Alkenes definition general formula. Chemical properties of alkenes

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

General formula

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

Features of the nomenclature

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

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

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

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

Hybridization in alkenes

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

Physical properties

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

Features of isomerism

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

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

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

Specifics of chemical properties

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

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

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

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

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

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

Let us give several specific examples of such transformations. What does the hydration of alkenes lead to? Examples offered in school course chemistry, indicate that propanol-2 can be obtained from propene by reacting with water, and butanol-2 can be obtained from butene-1.

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

Halogenation

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

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

Hydrohalogenation

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

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

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

Oxidation

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

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

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

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

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

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

Polymerization

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

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

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

Among the products obtained by the polymerization of alkenes having practical significance, let's highlight polyvinyl chloride. This connection obtained by polymerization of vinyl chloride. The resulting product has valuable performance characteristics. It is characterized by increased resistance to aggressive influences chemicals, non-flammable, easy to paint. What can be made from polyvinyl chloride? Briefcases, raincoats, oilcloth, artificial leather, cables, electrical wire insulation.

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

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

Features of obtaining alkenes

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

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

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

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

Some Applications

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

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

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

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

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

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

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

The physical properties of alkenes are similar to those of alkanes, although they all have slightly more low temperatures melting and boiling than the corresponding alkanes. For example, pentane has a boiling point of 36 °C, and pentene-1 - 30 °C. Under normal conditions, alkenes C 2 - C 4 are gases. C 5 – C 15 are liquids, starting from C 16 are solids. Alkenes are insoluble in water but highly soluble in organic solvents.

Alkenes are rare in nature. Since alkenes are valuable raw materials for industrial organic synthesis, many methods for their preparation have been developed.

1. The main industrial source of alkenes is the cracking of alkanes that are part of oil:

3. In laboratory conditions, alkenes are obtained by elimination reactions, in which two atoms or two groups of atoms are eliminated from neighboring carbon atoms, and an additional p-bond is formed. Such reactions include the following.

1) Dehydration of alcohols occurs when they are heated with water-removing agents, for example with sulfuric acid at temperatures above 150 ° C:

When H 2 O is eliminated from alcohols, HBr and HCl from alkyl halides, the hydrogen atom is preferentially eliminated from that of the neighboring carbon atoms that is bonded to the smallest number hydrogen atoms (from the least hydrogenated carbon atom). This pattern is called Zaitsev's rule.

3) Dehalogenation occurs when dihalides that have halogen atoms at adjacent carbon atoms are heated with active metals:

CH 2 Br -CHBr -CH 3 + Mg → CH 2 =CH-CH 3 + Mg Br 2.

Chemical properties alkenes are determined by the presence of a double bond in their molecules. The electron density of the p-bond is quite mobile and easily reacts with electrophilic particles. Therefore, many reactions of alkenes proceed according to the mechanism electrophilic addition, designated by the symbol A E (from English, addition electrophilic). Electrophilic addition reactions are ionic processes that occur in several stages.

In the first stage, an electrophilic particle (most often this is an H + proton) interacts with the p-electrons of the double bond and forms a p-complex, which is then converted into a carbocation by forming a covalent s-bond between the electrophilic particle and one of the carbon atoms:

alkene p-complex carbocation

In the second stage, the carbocation reacts with the X - anion, forming a second s-bond due to the electron pair of the anion:

In electrophilic addition reactions, a hydrogen ion attaches to the carbon atom at the double bond that has a greater negative charge. The charge distribution is determined by the shift in p-electron density under the influence of substituents: .

Electron-donating substituents exhibiting the +I effect shift the p-electron density to a more hydrogenated carbon atom and create a partial negative charge on it. This explains Markovnikov's rule: when adding polar molecules like HX (X = Hal, OH, CN, etc.) to unsymmetrical alkenes, hydrogen preferentially attaches to the more hydrogenated carbon atom at the double bond.

Let's look at specific examples of addition reactions.

1) Hydrohalogenation. When alkenes interact with hydrogen halides (HCl, HBr), alkyl halides are formed:

CH 3 -CH = CH 2 + HBr ® CH 3 -CHBr-CH 3 .

The reaction products are determined by Markovnikov's rule.

It should, however, be emphasized that in the presence of any organic peroxide, polar HX molecules do not react with alkenes according to Markovnikov’s rule:

	R-O-O-R
CH 3 -CH = CH 2 + HBr		CH 3 -CH 2 -CH 2 Br

This is due to the fact that the presence of peroxide determines the radical rather than ionic mechanism of the reaction.

2) Hydration. When alkenes react with water in the presence of mineral acids (sulfuric, phosphoric), alcohols are formed. Mineral acids act as catalysts and are sources of protons. The addition of water also follows Markovnikov’s rule:

CH 3 -CH = CH 2 + HON ® CH 3 -CH (OH) -CH 3 .

3) Halogenation. Alkenes discolor bromine water:

CH 2 = CH 2 + Br 2 ® B-CH 2 -CH 2 Br.

This reaction is qualitative for a double bond.

4) Hydrogenation. The addition of hydrogen occurs under the action of metal catalysts:

where R = H, CH 3, Cl, C 6 H 5, etc. The CH 2 =CHR molecule is called a monomer, the resulting compound is called a polymer, the number n is the degree of polymerization.

Polymerization of various alkene derivatives produces valuable industrial products: polyethylene, polypropylene, polyvinyl chloride and others.

In addition to addition, alkenes also undergo oxidation reactions. During the mild oxidation of alkenes with an aqueous solution of potassium permanganate (Wagner reaction), dihydric alcohols are formed:

ZSN 2 =CH 2 + 2KMn O 4 + 4H 2 O ® ZNOSN 2 -CH 2 OH + 2MnO 2 ↓ + 2KOH.

As a result of this reaction, the purple solution of potassium permanganate quickly becomes discolored and a brown precipitate of manganese (IV) oxide precipitates. This reaction, like the decolorization reaction of bromine water, is qualitative for a double bond. During the severe oxidation of alkenes with a boiling solution of potassium permanganate in an acidic environment, complete cleavage of the double bond occurs with the formation of ketones, carboxylic acids or CO 2, for example:

	[ABOUT]
CH 3 -CH=CH-CH 3		2CH 3 -COOH

Based on the oxidation products, the position of the double bond in the original alkene can be determined.

Like all other hydrocarbons, alkenes burn and, with plenty of air, form carbon dioxide and water:

C n H 2 n + Zn /2O 2 ® n CO 2 + n H 2 O.

When air is limited, combustion of alkenes can lead to the formation of carbon monoxide and water:

C n H 2n + nO 2 ® nCO + nH 2 O .

If you mix an alkene with oxygen and pass this mixture over a silver catalyst heated to 200°C, an alkene oxide (epoxyalkane) is formed, for example:

At any temperature, alkenes are oxidized by ozone (ozone is a stronger oxidizing agent than oxygen). If ozone gas is passed through a solution of an alkene in methane tetrachloride at temperatures below room temperature, an addition reaction occurs and the corresponding ozonides (cyclic peroxides) are formed. Ozonides are very unstable and can explode easily. Therefore, they are usually not isolated, but immediately after production they are decomposed with water - this produces carbonyl compounds (aldehydes or ketones), the structure of which indicates the structure of the alkene that was subjected to ozonation.

Lower alkenes are important starting materials for industrial organic synthesis. Ethyl alcohol, polyethylene, and polystyrene are produced from ethylene. Propene is used for the synthesis of polypropylene, phenol, acetone, and glycerol.

Alkenes are unsaturated aliphatic hydrocarbons with one or more carbon-carbon double bonds. A double bond transforms two carbon atoms into a planar structure with bond angles between adjacent bonds of 120°C:

Homologous series alkenes have a general formula; its first two members are ethene (ethylene) and propene (propylene):

Members of the alkene series with four or a large number carbon atoms exhibit isomerism in bond positions. For example, an alkene with the formula has three isomers, two of which are bond position isomers:

Note that the alkene chain is numbered from the end closest to the double bond. The position of the double bond is indicated by the lower of the two numbers, which correspond to the two carbon atoms connected by the double bond. The third isomer has a branched structure:

The number of isomers of any alkene increases with the number of carbon atoms. For example, hexene has three bond position isomers:

The diene is buta-1,3-diene, or simply butadiene:

Compounds containing three double bonds are called trienes. Compounds with multiple double bonds have common name polyenes.

Physical properties

Alkenes have slightly lower melting and boiling points than their corresponding alkanes. For example, pentane has a boiling point. Ethylene, propene and three isomers of butene at room temperature and normal pressure are in a gaseous state. Alkenes with the number of carbon atoms from 5 to 15 are in a liquid state under normal conditions. Their volatility, like that of alkanes, increases in the presence of branching in the carbon chain. Alkenes with more than 15 carbon atoms are solids under normal conditions.

Obtained in laboratory conditions

The two main methods for producing alkenes in the laboratory are the dehydration of alcohols and the dehydrohalogenation of haloalkanes. For example, ethylene can be obtained by dehydration of ethanol under the action of an excess of concentrated sulfuric acid at a temperature of 170 ° C (see section 19.2):

Ethylene can also be produced from ethanol by passing ethanol vapor over the surface of heated alumina. For this purpose, you can use the installation schematically shown in Fig. 18.3.

The second common method for the preparation of alkenes is based on the dehydrohalogenation of halogenated alkanes under basic catalysis conditions

The mechanism of this type of elimination reaction is described in Section. 17.3.

Alkene reactions

Alkenes are much more reactive than alkanes. This is due to the ability of the -electrons of the double bond to attract electrophiles (see Section 17.3). Therefore, the characteristic reactions of alkenes are mainly electrophilic addition reactions at the double bond:

Many of these reactions have ionic mechanisms (see Section 17.3).

Hydrogenation

If some alkene, for example ethylene, is mixed with hydrogen and passed this mixture over the surface of a platinum catalyst at room temperature or a nickel catalyst at a temperature of about 150 ° C, then addition will occur

hydrogen at the double bond of the alkene. This produces the corresponding alkane:

This type of reaction is an example of heterogeneous catalysis. Its mechanism is described in Section. 9.2 and is shown schematically in Fig. 9.20.

Addition of halogens

Chlorine or bromine easily adds to the double bond of the alkene; this reaction occurs in non-polar solvents, such as tetrachloromethane or hexane. The reaction proceeds by an ionic mechanism, which involves the formation of a carbocation. The double bond polarizes the halogen molecule, turning it into a dipole:

Therefore, a solution of bromine in hexane or tetrachloromethane becomes colorless when shaken with an alkene. The same thing happens if you shake an alkene with bromine water. Bromine water is a solution of bromine in water. This solution contains hypobromous acid. A hypobromous acid molecule attaches to the double bond of the alkene, resulting in the formation of a brominated alcohol. For example

Addition of hydrogen halides

The mechanism of this type of reaction is described in Section. 18.3. As an example, consider the addition of hydrogen chloride to propene:

Note that the product of this reaction is 2-chloropropane, not 1-chloropropane:

In such addition reactions, the most electronegative atom or the most electronegative group always adds to the carbon atom bonded to

the smallest number of hydrogen atoms. This pattern is called Markovnikov's rule.

The preferential attachment of an electronegative atom or group to the carbon atom associated with the smallest number of hydrogen atoms is due to the increase in the stability of the carbocation as the number of alkyl substituents on the carbon atom increases. This increase in stability is in turn explained by the inductive effect that occurs in alkyl groups, since they are electron donors:

In the presence of any organic peroxide, propene reacts with hydrogen bromide, i.e., not according to Markovnikov’s rule. Such a product is called anti-Markovnikov. It is formed as a result of a reaction occurring by a radical rather than an ionic mechanism.

Hydration

Alkenes react with cold concentrated sulfuric acid to form alkyl hydrogen sulfates. For example

This reaction is an addition because it involves the addition of an acid at a double bond. It is the reverse reaction to the dehydration of ethanol to form ethylene. The mechanism of this reaction is similar to the mechanism of addition of hydrogen halides at the double bond. It involves the formation of a carbocation intermediate. If the product of this reaction is diluted with water and heated gently, it hydrolyzes to form ethanol:

The reaction of addition of sulfuric acid to alkenes obeys Markovnikov’s rule:

Reaction with acidified solution of potassium permanganate

The violet color of an acidified solution of potassium permanganate disappears if this solution is shaken in a mixture with any alkene. Hydroxylation of the alkene occurs (the introduction of a hydroxy group formed as a result of oxidation), which as a result is converted into a diol. For example, when an excess amount of ethylene is shaken with an acidified solution, ethane-1,2-diol (ethylene glycol) is formed.

If an alkene is shaken with an excess amount of -ion solution, oxidative cleavage of the alkene occurs, leading to the formation of aldehydes and ketones:

The aldehydes formed in this case undergo further oxidation to form carboxylic acids.

Hydroxylation of alkenes to form diols can also be carried out using an alkaline solution of potassium permanganate.

Reaction with perbenzoic acid

Alkenes react with peroxyacids (peracids), such as perbenzoic acid, to form cyclic ethers (epoxy compounds). For example

When epoxyethane is gently heated with a dilute solution of an acid, ethane-1,2-diol is formed:

Reactions with oxygen

Like all other hydrocarbons, alkenes burn and, with plenty of air, form carbon dioxide and water:

With limited air access, combustion of alkenes leads to the formation of carbon monoxide and water:

Because alkenes have a higher relative carbon content than the corresponding alkanes, they burn with a smokier flame. This is due to the formation of carbon particles:

If you mix any alkene with oxygen and pass this mixture over the surface of a silver catalyst, epoxyethane is formed at a temperature of about 200 ° C:

Ozonolysis

When ozone gas is passed through a solution of an alkene in trichloromethane or tetrachloromethane at temperatures below 20 °C, the ozonide of the corresponding alkene (oxirane) is formed.

Ozonides are unstable compounds and can be explosive. They undergo hydrolysis to form aldehydes or ketones. For example

In this case, part of the methanal (formaldehyde) reacts with hydrogen peroxide, forming methane (formic) acid:

Polymerization

The simplest alkenes can polymerize to form high molecular weight compounds that have the same empirical formula as the parent alkene:

This reaction occurs at high blood pressure, temperature 120°C and in the presence of oxygen, which plays the role of a catalyst. However, ethylene polymerization can be carried out at lower pressure if a Ziegler catalyst is used. One of the most common Ziegler catalysts is a mixture of triethylaluminum and titanium tetrachloride.

The polymerization of alkenes is discussed in more detail in Section. 18.3.

Knowledge Hypermarket >>Chemistry >>Chemistry 10th grade >> Chemistry: Alkenes

Unsaturated include hydrocarbons containing multiple bonds between carbon atoms in their molecules. Unsaturated 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

Alkenes are acyclic, 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.

Alkenes received their second name - “olefins” 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, as you know, are in a state of sp 2 hybridization. This means that one s and two p orbitals are involved in hybridization, and one p orbital remains unhybridized. The overlap of the hybrid orbitals leads to the formation of an a-bond, and due to the unhybridized -orbitals of the neighboring carbon atoms of the ethylene molecule, a second one is formed, n-connection. Thus, a double bond consists of one Þ-bond and one p-bond.

The hybrid orbitals of the atoms forming a double bond are in the same plane, and the orbitals forming an n-bond are located perpendicular to the plane of the molecule (see Fig. 5).

The double bond (0.132 nm) is shorter than the single bond, and its energy is higher, i.e. it is stronger. Nevertheless, the presence of a mobile, easily polarizable 7g-bond leads to the fact that alkenes are chemically more active than alkanes and are capable of entering into addition reactions.

Homologous series of ethene

Unbranched alkenes form the homologous series of ethene (ethylene).

C2H4 - ethene, C3H6 - propene, C4H8 - butene, C5H10 - pentene, C6H12 - hexene, etc.

Isomerism and nomenclature

Alkenes, like alkanes, are characterized by structural isomerism. Structural isomers, as you remember, differ from each other in the structure of the carbon skeleton. The simplest alkene, characterized by structural isomers, is butene.

CH3-CH2-CH=CH2 CH3-C=CH2
l
CH3
butene-1 methylpropene

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

CH3-CH2-CH=CH2 CH3-CH=CH-CH3
butene-1 butene-2

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 thorax isomers in the spatial arrangement of molecular fragments (in this case, methyl groups) relative to the plane n-connections, and therefore properties.

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

CH2 = CH-CH2-CH2-CH2-CH3
hexene-1 cyclohexane

Nomenclature alkenes, developed by IUPAC, is similar to the nomenclature of 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 atoms of the main chain

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

dn3-dn-dn2-dn=dn-dn3 dn3

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.

CH3- CH2-CH=CH-CH-CH3
l
CH3
2-methylhexene-3

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 indicating that the compound belongs to the class of alkenes, -ene.

Receipt

1. Cracking of petroleum products. In the process of thermal cracking of saturated hydrocarbons, along with the formation of alkanes, the formation of alkenes occurs.

2. Dehydrogenation of saturated hydrocarbons. When alkanes are passed over a catalyst at high temperatures (400-600 °C), a hydrogen molecule is eliminated and an alkene is formed:

3. Dehydration of alcohols (elimination of water). The effect of water-removing agents (H2804, Al203) on monohydric alcohols at high temperatures leads to the elimination of a water molecule and the formation of a double bond:

This reaction is called intramolecular dehydration (in contrast to intermolecular dehydration, which leads to the formation of ethers and will be studied in § 16 “Alcohols”).

4. Dehydrohalogenation (elimination of hydrogen halide).

When a haloalkane reacts with an alkali in an alcohol solution, a double bond is formed as a result of the elimination of a hydrogen halide molecule.

Note that this reaction produces predominantly butene-2 rather than butene-1, which corresponds to Zaitsev's rule:

When a hydrogen halide is eliminated from secondary and tertiary haloalkanes, a hydrogen atom is eliminated from the least hydrogenated carbon atom.

5. Dehalogenation. When zinc acts on a dibromo derivative of an alkane, halogen atoms located at neighboring carbon atoms are eliminated and a double bond is formed:

Physical properties

The first three representatives of the homologous series of alkenes are gases, substances of the composition C5H10-C16H32 are liquids, and higher alkenes are solids.

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

Chemical properties

Addition reactions

Let us remind you that distinctive feature representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed by the electrophilic addition mechanism.

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

CH3-CH2-CH=CH2 + H2 -> CH3-CH2-CH2-CH3

This reaction occurs at both atmospheric and elevated pressure and does not require high temperature, since 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 (CCl4) 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.

Markovnikov Vladimir Vasilievich

(1837-1904)

Russian organic chemist. Formulated (1869) rules on the direction of substitution, elimination, addition at a double bond and isomerization reactions depending on chemical structure. He studied (since 1880) the composition of oil and laid the foundations of petrochemistry as an independent science. Opened (1883) new class organic matter- cyclo-paraffins (naphthenes).

3. Hydrohalogenation (addition of hydrogen halide).

The hydrogen halide addition reaction will be discussed in more detail below. 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 (addition of water). Hydration of alkenes leads to the formation of alcohols. For example, the addition of water to ethene underlies one of the industrial methods obtaining ethyl alcohol:

CH2=CH2 + H2O -> CH3-CH2OH
ethene ethanol

Note that a primary alcohol (with a hydroxy 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 in accordance with Markovnikov's rule - a hydrogen cation attaches to a more hydrogenated carbon atom, and a hydroxy group attaches 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

Like any organic compounds, alkenes burn in oxygen to form CO2 and H20.

Unlike alkanes, which are resistant to oxidation in solutions, alkenes are easily oxidized by the action of aqueous solutions of potassium permanganate. In neutral or slightly alkaline solutions, oxidation of alkenes to diols (dihydric alcohols) occurs, and hydroxyl groups are added to those atoms between which a double bond existed before oxidation.

As you already know, unsaturated hydrocarbons - alkenes are capable of entering into addition reactions. Most of these reactions proceed by the electrophilic addition mechanism.

Electrophilic connection

Electrophilic reactions are reactions that occur under the influence of electrophiles - particles that have a lack of electron density, for example, an unfilled orbital. The simplest electrophilic particle is the hydrogen cation. It is known that the hydrogen atom has one electron in the 3rd orbital. A hydrogen cation is formed when an atom loses this electron, thus the hydrogen cation has no electrons at all:

Н· - 1е - -> Н +

In this case, the cation has a fairly high electron affinity. The combination of these factors makes the hydrogen cation a fairly strong electrophilic particle.

The formation of a hydrogen cation is possible during the electrolytic dissociation of acids:

НВr -> Н + + Вr -

It is for this reason that many electrophilic reactions occur in the presence and participation of acids.

Electrophilic particles, as mentioned earlier, act on systems containing areas of increased electron density. An example of such a system is a multiple (double or triple) carbon-carbon bond.

You already know that carbon atoms between which a double bond is formed are in a state of sp 2 hybridization. Unhybridized p-orbitals of neighboring carbon atoms located in the same plane overlap, forming n-bond, which is less strong than the Þ-bond, and, most importantly, is easily polarized under the influence of external electric field. This means that when a positively charged particle approaches, the electrons of the CS bond shift towards it and a so-called p- complex.

It turns out n-complex and upon addition of a hydrogen cation to n- connections. The hydrogen cation seems to bump into the electron density protruding from the plane of the molecule n-connection and joins it.

At the next stage, a complete displacement of the electron pair occurs n-bond to one of the carbon atoms, which leads to the appearance of a lone pair of electrons on it. The orbital of the carbon atom on which this pair is located and the unoccupied orbital of the hydrogen cation overlap, which leads to the formation of a covalent bond through the donor-acceptor mechanism. The second carbon atom still has an unfilled orbital, i.e., a positive charge.

The resulting particle is called a carbocation because it contains a positive charge on the carbon atom. This particle can combine with any anion, a particle that has a lone electron pair, i.e., a nucleophile.

Let us consider the mechanism of the electrophilic addition reaction using the example of hydrobromination (addition of hydrogen bromide) of ethene:

СН2= СН2 + НВг --> СНВr-СН3

The reaction begins with the formation of an electrophilic particle - a hydrogen cation, which occurs as a result of the dissociation of a hydrogen bromide molecule.

Hydrogen cation attacks n- connection, forming n- a complex that is quickly converted into a carbocation:

Now let's look at a more complex case.

The reaction of the addition of hydrogen bromide to ethene proceeds unambiguously, and the interaction of hydrogen bromide with propene can theoretically give two products: 1-bromopropane and 2-bromopropane. Experimental data show that 2-bromopropane is mainly produced.

In order to explain this, we will have to consider the intermediate particle - the carbocation.

The addition of a hydrogen cation to propene can lead to the formation of two carbocations: if a hydrogen cation joins the first carbon atom, the atom located at the end of the chain, then the second one will have a positive charge, i.e., in the center of the molecule (1); if it joins the second one, then the first atom (2) will have a positive charge.

The preferential direction of the reaction will depend on which carbocation is more abundant in the reaction medium, which, in turn, is determined by the stability of the carbocation. The experiment shows the predominant formation of 2-bromopropane. This means that in to a greater extent a carbocation (1) with a positive charge on the central atom is formed.

The greater stability of this carbocation is explained by the fact that the positive charge on the central carbon atom is compensated by the positive inductive effect of two methyl groups, the total effect of which is higher than the +/- effect of one ethyl group:

The laws of the reactions of hydrohalogenation of alkenes were studied by the famous Russian chemist V.V. Markovnikov, a student of A.M. Butlerov, who, as mentioned above, formulated the rule that bears his name.

This rule was established empirically, that is, experimentally. At present, we can give a completely convincing explanation for it.

Interestingly, other electrophilic addition reactions also obey Markovnikov’s rule, so it would be correct to formulate it in a more general form.

In electrophilic addition reactions, an electrophile (a particle with an unfilled orbital) adds to a more hydrogenated carbon atom, and a nucleophile (a particle with a lone pair of electrons) adds to a less hydrogenated one.

Polymerization

A special case of addition reaction is the polymerization reaction of alkenes and their derivatives. This reaction proceeds by the free radical addition mechanism:

Polymerization is carried out in the presence of initiators - peroxide compounds, which are a source free radicals. Peroxide compounds are substances whose molecules include the -O-O- group. The simplest peroxide compound is hydrogen peroxide HOOH.

At a temperature of 100 °C and a pressure of 100 MPa, homolysis of the unstable oxygen-oxygen bond and the formation of radicals - initiators of polymerization - occur. Under the influence of KO- radicals, polymerization is initiated, which develops as a free radical addition reaction. Chain growth stops when recombination of radicals occurs in the reaction mixture - the polymer chain and radicals or COCH2CH2-.

Using the reaction of free radical polymerization of substances containing a double bond, we obtain large number high molecular weight compounds:

The use of alkenes with various substituents makes it possible to synthesize a wide range of polymeric materials with a wide range of properties.

All these polymer compounds are found wide application in a variety of areas human activity- industry, medicine, used for the manufacture of equipment for biochemical laboratories, some are intermediates for the synthesis of other high-molecular compounds.

Oxidation

You already know that in neutral or slightly alkaline solutions, oxidation of alkenes to diols (dihydric alcohols) occurs. In an acidic environment (a solution acidified with sulfuric acid), the double bond is completely destroyed and the carbon atoms between which the double bond existed are converted into carbon atoms of the carboxyl group:

Destructive oxidation of alkenes can be used to determine their structure. So, for example, if acetic and propionic acids are obtained during the oxidation of a certain alkene, this means that pentene-2 has undergone oxidation, and if butyric acid and carbon dioxide, then the starting hydrocarbon is pentene-1.

Application

Alkenes are widely used in the chemical industry as raw materials for the production of a variety of organic substances and materials.

For example, ethene is the starting material for the production of ethanol, ethylene glycol, epoxides, and dichloroethane.

A large amount of ethene is processed into polyethylene, which is used to make packaging film, tableware, pipes, and electrical insulating materials.

Glycerin, acetone, isopropanol, and solvents are obtained from propene. By polymerizing propene, polypropylene is obtained, which is superior to polyethylene in many respects: it has more high temperature melting, chemical resistance.

Currently, polymers - analogues of polyethylene are used to produce fibers with unique properties. For example, polypropylene fiber is stronger than all known synthetic fibers.

Materials made from these fibers are promising and are increasingly used in various areas of human activity.

1. What types of isomerism are characteristic of alkenes? Write the formulas for possible isomers of pentene-1.
2. From what compounds can be obtained: a) isobutene (2-methylpropene); b) butene-2; c) butene-1? Write the equations for the corresponding reactions.
3. Decipher the following chain of transformations. Name compounds A, B, C. 4. Suggest a method for obtaining 2-chloropropane from 1-chloropropane. Write the equations for the corresponding reactions.
5. Suggest a method for purifying ethane from ethylene impurities. Write the equations for the corresponding reactions.
6. Give examples of reactions that can be used to distinguish between saturated and unsaturated hydrocarbons.
7. For complete hydrogenation of 2.8 g of alkene, 0.896 liters of hydrogen (n.e.) were consumed. What is the molecular weight and structural formula of this compound, which has a normal chain of carbon atoms?
8. What gas is in the cylinder (ethene or propene), if it is known that the complete combustion of 20 cm3 of this gas required 90 cm3 (n.s.) of oxygen?
9*. When an alkene reacts with chlorine in the dark, 25.4 g of dichloride is formed, and when this alkene of the same mass reacts with bromine in carbon tetrachloride, 43.2 g of dibromide is formed. Determine all possible structural formulas of the starting alkene.

History of discovery

From the above material, we have already understood that ethylene is the ancestor of the homologous series of unsaturated hydrocarbons, which has one double bond. Their formula is C n H 2n and they are called alkenes.

In 1669, the German physician and chemist Becher was the first to obtain ethylene by reacting sulfuric acid with ethyl alcohol. Becher found that ethylene is more chemically active than methane. But, unfortunately, at that time the scientist could not identify the resulting gas, and therefore did not assign any name to it.

A little later, Dutch chemists used the same method for producing ethylene. And since, when interacting with chlorine, it tended to form an oily liquid, it accordingly received the name “oil gas.” Later it became known that this liquid was dichloroethane.

In French the term “oil-bearing” sounds like oléfiant. And after other hydrocarbons of this type were discovered, Antoine Fourcroix, a French chemist and scientist, introduced a new term that became common to the entire class of olefins or alkenes.

But already at the beginning of the nineteenth century, the French chemist J. Gay-Lussac discovered that ethanol consists not only of “oil” gas, but also of water. In addition, the same gas was discovered in ethyl chloride.

And although chemists determined that ethylene consists of hydrogen and carbon, and already knew the composition of the substances, they could not find its real formula for a long time. And only in 1862 E. Erlenmeyer managed to prove the presence of a double bond in the ethylene molecule. This was also recognized by the Russian scientist A.M. Butlerov and confirmed the correctness of this point of view experimentally.

Occurrence in nature and physiological role of alkenes

Many people are interested in the question of where alkenes can be found in nature. So, it turns out that they practically do not occur in nature, since its simplest representative, ethylene, is a hormone for plants and is synthesized in them only in small quantities.

It is true that in nature there is such an alkene as muskalur. This one of the natural alkenes is a sexual attractant of the female house fly.

It is worth paying attention to the fact that, having a high concentration, lower alkenes have a narcotic effect, which can cause convulsions and irritation of the mucous membranes.

Applications of alkenes

Life modern society Today it is difficult to imagine without the use of polymer materials. Since, unlike natural materials, polymers have different properties, they are easy to process, and if you look at the price, they are relatively cheap. Another important aspect in favor of polymers is that many of them can be recycled.

Alkenes have found their use in the production of plastics, rubbers, films, Teflon, ethyl alcohol, acetaldehyde and other organic compounds.

IN agriculture it is used as a means that accelerates the ripening process of fruits. Propylene and butylenes are used to produce various polymers and alcohols. But in the production of synthetic rubber, isobutylene is used. Therefore, we can conclude that it is impossible to do without alkenes, since they are the most important chemical raw materials.

Industrial uses of ethylene

IN industrial scale propylene is usually used for the synthesis of polypropylene and for the production of isopropanol, glycerol, butyraldehydes, etc. Every year the demand for propylene increases.

UNSATURATED OR UNSATURATED HYDROCARBONS OF THE ETHYLENE SERIES

(ALKENES OR OLEFINS)

Alkenes, or olefins(from Latin 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 the carbon atoms.

Alkenes contain fewer hydrogen atoms in their molecule than their corresponding alkanes (with the same number of carbon atoms), therefore such hydrocarbons are called unlimited or unsaturated.

Alkenes form a homologous series with the general formula CnH2n

1. Homologous series of alkenes

WITH n H 2 n alkene	Names, suffix *EH, ILENE*
C2H4	this en, this Ilen
C3H6	propene
C4H8	butene
C5H10	penten
C6H12	hexene

Homologs:

WITHH 2 = CH 2 ethene

WITHH 2 = CH- CH 3 propene

WITHH 2 =CH-CH 2 -CH 3butene-1

WITHH 2 =CH-CH 2 -CH 2 -CH 3 penten-1

2. Physical properties

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

C 2 – C 4 (gases)

C 5 – C 17 (liquids)

C 18 – (solid)

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

Lighter than water

With increasing Mr, the melting and boiling points increase

3. The simplest alkene is ethylene - C2H4

Structural and electronic formula ethylene have the form:

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

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

σ - bond. The remaining four hybrid orbitals of the 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 located perpendicular to the σ-bond plane, i.e. one is formed P- connection.

By nature P- connection is sharply different 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 bond angles are close to 120°; the distance between the centers of C atoms is 0.134 nm.

If atoms are connected by a double bond, then their rotation is impossible without electron clouds P- the connection was 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 there is spatial isomerism related to different position substituents relative to a double bond around which intramolecular rotation is impossible.

Structural isomerism of alkenes

1. Isomerism of the carbon skeleton (starting from C 4 H 8):

2. Isomerism of the position of the double bond (starting from C 4 H 8):

3. Interclass isomerism with cycloalkanes, starting with C 3 H 6:

Spatial isomerism of alkenes

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 fixation of the atoms, rotational isomerism with respect to the double bond does not appear. But it becomes possible cis-trance-isomerism.

Alkenes, which have different substituents on each of the two carbon atoms at the double bond, can exist in the form of two spatial isomers, differing in the location of the substituents relative to the plane of the p-bond. So, in the butene-2 molecule CH 3 –CH=CH–CH 3 CH 3 groups can be located either on one side of the double bond in cis-isomer, or different sides V trance-isomer.

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

For example,

butene-1 CH 2 = CH – CH 2 – CH 3 doesn't have cis- And trance-isomers, because The 1st C atom is bonded to two identical H atoms.

Isomers cis- And trance- differ not only physically

but also chemical properties, because bringing parts of a molecule closer or further away from each other in space promotes or hinders chemical interaction.

Sometimes cis-trans-isomerism is not quite accurately called geometric isomerism. The inaccuracy is that All spatial isomers differ in their geometry, and not only cis- And trance-.

5. Nomenclature

Alkenes simple structure often called by replacing the suffix -ane in alkanes with -ylene: ethane - ethylene, propane - propylene, etc.

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