Write down the chemical properties characteristic of alkanes. Alkanes: structure, nomenclature, isomerism
Heating sodium salt acetic acid(sodium acetate) with an excess of alkali leads to the elimination of the carboxyl group and the formation of methane:
CH3CONa + NaOH CH4 + Na2C03
If you take sodium propionate instead of sodium acetate, then ethane is formed, from sodium butanoate - propane, etc.
RCH2CONa + NaOH -> RCH3 + Na2C03
5. Wurtz synthesis. When haloalkanes interact with the alkali metal sodium, saturated hydrocarbons and an alkali metal halide are formed, for example:
The action of an alkali metal on a mixture of halocarbons (eg bromoethane and bromomethane) will result in the formation of a mixture of alkanes (ethane, propane and butane).
The reaction on which the Wurtz synthesis is based proceeds well only with haloalkanes in the molecules of which a halogen atom is attached to a primary carbon atom.
6. Hydrolysis of carbides. When some carbides containing carbon in the -4 oxidation state (for example, aluminum carbide) are treated with water, methane is formed:
Al4C3 + 12H20 = 3CH4 + 4Al(OH)3 Physical properties
The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a gas without color, taste and smell (the smell of “gas”, which you need to call 04, is determined by the smell of mercaptans - sulfur-containing compounds, specially added to methane used in household and industrial gas appliances, for so that people nearby can detect a leak by smell).
Hydrocarbons of composition from C5H12 to C15H32 are liquids, heavier hydrocarbons are solids.
The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents.
Chemical properties
1. Substitution reactions. The most characteristic reactions for alkanes are free radical substitution reactions, during which a hydrogen atom is replaced by a halogen atom or some group.
Let us present the equations of the most characteristic reactions.
Halogenation:
СН4 + С12 -> СН3Сl + HCl
In case of excess halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms with chlorine:
СН3Сl + С12 -> HCl + СН2Сl2
dichloromethane methylene chloride
СН2Сl2 + Сl2 -> HCl + CHCl3
trichloromethane chloroform
СНСl3 + Сl2 -> HCl + СCl4
carbon tetrachloride carbon tetrachloride
The resulting substances are widely used as solvents and starting materials in organic syntheses.
2. Dehydrogenation (elimination of hydrogen). When passing alkanes over a catalyst (Pt, Ni, Al2O3, Cr2O3) at high temperature(400-600 °C) a hydrogen molecule is eliminated and an alkene is formed:
CH3-CH3 -> CH2=CH2 + H2
3. Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons burn to form carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode. The combustion of saturated hydrocarbons is a free-radical exothermic reaction, which is very important when using alkanes as fuel.
CH4 + 2O2 -> C02 + 2H2O + 880kJ
IN general view The combustion reaction of alkanes can be written as follows:
Thermal decomposition reactions underlie the industrial process of hydrocarbon cracking. This process is the most important stage oil refining.
When methane is heated to a temperature of 1000 °C, methane pyrolysis begins - decomposition into simple substances. When heated to a temperature of 1500 °C, the formation of acetylene is possible.
4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton are formed: 
5. Flavoring. Alkanes with six or more carbon atoms in the chain cyclize in the presence of a catalyst to form benzene and its derivatives: 
What is the reason that alkanes undergo free radical reactions? All carbon atoms in alkane molecules are in a state of sp 3 hybridization. The molecules of these substances are built using covalent nonpolar C-C (carbon-carbon) bonds and weakly polar C-H (carbon-hydrogen) bonds. They do not contain areas with increased or decreased electron density, or easily polarizable bonds, i.e., such bonds in which the electron density can shift under the influence of external influences (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since the bonds in alkane molecules are not broken by a heterolytic mechanism.
The most characteristic reactions of alkanes are free radical substitution reactions. During these reactions, a hydrogen atom is replaced by a halogen atom or some group.
Kinetics and mechanism of free radicals chain reactions, i.e., reactions occurring under the influence of free radicals - particles with unpaired electrons - were studied by the remarkable Russian chemist N. N. Semenov. It was for these studies that he was awarded the Nobel Prize in Chemistry.
Typically, the mechanism of free radical substitution reactions is represented by three main stages:
1. Initiation (nucleation of a chain, formation of free radicals under the influence of an energy source - ultraviolet light, heating).
2. Chain development (a chain of sequential interactions of free radicals and inactive molecules, as a result of which new radicals and new molecules are formed).
3. Chain termination (combination of free radicals into inactive molecules (recombination), “death” of radicals, cessation of the development of a chain of reactions).
Scientific research by N.N. Semenov
Semenov Nikolay Nikolaevich
(1896 - 1986)
Soviet physicist and physical chemist, academician. Nobel Prize winner (1956). Scientific research relate to the doctrine of chemical processes, catalysis, chain reactions, theory of thermal explosion and combustion of gas mixtures.
Let's consider this mechanism using the example of the methane chlorination reaction:
CH4 + Cl2 -> CH3Cl + HCl
The initiation of the chain occurs as a result of the fact that under the influence ultraviolet irradiation or when heated, a homolytic cleavage of the Cl-Cl bond occurs and the chlorine molecule disintegrates into atoms:
Сl: Сl -> Сl· + Сl·
The resulting free radicals attack methane molecules, tearing off their hydrogen atom:
CH4 + Cl· -> CH3· + HCl
and transforming into CH3· radicals, which, in turn, colliding with chlorine molecules, destroy them with the formation of new radicals:
CH3 + Cl2 -> CH3Cl + Cl etc.
The chain develops.
Along with the formation of radicals, their “death” occurs as a result of the process of recombination - the formation of an inactive molecule from two radicals:
СН3+ Сl -> СН3Сl
Сl· + Сl· -> Сl2
CH3 + CH3 -> CH3-CH3
It is interesting to note that during recombination, only as much energy is released as is necessary to break the newly formed bond. In this regard, recombination is possible only if a third particle (another molecule, the wall of the reaction vessel) participates in the collision of two radicals, which absorbs excess energy. This makes it possible to regulate and even stop free radical chain reactions.
Note the last example of a recombination reaction - the formation of an ethane molecule. This example shows that a reaction involving organic compounds is a rather complex process, as a result of which, along with the main reaction product, by-products are very often formed, which leads to the need to develop complex and expensive methods for the purification and isolation of target substances.
The reaction mixture obtained from the chlorination of methane, along with chloromethane (CH3Cl) and hydrogen chloride, will contain: dichloromethane (CH2Cl2), trichloromethane (CHCl3), carbon tetrachloride (CCl4), ethane and its chlorination products.
Now let's try to consider the halogenation reaction (for example, bromination) of a more complex organic compound - propane.
If in the case of methane chlorination only one monochloro derivative is possible, then in this reaction two monobromo derivatives can be formed: 
It can be seen that in the first case, the hydrogen atom is replaced at the primary carbon atom, and in the second case, at the secondary one. Are the rates of these reactions the same? It turns out that the product of substitution of the hydrogen atom, which is located at the secondary carbon, predominates in the final mixture, i.e. 2-bromopropane (CH3-CHBg-CH3). Let's try to explain this.
In order to do this, we will have to use the idea of the stability of intermediate particles. Did you notice that when describing the mechanism of the methane chlorination reaction we mentioned the methyl radical - CH3·? This radical is an intermediate particle between methane CH4 and chloromethane CH3Cl. The intermediate particle between propane and 1-bromopropane is a radical with an unpaired electron at the primary carbon, and between propane and 2-bromopropane at the secondary carbon.
A radical with an unpaired electron at the secondary carbon atom (b) is more stable compared to a free radical with an unpaired electron at the primary carbon atom (a). It is formed in greater quantities. For this reason, the main product of the propane bromination reaction is 2-bromopropane, a compound whose formation occurs through a more stable intermediate species.
Here are some examples of free radical reactions:
Nitration reaction (Konovalov reaction) 
The reaction is used to obtain nitro compounds - solvents, starting materials for many syntheses.
Catalytic oxidation of alkanes with oxygen
These reactions are the basis of the most important industrial processes for the production of aldehydes, ketones, and alcohols directly from saturated hydrocarbons, for example:
CH4 + [O] -> CH3OH
Application
Saturated hydrocarbons, especially methane, are very wide application in industry (Scheme 2). They are simple and fairly cheap fuel, raw materials for obtaining large quantity the most important connections.
Compounds obtained from methane, the cheapest hydrocarbon raw material, are used to produce many other substances and materials. Methane is used as a source of hydrogen in the synthesis of ammonia, as well as to produce synthesis gas (a mixture of CO and H2), used for the industrial synthesis of hydrocarbons, alcohols, aldehydes and other organic compounds. 
Hydrocarbons of higher boiling oil fractions are used as fuel for diesel and turbojet engines, as the basis of lubricating oils, as raw materials for the production of synthetic fats, etc.
Here are several industrially significant reactions that occur with the participation of methane. Methane is used to produce chloroform, nitromethane, and oxygen-containing derivatives. Alcohols, aldehydes, carboxylic acids can be formed by the direct interaction of alkanes with oxygen, depending on the reaction conditions (catalyst, temperature, pressure): 
As you already know, hydrocarbons of the composition from C5H12 to C11H24 are included in the gasoline fraction of oil and are used mainly as fuel for internal combustion engines. It is known that the most valuable components of gasoline are isomeric hydrocarbons, since they have maximum detonation resistance.
When hydrocarbons come into contact with atmospheric oxygen, they slowly form compounds with it - peroxides. This is a slowly occurring free radical reaction, initiated by an oxygen molecule: 
Please note that the hydroperoxide group is formed at secondary carbon atoms, which are most abundant in linear, or normal, hydrocarbons.
At sharp increase pressure and temperature occurring at the end of the compression stroke, the decomposition of these peroxide compounds begins with the formation large number free radicals that “start” the free radical chain reaction of combustion earlier than necessary. The piston still goes up, and the combustion products of gasoline, which have already formed as a result of premature ignition of the mixture, push it down. This leads to a sharp decrease in engine power and wear.
Thus, the main cause of detonation is the presence of peroxide compounds, the ability to form which is maximum in linear hydrocarbons.
C-heptane has the lowest detonation resistance among the hydrocarbons of the gasoline fraction (C5H14 - C11H24). The most stable (i.e., forms peroxides to the least extent) is the so-called isooctane (2,2,4-trimethylpentane).
A generally accepted characteristic of the knock resistance of gasoline is the octane number. An octane number of 92 (for example, A-92 gasoline) means that this gasoline has the same properties as a mixture consisting of 92% isooctane and 8% heptane.
In conclusion, we can add that the use of high-octane gasoline makes it possible to increase the compression ratio (pressure at the end of the compression stroke), which leads to increased power and Engine efficiency internal combustion.
Being in nature and receiving
In today's lesson, you were introduced to the concept of alkanes, and also learned about its chemical composition and methods of obtaining. Therefore, let's now dwell in more detail on the topic of the presence of alkanes in nature and find out how and where alkanes have found application.
The main sources for the production of alkanes are natural gas and oil. They make up the bulk of oil refining products. Methane, widespread in sedimentary rock deposits, is also gas hydrate alkanes.
The main component of natural gas is methane, but it also contains a small proportion of ethane, propane and butane. Methane can be found in emissions from coal seams, swamps and associated petroleum gases.
Ankans can also be obtained by coking coal. In nature, there are also so-called solid alkanes - ozokerites, which are presented in the form of deposits mountain wax. Ozokerite can be found in the waxy coatings of plants or their seeds, as well as in beeswax.
The industrial isolation of alkanes is taken from natural sources, which, fortunately, are still inexhaustible. They are obtained by the catalytic hydrogenation of carbon oxides. Methane can also be produced in the laboratory using the method of heating sodium acetate with solid alkali or hydrolysis of certain carbides. But alkanes can also be obtained by decarboxylation of carboxylic acids and by their electrolysis.
Applications of alkanes
Alkanes at the household level are widely used in many areas of human activity. After all, it is very difficult to imagine our life without natural gas. And it will not be a secret to anyone that the basis of natural gas is methane, from which carbon black is produced, which is used in the production of topographic paints and tires. The refrigerator that everyone has in their home also works thanks to alkane compounds used as refrigerants. Acetylene obtained from methane is used for welding and cutting metals.
Now you already know that alkanes are used as fuel. They are present in gasoline, kerosene, diesel oil and fuel oil. In addition, they are also found in lubricating oils, petroleum jelly and paraffin.
Cyclohexane has found wide use as a solvent and for the synthesis of various polymers. Cyclopropane is used in anesthesia. Squalane, as a high-quality lubricating oil, is a component of many pharmaceutical and cosmetic preparations. Alkanes are the raw materials used to produce organic compounds such as alcohol, aldehydes and acids.
Paraffin is a mixture of higher alkanes, and since it is non-toxic, it is widely used in Food Industry. It is used for impregnation of packaging for dairy products, juices, cereals, etc., but also in the manufacture chewing gum. And heated paraffin is used in medicine for paraffin treatment.
In addition to the above, the heads of matches are impregnated with paraffin for better burning, pencils, and candles are made from it.
By oxidizing paraffin, oxygen-containing products are obtained, mainly organic acids. When mixing liquid hydrocarbons with a certain number Vaseline is obtained from carbon atoms, which is widely used in perfumery and cosmetology, as well as in medicine. It is used for cooking various ointments, creams and gels. They are also used for thermal procedures in medicine.
Practical tasks
1. Write down the general formula of hydrocarbons of the homologous series of alkanes.
2. Write the formulas of possible isomers of hexane and name them according to systematic nomenclature.
3. What is cracking? What types of cracking do you know?
4. Write the formulas of possible products of hexane cracking.
5. Decipher the following chain of transformations. Name the compounds A, B and C.
6. Give the structural formula of the hydrocarbon C5H12, which forms only one monobromine derivative upon bromination.
7. For the complete combustion of 0.1 mol of an alkane of unknown structure, 11.2 liters of oxygen were consumed (at ambient conditions). What is the structural formula of an alkane?
8. What is the structural formula of a gaseous saturated hydrocarbon if 11 g of this gas occupy a volume of 5.6 liters (at standard conditions)?
9. Recall what you know about the use of methane and explain why a domestic gas leak can be detected by smell, although its components are odorless.
10*. What compounds can be obtained by catalytic oxidation of methane to different conditions? Write the equations for the corresponding reactions.
eleven*. Products of complete combustion (in excess oxygen) 10.08 liters (N.S.) of a mixture of ethane and propane were passed through an excess of lime water. In this case, 120 g of sediment was formed. Determine the volumetric composition of the initial mixture.
12*. The ethane density of a mixture of two alkanes is 1.808. Upon bromination of this mixture, only two pairs of isomeric monobromoalkanes were isolated. The total mass of lighter isomers in the reaction products is equal to the total mass of heavier isomers. Determine the volume fraction of the heavier alkane in the initial mixture.
The table shows some representatives of a number of alkanes and their radicals.
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Formula |
Name |
Radical name |
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CH3 methyl |
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C3H7 cut |
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C4H9 butyl |
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isobutane |
isobutyl |
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isopentane |
isopentyl |
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neopentane |
neopentyl |
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The table shows that these hydrocarbons differ from each other in the number of groups - CH2 -. Such a series of similar in structure, having similar chemical properties and differing from each other in the number of these groups is called homologous series. And the substances that make it up are called homologues. Homologues - substances similar in structure and properties, but differing in composition by one or more homologous differences (- CH2 -)
Carbon chain - zigzag (if n ≥ 3) σ - bonds (free rotation around bonds) length (-C-C-) 0.154 nm binding energy (-C-C-) 348 kJ/mol All carbon atoms in alkane molecules are in a state of sp3 hybridization
angle between C-C connections is 109°28", so molecules of normal alkanes with a large number carbon atoms have a zigzag structure (zigzag). Length S-S connections in saturated hydrocarbons it is equal to 0.154 nm (1 nm = 1*10-9 m). a) electronic and structural formulas; b) spatial structure
4. Isomerism- STRUCTURAL isomerism of the chain with C4 is characteristic One of these isomers ( n-butane) contains an unbranched carbon chain, and the other, isobutane, contains a branched one (isostructure). The carbon atoms in a branched chain differ in the type of connection with other carbon atoms. Thus, a carbon atom bonded to only one other carbon atom is called primary, with two other carbon atoms - secondary, with three - tertiary, with four - quaternary. With an increase in the number of carbon atoms in the molecules, the possibilities for chain branching increase, i.e. the number of isomers increases with the number of carbon atoms. Comparative characteristics of homologues and isomers
1. They have their own nomenclature radicals(hydrocarbon radicals)
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I. ALKANES (saturated hydrocarbons, paraffins)
Alkanes are aliphatic (acyclic) saturated hydrocarbons in which the carbon atoms are linked together by simple (single) bonds in straight or branched chains.
Alkanes– the name of saturated hydrocarbons according to the international nomenclature.
Paraffins– a historically established name reflecting the properties of these compounds (from Lat. parrum affinis– having little affinity, low activity).
Limit, or saturated, these hydrocarbons are named due to the complete saturation of the carbon chain with hydrogen atoms.
The simplest representatives of alkanes:
When comparing these compounds, it is clear that they differ from each other by a group -CH 2 - (methylene). Adding another group to propane -CH 2 -, we get butane C 4 H 10, then alkanes C 5 H 12, C 6 H 14 etc.
Now we can derive the general formula of alkanes. The number of carbon atoms in the series of alkanes is taken to be n
, then the number of hydrogen atoms will be 2n+2
. Therefore, the composition of alkanes corresponds to the general formula C n H 2n+2.
Therefore, the following definition is often used:
- Alkanes- hydrocarbons, the composition of which is expressed by the general formula C n H 2n+2, Where n – number of carbon atoms.
II. Structure of alkanes
Chemical structure(the order of connection of atoms in molecules) of the simplest alkanes - methane, ethane and propane - are shown by their structural formulas. From these formulas it is clear that there are two types of chemical bonds in alkanes:
S–S And S–H.The C–C bond is covalent nonpolar. The C–H bond is covalent, weakly polar, because carbon and hydrogen are close in electronegativity (2.5 for carbon and 2.1 for hydrogen). The formation of covalent bonds in alkanes due to shared electron pairs of carbon and hydrogen atoms can be shown using electronic formulas:
Electronic and structural formulas reflect chemical structure, but do not give an idea about spatial structure of molecules, which significantly affects the properties of the substance.
Spatial structure, i.e. the relative arrangement of the atoms of a molecule in space depends on the direction atomic orbitals(AO) of these atoms. In hydrocarbons, the main role is played by the spatial orientation of the atomic orbitals of carbon, since the spherical 1s-AO of the hydrogen atom lacks a specific orientation.
The spatial arrangement of carbon AO, in turn, depends on the type of its hybridization. The saturated carbon atom in alkanes is bonded to four other atoms. Therefore, its state corresponds to sp 3 hybridization. In this case, each of the four sp 3 -hybrid carbon AOs participates in axial (σ-) overlap with the s-AO of hydrogen or with the sp 3 -AO of another carbon atom, forming σ-CH or C-C bonds.
The four σ-bonds of carbon are directed in space at an angle of 109 about 28", which corresponds to the least repulsion of electrons. Therefore, the molecule of the simplest representative of alkanes - methane CH4 - has the shape of a tetrahedron, in the center of which there is a carbon atom, and at the vertices there are hydrogen atoms:
Valence angle N-S-N equals 109 o 28". The spatial structure of methane can be shown using volumetric (scale) and ball-and-stick models.
For recording, it is convenient to use a spatial (stereochemical) formula.
In the molecule of the next homologue - ethane C 2 H 6 - two tetrahedral sp 3 carbon atoms form a more complex spatial structure:
2. If the molecules have the same composition and the same chemical structure different relative positions of atoms in space are possible, then it is observed spatial isomerism (stereoisomerism). In this case, the use of structural formulas is not enough and molecular models or special formulas - stereochemical (spatial) or projection - should be used.
Alkanes, starting with ethane H 3 C–CH 3, exist in various spatial forms ( conformations), caused by intramolecular rotation along C–C σ bonds, and exhibit the so-called rotational (conformational) isomerism.
Various spatial forms of a molecule that transform into each other by rotating around C–C σ bonds are called conformations or rotary isomers(conformers).
Rotational isomers of a molecule are its energetically unequal states. Their interconversion occurs quickly and constantly as a result of thermal movement. Therefore, rotary isomers cannot be isolated individually, but their existence has been proven by physical methods. Some conformations are more stable (energetically favorable) and the molecule remains in such states more long time.
3. In addition, if a molecule contains a carbon atom bonded to 4 different substituents, another type of spatial isomerism is possible -
optical isomerism.For example:
then the existence of two compounds with the same structural formula, but differing in spatial structure, is possible. The molecules of such compounds relate to each other as an object and its mirror image and are spatial isomers.
This type of isomerism is called optical; isomers are called optical isomers or optical antipodes:
Molecules of optical isomers are incompatible in space (both left and right hand), they lack a plane of symmetry.
Thus,optical isomers are called spatial isomers, the molecules of which are related to each other as an object and an incompatible mirror image.
Optical isomers have the same physical and Chemical properties, but differ in their attitude to polarized light. Such isomers have optical activity (one of them rotates the plane of polarized light to the left, and the other by the same angle to the right). Differences in chemical properties are observed only in reactions with optically active reagents.
Optical isomerism manifests itself in organic substances of various classes and plays a very important role important role in the chemistry of natural compounds.
Saturated hydrocarbons are compounds that are molecules consisting of carbon atoms in a state of sp 3 hybridization. They are connected to each other exclusively by covalent sigma bonds. The name "saturated" or "saturated" hydrocarbons comes from the fact that these compounds do not have the ability to attach any atoms. They are extreme, completely saturated. The exception is cycloalkanes.
What are alkanes?
Alkanes are saturated hydrocarbons, and their carbon chain is open and consists of carbon atoms connected to each other using single bonds. It does not contain other (that is, double, like alkenes, or triple, like alkyls) bonds. Alkanes are also called paraffins. They received this name because well-known paraffins are a mixture of predominantly these saturated hydrocarbons C 18 -C 35 with particular inertness.
General information about alkanes and their radicals
Their formula: C n P 2 n +2, here n is greater than or equal to 1. The molar mass is calculated by the formula: M = 14n + 2. Feature: The endings in their names are “-an”. The residues of their molecules, which are formed as a result of the replacement of hydrogen atoms with other atoms, are called aliphatic radicals, or alkyls. They are designated by the letter R. The general formula of monovalent aliphatic radicals: C n P 2 n +1, here n is greater than or equal to 1. Molar mass aliphatic radicals are calculated by the formula: M = 14n + 1. A characteristic feature of aliphatic radicals: the endings in the names are “-yl”. Alkane molecules have their own structural features:
- The C-C bond is characterized by a length of 0.154 nm;
- The C-H bond is characterized by a length of 0.109 nm;
- the bond angle (the angle between carbon-carbon bonds) is 109 degrees and 28 minutes.
Alkanes begin the homologous series: methane, ethane, propane, butane, and so on.
Physical properties of alkanes
Alkanes are substances that are colorless and insoluble in water. The temperature at which alkanes begin to melt and the temperature at which they boil increase in accordance with the increase in molecular weight and hydrocarbon chain length. From less branched to more branched alkanes, the boiling and melting points decrease. Gaseous alkanes can burn with a pale blue or colorless flame and produce quite a lot of heat. CH 4 -C 4 H 10 are gases that also have no odor. C 5 H 12 -C 15 H 32 are liquids that have a specific odor. C 15 H 32 and so on are solids that are also odorless.
Chemical properties of alkanes
These compounds are chemically inactive, which can be explained by the strength of difficult-to-break sigma bonds - C-C and C-H. It is also worth considering that C-C bonds are non-polar, and C-H bonds are low-polar. These are low-polarized types of bonds belonging to the sigma type and, accordingly, they are most likely to be broken by a homolytic mechanism, as a result of which radicals will be formed. Thus, the chemical properties of alkanes are mainly limited to radical substitution reactions.
Nitration reactions
Alkanes react only with nitric acid with a concentration of 10% or with tetravalent nitrogen oxide in a gaseous environment at a temperature of 140°C. The nitration reaction of alkanes is called the Konovalov reaction. As a result, nitro compounds and water are formed: CH 4 + nitric acid (diluted) = CH 3 - NO 2 (nitromethane) + water.
Combustion reactions
Saturated hydrocarbons are very often used as fuel, which is justified by their ability to burn: C n P 2n+2 + ((3n+1)/2) O 2 = (n+1) H 2 O + n CO 2.
Oxidation reactions
The chemical properties of alkanes also include their ability to oxidize. Depending on what conditions accompany the reaction and how they are changed, different end products can be obtained from the same substance. Mild oxidation of methane with oxygen in the presence of a catalyst accelerating the reaction and a temperature of about 200 ° C can result in the following substances:
1) 2CH 4 (oxidation with oxygen) = 2CH 3 OH (alcohol - methanol).
2) CH 4 (oxidation with oxygen) = CH 2 O (aldehyde - methanal or formaldehyde) + H 2 O.
3) 2CH 4 (oxidation with oxygen) = 2HCOOH (carboxylic acid - methane or formic) + 2H 2 O.
Also, the oxidation of alkanes can be carried out in a gaseous or liquid medium with air. Such reactions lead to the formation of higher fatty alcohols and corresponding acids.
Relation to heat
At temperatures not exceeding +150-250°C, always in the presence of a catalyst, a structural rearrangement of organic substances occurs, which consists of a change in the order of connection of atoms. This process is called isomerization, and the substances resulting from the reaction are called isomers. Thus, from normal butane, its isomer is obtained - isobutane. At temperatures of 300-600°C and the presence of a catalyst, C-H bonds are broken with the formation of hydrogen molecules (dehydrogenation reactions), hydrogen molecules with the closure of the carbon chain into a cycle (cyclization or aromatization reactions of alkanes):
1) 2CH 4 = C 2 H 4 (ethene) + 2H 2.
2) 2CH 4 = C 2 H 2 (ethyne) + 3H 2.
3) C 7 H 16 (normal heptane) = C 6 H 5 - CH 3 (toluene) + 4 H 2.
Halogenation reactions
Such reactions involve the introduction of halogens (their atoms) into the molecule of an organic substance, resulting in the formation of a C-halogen bond. When alkanes react with halogens, halogen derivatives are formed. This reaction has specific features. It proceeds according to a radical mechanism, and in order to initiate it, it is necessary to expose the mixture of halogens and alkanes to ultraviolet radiation or simply heat it. The properties of alkanes allow the halogenation reaction to proceed until complete replacement with halogen atoms is achieved. That is, the chlorination of methane will not end in one stage and the production of methyl chloride. The reaction will go further, all possible substitution products will be formed, starting with chloromethane and ending with carbon tetrachloride. Exposure of other alkanes to chlorine under these conditions will result in the formation various products, obtained by replacing hydrogen at various carbon atoms. The temperature at which the reaction occurs will determine the ratio of the final products and the rate of their formation. The longer the hydrocarbon chain of the alkane, the easier the reaction will be. During halogenation, the least hydrogenated (tertiary) carbon atom will be replaced first. The primary one will react after all the others. The halogenation reaction will occur in stages. In the first stage, only one hydrogen atom is replaced. Alkanes do not interact with halogen solutions (chlorine and bromine water).
Sulfochlorination reactions
The chemical properties of alkanes are also complemented by the sulfochlorination reaction (called the Reed reaction). When exposed to ultraviolet radiation, alkanes are able to react with a mixture of chlorine and sulfur dioxide. As a result, hydrogen chloride is formed, as well as an alkyl radical, which adds sulfur dioxide. The result is a complex compound that becomes stable due to the capture of a chlorine atom and the destruction of its next molecule: R-H + SO 2 + Cl 2 + ultraviolet radiation= R-SO 2 Cl + HCl. The sulfonyl chlorides formed as a result of the reaction are widely used in the production of surfactants.