The structure of alkanes. International nomenclature of alkanes

Heating the sodium salt of 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 + Na2CO3

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

RCH2CONa + NaOH -> RCH3 + Na2CO3

5. Wurtz synthesis. In the interaction of haloalkanes with an alkali metal sodium, saturated hydrocarbons and a halide are formed alkali metal, 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 the halogen atom is attached to the primary carbon atom.

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

Al4C3 + 12H20 = ZCH4 + 4Al(OH)3 Physical properties

The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a colorless, tasteless and odorless gas (the smell of "gas", having felt 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 near them can smell the leak).

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 of 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 typical reactions.

Halogenation:

CH4 + C12 -> CH3Cl + HCl

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

CH3Cl + C12 -> HCl + CH2Cl2
dichloromethane methylene chloride

CH2Cl2 + Cl2 -> HCl + CHCl3
trichloromethane chloroform

CHCl3 + Cl2 -> HCl + CCl4
carbon tetrachloride carbon tetrachloride

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

2. Dehydrogenation (hydrogen elimination). When alkanes are passed over a catalyst (Pt, Ni, A12O3, Cr2O3) at a high temperature (400-600 °C), a hydrogen molecule is split off and an alkene is formed:

CH3-CH3 -> CH2=CH2 + H2

3. Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons burn with the formation of 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 that has a very great importance using alkanes as fuel.

CH4 + 2O2 -> CO2 + 2H2O + 880kJ

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

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

When methane is heated to a temperature of 1000 ° C, pyrolysis of methane 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. Aromatization. Alkanes with six or more carbon atoms in the chain in the presence of a catalyst are cyclized to form benzene and its derivatives:

What is the reason that alkanes enter into reactions proceeding according to the free radical mechanism? All carbon atoms in alkane molecules are in a state of sp 3 hybridization. The molecules of these substances are built using covalent non-polar C-C (carbon-carbon) bonds and weakly polar C-H (carbon-hydrogen) bonds. They do not have areas with increased and decreased electron density, easily polarizable bonds, i.e., such bonds, the electron density in which can be shifted under the influence of external influences (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since 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 action 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.

Usually, the reaction mechanism of free radical substitution is represented by three main stages:

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

2. Development of a chain (a chain of successive 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 chain of reactions).

Scientific research by N.N. Semenov

Semenov Nikolay Nikolaevich

(1896 - 1986)

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

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 action of ultraviolet radiation or heating, a homolytic cleavage of the Cl-Cl bond occurs and the chlorine molecule decomposes into atoms:

Cl: Cl -> Cl + + Cl

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

CH4 + Cl -> CH3 + HCl

and converting 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 recombination process - the formation of an inactive molecule from two radicals:

CH3 + Cl -> CH3Cl

Cl+ + Cl+ -> Cl2

CH3 + CH3 -> CH3-CH3

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

Pay attention to the last example of a recombination reaction - the formation of an ethane molecule. This example shows that the 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 purification and isolation of target substances.

The reaction mixture obtained by methane chlorination, 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 monochlorine derivative is possible, then two monobromo derivatives can already be formed in this reaction:

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. Are the rates of these reactions the same? It turns out that in the final mixture, the product of substitution of the hydrogen atom, which is located at the secondary carbon, predominates, i.e. 2-bromopropane (CH3-CHBr-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. An 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.

A radical with an unpaired electron at the secondary carbon atom (b) is more stable than a free radical with an unpaired electron at the primary carbon atom (a). It is formed in more. For this reason, the main product of the propane bromination reaction is 2-bromo-propane, a compound whose formation proceeds through a more stable intermediate particle.

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 obtaining aldehydes, ketones, alcohols directly from saturated hydrocarbons, for example:

CH4 + [O] -> CH3OH

Application

Saturated hydrocarbons, especially methane, are widely used in industry (Scheme 2). They are a simple and fairly cheap fuel, a raw material for obtaining a large number of the most important compounds.

Compounds derived from methane, the cheapest hydrocarbon feedstock, 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 a fuel for diesel and turbojet engines, as a base for lubricating oils, as a raw material for the production of synthetic fats, etc.

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

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

Hydrocarbons, when in contact with atmospheric oxygen, slowly form compounds with it - peroxides. This is a slow free radical reaction initiated by an oxygen molecule:

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

With a sharp increase in pressure and temperature, which occurs at the end of the compression stroke, the decomposition of these peroxide compounds begins with the formation a large number free radicals, which "trigger" the free radical chain reaction burning earlier than necessary. The piston is still going 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, its wear.

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

k-heptane has the lowest detonation resistance among 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).

The generally accepted characteristic of the knock resistance of gasoline is the octane number. An octane rating 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, it can be added 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 an increase in the power and efficiency of the internal combustion engine.

Being in nature and getting

In today's lesson, you got acquainted with such a concept as alkanes, and also learned about it. chemical composition and methods of obtaining. Therefore, let's now dwell in more detail on the topic of finding alkanes in nature and find out how and where alkanes have found application.

The main sources for obtaining alkanes are natural gas and oil. They make up the bulk of products from oil refining. Methane, common in deposits of sedimentary rocks, is also a gas hydrate of 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 coal seam emissions, swamps and associated petroleum gases.

Ankans can also be obtained by coking coal. In nature, there are also so-called solid alkanes - ozocerites, which are presented in the form of deposits of mountain wax. Ozokerite can be found in the wax coatings of plants or their seeds, as well as in the composition of 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. Also, methane can be obtained in the laboratory using the method of heating sodium acetate with solid alkali or hydrolysis of some carbides. But also alkanes can be obtained by decarboxylation of carboxylic acids and by their electrolysis.

Application of alkanes

Alkanes at the household level are widely used in many areas of human activity. 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. And 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 the composition of gasoline, kerosene, solar oil and fuel oil. In addition, they are also in the composition of lubricating oils, petroleum jelly and paraffin.

As a solvent and for the synthesis of various polymers, cyclohexane has found wide application. 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 with which organic compounds such as alcohol, aldehydes and acids are obtained.

Paraffin is a mixture of higher alkanes, and since it is non-toxic, it is widely used in Food Industry. It is used to impregnate packages for dairy products, juices, cereals, and so on, but also in the manufacture of chewing gums. And heated paraffin is used in medicine for paraffin treatment.

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

By oxidizing paraffin, oxygen-containing products, mainly organic acids, are obtained. When mixing liquid hydrocarbons with certain number Vaseline is obtained from carbon atoms, which has found wide application both in perfumery and cosmetology, and in medicine. It is used to prepare various ointments, creams and gels. And also used for thermal procedures in medicine.

Practical tasks

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

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

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

4. Write formulas for possible products of hexane cracking.

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

6. Lead structural formula hydrocarbon С5Н12, which forms only one monobromo derivative during bromination.

7. For the complete combustion of 0.1 mol of an alkane of an unknown structure, 11.2 liters of oxygen were consumed (at n.a.). 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 n.a.)?

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

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

eleven*. Products of complete combustion (in excess of oxygen) 10.08 liters (n.a.) of a mixture of ethane and propane were passed through an excess of lime water. This formed 120 g of sediment. 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.

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, application.

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 forms, 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, with 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

IN vivo 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 are formed 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 liquid state 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 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 quantities in industry, for consumption in living conditions. 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 color and without fragrance. 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

Output

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.

It would be useful to start with a definition of the concept of alkanes. These are saturated or limiting. We can also say that these are carbons in which the connection of C atoms is carried out through simple bonds. The general formula is: CnH₂n+ 2.

It is known that the ratio of the number of H and C atoms in their molecules is maximum when compared with other classes. Due to the fact that all valences are occupied by either C or H, the chemical properties of alkanes are not expressed clearly enough, so the phrase saturated or saturated hydrocarbons is their second name.

There is also an older name that best reflects their relative chemical inertness - paraffins, which means "devoid of affinity".

So, the topic of our today's conversation: "Alkanes: homologous series, nomenclature, structure, isomerism." Data regarding their physical properties will also be presented.

Alkanes: structure, nomenclature

In them, the C atoms are in such a state as sp3 hybridization. In this regard, the alkanes molecule can be demonstrated as a set of tetrahedral structures C, which are connected not only to each other, but also to H.

There are strong, very low polarity s bonds between the C and H atoms. Atoms, on the other hand, always rotate around simple bonds, which is why alkane molecules take on various forms, and the bond length and the angle between them are constant values. Forms that are transformed into each other due to the rotation of the molecule around σ-bonds are commonly called its conformations.

In the process of detachment of the H atom from the molecule under consideration, 1-valent particles are formed, called hydrocarbon radicals. They appear as a result of compounds not only but also inorganic. If we subtract 2 hydrogen atoms from a saturated hydrocarbon molecule, we get 2-valent radicals.

Thus, the nomenclature of alkanes can be:

• radial (old version);
• substitution (international, systematic). It has been proposed by IUPAC.

Features of the radial nomenclature

In the first case, the nomenclature of alkanes is characterized by the following:

1. Consideration of hydrocarbons as derivatives of methane, in which 1 or more H atoms are replaced by radicals.
2. A high degree of convenience in the case of not very complex connections.

Features of the replacement nomenclature

The substitutional nomenclature of alkanes has the following features:

1. The basis for the name is 1 carbon chain, while the rest of the molecular fragments are considered as substituents.
2. If there are several identical radicals, the number is indicated before their name (strictly in words), and the radical numbers are separated by commas.

Chemistry: alkanes nomenclature

For convenience, the information is presented in the form of a table.

Substance name	Name base (root)	Molecular formula	Name of the carbon substituent	Formula of the carbon substituent

The above nomenclature of alkanes includes names that have developed historically (the first 4 members of the series of saturated hydrocarbons).

The names of unfolded alkanes with 5 or more C atoms are derived from Greek numerals that reflect the given number of C atoms. Thus, the suffix -an indicates that the substance is from a series of saturated compounds.

When naming unfolded alkanes, the one that contains the maximum number of C atoms is chosen as the main chain. It is numbered so that the substituents are with the smallest number. In the case of two or more chains of the same length, the main one becomes the one that contains the largest number deputies.

Isomerism of alkanes

Methane CH₄ acts as the hydrocarbon-ancestor of their series. With each subsequent representative of the methane series, there is a difference from the previous one in the methylene group - CH₂. This regularity can be traced in the entire series of alkanes.

The German scientist Schiel put forward a proposal to call this series homological. Translated from Greek means "similar, similar."

Thus, a homologous series is a set of related organic compounds that have the same type of structure with similar chemical properties. Homologues are members of a given series. The homologous difference is the methylene group by which 2 neighboring homologues differ.

As mentioned earlier, the composition of any saturated hydrocarbon can be expressed using the general formula CnH₂n + 2. Thus, the next member of the homologous series after methane is ethane - C₂H₆. To derive its structure from methane, it is necessary to replace 1 H atom with CH₃ (figure below).

The structure of each subsequent homologue can be derived from the previous one in the same way. As a result, propane is formed from ethane - C₃H₈.

What are isomers?

These are substances that have an identical qualitative and quantitative molecular composition (identical molecular formula), but different chemical structure, as well as having different chemical properties.

The above hydrocarbons differ in such a parameter as the boiling point: -0.5 ° - butane, -10 ° - isobutane. This type isomerism is referred to as carbon skeletal isomerism, it refers to the structural type.

The number of structural isomers grows rapidly with the increase in the number of carbon atoms. Thus, C₁₀H₂₂ will correspond to 75 isomers (not including spatial ones), and for C₁₅H₃₂ 4347 isomers are already known, for C₂₀H₄₂ - 366,319.

So, it has already become clear what alkanes are, a homologous series, isomerism, nomenclature. Now it's time to move on to the IUPAC naming conventions.

IUPAC nomenclature: rules for the formation of names

First, it is necessary to find in the hydrocarbon structure the carbon chain that is the longest and contains the maximum number of substituents. Then it is required to number the C atoms of the chain, starting from the end to which the substituent is closest.

Secondly, the base is the name of a straight-chain saturated hydrocarbon, which corresponds to the most main chain in terms of the number of C atoms.

Thirdly, before the base it is necessary to indicate the numbers of locants near which the substituents are located. They are followed by the names of the substitutes with a hyphen.

Fourth, in the case of the presence of identical substituents at different atoms C locants are combined, and a multiplying prefix appears in front of the name: di - for two identical substituents, three - for three, tetra - four, penta - for five, etc. The numbers must be separated from each other by a comma, and from words - hyphen.

If the same C atom contains two substituents at once, the locant is also written twice.

According to these rules, the international nomenclature of alkanes is formed.

Newman projections

This American scientist proposed special projection formulas for the graphical demonstration of conformations - Newman projections. They correspond to forms A and B and are shown in the figure below.

In the first case, this is an A-shielded conformation, and in the second, it is a B-inhibited conformation. In position A, H atoms are located on minimum distance from each other. This form corresponds to the largest value of energy, due to the fact that the repulsion between them is the largest. This is an energetically unfavorable state, as a result of which the molecule tends to leave it and move to a more stable position B. Here, the H atoms are as far apart as possible. So, the energy difference between these positions is 12 kJ / mol, due to which the free rotation around the axis in the ethane molecule, which connects the methyl groups, is uneven. After getting into an energetically favorable position, the molecule lingers there, in other words, “slows down”. That is why it is called inhibited. The result - 10 thousand molecules of ethane are in a hindered form of conformation at room temperature. Only one has a different shape - obscured.

Getting saturated hydrocarbons

It has already become known from the article that these are alkanes (their structure, nomenclature are described in detail earlier). It would be useful to consider how to obtain them. They are isolated from such natural sources as oil, natural, coal. They also apply synthetic methods. For example, H₂ 2H₂:

1. Hydrogenation process CnH₂n (alkenes)→ CnH₂n+2 (alkanes)← CnH₂n-2 (alkynes).
2. From a mixture of monoxide C and H - synthesis gas: nCO+(2n+1)H₂→ CnH₂n+2+nH₂O.
3. From carboxylic acids (their salts): electrolysis at the anode, at the cathode:

• Kolbe electrolysis: 2RCOONa+2H₂O→R-R+2CO₂+H₂+2NaOH;
• Dumas reaction (alkali alloy): CH₃COONa+NaOH (t)→CH₄+Na₂CO₃.

1. Oil cracking: CnH₂n+2 (450-700°)→ CmH₂m+2+ Cn-mH₂(n-m).
2. Fuel gasification (solid): C+2H₂→CH₄.
3. Synthesis of complex alkanes (halogen derivatives) that have fewer C atoms: 2CH₃Cl (chloromethane) +2Na →CH₃- CH₃ (ethane) +2NaCl.
4. Water decomposition of methanides (metal carbides): Al₄C₃+12H₂O→4Al(OH₃)↓+3CH₄.

Physical properties of saturated hydrocarbons

For convenience, the data is grouped in a table.

Formula	Alkan	Melting point in °C	Boiling point in °C	Density, g/ml
				0.415 at t = -165°C
				0.561 at t= -100°C
				0.583 at t = -45°C
				0.579 at t =0°C
	2-methyl propane			0.557 at t = -25°C
	2,2-Dimethyl propane
	2-methylbutane
	2-Methylpentane
	2,2,3,3-Tetra-methylbutane
	2,2,4-trimethyl-pentane
n-C₁₀H₂₂
n-C₁₁H₂₄	n-undecan
n-C₁₂H₂₆	n-Dodecane
n-C₁₃H₂₈	n-Tridecan
n-C₁₄H₃₀	n-Tetradecane
n-C₁₅H₃₂	n-Pentadecan
n-C₁₆H₃₄	n-Hexadecane
n-C₂₀H₄₂	n-Eikosan
n-C₃₀H₆₂	n-Triacontan		1 mmHg st
n-C₄₀H₈₂	n-Tetracontane		3 mmHg Art.
n-C₅₀H₁₀₂	n-Pentacontan		15 mmHg Art.
n-C₆₀H₁₂₂	n-Hexacontan
n-C₇₀H₁₄₂	n-Heptacontane
n-C₁₀₀H₂₀₂

Conclusion

The article considered such a concept as alkanes (structure, nomenclature, isomerism, homologous series, etc.). A little is told about the features of the radial and substitution nomenclature. Methods for obtaining alkanes are described.

In addition, the entire nomenclature of alkanes is listed in detail in the article (the test can help to assimilate the information received).

One of the first types 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.

Homologs are called chemical substances, which 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 molecular formula which are 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 an alkane 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, starting from C5H12 and up to C13H28 - liquid and having a specific odor, all subsequent ones are solid. 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 σ-bonds between atoms C-C and C-H. 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 salt carboxylic acid with alkali when heated:

C2H5COONa + NaOH = Na2CO3 + C2H6.

In addition, methane is sometimes obtained in electric arc(C + 2H2 = CH4) or when aluminum carbide interacts 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.

Hydrocarbons are the simplest organic compounds. They are made up of carbon and hydrogen. Compounds of these two elements are called saturated hydrocarbons or alkanes. Their composition is expressed by the formula CnH2n+2 common to alkanes, where n is the number of carbon atoms.

Alkanes - the international name for these compounds. Also, these compounds are called paraffins and saturated hydrocarbons. The bond in alkane molecules is simple (or single). The remaining valences are saturated with hydrogen atoms. All alkanes are saturated with hydrogen to the limit, its atoms are in a state of sp3 hybridization.

Homologous series of saturated hydrocarbons

The first in the homologous series of saturated hydrocarbons is methane. Its formula is CH4. The ending -an in the name of saturated hydrocarbons is hallmark. Further, in accordance with the above formula, ethane - C2H6, propane C3H8, butane - C4H10 are located in the homologous series.

From the fifth alkane in the homologous series, the names of compounds are formed as follows: Greek number indicating the number of hydrocarbon atoms in the molecule + ending -an. So, in Greek, the number 5 is pende, respectively, butane is followed by pentane - C5H12. Next - hexane C6H14. heptane - C7H16, octane - C8H18, nonane - C9H20, decane - C10H22, etc.

The physical properties of alkanes change markedly in the homologous series: the melting point and boiling point increase, and the density increases. Methane, ethane, propane, butane under normal conditions, i.e. at a temperature of approximately 22 degrees Celsius, are gases, from pentane to hexadecane inclusive - liquids, from heptadecane - solids. Starting with butane, alkanes have isomers.

There are tables showing changes in the homologous series of alkanes, which clearly reflect their physical properties.

Nomenclature of saturated hydrocarbons, their derivatives

If a hydrogen atom is detached from a hydrocarbon molecule, then monovalent particles are formed, which are called radicals (R). The name of the radical is given by the hydrocarbon from which this radical is derived, while the ending -an changes to the ending -yl. For example, from methane, when a hydrogen atom is removed, a methyl radical is formed, from ethane - ethyl, from propane - propyl, etc.

Radicals are also formed in inorganic compounds. For example, by taking away the hydroxyl group OH from nitric acid, one can obtain a monovalent radical -NO2, which is called a nitro group.

When detached from a molecule an alkane of two hydrogen atoms, divalent radicals are formed, the names of which are also formed from the names of the corresponding hydrocarbons, but the ending changes to:

• ilien, in the event that hydrogen atoms are torn off from one carbon atom,
• ilene, in the event that two hydrogen atoms are torn off from two neighboring carbon atoms.

Alkanes: chemical properties

Consider the reactions characteristic of alkanes. All alkanes share common chemical properties. These substances are inactive.

All known reactions involving hydrocarbons are divided into two types:

• gap S-N connections(an example is a substitution reaction);
• rupture of the C-C bond (cracking, formation of separate parts).

Very active at the time of radical formation. By themselves, they exist for a fraction of a second. Radicals easily react with each other. Their unpaired electrons form a new covalent bond. Example: CH3 + CH3 → C2H6

Radicals readily react with organic molecules. They either attach to them or tear off an atom with an unpaired electron from them, as a result of which new radicals appear, which, in turn, can react with other molecules. With such a chain reaction, macromolecules are obtained that stop growing only when the chain breaks (example: the connection of two radicals)

Free radical reactions explain many important chemical processes such as:

• Explosions;
• oxidation;
• Oil cracking;
• Polymerization of unsaturated compounds.

in detail chemical properties can be considered saturated hydrocarbons on the example of methane. Above, we have already considered the structure of the alkane molecule. The carbon atoms are in the sp3 hybridization state in the methane molecule, and a sufficiently strong bond is formed. Methane is a gas of odor and color bases. It is lighter than air. It is slightly soluble in water.

Alkanes can burn. Methane burns with a bluish pale flame. In this case, the result of the reaction will be carbon monoxide and water. When mixed with air, as well as in a mixture with oxygen, especially if the volume ratio is 1:2, these hydrocarbons form explosive mixtures, which is why it is extremely dangerous for use in everyday life and mines. If methane does not burn completely, then soot is formed. In industry, it is obtained in this way.

Formaldehyde and methyl alcohol are obtained from methane by its oxidation in the presence of catalysts. If methane is strongly heated, then it decomposes according to the formula CH4 → C + 2H2

Methane decay can be carried out to an intermediate product in specially equipped furnaces. intermediate product will be acetylene. Reaction formula 2CH4 → C2H2 + 3H2. Separation of acetylene from methane reduces production costs by almost half.

Hydrogen is also produced from methane by converting methane with steam. Methane is characterized by substitution reactions. So, at ordinary temperature, in the light, halogens (Cl, Br) displace hydrogen from the methane molecule in stages. In this way, substances called halogen derivatives are formed. Chlorine atoms, substituting hydrogen atoms in a hydrocarbon molecule, form a mixture different compounds.

Such a mixture contains chloromethane (CH3 Cl or methyl chloride), dichloromethane (CH2Cl2 or methylene chloride), trichloromethane (CHCl3 or chloroform), carbon tetrachloride (CCl4 or carbon tetrachloride).

Any of these compounds can be isolated from a mixture. In production, chloroform and carbon tetrachloride are of great importance, due to the fact that they are solvents of organic compounds (fats, resins, rubber). Halogen derivatives of methane are formed by a chain free radical mechanism.

Light affects chlorine molecules, causing them to fall apart into inorganic radicals that abstract a hydrogen atom with one electron from a methane molecule. This produces HCl and methyl. Methyl reacts with a chlorine molecule, resulting in a halogen derivative and a chlorine radical. Further, the chlorine radical continues the chain reaction.

At ordinary temperatures, methane has sufficient resistance to alkalis, acids, and many oxidizing agents. An exception - Nitric acid. In the reaction with it, nitromethane and water are formed.

Addition reactions are not typical for methane, since all valences in its molecule are saturated.

Reactions involving hydrocarbons can take place not only with the splitting of the C-H bond, but also with the breaking of the C-C bond. These transformations take place at high temperatures. and catalysts. These reactions include dehydrogenation and cracking.

From saturated hydrocarbons, acids are obtained by oxidation - acetic (from butane), fatty acids (from paraffin).

Getting methane

In nature, methane widely distributed. He is the main component most combustible natural and artificial gases. It is released from the coal seams in the mines, from the bottom of the swamps. natural gases(which is very noticeable in the associated gases of oil fields) contain not only methane, but also other alkanes. The use of these substances is varied. They are used as fuel for various industries, in medicine and technology.

Under laboratory conditions, this gas is released by heating a mixture of sodium acetate + sodium hydroxide, as well as by the reaction of aluminum carbide and water. Methane is also obtained from simple substances. For this, the prerequisites are heating and catalyst. Of industrial importance is the production of methane by synthesis based on steam.

Methane and its homologues can be obtained by calcining salts of the corresponding organic acids with alkalis. Another way to obtain alkanes is the Wurtz reaction, in which monohalogen derivatives are heated with sodium metal.