AT organic chemistry hydrocarbons can be found different amount carbon in the chain and a C=C bond. They are homologues and are called alkenes. Because of their structure, they are chemically more reactive than alkanes. But what exactly are their reactions? 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 -an changes to -ene. The Arabic numeral at the end of the name through a hyphen indicates the carbon number from which the double bond begins. Consider 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 they be found?

Since the reactivity of alkenes is very high, their representatives in nature are extremely rare. The principle of life of the olefin molecule is "let's be friends." There are no other substances around - it does not matter, 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 oil produced in Canada.

The very first representative of alkenes, ethene, is a hormone that stimulates the ripening of fruits; therefore, representatives of the flora synthesize it in small quantities. There is an alkene cis-9-tricosene, which in female houseflies plays the role of a sexual attractant. It is also called Muscalur. (Attractant - a substance of natural or synthetic origin, which causes attraction to the source of the smell in another organism). From the point of view of chemistry, this alkene looks like this:

Since all alkenes are very valuable raw materials, the methods for obtaining them artificially are very diverse. Let's consider the most common.

What if you need a lot?

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

C 7 H 16 -> CH 3 -CH \u003d CH 2 + C 4 H 10.

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

Under laboratory conditions, alkenes and methods of preparation are different, they are based on elimination reactions (elimination of a group of atoms without replacing them). Most often, water atoms are eliminated from alcohols, halogens, hydrogen or hydrogen halide. 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 the Zaitsev rule, which says:

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

Applying the rule, answer which reaction product will prevail? Later you will know if you answered correctly.

Chemical properties

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

• addition of hydrogen (hydrogenation). The presence of a catalyst and heating is needed for its passage;
• addition of halogen molecules (halogenation). Is one of qualitative reactions on a pi connection. Indeed, in the reaction of alkenes with bromine water, it becomes transparent from brown;
• reaction with hydrogen halides (hydrohalogenation);
• addition of water (hydration). The reaction conditions are heating and the presence of a catalyst (acid);

The reactions of unsymmetrical olefins with hydrogen halides and water follow the Markovnikov rule. This means that hydrogen will join that carbon from the carbon-carbon double bond, which already has more atoms hydrogen.

• combustion;
• partial oxidation catalytic. The product is cyclic oxides;
• Wagner reaction (oxidation with permanganate in neutral environment). This alkene reaction is another high quality C=C bond. When flowing, the pink solution of potassium permanganate discolors. 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-, movement double bond, cyclization, skeletal isomerization;
• polymerization is the main property of olefins for industry.

Application in medicine

big practical value have alkene reaction products. Many of them are used in medicine. Glycerin is obtained from propene. This polyhydric alcohol is an excellent solvent, and if used instead of water, the solutions will be more concentrated. For 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. By itself, glycerin is an antiseptic.

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

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

Application in industry

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

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

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

Alkenes (olefins, ethylene hydrocarbons C n H 2n

homologous series.

ethene (ethylene)

The simplest alkene is ethylene (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.

Hydrocarbon radicals derived from alkenes have the suffix "-enyl". Trivial names: CH 2 =CH- "vinyl", CH 2 =CH-CH 2 - "allyl".

The carbon atoms in the double bond are in a state of sp² hybridization, and have a bond angle of 120°.

Alkenes are characterized by isomerism of the carbon skeleton, double bond positions, interclass and spatial.

Physical Properties

The melting and boiling points of alkenes (simplified) increase with molecular weight and length of the main carbon chain.

Under normal conditions, alkenes from C 2 H 4 to C 4 H 8 are gases; from pentene C 5 H 10 to hexadecene C 17 H 34 inclusive - liquids, and starting from octadecene C 18 H 36 - solids. Alkenes are insoluble in water, but readily soluble in organic solvents.

Dehydrogenation of alkanes

This is one of the industrial methods for obtaining alkenes.

Alkyne hydrogenation

Partial hydrogenation of alkynes requires special conditions and the presence of a catalyst

A double bond is a combination of sigma and pi bonds. Sigma bond occurs with axial overlap of sp2 orbitals, and pi-bond with lateral overlap

Zaitsev's rule:

The elimination of a hydrogen atom in elimination reactions occurs predominantly from the least hydrogenated carbon atom.

13. Alkenes. Structure. sp 2 hybridization, multiple bond parameters. Reactions of electrophilic addition of halogens, hydrogen halides, hypochlorous acid. Hydration of alkenes. Morkovnikov's rule. Reaction mechanisms.

Alkenes (olefins, ethylene hydrocarbons) - acyclic unsaturated hydrocarbons containing one double bond between carbon atoms, forming a homologous series with the general formula C n H 2n

One s- and 2 p-orbitals mix and form 2 equivalent sp2-hybrid orbitals located in the same plane at an angle of 120.

If a bond is formed by more than one pair of electrons, then it is called multiple.

A multiple bond is formed when there are too few electrons and bonding atoms for each bondable valence orbital of the central atom to overlap with any orbital of the surrounding atom.

Electrophilic addition reactions

In these reactions, the attacking particle is an electrophile.

Halogenation:

Hydrohalogenation

Electrophilic addition of hydrogen halides to alkenes occurs according to Markovnikov's rule

Markovnikov rule

Addition of hypochlorous acid to form chlorohydrins:

Hydration

The addition reaction of water to alkenes proceeds in the presence of sulfuric acid:

carbocation- a particle in which a carbon atom is concentrated positive charge, the carbon atom has a vacant p-orbital.

14. Ethylene hydrocarbons. Chemical properties: reactions with oxidizing agents. Catalytic oxidation, reaction with peracids, oxidation reaction to glycols, with carbon-carbon bond breaking, ozonation. Wacker process. substitution reactions.

Alkenes (olefins, ethylene hydrocarbons) - acyclic unsaturated hydrocarbons containing one double bond between carbon atoms, forming a homologous series with the general formula C n H 2n

Oxidation

The oxidation of alkenes can occur, depending on the conditions and types of oxidizing reagents, both with the breaking of the double bond and with the preservation of the carbon skeleton.

When burned in air, olefins produce carbon dioxide and water.

H 2 C \u003d CH 2 + 3O 2 \u003d\u003e 2CO 2 + 2H 2 O

C n H 2n+ 3n/O 2 => nCO 2 + nH 2 O - general formula

catalytic oxidation

In the presence of palladium salts, ethylene is oxidized to acetaldehyde. Similarly, acetone is formed from propene.

When strong oxidizing agents (KMnO 4 or K 2 Cr 2 O 7 in H 2 SO 4 medium) act on alkenes, the double bond breaks when heated:

During the oxidation of alkenes with a dilute solution of potassium permanganate, dihydric alcohols are formed - glycols (E.E. Wagner reaction). The reaction takes place in the cold.

Acyclic and cyclic alkenes, when interacting with RCOOOH peracids in a nonpolar medium, form epoxides (oxiranes), therefore the reaction itself is called the epoxidation reaction.

Ozonation of alkenes.

When alkenes react with ozone, peroxide compounds are formed, which are called ozonides. The reaction of alkenes with ozone is the most important method for the oxidative cleavage of alkenes at the double bond.

Alkenes do not undergo substitution reactions.

Wacker process- the process of obtaining acetaldehyde by direct oxidation of ethylene.

The Wacker process is based on the oxidation of ethylene with palladium dichloride:

CH 2 \u003d CH 2 + PdCl 2 + H 2 O \u003d CH 3 CHO + Pd + 2HCl

15. Alkenes: chemical properties. Hydrogenation. Lebedev's rule. Isomerization and oligomerization of alkenes. Radical and ionic polymerization. The concept of polymer, oligomer, monomer, elementary link, degree of polymerization. Telomerization and copolymerization.

hydrogenation

Hydrogenation of alkenes directly with hydrogen occurs only in the presence of a catalyst. Hydrogenation catalysts are platinum, palladium, nickel

Hydrogenation can also be carried out in liquid phase with homogeneous catalysts

Isomerization reactions

When heated, isomerization of alkene molecules is possible, which

can lead to both displacement of the double bond and changes in the skeleton

hydrocarbon.

CH2=CH-CH2-CH3 CH3-CH=CH-CH3

polymerization reactions

This is a type of addition reaction. Polymerization is a reaction of sequential connection of identical molecules into larger molecules, without isolating any low molecular weight product. During polymerization, a hydrogen atom is attached to the most hydrogenated carbon atom located at the double bond, and the rest of the molecule is attached to the other carbon atom.

CH2=CH2 + CH2=CH2 + ... -CH2-CH2-CH2-CH2- ...

or n CH2=CH2 (-CH2-CH2-)n (polyethylene)

A substance whose molecules undergo a polymerization reaction is called monomer. A monomer molecule must have at least one double bond. The resulting polymers consist of a large number of repeating chains having the same structure ( elementary links). The number showing how many times a structural (elementary) unit is repeated in a polymer is called degree of polymerization(n).

Depending on the type of intermediate particles formed during polymerization, there are 3 polymerization mechanisms: a) radical; b) cationic; c) anionic.

According to the first method, high-pressure polyethylene is obtained:

The reaction is catalyzed by peroxides.

The second and third methods involve the use of acids (cationic polymerization) and organometallic compounds as catalysts.

In chemistry oligomer) - a molecule in the form of a chain of small the number of identical components.

Telomerization

Telomerization - oligomerization of alkenes in the presence of substances - chain transmitters (telogens). As a result of the reaction, a mixture of oligomers (telomeres) is formed, the end groups of which are parts of the telogen. For example, in the reaction of CCl 4 with ethylene, the telogen is CCl 4 .

CCl 4 + nCH 2 \u003d CH 2 \u003d\u003e Cl (CH 2 CH 2) n CCl 3

These reactions can be initiated by radical initiators or by gamma radiation.

16. Alkenes. Radical addition reactions of halogens and hydrogen halides (mechanism). addition of carbenes to olefins. Ethylene, propylene, butylenes. Industrial sources and main uses.

Alkenes easily add halogens, especially chlorine and bromine (halogenation).

A typical reaction of this type is the discoloration bromine water

CH2=CH2 + Br2 → СH2Br-CH2Br (1,2-dibromoethane)

Electrophilic addition of hydrogen halides to alkenes occurs according to Markovnikov's rule:

Markovnikov rule: when protic acids or water are added to unsymmetrical alkenes or an alkynamate, hydrogen is attached to the most hydrogenated carbon atom

A hydrogenated carbon atom is one to which hydrogen is attached. The most hydrogenated - where there is the most H

Carben addition reactions

CR 2 carbenes: - highly reactive short-lived particles that can easily add to the double bond of alkenes. As a result of the carbene addition reaction, cyclopropane derivatives are formed

Ethylene is an organic chemical described by the formula C 2 H 4. Is the simplest malken ( olefin)compound. At normal conditions It is a colorless flammable gas with a slight odour. Partially soluble in water. Contains a double bond and therefore refers to unsaturated or unsaturated hydrocarbons. Plays extremely important role in industry. Ethylene is the world's most produced organic compound: Ethylene oxide; polyethylene, acetic acid, ethyl alcohol.

Basic chemical properties(do not teach, just let them be just in case, suddenly it will be possible to write off)

Ethylene - chemically active substance. Since there is a double bond between the carbon atoms in the molecule, one of them, less strong, is easily broken, and at the place of the bond breaking, the molecules are joined, oxidized, and polymerized.

Halogenation:

CH 2 \u003d CH 2 + Br 2 → CH 2 Br-CH 2 Br

Bromine water becomes decolorized. This is a qualitative reaction to unsaturated compounds.

Hydrogenation:

CH 2 \u003d CH 2 + H - H → CH 3 - CH 3 (under the action of Ni)

Hydrohalogenation:

CH 2 \u003d CH 2 + HBr → CH 3 - CH 2 Br

Hydration:

CH 2 \u003d CH 2 + HOH → CH 3 CH 2 OH (under the action of a catalyst)

This reaction was discovered by A.M. Butlerov, and it is used for the industrial production of ethyl alcohol.

Oxidation:

Ethylene is easily oxidized. If ethylene is passed through a solution of potassium permanganate, it will become colorless. This reaction is used to distinguish between saturated and unsaturated compounds. Ethylene oxide is a fragile substance, the oxygen bridge breaks and water joins, resulting in the formation of ethylene glycol. Reaction equation:

3CH 2 \u003d CH 2 + 2KMnO 4 + 4H 2 O → 3HOH 2 C - CH 2 OH + 2MnO 2 + 2KOH

C 2 H 4 + 3O 2 → 2CO 2 + 2H 2 O

Polymerization (obtaining polyethylene):

nCH 2 \u003d CH 2 → (-CH 2 -CH 2 -) n

Propylene(propene) CH 2 \u003d CH-CH 3 - unsaturated (unsaturated) hydrocarbon of the ethylene series, combustible gas. Propylene is a gaseous substance with a low boiling point t bp = -47.6 ° C

Typically, propylene is isolated from refinery gases (during the cracking of crude oil, pyrolysis of gasoline fractions) or associated gases, as well as from coal coking gases.

Alkenes are a more active class of substances than alkanes. Chemical properties alkenes are due to the structure of their molecules.

Structure

Unsaturated hydrocarbons - alkenes or olefins - differ from other classes of organic substances by the presence of a double or π-bond between carbon atoms. The double bond can be anywhere in the molecule.

A pi bond is formed by overlapping p orbitals. Due to the fact that the p-orbital has extensions in both directions from the axis and resembles a dumbbell, the pi-bond occurs in two places. Unlike the σ-bond, which occurs when the s-orbitals overlap in the form of a sphere, the π-bond is less strong and is easily destroyed by other compounds. This causes the activity of alkenes.

Rice. 1. π-bond and σ-bond.

The double bond acts as an electron donor in addition reactions. Therefore, alkenes are characterized by electrophilic addition reactions.

Physical Properties

General physical properties alkenes:

• melting and boiling points increase with increasing molecular weight in the homologous series;
• do not dissolve in water;
• hold on water surface, since they have a density many times less density water;
• dissolve in organic solvents - alcohols, ethers.

The aggregate state of substances varies from the number of carbon atoms in the homologous series. Alkenes with 2-4 carbon atoms are gases. From peptene (C 5 H 10) to heptadecene (C 17 H3 4), substances are in liquid state. Alkenes containing more than 17 carbon atoms are solids.

Rice. 2. Homologous series of alkenes.

Chemical properties

Features and examples of chemical properties of alkenes are given in the table.

Reaction	Description	Reaction equations
Hydrogenation - addition of hydrogen	It proceeds at high pressure in the presence of a catalyst - nickel, palladium or platinum. Alkanes are formed - saturated hydrocarbons	CH 2 \u003d CH-CH 3 + H 2 → CH 3 -CH 2 -CH 3
Halogenation - addition of halogens	Leaks at normal conditions. Halogens join at the double bond. Dihaloalkanes are formed	CH 2 \u003d CH 2 + Cl 2 → Cl-CH 2 -CH 2 -Cl (1,2-dichloroethane); CH 3 -CH \u003d CH-CH 3 + Br 2 → CH 3 -CH-Br-CH-Br-CH 3 (2,3-dibromobutane)
Hydrohalogenation - addition of hydrogen halides	Electrophilic addition reaction. An electrophile is a hydrogen proton in a halogen. haloalkanes are formed	CH 2 \u003d CH 2 + HCl → CH 3 -CH 2 -Cl (chloroethane)
Hydration - adding water	The reaction takes place in the presence inorganic acids- sulfuric, phosphoric. They act as a catalyst and are sources of hydrogen. Monohydric alcohols are formed	CH 2 \u003d CH 2 + H 2 O → CH 3 -CH 2 OH
Polymerization - an increase in the number of atoms	It proceeds in the presence of a catalyst, at elevated pressure and temperature. In this way, polyethylene, polyvinyl chloride, polypropylene are obtained	nCH 2 \u003d CH 2 → (-CH 2 -CH 2 -) n
	Occurs with excess oxygen	CH 2 \u003d CH 2 + 3O 2 → 2CO 2 + H 2 O
incomplete oxidation	Runs in the presence of a catalyst. Alkene mixed with oxygen is passed over heated silver. Epoxide is formed - alkene oxide	2CH 2 \u003d CH 2 + O 2 → 2CH 2 -O-CH 2
Wagner reaction	Oxidation with potassium permanganate in an alkaline or neutral medium. Alcohols are formed	3CH 2 \u003d CH 2 + 2KMnO 4 + 4H 2 O → 3CH 2 OH-CH 2 OH + 2KOH + 2MnO 2
Oxidation with boiling potassium permanganate in an acidic medium	carboxylic acids are formed	CH 3 -CH \u003d CH-CH 3 + 4 [O] → 2CH 3 COOH

When heated in the presence of a catalyst, alkenes enter into an isomerization reaction. The position of the double bond or the structure of the carbon skeleton changes. For example, butene-1 (the position of the double bond between the first and second atoms) is converted to butene-2 ​​(the double bond is "shifted" to the second atom).

Rice. 3. Isomerization of alkenes.

What have we learned?

From the 10th grade chemistry lesson, they learned about the chemical properties of alkenes. The double bond makes these substances more reactive than alkanes. Alkenes interact with halogens, oxygen, water, hydrogen, and hydrogen halides. Most reactions proceed in the presence of a catalyst at high temperature or at high blood pressure. Polymers are made from alkenes. Isomers are also formed under the action of catalysts.

Topic quiz

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

Goals and objectives of the lesson:

• consider the specific chemical properties of ethylene and general properties alkenes;
• to deepen and concretize the concepts of?-connection, about the mechanisms 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, alcohol lamp, sand, tables "Structure of the molecule of ethylene", "Basic chemical properties of alkenes", demo samples"Polymers".

DURING THE CLASSES

I. Organizational moment

We continue to study homologous series 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, obtain an initial understanding of polymerization reactions, consider laboratory and industrial methods 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, which have only single bonds, addition reactions - to 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 (at complete oxidation 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.

ALKENES

Hydrocarbons, in the molecule of which, in addition to simple carbon-carbon and carbon-hydrogen σ-bonds, there are carbon-carbon π-bonds, are called unlimited. Since the formation of a π bond is formally equivalent to the loss of two hydrogen atoms by a molecule, unsaturated hydrocarbons contain 2p fewer hydrogen atoms than the limit, where P - number of π-bonds:

A series whose members differ from each other by (2H) n is called isological side. So, in the above scheme, the isologues are hexanes, hexenes, hexadienes, hexines, hexatrienes, etc.

Hydrocarbons containing one π-bond (i.e. double bond) are called alkenes (olefins) or, according to the first member of the series - ethylene, ethylene hydrocarbons. The general formula for their homologous series C p H 2l.

1. Nomenclature

In accordance with the rules of IUPAC, when constructing the names of alkenes, the longest carbon chain containing a double bond receives the name of the corresponding alkane, in which the ending -en changed to -en. This chain is numbered in such a way that the carbon atoms involved in the formation of a double bond receive the smallest number possible:

Radicals are named and numbered as in the case of alkanes.

For alkenes, relatively simple structure simpler names are allowed. So, some of the most common alkenes are called by adding the suffix -en to the name of a hydrocarbon radical with the same carbon skeleton:

Hydrocarbon radicals formed from alkenes receive the suffix -enyl. The numbering in the radical starts from the carbon atom that has a free valence. However, for the simplest alkenyl radicals, instead of systematic names, it is allowed to use trivial ones:

Hydrogen atoms directly bonded to unsaturated carbon atoms forming a double bond are often referred to as vinyl hydrogen atoms,

2. Isomerism

In addition to the isomerism of the carbon skeleton, in the series of alkenes there is also the isomerism of the position of the double bond. In general, isomerism of this type - substituent position isomerism (functions)- is observed in all cases when there are any functional groups in the molecule. For alkane C 4 H 10, two structural isomers are possible:

For alkene C 4 H 8 (butene), three isomers are possible:

Butene-1 and butene-2 ​​are position function isomers (in this case its role is played by a double bond).

Spatial isomers differ in the spatial arrangement of substituents relative to each other and are called cis isomers, if the substituents are on the same side of the double bond, and trans isomers, if on opposite sides:

3. Double bond structure

The breaking energy of a molecule at the C=C double bond is 611 kJ/mol; since the energy of the σ-bond C-C is 339 kJ / mol, the energy of breaking the π bond is only 611-339 = 272 kJ / mol. π-electrons are much easier than σ-electrons to be influenced, for example, by polarizing solvents or by any attacking reagents. This is explained by the difference in the symmetry of the distribution of the electron cloud of σ- and π-electrons. The maximum overlap of p-orbitals and, consequently, the minimum free energy of the molecule are realized only with a planar structure of the vinyl fragment and with a shortened distance s-s, equal to 0.134 nm, i.e. much smaller than the distance between carbon atoms connected by a single bond (0.154 nm). With the rotation of the "halves" of the molecule relative to each other along the axis of the double bond, the degree of overlapping of the orbitals decreases, which is associated with the expenditure of energy. The consequence of this is the absence of free rotation along the axis of the double bond and the existence of geometric isomers with the corresponding substitution at carbon atoms.

4. Physical properties

Like alkanes, the lower homologues of a number of the simplest alkenes under normal conditions are gases, and starting from C 5 they are low-boiling liquids.

All alkenes, like alkanes, are practically insoluble in water and readily soluble in other organic solvents, with the exception of methyl alcohol; they all have a lower density than water.

5. Chemical properties

By revising reactivity complex organic compounds acts general principle. In most reactions, not an "inert" hydrocarbon radical is involved, but the existing functional groups and their immediate environment. This is natural, since most bonds are less strong than C-C connections and CH, and, in addition, the bonds in the functional group and near it are the most polarized.

It is natural to expect that the reactions of alkenes will pass through a double bond, which can also be considered a functional group, and therefore, they will be addition reactions, and not substitution reactions, characteristic of the previously considered alkanes.

Addition of hydrogen

The addition of hydrogen to alkenes leads to the formation of alkanes:

The addition of hydrogen to ethylene compounds in the absence of catalysts occurs only at high temperatures, at which the decomposition of organic substances often begins. Hydrogen addition is much easier in the presence of catalysts. The catalysts are platinum group metals in a finely dispersed state, platinum itself and especially palladium - already at ordinary temperature. Of great practical importance was the discovery of Sabatier, who used specially prepared finely divided nickel at a temperature of 150-300°C and in numerous works showed the versatility of this catalyst for a number of reduction reactions.

Addition of halogens

Halogens add to alkenes to form dihalogen derivatives containing halogen atoms at neighboring carbon atoms:

At the first stage of this reaction, the interaction between the π-electrons of the double bond and the electrophilic halogen particle occurs with the formation of the π-complex (I). Further, the π-complex rearranges into the onium (bromonium) ion (II) with the elimination of the halogen anion, which is in equilibrium with the carbocation (III). The anion then attacks the onium ion to form the addition product (IV):

The anion attack of the bromonium ion (II) with the formation of dibromide (IV) occurs in the trans position. Thus, in the case of addition of Br 2 to cyclopentene, only trans-1,2-dibromodiclo-pentane is formed:

Evidence of the two-stage addition of a halogen to alkenes is the fact that when Br 2 is added to cyclohexene in the presence of MaCl, not only trans-1,2-dibromocyclohexane is formed, but also trans-1-bromo-2-chlorocyclohexane:

Radical halogenation

Under harsh conditions (gas phase, 500°C), halogens do not add to the double bond, but the α-position is halogenated:

In this case, the reaction proceeds by a radical mechanism.

Addition of hydrogen halides

Hydrogen halides are added to alkenes to form haloalkyls. Attachment in the case of asymmetric molecules follows the Markovnikov rule, i.e., hydrogen attaches to the most hydrogenated carbon atom (with largest number hydrogen atoms):

This reaction, like the addition of bromine to ethylene, proceeds after the formation of the π-complex through the stage of formation of the protonium ion:

In the presence of peroxides, hydrogen bromide does not add according to Markovnikov's rule (Harash effect):

In the presence of peroxides, the reaction proceeds not by the mechanism of electrophilic addition, as above, but by a radical mechanism. The first stage is the attack of the peroxide radical on the HBr molecule:

The resulting bromine radical is added to propylene to form a new radical:

The latter is stabilized by pulling out hydrogen from the new HBr molecule with the regeneration of a new bromine radical, etc.:

In this case, too, the direction of the process is determined by the stability of the bromopropane radicals: a more stable one is predominantly formed, leading to 1-bromopropane.

Addition of water and sulfuric acid

In the presence of acids, water is added at the double bond according to Markovnikov's rule:

The same reaction occurs with sulfuric acid:

Oxidation with potassium permanganate in a neutral or slightly alkaline medium (Wagner reaction)

At the first stage, according to the cis-addition mechanism, the MnO 4 ion is added to the multiple bond, followed by hydrolytic cleavage of the unstable addition product and the release of the MnO 3 ion -

The reaction proceeds according to the cis-addition scheme:

Acidic solutions of permanganate oxidize alkenes with a chain break at the C=C bond and the formation of acids or ketones:

The effect of ozone on alkenes

This reaction leads to highly explosive crystalline ozonides, which upon hydrolysis form aldehydes or ketones:

The reaction is often used to determine the position of a double bond in a molecule, since the structure of the initial alkene can also be imagined from the resulting carbonyl compounds.

The reaction proceeds by cis-cycloaddition through the stage of unstable molozonide, which undergoes dissociation and subsequent recombination:

Polymerization of alkenes

Of particular importance is the polymerization of ethylene and propylene into polymers with a molecular weight of about 10 5 . Until 1953, radical (free-radical-initiated) polymerization was mainly used, although both anionic and cationic initiation of the process were used in principle.

After the work of Ziegler and Nutt, who received the Nobel Prize for these studies, the so-called coordination polymerization. The simplest "Ziegler" catalyst of this type consists of triethylaluminum and titanium (IV) compounds. This results in the formation of polymers a high degree stereoregularity. For example, during the polymerization of propylene, isotactic polypropylene is formed - a polymer in which all side CH 3 groups occupy the same spatial position:

This gives the polymer greater strength, and it can even be used to make synthetic fibers.

The polyethylene produced by this process is saturated hydrocarbon with an unbranched chain. It is less elastic than high-pressure polyethylene, but has greater hardness and is able to withstand higher temperatures.

Due to the combination of many valuable properties, polyethylene has a very wide application. It is one of the best materials for cable insulation, for use in radar technology, radio engineering, agriculture, etc. Pipes, hoses, vessels, containers for agricultural products and fertilizers, films of various thicknesses and many household items are made from it. Durable polyethylene films have even begun to be used as a cover for the bottom of artificial channels to make them waterproof.

Telomerization

An interesting industrial application is the process of copolymerization of ethylene with carbon tetrachloride, called telomerization. If benzoyl peroxide or another initiator that decomposes with the formation of free radicals is added to a mixture of ethylene with CC1 4, the following process occurs:

Radicals СС1 3 "initiate the chain polymerization of ethylene:

When meeting with another CC1 4 molecule, chain growth stops:

Radical CC1 3 - gives rise to a new chain.

The resulting low molecular weight polymerization products containing halogen atoms at the ends of the chain are called telomeres. Telomeres obtained with values n = 2.3, 4, ..., 15.

During the hydrolysis of telomerization products, ω-chloro-substituted carboxylic acids are formed, which are valuable chemical products.