The homologous series of alkanes corresponds to the general formula. Alkanes: structure, nomenclature, isomerism

Alkanes are saturated hydrocarbons. In their molecules, the atoms have single bonds. The structure is determined by the formula CnH2n+2. Let's consider alkanes: Chemical properties, types, application.

In the structure of carbon, there are four orbits in which the atoms rotate. Orbitals have the same shape and energy.

Note! The angles between them are 109 degrees and 28 minutes, they are directed to the vertices of the tetrahedron.

The single carbon bond allows the alkane molecules to rotate freely, resulting in structures that various shapes, forming vertices at carbon atoms.

All alkane compounds are divided into two main groups:

1. Aliphatic hydrocarbons. Such structures have a linear connection. The general formula looks like this: CnH2n+2. A value of n equal to or greater than one indicates the number of carbon atoms.
2. Cycloalkanes with cyclic structure. The chemical properties of cyclic alkanes differ significantly from the properties of linear compounds. The formula of cycloalkanes makes them to some extent similar to hydrocarbons that have a triple atomic bond, that is, alkynes.

Types of alkanes

There are several types of alkane compounds, each of which has its own formula, structure, chemical properties and alkyl substituent. The table contains a homological series

Name of alkanes

The general formula of saturated hydrocarbons is CnH2n+2. By changing the value of n, a compound with a simple interatomic bond is obtained.

Useful video: alkanes - molecular structure, physical properties

Types of alkanes, reaction options

IN natural conditions Alkanes are chemically inert compounds. Hydrocarbons do not react to contact with nitric and sulfuric acid concentrate, alkali and potassium permanganate.

Single molecular bonds determine the reactions characteristic of alkanes. Alkane chains are characterized by nonpolar and weakly polarizable bonds. It is slightly longer than S-N.

General formula of alkanes

Substitution reaction

Paraffin substances are characterized by insignificant chemical activity. This is explained by the increased strength of the chain connection, which is not easy to break. For destruction, a homological mechanism is used, in which free radicals take part.

For alkanes, substitution reactions are more natural. They do not react to water molecules and charged ions. During substitution, hydrogen particles are replaced by halogen and other active elements. Among such processes are halogenation, nitridation and sulfochlorination. Such reactions are used to form alkane derivatives.

Free radical replacement occurs in three main stages:

1. The appearance of a chain on the basis of which free radicals are created. Heat and ultraviolet light are used as catalysts.
2. Development of a chain in the structure of which interactions of active and inactive particles occur. This is how molecules and radical particles are formed.
3. At the end, the chain breaks. Active elements create new combinations or disappear altogether. The chain reaction ends.

Halogenation

The process is carried out according to the radical type. Halogenation occurs under the influence of ultraviolet radiation and thermal heating of the hydrocarbon and halogen mixture.

The whole process follows Markovnikov's rule. Its essence lies in the fact that the hydrogen atom belonging to the hydrogenated carbon is the first to undergo halogenation. The process begins with a tertiary atom and ends with a primary carbon.

Sulfochlorination

Another name is the Reed reaction. It is carried out by the method of free radical substitution. Thus, alkanes react to the combination of sulfur dioxide and chlorine under the influence of ultraviolet radiation.

The reaction begins with the activation of a chain mechanism. At this time, two radicals are released from chlorine. The action of one is directed towards the alkane, resulting in the formation of a hydrogen chloride molecule and an alkyl element. Another radical combines with sulfur dioxide, creating a complex combination. To achieve equilibrium, one chlorine atom is removed from another molecule. The result is alkane sulfonyl chloride. This substance is used to produce surfactants.

Sulfochlorination

Nitration

The nitration process involves the combination of saturated carbons with gaseous tetravalent nitrogen oxide and nitric acid, brought to a 10% solution. For the reaction to take place it will be necessary low level pressure and high temperature, approximately 104 degrees. As a result of nitration, nitroalkanes are obtained.

Splitting off

Dehydrogenation reactions are carried out by separating atoms. The molecular particle of methane completely decomposes under the influence of temperature.

Dehydrogenation

If a hydrogen atom is separated from the carbon lattice of paraffin (except methane), unsaturated compounds are formed. These reactions are carried out under conditions of significant temperature conditions (400-600 degrees). Various metal catalysts are also used.

Alkanes are obtained by hydrogenation of unsaturated hydrocarbons.

Decomposition process

Under the influence of temperatures during alkane reactions, molecular bonds can be broken and active radicals can be released. These processes are known as pyrolysis and cracking.

When the reaction component is heated to 500 degrees, the molecules begin to decompose, and in their place complex radical alkyl mixtures are formed. Alkanes and alkenes are prepared industrially in this way.

Oxidation

These are chemical reactions based on the donation of electrons. Paraffins are characterized by auto-oxidation. The process uses the oxidation of saturated hydrocarbons by free radicals. Alkane compounds in the liquid state are converted into hydroperoxide. First, paraffin reacts with oxygen. Active radicals are formed. Then the alkyl species reacts with a second oxygen molecule. A peroxide radical is formed, which subsequently interacts with the alkane molecule. As a result of the process, hydroperoxide is released.

Alkanes oxidation reaction

Applications of alkanes

Carbon compounds have wide application in almost all major areas of human life. Some types of compounds are indispensable for certain industries and the comfortable existence of modern man.

Gaseous alkanes are the basis of valuable fuels. The main component of most gases is methane.

Methane has the ability to create and release large amounts of heat. Therefore, it is used in significant quantities in industry and for domestic consumption. By mixing butane and propane, a good household fuel is obtained.

Methane is used in the production of the following products:

• methanol;
• solvents;
• freon;
• ink;
• fuel;
• synthesis gas;
• acetylene;
• formaldehyde;
• formic acid;
• plastic.

Application of methane

Liquid hydrocarbons are intended to create fuel for engines and rockets, and solvents.

Higher hydrocarbons, where the number of carbon atoms exceeds 20, are involved in the production of lubricants, paints and varnishes, soaps and detergents.

A combination of fatty hydrocarbons containing less than 15 H atoms is Vaseline oil. This one is tasteless clear liquid used in cosmetics, in the creation of perfumes, and for medical purposes.

Vaseline is the result of a combination of solid and fatty alkanes with less than 25 carbon atoms. The substance is involved in the creation of medical ointments.

Paraffin obtained by combining solid alkanes is a solid, tasteless mass, white and without aroma. The substance is used to make candles, an impregnating substance for wrapping paper and matches. Paraffin is also popular for thermal procedures in cosmetology and medicine.

Note! Alkane mixtures are also used to make synthetic fibers, plastics, detergents and rubber.

Halogenated alkane compounds function as solvents, refrigerants, and also as the main substance for further synthesis.

Useful video: alkanes - chemical properties

Conclusion

Alkanes are acyclic hydrocarbon compounds with a linear or branched structure. A single bond is established between the atoms, which cannot be broken. Reactions of alkanes based on the substitution of molecules characteristic of this type of compound. The homologous series has the general structural formula CnH2n+2. Hydrocarbons belong to the saturated class because they contain the maximum permissible quantity hydrogen atoms.

Heating sodium salt acetic acid(sodium acetate) with an excess of alkali leads to the elimination of the carboxyl group and the formation of methane:

CH3CONa + NaOH CH4 + Na2C03

If you take sodium propionate instead of sodium acetate, then ethane is formed, from sodium butanoate - propane, etc.

RCH2CONa + NaOH -> RCH3 + Na2C03

5. Wurtz synthesis. When haloalkanes interact with the alkali metal sodium, saturated hydrocarbons and an alkali metal halide are formed, for example:

The action of an alkali metal on a mixture of halocarbons (eg bromoethane and bromomethane) will result in the formation of a mixture of alkanes (ethane, propane and butane).

The reaction on which the Wurtz synthesis is based proceeds well only with haloalkanes in the molecules of which a halogen atom is attached to a primary carbon atom.

6. Hydrolysis of carbides. When some carbides containing carbon in the -4 oxidation state (for example, aluminum carbide) are treated with water, methane is formed:

Al4C3 + 12H20 = 3CH4 + 4Al(OH)3 Physical properties

The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a gas without color, taste and smell (the smell of “gas”, which you need to call 04, is determined by the smell of mercaptans - sulfur-containing compounds, specially added to methane used in household and industrial gas appliances, for so that people nearby can detect a leak by smell).

Hydrocarbons of composition from C5H12 to C15H32 are liquids, heavier hydrocarbons are solids.

The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents.

Chemical properties

1. Substitution reactions. The most characteristic reactions for alkanes are free radical substitution reactions, during which a hydrogen atom is replaced by a halogen atom or some group.

Let us present the equations of the most characteristic reactions.

Halogenation:

СН4 + С12 -> СН3Сl + HCl

In case of excess halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms with chlorine:

СН3Сl + С12 -> HCl + СН2Сl2
dichloromethane methylene chloride

СН2Сl2 + Сl2 -> HCl + CHCl3
trichloromethane chloroform

СНСl3 + Сl2 -> HCl + СCl4
carbon tetrachloride carbon tetrachloride

The resulting substances are widely used as solvents and starting materials in organic syntheses.

2. Dehydrogenation (elimination of hydrogen). When alkanes are passed over a catalyst (Pt, Ni, Al2O3, Cr2O3) at high temperatures (400-600 °C), a hydrogen molecule is eliminated and an alkene is formed:

CH3-CH3 -> CH2=CH2 + H2

3. Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons burn to form carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode. The combustion of saturated hydrocarbons is a free-radical exothermic reaction, which is very important when using alkanes as fuel.

CH4 + 2O2 -> C02 + 2H2O + 880kJ

IN general view The combustion reaction of alkanes can be written as follows:

Thermal decomposition reactions underlie the industrial process of hydrocarbon cracking. This process is the most important stage oil refining.

When methane is heated to a temperature of 1000 °C, methane pyrolysis begins - decomposition into simple substances. When heated to a temperature of 1500 °C, the formation of acetylene is possible.

4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton are formed:

5. Flavoring. Alkanes with six or more carbon atoms in the chain cyclize in the presence of a catalyst to form benzene and its derivatives:

What is the reason that alkanes undergo free radical reactions? All carbon atoms in alkane molecules are in a state of sp 3 hybridization. The molecules of these substances are built using covalent nonpolar C-C (carbon-carbon) bonds and weakly polar C-H (carbon-hydrogen) bonds. They do not contain areas with increased or decreased electron density, or easily polarizable bonds, i.e., such bonds in which the electron density can shift under the influence of external influences (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since the bonds in alkane molecules are not broken by a heterolytic mechanism.

The most characteristic reactions of alkanes are free radical substitution reactions. During these reactions, a hydrogen atom is replaced by a halogen atom or some group.

The kinetics and mechanism of free radical chain reactions, i.e. reactions occurring under the influence of free radicals - particles with unpaired electrons - were studied by the remarkable Russian chemist N. N. Semenov. It was for these studies that he was awarded the Nobel Prize in Chemistry.

Typically, the mechanism of free radical substitution reactions is represented by three main stages:

1. Initiation (nucleation of a chain, formation of free radicals under the influence of an energy source - ultraviolet light, heating).

2. Chain development (a chain of sequential interactions of free radicals and inactive molecules, as a result of which new radicals and new molecules are formed).

3. Chain termination (combination of free radicals into inactive molecules (recombination), “death” of radicals, cessation of the development of a chain of reactions).

Scientific research by N.N. Semenov

Semenov Nikolay Nikolaevich

(1896 - 1986)

Soviet physicist and physical chemist, academician. Nobel Prize winner (1956). Scientific research relate to the doctrine of chemical processes, catalysis, chain reactions, theory of thermal explosion and combustion of gas mixtures.

Let's consider this mechanism using the example of the methane chlorination reaction:

CH4 + Cl2 -> CH3Cl + HCl

The initiation of the chain occurs as a result of the fact that under the influence ultraviolet irradiation or when heated, a homolytic cleavage of the Cl-Cl bond occurs and the chlorine molecule disintegrates into atoms:

Сl: Сl -> Сl· + Сl·

The resulting free radicals attack methane molecules, tearing off their hydrogen atom:

CH4 + Cl· -> CH3· + HCl

and transforming into CH3· radicals, which, in turn, colliding with chlorine molecules, destroy them with the formation of new radicals:

CH3 + Cl2 -> CH3Cl + Cl etc.

The chain develops.

Along with the formation of radicals, their “death” occurs as a result of the process of recombination - the formation of an inactive molecule from two radicals:

СН3+ Сl -> СН3Сl

Сl· + Сl· -> Сl2

CH3 + CH3 -> CH3-CH3

It is interesting to note that during recombination, only as much energy is released as is necessary to break the newly formed bond. In this regard, recombination is possible only if a third particle (another molecule, the wall of the reaction vessel) participates in the collision of two radicals, which absorbs excess energy. This makes it possible to regulate and even stop free radical chain reactions.

Note the last example of a recombination reaction - the formation of an ethane molecule. This example shows that a reaction involving organic compounds is a rather complex process, as a result of which, along with the main reaction product, by-products are very often formed, which leads to the need to develop complex and expensive methods for the purification and isolation of target substances.

The reaction mixture obtained from the chlorination of methane, along with chloromethane (CH3Cl) and hydrogen chloride, will contain: dichloromethane (CH2Cl2), trichloromethane (CHCl3), carbon tetrachloride (CCl4), ethane and its chlorination products.

Now let's try to consider the halogenation reaction (for example, bromination) of a more complex organic compound - propane.

If in the case of methane chlorination only one monochloro derivative is possible, then in this reaction two monobromo derivatives can be formed:

It can be seen that in the first case, the hydrogen atom is replaced at the primary carbon atom, and in the second case, at the secondary one. Are the rates of these reactions the same? It turns out that the product of substitution of the hydrogen atom, which is located at the secondary carbon, predominates in the final mixture, i.e. 2-bromopropane (CH3-CHBg-CH3). Let's try to explain this.

In order to do this, we will have to use the idea of ​​​​the stability of intermediate particles. Did you notice that when describing the mechanism of the methane chlorination reaction we mentioned the methyl radical - CH3·? This radical is an intermediate particle between methane CH4 and chloromethane CH3Cl. The intermediate particle between propane and 1-bromopropane is a radical with an unpaired electron at the primary carbon, and between propane and 2-bromopropane at the secondary carbon.

A radical with an unpaired electron at the secondary carbon atom (b) is more stable compared to a free radical with an unpaired electron at the primary carbon atom (a). It is formed in greater quantities. For this reason, the main product of the propane bromination reaction is 2-bromopropane, a compound whose formation occurs through a more stable intermediate species.

Here are some examples of free radical reactions:

Nitration reaction (Konovalov reaction)

The reaction is used to obtain nitro compounds - solvents, starting materials for many syntheses.

Catalytic oxidation of alkanes with oxygen

These reactions are the basis of the most important industrial processes for the production of aldehydes, ketones, and alcohols directly from saturated hydrocarbons, for example:

CH4 + [O] -> CH3OH

Application

Saturated hydrocarbons, especially methane, are widely used in industry (Scheme 2). They are simple and fairly cheap fuel, raw materials for obtaining large quantity the most important connections.

Compounds obtained from methane, the cheapest hydrocarbon raw material, are used to produce many other substances and materials. Methane is used as a source of hydrogen in the synthesis of ammonia, as well as to produce synthesis gas (a mixture of CO and H2), used for the industrial synthesis of hydrocarbons, alcohols, aldehydes and other organic compounds.

Hydrocarbons of higher boiling oil fractions are used as fuel for diesel and turbojet engines, as the basis of lubricating oils, as raw materials for the production of synthetic fats, etc.

Here are several industrially significant reactions that occur with the participation of methane. Methane is used to produce chloroform, nitromethane, and oxygen-containing derivatives. Alcohols, aldehydes, carboxylic acids can be formed by the direct interaction of alkanes with oxygen, depending on the reaction conditions (catalyst, temperature, pressure):

As you already know, hydrocarbons of the composition from C5H12 to C11H24 are included in the gasoline fraction of oil and are used mainly as fuel for internal combustion engines. It is known that the most valuable components of gasoline are isomeric hydrocarbons, since they have maximum detonation resistance.

When hydrocarbons come into contact with atmospheric oxygen, they slowly form compounds with it - peroxides. This is a slowly occurring free radical reaction, initiated by an oxygen molecule:

Please note that the hydroperoxide group is formed at secondary carbon atoms, which are most abundant in linear, or normal, hydrocarbons.

At sharp increase pressure and temperature occurring at the end of the compression stroke, the decomposition of these peroxide compounds begins with the formation large number free radicals that “launch” free radical chain reaction combustion earlier than necessary. The piston still goes up, and the combustion products of gasoline, which have already formed as a result of premature ignition of the mixture, push it down. This leads to a sharp decrease in engine power and wear.

Thus, the main cause of detonation is the presence of peroxide compounds, the ability to form which is maximum in linear hydrocarbons.

C-heptane has the lowest detonation resistance among the hydrocarbons of the gasoline fraction (C5H14 - C11H24). The most stable (i.e., forms peroxides to the least extent) is the so-called isooctane (2,2,4-trimethylpentane).

A generally accepted characteristic of the knock resistance of gasoline is the octane number. An octane number of 92 (for example, A-92 gasoline) means that this gasoline has the same properties as a mixture consisting of 92% isooctane and 8% heptane.

In conclusion, we can add that the use of high-octane gasoline makes it possible to increase the compression ratio (pressure at the end of the compression stroke), which leads to increased power and Engine efficiency internal combustion.

Being in nature and receiving

In today's lesson, you were introduced to the concept of alkanes, and also learned about its chemical composition and methods of obtaining. Therefore, let's now dwell in more detail on the topic of the presence of alkanes in nature and find out how and where alkanes have found application.

The main sources for the production of alkanes are natural gas and oil. They make up the bulk of oil refining products. Methane, widespread in sedimentary rock deposits, is also gas hydrate alkanes.

The main component of natural gas is methane, but it also contains a small proportion of ethane, propane and butane. Methane can be found in emissions from coal seams, swamps and associated petroleum gases.

Ankans can also be obtained by coking coal. In nature, there are also so-called solid alkanes - ozokerites, which are presented in the form of deposits mountain wax. Ozokerite can be found in the waxy coatings of plants or their seeds, as well as in beeswax.

The industrial isolation of alkanes is taken from natural sources, which, fortunately, are still inexhaustible. They are obtained by the catalytic hydrogenation of carbon oxides. Methane can also be produced in the laboratory using the method of heating sodium acetate with solid alkali or hydrolysis of certain carbides. But alkanes can also be obtained by decarboxylation of carboxylic acids and by their electrolysis.

Applications of alkanes

Alkanes at the household level are widely used in many areas of human activity. After all, it is very difficult to imagine our life without natural gas. And it will not be a secret to anyone that the basis of natural gas is methane, from which carbon black is produced, which is used in the production of topographic paints and tires. The refrigerator that everyone has in their home also works thanks to alkane compounds used as refrigerants. Acetylene obtained from methane is used for welding and cutting metals.

Now you already know that alkanes are used as fuel. They are present in gasoline, kerosene, diesel oil and fuel oil. In addition, they are also found in lubricating oils, petroleum jelly and paraffin.

Cyclohexane has found wide use as a solvent and for the synthesis of various polymers. Cyclopropane is used in anesthesia. Squalane, as a high-quality lubricating oil, is a component of many pharmaceutical and cosmetic preparations. Alkanes are the raw materials used to obtain such organic compounds, such as alcohol, aldehydes and acids.

Paraffin is a mixture of higher alkanes, and since it is non-toxic, it is widely used in Food Industry. It is used for impregnation of packaging for dairy products, juices, cereals, etc., but also in the manufacture chewing gum. And heated paraffin is used in medicine for paraffin treatment.

In addition to the above, the heads of matches are impregnated with paraffin for better burning, pencils, and candles are made from it.

By oxidizing paraffin, oxygen-containing products are obtained, mainly organic acids. When mixing liquid hydrocarbons with a certain number Vaseline is obtained from carbon atoms, which is widely used in perfumery and cosmetology, as well as in medicine. It is used for cooking various ointments, creams and gels. They are also used for thermal procedures in medicine.

Practical tasks

1. Write down the general formula of hydrocarbons of the homologous series of alkanes.

2. Write the formulas of possible isomers of hexane and name them according to systematic nomenclature.

3. What is cracking? What types of cracking do you know?

4. Write the formulas of possible products of hexane cracking.

5. Decipher the following chain of transformations. Name the compounds A, B and C.

6. Give the structural formula of the hydrocarbon C5H12, which forms only one monobromine derivative upon bromination.

7. For the complete combustion of 0.1 mol of an alkane of unknown structure, 11.2 liters of oxygen were consumed (at ambient conditions). What is the structural formula of an alkane?

8. What is the structural formula of a gaseous saturated hydrocarbon if 11 g of this gas occupy a volume of 5.6 liters (at standard conditions)?

9. Recall what you know about the use of methane and explain why a domestic gas leak can be detected by smell, although its components are odorless.

10*. What compounds can be obtained by catalytic oxidation of methane to different conditions? Write the equations for the corresponding reactions.

eleven*. Products of complete combustion (in excess oxygen) 10.08 liters (N.S.) of a mixture of ethane and propane were passed through an excess of lime water. In this case, 120 g of sediment was formed. Determine the volumetric composition of the initial mixture.

12*. The ethane density of a mixture of two alkanes is 1.808. Upon bromination of this mixture, only two pairs of isomeric monobromoalkanes were isolated. The total mass of lighter isomers in the reaction products is equal to the total mass of heavier isomers. Determine the volume fraction of the heavier alkane in the initial mixture.

Structure of alkanes

The chemical structure (the order of connection of atoms in molecules) of the simplest alkanes - methane, ethane and propane - is shown by their structural formulas given in section 2. From these formulas it is clear that there are two types of chemical bonds in alkanes:

S–S and S–N.

The C–C bond is covalent nonpolar. The C–H bond is covalent, weakly polar, because carbon and hydrogen are close in electronegativity (2.5 for carbon and 2.1 for hydrogen). The formation of covalent bonds in alkanes due to shared electron pairs of carbon and hydrogen atoms can be shown using electronic formulas:

Electronic and structural formulas reflect the chemical structure, but do not give an idea of ​​the spatial structure of molecules, which significantly affects the properties of the substance.

Spatial structure, i.e. the relative arrangement of the atoms of a molecule in space depends on the direction of the atomic orbitals (AO) of these atoms. In hydrocarbons, the main role is played by the spatial orientation of the atomic orbitals of carbon, since the spherical 1s-AO of the hydrogen atom lacks a specific orientation.

The spatial arrangement of carbon AO, in turn, depends on the type of its hybridization (Part I, Section 4.3). The saturated carbon atom in alkanes is bonded to four other atoms. Therefore, its state corresponds to sp3 hybridization (Part I, section 4.3.1). In this case, each of the four sp3-hybrid carbon AOs participates in axial (σ-) overlap with the s-AO of hydrogen or with the sp3-AO of another carbon atom, forming σ-CH or C-C bonds.

The four σ-bonds of carbon are directed in space at an angle of 109°28", which corresponds to the least repulsion of electrons. Therefore, the molecule of the simplest representative of alkanes - methane CH4 - has the shape of a tetrahedron, in the center of which there is a carbon atom, and at the vertices there are hydrogen atoms:

Bond angle N-C-H is equal 109о28". The spatial structure of methane can be shown using volumetric (scale) and ball-and-stick models.

For recording, it is convenient to use a spatial (stereochemical) formula.

In the molecule of the next homologue, ethane C2H6, two tetrahedral sp3 carbon atoms form a more complex spatial structure:

Alkane molecules containing more than 2 carbon atoms are characterized by curved shapes. This can be shown using the example of n-butane (VRML model) or n-pentane:

Isomerism of alkanes

Isomerism is the phenomenon of the existence of compounds that have the same composition (same molecular formula), but different structures. Such connections are called isomers.

Differences in the order in which atoms are combined in molecules (i.e., chemical structure) lead to structural isomerism. The structure of structural isomers is reflected by structural formulas. In the series of alkanes, structural isomerism manifests itself when the chain contains 4 or more carbon atoms, i.e. starting with butane C 4 H 10. If in molecules of the same composition and the same chemical structure different relative positions of atoms in space are possible, then we observe spatial isomerism (stereoisomerism). In this case, the use of structural formulas is not enough and molecular models or special formulas - stereochemical (spatial) or projection - should be used.

Alkanes, starting with ethane H 3 C–CH 3, exist in various spatial forms ( conformations), caused by intramolecular rotation along C–C σ bonds, and exhibit the so-called rotational (conformational) isomerism.

In addition, if a molecule contains a carbon atom bonded to 4 different substituents, another type of spatial isomerism is possible, when two stereoisomers relate to each other as an object and its mirror image (similar to how left hand refers to the right one). Such differences in the structure of molecules are called optical isomerism.

. Structural isomerism of alkanes

Structural isomers are compounds of the same composition that differ in the order of bonding of atoms, i.e. chemical structure molecules.

The reason for the manifestation of structural isomerism in the series of alkanes is the ability of carbon atoms to form chains of different structures. This type of structural isomerism is called carbon skeleton isomerism.

For example, an alkane of composition C 4 H 10 can exist in the form two structural isomers:

and alkane C 5 H 12 - in the form three structural isomers, differing in the structure of the carbon chain:

With an increase in the number of carbon atoms in the molecules, the possibilities for chain branching increase, i.e. the number of isomers increases with the number of carbon atoms.

Structural isomers differ in physical properties. Alkanes with a branched structure, due to the less dense packing of molecules and, accordingly, smaller intermolecular interactions, boil at a lower temperature than their unbranched isomers.

Techniques for constructing structural formulas of isomers

Let's look at the example of an alkane WITH 6 N 14 .

1. First, we depict the linear isomer molecule (its carbon skeleton)

2. Then we shorten the chain by 1 carbon atom and attach this atom to any carbon atom of the chain as a branch from it, excluding extreme positions:

If you attach a carbon atom to one of the extreme positions, the chemical structure of the chain does not change:

In addition, you need to ensure that there are no repetitions. Thus, the structure is identical to structure (2).

3. When all positions of the main chain have been exhausted, we shorten the chain by another 1 carbon atom:

Now there will be 2 carbon atoms in the side branches. The following combinations of atoms are possible here:

A side substituent can consist of 2 or more carbon atoms connected in series, but for hexane there are no isomers with such side branches, and the structure is identical to structure (3).

The side substituent - C-C can only be placed in a chain containing at least 5 carbon atoms and can only be attached to the 3rd and further atom from the end of the chain.

4. After constructing the carbon skeleton of the isomer, it is necessary to supplement all carbon atoms in the molecule with hydrogen bonds, given that carbon is tetravalent.

So, the composition WITH 6 N 14 corresponds to 5 isomers: 1) 2) 3)4)5)

Nomenclature

The nomenclature of organic compounds is a system of rules that allows us to give an unambiguous name to each individual substance.

This is the language of chemistry, which is used to convey information about their structure in the names of compounds. A compound of a certain structure corresponds to one systematic name, and by this name one can imagine the structure of the compound (its structural formula).

Currently, the IUPAC systematic nomenclature is generally accepted. International Union of the Pure and Applied Chemistry– International Union of Pure and Applied Chemistry).

Along with systematic names, trivial (ordinary) names are also used, which are associated with the characteristic property of a substance, the method of its preparation, natural source, area of ​​application, etc., but do not reflect its structure.

To apply the IUPAC nomenclature, you need to know the names and structure of certain fragments of molecules - organic radicals.

The term "organic radical" is a structural concept and should not be confused with the term "free radical", which characterizes an atom or group of atoms with an unpaired electron.

Radicals in the series of alkanes

If one hydrogen atom is “subtracted” from an alkane molecule, a monovalent “residue” is formed – a hydrocarbon radical ( R – ). The general name for monovalent alkane radicals is alkyls – formed by replacing the suffix - en on - silt : methane – methyl, ethane – ethyl, propane – drank it on drink etc.

Monovalent radicals are expressed by the general formula WITH n N 2n+1 .

A divalent radical is obtained by removing 2 hydrogen atoms from the molecule. For example, from methane you can form the divalent radical –CH 2 – methylene. The names of such radicals use the suffix - Ilen.

The names of radicals, especially monovalent ones, are used in the formation of the names of branched alkanes and other compounds. Such radicals can be considered as components of molecules, their structural details. To give a name to a compound, it is necessary to imagine what “parts”—radicals—its molecule is made up of.

Methane CH 4 corresponds to one monovalent radical methyl CH 3 .

From ethane WITH 2 N 6 it is also possible to produce only one radical - ethyl  CH 2  CH 3 (or - C 2 H 5 ).

Propane CH 3 –CH 2 –CH 3 correspond to two isomeric radicals  WITH 3 N 7 :

Radicals are divided into primary, secondary And tertiary depending on what carbon atom(primary, secondary or tertiary) is the free valence. On this basis n-propyl belongs to the primary radicals, and isopropyl– to secondary ones.

Two alkanes C 4 H 10 ( n-butane and isobutane) corresponds to 4 monovalent radicals -WITH 4 N 9 :

From n-butane are produced n-butyl(primary radical) and sec-butyl(secondary radical), - from isobutane – isobutyl(primary radical) and tert-butyl(tertiary radical).

Thus, the phenomenon of isomerism is also observed in the series of radicals, but the number of isomers is greater than that of the corresponding alkanes.

Construction of alkanes molecules from radicals

For example, a molecule

can be “assembled” in three ways from different pairs of monovalent radicals:

This approach is used in some syntheses of organic compounds, for example:

Where R– monovalent hydrocarbon radical (Wurtz reaction).

Rules for constructing the names of alkanes according to the IUPAC systematic international nomenclature

For the simplest alkanes (C 1 -C 4), trivial names are accepted: methane, ethane, propane, butane, isobutane.

Starting from the fifth homolog, the names normal(unbranched) alkanes are built according to the number of carbon atoms, using Greek numerals and suffix -an: pentane, hexane, heptane, octane, nonane, decane and Further...

At the heart of the name branched alkane is the name of the normal alkane included in its structure with the longest carbon chain. In this case, a branched-chain hydrocarbon is considered as a product of the replacement of hydrogen atoms in a normal alkane by hydrocarbon radicals.

For example, alkane

considered as substituted pentane, in which two hydrogen atoms are replaced by radicals –CH 3 (methyl).

The order in which the name of a branched alkane is constructed

Select the main carbon chain in the molecule. Firstly, it must be the longest. Secondly, if there are two or more chains of equal length, then the most branched one is selected. For example, in a molecule there are 2 chains with the same number (7) of C atoms (highlighted in color):

In case (a) the chain has 1 substituent, and in (b) - 2. Therefore, you should choose option (b).

Number the carbon atoms in the main chain so that the C atoms associated with the substituents receive the lowest numbers possible. Therefore, numbering begins from the end of the chain closest to the branch. For example:

Name all radicals (substituents), indicating in front the numbers indicating their location in the main chain. If there are several identical substituents, then for each of them a number (location) is written separated by a comma, and their number is indicated by prefixes di-, three-, tetra-, penta- etc. (For example, 2,2-dimethyl or 2,3,3,5-tetramethyl).

Place the names of all substituents in alphabetical order (as established by the latest IUPAC rules).

Name the main chain of carbon atoms, i.e. the corresponding normal alkane.

Thus, in the name of a branched alkane

root+suffix – name of a normal alkane (Greek numeral + suffix "an"), prefixes – numbers and names of hydrocarbon radicals.

Example of title construction:

Chemical properties of alkanes

The chemical properties of any compound are determined by its structure, i.e. the nature of the atoms included in its composition and the nature of the bonds between them.

Based on this position and reference data on C–C and C–H bonds, let’s try to predict what reactions are characteristic of alkanes.

Firstly, the extreme saturation of alkanes does not allow addition reactions, but does not prevent decomposition, isomerization and substitution reactions (see. Part I, Section 6.4 "Types of Reactions" ). Secondly, the symmetry of nonpolar C–C and weakly polar C–H covalent bonds (see the table for the values ​​of dipole moments) suggests their homolytic (symmetrical) cleavage into free radicals ( Part I, Section 6.4.3 ). Therefore, reactions of alkanes are characterized by radical mechanism. Since the heterolytic cleavage of C–C and C–H bonds in normal conditions does not occur, then alkanes practically do not enter into ionic reactions. This is manifested in their resistance to the action of polar reagents (acids, alkalis, ionic oxidizing agents: KMnO 4, K 2 Cr 2 O 7, etc.). This inertness of alkanes in ionic reactions previously served as the basis for considering them to be inactive substances and calling them paraffins. Video experience"Relation of methane to potassium permanganate solution and bromine water." So, alkanes exhibit their reactivity mainly in radical reactions.

Conditions for such reactions: elevated temperature (often the reaction is carried out in the gas phase), exposure to light or radioactive radiation, the presence of compounds that are sources of free radicals (initiators), non-polar solvents.

Depending on which bond in the molecule is broken first, alkane reactions are divided into the following types. When C–C bonds are broken, reactions occur decomposition(cracking of alkanes) and isomerization carbon skeleton. Reactions are possible at C–H bonds substitution hydrogen atom or its splitting off(dehydrogenation of alkanes). In addition, the carbon atoms in alkanes are in the most reduced form (the oxidation state of carbon, for example, in methane is –4, in ethane –3, etc.) and in the presence of oxidizing agents, reactions will occur under certain conditions oxidation alkanes involving C–C and C–H bonds.

Cracking of alkanes

Cracking is a process of thermal decomposition of hydrocarbons, which is based on the reactions of splitting the carbon chain of large molecules with the formation of compounds with a shorter chain.

Cracking of alkanes is the basis of oil refining in order to obtain products of lower molecular weight, which are used as motor fuels, lubricating oils, etc., as well as raw materials for the chemical and petrochemical industries. There are two ways to carry out this process: thermal cracking(when heated without air access) and catalytic cracking(more moderate heating in the presence of a catalyst).

Thermal cracking. At a temperature of 450–700 o C, alkanes decompose due to the cleavage of C–C bonds (stronger C–H bonds are retained at this temperature) and alkanes and alkenes with a smaller number of carbon atoms are formed.

For example:

C 6 H 14 C 2 H 6 +C 4 H 8

The breakdown of bonds occurs homolytically with the formation of free radicals:

Free radicals are very active. One of them (for example, ethyl) abstracts atomic hydrogen N from another ( n-butyl) and turns into alkane (ethane). Another radical, having become divalent, turns into an alkene (butene-1) due to the formation of a π-bond when two electrons are paired from neighboring atoms:

Animation(work by Alexey Litvishko, 9th grade student at school No. 124 in Samara)

C–C bond cleavage is possible at any random location in the molecule. Therefore, a mixture of alkanes and alkenes is formed with a molecular weight lower than that of the original alkane.

In general, this process can be expressed by the following diagram:

C n H 2n+2 C m H 2m +C p H 2p+2 , Where m + p = n

At higher temperatures (over 1000C), not only C–C bonds break, but also stronger C–H bonds. For example, thermal cracking of methane is used to produce soot (pure carbon) and hydrogen:

CH 4 C+2H 2

Thermal cracking was discovered by a Russian engineer V.G. Shukhov in 1891

Catalytic cracking carried out in the presence of catalysts (usually aluminum and silicon oxides) at a temperature of 500°C and atmospheric pressure. In this case, along with the rupture of molecules, isomerization and dehydrogenation reactions occur. Example: octane cracking(work by Alexey Litvishko, 9th grade student at school No. 124 in Samara). When alkanes are dehydrogenated, cyclic hydrocarbons are formed (reaction dehydrocyclization, section 2.5.3). The presence of branched and cyclic hydrocarbons in gasoline increases its quality (knock resistance, expressed by octane number). Cracking processes produce a large amount of gases, which contain mainly saturated and unsaturated hydrocarbons. These gases are used as raw materials for the chemical industry. Fundamental work on catalytic cracking in the presence of aluminum chloride has been carried out N.D. Zelinsky.

Isomerization of alkanes

Alkanes of normal structure under the influence of catalysts and upon heating are able to transform into branched alkanes without changing the composition of the molecules, i.e. enter into isomerization reactions. These reactions involve alkanes whose molecules contain at least 4 carbon atoms.

For example, the isomerization of n-pentane into isopentane (2-methylbutane) occurs at 100°C in the presence of an aluminum chloride catalyst:

The starting material and the product of the isomerization reaction have the same molecular formulas and are structural isomers (carbon skeleton isomerism).

Dehydrogenation of alkanes

When alkanes are heated in the presence of catalysts (Pt, Pd, Ni, Fe, Cr 2 O 3, Fe 2 O 3, ZnO), their catalytic dehydrogenation– abstraction of hydrogen atoms due to the breaking of C-H bonds.

The structure of dehydrogenation products depends on the reaction conditions and the length of the main chain in the starting alkane molecule.

1. Lower alkanes containing from 2 to 4 carbon atoms in the chain, when heated over a Ni catalyst, remove hydrogen from neighboring carbon atoms and turn into alkenes:

Along with butene-2 this reaction produces butene-1 CH 2 =CH-CH 2 -CH 3. In the presence of a Cr 2 O 3 /Al 2 O 3 catalyst at 450-650 °C from n-butane is also obtained butadiene-1,3 CH 2 =CH-CH=CH 2.

2. Alkanes containing more than 4 carbon atoms in the main chain are used to obtain cyclical connections. This happens dehydrocyclization– dehydrogenation reaction, which leads to the closure of the chain into a stable cycle.

If the main chain of an alkane molecule contains 5 (but not more) carbon atoms ( n-pentane and its alkyl derivatives), then when heated over a Pt catalyst, hydrogen atoms are split off from the terminal atoms of the carbon chain, and a five-membered cycle is formed (cyclopentane or its derivatives):

Alkanes with a main chain of 6 or more carbon atoms also undergo dehydrocyclization, but always form a 6-membered ring (cyclohexane and its derivatives). Under reaction conditions, this cycle undergoes further dehydrogenation and turns into the energetically more stable benzene ring of an aromatic hydrocarbon (arene). For example:

These reactions underlie the process reforming– processing of petroleum products to obtain arenes ( aromatization saturated hydrocarbons) and hydrogen. Transformation n- alkanes in the arena leads to an improvement in the detonation resistance of gasoline.

3. At 1500 С occurs intermolecular dehydrogenation methane according to the scheme:

This reaction ( methane pyrolysis ) is used for the industrial production of acetylene.

Alkane oxidation reactions

In organic chemistry, oxidation and reduction reactions are considered as reactions involving the loss and acquisition of hydrogen and oxygen atoms by an organic compound. These processes are naturally accompanied by a change in the oxidation states of atoms ( Part I, Section 6.4.1.6 ).

Oxidation of an organic substance is the introduction of oxygen into its composition and (or) the elimination of hydrogen. Reduction is the reverse process (introduction of hydrogen and elimination of oxygen). Considering the composition of alkanes (C n H 2n + 2), we can conclude that they are incapable of participating in reduction reactions, but can participate in oxidation reactions.

Alkanes are compounds with low oxidation states of carbon, and depending on the reaction conditions, they can be oxidized to form various compounds.

At ordinary temperatures, alkanes do not react even with strong oxidizing agents (H 2 Cr 2 O 7, KMnO 4, etc.). When introduced into an open flame, alkanes burn. In this case, in an excess of oxygen, they are completely oxidized to CO 2, where carbon has the highest oxidation state of +4, and water. The combustion of hydrocarbons leads to the rupture of all C-C connections and C-H and is accompanied by the release of a large amount of heat (exothermic reaction).

Lower (gaseous) homologues - methane, ethane, propane, butane - are easily flammable and form explosive mixtures with air, which must be taken into account when using them. As the molecular weight increases, alkanes are more difficult to ignite. Video experience"Explosion of a mixture of methane and oxygen." Video experience"Combustion of liquid alkanes". Video experience"Paraffin burning."

The combustion process of hydrocarbons is widely used to produce energy (in internal combustion engines, thermal power plants, etc.).

The equation for the combustion reaction of alkanes in general form:

From this equation it follows that with an increase in the number of carbon atoms ( n) in an alkane, the amount of oxygen required for its complete oxidation increases. When burning higher alkanes ( n>>1) the oxygen contained in the air may not be enough for their complete oxidation to CO 2 . Then partial oxidation products are formed: carbon monoxide CO (carbon oxidation state +2), soot(fine carbon, zero oxidation state). Therefore, higher alkanes burn in air with a smoky flame, and the toxic carbon monoxide released along the way (odorless and colorless) poses a danger to humans.

Alkanes are compounds of the homologous series of methane. These are saturated non-cyclic hydrocarbons. The chemical properties of alkanes depend on the structure of the molecule and the physical state of the substances.

Structure of alkanes

An alkane molecule consists of carbon and hydrogen atoms, which form methylene (-CH 2 -) and methyl (-CH 3) groups. Carbon can form four covalent nonpolar bonds with neighboring atoms. It is the presence of strong σ-bonds -C-C- and -C-H that determines the inertness of the homologous series of alkanes.

Rice. 1. The structure of an alkane molecule.

The compounds react when exposed to light or heat. Reactions proceed by a chain (free radical) mechanism. Thus, bonds can only be broken down by free radicals. As a result of hydrogen substitution, haloalkanes, salts, and cycloalkanes are formed.

Alkanes are classified as saturated or saturated carbons. This means that the molecules contain maximum amount hydrogen atoms. Due to the absence of free bonds, addition reactions are not typical for alkanes.

Chemical properties

General properties of alkanes are given in the table.

Types of chemical reactions	Description	The equation
Halogenation	React with F 2, Cl 2, Br 2. There is no reaction with iodine. Halogens replace a hydrogen atom. The reaction with fluorine is accompanied by an explosion. Chlorination and bromination occurs at a temperature of 300-400°C. As a result, haloalkanes are formed	CH 4 + Cl 2 → CH 3 Cl + HCl
Nitration (Konovalov reaction)	Interaction with dilute nitric acid at a temperature of 140°C. The hydrogen atom is replaced by the nitro group NO 2. As a result, nitroalkanes are formed	CH 3 -CH 3 +HNO 3 → CH 3 -CH 2 -NO 2 + H 2 O
Sulfochlorination	Accompanied by oxidation with the formation of alkanesulfonyl chlorides	R-H + SO 2 + Cl 2 → R-SO 3 Cl + HCl
Sulfoxidation	Formation of alkanesulfonic acids in excess oxygen. The hydrogen atom is replaced by SO 3 H group	C 5 H 10 + HOSO 3 H → C 5 H 11 SO 3 H + H 2 O
	Occurs in the presence of a catalyst at high temperatures. As a result of the cleavage of C-C bonds, alkanes and alkenes are formed	C 4 H 10 → C 2 H 6 + C 2 H 4
	In excess oxygen, complete oxidation to carbon dioxide occurs. With a lack of oxygen, incomplete oxidation occurs with the formation of carbon monoxide and soot	CH 4 + 2O 2 → CO 2 + 2H 2 O; 2CH 4 + 3O 2 → 2CO + 4H 2 O
Catalytic oxidation	Partial oxidation of alkanes occurs when low temperature and in the presence of catalysts. Ketones, aldehydes, alcohols, carboxylic acids can be formed	C 4 H 10 → 2CH 3 COOH + H 2 O
Dehydrogenation	The elimination of hydrogen as a result of the rupture of C-H bonds in the presence of a catalyst (platinum, aluminum oxide, chromium oxide) at a temperature of 400-600°C. Alkenes are formed	C 2 H 6 → C 2 H 4 + H 2
Aromatization	Dehydrogenation reaction to form cycloalkanes	C 6 H 14 → C 6 H 6 + 4H 2
Isomerization	Formation of isomers under the influence of temperature and catalysts	C 5 H 12 → CH 3 -CH(CH 3)-CH 2 -CH 3

To understand how the reaction proceeds and which radicals are replaced, it is recommended to write down the structural formulas.

Rice. 2. Structural formulas.

Application

Alkanes are widely used in industrial chemistry, cosmetology, and construction. The compounds are made from:

• fuel (gasoline, kerosene);
• asphalt;
• lubricating oils;
• petrolatum;
• paraffin;
• soap;
• varnishes;
• paints;
• enamels;
• alcohols;
• synthetic fabrics;
• rubber;
• addehydes;
• plastics;
• detergents;
• acids;
• propellants;
• cosmetical tools.

Rice. 3. Products obtained from alkanes.

What have we learned?

Learned about the chemical properties and uses of alkanes. Due to strong covalent bonds between carbon atoms, as well as between carbon and hydrogen atoms, alkanes are inert. Substitution and decomposition reactions are possible in the presence of a catalyst at high temperatures. Alkanes are saturated hydrocarbons, so addition reactions are impossible. Alkanes are used to produce materials, detergents, and organic compounds.

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Alkanes (methane and its homologues) have the general formula C n H 2 n+2. The first four hydrocarbons are called methane, ethane, propane, butane. Titles senior members This series consists of the root - the Greek numeral and the suffix -an. The names of alkanes are the basis of IUPAC nomenclature.

Rules for systematic nomenclature:

• Main chain rule.

The main circuit is selected based on the following criteria:

• Maximum number of functional substituents.
• Maximum number of multiple connections.
• Maximum length.
• Maximum number of side hydrocarbon groups.
• Rule of smallest numbers (locants).

The main circuit is numbered from one end to the other in Arabic numerals. Each substituent is assigned the number of the main chain carbon atom to which it is attached. The numbering sequence is chosen in such a way that the sum of the numbers of the substituents (locants) is the smallest. This rule also applies when numbering monocyclic compounds.

• Radical rule.

All hydrocarbon side groups are considered as monovalent (single-connected) radicals. If the side radical itself contains side chains, then according to the above rules, an additional main chain is selected, which is numbered starting from the carbon atom attached to the main chain.

• Alphabetical order rule.

The name of the compound begins with a list of substituents, indicating their names in alphabetical order. The name of each substituent is preceded by its number in the main chain. The presence of several substituents is indicated by numerator prefixes: di-, tri-, tetra-, etc. After this, the hydrocarbon corresponding to the main chain is named.

In table Table 12.1 shows the names of the first five hydrocarbons, their radicals, possible isomers and their corresponding formulas. The names of radicals end with the suffix -yl.

Formula Name hydrocarbon radical coal hydrogen radical Isopropyl Methylpropane (iso-butane) Methylpropyl (iso-butyl) Tert-butyl methylbutane (isopentane) methylbutyl (isopentyl) dimethylpropane (neopentane) dimethylpropyl (neopentyl)

Table 12.1. Alkanes of the acyclopean series C n H 2 n +2 .

Example. Name all isomers of hexane.

Example. Name the alkane with the following structure

In this example, from two twelve-atom chains, the one in which the sum of the numbers is the smallest is selected (rule 2).

Using the names of branched radicals given in table. 12.2,

Radical Name Radical Name isopropyl isopentyl isobutyl neopentyl sec-butyl tert-pentyl tert-butyl isohexyl

Table 12.2. Names of branched radicals.

The name of this alkane is somewhat simplified:

10-tert-butyl-2,2-(dimethyl)-7-propyl-4-isopropyl-3-ethyl-dodecane.

When a hydrocarbon chain closes into a cycle with the loss of two hydrogen atoms, monocycloalkanes are formed with the general formula C n H 2 n. Cyclization starts with C 3, names are formed from C n with the cyclo prefix:

Polycyclic alkanes. Their names are formed using the prefix bicyclo-, tricyclo-, etc. Bicyclic and tricyclic compounds contain, respectively, two and three rings in the molecule; to describe their structure, the number of carbon atoms in each of the chains connecting the node atoms is indicated in decreasing order in square brackets ; under the formula is the name of the atom:

This tricyclic hydrocarbon is commonly called adamantane (from the Czech adamant, diamond) because it is a combination of three fused cyclohexane rings in a form that results in the arrangement of carbon atoms in the crystal lattice that is characteristic of diamond.

Cyclic hydrocarbons with one common carbon atom are called spiranes, for example, spiro-5,5-undecane:

Planar cyclic molecules are unstable, so various conformational isomers are formed. Unlike configurational isomers (the spatial arrangement of atoms in a molecule without taking into account orientation), conformational isomers differ from each other only by the rotation of atoms or radicals around the formal simple connections while maintaining the configuration of the molecules. The energy of formation of a stable conformer is called conformational.

Conformers are in dynamic equilibrium and transform into each other through unstable forms. The instability of planar cycles is caused by significant deformation of bond angles. While maintaining the tetrahedral bond angles for cyclohexane C 6H 12, two stable conformations are possible: in the shape of a chair (a) and in the shape of a bath (b):

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