Alkenes- unsaturated hydrocarbons, which contain one double bond. Examples of alkenes:

Methods for obtaining alkenes.

1. Cracking of alkanes at 400-700°C. The reaction occurs via a free radical mechanism:

2. Dehydrogenation of alkanes:

3. Elimination reaction (elimination): 2 atoms or 2 groups of atoms are eliminated from neighboring carbon atoms, and a double bond is formed. Such reactions include:

A) Dehydration of alcohols (heating above 150°C, with the participation of sulfuric acid as a water-removing reagent):

B) Elimination of hydrogen halides when exposed to an alcoholic alkali solution:

The hydrogen atom is split off preferentially from the carbon atom that is bonded to fewer hydrogen atoms (the least hydrogenated atom) - Zaitsev's rule.

B) Dehalogenation:

Chemical properties of alkenes.

The properties of alkenes are determined by the presence of a multiple bond, therefore alkenes enter into electrophilic addition reactions, which occur in several stages (H-X - reagent):

1st stage:

2nd stage:

.

The hydrogen ion in this type of reaction belongs to the carbon atom that has a more negative charge. The density distribution is:

If the substituent is a donor, which manifests the +I- effect, then the electron density shifts towards the most hydrogenated carbon atom, creating a partially negative charge on it. Reactions go according to Markovnikov's rule: when joining polar molecules like NH (HCl, HCN, HOH etc.) to unsymmetrical alkenes, hydrogen attaches preferentially to the more hydrogenated carbon atom at the double bond.

A) Addition reactions:
1) Hydrohalogenation:

The reaction follows Markovnikov's rule. But if peroxide is present in the reaction, then the rule is not taken into account:

2) Hydration. The reaction follows Markovnikov's rule in the presence of phosphoric or sulfuric acid:

3) Halogenation. As a result, bromine water becomes discolored - this is a qualitative reaction to a multiple bond:

4) Hydrogenation. The reaction occurs in the presence of catalysts.

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

Goals and objectives of the lesson:

• review the specific chemical properties of ethylene and the general properties of alkenes;
• deepen and concretize the concepts of?-bonds and the mechanisms of 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”.

PROGRESS OF THE LESSON

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 substances enter. 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.

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 the molecule, unsaturated hydrocarbons contain 2p there are fewer hydrogen atoms than the limiting ones, where p - number of π bonds:

A series whose members differ from each other by (2H) n is called isological series. Thus, in the above scheme, the isologs 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 of their homologous series is C p H 2l.

1. Nomenclature

In accordance with the IUPAC rules, when naming alkenes, the longest carbon chain containing a double bond is given the name of the corresponding alkane in which the ending -an replaced by -en. This chain is numbered in such a way that the carbon atoms involved in the formation of the double bond receive the lowest possible numbers:

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

For alkenes of relatively simple structure, simpler names are allowed. Thus, some of the most frequently occurring alkenes are named 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 -enil. The numbering in the radical starts from the carbon atom having a free valency. 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 called vinyl hydrogen atoms,

2. Isomerism

In addition to isomerism of the carbon skeleton, isomerism of the position of the double bond also appears in the series of alkenes. In general, this type of isomerism is isomerism of substituent position (function)- observed in all cases when the molecule contains any functional groups. For the alkane C4H10, two structural isomers are possible:

For the C4H8 alkene (butene), three isomers are possible:

Butene-1 and butene-2 ​​are isomers of the position of the function (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 located on the same side of the double bond, and trans isomers, if on opposite sides:

3. Structure of a double bond

The energy of cleavage of a molecule at the C=C double bond is 611 kJ/mol; since the energy of the C-C σ bond is 339 kJ/mol, the energy of breaking the π bond is only 611-339 = 272 kJ/mol. π -electrons are much lighter than σ -electrons and are susceptible to the influence of, for example, polarizing solvents or the influence of 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 flat structure of the vinyl fragment and with a shortened C-C distance equal to 0.134 nm, i.e. significantly smaller than the distance between carbon atoms connected by a single bond (0.154 nm). As the “halves” of the molecule rotate relative to each other along the double bond axis, the degree of orbital overlap decreases, which is associated with energy consumption. The consequence of this is the absence of free rotation along the double bond axis and the existence of geometric isomers with appropriate substitution at the carbon atoms.

4. Physical properties

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

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

5. Chemical properties

When considering the reactivity of complex organic compounds, a general principle applies. In most reactions, it is not the “inert” hydrocarbon radical that participates, but the existing functional groups and their immediate environment. This is natural, since most bonds are less strong than C-C and C-H bonds, and, in addition, the bonds in and near the functional group are the most polarized.

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

Hydrogen addition

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. The addition of hydrogen occurs 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 temperatures. The discovery of Sabatier, who used specially prepared finely crushed nickel at a temperature of 150-300°C and in numerous works demonstrated the versatility of this catalyst for a number of reduction reactions, was of great practical importance.

Addition of halogens

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

In the first stage of this reaction, interaction occurs between the π-electrons of the double bond and the electrophilic halogen species to form the π-complex (I). Next, the π-complex is rearranged into 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):

Anion attack of the bromonium ion (II) to form 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 halogen to alkenes is the fact that when Br 2 is added to cyclohexene in the presence of NaCl, not only trans-1,2-dibromocyclohexane, but also trans-1-bromo-2-chlorocyclohexane is formed:

Radical halogenation

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

In this case, the reaction follows a radical mechanism.

Addition of hydrogen halides

Hydrogen halides add to alkenes to form alkyl halides. In the case of asymmetric molecules, addition follows Markovnikov’s rule, i.e., hydrogen attaches to the most hydrogenated carbon atom (with the largest number of hydrogen atoms):

This reaction, like the addition of bromine to ethylene, occurs 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 (Kharash effect):

In the presence of peroxides, the reaction does not proceed 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 adds to propylene to form a new radical:

The latter is stabilized due to the expulsion of hydrogen from a new HBr molecule with the regeneration of a new bromine radical, etc.:

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

Addition of water and sulfuric acid

In the presence of acids, water adds 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 environment (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 follows the cis addition scheme:

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

Effect of ozone on alkenes

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

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

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 in principle both anionic and cationic initiation of the process were used.

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. In this case, polymers with a high degree of stereoregularity are formed. For example, when polymerizing 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.

Polyethylene obtained by this method is a saturated hydrocarbon with a straight chain. It is less elastic than polyethylene produced at high pressures, but has greater hardness and can 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 coating for the bottom of artificial canals to make them waterproof.

Telomerization

An interesting industrially used process is the copolymerization of ethylene with carbon tetrachloride, called telomerization. If benzoyl peroxide or another initiator that decomposes to form free radicals is added to a mixture of ethylene and CC14, the following process occurs:

CC1 3" radicals initiate chain polymerization of ethylene:

When it encounters another CC1 4 molecule, the 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 with values ​​obtained n =2,3, 4, ...,15.

The hydrolysis of telomerization products produces ω-chloro-substituted carboxylic acids, which are valuable chemical products.

Alkenes are unsaturated hydrocarbons which have one double bond between atoms. Their other name is olefins, it is associated with the history of the discovery of this class of compounds. Basically, these substances are not found in nature, but are synthesized by humans for practical purposes. In the IUPAC nomenclature, the names of these compounds are formed according to the same principle as for alkanes, only the suffix “an” is replaced by “ene”.

Structure of alkenes

The two carbon atoms involved in the formation of a double bond are always in sp2 hybridization, and the angle between them is 120 degrees. A double bond is formed by overlapping π -π orbitals, but it is not very strong, so this bond can be easily broken, which is used in the chemical properties of substances.

Isomerism

Compared to the limit values, in these hydrocarbons it is possible more types, moreover, both spatial and structural. Structural isomerism can also be divided into several types.

The first also exists for alkanes, and consists in a different order of connection of carbon atoms. Thus, pentene-2 ​​and 2-methylbutene-2 ​​can be isomers. And the second is a change in the position of the double bond.

Spatial isomerism in these compounds is possible due to the appearance of a double bond. It comes in two types - geometric and optical.

Geometric isomerism is one of the most common types in nature, and almost always geometric isomers will have radically different physical and chemical properties. Distinguish cis and trans isomers. In the former, the substituents are located on one side of the multiple bond, while in trans isomers they are in different planes.

Preparation of alkenes

They were first obtained, like many other substances, completely by accident.

The German chemist and researcher Becher at the end of the 17th century studied the effect of sulfuric acid on ethyl alcohol and realized that received unknown gas, which is more reactive than methane.

Later, several more scientists conducted similar studies, and they also learned that this gas, when interacting with chlorine, forms an oily substance.

Therefore, initially this class of compounds was given the name olefins, which translates as oily. But still, scientists were unable to determine the composition and structure of this compound. This happened only almost two centuries later, at the end of the nineteenth century.

Currently, there are many ways to obtain alkenes.

Industrial methods

Receipt industrial methods:

1. Dehydrogenation of saturated hydrocarbons. This reaction is possible only under the action of high temperatures (about 400 degrees) and catalysts - either chromium oxide 3 or platinum-alumina catalysts.
2. Dehalogenation of dihaloalkanes. Occurs only in the presence of zinc or magnesium, and at high temperatures.
3. Dehydrohalogenation of halogenated alkanes. It is carried out using sodium or potassium salts of organic acids at elevated temperatures.

Important! These methods for producing alkenes do not give a pure product; the result of the reaction will be a mixture of unsaturated hydrocarbons. The predominant connection among them is determined using Zaitsev's rule. It states that hydrogen is most likely to be removed from the carbon atom that has the fewest bonds with hydrogens.

Dehydration of alcohols. It can only be carried out with heating and in the presence of solutions of strong mineral acids that have water-removing properties.

Hydrogenation of alkynes. Possible only in the presence of paladium catalysts.

Chemical properties of alkenes

Alkenes are very chemically active substances. This is largely due to the presence of a double bond. The most characteristic reactions for this class of compounds are electrophilic and radical addition.

1. Halogenation of alkenes is a classic electrophilic addition reaction. It occurs only in the presence of inert organic solvents, most often carbon tetrachloride.
2. Hydrohalogenation. This type of adjunction is carried out according to Markovnikov's rule. The ion attaches to the more hydrogenated carbon atom near the double bond, and accordingly, the halide ion attaches to the second carbon atom. This rule is violated in the presence of peroxide compounds - the Harrosh effect. The addition of hydrogen halide occurs completely opposite to Markovnikov's rule.
3. Hydroboration. This reaction is of significant practical importance. Therefore, the scientist who discovered and studied it even received the Nobel Prize. This reaction is carried out in several steps, and the addition of boron ion does not occur according to Markovnikov’s rule.
4. Alkene hydration or addition. This reaction also proceeds according to Markovnikov’s rule. The hydroxide ion attaches to the least hydrogenated carbon atom at the double bond.
5. Alkylation is another reaction often used in industry. It consists in the addition of saturated hydrocarbons to unsaturated hydrocarbons under the influence of low temperatures and catalysts in order to increase the atomic mass of the compounds. The catalyst is most often strong mineral acids. This reaction can also occur via a free radical mechanism.
6. Polymerization of alkenes is another reaction uncharacteristic of saturated hydrocarbons. It involves the connection of numerous molecules with each other in order to form a strong compound that differs in its physical properties.

n in a given reaction is the number of molecules that come into contact. A prerequisite for implementation is an acidic environment, elevated temperature and increased pressure.

Alkenes are also characterized by other electrophilic addition reactions, which have not received such widespread practical distribution.

For example, the addition reaction of alcohols to form ethers.

Or the addition of acid chlorides to produce unsaturated ketones - the Kondakov reaction.

Pay attention! This reaction is possible only in the presence of a zinc chloride catalyst.

The next large class of reactions characteristic of alkenes are radical addition reactions. These reactions are possible only when free radicals are formed under the influence of high temperatures, irradiation and other actions. The most characteristic radical addition reaction is hydrogenation with the formation of saturated hydrocarbons. It occurs exclusively under the influence of temperatures and in the presence of a platinum catalyst.

Due to the presence of a double bond, alkenes are characterized by different oxidation reactions.

• Combustion is a classic oxidation reaction. It runs well without catalysts. Depending on the amount of oxygen, different end products are possible: from carbon dioxide to carbon.
• Oxidation with potassium permanganate in a neutral environment. The products are polyhydric alcohols and a brown precipitate of manganese dioxide. This reaction is considered qualitative for alkenes.
• Also, mild oxidation can be carried out with hydrogen peroxide, osmium oxide 8, and other oxidizing agents in a neutral environment. The mild oxidation of alkenes is characterized by the cleavage of only one bond; the reaction product, as a rule, is polyhydric alcohols.
• Hard oxidation is also possible, in which both bonds are broken and acids or ketones are formed. An acidic environment is a prerequisite; sulfuric acid is most often used, since other acids can also undergo oxidation to form by-products.

Unsaturated include hydrocarbons containing multiple bonds between carbon atoms in their molecules. Unlimited are alkenes, alkynes, alkadienes (polyenes). Cyclic hydrocarbons containing a double bond in the ring (cycloalkenes), as well as cycloalkanes with a small number of carbon atoms in the ring (three or four atoms) also have an unsaturated character. The property of “unsaturation” is associated with the ability of these substances to enter into addition reactions, primarily hydrogen, with the formation of saturated, or saturated, hydrocarbons - alkanes.

Structure of alkenes

Acyclic hydrocarbons containing in the molecule, in addition to single bonds, one double bond between carbon atoms and corresponding to the general formula C n H 2n.

Its second name is olefins- alkenes were obtained by analogy with unsaturated fatty acids (oleic, linoleic), the remains of which are part of liquid fats - oils (from the English oil - oil).

Carbon atoms that have a double bond between them are in a state sp 2 -hybridization. This means that one s and two p orbitals participate in hybridization, and one p orbital remains unhybridized.

The overlap of hybrid orbitals leads to the formation of a σ bond, and due to the unhybridized p orbitals of neighboring carbon atoms, a second, π bond is formed. Thus, a double bond consists of one σ- and one π-bond.

The hybrid orbitals of the atoms forming a double bond are in the same plane, and the orbitals forming a π bond are perpendicular to the plane of the molecule.

The double bond (0.132 nm) is shorter than the single bond, and its energy is higher, because it is stronger. However, the presence of a mobile, easily polarizable π bond leads to the fact that alkenes are chemically more active than alkanes and are able to undergo addition reactions.

Homologous series of alkenes

The first three members of the homologous series of alkenes are gases, from C 5 H 10 to C 17 H 34 are liquids, and from C 18 H 36 are solids. Liquid and solid alkenes are practically insoluble in water, but are highly soluble in organic solvents.

In accordance with IUPAC rules, the suffix -ene is used in the names of homologs of a number of alkenes. The position of the double bond is indicated by a number indicating the location of the bond. The number is placed after the name of the main chain separated by a hyphen. The numbering of atoms in an alkene molecule begins from the end to which the bond is closest, for example, an alkene corresponding to the formula CH 3 −CH 2 −CH=CH−CH 3 should be called penten-2, since the bond begins at the second carbon atom, starting from the end chains.

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

Isomerism and nomenclature of alkenes

For alkenes, as well as for alkanes, it is characteristic structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkene, characterized by structural isomers, is butene.

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

Almost free rotation of carbon atoms is possible around a single carbon-carbon bond, so alkane molecules can take on a wide variety of shapes. Rotation around the double bond is impossible, which leads to the appearance of another type of isomerism in alkenes - geometric, or cis-trans isomerism.

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

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

The IUPAC nomenclature for alkenes is similar to that for alkanes.

1. Main circuit selection. The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in the molecule. In the case of alkenes, the main chain must contain a double bond.

2. Numbering of main chain atoms. The numbering of the atoms of the main chain begins from the end to which the double bond is closest. For example, the correct connection name is:

5-methylhexene-2, not 2-methylhexene-4, as one might expect.

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

3. Formation of the name. The names of alkenes are formed in the same way as the names of alkanes. At the end of the name, indicate the number of the carbon atom at which the double bond begins, and the suffix -ene, indicating that the compound belongs to the class of alkenes. For example:

Physical properties of alkenes

First three representatives of the homologous series of alkenes- gases; substances of the composition C 5 H 10 - C 16 H 32 - liquids; Higher alkenes are solids.

Boiling and melting points naturally increase with increasing molecular weight of compounds.

Chemical properties of alkenes

Addition reactions. Let us recall that a distinctive feature of representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed according to the mechanism electrophilic addition.

1. Hydrogenation of alkenes. Alkenes are capable of adding hydrogen in the presence of hydrogenation catalysts, metals - platinum, palladium, nickel:

This reaction occurs at atmospheric and elevated pressure and does not require high temperature, because it is exothermic. When the temperature increases, the same catalysts can cause a reverse reaction - dehydrogenation.

2. Halogenation(addition of halogens). The interaction of an alkene with bromine water or a solution of bromine in an organic solvent (CCl 4) leads to rapid discoloration of these solutions as a result of the addition of a halogen molecule to the alkene and the formation of dihaloalkanes:

3. Hydrohalogenation(addition of hydrogen halide).

This reaction obeys Markovnikov's rule:

When a hydrogen halide attaches to an alkene, the hydrogen attaches to the more hydrogenated carbon atom, i.e., the atom at which there are more hydrogen atoms, and the halogen to the less hydrogenated one.

4. Hydration(water connection). Hydration of alkenes leads to the formation of alcohols. For example, the addition of water to ethene underlies one of the industrial methods for producing ethyl alcohol:

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

This reaction also proceeds according to Markovnikov's rule- a hydrogen cation is attached to a more hydrogenated carbon atom, and a hydroxo group is added to a less hydrogenated one.

5. Polymerization. A special case of addition is the polymerization reaction of alkenes:

This addition reaction occurs via a free radical mechanism.

Oxidation reactions.

1. Combustion. Like any organic compounds, alkenes burn in oxygen to form CO 2 and H 2 O:

2. Oxidation in solutions. Unlike alkanes, alkenes are easily oxidized by potassium permanganate solutions. In neutral or alkaline solutions, alkenes are oxidized to diols (dihydric alcohols), and hydroxyl groups are added to those atoms between which a double bond existed before oxidation: