Rules for the composition of isomers of alkanes. Alkanes

In total, 390 substances belonging to the class of alkanes are known. The International Union of Pure and Applied Chemistry - IUPAC or IUPAC - has developed a nomenclature for alkanes to make it easy to name each compound, knowing only its formula.

General rules

The name of alkanes is characterized by the suffix -an. The first four representatives of the homologous series - methane, ethane, propane, butane - have historically established names. The prefix in the names of other alkanes indicates the number of carbon atoms in the substance:

• pent- - five;
• hex - six;
• hept- - seven;
• Oct- - eight;
• non- - nine;
• Dec- - ten.

Rice. 1. Homologous series of alkanes.

One is un- or gen-, two is do-, three is three-, four is tetra-.

Prefixes and suffixes are preserved for all substances. Starting from the dean, a root is added to the names. It changes in every ten substances, as indicated by the prefix. For example, eicosane contains 20 carbons, triacontane - 30, tetracontane - 40, pentacontane - 50.

The names of these substances move to the next ten with the addition of prefixes and suffixes. For example, eicosane is followed by heneicosane, docosane, tricosane, and triacontane - gentriacontan, dotriacontan, tritriacontan, tetratriacontan, etc.

Names of branched chains

Substances with the same number of atoms, but with different arrangements, are called isomers. Compare butane and isobutane. In both cases the formula is C 4 H 10, but the atoms are arranged differently. In the first case, the chain is longer, in the second, it is shorter by one link. The names of isomers correspond to the names of alkanes with the prefix iso-.

Rice. 2. Butane and isobutane.

To more accurately name and indicate the location of the radical, a separate nomenclature is used. Rules for determining the name of a branched chain:

• take the longest chain or the one with the greatest number of branches as the main chain - this will be the main name of the substance (in isobutane, the longest chain contains three carbons - this is propane);
• number the carbon atoms starting from the end to which the radical is adjacent (in isopentane the alkyl is shifted to the right end);
• if there are alkyls at both ends, choose the end with the radical containing fewer carbon atoms;
• if the number of carbon atoms in equidistant alkyls is the same, choose the end with the largest number of branches;
• name the compound, indicating, separated by commas, the numbers of atoms that have radicals (2,2,3-, 1,4-);
• indicate the prefix corresponding to the number of alkyls (di-, tri-);
• list the radicals (methyl-, chloromethyl-);
• complete with the name of the main chain (-propane, -butane, -pentane).

In structural formulas, alkyls are written with a vertical bar above and below the carbon atom. It is acceptable to write radicals in parentheses after the carbon atom. For example, 2-methylbutane - CH 3 -CH(CH 3) -CH 2 -CH 3.

Examples

Several examples of the nomenclature of alkanes with a branched structure are presented in the table.

Rice. 3. Examples of structural formulas with names.

What have we learned?

The names of alkanes in accordance with the IUPAC nomenclature are composed of the suffix -ane, a prefix indicating the number of carbon atoms, and the root of the name of every tenth homologue. The names of the first four alkanes should be remembered. Branched molecules include a list of the numbers of atoms that contain radicals, a prefix indicating their number, a list of radicals and the name of the main chain.

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Alkanes or aliphatic saturated hydrocarbons are compounds with an open (non-cyclic) chain, in the molecules of which the carbon atoms are connected to each other by a σ bond. The carbon atom in alkanes is in a state of sp 3 hybridization.

Alkanes form a homologous series in which each member differs by a constant structural unit -CH 2 -, which is called a homological difference. The simplest representative is methane CH4.

• General formula of alkanes: C n H 2n+2

For alkanes, in addition to structural isomerism, there is conformational isomerism and, starting with heptane, enantiomerism:

IUPAC nomenclature Prefixes are used in the names of alkanes n-, second-, iso, tert-, neo:

• n- means normal (uncorroded) structure of the hydrocarbon chain;
• second- applies only to recycled butyl;
• tert- means alkyl of tertiary structure;
• iso branches at the end of the chain;
• neo used for alkyl with a quaternary carbon atom.

The nomenclature of branched alkanes is based on the following basic rules:

• To construct a name, a long chain of carbon atoms is selected and numbered with Arabic numerals (locants), starting from the end closer to which the substituent is located, for example:

• If the same alkyl group occurs more than once, then multiplying prefixes are placed in front of it in the name di-(before a vowel di-), three-, tetra- etc. and designate each alkyl separately with a number, for example:

• If the side branches of the main chain contain various alkyl substituents, then they are rearranged alphabetically (with multiplying prefixes di-, tetra- etc., as well as prefixes n-, second-, tert- are not taken into account), for example:

• If two or more options for the longest chain are possible, then choose the one that has the maximum number of side branches.
• The names of complex alkyl groups are constructed according to the same principles as the names of alkanes, but the numbering of the alkyl chain is always autonomous and begins with the carbon atom having free valence, for example:

• When used in the name of such a group, it is put in brackets and the first letter of the name of the entire group is taken into account in alphabetical order:

Industrial extraction methods 1. Extraction of alkanes gas. Natural gas consists mainly of methane and small admixtures of ethane, propane, and butane. Gas under pressure at low temperatures is divided into appropriate fractions.

2. Extraction of alkanes from oil. Crude oil is purified and processed (distillation, fractionation, cracking). Mixtures or individual compounds are obtained from processed products.

3. Hydrogenation of coal (method of F. Bergius, 1925). Hard or brown coal in autoclaves at 30 MPa in the presence of catalysts (oxides and sulfides of Fe, Mo, W, Ni) in a hydrocarbon environment is hydrogenated and converted into alkanes, the so-called motor fuel:

nC + (n+1)H 2 = C n H 2n+2

4. Oxosynthesis of alkanes (method of F. Fischer - G. Tropsch, 1922). Using the Fischer-Tropsch method, alkanes are obtained from synthesis gas. Synthesis gas is a mixture of CO and H 2 with different ratios. It is obtained from methane by one of the reactions that occur at 800-900°C in the presence of nickel oxide NiO supported on Al 2 O 3:

CH 4 + H 2 O ⇄ CO + 3H 2

CH 4 + CO 2 ⇄ 2CO + 2H 2

2CH 4 + O 2 ⇄ 2CO + 4H 2

Alkanes are obtained by the reaction (temperature about 300°C, Fe-Co catalyst):

nCO + (2n+1)H 2 → C n H 2n+2 + nH 2 O

The resulting mixture of hydrocarbons, consisting mainly of alkanes of the structure (n = 12-18), is called “syntin”.

5. Dry distillation. Alkanes are obtained in relatively small quantities by dry distillation or heating of coal, shale, wood, and peat without access to air. The approximate composition of the resulting mixture is 60% hydrogen, 25% methane and 3-5% ethylene.

Laboratory extraction methods 1. Preparation from haloalkyls

1.1. Reaction with metallic sodium (Wurz, 1855). The reaction consists of the interaction of an alkali metal with a haloalkyl and is used for the synthesis of higher symmetrical alkanes:

2CH 3 -I + 2Na ⇄ CH 3 -CH 3 + 2NaI

If two different haloalkyls participate in the reaction, a mixture of alkanes is formed:

3CH 3 -I + 3CH 3 CH 2 -I + 6Na → CH 3 -CH 3 + CH 3 CH 2 CH 3 + CH 3 CH 2 CH 2 CH 3 + 6NaI

1.2 Interaction with lithium dialkyl cuprates. The method (sometimes called the E. Core - H. House reaction) involves the interaction of reactive lithium dialkyl cuprates R 2 CuLi with haloalkyls. First, lithium metal reacts with a haloalkane in an ether environment. Next, the corresponding alkyl lithium reacts with copper(I) halide to form a soluble lithium dialkyl cuprate:

CH 3 Cl + 2Li → CH 3 Li + LiCl

2CH 3 Li + CuI → (CH 3 ) 2 CuLi + LiI

When such a lithium dialkyl cuprate reacts with the corresponding haloalkyl, the final compound is formed:

(CH 3 ) 2 CuLi + 2CH 3 (CH 2 ) 6 CH 2 -I → 2CH 3 (CH 2 ) 6 CH 2 -CH 3 + LiI + CuI

The method makes it possible to achieve a yield of alkanes of almost 100% when using primary haloalkyls. With their secondary or tertiary structure, the yield is 30-55%. The nature of the alkyl component in lithium dialkyl cuprate has little effect on the yield of the alkane.

CH 3 I + H 2 → CH 4 + HI (Pd catalyst)

CH 3 CH 2 I + 2H → CH 3 CH 3 + HI

CH 3 I + HI → CH 4 + I 2

The method has no preparative value; a strong reducing agent is often used - iodine.

2. Preparation from salts of carboxylic acids.
2.1 Electrolysis of salts (Kolbe, 1849). The Kolbe reaction involves the electrolysis of aqueous solutions of carboxylic acid salts:

R-COONa ⇄ R-COO - + Na +

At the anode, the carboxylic acid anion is oxidized, forming a free radical, and is easily decarboxylated or eliminated by CO 2 . Alkyl radicals are further converted into alkanes due to recombination:

R-COO - → R-COO . + e -

R-COO. →R. +CO2

R. +R. → R-R

2.2 Fusion of salts of carboxylic acids with alkali. Alkali metal salts of carboxylic acids, when combined with alkali, form alkanes:

CH 3 CH 2 COONa + NaOH → Na 2 CO 3 + CH 3 CH 3

Alkanes are much lighter than water, non-polar and difficult to polarize, but they are soluble in most non-polar solvents, due to which they themselves can be a solvent for many organic compounds.

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:

The H-C-H bond angle is 109°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 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 structures. Such connections are called isomers.

Differences in the order in which atoms are combined in molecules (i.e., chemical structure) lead to structural isomerism. The structure of structural isomers is 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 the left hand relates to the right). 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 of 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.

Techniques for constructing structural formulas of 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 the extreme positions, the chemical structure of the chain does 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

The nomenclature of organic compounds is a system of rules that allows us 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 valency. 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 nonpolar C–C and weakly polar C–H covalent bonds (see the table for the values ​​of dipole moments) suggests their homolytic (symmetrical) cleavage into free radicals ( Part I, Section 6.4.3 ). Therefore, reactions of alkanes are characterized by radical mechanism. Since heterolytic cleavage of C–C and C–H bonds does not occur under normal conditions, alkanes practically do not enter into ionic reactions. This is manifested in their resistance to the action of polar reagents (acids, alkalis, ionic 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 with a smaller number of carbon atoms are formed.

For example:

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

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

Free radicals are very active. One of them (for example, ethyl) 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 and C-H bonds 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"Paraffin burning."

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 oxidation state). 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.

Acyclic hydrocarbons are called alkanes. There are 390 alkanes in total. Nonacontatrictan has the longest structure (C 390 H 782). Halogens can attach to carbon atoms to form haloalkanes.

Structure and nomenclature

By definition, alkanes are saturated or saturated hydrocarbons that have a linear or branched structure. Also called paraffins. Alkane molecules contain only single covalent bonds between carbon atoms. General formula -

To name a substance, you must follow the rules. According to international nomenclature, names are formed using the suffix -an. The names of the first four alkanes were formed historically. Starting from the fifth representative, the names are composed of a prefix indicating the number of carbon atoms and the suffix -an. For example, okta (eight) forms octane.

For branched chains, the names are added up:

• from numbers indicating the numbers of carbon atoms near which the radicals are located;
• from the name of radicals;
• from the name of the main circuit.

Example: 4-methylpropane - the fourth carbon atom in the propane chain has a radical (methyl).

Rice. 1. Structural formulas with the names of alkanes.

Every tenth alkane gives the name to the next nine alkanes. After the decan come undecane, dodecane and then, after eicosane - heneicosane, docosane, tricosane, etc.

Homologous series

The first representative is methane, which is why alkanes are also called the homologous series of methane. The table of alkanes shows the first 20 representatives.

Starting with butane, all alkanes have structural isomers. The name is appended with the prefix iso-: isobutane, isopentane, isohexane.

Rice. 2. Examples of isomers.

Physical properties

The state of aggregation of substances changes in the list of homologues from top to bottom. The more carbon atoms it contains and, accordingly, the greater the molecular weight of the compounds, the higher the boiling point and the harder the substance.

The remaining substances containing more than 15 carbon atoms are in the solid state.

Gaseous alkanes burn with a blue or colorless flame.

Receipt

Alkanes, like other classes of hydrocarbons, are obtained from oil, gas, and coal. For this purpose, laboratory and industrial methods are used:

• gasification of solid fuel:

  C + 2H 2 → CH 4;

• hydrogenation of carbon monoxide (II):

  CO + 3H 2 → CH 4 + H 2 O;

• hydrolysis of aluminum carbide:

  Al 4 C 3 + 12H 2 O → 4Al(OH) 3 + 3CH 4;

• reaction of aluminum carbide with strong acids:

  Al 4 C 3 + H 2 Cl → CH 4 + AlCl 3;

• reduction of haloalkanes (substitution reaction):

  2CH 3 Cl + 2Na → CH 3 -CH 3 + 2NaCl;

• hydrogenation of haloalkanes:

  CH 3 Cl + H 2 → CH 4 + HCl;

• fusion of salts of acetic acid with alkalis (Dumas reaction):

  CH 3 COONa + NaOH → Na 2 CO 3 + CH 4.

Alkanes can be obtained by hydrogenation of alkenes and alkynes in the presence of a catalyst - platinum, nickel, palladium.

Chemical properties

Alkanes react with inorganic substances:

• combustion:

  CH 4 + 2O 2 → CO 2 + 2H 2 O;

• halogenation:

  CH 4 + Cl 2 → CH 3 Cl + HCl;

• nitration (Konovalov reaction):

  CH 4 + HNO 3 → CH 3 NO 2 + H 2 O;

• accession:

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

In contact with

Classmates

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

Homologous series of saturated hydrocarbons

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

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

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

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

Nomenclature of saturated hydrocarbons, their derivatives

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

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

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

• ylen, if the hydrogen atoms are removed from one carbon atom,
• ylen, in the case where two hydrogen atoms are removed from two adjacent carbon atoms.

Alkanes: chemical properties

Let's consider reactions characteristic of alkanes. All alkanes share common chemical properties. These substances are inactive.

All known reactions involving hydrocarbons are divided into two types:

• cleavage of the C-H bond (an example is a substitution reaction);
• rupture of the C-C bond (cracking, formation of separate parts).

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

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

Free radical reactions explain many important chemical processes, such as:

• Explosions;
• Oxidation;
• Petroleum cracking;
• Polymerization of unsaturated compounds.

Details chemical properties can be considered saturated hydrocarbons using methane as an example. Above we have already considered the structure of an alkane molecule. The carbon atoms in the methane molecule are in a state of sp3 hybridization, and a fairly strong bond is formed. Methane is a gas with odor and color. It is lighter than air. Slightly soluble in water.

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

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

Methane decay can be carried out to the intermediate product in specially equipped ovens. The intermediate product will be acetylene. The reaction formula is 2CH4 → C2H2 + 3H2. The separation of acetylene from methane reduces production costs by almost half.

Hydrogen is also produced from methane by converting methane with water vapor. Substitution reactions are characteristic of methane. Thus, at ordinary temperatures, in the light, halogens (Cl, Br) displace hydrogen from the methane molecule in stages. In this way, substances called halogen derivatives are formed. Chlorine atoms By replacing hydrogen atoms in a hydrocarbon molecule, they form a mixture of different compounds.

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

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

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

At ordinary temperatures, methane is sufficiently resistant to alkalis, acids, and many oxidizing agents. The exception is nitric acid. In reaction with it, nitromethane and water are formed.

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

Reactions in which hydrocarbons participate can occur not only with the cleavage of the C-H bond, but also with the cleavage of the C-C bond. Such transformations occur in the presence of high temperatures and catalysts. These reactions include dehydrogenation and cracking.

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

Methane production

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

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

Methane and its homologues can be obtained by calcination of salts of the corresponding organic acids with alkalis. Another method for producing alkanes is the Wurtz reaction, in which monohalogen derivatives are heated with sodium metal.