible:

a) if alkenes and the reagent are non-polar compounds, then the -bond is broken to form a free radical:

H 2 C = CH 2 + H: H ––> + +

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

c) when reagents containing hydrogen atoms in the molecule join at the site of a broken -bond, 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 (instructions 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 released gas through a solution with potassium permanganate. Note the change in color of the solution.
4. Light the gas at the end of the gas outlet 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 = CH 2 + 4 [O] ––> CH 3 COOH + HCOOH

– catalytic oxidation

Remember the main thing!

1. Unsaturated hydrocarbons actively participate in addition reactions.
2. The reactivity of alkenes is due to the fact that the bond is easily broken under the influence of reagents.
3. As a result of addition, the transition of carbon atoms from sp 2 to sp 3 - a 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. Application of alkenes(student message according to the following plan).

1 – production of fuel with a high octane number;
2 – plastics;
3 – explosives;
4 – antifreeze;
5 – solvents;
6 – to accelerate fruit ripening;
7 – production of acetaldehyde;
8 – synthetic rubber.

III. Reinforcing the material learned

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

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 “-ane” 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 simple structure are often named by replacing the suffix -ane in alkanes with -ylene: ethane - ethylene, propane - propylene, etc.

According to systematic nomenclature, the names of ethylene hydrocarbons are made by replacing the suffix -ane in the corresponding alkanes with the suffix -ene (alkane - alkene, ethane - ethene, propane - propene, etc.). The choice of the main chain and the naming order are the same as for alkanes. However, the chain must necessarily include a double bond. The numbering of the chain begins from the end to which this connection is located closest. For example:

Sometimes rational names are also used. In this case, all alkene hydrocarbons are considered as substituted ethylene:

Unsaturated (alkene) radicals are called by trivial names or by systematic nomenclature:

H 2 C = CH - - vinyl (ethenyl)

H 2 C = CH - CH 2 - -allyl (propenyl-2)

Isomerism

Alkenes are characterized by two types of structural isomerism. In addition to isomerism associated with the structure of the carbon skeleton (as in alkanes), isomerism appears depending on the position of the double bond in the chain. This leads to an increase in the number of isomers in the series of alkenes.

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 = CH 2 ethylene (ethene)

H 2 C = CH - CH 3 propylene (propene)

Multiple bond position isomerism

H 2 C = CH - CH 2 - CH 3 butene-1

H 3 C - CH = CH - CH 3 butene-2

Geometric isomerism - cis-, trans-isomerism.

This isomerism is typical for 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 on the opposite side - trans-isomers:

Cis- And trance- isomers differ not only in their spatial structure, but also in many physical and chemical properties. Trance- isomers are more stable than cis- isomers.

Preparation of alkenes

Alkenes are rare in nature. Typically, gaseous alkenes (ethylene, propylene, butylenes) are isolated from oil refining 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 = CH - CH 2 - CH 3 + H 2 (butene-1)

H 3 C - CH 2 - CH 2 - CH 3 → H 3 C - CH = CH - CH 3 + H 2 (butene-2)

Among the laboratory methods of production, the following can be noted:

1. Elimination of hydrogen halide from alkyl halides under the action of an alcoholic alkali solution on them:

2. Hydrogenation of acetylene in the presence of a catalyst (Pd):

H-C ≡ C-H + H 2 → H 2 C = CH 2

3. Dehydration of alcohols (elimination of water).
Acids (sulfuric 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 molecular weight increases, melting and boiling points increase. Alkenes with a normal structure boil at a higher temperature than their isomers, which have an iso structure. Boiling points cis-isomers higher than trance-isomers, and the melting points are the opposite.

Alkenes are poorly soluble in water (however, better than the corresponding alkanes), but well soluble in organic solvents. Ethylene and propylene burn with a smoky flame.

Physical properties of some alkenes

Name		t pl,°С	t kip, °C
Ethylene (ethene)
Propylene (propene)
Butylene (butene-1)
Cis-butene-2
Trans-butene-2
Isobutylene (2-methylpropene)
Amylene (pentene-1)
Hexylene (hexene-1)
Heptylene (heptene-1)
Octylene (octene-1)
Nonylene (nonene-1)
Decylene (decene-1)

Alkenes are slightly polar, 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, being the least strong and more accessible, is broken by the action of the reagent, and the released valences of the carbon atoms are spent on attaching the atoms that make up the reagent molecule. This can be represented as a diagram:

Thus, during addition reactions, the double bond is broken as if by half (with the σ bond remaining).

In addition to addition, alkenes also undergo oxidation and polymerization reactions.

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), transform into saturated hydrocarbons - alkanes:

H 2 C = CH 2 + H 2 → H 3 C - CH 3 (ethane)

2. Halogenation (addition of halogens). Halogens easily add at the site of double bond cleavage to form dihalogen derivatives:

H 2 C = 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 reacts with alkenes, as well as with alkanes, explosively.

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, including several sequential stages. At the first stage, polarization of the halogen molecule occurs under the action of π-bond electrons. The halogen atom, which acquires a certain fractional positive charge, forms an unstable intermediate with the electrons of the π bond, called a π complex or a charge transfer complex. It should be noted that in the π-complex the halogen does not form a directional bond with any specific carbon atom; in this complex the donor-acceptor interaction of the electron pair of the π bond as the donor and the halogen as the acceptor is simply realized.

The π-complex then transforms into a cyclic bromonium ion. During the formation of this cyclic cation, heterolytic cleavage of the Br-Br bond occurs and an empty R-the sp 2 orbital of the hybridized carbon atom overlaps with R-orbital of the “lone pair” of electrons of the halogen atom, forming a cyclic bromonium ion.

In 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 stage can formally be considered as a nucleophilic substitution of SN 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 decolorization of a solution of bromine in CCl4 serves as one of the simplest tests for unsaturation, since alkenes, alkynes, and dienes react quickly 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 saturated hydrocarbons, it remains).

3. Hydrohalogenation (addition of hydrogen halides). Alkenes easily add hydrogen halides:

H 2 C = 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 added at the site of the double bond to the most hydrogenated carbon atom, and the halogen to the less hydrogenated one:

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 bonded directly to the double bond, the electron density shifts towards this bond (to the outermost carbon atom).

As a result of this displacement, the p-bond is polarized and partial charges arise on the carbon atoms. It is easy to imagine that a positively charged hydrogen ion (proton) will attach to a carbon atom (electrophilic addition) that has a partial negative charge, and a bromine anion will attach to a carbon that has a partial positive charge.

This addition 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 there is some polarization of C-H σ bonds associated with a shift in 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 outermost 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 Markovnikov’s rule.

For the same reasons, Markovnikov’s rule is observed when adding not only hydrogen halides, but also other electrophilic reagents (H 2 O, H 2 SO 4, HOCl, ICl, etc.) to unsymmetrical alkenes.

4. Hydration (addition of water). In the presence of catalysts, water adds to alkenes to form alcohols. For example:

H 3 C - CH = CH 2 + H - OH → H 3 C - CHOH - CH 3 (isopropyl alcohol)

Oxidation reactions

Alkenes are oxidized more easily than alkanes. The products formed during the oxidation of alkenes and their structure depend on the structure of the alkenes and on the conditions of this reaction.

1. Combustion

H 2 C = CH 2 + 3O 2 → 2СO 2 + 2H 2 O

2. Incomplete catalytic oxidation

3. Oxidation at ordinary temperature. When ethylene is exposed to an aqueous solution of KMnO4 (under normal conditions, in neutral or alkaline environment- Wagner reaction) the formation of diatomic alcohol - ethylene glycol occurs:

3H 2 C = CH 2 + 2KMnO 4 + 4H 2 O → 3HOCH 2 - CH 2 OH (ethylene glycol) + 2MnO 2 + KOH

This reaction is qualitative: the purple color of the potassium permanganate solution changes when an unsaturated compound is added to it.

Under more severe conditions (oxidation of KMnO4 in the presence of sulfuric acid or a chromium mixture), the double bond in the alkene breaks to form oxygen-containing products:

H 3 C - CH = CH - CH 3 + 2O 2 → 2H 3 C - COOH (acetic acid)

Isomerization reaction

When heated or in the presence of catalysts, alkenes are capable of isomerization - the movement of the double bond occurs or the establishment of isostructure.

Polymerization reactions

By breaking π bonds, alkene molecules can connect with each other, forming long chain molecules.

Occurrence in nature and physiological role of alkenes

Acyclic alkenes are practically never found in nature. 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 natural alkenes is muskalur ( cis- tricosen-9) is a sexual attractant of the female house fly (Musca domestica).

Lower alkenes in high concentrations have a narcotic effect. 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 belongs 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; general world production ethylene in 2008 amounted to 113 million tons and continues to grow by 2-3% per year.

Ethylene is the leading product of basic 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 white (thin sheets transparent, colorless). Chemical and frost-resistant, insulator, not sensitive to shock (shock absorber), softens when heated (80-120°C), hardens when cooled, adhesion (adhesion of surfaces of dissimilar solids and/or liquid bodies) is extremely low. Sometimes in the popular consciousness it is identified with cellophane - a similar material of plant origin.

Propylene - causes anesthesia (more powerful 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.

Plastic film (especially packaging film, such as bubble wrap or tape).

Containers (bottles, jars, boxes, canisters, garden watering cans, seedling pots.

Polymer pipes for sewerage, drainage, water and gas supply.

Electrical insulating material.

Polyethylene powder is used as hot melt adhesive.

Butene-2 ​​- causes anesthesia and has an irritating effect.

In organic chemistry you can find hydrocarbon substances with different quantities carbon in the chain and C=C bond. They are homologues and are called alkenes. Due to their structure, they are chemically more reactive than alkanes. But what kind of reactions are typical for them? Let's consider their distribution in nature, different ways receipt and application.

What are they?

Alkenes, which are also called olefins (oily), get their name from ethene chloride, a derivative of the first member of this group. All alkenes have at least one C=C double bond. C n H 2n is the formula of all olefins, and the name is formed from an alkane with the same number of carbons in the molecule, only the suffix -ane changes to -ene. Arabic numeral at the end of the name, a hyphen indicates the number of the carbon from which the double bond begins. Let's look at the main alkenes, the table will help you remember them:

If the molecules have a simple, unbranched structure, then the suffix -ylene is added, this is also reflected in the table.

Where can you find them?

Since the reactivity of alkenes is very high, their representatives are extremely rare in nature. The principle of life of an olefin molecule is “let’s be friends.” There are no other substances around - no problem, we will be friends with each other, forming polymers.

But they exist, and a small number of representatives are included in the accompanying petroleum gas, and higher ones are in the oil produced in Canada.

The very first representative of alkenes, ethene, is a hormone that stimulates fruit ripening, so it is synthesized in small quantities by representatives of the flora. There is an alkene, cis-9-tricosene, which plays the role of a sexual attractant in female house flies. It is also called muscalur. (An attractant is a substance of natural or synthetic origin that causes attraction to the source of odor in another organism). From a chemical point of view, this alkene looks like this:

Since all alkenes are very valuable raw materials, the methods for producing them artificially are very diverse. Let's look at the most common ones.

What if you need a lot?

In industry, the class of alkenes is mainly obtained by cracking, i.e. cleavage of a molecule under the influence high temperatures, higher alkanes. The reaction requires heating in the range of 400 to 700 °C. The alkane splits the way it wants, forming alkenes, the methods of obtaining which we are considering, with a large number of molecular structure options:

C 7 H 16 -> CH 3 -CH=CH 2 + C 4 H 10.

Another common method is called dehydrogenation, in which a hydrogen molecule is separated from a representative of an alkane series in the presence of a catalyst.

In laboratory conditions, alkenes and methods of preparation differ; they are based on elimination reactions (elimination of a group of atoms without their substitution). The most commonly eliminated water atoms from alcohols are halogens, hydrogen or hydrogen halides. The most common way to obtain alkenes is from alcohols in the presence of an acid as a catalyst. It is possible to use other catalysts

All elimination reactions are subject to Zaitsev’s rule, which states:

A hydrogen atom is split off from the carbon adjacent to the carbon bearing the -OH group, which has fewer hydrogens.

Having applied the rule, answer which reaction product will predominate? Later you will find out if you answered correctly.

Chemical properties

Alkenes react actively with substances, breaking their pi bond (another name for the C=C bond). After all, it is not as strong as a single bond (sigma bond). A hydrocarbon is converted from unsaturated to saturated without forming other substances after the reaction (addition).

• addition of hydrogen (hydrogenation). The presence of a catalyst and heating is necessary for its passage;
• addition of halogen molecules (halogenation). It is one of the qualitative reactions to the pi bond. After all, when alkenes react with bromine water, it turns from brown to transparent;
• reaction with hydrogen halides (hydrohalogenation);
• addition of water (hydration). The conditions for the reaction to occur are heating and the presence of a catalyst (acid);

Reactions of unsymmetrical olefins with hydrogen halides and water obey Markovnikov's rule. This means that hydrogen will attach itself to the carbon from the carbon-carbon double bond that already has more hydrogen atoms.

• combustion;
• incomplete oxidation catalytic. The product is cyclic oxides;
• Wagner reaction (oxidation with permanganate in a neutral environment). This alkene reaction is another qualitative C=C bond. As it flows, the pink solution of potassium permanganate becomes discolored. If the same reaction is carried out in a combined acidic environment, the products will be different (carboxylic acids, ketones, carbon dioxide);
• isomerization. All types are characteristic: cis- and trans-, double bond movement, cyclization, skeletal isomerization;
• Polymerization is the main property of olefins for industry.

Application in medicine

The reaction products of alkenes are of great practical importance. Many of them are used in medicine. Glycerin is obtained from propene. This polyhydric alcohol is an excellent solvent, and if it is used instead of water, the solutions will be more concentrated. IN medical purposes Alkaloids, thymol, iodine, bromine, etc. are dissolved in it. Glycerin is also used in the preparation of ointments, pastes and creams. It prevents them from drying out. Glycerin itself is an antiseptic.

When reacted with hydrogen chloride, derivatives are obtained that are used as local anesthesia when applied to the skin, as well as for short-term anesthesia during minor surgical interventions, using inhalation.

Alkadienes are alkenes with two double bonds in one molecule. Their main use is the production of synthetic rubber, from which various heating pads and syringes, probes and catheters, gloves, pacifiers and much more are then made, which are simply irreplaceable when caring for the sick.

Industrial Applications

Type of industry	What is used	How can they use
Agriculture	ethene	accelerates the ripening of vegetables and fruits, defoliation of plants, films for greenhouses
Varnish and colorful	ethene, butene, propene, etc.	for the production of solvents, ethers, solvents
Mechanical engineering	2-methylpropene, ethene	production of synthetic rubber, lubricating oils, antifreeze
Food industry	ethene	production of teflon, ethyl alcohol, acetic acid
Chemical industry	ethene, polypropylene	alcohols, polymers (polyvinyl chloride, polyethylene, polyvinyl acetate, polyisobtylene, acetaldehyde) are obtained
Mining	ethene etc.	explosives

More wide application found alkenes and their derivatives in industry. (Where and how are alkenes used, table above).

This is only a small part of the use of alkenes and their derivatives. Every year the demand for olefins only increases, which means that the need for their production also increases.

DEFINITION

Alkenes are called unsaturated hydrocarbons whose molecules contain one double bond. The structure of the alkene molecule using ethylene as an example is shown in Fig. 1.

Rice. 1. The structure of the ethylene molecule.

By physical properties alkenes differ little from alkanes with the same number of carbon atoms in the molecule. Lower homologs C 2 - C 4 under normal conditions are gases; C 5 - C 17 - liquids; higher homologues are solids. Alkenes are insoluble in water. Highly soluble in organic solvents.

Preparation of alkenes

In industry, alkenes are obtained during oil refining: cracking and dehydrogenation of alkanes. We divided laboratory methods for obtaining alkenes into two groups:

• Elimination reactions

– dehydration of alcohols

CH 3 -CH 2 -OH → CH 2 =CH 2 + H 2 O (H 2 SO 4 (conc), t 0 = 170).

— dehydrohalogenation of monohaloalkanes

CH 3 -CH(Br)-CH 2 -CH 3 + NaOH alcohol → CH 3 -CH=CH-CH 3 + NaBr + H 2 O (t 0).

— dehalogenation of dihaloalkanes

CH 3 -CH(Cl)-CH(Cl)-CH 2 -CH 3 + Zn(Mg) → CH 3 -CH=CH-CH 2 -CH 3 + ZnCl 2 (MgCl 2).

• Incomplete hydrogenation of alkynes

CH≡CH + H 2 →CH 2 =CH 2 (Pd, t 0).

Chemical properties of alkenes

Alkenes are highly reactive organic compounds. This is explained by their structure. The chemistry of alkenes is the chemistry of double bonds. Typical reactions for alkenes are electrophilic addition reactions.

Chemical transformations of alkenes proceed with splitting:

1) π-C-C bonds (addition, polymerization and oxidation)

- hydrogenation

CH 3 -CH=CH 2 + H 2 → CH 3 -CH 2 -CH 2 (kat = Pt).

- halogenation

CH 3 -CH 2 -CH=CH 2 + Br 2 → CH 3 -CH 2 -CH(Br)-CH 2 Br.

— hydrohalogenation (proceeds according to Markovnikov’s rule: a hydrogen atom attaches preferentially to a more hydrogenated carbon atom)

CH 3 -CH=CH 2 + H-Cl → CH 3 -CH(Cl)-CH 3 .

- hydration

CH 2 =CH 2 + H-OH → CH 3 -CH 2 -OH (H + , t 0).

- polymerization

nCH 2 =CH 2 → -[-CH 2 -CH 2 -]- n (kat, t 0).

- oxidation

CH 2 =CH 2 + 2KMnO 4 + 2KOH → HO-CH 2 -CH 2 -OH + 2K 2 MnO 4;

2CH 2 =CH 2 + O 2 → 2C 2 OH 4 (epoxide) (kat = Ag,t 0);

2CH 2 =CH 2 + O 2 → 2CH 3 -C(O)H (kat = PdCl 2, CuCl).

2) σ- and π-C-C bonds

CH 3 -CH=CH-CH 2 -CH 3 + 4[O] → CH 3 COOH + CH 3 CH 2 COOH (KMnO 4, H +, t 0).

3) bonds C sp 3 -H (in the allylic position)

CH 2 =CH 2 + Cl 2 → CH 2 =CH-Cl + HCl (t 0 =400).

4) Breaking all ties

C 2 H 4 + 2O 2 → 2CO 2 + 2H 2 O;

C n H 2n + 3n/2 O 2 → nCO 2 + nH 2 O.

Applications of alkenes

Alkenes have found application in various sectors of the national economy. Let's look at the example of individual representatives.

Ethylene is widely used in industrial organic synthesis to produce a variety of organic compounds, such as halogen derivatives, alcohols (ethanol, ethylene glycol), acetaldehyde, acetic acid, etc. Ethylene is consumed in large quantities for the production of polymers.

Propylene is used as a raw material for the production of some alcohols (for example, 2-propanol, glycerin), acetone, etc. Polypropylene is produced by polymerization of propylene.

Examples of problem solving

EXAMPLE 1

Exercise	During hydrolysis aqueous solution sodium hydroxide NaOH dichloride, obtained by adding 6.72 liters of chlorine to ethylene hydrocarbon, produced 22.8 g of dihydric alcohol. What is the formula of the alkene if it is known that the reactions proceed in quantitative yields (without losses)?
Solution	Let us write the equation for alkene chlorination in general view, as well as the reaction to produce dihydric alcohol: C n H 2 n + Cl 2 = C n H 2 n Cl 2 (1); C n H 2 n Cl 2 + 2NaOH = C n H 2 n (OH) 2 + 2HCl (2). Let's calculate the amount of chlorine: n(Cl 2) = V(Cl 2) / V m; n(Cl 2) = 6.72 / 22.4 = 0.3 mol, therefore, ethylene dichloride will also be 0.3 mol (equation 1), dihydric alcohol should also be 0.3 mol, and according to the conditions of the problem this is 22.8 g. This means its molar mass will be equal to: M(C n H 2 n (OH) 2) = m(C n H 2 n (OH) 2) / n(C n H 2 n (OH) 2); M(C n H 2 n (OH) 2) = 22.8 / 0.3 = 76 g/mol. Let's find the molar mass of the alkene: M(C n H 2 n) = 76 - (2×17) = 42 g/mol, which corresponds to the formula C 3 H 6 .
Answer	Alkene formulaC 3 H 6

EXAMPLE 2

Exercise	How many grams will be required to brominate 16.8 g of an alkene, if it is known that during the catalytic hydrogenation of the same amount of alkene, 6.72 liters of hydrogen were added? What is the composition and possible structure of the original hydrocarbon?
Solution	Let us write in general form the equations for the bromination and hydrogenation of an alkene: C n H 2 n + Br 2 = C n H 2 n Br 2 (1); C n H 2 n + H 2 = C n H 2 n +2 (2). Let's calculate the amount of hydrogen substance: n(H 2) = V(H 2) / V m; n(H 2) = 6.72 / 22.4 = 0.3 mol, therefore, the alkene will also be 0.3 mol (equation 2), and according to the conditions of the problem it is 16.8 g. This means its molar mass will be equal to: M(C n H 2n) = m(C n H 2n) / n(C n H 2n); M(C n H 2 n) = 16.8 / 0.3 = 56 g/mol, which corresponds to the formula C 4 H 8 . According to equation (1) n(C n H 2 n) : n(Br 2) = 1:1, i.e. n(Br 2) = n(C n H 2 n) = 0.3 mol. Let's find the mass of bromine: m(Br 2) = n(Br 2) × M(Br 2); M(Br 2) = 2×Ar(Br) = 2×80 = 160 g/mol; m(MnO 2) = 0.3 × 160 = 48 g. Let's create the structural formulas of the isomers: butene-1 (1), butene-2 ​​(2), 2-methylpropene (3), cyclobutane (4). CH 2 =CH-CH 2 -CH 3 (1); CH 3 -CH=CH-CH 3 (2); CH 2 =C(CH 3)-CH 3 (3);
Answer	The mass of bromine is 48 g

Let's find out what the alkene hydration reaction is. For this we will give brief description this class of hydrocarbons.

General formula

Alkenes are unsaturated organic compounds with the general formula SpH2n, the molecules of which have one double bond and also contain single (simple) bonds. The carbon atoms are in the sp2 hybrid state. Representatives of this class are called ethylene, since the ancestor this series is ethylene.

Features of the nomenclature

In order to understand the mechanism of alkene hydration, it is necessary to highlight the features of their names. According to systematic nomenclature, when naming an alkene, a certain algorithm of actions is used.

First, you need to determine the longest carbon chain that includes a double bond. The numbers indicate the location of hydrocarbon radicals, starting with the smallest in the Russian alphabet.

If there are several identical radicals in the molecule, the qualifying prefixes di-, tri-, and tetra are added to the name.

Only after this the chain of carbon atoms itself is named, adding the suffix -ene at the end. To clarify the location of an unsaturated (double) bond in a molecule, it is indicated by a number. For example, 2methylpentene-2.

Hybridization in alkenes

To cope with a task of the following type: “Establish the molecular formula of an alkene, the hydration of which produced a secondary alcohol,” it is necessary to find out the structural features of representatives of this class of hydrocarbons. The presence of a double bond explains the ability of CxHy to enter into addition reactions. The angle between double bonds is 120 degrees. No rotation is observed in the unsaturated bond, so representatives of this class are characterized by geometric isomerism. The main reaction site in alkene molecules is the double bond.

Physical properties

They are similar to saturated hydrocarbons. The lower representatives of this class of organic hydrocarbons are gaseous substances under normal conditions. Next, a gradual transition to liquids is observed, and alkenes, whose molecules contain more than seventeen carbon atoms, are characterized by a solid state. All compounds of this class have insignificant solubility in water, while they are perfectly soluble in polar organic solvents.

Features of isomerism

The presence of ethylene compounds in molecules explains their diversity structural formulas. In addition to the isomerization of the carbon skeleton, which is characteristic of representatives of all classes of organic compounds, they have interclass isomers. They are cycloparaffins. For example, for propene the interclass isomer is cyclopropane.

The presence of a double bond in molecules of this class explains the possibility of geometric cis- and trans-isomerism. Such structures are possible only for symmetrical unsaturated hydrocarbons containing a double bond.

The existence of this variant of isomerism is determined by the impossibility of free rotation of carbon atoms along the double bond.

Specifics of chemical properties

The mechanism of alkene hydration has certain features. This reaction refers to electrophilic addition.

How does the hydration reaction of an alkene proceed? To answer this question, consider Markovnikov's rule. Its essence is that the hydration of alkenes of asymmetrical structure is carried out in a certain way. The hydrogen atom will attach to the carbon that is more hydrogenated. The hydroxyl group is attached to a carbon atom that has less H. Hydration of alkenes leads to the formation of secondary monohydric alcohols.

In order for the reaction to proceed fully, mineral acids are used as catalysts. They guarantee the introduction into the reaction mixture required quantity hydrogen cations.

It is impossible to obtain primary monohydric alcohols by hydration of alkenes, since Markovnikov’s rule will not be observed. This feature used in the organic synthesis of secondary alcohols. Any hydration of alkenes is carried out without the use of harsh conditions, so the process has found its practical use.

If ethylene is taken as the initial representative of the SpH2n class, Markovnikov’s rule does not work. What alcohols cannot be obtained by hydration of alkenes? It is impossible to obtain as a result of this chemical process primary alcohols from unsymmetrical alkenes. How is the hydration of alkenes used? The production of secondary alcohols is carried out in exactly this way. If a representative of the acetylene series (alkynes) is chosen as a hydrocarbon, hydration leads to the production of ketones and aldehydes.

According to Markovnikov's rule, the hydration of alkenes is carried out. The reaction has an electrophilic addition mechanism, the essence of which is well studied.

Here are a few specific examples similar transformations. What does the hydration of alkenes lead to? Examples offered in a school chemistry course indicate that propanol-2 can be obtained from propene by reacting with water, and butanol-2 can be obtained from butene-1.

Alkene hydration is used commercially. Secondary alcohols are obtained in this way.

Halogenation

The interaction of unsaturated hydrocarbons with halogen molecules is considered a qualitative reaction to a double bond. We have already analyzed how the hydration of alkenes occurs. The mechanism of halogenation is similar.

Halogen molecules have a covalent nonpolar chemical bond. When temporary fluctuations occur, each molecule becomes electrophilic. As a result, the probability of addition increases, accompanied by the destruction of the double bond in the molecules of unsaturated hydrocarbons. After completion of the process, the reaction product is a dihalogen derivative of the alkane. Bromination is considered a qualitative reaction to unsaturated hydrocarbons, since the brown color of the halogen gradually disappears.

Hydrohalogenation

We have already looked at what the formula for the hydration of alkenes is. Reactions with hydrogen bromide have a similar option. In a given inorganic compound, the covalent polar chemical bond, therefore, there is a shift in electron density to the more electronegative bromine atom. Hydrogen acquires a partial positive charge, giving an electron to the halogen and attacks the alkene molecule.

If an unsaturated hydrocarbon has an asymmetric structure, when it reacts with a hydrogen halide, two products are formed. Thus, from propene during hydrohalogenation, 1-bromoproane and 2-bromopropane are obtained.

For a preliminary assessment of interaction options, the electronegativity of the selected substituent is taken into account.

Oxidation

The double bond inherent in unsaturated hydrocarbon molecules is exposed to strong oxidizing agents. They are also electrophilic in nature and are used in the chemical industry. Of particular interest is the oxidation of alkenes with an aqueous (or weakly alkaline) solution of potassium permanganate. It is called the hydroxylation reaction because it results in dihydric alcohols.

For example, when ethylene molecules are oxidized with an aqueous solution of potassium permanganate, ethinediol-1,2 (ethylene glycol) is obtained. This interaction is considered a qualitative reaction to a double bond, since during the interaction, discoloration of the potassium permanganate solution is observed.

In an acidic environment (under harsh conditions), aldehyde can be noted among the reaction products.

When interacting with atmospheric oxygen, the corresponding alkene is oxidized to carbon dioxide and water vapor. The process is accompanied by the release of thermal energy, so in industry it is used to generate heat.

The presence of a double bond in an alkene molecule indicates the possibility of hydrogenation reactions occurring in this class. The interaction of SpH2n with hydrogen molecules occurs when platinum and nickel are used thermally as catalysts.

Many representatives of the class of alkenes are prone to ozonation. At low temperatures, representatives of this class react with ozone. The process is accompanied by the cleavage of the double bond, the formation of cyclic peroxide compounds called ozonides. Their molecules contain O-O communications, therefore the substances are explosive. Ozonides are not synthesized in pure form, they are decomposed using a process of hydrolysis, then reduced with zinc. The products of this reaction are carbonyl compounds, which are isolated and identified by researchers.

Polymerization

This reaction involves the sequential combination of several alkene molecules (monomers) into a large macromolecule (polymer). From the initial ethene, polyethylene is produced, which has industrial applications. A polymer is a substance that has a high molecular weight.

Inside the macromolecule there is a certain number of repeating fragments called structural units. For the polymerization of ethylene, the group - CH2—CH2- is considered as a structural unit. The degree of polymerization indicates the number of units repeated in the polymer structure.

The degree of polymerization determines the properties of polymer compounds. For example, short chain polyethylene is a liquid that has lubricating properties. A macromolecule with long chains is characterized by a solid state. The flexibility and plasticity of the material is used in the manufacture of pipes, bottles, and films. Polyethylene, in which the degree of polymerization is five to six thousand, has increased strength, therefore it is used in the production of strong threads, rigid pipes, and cast products.

Among the products obtained by the polymerization of alkenes that are of practical importance, we highlight polyvinyl chloride. This compound is obtained by polymerization of vinyl chloride. The resulting product has valuable performance characteristics. It is characterized by increased resistance to aggressive influences chemical substances, non-flammable, easy to paint. What can be made from polyvinyl chloride? Briefcases, raincoats, oilcloth, artificial leather, cables, electrical wire insulation.

Teflon is a product of the polymerization of tetrafluoroethylene. This organic inert compound is resistant to sudden temperature changes.

Polystyrene is an elastic transparent substance formed by polymerization of the original styrene. It is indispensable in the manufacture of dielectrics in radio and electrical engineering. In addition, polystyrene is used in large quantities for the production of acid-resistant pipes, toys, combs, and porous plastics.

Features of obtaining alkenes

Representatives of this class are in demand in the modern chemical industry, so various methods for their industrial and laboratory production have been developed. Ethylene and its homologues do not exist in nature.

Many laboratory options for obtaining representatives of this class of hydrocarbons involve reverse addition reactions called elimination. For example, the dehydrogenation of paraffins (saturated hydrocarbons) produces the corresponding alkenes.

By reacting halogen derivatives of alkanes with metallic magnesium, it is also possible to obtain compounds with the general formula SpH2n. Elimination is carried out according to Zaitsev’s rule, reverse rule Markovnikova.

In industrial quantities, unsaturated hydrocarbons of the ethylene series are produced by cracking oil. Gases from cracking and pyrolysis of oil and gas contain from ten to twenty percent of unsaturated hydrocarbons. The mixture of reaction products contains both paraffins and alkenes, which are separated from each other by fractional distillation.

Some Applications

Alkenes are an important class of organic compounds. The possibility of their use is explained by their excellent reactivity, ease of preparation, and reasonable cost. Among the numerous industrial sectors that use alkenes, we highlight the polymer industry. A huge amount of ethylene, propylene, and their derivatives is spent on the production of polymer compounds.

That is why questions regarding the search for new ways to produce alkene hydrocarbons are so relevant.

Polyvinyl chloride is considered one of the most important products obtained from alkenes. It is characterized by chemical and thermal stability and low flammability. Since this substance is insoluble in mineral solvents, but soluble in organic solvents, it can be used in various industrial sectors.

Its molecular weight is several hundred thousand. When the temperature rises, the substance is capable of decomposition, accompanied by the release of hydrogen chloride.

Of particular interest are its dielectric properties, used in modern electrical engineering. Among the industries in which polyvinyl chloride is used, we highlight the production of artificial leather. The resulting material operational characteristics in no way inferior natural material, while having a much lower cost. Clothing made from such material is becoming increasingly popular among fashion designers who create bright and colorful collections of youth clothing made from polyvinyl chloride in different colors.

Polyvinyl chloride is used in large quantities as a sealant in refrigerators. Due to its elasticity and resilience, this chemical compound is in demand in the manufacture of films and modern suspended ceilings. Washable wallpaper is additionally covered with a thin PVC film. This allows you to add them mechanical strength. Such finishing materials will become ideal option for cosmetic repairs in office premises.

In addition, hydration of alkenes leads to the formation of primary and secondary monohydric alcohols, which are excellent organic solvents.