Lower alkenes (C 2 - C 5), in industrial scale obtained from gases formed during the thermal processing of oil and petroleum products. Alkenes can also be prepared using laboratory synthesis methods.

4.5.1. Dehydrohalogenation

When haloalkanes are treated with bases in anhydrous solvents, for example, an alcoholic solution of potassium hydroxide, hydrogen halide is eliminated.

4.5.2. Dehydration

When alcohols are heated with sulfuric or phosphoric acids, intramolecular dehydration occurs (  - elimination).

The predominant direction of the reaction, as in the case of dehydrohalogenation, is the formation of the most stable alkene (Zaitsev’s rule).

Dehydration of alcohols can be carried out by passing alcohol vapor over a catalyst (aluminum or thorium oxides) at 300 - 350 o C.

4.5.3. Dehalogenation of vicinal dihalides

By the action of zinc in alcohol, dibromides containing halogens at neighboring atoms (vicinal) can be converted into alkenes.

4.5.4. Hydrogenation of alkynes

When alkynes are hydrogenated in the presence of platinum or nickel catalysts, the activity of which is reduced by the addition of a small amount of lead compounds (catalytic poison), an alkene is formed that does not undergo further reduction.

4.5.5. Reductive combination of aldehydes and ketones

When treated with lithium aluminum hydride and titanium(III) chloride, di- or tetra-substituted alkenes are formed from two molecules of aldehyde or ketone in good yields.

5. ALKYNE

Alkynes are hydrocarbons containing a triple carbon-carbon bond –СС–.

The general formula of simple alkynes is C n H 2n-2. The simplest representative of the class of alkynes is acetylene H–СС–H, therefore alkynes are also called acetylene hydrocarbons.

5.1. The structure of acetylene

The carbon atoms of acetylene are in sp- hybrid state. Let us depict the orbital configuration of such an atom. During hybridization 2s-orbitals and 2p-orbitals are formed into two equal ones sp-hybrid orbitals located on the same straight line, leaving two unhybridized ones r-orbitals.

Rice. 5.1 Schemeformationsp -hybrid orbitals of the carbon atom

Directions and shapes of orbitals sr-hybridized carbon atom: hybridized orbitals are equivalent, maximally distant from each other

In the acetylene molecule there is a single bond (  - bond) between carbon atoms is formed by the overlap of two sp-hybridized orbitals. Two mutually perpendicular  - bonds arise when two pairs of unhybridized pairs overlap laterally 2p- orbitals,  - electron clouds cover the skeleton so that the electron cloud has symmetry close to cylindrical. Bonds with hydrogen atoms are formed due to sp-hybrid orbitals of the carbon atom and 1 s-orbitals of the hydrogen atom, the acetylene molecule is linear.

Rice. 5.2 Acetylene molecule

a - lateral overlap 2p gives two orbitals  - communications;

b - the molecule is linear,  -the cloud has a cylindrical shape

In propyne there is a simple connection (  - connection) C sp-WITH sp3 shorter than a similar connection C sp-WITH sp2 in alkenes, this is explained by the fact that sp- the orbital is closer to the nucleus than sp 2 - orbital .

The carbon-carbon triple bond C  C is shorter than the double bond, and the total energy of the triple bond is approximately equal to the sum of the energies of one single C–C bond (347 kJ/mol) and two  bonds (259 2 kJ/mol) (Table 5.1 ).

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

Unsaturated include hydrocarbons containing multiple bonds between carbon atoms in their molecules. Alkenes, alkynes, and alkadienes (polyenes) are unsaturated. 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 participate 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 the 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 undergoing 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, the correct connection name is

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

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 impact 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) a 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 is the basis of one of the industrial methods for 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 undergoing 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 an external electric field. This means that when a positively charged particle approaches, the electrons of the CS bond shift towards it and the 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 the formation of carbocation (1) with a positive charge on the central atom occurs to a greater extent.

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 of 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 - polymerization initiators - 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 widely used in the most different 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 are obtained, then the original 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 a higher melting point and chemical resistance.

Currently, fibers are produced from polymers - analogues of polyethylene, which have 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. Install all possible structural formulas the original 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 found 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 that 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 various 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

On an 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.

The simplest organic compounds are saturated and unsaturated hydrocarbons. These include substances of the class of alkanes, alkynes, alkenes.

Their formulas include hydrogen and carbon atoms in a certain sequence and quantity. They are often found in nature.

Determination of alkenes

Another name for them is olefins or ethylene hydrocarbons. This is exactly what this class of compounds was called in the 18th century when an oily liquid, ethylene chloride, was discovered.

Alkenes include substances consisting of hydrogen and carbon elements. They belong to acyclic hydrocarbons. Their molecule contains a single double (unsaturated) bond connecting two carbon atoms to each other.

Alkene formulas

Each class of compounds has its own chemical designation. In them the symbols of the elements periodic table the composition and bond structure of each substance is indicated.

The general formula of alkenes is denoted as follows: C n H 2n, where the number n is greater than or equal to 2. When deciphering it, it is clear that for each carbon atom there are two hydrogen atoms.

The molecular formulas of alkenes from the homologous series are represented by the following structures: C 2 H 4, C 3 H 6, C 4 H 8, C 5 H 10, C 6 H 12, C 7 H 14, C 8 H 16, C 9 H 18, C10H20. It can be seen that each subsequent hydrocarbon contains one more carbon and 2 more hydrogen.

There is a graphic designation of the location and order of chemical compounds between atoms in a molecule, which is shown by the structural formula of alkenes. Using valence bars, the bond of carbons with hydrogens is indicated.

The structural formula of alkenes can be depicted in expanded form, when all chemical elements and connections. The more concise expression of olefins does not show the combination of carbon and hydrogen using valence bars.

The skeletal formula denotes the simplest structure. The broken line represents the base of the molecule, in which the carbon atoms are represented by its tips and ends, and the links indicate hydrogen.

How are the names of olefins formed?

CH 3 -HC=CH 2 + H 2 O → CH 3 -OHCH-CH 3.

When alkenes are exposed to sulfuric acid, the process of sulfonation occurs:

CH 3 -HC=CH 2 + HO−OSO−OH → CH 3 -CH 3 CH-O−SO 2 −OH.

The reaction proceeds with the formation of acid esters, for example, isopropyl sulfuric acid.

Alkenes are subject to oxidation during their combustion under the influence of oxygen to form water and carbon dioxide:

2CH 3 -HC=CH 2 + 9O 2 → 6CO 2 + 6H 2 O.

The interaction of olefinic compounds and dilute potassium permanganate in the form of a solution leads to the formation of glycols or alcohols of a diatomic structure. This reaction is also oxidative with the formation of ethylene glycol and discoloration of the solution:

3H 2 C=CH 2 + 4H 2 O+ 2KMnO 4 → 3OHCH-CHOH+ 2MnO 2 +2KOH.

Alkene molecules can be involved in the polymerization process with a free radical or cation-anion mechanism. In the first case, under the influence of peroxides, a polyethylene-type polymer is obtained.

According to the second mechanism, acids act as cationic catalysts, and organometallic substances act as anionic catalysts, releasing a stereoselective polymer.

What are alkanes

They are also called paraffins or saturated acyclic hydrocarbons. They have a linear or branched structure, which contains only saturated simple bonds. All representatives of this class have the general formula C n H 2n+2 .

They contain only carbon and hydrogen atoms. The general formula of alkenes is formed from the designation of saturated hydrocarbons.

Names of alkanes and their characteristics

The simplest representative of this class is methane. It is followed by substances such as ethane, propane and butane. Their name is based on the root of the numeral in Greek, to which the suffix -an is added. The names of alkanes are listed in the IUPAC nomenclature.

The general formula of alkenes, alkynes, alkanes includes only two types of atoms. These include the elements carbon and hydrogen. The number of carbon atoms in all three classes is the same, the difference is only in the number of hydrogen, which can be eliminated or added. Unsaturated compounds are obtained from it. Representatives of paraffins contain 2 more hydrogen atoms in their molecule than olefins, which is confirmed by the general formula of alkanes and alkenes. The alkene structure is considered unsaturated due to the presence of a double bond.

If we compare the number of water and carbon atoms in al-cans, then the value will be maximal in comparison with other classes of carbon -ro-dov.

Starting from methane and ending with butane (from C 1 to C 4), substances exist in gaseous form.

Hydrocarbons of the homologous range from C 5 to C 16 are presented in liquid form. Starting with an alkane, which has 17 carbon atoms in the main chain, a transition from the physical state to a solid form occurs.

They are characterized by isomerism in the carbon skeleton and optical modifications of the molecule.

In paraffins, carbon valences are considered to be completely occupied by neighboring carbons or waters with the formation of a σ-type bond. From a chemical point of view, this determines their weak properties, which is why alkanes are called limiting or saturated coals. dovs devoid of affinity.

They undergo substitution reactions associated with radical halogenation, sulfochlorination or nitration of the molecule.

Paraffins undergo a process of oxidation, combustion or decomposition when high temperatures. Under the influence of reaction accelerators, hydrogen atoms are removed or alkanes are dehydrogenated.

What are alkynes

They are also called acetylene hydrocarbons, which have a triple bond in the carbon chain. The structure of alkynes is described by the general formula C n H 2 n-2. It shows that, unlike alkanes, acetylene hydrocarbons lack four hydrogen atoms. They are replaced by a triple bond formed by two π compounds.

This structure determines the chemical properties of this class. The structural formula of alkenes and alkynes clearly shows the unsaturation of their molecules, as well as the presence of a double (H 2 C꞊CH 2) and triple (HC≡CH) bond.

Name of alkynes and their characteristics

The simplest representative is acetylene or HC≡CH. It is also called ethin. It comes from the name of a saturated hydrocarbon, in which the suffix -an is removed and -in is added. In the names of long alkynes, the number indicates the location of the triple bond.

Knowing the structure of saturated and unsaturated hydrocarbons, you can determine which letter indicates the general formula of alkynes: a) CnH2n; c) CnH2n+2; c) CnH2n-2; d) CnH2n-6. The correct answer is the third option.

Starting from acetylene and ending with butane (from C 2 to C 4), the substances are gaseous in nature.

In liquid form there are hydrocarbons of the homologous range from C 5 to C 17. Starting with an alkyne, which has 18 carbon atoms in the main chain, a transition from the physical state to a solid form occurs.

They are characterized by isomerism in the carbon skeleton, in the position of the triple bond, as well as interclass modifications of the molecule.

By chemical characteristics acetylene hydrocarbons are similar to alkenes.

If alkynes have a terminal triple bond, then they perform the function of an acid with the formation of alkinide salts, for example, NaC≡CNa. The presence of two π bonds makes the sodium acetylidene molecule a strong nucleophile that undergoes substitution reactions.

Acetylene undergoes chlorination in the presence of copper chloride to produce dichloroacetylene, condensation under the action of haloalkynes to release diacetylene molecules.

Alkynes participate in reactions whose principles underlie halogenation, hydrohalogenation, hydration and carbonylation. However, such processes are weaker than in alkenes with a double bond.

For acetylene hydrocarbons, nucleophilic addition reactions of an alcohol, primary amine, or hydrogen sulfide molecule are possible.

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 XVIII century, - liquid oily substance.) - aliphatic unsaturated hydrocarbons, in the molecules of which there is one double bond between the carbon atoms.

Alkenes form a homologous series with the general formula CnH2n

1. Homologous series of alkenes

Homologues:

WITHH2 = CH2 ethene

WITHH2 = CH- CH3 propene

WITHH2=CH-CH2-CH3butene-1

WITHH2=CH-CH2-CH2-CH3 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.

C2 - C4 (gases)

C5 - C17 (liquids)

C18 - (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

The structural and electronic formulas of ethylene are:

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 , associated with different positions of substituents relative to the double bond, around which intramolecular rotation is impossible.

Structural isomerism of alkenes

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

2. Isomerism of the position of the double bond (starting from C4H8):

3. Interclass isomerism with cycloalkanes, starting with C3H6:

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 CH3-CH=CH-CH3 CH3 groups can be located either on one side of the double bond in cis-isomer, or on opposite sides in trance-isomer.

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

For example,

butene-1 CH2=CH-CH2-CH3 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 of simple structure are often named 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:

Unsaturated (alkene) radicals are called by trivial names or by systematic nomenclature:

(H2C=CH—) vinyl or ethenyl

(H2C=CH—CH2) allyl

General formula of alkenes: CnH2n(n 2)

The first representatives of the homologous series of alkenes:

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

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

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

Alkene isomers

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

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

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

Qualitative reaction to a double bond – discoloration of bromine water:

Examples of solving problems on the topic “formula of alkenes”

EXAMPLE 1

Exercise	How many isomers capable of decolorizing bromine water does a substance with the composition C 3 H 5 Cl have? Write the structural formulas of these isomers
Solution	C 3 H 5 Cl is a monochlor derivative of the hydrocarbon C 3 H 6 . This formula corresponds to either propene, a hydrocarbon with one double bond, or cyclopropane (a cyclic hydrocarbon). This substance discolors bromine water, which means it contains a double bond. Three carbon atoms can only form this structure: since isomerism of the carbon skeleton and the position of the double bond is impossible with such a number of carbon atoms. Structural isomerism in a given molecule is possible only due to a change in the position of the chlorine atom relative to the double bond: For 1-chloropropene, cis-trans isomerism is possible:
Answer	The problem conditions are satisfied by 4 isomers

EXAMPLE 2

Exercise

A mixture of isomeric hydrocarbons (gases with a hydrogen density of 21) with a volume of 11.2 liters (n.s.) reacted with bromine water. The result was 40.4 g of the corresponding dibromo derivative. What structure do these hydrocarbons have? Determine their volumetric content in the mixture (in%).

Solution

The general formula of hydrocarbons is C x H y. Let's calculate molar mass hydrocarbons: Therefore, the formula of hydrocarbons is C 3 H 6. Only two substances have this formula - propene and cyclopropane. Only propene reacts with bromine water: Let's calculate the amount of dibromo derivative substance: According to the reaction equation: n(propene) mol The total amount of hydrocarbons in the mixture is equal to: