Chemistry Alkenes chemical properties. Physical and chemical properties of alkenes

General formula of alkenes: C n H 2n(n 2)

The first representatives of the homologous series of alkenes:

The formulas of alkenes can be compiled from the corresponding formulas of alkanes (saturated hydrocarbons). The names of alkenes are formed by replacing the suffix -an of the corresponding alkane with -ene or -ylene: butane - butylene, pentane -pentene, etc. The number of a carbon atom with a double bond is indicated by an Arabic numeral after the name.

The carbon atoms involved in the formation of a double bond are in a state of sp hybridization. Three -bonds formed by hybrid orbitals and are located in the same plane at an angle of 120° to each other. An additional -bond is formed by lateral overlap of non-hybrid p-orbitals:

The length of the C=C double bond (0.133 nm) is less than the length of the single bond (0.154 nm). The energy of a double bond is less than twice the energy of a single bond, since the bond energy less energy-connections.

Alkene isomers

All alkenes except ethylene have isomers. Alkenes are characterized by isomerism of the carbon skeleton, isomerism of the position of the double bond, interclass and spatial isomerism.

The interclass isomer of propene (C 3 H 6) is cyclopropane. Starting with butene (C 4 H 8), isomerism appears at the position of the double bond (butene-1 and butene-2), carbon skeleton isomerism (methylpropene or isobutylene), as well as spatial isomerism (cis-butene-2 ​​and trans-butene-2 ). In cis-isomers, substituents are located on one side, and in trans-isomers, on opposite sides of the double bond.

The chemical properties and chemical activity of alkenes are determined by the presence of a double bond in their molecules. For alkenes, electrophilic addition reactions are most characteristic: hydrohalogenation, hydration, halogenation, hydrogenation, polymerization.

Qualitative reaction to a double bond - decolorization of bromine water:

Examples of solving problems on the topic "formula of alkenes"

EXAMPLE 1

Exercise	How many isomers capable of decolorizing bromine water does a substance of the composition C 3 H 5 Cl have? Write the structural formulas of these isomers
Decision	C 3 H 5 Cl is a monochloro derivative of the hydrocarbon C 3 H 6 . This formula corresponds to either propene - a hydrocarbon with one double bond, or cyclopropane (a cyclic hydrocarbon). This substance decolorizes bromine water, which means that it contains a double bond. Three carbon atoms can only form the following structure: since the isomerism of the carbon skeleton and the position of the double bond with such a number of carbon atoms is not possible. Structural isomerism in this molecule is possible only by changing the position of the chlorine atom relative to the double bond: For 1-chloropropene, cis-trans isomerism is possible:
Answer	The condition of the problem is satisfied by 4 isomers

EXAMPLE 2

Exercise	A mixture of isomeric hydrocarbons (gases with a hydrogen density of 21) with a volume of 11.2 liters (N.O.) reacted with bromine water. As a result, 40.4 g of the corresponding dibromo derivative was obtained. What is the structure of these hydrocarbons? Determine their volume content in the mixture (in %).
Decision	The general formula of hydrocarbons is C x H y. Calculate molar mass hydrocarbons: Therefore, the formula of hydrocarbons is C 3 H 6. Only two substances have such a formula - propene and cyclopropane. Only propene reacts with bromine water: Calculate the amount of dibromo derivative substance: According to the reaction equation: n(propene) mol The total amount of hydrocarbons in the mixture is:

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. Arabic numeral at the end of the name, a hyphen denotes 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 are, and a small number of representatives are part of the accompanying petroleum gas, and higher - 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. cleavage of the molecule under the influence 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. After all, when alkenes react 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 hydrogen atoms.

• combustion;
• partial oxidation catalytic. The product is cyclic oxides;
• Wagner reaction (oxidation with permanganate in a neutral medium). 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 medium, 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

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. AT 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 by means of inhalation.

Alkadienes are alkenes with two double bonds in one molecule. Their main application is the production of synthetic rubber, from which various heating pads and syringes, probes and catheters, gloves, nipples and much more are then made, which is simply indispensable in 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	production of teflon, ethyl alcohol, acetic acid
Chemical industry	ethene, polypropylene	get alcohols, polymers (polyvinyl chloride, polyethylene, polyvinyl acetate, polyisobtylene, acetaldehyde
Mining	ethene etc.	explosives

Alkenes and their derivatives have found wider application in industry. (Where and how alkenes are used, table above).

It's only small part use of alkenes and their derivatives. Every year the need for olefins only increases, which means that the need for their production also increases.

Lesson topic: Alkenes. receiving, Chemical properties and the use 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 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 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.

4. Chemical properties of alkenes

The energy of the double carbon-carbon bond in ethylene (146 kcal/mol) is significantly lower than the doubled energy of a single C-C bond in ethane (288=176 kcal/mol). -Communication C-Cα-bonds are stronger in ethylene, so the reactions of alkenes, accompanied by the breaking of α-bonds with the formation of two new simple α-bonds, are a thermodynamically favorable process. For example, in the gas phase, according to the calculated data, all the reactions below are exothermic with a significant negative enthalpy, regardless of their real mechanism.

From the point of view of the theory of molecular orbitals, one can also conclude that the -bond is more reactive than the -bond. Consider the molecular orbitals of ethylene (Fig. 2).

Indeed, the bonding -orbital of ethylene has a higher energy than the bonding -orbital, and vice versa, the antibonding *-orbital of ethylene lies below the antibonding *-orbital of the C=C bond. Under normal conditions, *- and *-orbitals of ethylene are vacant. Therefore, the frontier orbitals of ethylene and other alkenes that determine them reactivity will be orbitals.

4.1. Catalytic hydrogenation of alkenes

Despite the fact that the hydrogenation of ethylene and other alkenes to alkanes is accompanied by the release of heat, this reaction proceeds at a noticeable rate only in the presence of certain catalysts. The catalyst, by definition, does not affect the thermal effect of the reaction, and its role is reduced to lowering the activation energy. One should distinguish between heterogeneous and homogeneous catalytic hydrogenation of alkenes. In heterogeneous hydrogenation, finely divided metal catalysts are used - platinum, palladium, ruthenium, rhodium, osmium and nickel, either in pure form or supported on inert carriers - BaSO 4 , CaCO 3 , activated carbon, Al 2 O 3, etc. All of them insoluble in organic media and act as heterogeneous catalysts. The most active among them are ruthenium and rhodium, but most widespread received platinum and nickel. Platinum is usually used in the form of black dioxide PtO 2 , commonly known as "Adams catalyst". Platinum dioxide is obtained by fusing chloroplatinic acid H 2 PtCl 6 . 6H 2 O or ammonium hexachloroplatinate (NH 4) 2 PtCl 6 with sodium nitrate. Hydrogenation of alkenes with an Adams catalyst is usually carried out at normal pressure and a temperature of 20-50 0 C in alcohol, acetic acid, ethyl acetate. When hydrogen is passed through, platinum dioxide is reduced directly in the reaction vessel to platinum black, which catalyzes the hydrogenation. Other more active platinum group metals are used on inert supports, such as Pd/C or Pd/BaSO 4 , Ru/Al 2 O 3 ; Rh/C, etc. Palladium deposited on coal catalyzes the hydrogenation of alkenes to alkanes in an alcohol solution at 0-20 0 C and normal pressure. Nickel is usually used in the form of the so-called "Raney nickel". To obtain this catalyst, the nickel-aluminum alloy is treated with hot aqueous alkali to remove almost all of the aluminum and then with water until neutral. The catalyst has a porous structure and is therefore also called a skeletal nickel catalyst. Typical conditions for the hydrogenation of alkenes over Raney nickel require a pressure of about 5-10 atm and a temperature of 50-100 0 C, i.e. this catalyst is much less active than platinum group metals, but it is whiter and cheaper. The following are some typical examples of heterogeneous catalytic hydrogenation of acyclic and cyclic alkenes:

Since both hydrogen atoms are attached to the carbon atoms of the double bond from the surface of the catalyst metal, the addition usually occurs on one side of the double bond. This type of attachment is called syn- connection. In those cases when two fragments of the reagent are attached from different sides of the multiple bond (double or triple) anti- accession. Terms syn- and anti- are equivalent in meaning to terms cis- and trance-. To avoid confusion and misunderstandings, the terms syn- and anti- refer to the type of connection, and the terms cis- and trance- to the structure of the substrate.

The double bond in alkenes is hydrogenated at a faster rate than many other functional groups (C=O, COOR, CN, etc.) and therefore the hydrogenation of the C=C double bond is often a selective process if the hydrogenation is carried out under mild conditions (0- 20 0 C and at atmospheric pressure). Below are some typical examples:

The benzene ring is not reduced under these conditions.

A great and fundamentally important achievement in catalytic hydrogenation is the discovery of soluble metal complexes that catalyze hydrogenation in a homogeneous solution. Heterogeneous hydrogenation on the surface of metal catalysts has a number of significant disadvantages, such as isomerization of alkenes and splitting of single carbon-carbon bonds (hydrogenolysis). Homogeneous hydrogenation is devoid of these disadvantages. Behind last years a large group of catalysts for homogeneous hydrogenation - transition metal complexes containing various ligands - has been obtained. The best catalysts for homogeneous hydrogenation are complexes of rhodium (I) and ruthenium (III) chlorides with triphenylphosphine - tris (triphenylphosphine) rhodium chloride (Ph 3 P) 3 RhCl (Wilkinson's catalyst) and tris (triphenylphosphine) ruthenium hydrochloride (Ph 3 P) 3 RuHCl. The most accessible rhodium complex, which is obtained by the interaction of rhodium (III) chloride with triphenylphosphine. Wilkinson's rhodium complex is used to hydrogenate the double bond under normal conditions.

An important advantage of homogeneous catalysts is the possibility of selective reduction of a mono- or disubstituted double bond in the presence of a tri- and tetrasubstituted double bond due to large differences in their hydrogenation rates.

In the case of homogeneous catalysts, hydrogen addition also occurs as syn- accession. So recovery cis-butene-2 ​​deuterium under these conditions leads to meso-2,3-dideuterobutane.

4.2. Double bond reduction with diimide

The reduction of alkenes to the corresponding alkanes can be successfully carried out using diimide NH=NH.

Diimide is obtained by two main methods: oxidation of hydrazine with hydrogen peroxide in the presence of Cu 2+ ions or by the interaction of hydrazine with Ni-Raney (hydrazine dehydrogenation). If an alkene is present in the reaction mixture, its double bond undergoes hydrogenation under the action of a very unstable diimide. A distinctive feature of this method is the strict syn-stereospecificity of the recovery process. It is believed that this reaction proceeds through a cyclic activated complex with a strict orientation of both reacting molecules in space.

4.3. Electrophilic addition reactions at the double bond of alkenes

The boundary orbitals of the HOMO and LUMO of alkenes are the occupied - and empty *-orbitals. Consequently, in reactions with electrophiles (E +), the -orbital will participate, and in reactions with nucleophiles (Nu -), the *-orbital of the C=C bond will participate (see Fig. 3). In most cases, simple alkenes easily react with electrophiles, and react with nucleophiles with great difficulty. This is explained by the fact that usually the LUMO of most electrophiles is close in energy to the energy of -HOMO of alkenes, while the HOMO of most nucleophiles lies much lower than *-LUMO.

Simple alkenes react only with very strong nucleophilic agents (carbanions) under harsh conditions, however, the introduction of electron-withdrawing groups into alkenes, for example, NO 2 , COR, etc., leads to a decrease in the *-level, due to which the alkene acquires the ability to react with medium-strength nucleophiles (ammonia, RO - , NєC - , enolate anion, etc.).

As a result of the interaction of the electrophilic agent E + with an alkene, a highly reactive carbocation is formed. The carbocation is further stabilized by the rapid addition of the nucleophilic agent Nu - :

Since the addition of an electrophile is a slow step, the process of addition of any polar agent E + Nu - should be considered precisely as an electrophilic addition to the multiple bond of the alkene. A large number of reactions of this type are known, where the role of the electrophilic agent is played by halogens, hydrogen halides, water, divalent mercury salts, and other polar reagents. Electrophilic addition to a double bond in the classification of organic reaction mechanisms has the symbol Ad E ( Addition Electrophilic) and, depending on the number of reacting molecules, is designated as Ad E 2 (bimolecular reaction) or Ad E 3 (trimolecular reaction).

4.3.a. Addition of halogens

Alkenes react with bromine and chlorine to form addition products at the double bond of one halogen molecule in a yield close to quantitative. Fluorine is too active and causes the destruction of alkenes. The addition of iodine to alkenes in most cases is a reversible reaction, the equilibrium of which is shifted towards the starting reagents.

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

Further, the -complex is converted 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 formally be considered as a nucleophilic substitution of S N 2 at the carbon atom, where the leaving group is Br+ .

The addition of halogens to the double bond of alkenes is one of the formally simple model reactions, on the example of which one can consider the influence of the main factors that allow one to draw well-reasoned conclusions about the detailed mechanism of the process. For reasonable conclusions about the mechanism of any reaction, one should have data on: 1) reaction kinetics; 2) stereochemistry (stereochemical result of the reaction); 3) the presence or absence of an associated, competing process; 4) the effect of substituents in the initial substrate on the reaction rate; 5) the use of labeled substrates and (or) reagents; 6) the possibility of rearrangements during the reaction; 7) the influence of the solvent on the reaction rate.

Let us consider these factors using the example of halogenation of alkenes. Kinetic data make it possible to establish the order of the reaction for each component and, on this basis, to draw a conclusion about the overall molecularity of the reaction, i.e., about the number of reacting molecules.

For the bromination of alkenes, the reaction rate is generally described by the following equation:

v = k`[alkene] + k``[alkene] 2 ,

which in rare cases is simplified to

v = k`[alkene].

Based on the kinetic data, it can be concluded that one or two bromine molecules are involved in the rate-determining step. The second order in terms of bromine means that it is not the bromide ion Br - that reacts with the bromonium ion, but the tribromide ion formed during the interaction of bromine and the bromide ion:

This balance is shifted to the right. The kinetic data do not allow any other conclusions to be drawn about the structure of the transition state and the nature of the electrophilic species in the addition of a halogen to the double bond. The most valuable information about the mechanism of this reaction is provided by data on the stereochemistry of addition. The addition of a halogen to a double bond is a stereospecific process (a process in which only one of the possible stereoisomers is formed; in a stereoselective process, one stereomer is predominantly formed) anti-additions for alkenes and cycloalkenes, in which the double bond is not conjugated to the benzene ring. For cis- and trance-isomers of butene-2, pentene-2, hexene-3, cyclohexene, cyclopentene and other alkenes, the addition of bromine occurs exclusively as anti- accession. In the case of cyclohexene, only trance-1,2-dibromocyclohexane (mixture of enantiomers).

The trans arrangement of bromine atoms in 1,2-dibromocyclohexane can be depicted in a simplified way with respect to the average plane of the cyclohexane ring (without taking into account conformations):

When bromine is added to cyclohexene, it initially forms trance-1,2-dibromocyclohexane in a, a-conformation, which then immediately passes into an energetically more favorable her-conformation. Anti- addition of halogens to the double bond makes it possible to reject the mechanism of one-step synchronous addition of one halogen molecule to the double bond, which can only be carried out as syn- accession. Anti- addition of a halogen is also inconsistent with the formation of an open carbocation RCH + -CH 2 Hal as an intermediate. In an open carbocation, free rotation around the C-C bond is possible, which should lead after the attack of the Br anion - to form a mixture of products anti- and so syn- connections. stereospecific anti- the addition of halogens was main reason creation of the concept of bromonium or chloronium ions as discrete intermediate particles. This concept perfectly satisfies the rule anti-addition, since the nucleophilic attack of the halide ion is possible with anti-sides on any of the two carbon atoms of the halonium ion by the S N 2 mechanism.

In the case of unsymmetrically substituted alkenes, this should result in two enantiomers treo-form upon addition of bromine to cis-isomer or to enantiomers erythro-forms during halogenation trance-isomer. This is indeed observed when bromine is added, for example, to cis- and trance-pentene-2 ​​isomers.

In the case of bromination of symmetrical alkenes, for example, cis- or trance-hexenes-3 should form or a racemate ( D,L-form), or meso-form of the final dibromide, which is actually observed.

There is independent, direct evidence for the existence of halonium ions in a non-nucleophilic, indifferent medium at low temperatures. Using NMR spectroscopy, the formation of bromonium ions was recorded during the ionization of 3-bromo-2-methyl-2-fluorobutane under the action of a very strong Lewis acid of antimony pentafluoride in a solution of liquid sulfur dioxide at -80 0 C.

This cation is quite stable at -80 0 C in a non-nucleophilic medium, but is instantly destroyed by the action of any nucleophilic agents or by heating.

Cyclic bromonium ions can sometimes be isolated in pure form if spatial obstacles prevent their opening under the action of nucleophiles:

It is clear that the possibility of the existence of bromonium ions, which are quite stable under special conditions, cannot serve as direct evidence of their formation in the reaction of the addition of bromine to the double bond of an alkene in alcohol, acetic acid, and other electron-donating solvents. Such data should be considered only as an independent confirmation of the fundamental possibility of the formation of halonium ions in the process of electrophilic addition to the double bond.

The concept of the halonium ion provides a rational explanation for the reversibility of the addition of iodine to the double bond. The halonium cation has three electrophilic centers accessible to nucleophilic attack by the halide anion: two carbon atoms and a halogen atom. In the case of chloronium ions, the Cl - anion appears to predominantly or even exclusively attack the carbon centers of the cation. For the bromonium cation, both directions of opening of the halonium ion are equally probable, both due to the attack of the bromide ion on both carbon atoms and on the bromine atom. Nucleophilic attack on the bromine atom of the bromonium ion leads to the initial reagents bromine and alkene:

The iodonium ion opens mainly as a result of the attack of the iodide ion on the iodine atom, and therefore the equilibrium between the initial reagents and the iodonium ion is shifted to the left.

In addition, the end product of the addition, vicinal diiodide, can undergo a nucleophilic attack on the iodine atom by the triiodide anion present in the solution, which also leads to the formation of the initial reagents alkene and iodine. In other words, under the conditions of the addition reaction, the resulting vicinal diiodide is deiodinated under the action of the triiodide anion. The vicinal dichlorides and dibromides are not dehalogenated under the conditions of the addition reaction of chlorine or bromine, respectively, to alkenes.

Anti-addition of chlorine or bromine is characteristic of alkenes, in which the double bond is not conjugated with the -electrons of the benzene ring. For styrene, stilbene and their derivatives along with anti-addition takes place and syn-addition of a halogen, which in polar environment may even become dominant.

In cases where the addition of a halogen to a double bond is carried out in a medium of nucleophilic solvents, the solvent effectively competes with the halide ion in opening the three-membered ring of the halonium ion:

The formation of addition products with the participation of a solvent or some other "external" nucleophilic agent is called a conjugated addition reaction. When bromine and styrene react in methanol, two products are formed: vicinal dibromide and bromoether, the ratio of which depends on the concentration of bromine in methanol

In a highly dilute solution, the product of conjugated addition dominates, while in a concentrated solution, on the contrary, the predominant vicinal dibromide. In an aqueous solution, halohydrin (an alcohol containing a halogen at the -carbon atom) always predominates - the product of conjugated addition.

her-conformer trance-2-chlorocyclohexanol additionally stabilized by O-H hydrogen bond . . . Cl. In the case of unsymmetrical alkenes, in conjugated addition reactions, the halogen always adds to the carbon atom containing the largest number of hydrogen atoms, and the nucleophilic agent to the carbon with fewer hydrogen atoms. An isomeric product with a different arrangement of the joining groups is not formed. This means that the cyclic halonium ion formed as an intermediate must have an asymmetric structure with two C 1 -Hal and C 2 -Hal bonds differing in energy and strength and a large positive charge on the internal C 2 carbon atom, which can be expressed graphically in two ways:

Therefore, the carbon atom C 2 of the halonium ion is subjected to nucleophilic attack by the solvent, despite the fact that it is more substituted and sterically less accessible.

One of the best preparative methods for the synthesis of bromhydrins is the hydroxybromination of alkenes with N-bromosuccinimide ( NBS) in a binary mixture of dimethyl sulfoxide ( DMSO) and water.

This reaction can be carried out in water and without DMSO, however, the yields of bromhydrins in this case are somewhat lower.

The formation of conjugated addition products in the alkene halogenation reaction also makes it possible to reject the synchronous mechanism of addition of one halogen molecule. The conjugated addition to the double bond is in good agreement with the two-step mechanism involving the halonium cation as an intermediate.

For the reaction of electrophilic addition to the double bond, one should expect an increase in the reaction rate in the presence of electron-donating alkyl substituents and its decrease in the presence of electron-withdrawing substituents at the double bond. Indeed, the rate of addition of chlorine and bromine to the double bond increases sharply upon passing from ethylene to its methyl-substituted derivatives. For example, the rate of addition of bromine to tetramethylethylene is 10 5 times higher than the rate of its addition to butene-1. Such a huge acceleration definitely indicates a high polarity of the transition state and a high degree of charge separation in the transition state and is consistent with an electrophilic addition mechanism.

In some cases, the addition of chlorine to alkenes containing electron-donating substituents is accompanied by the elimination of a proton from the intermediate instead of the addition of a chloride ion. Elimination of a proton leads to the formation of a chlorine-substituted alkene, which can formally be considered as a direct substitution with double bond migration. However, experiments with isotopic labeling indicate a more complex nature of the transformations occurring here. When isobutylene is chlorinated at 0 0 C, 2-methyl-3-chloropropene (metallyl chloride) is formed instead of the expected dichloride - the product of addition to the double bond.

Formally, it seems that there is a substitution, not an addition. The study of this reaction using isobutylene labeled at position 1 with the 14 C isotope showed that direct replacement of hydrogen by chlorine does not occur, since the label is in the 14 CH 2 Cl group in the formed metallyl chloride. This result can be explained by the following sequence of transformations:

In some cases, 1,2-migration of the alkyl group can also occur

In CCl 4 (non-polar solvent) this reaction gives almost 100% dichloride B- the product of a conventional double bond addition (without rearrangement).

Skeletal rearrangements of this type are most characteristic of processes involving open carbocations as intermediate particles. It is possible that the addition of chlorine in these cases does not occur through the chloronium ion, but through a cationic particle close to an open carbocation. At the same time, it should be noted that skeletal rearrangements are a rather rare phenomenon in the addition of halogens and mixed halogens to the double bond: they are more often observed in the addition of chlorine and much less frequently in the addition of bromine. The probability of such rearrangements increases upon passing from nonpolar solvents (СCl 4) to polar ones (nitromethane, acetonitrile).

Summarizing the above data on stereochemistry, conjugated addition, the effect of substituents in the alkene, and rearrangements in the double bond addition reactions of halogens, it should be noted that they are in good agreement with the electrophilic addition mechanism involving the cyclic halonium ion. The data on the addition of mixed halogens to alkenes, for which the addition steps are determined by the polarity of the bond of two halogen atoms, can be interpreted in the same way.

UNSATURATED OR UNSATURATED HYDROCARBONS OF THE ETHYLENE SERIES (ALKENES OR OLEFINS)

Alkenes, or olefins(from lat. olefiant - oil - an old name, but widely used in chemical literature. The reason for this name was ethylene chloride, obtained in the 18th century, is a liquid oily substance.) - aliphatic unsaturated hydrocarbons, in the molecules of which there is one double bond between carbon atoms.

Alkenes form a homologous series with general formula CnH2n

1. Homologous series of alkenes

Homologs:

WithH2 = CH2 ethene

WithH2 = CH- CH3 propene

WithH2=CH-CH2-CH3butene-1

WithH2=CH-CH2-CH2-CH3 pentene-1

2. Physical Properties

Ethylene (ethene) is a colorless gas with a very faint sweet smell, slightly lighter than air, slightly soluble in water.

C2 - C4 (gases)

C5 - C17 (liquids)

С18 - (solid)

Alkenes are insoluble in water, soluble in organic solvents (gasoline, benzene, etc.)

Lighter than water

With an increase in Mr, the melting and boiling points increase

3. The simplest alkene is ethylene - C2H4

Structural and electronic formula ethylene look like:

In the ethylene molecule, one s- and two p-orbitals of C atoms ( sp 2-hybridization).

Thus, each C atom has three hybrid orbitals and one non-hybrid orbital. p-orbitals. Two of the hybrid orbitals of C atoms mutually overlap and form between C atoms

σ - connection. The remaining four hybrid orbitals of C atoms overlap in the same plane with four s-orbitals of H atoms and also form four σ-bonds. Two non-hybrid p-orbitals of C atoms mutually overlap in a plane that is perpendicular to the plane σ - bond, i.e. one is formed P- connection.

By it's nature P- connection sharply differs from σ - connection; P- the bond is less strong due to the overlap of electron clouds outside the plane of the molecule. Under the influence of reagents P- the connection is easily broken.

The ethylene molecule is symmetrical; the nuclei of all atoms are located in the same plane and the bond angles are close to 120°; the distance between the centers of C atoms is 0.134 nm.

If the atoms are connected by a double bond, then their rotation is impossible without electron clouds P- the connection is not opened.

4. Isomerism of alkenes

Along with structural isomerism of the carbon skeleton alkenes are characterized, firstly, by other types of structural isomerism - multiple bond position isomerism and interclass isomerism.

Secondly, in the series of alkenes, spatial isomerism , associated with the different position of the substituents relative to the double bond, around which intramolecular rotation is impossible.

Structural isomerism of alkenes

1. Isomerism of the carbon skeleton (starting from C4H8):

2. Isomerism of the position of the double bond (starting from С4Н8):

3. Interclass isomerism with cycloalkanes, starting with C3H6:

Spatial isomerism of alkenes

The rotation of atoms around a double bond is impossible without breaking it. This is due to the structural features of the p-bond (the p-electron cloud is concentrated above and below the plane of the molecule). Due to the rigid attachment of atoms, rotational isomerism with respect to the double bond does not appear. But it becomes possible cis-trance-isomerism.

Alkenes that have different substituents on each of the two carbon atoms in the double bond can exist as two spatial isomers that differ in the arrangement of substituents relative to the p-bond plane. So, in the butene-2 ​​molecule CH3-CH=CH-CH3 CH3 groups can be either on one side of the double bond in cis-isomer, or on opposite sides in trance-isomer.

ATTENTION! cis-trans- Isomerism does not appear if at least one of the C atoms in the double bond has 2 identical substituents.

For example,

butene-1 CH2=CH-CH2-CH3 does not have cis- and trance-isomers, because The 1st C atom is bonded to two identical H atoms.

Isomers cis- and trance- differ not only in physical

,

but also chemical properties, tk. the approach or removal of parts of the molecule from each other in space promotes or hinders the chemical interaction.

Sometimes cis-trans isomerism is not exactly called geometric isomerism. The inaccuracy is that all spatial isomers differ in their geometry, and not only cis- and trance-.

5. Nomenclature

Simple alkenes are often named 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:

Unsaturated (alkene) radicals are called trivial names or according to the systematic nomenclature:

(H2C=CH-) vinyl or ethenyl

(Н2С=CH—CH2) allyl