It would be useful to start with a definition of the concept of alkanes. These are saturated or saturated. 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 clearly expressed, so their second name is the phrase saturated or saturated hydrocarbons.
There is also an older name that best reflects their relative chemical inertness - paraffins, which means “devoid of affinity.”
So, the topic of our conversation today is: “Alkanes: homological series, nomenclature, structure, isomerism.” Data regarding their physical properties will also be presented.
Alkanes: structure, nomenclature
In them, the C atoms are in a state called sp3 hybridization. In this regard, the alkane molecule can be demonstrated as a set of tetrahedral C structures that are connected not only to each other, but also to H.
Between the C and H atoms there are strong, very low-polar s-bonds. Atoms always rotate around simple bonds, which is why alkane molecules take on various shapes, and the bond length and the angle between them are constant values. Shapes that transform into each other due to the rotation of the molecule around σ bonds are usually called conformations.
In the process of abstraction of an H atom from the molecule in question, 1-valent species called hydrocarbon radicals are formed. They appear as a result of not only but also inorganic compounds. If you subtract 2 hydrogen atoms from a saturated hydrocarbon molecule, you get 2-valent radicals.
Thus, the nomenclature of alkanes can be:
- radial (old version);
- substitution (international, systematic). It was proposed by IUPAC.
Features of radial nomenclature
In the first case, the nomenclature of alkanes is characterized as follows:
- Consideration of hydrocarbons as derivatives of methane, in which 1 or several H atoms are replaced by radicals.
- High degree of convenience in the case of not very complex connections.
Features of substitution nomenclature
The substitutive nomenclature of alkanes has the following features:
- The basis for the name is 1 carbon chain, while the remaining molecular fragments are considered as substituents.
- 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: nomenclature of alkanes
For convenience, the information is presented in table form.
Substance name | The basis of the name (root) | Molecular formula | Name of carbon substituent | Carbon Substituent Formula |
The above nomenclature of alkanes includes names that have developed historically (the first 4 members of the series of saturated hydrocarbons).
The names of unexpanded 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 composing the names of unfolded alkanes, the main chain is the one that contains the maximum number of C atoms. It is numbered so that the substituents have the lowest number. In the case of two or more chains of the same length, the main one becomes the one that contains the largest number of substituents.
Isomerism of alkanes
The parent hydrocarbon of their series is methane CH₄. With each subsequent representative of the methane series, a difference from the previous one is observed in the methylene group - CH₂. This pattern can be traced throughout the entire series of alkanes.
The German scientist Schiel put forward a proposal to call this series homological. Translated from Greek it means “similar, similar.”
Thus, a homologous series is a set of related organic compounds that have the same structure and similar chemical properties. Homologues are members of a given series. Homologous difference is a methylene group in 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 convert its structure from methane, it is necessary to replace 1 H atom with CH₃ (figure below).
The structure of each subsequent homolog can be deduced 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 a different chemical structure, and also have different chemical properties.
The hydrocarbons discussed above differ in such a parameter as boiling point: -0.5° - butane, -10° - isobutane. This type of isomerism is called carbon skeleton isomerism; it belongs to the structural type.
The number of structural isomers increases rapidly as the number of carbon atoms increases. 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, homologous series, isomerism, nomenclature. Now it’s worth moving on to the rules for compiling names according to IUPAC.
IUPAC nomenclature: rules for the formation of names
First, it is necessary to find in the hydrocarbon structure the carbon chain that is longest and contains the maximum number of substituents. Then you need 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 an unbranched saturated hydrocarbon, which, in terms of the number of C atoms, corresponds to the main chain.
Thirdly, before the base it is necessary to indicate the numbers of the locants near which the substituents are located. The names of the substituents are written after them with a hyphen.
Fourthly, in the case of the presence of identical substituents at different C atoms, the locants are combined, and a multiplying prefix appears before the name: di - for two identical substituents, three - for three, tetra - four, penta - for five, etc. Numbers must be separated from each other by a comma, and from words by a 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 graphical demonstration of conformations - Newman projections. They correspond to forms A and B and are presented in the figure below.
In the first case, this is an A-occluded conformation, and in the second, it is a B-inhibited conformation. In position A, the H atoms are located at a minimum distance from each other. This form corresponds to the highest energy value, due to the fact that the repulsion between them is greatest. 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 from each other. Thus, 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 entering an energetically favorable position, the molecule lingers there, in other words, “slows down.” That is why it is called inhibited. The result is that 10 thousand ethane molecules are in the inhibited form of conformation at room temperature. Only one has a different shape - obscured.
Obtaining saturated hydrocarbons
From the article it has already become known that these are alkanes (their structure and nomenclature were described in detail earlier). It would be useful to consider ways to obtain them. They are released from natural sources such as oil, natural, and coal. Synthetic methods are also used. For example, H₂ 2H₂:
- Hydrogenation process CnH₂n (alkenes)→ CnH₂n+2 (alkanes)← CnH₂n-2 (alkynes).
- From a mixture of C and H monoxide - synthesis gas: nCO+(2n+1)H₂→ CnH₂n+2+nH₂O.
- From carboxylic acids (their salts): electrolysis at the anode, at the cathode:
- Kolbe electrolysis: 2RCOONa+2H₂O→R-R+2CO₂+H₂+2NaOH;
- Dumas reaction (alloy with alkali): CH₃COONa+NaOH (t)→CH₄+Na₂CO₃.
- Oil cracking: CnH₂n+2 (450-700°)→ CmH₂m+2+ Cn-mH₂(n-m).
- Gasification of fuel (solid): C+2H₂→CH₄.
- Synthesis of complex alkanes (halogen derivatives) that have fewer C atoms: 2CH₃Cl (chloromethane) +2Na →CH₃- CH₃ (ethane) +2NaCl.
- Decomposition of methanides (metal carbides) by water: Al₄C₃+12H₂O→4Al(OH₃)↓+3CH₄.
Physical properties of saturated hydrocarbons
For convenience, the data is grouped into a table.
Formula | Alkane | Melting point in °C | Boiling point in °C | Density, g/ml |
0.415 at t = -165°С |
||||
0.561 at t= -100°C |
||||
0.583 at t = -45°C |
||||
0.579 at t =0°C |
||||
2-Methylpropane | 0.557 at t = -25°C |
|||
2,2-Dimethylpropane | ||||
2-Methylbutane | ||||
2-Methylpentane | ||||
2,2,3,3-Tetra-methylbutane | ||||
2,2,4-Trimethylpentane | ||||
n-C₁₀H₂₂ | ||||
n-C₁₁H₂₄ | n-Undecane | |||
n-C₁₂H₂₆ | n-Dodecane | |||
n-C₁₃H₂₈ | n-Tridecan | |||
n-C₁₄H₃₀ | n-Tetradecane | |||
n-C₁₅H₃₂ | n-Pentadecan | |||
n-C₁₆H₃₄ | n-Hexadecane | |||
n-C₂₀H₄₂ | n-Eicosane | |||
n-C₃₀H₆₂ | n-Triacontan | 1 mmHg st | ||
n-C₄₀H₈₂ | n-Tetracontane | 3 mmHg Art. | ||
n-C₅₀H₁₀₂ | n-Pentacontan | 15 mmHg Art. | ||
n-C₆₀H₁₂₂ | n-Hexacontane | |||
n-C₇₀H₁₄₂ | n-Heptacontane | |||
n-C₁₀₀H₂₀₂ |
Conclusion
The article examined such a concept as alkanes (structure, nomenclature, isomerism, homologous series, etc.). A little is said about the features of radial and substitutive nomenclatures. Methods for obtaining alkanes are described.
In addition, the article lists in detail the entire nomenclature of alkanes (the test can help you assimilate the information received).
Alkanes (paraffins or saturated hydrocarbons)– the simplest class of organic compounds in terms of elemental composition. They are composed of carbon and hydrogen. The ancestor of this class is methane CH 4. All other hydrocarbons classified as alkanes are members of the homologous methane series. General formula of alkanes C n H 2 n +2
Carbon has four valence electrons on its outer shell, so it can form four two-electron covalent bonds with hydrogen atoms:
When moving to higher homologs, the number of isomers increases sharply (see above).
A carbon atom bonded to one adjacent carbon atom is called primary, with two - secondary, with three – tertiary and with four - quaternary:
Several nomenclatures can be used to name alkanes: historical or trivial nomenclature - This is a summary of historically established names for commonly used organic compounds. – rational nomenclature. When compiling a name according to this nomenclature, the compound is considered as obtained from the simplest representative of the series as a result of the replacement of hydrogen atoms in it by alkyl radicals.
– IUPAC nomenclature
Homologous series, a sequence of organic compounds with the same functional groups and the same structure, each member of which differs from its neighbor by a permanent structural unit (homologous difference), most often a methylene group -CH 2 -. Members of a homologous series are called homologues. In homologous series, many physical properties change naturally. For example, the boiling points in the middle of a series of straight-chain compounds (C 5 -C 14) differ between neighboring homologues by 20-30 ° C; the homologous difference -CH 2 - corresponds to an increase in the heat of combustion by 630-640 kJ/mol and molecular refraction by 4.6 for the sodium D-line. In the higher members of the homologous series, these differences are gradually smoothed out.
Physical and chemical properties of alkanes. Methods for obtaining and identifying alkanes. Individual representatives.
Physical properties of alkanes.
The first four members of the series - methane, ethane, propane and butane - are gases at room conditions. Alkanes C 5 – C 15 are liquid, and C 16 and beyond are solid.
Under normal conditions
Chemical properties of alkanes
Hydrocarbons of the methane series are chemically very inert at ordinary temperatures. They do not add hydrogen (hence the limiting ones), do not react without initiation with Cl 2 and Br 2, and are not oxidized in the cold by such strong oxidizing agents as potassium permanganate and chromic acid. At the same time, these bonds are relatively easily subject to homolytic cleavage with the formation of radicals. That's why radical substitution reactions are more common for alkanes.
– Halogenation
In the light, alkanes can successively replace hydrogen atoms with halogen atoms, for example:
At temperature » 500 °C Methane is nitrated under the influence of nitric acid and nitrogen dioxide:
– Sulfonation
Sulfuric acid (oleum) slowly sulfonates alkanes when heated with tertiary carbon atom:
– Sulfochlorination
Under the influence of ultraviolet lighting, alkanes undergo a substitution reaction with a mixture of SO 2 + Cl 2:
– Oxidation
In isoalkanes, the tertiary group CH is relatively easily oxidized. Of industrial interest is the catalytic oxidation of a mixture of higher saturated hydrocarbons C 8 - C 18:
– Dehydrogenation
At t = 300 °C...400 °C, alkanes passed over the catalyst lose two hydrogen atoms and turn into alkenes:
– Isomerization
Under the influence of acid catalysts (for example, AlCl 3, H 2 SO 4, etc.), alkanes are capable of restructuring the carbon skeleton:
Methods for producing alkanes
– Hydrogenation of unsaturated hydrocarbons
– From alkyl halides ( Wurtz reaction, 1870)
– From carboxylic acids
– Cracking and pyrolysis of petroleum alkanes:
5. Alkenes. General characteristics: structure, isomerism, nomenclature.
The homologous series of alkenes begins with ethylene. Alkenes(olefins, ethylene hydrocarbons) – hydrocarbons that contain one double bond in the molecule. The general formula is C n H 2n.
Isomerism. Nomenclature
As in the series of saturated hydrocarbons, the structural isomerism of alkenes begins with the fourth member of the series. However, the number of isomers is much larger. The isomerism of olefins is determined by the structure of the carbon chain, secondly, by the position of the double bond in the chain, and thirdly, by the spatial arrangement of atoms or groups at carbons with a double bond
Alkenes are called by different nomenclatures. In trivial nomenclature, the suffix –ene is added to the name of the corresponding saturated hydrocarbon radical: ethylene, propylene, butylene, isobutylene, amylene, etc. According to rational nomenclature, olefins are called derivatives of ethylene. When naming the compound using IUPAC nomenclature, the longest carbon chain containing a double bond is selected as the backbone of the compound. The name is based on the name of the alkane with the ending -an replaced by
-en. The number indicates the number of the carbon atom followed by the double bond. The carbon atoms of the main chain should be numbered from the end to which the double bond is closest.
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 + Na2C03
If you take sodium propionate instead of sodium acetate, then ethane is formed, from sodium butanoate - propane, etc.
RCH2CONa + NaOH -> RCH3 + Na2C03
5. Wurtz synthesis. When haloalkanes interact with the alkali metal sodium, saturated hydrocarbons and an alkali metal halide are formed, for example:
The action of an alkali metal on a mixture of halocarbons (eg bromoethane and bromomethane) will result in the formation of a mixture of alkanes (ethane, propane and butane).
The reaction on which the Wurtz synthesis is based proceeds well only with haloalkanes in the molecules of which a halogen atom is attached to a primary carbon atom.
6. Hydrolysis of carbides. When some carbides containing carbon in the -4 oxidation state (for example, aluminum carbide) are treated with water, methane is formed:
Al4C3 + 12H20 = 3CH4 + 4Al(OH)3 Physical properties
The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a gas without color, taste and smell (the smell of “gas”, which you need to call 04, is determined by the smell of mercaptans - sulfur-containing compounds, specially added to methane used in household and industrial gas appliances, for so that people nearby can detect a leak by smell).
Hydrocarbons of composition from C5H12 to C15H32 are liquids, heavier hydrocarbons are solids.
The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents.
Chemical properties
1. Substitution reactions. The most characteristic reactions for alkanes are free radical substitution reactions, during which a hydrogen atom is replaced by a halogen atom or some group.
Let us present the equations of the most characteristic reactions.
Halogenation:
СН4 + С12 -> СН3Сl + HCl
In case of excess halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms with chlorine:
СН3Сl + С12 -> HCl + СН2Сl2
dichloromethane methylene chloride
СН2Сl2 + Сl2 -> HCl + CHCl3
trichloromethane chloroform
СНСl3 + Сl2 -> HCl + СCl4
carbon tetrachloride carbon tetrachloride
The resulting substances are widely used as solvents and starting materials in organic syntheses.
2. Dehydrogenation (elimination of hydrogen). When alkanes are passed over a catalyst (Pt, Ni, Al2O3, Cr2O3) at high temperatures (400-600 °C), a hydrogen molecule is eliminated and an alkene is formed:
CH3-CH3 -> CH2=CH2 + H2
3. Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons burn to form carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode. The combustion of saturated hydrocarbons is a free-radical exothermic reaction, which is very important when using alkanes as fuel.
CH4 + 2O2 -> C02 + 2H2O + 880kJ
In general, 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 of oil refining.
When methane is heated to a temperature of 1000 ° C, methane pyrolysis begins - decomposition into simple substances. When heated to a temperature of 1500 °C, the formation of acetylene is possible.
4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton are formed: 
5. Flavoring. Alkanes with six or more carbon atoms in the chain cyclize in the presence of a catalyst to form benzene and its derivatives: 
What is the reason that alkanes undergo free radical reactions? All carbon atoms in alkane molecules are in a state of sp 3 hybridization. The molecules of these substances are built using covalent nonpolar C-C (carbon-carbon) bonds and weakly polar C-H (carbon-hydrogen) bonds. They do not contain areas with increased or decreased electron density, or easily polarizable bonds, i.e., such bonds in which the electron density can shift under the influence of external influences (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since the bonds in alkane molecules are not broken by a heterolytic mechanism.
The most characteristic reactions of alkanes are free radical substitution reactions. During these reactions, a hydrogen atom is replaced by a halogen atom or some group.
The kinetics and mechanism of free radical chain reactions, i.e. reactions occurring under the influence of free radicals - particles with unpaired electrons - were studied by the remarkable Russian chemist N. N. Semenov. It was for these studies that he was awarded the Nobel Prize in Chemistry.
Typically, the mechanism of free radical substitution reactions is represented by three main stages:
1. Initiation (nucleation of a chain, formation of free radicals under the influence of an energy source - ultraviolet light, heating).
2. Chain development (a chain of sequential interactions of free radicals and inactive molecules, as a result of which new radicals and new molecules are formed).
3. Chain termination (combination of free radicals into inactive molecules (recombination), “death” of radicals, cessation of the development of a chain of reactions).
Scientific research by N.N. Semenov
Semenov Nikolay Nikolaevich
(1896 - 1986)
Soviet physicist and physical chemist, academician. Nobel Prize winner (1956). Scientific research relates to the study of chemical processes, catalysis, chain reactions, the theory of thermal explosion and the combustion of gas mixtures.
Let's consider this mechanism using the example of the methane chlorination reaction:
CH4 + Cl2 -> CH3Cl + HCl
Chain initiation occurs as a result of the fact that under the influence of ultraviolet irradiation or heating, homolytic cleavage of the Cl-Cl bond occurs and the chlorine molecule disintegrates into atoms:
Сl: Сl -> Сl· + Сl·
The resulting free radicals attack methane molecules, tearing off their hydrogen atom:
CH4 + Cl· -> CH3· + HCl
and transforming into CH3· radicals, which, in turn, colliding with chlorine molecules, destroy them with the formation of new radicals:
CH3 + Cl2 -> CH3Cl + Cl etc.
The chain develops.
Along with the formation of radicals, their “death” occurs as a result of the process of recombination - the formation of an inactive molecule from two radicals:
СН3+ Сl -> СН3Сl
Сl· + Сl· -> Сl2
CH3 + CH3 -> CH3-CH3
It is interesting to note that during recombination, only as much energy is released as is necessary to break the newly formed bond. In this regard, recombination is possible only if a third particle (another molecule, the wall of the reaction vessel) participates in the collision of two radicals, which absorbs excess energy. This makes it possible to regulate and even stop free radical chain reactions.
Note the last example of a recombination reaction - the formation of an ethane molecule. This example shows that a reaction involving organic compounds is a rather complex process, as a result of which, along with the main reaction product, by-products are very often formed, which leads to the need to develop complex and expensive methods for the purification and isolation of target substances.
The reaction mixture obtained from the chlorination of methane, along with chloromethane (CH3Cl) and hydrogen chloride, will contain: dichloromethane (CH2Cl2), trichloromethane (CHCl3), carbon tetrachloride (CCl4), ethane and its chlorination products.
Now let's try to consider the halogenation reaction (for example, bromination) of a more complex organic compound - propane.
If in the case of methane chlorination only one monochloro derivative is possible, then in this reaction two monobromo derivatives can be formed: 
It can be seen that in the first case, the hydrogen atom is replaced at the primary carbon atom, and in the second case, at the secondary one. Are the rates of these reactions the same? It turns out that the product of substitution of the hydrogen atom, which is located at the secondary carbon, predominates in the final mixture, i.e. 2-bromopropane (CH3-CHBg-CH3). Let's try to explain this.
In order to do this, we will have to use the idea of the stability of intermediate particles. Did you notice that when describing the mechanism of the methane chlorination reaction we mentioned the methyl radical - CH3·? This radical is an intermediate particle between methane CH4 and chloromethane CH3Cl. The intermediate particle between propane and 1-bromopropane is a radical with an unpaired electron at the primary carbon, and between propane and 2-bromopropane at the secondary carbon.
A radical with an unpaired electron at the secondary carbon atom (b) is more stable compared to a free radical with an unpaired electron at the primary carbon atom (a). It is formed in greater quantities. For this reason, the main product of the propane bromination reaction is 2-bromopropane, a compound whose formation occurs through a more stable intermediate species.
Here are some examples of free radical reactions:
Nitration reaction (Konovalov reaction) 
The reaction is used to obtain nitro compounds - solvents, starting materials for many syntheses.
Catalytic oxidation of alkanes with oxygen
These reactions are the basis of the most important industrial processes for the production of aldehydes, ketones, and alcohols directly from saturated hydrocarbons, for example:
CH4 + [O] -> CH3OH
Application
Saturated hydrocarbons, especially methane, are widely used in industry (Scheme 2). They are a simple and fairly cheap fuel, a raw material for the production of a large number of important compounds.
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.
With a sharp increase in pressure and temperature that occurs at the end of the compression stroke, the decomposition of these peroxide compounds begins with the formation of a large number of free radicals, which “trigger” the free radical combustion chain reaction 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 efficiency of the internal combustion engine.
Being in nature and receiving
In today's lesson, you became acquainted with the concept of alkanes, and also learned about its chemical composition and methods of preparation. 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, common in sedimentary rock deposits, 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 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 of 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 the food industry. It is used for impregnation of packaging for dairy products, juices, cereals, etc., but also in the manufacture of 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, mainly organic acids, are obtained. When liquid hydrocarbons with a certain number of carbon atoms are mixed, Vaseline is obtained, which is widely used in perfumery and cosmetology, as well as in medicine. It is used to prepare 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 under various conditions? Write the equations for the corresponding reactions.
eleven*. Products of complete combustion (in excess oxygen) 10.08 liters (N.S.) of a mixture of ethane and propane were passed through an excess of lime water. In this case, 120 g of sediment was formed. Determine the volumetric composition of the initial mixture.
12*. The ethane density of a mixture of two alkanes is 1.808. Upon bromination of this mixture, only two pairs of isomeric monobromoalkanes were isolated. The total mass of lighter isomers in the reaction products is equal to the total mass of heavier isomers. Determine the volume fraction of the heavier alkane in the initial mixture.
Structure of alkanes
Alkanes are hydrocarbons in whose molecules the atoms are connected by single bonds and which correspond to the general formula C n H 2n+2. In alkane molecules, all carbon atoms are in the state sp 3 -hybridization.
This means that all four hybrid orbitals of the carbon atom are identical in shape, energy and are directed towards the corners of an equilateral triangular pyramid - tetrahedron. The angles between the orbitals are 109° 28′. Almost free rotation is possible around a single carbon-carbon bond, and alkane molecules can take on a wide variety of shapes with angles at the carbon atoms close to tetrahedral (109° 28′), for example, in the n-pentane molecule.
It is especially worth recalling the bonds in alkane molecules. All bonds in the molecules of saturated hydrocarbons are single. The overlap occurs along the axis connecting the nuclei of atoms, i.e. it σ bonds. Carbon-carbon bonds are non-polar and poorly polarizable. The length of the C-C bond in alkanes is 0.154 nm (1.54 10 10 m). C-H bonds are somewhat shorter. The electron density is slightly shifted towards the more electronegative carbon atom, i.e. the C-H bond is weakly polar.
Homologous series of methane
Homologues- substances that are similar in structure and properties and differ in one or more CH groups 2 .
Saturated hydrocarbons constitute the homologous series of methane.
Isomerism and nomenclature of alkanes
Alkanes are characterized by the so-called structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkane, which is characterized by structural isomers, is butane.
Let us consider in more detail the basic nomenclature for alkanes IUPAC.
1. Main circuit selection. The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in the molecule, which is, as it were, its basis.
2. Numbering of main chain atoms. The atoms of the main chain are assigned numbers. The numbering of the atoms of the main chain begins from the end to which the substituent is closest (structures A, B). If the substituents are located at an equal distance from the end of the chain, then numbering starts from the end at which there are more of them (structure B). If different substituents are located at equal distances from the ends of the chain, then numbering begins from the end to which the senior one is closest (structure D). The seniority of hydrocarbon substituents is determined by the order in which the letter with which their name begins appears in the alphabet: methyl (-CH 3), then propyl (-CH 2 -CH 2 -CH 3), ethyl (-CH 2 -CH 3 ) etc.
Please note that the name of the substituent is formed by replacing the suffix -ane with the suffix -yl in the name of the corresponding alkane.
3. Formation of the name. At the beginning of the name, numbers are indicated - the numbers of the carbon atoms at which the substituents are located. If there are several substituents at a given atom, then the corresponding number in the name is repeated twice separated by a comma (2,2-). After the number, a hyphen indicates the number of substituents (di - two, three - three, tetra - four, penta - five) and the name of the substituent (methyl, ethyl, propyl). Then, without spaces or hyphens, the name of the main chain. The main chain is called a hydrocarbon - a member of the homologous series of methane (methane, ethane, propane, etc.).
The names of substances whose structural formulas are given above are as follows:
Structure A: 2-methylpropane;
Structure B: 3-ethylhexane;
Structure B: 2,2,4-trimethylpentane;
Structure D: 2-methyl 4-ethylhexane.
Absence of saturated hydrocarbons in molecules polar bonds leads to them poorly soluble in water, do not interact with charged particles (ions). The most characteristic reactions for alkanes are those involving free radicals.
Physical properties of alkanes
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”, when you smell it, 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 so that people , located next to them, could detect the leak by smell).
Hydrocarbons of composition from WITH 5 N 12 before WITH 15 N 32 - 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 of alkanes
Substitution reactions.
The most characteristic reactions for alkanes are free radical substitution, during which a hydrogen atom is replaced by a halogen atom or some group.
Let us present the characteristic equations halogenation reactions:
In case of excess halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms with chlorine:
The resulting substances are widely used as solvents and starting materials in organic syntheses.
Dehydrogenation reaction(hydrogen abstraction).
When alkanes are passed over a catalyst (Pt, Ni, Al 2 O 3, Cr 2 O 3) at high temperatures (400-600 °C), a hydrogen molecule is eliminated and a alkene:
Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons are burning with the formation of carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode.
1. Combustion of saturated hydrocarbons is a free radical exothermic reaction, which is very important when using alkanes as fuel:
In general, the combustion reaction of alkanes can be written as follows:
2. Thermal splitting of hydrocarbons.
The process proceeds according to free radical mechanism. An increase in temperature leads to homolytic cleavage of the carbon-carbon bond and the formation of free radicals.
These radicals interact with each other, exchanging a hydrogen atom, to form a molecule alkane and alkene molecule:
Thermal decomposition reactions underlie the industrial process - hydrocarbon cracking. This process is the most important stage of oil refining.
3. Pyrolysis. When methane is heated to a temperature of 1000 °C, methane pyrolysis- decomposition into simple substances:
When heated to a temperature of 1500 °C, the formation of acetylene:
4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with branched carbon skeleton:
5. Aromatization. Alkanes with six or more carbon atoms in the chain cyclize in the presence of a catalyst to form benzene and its derivatives:
Alkanes enter into reactions that proceed according to the free radical mechanism, since 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 factors (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since the bonds in alkane molecules are not broken by the heterolytic mechanism.
Alkanes- saturated (saturated) hydrocarbons. A representative of this class is methane ( CH 4). All subsequent saturated hydrocarbons differ by CH 2- a group that is called a homologous group, and compounds are called homologues.
General formula - WITHnH 2 n +2 .
Structure of alkanes.
Each carbon atom is in sp 3- hybridization, forms 4 σ - communications (1 S-S and 3 S-N). The shape of the molecule is in the form of a tetrahedron with an angle of 109.5°.
The bond is formed through the overlap of hybrid orbitals, with the maximum area of overlap lying in space on the straight line connecting the atomic nuclei. This is the most efficient overlap, so the σ bond is considered the strongest.
Isomerism of alkanes.
For alkanes isomerism of the carbon skeleton is characteristic. Limit connections can take on different geometric shapes while maintaining the angle between the connections. For example,
The different positions of the carbon chain are called conformations. Under normal conditions, the conformations of alkanes freely transform into each other through the rotation of C-C bonds, which is why they are often called rotary isomers. There are 2 main conformations - “inhibited” and “eclipsed”:
Isomerism of the carbon skeleton of alkanes.
The number of isomers increases with increasing carbon chain growth. For example, butane has 2 isomers:
For pentane - 3, for heptane - 9, etc.
If a molecule alkane subtract one proton (hydrogen atom), you get a radical:
Physical properties of alkanes.
Under normal conditions - C 1 -C 4- gases , From 5 to From 17- liquids, and hydrocarbons with more than 18 carbon atoms - solids.
As the chain grows, the boiling and melting points increase. Branched alkanes have lower boiling points than normal ones.
Alkanes insoluble in water, but soluble in non-polar organic solvents. Mix easily with each other.
Preparation of alkanes.
Synthetic methods for producing alkanes:
1. From unsaturated hydrocarbons - the “hydrogenation” reaction occurs under the influence of a catalyst (nickel, platinum) and at a temperature:
2. From halogen derivatives - Wurtz reaction: the interaction of monohaloalkanes with sodium metal, resulting in alkanes with double the number of carbon atoms in the chain:
3. From salts of carboxylic acids. When a salt reacts with an alkali, alkanes are obtained that contain 1 less carbon atom compared to the original carboxylic acid:
4. Production of methane. In an electric arc in a hydrogen atmosphere:
C + 2H 2 = CH 4.
In the laboratory, methane is obtained as follows:
Al 4 C 3 + 12H 2 O = 3CH 4 + 4Al(OH) 3.
Chemical properties of alkanes.
Under normal conditions, alkanes are chemically inert compounds; they do not react with concentrated sulfuric and nitric acid, with concentrated alkali, or with potassium permanganate.
Stability is explained by the strength of the bonds and their non-polarity.
Compounds are not prone to bond breaking reactions (addition reactions); they are characterized by substitution.
1. Halogenation of alkanes. Under the influence of a light quantum, radical substitution (chlorination) of the alkane begins. General scheme:
The reaction follows a chain mechanism, in which there are:
A) Initiating the circuit:
B) Chain growth:
B) Open circuit:
In total it can be presented as:
2. Nitration (Konovalov reaction) of alkanes. The reaction occurs at 140 °C:
The reaction proceeds most easily with the tertiary carbon atom than with the primary and secondary ones.
3. Isomerization of alkanes. Under specific conditions, alkanes of normal structure can transform into branched ones:
4. Cracking alkane. Under the action of high temperatures and catalysts, higher alkanes can break their bonds, forming alkenes and lower alkanes:
5. Oxidation of alkanes. Under different conditions and with different catalysts, alkane oxidation can lead to the formation of alcohol, aldehyde (ketone) and acetic acid. Under conditions of complete oxidation, the reaction proceeds to completion - until water and carbon dioxide are formed:
Application of alkanes.
Alkanes have found wide application in industry, in the synthesis of oil, fuel, etc.
