Table 12.1.

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

Table 12.2.

Names of branched radicals.

Structure of alkanes

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

S–S and S–N.

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

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

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

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

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

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

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

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

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

Isomerism of alkanes

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

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

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

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

. Structural isomerism of alkanes

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

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

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

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

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

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

Construction techniques structural formulas isomers

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

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

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

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

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

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

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

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

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

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

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

Nomenclature

Nomenclature organic compounds- a system of rules that allows you to give an unambiguous name to each individual substance.

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

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

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

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

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

Radicals in the series of alkanes

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

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

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

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

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

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

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

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

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

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

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

Construction of alkanes molecules from radicals

For example, a molecule

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

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

Where R– monovalent hydrocarbon radical (Wurtz reaction).

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

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

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

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

For example, alkane

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

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

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

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

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

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

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

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

Thus, in the name of a branched alkane

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

Example of title construction:

Chemical properties of alkanes

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

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

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

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

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

Cracking of alkanes

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

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

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

For example:

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

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

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

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

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

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

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

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

CH 4 C+2H 2

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

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

Isomerization of alkanes

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

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

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

Dehydrogenation of alkanes

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

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

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

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

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

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

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

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

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

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

Alkane oxidation reactions

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

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

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

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

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

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

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

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

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

Structure of alkanes

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

Rice. 1. The structure of an alkane molecule.

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

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

Chemical properties

General properties of alkanes are given in the table.

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

Rice. 2. Structural formulas.

Application

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

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

Rice. 3. Products obtained from alkanes.

What have we learned?

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

Test on the topic

Evaluation of the report

average rating: 4.1. Total ratings received: 227.

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 various forms, and the length of the connection, the angle between them - constants. 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 molecule saturated hydrocarbon, then 2-valent radicals will be obtained.

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:

1. Consideration of hydrocarbons as derivatives of methane, in which 1 or several H atoms are replaced by radicals.
2. 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:

1. The basis for the name is 1 carbon chain, while the remaining 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: nomenclature of alkanes

For convenience, the information is presented in table form.

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, which reflect given number atoms of C. 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 circuits same length the main one becomes the one that contains greatest number deputies

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 - members this 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 by 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 identical qualitative and quantitative molecular composition(identical molecular formula), but different chemical structure, as well as having different chemical properties.

The hydrocarbons discussed above differ in such a parameter as boiling point: -0.5° - butane, -10° - isobutane. This type isomerism is referred to as carbon skeleton isomerism, it refers 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 most great importance energy, 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 sustainable situation 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. Result - 10 thousand ethane molecules are in the inhibited form of conformation under the condition 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 stand out from these natural sources, like oil, natural, coal. Synthetic methods are also used. For example, H₂ 2H₂:

1. Hydrogenation process CnH₂n (alkenes)→ CnH₂n+2 (alkanes)← CnH₂n-2 (alkynes).
2. From a mixture of C and H monoxide - 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 (alloy with alkali): CH₃COONa+NaOH (t)→CH₄+Na₂CO₃.

1. Oil cracking: CnH₂n+2 (450-700°)→ CmH₂m+2+ Cn-mH₂(n-m).
2. Gasification of fuel (solid): C+2H₂→CH₄.
3. Synthesis of complex alkanes (halogen derivatives) that have fewer C atoms: 2CH₃Cl (chloromethane) +2Na →CH₃- CH₃ (ethane) +2NaCl.
4. 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.

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).