The structure of alkanes

The chemical structure (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 can be seen 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 the common 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 a substance.

Spatial structure, i.e. the mutual 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 is devoid of a definite orientation.

The spatial arrangement of carbon AOs, 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 C-H or C-C σ-bonds.

Four σ-bonds of carbon are directed in space at an angle of 109o28 ", which corresponds to the smallest 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 - hydrogen atoms:

The H-C-H bond angle is 109o28". The spatial structure of methane can be shown using volumetric (scale) and ball-and-stick models.

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

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

Alkanes 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 (the same molecular formula), but a different structure. Such connections are called isomers.

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

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

In addition, if there is a carbon atom in the molecule associated with 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 the left hand relates to the right). Such differences in the structure of molecules are called optical isomerism.

. Structural isomerism of alkanes

Structural isomers - compounds of the same composition, differing in the order of binding atoms, i.e. the chemical structure of the molecules.

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

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 that differ in the structure of the carbon chain:

With an increase in the number of carbon atoms in the composition of 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 a 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

Consider the example of an alkane WITH 6 H 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, then the chemical structure of the chain will not change:

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

3. When all the positions of the main chain are exhausted, we shorten the chain by 1 more carbon atom:

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

The side substituent may consist of 2 or more sequentially connected carbon atoms, 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 H 14 corresponds to 5 isomers: 1) 2) 3)4)5)

Nomenclature

The nomenclature of organic compounds is a system of rules that allows you 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 this name can be used to represent the structure of the compound (its structural formula).

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

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

To apply the IUPAC nomenclature, it is necessary 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 alkane series

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

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

A divalent radical is obtained by removing 2 hydrogen atoms from a molecule. For example, from methane it is possible to form a divalent radical -CH 2 - methylene. The names of such radicals use the suffix - ilene.

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 constituent parts of molecules, their structural details. To give a name to a compound, it is necessary to imagine what "details"-radicals make up its molecule.

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

From ethane WITH 2 H 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 H 7 :

Radicals are divided into primary, secondary And tertiary depending on whether you what carbon atom(primary, secondary or tertiary) is a free valency. On this basis n-propyl refers to primary radicals, and isopropyl- to the secondary.

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

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

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

Construction of alkane 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 (С 1 -С 4), trivial names are accepted: methane, ethane, propane, butane, isobutane.

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

Based on the name branched alkane is the name of the normal alkane included in its construction with the longest carbon chain. In this case, a branched-chain hydrocarbon is considered as a product of substitution of hydrogen atoms in a normal alkane by hydrocarbon radicals.

For example, an alkane

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

The order of construction of the name of a branched alkane

Select the main carbon chain in the molecule. First, it must be the longest. Secondly, if there are two or more chains of the same length, then the most branched one is selected from them. 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 case (b), it has 2. Therefore, option (b) should be chosen.

Number the carbon atoms in the main chain so that the C atoms associated with the substituents get the lowest possible numbers. Therefore, the numbering starts 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).

The names of all substituents are arranged 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 the normal alkane (Greek numeral + suffix "an"), prefixes - numbers and names of hydrocarbon radicals.

Name construction example:

Chemical properties of alkanes

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

Based on this position and reference data on C–C and C–H bonds, we will try to predict which reactions are characteristic of alkanes.

Firstly, the limiting 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 (symmetric) rupture into free radicals ( part I, section 6.4.3 ). Therefore, reactions of alkanes are characterized by radical mechanism. Since heterolytic cleavage of C–C and C–H bonds does not occur under normal conditions, alkanes practically do not enter into ionic reactions. This is manifested in their resistance to the action of polar reagents (acids, alkalis, ionic type oxidizing agents: KMnO 4 , K 2 Cr 2 O 7, etc.). This inertness of alkanes in ionic reactions served as the earlier basis for considering them inactive substances and calling them paraffins. video experience"Ratio of Methane to Potassium Permanganate Solution and Bromine Water". So, alkanes show their reactivity mainly in radical reactions.

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

Depending on which bond in the molecule breaks first, the reactions of alkanes are divided into the following types. With the cleavage of C–C bonds, reactions occur decomposition(cracking of alkanes) and isomerization carbon skeleton. Reactions are possible through C–H bonds substitution a hydrogen atom or splitting off(dehydrogenation of alkanes). In addition, 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 with the participation of 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. Two methods are used to carry out this process: thermal cracking(when heated without air) 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 breaking 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) splits off atomic hydrogen H from another ( n-butyl) and turns into an alkane (ethane). Another radical, becoming divalent, turns into an alkene (butene-1) due to the formation of a π-bond during the pairing of two electrons from neighboring atoms:

Animation(the work of Litvishko Alexei, a student of the 9th grade of school No. 124 in Samara)

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

In general, this process can be expressed by the scheme:

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

At a higher temperature (above 1000°C), not only C–C bonds are broken, 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 oxides of aluminum and silicon) 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(the work of Litvishko Alexei, a student of the 9th grade of school No. 124 in Samara). During the dehydrogenation of alkanes, cyclic hydrocarbons are formed (reaction dehydrocyclization, section 2.5.3). The presence of branched and cyclic hydrocarbons in the composition of gasoline improves its quality (knock resistance, expressed by octane number). During cracking processes, a large amount of gases is formed, which mainly contain 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 when heated are able to turn 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 to 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– splitting off of hydrogen atoms due to the breaking of C-H bonds.

The structure of the 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 \u003d CH-CH 2 -CH 3. In the presence of a Cr 2 O 3 /Al 2 O 3 catalyst at 450-650 С from n-butane is also received 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. At the same time, it 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 enter into the dehydrocyclization reaction, but always form a 6-membered cycle (cyclohexane and its derivatives). Under the reaction conditions, this cycle undergoes further dehydrogenation and turns into an energetically more stable benzene cycle of an aromatic hydrocarbon (arene). For example:

These reactions underlie the process reforming– processing of petroleum products in order to obtain arenes ( aromatization saturated hydrocarbons) and hydrogen. transformation n- alkanes in arenas leads to improved knock resistance of gasoline.

3. At 1500 С, 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 associated with 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 organic matter - the introduction of oxygen into its composition and (or) the elimination of hydrogen. Recovery is the reverse process (the introduction of hydrogen and the elimination of oxygen). Given the composition of alkanes (C n H 2n + 2), we can conclude that they are incapable of participating in reduction reactions, but the possibility of participating in oxidation reactions.

Alkanes are compounds with low degrees of carbon oxidation, 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. At the same time, 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 breaking of all C-C and C-H bonds and is accompanied by the release of a large amount of heat (exothermic reaction).

The lower (gaseous) homologues - methane, ethane, propane, butane - are highly flammable and form explosive mixtures with air, which must be taken into account when using them. With an increase in molecular weight, alkanes are more difficult to ignite. video experience"Explosion of a mixture of methane and oxygen". video experience"Combustion of liquid alkanes". video experience"Burning of paraffin".

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

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

It follows from this equation that with an increase in the number of carbon atoms ( n) in the alkane, the amount of oxygen necessary 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 (oxidation state of carbon +2), soot(fine carbon, zero oxidation state). Therefore, higher alkanes burn in the air with a smoky flame, and the toxic carbon monoxide released along the way (odorless and colorless) is dangerous to humans.

Alkanes are saturated hydrocarbons. In their molecules, atoms have single bonds. The structure is determined by the formula CnH2n+2. Consider alkanes: chemical properties, types, applications.

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

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

A simple carbon bond allows alkane molecules to rotate freely, as a result of which the structures take on various shapes, forming vertices at carbon atoms.

All alkane compounds are divided into two main groups:

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

Name of alkanes

The general formula for 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

Varieties of alkanes, reaction options

Under natural conditions, alkanes are chemically inert compounds. Hydrocarbons do not react to contact with a concentrate of nitric and sulfuric acid, alkali and potassium permanganate.

Single molecular bonds determine the reactions characteristic of alkanes. Alkane chains are characterized by a non-polar and weakly polarizable bond. It is somewhat longer than S-N.

General formula of alkanes

substitution reaction

Paraffin substances differ in insignificant chemical activity. This is explained by the increased strength of the chain bond, 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 these processes are halogenation, nitration and sulfochlorination. Such reactions are used to form alkane derivatives.

Free radical substitution occurs in three main steps:

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

Halogenation

The process is radical. Halogenation occurs under the influence of ultraviolet radiation and thermal heating of the hydrocarbon and halogen mixture.

The whole process occurs according to Markovnikov's rule. Its essence lies in the fact that the hydrogen atom belonging to hydrogenated carbon is the first to be halogenated. The process starts with a tertiary atom and ends with 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 action of a combination of sulfur dioxide and chlorine under the influence of ultraviolet radiation.

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

Sulfochlorination

Nitration

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

splitting off

By separating the atoms, dehydrogenation reactions are carried out. 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.

Obtaining alkanes occurs by carrying out the hydrogenation of unsaturated hydrocarbons.

decomposition process

Under the influence of temperatures during alkane reactions, ruptures of molecular bonds and the release of active radicals can occur. These processes are known as pyrolysis and cracking.

When the reaction component is heated to 500 degrees, the molecules begin to decompose, and complex radical alkyl mixtures form in their place. In this way, alkanes and alkenes are obtained in industry.

Oxidation

These are chemical reactions based on the donation of electrons. Paraffins are characterized by autoxidation. The process uses the oxidation of saturated hydrocarbons by free radicals. Alkane compounds in the liquid state are converted to hydroperoxide. First, the paraffin reacts with oxygen. Active radicals are formed. Then the alkyl particle 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.

Alkane oxidation reaction

Application of alkanes

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

Gaseous alkanes are the basis of valuable fuel. 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 volumes in industry, for consumption at home. When mixing butane and propane, a good household fuel is obtained.

Methane is used in the production of such products:

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

Methane application

Liquid hydrocarbons are designed to create fuel for engines and rockets, 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 with less than 15 H atoms is paraffin oil. This tasteless transparent liquid is used in cosmetics, in the creation of perfumes, and for medical purposes.

Vaseline is the result of the 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 in color and odorless. The substance is used to produce candles, an impregnating substance for wrapping paper and matches. Paraffin is also popular in the implementation of thermal procedures in cosmetology and medicine.

Note! Synthetic fibers, plastics, detergent chemicals and rubber are also made from alkane mixtures.

Halogenated alkane compounds act 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 is indestructible. Reactions of alkanes based on the substitution of molecules, characteristic of this type of compounds. The homologous series has the general structural formula CnH2n+2. Hydrocarbons belong to the saturated class because they contain the maximum allowable number of hydrogen atoms.

One of the first types of chemical compounds studied in the school curriculum in organic chemistry are alkanes. They belong to the group of saturated (otherwise - aliphatic) hydrocarbons. Their molecules contain only single bonds. Carbon atoms are characterized by sp³ hybridization.

Homologues are chemical substances that have common properties and chemical structure, but differ by one or more CH2 groups.

In the case of methane CH4, the general formula for alkanes can be given: CnH (2n+2), where n is the number of carbon atoms in the compound.

Here is a table of alkanes, in which n is in the range from 1 to 10.

Isomerism of alkanes

Isomers are those substances whose molecular formula is the same, but the structure or structure is different.

The class of alkanes is characterized by 2 types of isomerism: carbon skeleton and optical isomerism.

Let us give an example of a structural isomer (i.e., a substance that differs only in the structure of the carbon skeleton) for butane C4H10.

Optical isomers are called such 2 substances, the molecules of which have a similar structure, but cannot be combined in space. The phenomenon of optical or mirror isomerism occurs in alkanes, starting with heptane C7H16.

To give the alkane the correct name, use the IUPAC nomenclature. To do this, use the following sequence of actions:

According to the above plan, let's try to give a name to the next alkane.

Under normal conditions, unbranched alkanes from CH4 to C4H10 are gaseous substances, from C5H12 to C13H28 they are liquid and have a specific odor, all subsequent ones are solid. It turns out that as the length of the carbon chain increases, the boiling and melting points increase. The more branched the structure of an alkane, the lower the temperature at which it boils and melts.

Gaseous alkanes are colorless. And also all representatives of this class cannot be dissolved in water.

Alkanes having a state of aggregation of a gas can burn, while the flame will either be colorless or have a pale blue tint.

Chemical properties

Under normal conditions, alkanes are rather inactive. This is explained by the strength of the σ-bonds between the C-C and C-H atoms. Therefore, it is necessary to provide special conditions (for example, a fairly high temperature or light) to make the chemical reaction possible.

Substitution reactions

Reactions of this type include halogenation and nitration. Halogenation (reaction with Cl2 or Br2) occurs when heated or under the influence of light. During the reaction proceeding sequentially, haloalkanes are formed.

For example, you can write the reaction of chlorination of ethane.

Bromination will proceed in a similar manner.

Nitration is a reaction with a weak (10%) solution of HNO3 or with nitric oxide (IV) NO2. Conditions for carrying out reactions - temperature 140 °C and pressure.

C3H8 + HNO3 = C3H7NO2 + H2O.

As a result, two products are formed - water and an amino acid.

Decomposition reactions

Decomposition reactions always require a high temperature. This is necessary to break bonds between carbon and hydrogen atoms.

So, when cracking temperature required between 700 and 1000 °C. During the reaction, -C-C- bonds are destroyed, a new alkane and alkene are formed:

C8H18 = C4H10 + C4H8

An exception is the cracking of methane and ethane. As a result of these reactions, hydrogen is released and alkyne acetylene is formed. Prerequisite is heating up to 1500 °C.

C2H4 = C2H2 + H2

If you exceed the temperature of 1000 ° C, you can achieve pyrolysis with a complete rupture of bonds in the compound:

During the pyrolysis of propyl, carbon C was obtained, and hydrogen H2 was also released.

Dehydrogenation reactions

Dehydrogenation (hydrogen elimination) occurs differently for different alkanes. The reaction conditions are a temperature in the range from 400 to 600 ° C, as well as the presence of a catalyst, which can be nickel or platinum.

From a compound with 2 or 3 C atoms in the carbon skeleton, an alkene is formed:

C2H6 = C2H4 + H2.

If there are 4-5 carbon atoms in the chain of the molecule, then after dehydrogenation, alkadiene and hydrogen will be obtained.

C5H12 = C4H8 + 2H2.

Starting with hexane, during the reaction, benzene or its derivatives are formed.

C6H14 = C6H6 + 4H2

We should also mention the conversion reaction carried out for methane at a temperature of 800 °C and in the presence of nickel:

CH4 + H2O = CO + 3H2

For other alkanes, the conversion is uncharacteristic.

Oxidation and combustion

If an alkane heated to a temperature of not more than 200 ° C interacts with oxygen in the presence of a catalyst, then the products obtained will differ depending on other reaction conditions: these may be representatives of the classes of aldehydes, carboxylic acids, alcohols or ketones.

In the case of complete oxidation, the alkane burns to the final products - water and CO2:

C9H20 + 14O2 = 9CO2 + 10H2O

If there is insufficient oxygen during oxidation, the end product will be coal or CO instead of carbon dioxide.

Carrying out isomerization

If a temperature of about 100-200 degrees is provided, a rearrangement reaction becomes possible for unbranched alkanes. The second mandatory condition for isomerization is the presence of an AlCl3 catalyst. In this case, the structure of the molecules of the substance changes and its isomer is formed.

Significant the share of alkanes is obtained by separating them from natural raw materials. Most often, natural gas is processed, the main component of which is methane, or oil is subjected to cracking and rectification.

You should also remember about the chemical properties of alkenes. In grade 10, one of the first laboratory methods studied in chemistry lessons is the hydrogenation of unsaturated hydrocarbons.

C3H6 + H2 = C3H8

For example, as a result of the addition of hydrogen to propylene, a single product is obtained - propane.

Using the Wurtz reaction, alkanes are obtained from monohaloalkanes, in the structural chain of which the number of carbon atoms is doubled:

2CH4H9Br + 2Na = C8H18 + 2NaBr.

Another way to obtain is the interaction of a salt of a carboxylic acid with an alkali when heated:

C2H5COONa + NaOH = Na2CO3 + C2H6.

In addition, methane is sometimes produced in an electric arc (C + 2H2 = CH4) or by reacting aluminum carbide with water:

Al4C3 + 12H2O = 3CH4 + 4Al(OH)3.

Alkanes are widely used in industry as a low cost fuel. And they are also used as raw materials for the synthesis of other organic substances. For this purpose, methane is usually used, which is necessary for and synthesis gas. Some other saturated hydrocarbons are used to obtain synthetic fats, and also as a base for lubricants.

For the best understanding of the topic "Alkanes", more than one video lesson has been created, which discusses in detail such topics as the structure of matter, isomers and nomenclature, and also shows the mechanisms of chemical reactions.

Chemical properties. Physical properties of alkanes

Physical properties of alkanes

Under normal conditions, the first four members of the homologous series of alkanes (C 1 - C 4) are gases. Normal alkanes from pentane to heptadecane (C 5 - C 17) are liquids, starting from C 18 and above are solids. As the number of carbon atoms in the chain increases, i.e. with an increase in the relative molecular weight, the boiling and melting points of alkanes increase.

With the same number of carbon atoms in a molecule, branched alkanes have lower boiling points than normal alkanes.

Alkanes are practically insoluble in water, tk. their molecules are low-polar and do not interact with water molecules. Liquid alkanes mix easily with each other. They dissolve well in non-polar organic solvents such as benzene, carbon tetrachloride, etc.

Structure

The molecule of the simplest alkane - methane - has the shape of a regular tetrahedron, in the center of which there is a carbon atom, and at the vertices - hydrogen atoms. The angles between the axes of the C-H bonds are 109°28" (Fig. 29).

In the molecules of other saturated hydrocarbons, the angles between bonds (both C-H and C-C) have the same meaning. used to describe the shape of molecules. concept of hybridization of atomic orbitals(See part I, §6).

In alkanes, all carbon atoms are in the state sp3- hybridization (Fig. 30).

Thus, the carbon atoms in the carbon chain are not in a straight line. The distance between neighboring carbon atoms (between the nuclei of atoms) is strictly fixed - this is chemical bond length(0.154 nm). Distance C 1 - C 3, C 2 - C 4, etc. (through one atom) are also constant, because constant angle between bonds - valence angle.

Distances between more distant carbon atoms can change (within some limits) as a result of rotation around s-bonds. Such a rotation does not break the overlap of the orbitals forming the s-bond, since this bond has axial symmetry.

Different spatial forms of one molecule, formed during the rotation of groups of atoms around s-bonds, are called conformations(Fig. 31).

Conformations are distinguished by energy, but this difference is small (12-15 kJ/mol). More stable are those conformations of alkanes in which the atoms are located as far apart as possible (repulsion of electron shells). The transition from one conformation to another is carried out due to the energy of thermal motion. To depict the conformation, special spatial formulas (Newman's formulas) are used.

Do not confuse!

It is necessary to distinguish between the concepts of conformation and configuration.

Different conformations can transform into each other without breaking chemical bonds. For the transformation of a molecule with one configuration into a molecule with another configuration, the breaking of chemical bonds is required.

Of the four types isomerism alkanes are characterized by two: isomerism of the carbon skeleton and optical isomerism (see part

Chemical bonds in alkanes, their breaking and formation determine the chemical properties of alkanes. C-C and C-H bonds are covalent, simple (s-bonds), practically non-polar, strong enough, therefore:

1) alkanes most often enter into such reactions that go with hemolytic cleavage of bonds;

2) in comparison with organic compounds of other classes, alkanes have a low reactivity (for this they are called paraffins- "devoid of properties"). So, alkanes are resistant to the action of aqueous solutions of acids, alkalis and oxidizing agents (for example, potassium permanganate) even when boiled.

Alkanes do not enter into reactions of addition of other molecules to them, because Alkanes do not have multiple bonds in their molecules.

Alkanes decompose under strong heating in the presence of catalysts in the form of platinum or nickel, while hydrogen is split off from alkanes.

Alkanes can enter into isomerization reactions. Their typical response is substitution reaction, proceeding by a radical mechanism.

Chemical properties

Radical substitution reactions

As an example, consider interaction of alkanes with halogens. Fluorine reacts very vigorously (usually with an explosion) - in this case, all C-H and C-C bonds are broken, and as a result, CF 4 and HF compounds are formed. The reaction has no practical significance. Iodine does not react with alkanes. Reactions with chlorine or bromine take place either under illumination or under strong heating; in this case, from mono- to polyhalo-substituted alkanes are formed, for example:

CH 3 -CH 3 + Cl 2 ® hv CH 3 -CH 2 -Cl + Hcl

The formation of halogen derivatives of methane proceeds along the chain free radical mechanism. Under the action of light, chlorine molecules decompose into inorganic radicals:

Inorganic radical Cl. detaches a hydrogen atom with one electron from a methane molecule, forming HC1 and a free radical CH 3

The free radical interacts with the Cl 2 chlorine molecule, forming a halogen derivative and a chlorine radical.

The oxidation reaction begins with the abstraction of a hydrogen atom by an oxygen molecule (which is a biradical) and then proceeds as a branched chain reaction. The number of radicals increases during the reaction. The process is accompanied

by the release of a large amount of heat, not only C-H, but also C-C bonds are torn, so that carbon monoxide (IV) and water are formed as a result. The reaction can proceed as combustion or lead to an explosion.

2C n H2 n + 2 + (3n + 1) O 2 ®2nCO 2 + (2n + 2) H 2 O

At ordinary temperature, the oxidation reaction does not take place; it can be initiated either by ignition or by the action of an electric discharge.

With strong heating (over 1000 ° C), alkanes completely decompose into carbon and hydrogen. This reaction is called pyrolysis.

CH 4 ® 1200 ° C + 2H 2

With the mild oxidation of alkanes, in particular methane, with atmospheric oxygen in the presence of various catalysts, methyl alcohol, formaldehyde, and formic acid can be obtained.

If methane is passed through a heated zone very quickly and then immediately cooled with water, the result is acetylene.

This reaction is the basis of industrial synthesis, which is called cracking(incomplete decomposition) of methane.

Cracking of methane homologues is carried out at a lower temperature (about 600°C). For example, propane cracking includes the following steps:

So, cracking of alkanes leads to the formation of a mixture of alkanes and alkenes of lower molecular weight.

Heating alkanes to 300–350°C (cracking is not yet in progress) in the presence of a catalyst (Pt or Ni) leads to dehydrogenation- elimination of hydrogen.

Under the action of dilute nitric acid on alkanes at 140 ° C and low pressure, a radical reaction occurs:

CH 3 -CH 3 + HNO 3 ®CH 3 -CH 2 -NO 2 + H 2 O Isomerization

Under certain conditions, normal alkanes can be converted into branched-chain alkanes.

Obtaining alkanes

Consider the production of alkanes using the example of methane production. Methane is widely distributed in nature. It is the main component of many combustible gases, both natural (90-98%) and artificial, released during the dry distillation of wood, peat, coal, and also during oil cracking. Natural gases, especially associated gases from oil fields, contain ethane, propane, butane and pentane in addition to methane.

Methane is emitted from the bottom of swamps and from coal seams in mines, where it is formed during the slow decomposition of plant residues without air access. Therefore, methane is often referred to as swamp gas or firedamp.

In the laboratory, methane is produced by heating a mixture of sodium acetate and sodium hydroxide:

CH 3 COONa+NaOH® 200 ° Na 2 CO 3 +CH 4

or when aluminum carbide interacts with water: Al 4 Cl 3 + 12H 2 O®4Al (OH) 3 + 3CH 4

In the latter case, methane is very pure.

Methane can be obtained from simple substances when heated in the presence of a catalyst:

С+2Н 2 ® Ni CH 4 8 also by synthesis based on water gas

CO + 3H 2 ® Ni CH 4 + H 2 O

This method is of industrial importance. However, methane is usually used in natural gases or gases formed during the coking of coal and during oil refining.

Methane homologues, like methane, are obtained under laboratory conditions by calcining salts of the corresponding organic acids with alkalis. Another way is the Wurtz reaction, i.e. heating monohalogen derivatives with sodium metal, for example:

C 2 H 5 Br + 2Na + BrC 2 H 6 ® C 2 H 5 -C 2 H 5 + 2NaBr

In technology, to obtain technical gasoline (a mixture of hydrocarbons containing 6-10 carbon atoms), synthesis is used

from carbon monoxide (II) and hydrogen in the presence of a catalyst (cobalt compounds) and at elevated pressure. Process

can be expressed by the equation

nСО+(2n+1)Н 2 ® 200° C n H 2n+2 + nН 2 O

I So, the main sources of alkanes are natural gas and oil. However, some saturated hydrocarbons are synthesized from other compounds.

Application of alkanes

Most of the alkanes are used as fuel. Cracking and

Their dehydrogenation leads to unsaturated hydrocarbons, on

on the basis of which many other organic substances are obtained.

Methane is the main part of natural gases (60-99%). Part

Natural gases include propane and butane. Liquid hydrocarbons

are used as a fuel in internal combustion engines in cars, aircraft, etc. The purified mixture of liquid

and solid alkanes forms vaseline. The higher alkanes are

starting materials in the production of synthetic detergents. Alkanes obtained by isomerization are used in the production of high-quality gasolines and rubber. Below is a diagram of the use of methane

Cycloalkanes

Structure

Cycloalkanes are saturated hydrocarbons whose molecules contain a closed ring of carbon atoms.

Cycloalkanes (cycloparaffins) form a homologous series with the general formula C n H 2 n, in which the first member is

cyclopropane C 3 H 6, because At least three carbon atoms are required to form a ring.

Cycloalkanes have several names: cycloparaffins, naphthenes, cyclanes, polymethylenes. Examples of some connections:

The formula C n H 2 n is typical for cycloparaffins, and exactly the same formula describes the homologous series of alkenes (unsaturated hydrocarbons having one multiple bond). From this we can conclude that each cycloalkane is isomerized by the corresponding alkene - this is an example of "interclass" isomerism.

Cycloalkanes are divided into a number of groups according to the ring size, of which we will consider two: small (C 3 , C 4) and ordinary (C 5 -C 7) cycles.

The names of cycloalkanes are built by adding the prefix cyclo- to the name of the alkane with the appropriate number of carbon atoms. The numbering in the cycle is carried out so that the substituents receive the smallest numbers.

Structural formulas of cycloalkanes are usually written in abbreviated form, using the geometric form of the cycle and omitting the symbols for carbon and hydrogen atoms. For example:

The structural isomerism of cycloalkanes is determined by the ring size (cyclobutane and methylcyclopropane are isomers) and the position of the substituents in the ring (for example, 1,1- and 1,2-dimethylbutane), as well as their structure.

Spatial isomerism is also characteristic of cycloalkanes, since it is associated with a different arrangement of substituents relative to the ring plane. When the substituents are located on one side of the ring plane, cis-isomers are obtained, on opposite sides - trans-isomers.

Alkanes :

Alkanes are saturated hydrocarbons, in the molecules of which all atoms are connected by single bonds. Formula -

Physical Properties :

• Melting and boiling points increase with molecular weight and main carbon chain length
• Under normal conditions, unbranched alkanes from CH 4 to C 4 H 10 are gases; from C 5 H 12 to C 13 H 28 - liquids; after C 14 H 30 - solids.
• The melting and boiling points decrease from less branched to more branched. So, for example, at 20 °C, n-pentane is a liquid, and neopentane is a gas.

Chemical properties:

· Halogenation

this is one of the substitution reactions. The least hydrogenated carbon atom is halogenated first (tertiary atom, then secondary, primary atoms are halogenated last). Halogenation of alkanes takes place in stages - no more than one hydrogen atom is replaced in one stage:

1. CH 4 + Cl 2 → CH 3 Cl + HCl (chloromethane)
2. CH 3 Cl + Cl 2 → CH 2 Cl 2 + HCl (dichloromethane)
3. CH 2 Cl 2 + Cl 2 → CHCl 3 + HCl (trichloromethane)
4. CHCl 3 + Cl 2 → CCl 4 + HCl (tetrachloromethane).

Under the action of light, the chlorine molecule decomposes into radicals, then they attack the alkane molecules, taking a hydrogen atom from them, as a result of which methyl radicals CH 3 are formed, which collide with chlorine molecules, destroying them and forming new radicals.

· Combustion

The main chemical property of saturated hydrocarbons, which determine their use as a fuel, is the combustion reaction. Example:

CH 4 + 2O 2 → CO 2 + 2H 2 O + Q

In the event of a lack of oxygen, instead of carbon dioxide, carbon monoxide or coal is obtained (depending on the oxygen concentration).

In general, the combustion reaction of alkanes can be written as follows:

WITH n H 2 n +2 +(1,5n+0.5)O 2 \u003d n CO 2 + ( n+1) H 2 O

· Decomposition

Decomposition reactions occur only under the influence of high temperatures. An increase in temperature leads to the breaking of the carbon bond and the formation of free radicals.

Examples:

CH 4 → C + 2H 2 (t > 1000 °C)

C 2 H 6 → 2C + 3H 2

Alkenes :

Alkenes are unsaturated hydrocarbons containing in the molecule, in addition to single bonds, one double carbon-carbon bond. The formula is C n H 2n

The belonging of a hydrocarbon to the class of alkenes is reflected by the generic suffix -ene in its name.

Physical Properties :

• The melting and boiling points of alkenes (simplified) increase with molecular weight and length of the main carbon chain.
• Under normal conditions, alkenes from C 2 H 4 to C 4 H 8 are gases; from C 5 H 10 to C 17 H 34 - liquids, after C 18 H 36 - solids. Alkenes are insoluble in water, but readily soluble in organic solvents.

Chemical properties :

· Dehydration is the process of splitting a water molecule from an organic compound molecule.

· Polymerization- this is a chemical process of combining many initial molecules of a low molecular weight substance into large polymer molecules.

Polymer is a high molecular weight compound, the molecules of which consist of many identical structural units.

Alkadienes :

Alkadienes are unsaturated hydrocarbons containing in the molecule, in addition to single bonds, two double carbon-carbon bonds. The formula is

. Dienes are structural isomers of alkynes.

Physical Properties :

Butadiene is a gas (tboiling −4.5 °C), isoprene is a liquid boiling at 34 °C, dimethylbutadiene is a liquid boiling at 70 °C. Isoprene and other diene hydrocarbons are able to polymerize into rubber. Natural rubber in its purified state is a polymer with the general formula (C5H8)n and is obtained from the latex of certain tropical plants.

Rubber is highly soluble in benzene, gasoline, carbon disulfide. At low temperature it becomes brittle, when heated it becomes sticky. To improve the mechanical and chemical properties of rubber, it is converted into rubber by vulcanization. To obtain rubber products, they are first molded from a mixture of rubber with sulfur, as well as with fillers: soot, chalk, clay, and some organic compounds that serve to accelerate vulcanization. Then the products are heated - hot vulcanization. During vulcanization, sulfur chemically bonds with rubber. In addition, in vulcanized rubber, sulfur is contained in a free state in the form of tiny particles.

Diene hydrocarbons are easily polymerized. The polymerization reaction of diene hydrocarbons underlies the synthesis of rubber. Enter into addition reactions (hydrogenation, halogenation, hydrohalogenation):

H 2 C \u003d CH-CH \u003d CH 2 + H 2 -> H 3 C-CH \u003d CH-CH 3

Alkynes :

Alkynes are unsaturated hydrocarbons whose molecules contain, in addition to single bonds, one triple carbon-carbon bond. Formula-C n H 2n-2

Physical Properties :

Alkynes are similar in physical properties to the corresponding alkenes. Lower (up to C 4) - gases without color and odor, having higher boiling points than their counterparts in alkenes.

Alkynes are poorly soluble in water, better in organic solvents.

Chemical properties :

halogenation reactions

Alkynes are capable of adding one or two halogen molecules to form the corresponding halogen derivatives:

Hydration

In the presence of mercury salts, alkynes add water to form acetaldehyde (for acetylene) or ketone (for other alkynes)