Chemical properties of butene alkenes. Alkenes: methods of preparation, chemical properties and applications

Alkenic hydrocarbons (olefins) are one of the classes organic matter, which have their own . Types of isomerism of alkenes in representatives of this class do not repeat with the isomerism of other organic substances.

In contact with

Characteristic features of the class

Ethylene olefins are called one of the classes of unsaturated hydrocarbons containing one double bond.

According to physical properties, representatives of this category of unsaturated compounds are:

• gases,
• liquids,
• solid compounds.

In the composition of the molecules there is not only a "sigma" bond, but also a "pi" bond. The reason for this is the presence in structural formula hybridization " sp2”, which is characterized by the arrangement of atoms of the compound in the same plane.

At the same time, an angle of at least one hundred and twenty degrees is formed between them. unhybridized orbitals " R» is characteristic of the location both above the molecular plane and below it.

This feature of the structure leads to the formation of additional bonds - "pi" or " π ».

The described connection is less strong compared to the "sigma"-bonds, since the side overlap has a weak adhesion. The total distribution of the electron densities of the formed bonds is characterized by inhomogeneity. When rotating near the carbon-carbon bond, there is a violation of the overlap of "p" orbitals. For each alkene (olefin), such a pattern is a distinctive feature.

Almost all ethylene compounds have high boiling and melting points, which are not characteristic of all organic substances. Representatives of this class of unsaturated carbohydrates quickly dissolve in other organic solvents.

Attention! Acyclic unsaturated compounds ethylene hydrocarbons have the general formula - C n H 2n.

Homology

Based on the fact that the general formula of alkenes is C n H 2n, they have a certain homology. The homologous series of alkenes begins with the first representative ethylene or ethene. This substance in normal conditions is a gas and contains two carbon atoms and four hydrogen atoms -C 2 H 4. Behind ethene, the homologous series of alkenes continues with propene and butene. Their formulas are as follows: "C 3 H 6" and "C 4 H 8". Under normal conditions, they are also gases that are heavier, which means that they must be collected with a test tube turned upside down.

The general formula of alkenes allows you to calculate the next representative of this class, having at least five carbon atoms in the structural chain. This is a pentene with the formula "C 5 H 10".

By physical characteristics the specified substance belongs to liquids, as well as the twelve following compounds of the homologous line.

Among the alkenes with the indicated characteristics, there are also solids, which begin with the formula C 18 H 36 . Liquid and solid ethylene hydrocarbons do not tend to dissolve in water, but when they enter organic solvents, they react with them.

The described general formula for alkenes implies the replacement of the previously standing suffix "an" with "en". This is enshrined in IUPAC rules. Whichever representative of this category of compounds we take, they all have the described suffix.

In the name of ethylene compounds, there is always a certain number that indicates the location of the double bond in the formula. Examples of this are: "butene-1" or "pentene-2". Atomic numbering starts from the edge closest to the double configuration. This rule is "iron" in all cases.

isomerism

Depending on the existing type of hybridization of alkenes, they have certain types of isomerism, each of which has its own characteristics and structure. Consider the main types of isomerism of alkenes.

structural type

Structural isomerism is subdivided into isomers according to:

• carbon skeleton;
• location of the double bond.

Structural isomers of the carbon skeleton arise in the case of the appearance of radicals (branches from the main chain).

Isomers of alkenes of the indicated isomerism will be:

CH 2 \u003d CH — CH 2 — CH 3.

2-methylpropene-1:

CH2=C — CH 3

│

The presented compounds have a total number of carbon and hydrogen atoms (C 4 H 8), but different structure hydrocarbon skeleton. These are structural isomers, although their properties are not the same. Butene-1 (butylene) has a characteristic odor and narcotic properties that irritate Airways. These features do not have 2-methylpropene-1.

AT this case ethylene has no isomers (C 2 H 4), since it consists of only two carbon atoms, where radicals cannot be substituted.

Advice! The radical is allowed to be placed on the middle and penultimate carbon atoms, but it is not allowed to place them near the extreme substituents. This rule works for all unsaturated hydrocarbons.

Regarding the location of the double bond, isomers are distinguished:

CH 2 \u003d CH — CH 2 — CH 2 -CH 3.

CH 3 -CH = CH — CH 2 -CH 3.

The general formula for alkenes in the examples presented is:C 5 H 10,, but the location of one double bond is different. The properties of these compounds will vary. This is structural isomerism.

isomerism

Spatial type

Spatial isomerism of alkenes is associated with the nature of the arrangement of hydrocarbon substituents.

Based on this, isomers are distinguished:

• "cis";
• "Trance".

The general formula of alkenes allows the creation of "trans-isomers" and "cis-isomers" of the same compound. Take, for example, butylene (butene). For it, it is possible to create isomers of the spatial structure by arranging the substituents in different ways relative to the double bond. With examples, the isomerism of alkenes would look like this:

"cis-isomer" "trans-isomer"

Butene-2 ​​Butene-2

From this example, it can be seen that the "cis-isomers" have two identical radicals on one side of the plane of the double bond. For "trans-isomers" this rule does not work, since they have two dissimilar substituents relative to the "C \u003d C" carbon chain. Given this regularity, it is possible to build "cis" and "trans" isomers for various acyclic ethylene hydrocarbons.

The presented "cis-isomer" and "trans-isomer" for butene-2 ​​cannot be converted into one another, since this requires rotation around the existing carbon double chain (C=C). To carry out this rotation, a certain amount of energy is needed to break the existing “p-bond”.

Based on the foregoing, it can be concluded that the "trans" and "cis" isomers of the species are individual compounds with a certain set of chemical and physical properties.

Which alkene has no isomers. Ethylene has no spatial isomers due to the same arrangement of hydrogen substituents relative to the double chain.

Interclass

Interclass isomerism in alkene hydrocarbons is widespread. The reason for this is the similarity general formula representatives of this class with the formula of cycloparaffins (cycloalkanes). These categories of substances have the same number of carbon and hydrogen atoms, a multiple of the composition (C n H 2n).

Interclass isomers would look like this:

CH 2 \u003d CH — CH 3.

Cyclopropane:

It turns out that the formulaC 3 H 6two compounds are responsible: propene-1 and cyclopropane. From structural structure it is seen different arrangement carbon relative to each other. The properties of these compounds are also different. Propene-1 (propylene) is a gaseous compound with a low boiling point. Cyclopropane is characterized by a gaseous state with a pungent odor and a pungent taste. The chemical properties of these substances also differ, but their composition is identical. To organic this species isomers are called interclass.

Alkenes. Isomerism of alkenes. USE. Organic chemistry.

Alkenes: Structure, nomenclature, isomerism

Conclusion

Alkene isomerism is their important characteristic, due to which new compounds with other properties appear in nature, which are used in industry and everyday life.

Chemical properties of alkanes

Alkanes (paraffins) are non-cyclic hydrocarbons, in the molecules of which all carbon atoms are connected only by single bonds. In other words, there are no multiple, double or triple bonds in the molecules of alkanes. In fact, alkanes are hydrocarbons containing the maximum possible number of hydrogen atoms, and therefore they are called limiting (saturated).

Due to saturation, alkanes cannot enter into addition reactions.

Since carbon and hydrogen atoms have fairly close electronegativity, this leads to the fact that the CH bonds in their molecules are extremely low polarity. In this regard, for alkanes, reactions proceeding according to the mechanism of radical substitution, denoted by the symbol S R, are more characteristic.

1. Substitution reactions

In reactions of this type carbon-hydrogen bonds are broken

RH + XY → RX + HY

Halogenation

Alkanes react with halogens (chlorine and bromine) under the action of ultraviolet light or with strong heat. In this case, a mixture of halogen derivatives with different degrees of substitution of hydrogen atoms is formed - mono-, di-tri-, etc. halogen-substituted alkanes.

On the example of methane, it looks like this:

By changing the halogen/methane ratio in the reaction mixture, it is possible to ensure that any particular methane halogen derivative predominates in the composition of the products.

2. Oxidation reactions

Under normal conditions, alkanes are inert with respect to such strong oxidizers, as concentrated sulfuric and nitric acids, potassium permanganate and dichromate (KMnO 4, K 2 Cr 2 O 7).

Combustion in oxygen

A) complete combustion with an excess of oxygen. Leads to the formation of carbon dioxide and water:

CH 4 + 2O 2 \u003d CO 2 + 2H 2 O

B) incomplete combustion with a lack of oxygen:

2CH 4 + 3O 2 \u003d 2CO + 4H 2 O

CH 4 + O 2 \u003d C + 2H 2 O

Catalytic oxidation with oxygen

As a result of heating alkanes with oxygen (~200 o C) in the presence of catalysts, a wide variety of organic products can be obtained from them: aldehydes, ketones, alcohols, carboxylic acids.

For example, methane, depending on the nature of the catalyst, can be oxidized to methyl alcohol, formaldehyde, or formic acid:

3. Thermal transformations of alkanes

Cracking

Cracking (from English to crack - to tear) is chemical process proceeding at high temperature, as a result of which the carbon skeleton of alkane molecules breaks with the formation of alkene and alkane molecules with lower molecular weights compared to the original alkanes. For example:

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

Cracking can be thermal or catalytic. For the implementation of catalytic cracking, due to the use of catalysts, significantly lower temperatures are used compared to thermal cracking.

Dehydrogenation

The elimination of hydrogen occurs as a result of the rupture C-H connections; carried out in the presence of catalysts elevated temperatures. Dehydrogenation of methane produces acetylene:

2CH 4 → C 2 H 2 + 3H 2

Heating methane to 1200 ° C leads to its decomposition into simple substances:

CH 4 → C + 2H 2

Dehydrogenation of other alkanes gives alkenes:

C 2 H 6 → C 2 H 4 + H 2

When dehydrogenating n-butane, butene or butene-2 ​​is formed (a mixture cis- and trance-isomers):

Dehydrocyclization

Isomerization

Chemical properties of cycloalkanes

The chemical properties of cycloalkanes with more than four carbon atoms in the cycles are generally almost identical to those of alkanes. For cyclopropane and cyclobutane, oddly enough, addition reactions are characteristic. This is due to the high tension within the cycle, which leads to the fact that these cycles tend to break. So cyclopropane and cyclobutane easily add bromine, hydrogen or hydrogen chloride:

Chemical properties of alkenes

1. Addition reactions

Since the double bond in alkene molecules consists of one strong sigma bond and one weak pi bond, they are quite active compounds that easily enter into addition reactions. Alkenes often enter into such reactions even in mild conditions- in the cold, in aqueous solutions and organic solvents.

Hydrogenation of alkenes

Alkenes are able to add hydrogen in the presence of catalysts (platinum, palladium, nickel):

CH 3 -CH \u003d CH 2 + H 2 → CH 3 -CH 2 -CH 3

Hydrogenation of alkenes proceeds easily even at normal pressure and slight heating. An interesting fact is that the same catalysts can be used for the dehydrogenation of alkanes to alkenes, only the dehydrogenation process proceeds at a higher temperature and lower pressure.

Halogenation

Alkenes easily enter into an addition reaction with bromine both in aqueous solution and in organic solvents. As a result of the interaction, initially yellow solutions of bromine lose their color, i.e. discolor.

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

Hydrohalogenation

It is easy to see that the addition of a hydrogen halide to an unsymmetrical alkene molecule should theoretically lead to a mixture of two isomers. For example, when hydrogen bromide is added to propene, the following products should be obtained:

Nevertheless, in the absence of specific conditions (for example, the presence of peroxides in the reaction mixture), the addition of a hydrogen halide molecule will occur strictly selectively in accordance with the Markovnikov rule:

The addition of a hydrogen halide to an alkene occurs in such a way that hydrogen is attached to a carbon atom with a larger number of hydrogen atoms (more hydrogenated), and a halogen is attached to a carbon atom with a smaller number of hydrogen atoms (less hydrogenated).

Hydration

This reaction leads to the formation of alcohols, and also proceeds in accordance with the Markovnikov rule:

As you might guess, due to the fact that the addition of water to the alkene molecule occurs according to the Markovnikov rule, the formation of primary alcohol is possible only in the case of ethylene hydration:

CH 2 \u003d CH 2 + H 2 O → CH 3 -CH 2 -OH

It is by this reaction that the main amount of ethyl alcohol is carried out in the large-capacity industry.

Polymerization

A specific case of the addition reaction is the polymerization reaction, which, unlike halogenation, hydrohalogenation and hydration, proceeds through a free radical mechanism:

Oxidation reactions

Like all other hydrocarbons, alkenes burn easily in oxygen to form carbon dioxide and water. The equation for the combustion of alkenes in excess oxygen has the form:

C n H 2n+2 + O 2 → nCO 2 + (n+1)H 2 O

Unlike alkanes, alkenes are easily oxidized. When acting on alkenes aqueous solution KMnO 4 discoloration, which is a qualitative reaction to double and triple CC bonds in organic molecules.

Oxidation of alkenes with potassium permanganate in a neutral or slightly alkaline solution leads to the formation of diols (dihydric alcohols):

C 2 H 4 + 2KMnO 4 + 2H 2 O → CH 2 OH–CH 2 OH + 2MnO 2 + 2KOH (cooling)

In an acidic environment, complete break double bond with the transformation of carbon atoms that formed a double bond into carboxyl groups:

5CH 3 CH=CHCH 2 CH 3 + 8KMnO 4 + 12H 2 SO 4 → 5CH 3 COOH + 5C 2 H 5 COOH + 8MnSO 4 + 4K 2 SO 4 + 17H 2 O (heating)

If the double C=C bond is at the end of the alkene molecule, then carbon dioxide is formed as the oxidation product of the extreme carbon atom at the double bond. This is due to the fact that the intermediate oxidation product, formic acid, is easily oxidized by itself in an excess of an oxidizing agent:

5CH 3 CH=CH 2 + 10KMnO 4 + 15H 2 SO 4 → 5CH 3 COOH + 5CO 2 + 10MnSO 4 + 5K 2 SO 4 + 20H 2 O (heating)

In the oxidation of alkenes, in which the C atom at the double bond contains two hydrocarbon substituents, a ketone is formed. For example, the oxidation of 2-methylbutene-2 ​​produces acetone and acetic acid.

The oxidation of alkenes, which breaks the carbon skeleton at the double bond, is used to establish their structure.

Chemical properties of alkadienes

Addition reactions

For example, the addition of halogens:

Bromine water becomes colorless.

Under normal conditions, the addition of halogen atoms occurs at the ends of the butadiene-1,3 molecule, while π bonds are broken, bromine atoms are attached to the extreme carbon atoms, and free valences form a new π bond. Thus, as if there is a "movement" of the double bond. With an excess of bromine, one more bromine molecule can be added at the site of the formed double bond.

polymerization reactions

Chemical properties of alkynes

Alkynes are unsaturated (unsaturated) hydrocarbons and therefore are capable of entering into addition reactions. Among the addition reactions for alkynes, electrophilic addition is the most common.

Halogenation

Since the triple bond of alkyne molecules consists of one stronger sigma bond and two weaker pi bonds, they are able to attach either one or two halogen molecules. The addition of two halogen molecules by one alkyne molecule proceeds by the electrophilic mechanism sequentially in two stages:

Hydrohalogenation

The addition of hydrogen halide molecules also proceeds by the electrophilic mechanism and in two stages. In both stages, the addition proceeds in accordance with the Markovnikov rule:

Hydration

The addition of water to alkynes occurs in the presence of ruthium salts in an acidic medium and is called the Kucherov reaction.

As a result of the hydration of the addition of water to acetylene, acetaldehyde (acetic aldehyde) is formed:

For acetylene homologues, the addition of water leads to the formation of ketones:

Alkyne hydrogenation

Alkynes react with hydrogen in two steps. Metals such as platinum, palladium, nickel are used as catalysts:

Alkyne trimerization

By passing acetylene over activated carbon at a high temperature, a mixture of various products is formed from it, the main of which is benzene, a product of acetylene trimerization:

Dimerization of alkynes

Acetylene also enters into a dimerization reaction. The process proceeds in the presence of copper salts as catalysts:

Alkyne oxidation

Alkynes burn in oxygen:

C n H 2n-2 + (3n-1) / 2 O 2 → nCO 2 + (n-1) H 2 O

The interaction of alkynes with bases

Alkynes with a triple C≡C at the end of the molecule, unlike other alkynes, are able to enter into reactions in which the hydrogen atom in the triple bond is replaced by a metal. For example, acetylene reacts with sodium amide in liquid ammonia:

HC≡CH + NaNH 2 → NaC≡CNa + 2NH 3,

and also with an ammonia solution of silver oxide, forming insoluble salt-like substances called acetylenides:

Thanks to this reaction, it is possible to recognize alkynes with a terminal triple bond, as well as to isolate such an alkyne from a mixture with other alkynes.

It should be noted that all silver and copper acetylenides are explosive substances.

Acetylides are able to react with halogen derivatives, which is used in the synthesis of more complex organic compounds with a triple bond:

CH 3 -C≡CH + NaNH 2 → CH 3 -C≡CNa + NH 3

CH 3 -C≡CNa + CH 3 Br → CH 3 -C≡C-CH 3 + NaBr

Chemical properties of aromatic hydrocarbons

The aromatic nature of the bond affects Chemical properties benzene and others aromatic hydrocarbons.

A single 6pi electron system is much more stable than conventional pi bonds. Therefore, for aromatic hydrocarbons, substitution reactions are more characteristic than addition reactions. Arenes enter into substitution reactions by an electrophilic mechanism.

Substitution reactions

Halogenation

Nitration

The nitration reaction proceeds best under the action of not pure nitric acid, but its mixture with concentrated sulfuric acid, the so-called nitrating mixture:

Alkylation

The reaction in which one of the hydrogen atoms at the aromatic nucleus is replaced by a hydrocarbon radical:

Alkenes can also be used instead of halogenated alkanes. Aluminum halides, ferric iron halides or inorganic acids can be used as catalysts.<

Addition reactions

hydrogenation

Accession of chlorine

It proceeds by a radical mechanism under intense irradiation with ultraviolet light:

Similarly, the reaction can proceed only with chlorine.

Oxidation reactions

Combustion

2C 6 H 6 + 15O 2 \u003d 12CO 2 + 6H 2 O + Q

incomplete oxidation

The benzene ring is resistant to oxidizing agents such as KMnO 4 and K 2 Cr 2 O 7 . The reaction does not go.

Division of substituents in the benzene ring into two types:

Consider the chemical properties of benzene homologues using toluene as an example.

Chemical properties of toluene

Halogenation

The toluene molecule can be considered as consisting of fragments of benzene and methane molecules. Therefore, it is logical to assume that the chemical properties of toluene should to some extent combine the chemical properties of these two substances taken separately. In particular, this is precisely what is observed during its halogenation. We already know that benzene enters into a substitution reaction with chlorine by an electrophilic mechanism, and catalysts (aluminum or ferric halides) must be used to carry out this reaction. At the same time, methane is also capable of reacting with chlorine, but by a free radical mechanism, which requires irradiation of the initial reaction mixture with UV light. Toluene, depending on the conditions under which it undergoes chlorination, is able to give either the products of substitution of hydrogen atoms in the benzene ring - for this you need to use the same conditions as in the chlorination of benzene, or the products of substitution of hydrogen atoms in the methyl radical, if on it, how to act on methane with chlorine when irradiated with ultraviolet light:

As you can see, the chlorination of toluene in the presence of aluminum chloride led to two different products - ortho- and para-chlorotoluene. This is due to the fact that the methyl radical is a substituent of the first kind.

If the chlorination of toluene in the presence of AlCl 3 is carried out in excess of chlorine, the formation of trichlorine-substituted toluene is possible:

Similarly, when toluene is chlorinated in the light at a higher chlorine / toluene ratio, dichloromethylbenzene or trichloromethylbenzene can be obtained:

Nitration

The substitution of hydrogen atoms for nitrogroup, during the nitration of toluene with a mixture of concentrated nitric and sulfuric acids, leads to substitution products in the aromatic nucleus, and not in the methyl radical:

Alkylation

As already mentioned, the methyl radical is an orientant of the first kind, therefore, its Friedel-Crafts alkylation leads to substitution products in the ortho and para positions:

Addition reactions

Toluene can be hydrogenated to methylcyclohexane using metal catalysts (Pt, Pd, Ni):

C 6 H 5 CH 3 + 9O 2 → 7CO 2 + 4H 2 O

incomplete oxidation

Under the action of such an oxidizing agent as an aqueous solution of potassium permanganate, the side chain undergoes oxidation. The aromatic nucleus cannot be oxidized under such conditions. In this case, depending on the pH of the solution, either a carboxylic acid or its salt will be formed.

Those containing a pi bond are unsaturated hydrocarbons. They are derivatives of alkanes, in the molecules of which two hydrogen atoms have been split off. The resulting free valences form a new type of bond, which is located perpendicular to the plane of the molecule. This is how a new group of compounds arises - alkenes. We will consider the physical properties, preparation and use of substances of this class in everyday life and industry in this article.

Homologous series of ethylene

The general formula for all compounds called alkenes, reflecting their qualitative and quantitative composition, is C n H 2 n. The names of hydrocarbons according to the systematic nomenclature are as follows: in the term of the corresponding alkane, the suffix changes from -an to -ene, for example: ethane - ethene, propane - propene, etc. In some sources, you can find another name for compounds of this class - olefins. Next, we will study the process of double bond formation and the physical properties of alkenes, and also determine their dependence on the structure of the molecule.

How is a double bond formed?

The electronic nature of the pi bond using the example of ethylene can be represented as follows: carbon atoms in its molecule are in the form of sp 2 hybridization. In this case, a sigma bond is formed. Two more hybrid orbitals, one each from carbon atoms, form simple sigma bonds with hydrogen atoms. The two remaining free hybrid clouds of carbon atoms overlap above and below the plane of the molecule - a pi bond is formed. It is she who determines the physical and chemical properties of alkenes, which will be discussed later.

Spatial isomerism

Compounds that have the same quantitative and qualitative composition of molecules, but a different spatial structure, are called isomers. Isomerism occurs in a group of substances called organic. The characterization of olefins is greatly influenced by the phenomenon of optical isomerism. It is expressed in the fact that ethylene homologues containing different radicals or substituents at each of the two carbon atoms in the double bond can occur in the form of two optical isomers. They differ from each other by the position of the substituents in space relative to the plane of the double bond. The physical properties of alkenes in this case will also be different. For example, this applies to the boiling and melting points of substances. Thus, straight chain olefins have higher boiling points than isomer compounds. Also, the boiling points of cis isomers of alkenes are higher than those of trans isomers. With regard to melting temperatures, the picture is opposite.

Comparative characteristics of the physical properties of ethylene and its homologues

The first three representatives of olefins are gaseous compounds, then, starting from the pentene C 5 H 10 and up to the alkene with the formula C 17 H 34, they are liquids, and then there are solids. The ethene homologues show the following trend: the boiling points of the compounds decrease. For example, for ethylene this indicator is -169.1°C, and for propylene -187.6°C. But the boiling points increase with increasing molecular weight. So, for ethylene it is -103.7°C, and for propene -47.7°C. Summing up what has been said, we can conclude that the physical properties of alkenes depend on their molecular weight. With its increase, the aggregate state of the compounds changes in the direction: gas - liquid - solid, and the melting point also decreases, and the boiling points increase.

Characteristics of ethene

The first representative of the homologous series of alkenes is ethylene. It is a colorless gas, slightly soluble in water, but highly soluble in organic solvents. Molecular weight - 28, ethene is slightly lighter than air, has a subtle sweet smell. It easily reacts with halogens, hydrogen and hydrogen halides. The physical properties of alkenes and paraffins, however, are quite close. For example, the state of aggregation, the ability of methane and ethylene to undergo severe oxidation, etc. How can alkenes be distinguished? How to reveal the unsaturated character of an olefin? For this, there are qualitative reactions, on which we will dwell in more detail. Recall what feature in the structure of the molecule alkenes have. The physical and chemical properties of these substances are determined by the presence of a double bond in their composition. To prove its presence, gaseous hydrocarbon is passed through a purple solution of potassium permanganate or bromine water. If they are discolored, then the compound contains pi bonds in the composition of the molecules. Ethylene enters into an oxidation reaction and decolorizes solutions of KMnO 4 and Br 2 .

Mechanism of addition reactions

The breaking of the double bond ends with the addition of atoms of other chemical elements to the free valences of the carbon. For example, the reaction of ethylene with hydrogen, called hydrogenation, produces ethane. A catalyst is needed, such as powdered nickel, palladium or platinum. The reaction with HCl ends with the formation of chloroethane. Alkenes containing more than two carbon atoms in their molecules undergo the addition reaction of hydrogen halides, taking into account V. Markovnikov's rule.

How ethene homologues interact with hydrogen halides

If we are faced with the task "Characterize the physical properties of alkenes and their preparation", we need to consider V. Markovnikov's rule in more detail. It has been established in practice that ethylene homologues react with hydrogen chloride and other compounds at the site of double bond rupture, obeying a certain pattern. It consists in the fact that the hydrogen atom is attached to the most hydrogenated carbon atom, and the chlorine, bromine or iodine ion is attached to the carbon atom containing the smallest number of hydrogen atoms. This feature of the course of addition reactions is called V. Markovnikov's rule.

Hydration and polymerization

Let us continue to consider the physical properties and application of alkenes using the example of the first representative of the homologous series - ethene. Its reaction with water is used in the organic synthesis industry and is of great practical importance. The process was first carried out in the 19th century by A.M. Butlerov. The reaction requires a number of conditions to be met. This is, first of all, the use of concentrated sulfuric acid or oleum as a catalyst and solvent for ethene, a pressure of about 10 atm and a temperature within 70 °. The hydration process occurs in two phases. At first, sulfate molecules are added to ethene at the point of rupture of the pi bond, and ethylsulfuric acid is formed. Then the resulting substance reacts with water, ethyl alcohol is obtained. Ethanol is an important product used in the food industry for the production of plastics, synthetic rubbers, varnishes and other organic chemicals.

Olefin based polymers

Continuing to study the issue of the use of substances belonging to the class of alkenes, we will study the process of their polymerization, in which compounds containing unsaturated chemical bonds in the composition of their molecules can participate. Several types of polymerization reactions are known, according to which high-molecular products are formed - polymers, for example, such as polyethylene, polypropylene, polystyrene, etc. The free radical mechanism leads to the production of high-pressure polyethylene. It is one of the most widely used compounds in industry. The cationic-ionic type provides a polymer with a stereoregular structure, such as polystyrene. It is considered one of the safest and most convenient polymers to use. Products made of polystyrene are resistant to aggressive substances: acids and alkalis, non-flammable, easily painted. Another type of polymerization mechanism is dimerization, which leads to the production of isobutene, which is used as an antiknock additive for gasoline.

How to get

Alkenes, the physical properties of which we study, are obtained in the laboratory and industry by various methods. In experiments in the school course of organic chemistry, the process of dehydration of ethyl alcohol is used with the help of water-removing agents, such as phosphorus pentoxide or sulfate acid. The reaction is carried out when heated and is the reverse of the process of obtaining ethanol. Another common method for obtaining alkenes has found its application in industry, namely: heating halogen derivatives of saturated hydrocarbons, such as chloropropane with concentrated alcoholic solutions of alkalis - sodium or potassium hydroxide. In the reaction, a hydrogen chloride molecule is split off, a double bond is formed at the place where free valences of carbon atoms appear. The end product of the chemical process will be an olefin - propene. Continuing to consider the physical properties of alkenes, let us dwell on the main process for obtaining olefins - pyrolysis.

Industrial production of unsaturated hydrocarbons of the ethylene series

Cheap raw materials - gases formed in the process of oil cracking, serve as a source of olefins in the chemical industry. For this, a technological scheme of pyrolysis is used - the splitting of a gas mixture, which goes with the breaking of carbon bonds and the formation of ethylene, propene and other alkenes. Pyrolysis is carried out in special furnaces, consisting of individual pyro-coils. They create a temperature of the order of 750-1150°C and there is water vapor as a diluent. Reactions proceed by a chain mechanism that proceeds with the formation of intermediate radicals. The final product is ethylene or propene, and they are produced in large volumes.

We studied in detail the physical properties, as well as the application and methods for obtaining alkenes.

Alkenes are chemically active. Their chemical properties are largely determined by the presence of a double bond. For alkenes, electrophilic addition reactions and radical addition reactions are most characteristic. Nucleophilic addition reactions usually require a strong nucleophile and are not typical of alkenes. Alkenes easily enter into reactions of oxidation, addition, and are also capable of allyl radical substitution.

Addition reactions

Hydrogenation Hydrogen addition (hydrogenation reaction) to alkenes is carried out in the presence of catalysts. Most often, crushed metals are used - platinum, nickel, palladium, etc. As a result, the corresponding alkanes (saturated hydrocarbons) are formed.

$CH_2=CH_2 + H2 → CH_3–CH_3$

addition of halogens. Alkenes easily react with chlorine and bromine under normal conditions to form the corresponding dihaloalkanes, in which the halogen atoms are located at neighboring carbon atoms.

Remark 1

When alkenes interact with bromine, the yellow-brown color of bromine is discolored. This is one of the oldest and simplest qualitative reactions for unsaturated hydrocarbons, since alkynes and alkadienes also react similarly.

$CH_2=CH_2 + Br_2 → CH_2Br–CH_2Br$

addition of hydrogen halides. When ethylene hydrocarbons react with hydrogen halides ($HCl$, $HBr$), haloalkanes are formed, the direction of the reaction depends on the structure of alkenes.

In the case of ethylene or symmetrical alkenes, the addition reaction occurs unambiguously and leads to the formation of only one product:

$CH_2=CH_2 + HBr → CH_3–CH_2Br$

In the case of unsymmetrical alkenes, the formation of two different addition reaction products is possible:

Remark 2

In fact, basically only one reaction product is formed. The regularity of the direction of passage of such reactions was established by the Russian chemist V.V. Markovnikov in 1869 It is called Markovnikov's rule. In the interaction of hydrogen halides with unsymmetrical alkenes, the hydrogen atom joins at the place where the double bond is broken in the most hydrogenated carbon atom, that is, before it is connected to a large number of hydrogen atoms.

Markovnikov formulated this rule on the basis of experimental data, and only much later did it receive a theoretical justification. Consider the reaction of propylene with hydrogen chloride.

One of the features of the $p$ bond is its ability to be easily polarized. Under the influence of the methyl group (positive inductive effect + $I$) in the propene molecule, the electron density of the $p$ bond is shifted to one of the carbon atoms (= $CH_2$). As a result, a partial negative charge ($\delta -$) appears on it. On the other carbon atom of the double bond, a partial positive charge arises ($\delta +$).

This distribution of electron density in the propylene molecule determines the location of the future attack by the proton. This is the carbon atom of the methylene group (= $CH_2$), which carries a partial negative charge $\delta-$. And chlorine, accordingly, attacks the carbon atom with a partial positive charge $\delta+$.

As a consequence, the main reaction product of propylene with hydrogen chloride is 2-chloropropane.

Hydration

Hydration of alkenes occurs in the presence of mineral acids and obeys the Markovnikov rule. The reaction products are alcohols

$CH_2=CH_2 + H_2O → CH_3–CH_2–OH$

Alkylation

The addition of alkanes to alkenes in the presence of an acid catalyst ($HF$ or $H_2SO_4$) at low temperatures leads to the formation of hydrocarbons with a higher molecular weight and is often used in industry to produce motor fuel

$R–CH_2=CH_2 + R’–H → R–CH_2–CH_2–R’$

Oxidation reactions

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

polymerization reactions

Alkene molecules are capable of adding to each other under certain conditions with the opening of $\pi$-bonds and the formation of dimers, trimers or high-molecular compounds - polymers. The polymerization of alkenes can proceed both by free radical and cation-anion mechanisms. Acids, peroxides, metals, etc. are used as polymerization initiators. The polymerization reaction is also carried out under the influence of temperature, irradiation, and pressure. A typical example is the polymerization of ethylene to form polyethylene

$nCH_2=CH_2 → (–CH_2–CH_(2^–))_n$

Substitution reactions

Substitution reactions for alkenes are not typical. However, at high temperatures (above 400 °C), radical addition reactions, which are reversible, are suppressed. In this case, it becomes possible to carry out the substitution of the hydrogen atom in the allyl position while maintaining the double bond

$CH_2=CH–CH_3 + Cl_2 – CH_2=CH–CH_2Cl + HCl$

Unsaturated hydrocarbons include hydrocarbons containing multiple bonds between carbon atoms in molecules. Unlimited are alkenes, alkynes, alkadienes (polyenes). Cyclic hydrocarbons containing a double bond in the cycle also have an unsaturated character ( cycloalkenes), as well as cycloalkanes with a small number of carbon atoms in the cycle (three or four atoms). 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.

The 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 СnН2n. Its second name olefins- alkenes were obtained by analogy with unsaturated fatty acids (oleic, linoleic), the remains of which are part of liquid fats - oils.
Carbon atoms between which there is a double bond are in a state of sp 2 hybridization. This means that one s- and two p-orbitals participate in hybridization, while one p-orbital remains unhybridized. The overlap of hybrid orbitals leads to the formation of a σ-bond, and due to unhybridized p-orbitals
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 that form a double bond are in the same plane, and the orbitals that form a π bond are located perpendicular to the plane of the molecule. A double bond (0.132 im) is shorter than a single bond, and its energy is greater, since it is more durable. 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 enter into addition reactions.

The structure of ethylene

Double bond formation in alkenes

Homologous series of ethene

Unbranched alkenes form 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, C 7 H 14 - heptene, etc.

Isomerism of alkenes

Alkenes are characterized by structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkene, which is characterized by structural isomers, is butene:

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

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

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 and transisomerism.

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

Alkene nomenclature

1. Selecting the main circuit. The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in a 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 starts from the end to which the double bond is closest.
For example, the correct connection name is:

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

3. Name formation. At the end of the name indicate the number of the carbon atom at which the double bond begins, and the suffix -en, denoting that the compound belongs to the class of alkenes. For example:

Physical properties of alkenes

The first three representatives of the homologous series of alkenes are gases; substances of the composition C5H10 - C16H32 - liquids; higher alkenes are solids.
The boiling and melting points naturally increase with an increase in the molecular weight of the compounds.

Chemical properties of alkenes

Addition reactions. Recall that a distinctive feature of the representatives of unsaturated hydrocarbons - alkenes is the ability to enter into addition reactions. Most of these reactions proceed by the mechanism electrophilic addition.
1. Hydrogenation of alkenes. Alkenes are able to add hydrogen in the presence of hydrogenation catalysts, metals - platinum, palladium, nickel:

This reaction proceeds at atmospheric and elevated pressure and does not require high temperature, since it is exothermic. With an increase in temperature on the same catalysts, the reverse reaction, dehydrogenation, can occur.

2. Halogenation (addition of halogens). The interaction of an alkene with bromine water or a solution of bromine in an organic solvent (CC14) leads to a 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 is subject to
When a hydrogen halide is added to an alkene, hydrogen is attached to a more hydrogenated carbon atom, i.e., an atom at which there are more hydrogen atoms, and a halogen to a less hydrogenated one.

4. Hydration (water addition). 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 at the primary carbon) is formed only when ethene is hydrated. When propene or other alkenes are hydrated, secondary alcohols.

This reaction also proceeds in accordance with Markovnikov's rule - the hydrogen cation is added to the more hydrogenated carbon atom, and the hydroxo group to the less hydrogenated one.
5. Polymerization. A special case of addition is the polymerization reaction of alkenes:

This addition reaction proceeds by a free radical mechanism.
Oxidation reactions.
1. Combustion. Like any organic compounds, alkenes burn in oxygen to form CO2 and H2O:

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