The simplest organic compounds are hydrocarbons, consisting of carbon and hydrogen. Depending on the nature of the chemical bonds in hydrocarbons and the ratio between carbon and hydrogen, they are divided into saturated and unsaturated (alkenes, alkynes, etc.)

Limit hydrocarbons (alkanes, methane hydrocarbons) are compounds of carbon with hydrogen, in the molecules of which each carbon atom spends no more than one valence on combining with any other neighboring atom, and all valences not spent on combining with carbon are saturated with hydrogen. All carbon atoms in alkanes are in the sp 3 state. Saturated hydrocarbons form a homologous series characterized by the general formula WITH n N 2n+2. The ancestor of this series is methane.

Isomerism. Nomenclature.

Alkanes with n=1,2,3 can only exist as one isomer

Starting from n=4, the phenomenon of structural isomerism appears.

The number of structural isomers of alkanes grows rapidly with increasing number of carbon atoms, for example, pentane has 3 isomers, heptane has 9, etc.

The number of isomers of alkanes also increases due to possible stereoisomers. Starting from C 7 H 16, the existence of chiral molecules is possible, which form two enantiomers.

Nomenclature of alkanes.

The dominant nomenclature is the IUPAC nomenclature. At the same time, it contains elements of trivial names. Thus, the first four members of the homologous series of alkanes have trivial names.

CH 4 - methane

C 2 H 6 - ethane

C 3 H 8 - propane

C 4 H 10 - butane.

The names of the remaining homologues are derived from Greek Latin numerals. Thus, for the following members of a series of normal (unbranched) structure, the names are used:

C 5 H 12 - pentane, C 6 H 14 - hexane, C 7 H 18 - heptane,

C 14 H 30 - tetradecane, C 15 H 32 - pentadecane, etc.

Basic IUPAC Rules for Branched Alkanes

a) choose the longest unbranched chain, the name of which forms the base (root). The suffix “an” is added to this stem.

b) number this chain according to the principle of smallest locants,

c) the substituent is indicated in the form of prefixes in alphabetical order indicating the location. If there are several identical substituents in the original structure, then their number is indicated by Greek numerals.

Depending on the number of other carbon atoms to which the carbon atom in question is directly bonded, there are primary, secondary, tertiary and quaternary carbon atoms.

Alkyl groups or alkyl radicals appear as substituents in branched alkanes, which are considered as a result of the elimination of one hydrogen atom from the alkane molecule.

The name of alkyl groups is formed from the name of the corresponding alkanes by replacing the latter suffix “an” with the suffix “yl”.

CH 3 - methyl

CH 3 CH 2 - ethyl

CH 3 CH 2 CH 2 - cut

To name branched alkyl groups, chain numbering is also used:

Starting from ethane, alkanes are able to form conformers that correspond to a inhibited conformation. The possibility of transition from one inhibited conformation to another through an eclipsed one is determined by the rotation barrier. Determination of the structure, composition of conformers and rotation barriers are the tasks of conformational analysis. Methods for obtaining alkanes.

1. Fractional distillation of natural gas or gasoline fraction of oil. In this way, individual alkanes up to 11 carbon atoms can be isolated.

2. Hydrogenation of coal. The process is carried out in the presence of catalysts (oxides and sulfides of molybdenum, tungsten, nickel) at 450-470 o C and pressures up to 30 MPa. Coal and catalyst are ground into powder and hydrogenated in suspended form, hydrogen boronation through the suspension. The resulting mixtures of alkanes and cycloalkanes are used as motor fuel.

3. Hydrogenation of CO and CO 2 .

CO + H 2  alkanes

CO 2 + H 2  alkanes

Co, Fe, and other d-elements are used as catalysts for these reactions.

4.Hydrogenation of alkenes and alkynes.

5.Organometallic synthesis.

A). Wurtz synthesis.

2RHal + 2Na  R R + 2NaHal

This synthesis is of little use if two different haloalkanes are used as organic reagents.

b). Protolysis of Grignard reagents.

R Hal + Mg  RMgHal

RMgHal + HOH  RH + Mg(OH)Hal

V). Interaction of lithium dialkyl cuprates (LiR 2 Cu) with alkyl halides

LiR 2 Cu + R X  R R + RCu + LiX

Lithium dialkylcuprates themselves are produced in a two-step process

2R Li + CuI  LiR 2 Cu + LiI

6. Electrolysis of salts of carboxylic acids (Kolbe synthesis).

2RCOONa + 2H 2 O  R R + 2CO 2 + 2NaOH + H 2

7. Fusion of salts of carboxylic acids with alkalis.

The reaction is used for the synthesis of lower alkanes.

8.Hydrogenolysis of carbonyl compounds and haloalkanes.

A). Carbonyl compounds. Clemmens synthesis.

b). Haloalkanes. Catalytic hydrogenolysis.

Ni, Pt, Pd are used as catalysts.

c) Haloalkanes. Reagent recovery.

RHal + 2HI  RH + HHal + I 2

Chemical properties of alkanes.

All bonds in alkanes are low-polar, which is why they are characterized by radical reactions. The absence of pi bonds makes addition reactions impossible. Alkanes are characterized by substitution, elimination, and combustion reactions.

Mechanism of halogenation reaction:

Halogenation

The halogenation of alkanes occurs via a radical mechanism. To initiate the reaction, the mixture of alkane and halogen must be irradiated with UV light or heated. Methane chlorination does not stop at the stage of obtaining methyl chloride (if equimolar amounts of chlorine and methane are taken), but leads to the formation of all possible substitution products, from methyl chloride to carbon tetrachloride. Chlorination of other alkanes results in a mixture of hydrogen substitution products at different carbon atoms. The ratio of chlorination products depends on temperature. The rate of chlorination of primary, secondary and tertiary atoms depends on temperature; at low temperatures the rate decreases in the order: tertiary, secondary, primary. As the temperature increases, the difference between the speeds decreases until they become the same. In addition to the kinetic factor, the distribution of chlorination products is influenced by a statistical factor: the probability of chlorine attacking a tertiary carbon atom is 3 times less than the primary one and two times less than the secondary one. Thus, the chlorination of alkanes is a non-stereoselective reaction, except in cases where only one monochlorination product is possible.

Halogenation is one of the substitution reactions. The halogenation of alkanes obeys Markovnik's rule (Markovnikov's Rule) - the least hydrogenated carbon atom is halogenated first. The halogenation of alkanes occurs in stages - no more than one hydrogen atom is halogenated in one stage.

CH 4 + Cl 2 → CH 3 Cl + HCl (chloromethane)

CH 3 Cl + Cl 2 → CH 2 Cl 2 + HCl (dichloromethane)

CH 2 Cl 2 + Cl 2 → CHCl 3 + HCl (trichloromethane)

CHCl 3 + Cl 2 → CCl 4 + HCl (carbon tetrachloride).

Under the influence of light, a chlorine molecule breaks down into atoms, then they attack methane molecules, tearing off their hydrogen atom, as a result of which methyl radicals CH 3 are formed, which collide with chlorine molecules, destroying them and forming new radicals.

Nitration (Konovalov reaction)

Alkanes react with a 10% solution of nitric acid or nitrogen oxide N 2 O 4 in the gas phase at a temperature of 140° and low pressure to form nitro derivatives. The reaction also obeys Markovnikov's rule.

RH + HNO 3 = RNO 2 + H 2 O

i.e., one of the hydrogen atoms is replaced by the NO 2 residue (nitro group) and water is released.

The structural features of the isomers strongly affect the course of this reaction, since it most easily leads to the replacement of the hydrogen atom in the SI residue (present only in some isomers) with a nitro group; it is less easy to replace hydrogen in the CH 2 group and even more difficult in the CH 3 residue.

Paraffins are quite easily nitrated in the gas phase at 150-475°C with nitrogen dioxide or nitric acid vapor; in this case, partially happens. oxidation. The nitration of methane produces almost exclusively nitromethane:

All available data point to a free radical mechanism. As a result of the reaction, mixtures of products are formed. Nitric acid at ordinary temperatures has almost no effect on paraffin hydrocarbons. When heated, it acts mainly as an oxidizing agent. However, as M.I. Konovalov found (1889), when heated, nitric acid acts partly in a “nitrating” manner; The nitration reaction with weak nitric acid occurs especially well when heated and under elevated pressure. The nitration reaction is expressed by the equation.

Homologues following methane give a mixture of various nitroparaffins due to the accompanying cleavage. When ethane is nitrated, nitroethane CH 3 -CH 2 -NO 2 and nitromethane CH 3 -NO 2 are obtained. A mixture of nitroparaffins is formed from propane:

Nitration of paraffins in the gas phase is now carried out on an industrial scale.

Sulfachlorination:

A practically important reaction is the sulfochlorination of alkanes. When an alkane reacts with chlorine and sulfur dioxide during irradiation, hydrogen is replaced by a chlorosulfonyl group:

The stages of this reaction are:

Cl +R:H→R +HCl

R+SO 2 →RSO 2

RSO 2 + Cl:Cl→RSO 2 Cl+Cl

Alkanesulfonyl chlorides are easily hydrolyzed to alkanesulfoxylost (RSO 2 OH), the sodium salts of which (RSO 3¯ Na + - sodium alkanesulfonate) exhibit properties similar to soaps and are used as detergents.

Each class of chemical compounds is capable of exhibiting properties determined by their electronic structure. Alkanes are characterized by substitution, elimination or oxidation reactions of molecules. All have their own characteristics, which will be discussed further.

What are alkanes

These are saturated hydrocarbon compounds called paraffins. Their molecules consist only of carbon and hydrogen atoms, have a linear or branched acyclic chain, in which there are only single compounds. Given the characteristics of the class, it is possible to calculate which reactions are characteristic of alkanes. They obey the formula for the entire class: H 2n+2 C n.

Chemical structure

The paraffin molecule contains carbon atoms that exhibit sp 3 hybridization. All four valence orbitals have the same shape, energy and direction in space. The angle between the energy levels is 109° and 28".

The presence of single bonds in molecules determines which reactions are characteristic of alkanes. They contain σ-compounds. The bond between carbons is nonpolar and weakly polarizable, slightly longer than in C−H. There is also a shift in electron density towards the carbon atom, as the most electronegative. As a result, the C−H compound is characterized by low polarity.

Substitution reactions

Substances of the paraffin class have weak chemical activity. This can be explained by the strength of the bonds between C−C and C−H, which are difficult to break due to non-polarity. Their destruction is based on a homolytic mechanism, in which free-type radicals participate. This is why alkanes are characterized by substitution reactions. Such substances are not able to interact with water molecules or charge-carrying ions.

They are considered free radical substitution, in which hydrogen atoms are replaced by halogen elements or other active groups. Such reactions include processes associated with halogenation, sulfochlorination and nitration. Their result is the production of alkane derivatives.

The mechanism of free radical substitution reactions is based on three main stages:

1. The process begins with the initiation or nucleation of a chain, as a result of which free radicals are formed. The catalysts are ultraviolet light sources and heat.
2. Then a chain develops in which sequential interactions of active particles with inactive molecules take place. They are converted into molecules and radicals, respectively.
3. The final stage will be breaking the chain. Recombination or disappearance of active particles is observed. This stops the development of the chain reaction.

Halogenation process

It is based on a radical type mechanism. The halogenation reaction of alkanes occurs upon irradiation with ultraviolet light and heating of a mixture of halogens and hydrocarbons.

All stages of the process obey the rule expressed by Markovnikov. It indicates that it is being replaced by a halogen, which primarily belongs to the hydrogenated carbon itself. Halogenation occurs in the following sequence: from the tertiary atom to the primary carbon.

The process works better for alkane molecules with a long backbone carbon chain. This is due to a decrease in ionizing energy in a given direction; an electron is more easily split off from a substance.

An example is the chlorination of a methane molecule. The action of ultraviolet radiation leads to the breakdown of chlorine into radical particles that attack the alkane. Atomic hydrogen is abstracted and H 3 C or a methyl radical is formed. Such a particle, in turn, attacks molecular chlorine, leading to the destruction of its structure and the formation of a new chemical reagent.

At each stage of the process, only one hydrogen atom is replaced. The halogenation reaction of alkanes leads to the gradual formation of chloromethane, dichloromethane, trichloromethane and tetrachloromethane molecules.

Schematically the process looks like this:

H 4 C + Cl:Cl → H 3 CCl + HCl,

H 3 CCl + Cl:Cl → H 2 CCl 2 + HCl,

H 2 CCl 2 + Cl:Cl → HCCl 3 + HCl,

HCCl 3 + Cl:Cl → CCl 4 + HCl.

Unlike the chlorination of a methane molecule, carrying out such a process with other alkanes is characterized by the production of substances in which the replacement of hydrogen occurs not at one carbon atom, but at several. Their quantitative relationship is related to temperature indicators. Under cold conditions, a decrease in the rate of formation of derivatives with tertiary, secondary and primary structures is observed.

With increasing temperature, the rate of formation of such compounds levels off. The halogenation process is influenced by a static factor, which indicates a different probability of a radical colliding with a carbon atom.

The process of halogenation with iodine does not occur under normal conditions. It is necessary to create special conditions. When methane is exposed to this halogen, hydrogen iodide appears. It is affected by methyl iodide, as a result of which the initial reagents are released: methane and iodine. This reaction is considered reversible.

Wurtz reaction for alkanes

It is a production method with a symmetrical structure. Sodium metal, alkyl bromides or alkyl chlorides are used as reactants. When they react, they produce sodium halide and an enlarged hydrocarbon chain, which is the sum of two hydrocarbon radicals. Schematically, the synthesis looks like this: R−Cl + Cl−R + 2Na → R−R + 2NaCl.

The Wurtz reaction for alkanes is possible only if the halogens in their molecules are located at the primary carbon atom. For example, CH 3 -CH 2 -CH 2 Br.

If a halohydrocarbon mixture of two compounds is involved in the process, then when their chains condense, three different products are formed. An example of such a reaction of alkanes is the interaction of sodium with chloromethane and chloroethane. The output is a mixture containing butane, propane and ethane.

In addition to sodium, other alkali metals can be used, which include lithium or potassium.

Sulfochlorination process

It is also called the Reed reaction. It proceeds according to the principle of free radical substitution. the type of reaction of alkanes to the action of a mixture of sulfur dioxide and molecular chlorine in the presence of ultraviolet radiation.

The process begins with the initiation of a chain mechanism in which two radicals are produced from chlorine. One of them attacks the alkane, which leads to the formation of an alkyl species and a hydrogen chloride molecule. Sulfur dioxide attaches to the hydrocarbon radical to form a complex particle. To stabilize, one chlorine atom is captured from another molecule. The final substance is alkane sulfonyl chloride; it is used in the synthesis of surfactants.

Schematically the process looks like this:

ClCl → hv ∙Cl + ∙Cl,

HR + ∙Cl → R∙ + HCl,

R∙ + OSO → ∙RSO 2 ,

∙RSO 2 + ClCl → RSO 2 Cl + ∙Cl.

Processes associated with nitration

Alkanes react with nitric acid in the form of a 10% solution, as well as with tetravalent nitrogen oxide in the gaseous state. The conditions for its occurrence are high temperatures (about 140 °C) and low pressures. The output produces nitroalkanes.

This free radical type process was named after the scientist Konovalov, who discovered the synthesis of nitration: CH 4 + HNO 3 → CH 3 NO 2 + H 2 O.

Cleavage mechanism

Alkanes are characterized by dehydrogenation and cracking reactions. The methane molecule undergoes complete thermal decomposition.

The main mechanism of the above reactions is the abstraction of atoms from alkanes.

Dehydrogenation process

When hydrogen atoms are separated from the carbon skeleton of paraffins, with the exception of methane, unsaturated compounds are obtained. Such chemical reactions of alkanes take place under high temperature conditions (from 400 to 600 °C) and under the influence of accelerators in the form of platinum, nickel, and aluminum.

If propane or ethane molecules are involved in the reaction, then its products will be propene or ethene with one double bond.

Dehydrogenation of a four- or five-carbon skeleton produces diene compounds. Butadiene-1,3 and butadiene-1,2 are formed from butane.

If the reaction contains substances with 6 or more carbon atoms, benzene is formed. It has an aromatic ring with three double bonds.

Process associated with decomposition

At high temperatures, reactions of alkanes can occur with the breaking of carbon bonds and the formation of active particles of the radical type. Such processes are called cracking or pyrolysis.

Heating reactants to temperatures exceeding 500 °C leads to the decomposition of their molecules, during which complex mixtures of alkyl-type radicals are formed.

Carrying out the pyrolysis of alkanes with long carbon chains under strong heating is associated with the production of saturated and unsaturated compounds. It is called thermal cracking. This process was used until the mid-20th century.

The disadvantage was the production of hydrocarbons with a low octane number (no more than 65), so it was replaced. The process takes place under temperature conditions that are below 440 ° C and pressure values ​​less than 15 atmospheres, in the presence of an aluminosilicate accelerator with the release of alkanes having a branched structure. An example is methane pyrolysis: 2CH 4 → t ° C 2 H 2 + 3H 2. During this reaction, acetylene and molecular hydrogen are formed.

The methane molecule can undergo conversion. This reaction requires water and a nickel catalyst. The output is a mixture of carbon monoxide and hydrogen.

Oxidative processes

Chemical reactions characteristic of alkanes involve the loss of electrons.

There is auto-oxidation of paraffins. It involves the free radical mechanism of oxidation of saturated hydrocarbons. During the reaction, hydroperoxides are obtained from the liquid phase of alkanes. At the initial stage, the paraffin molecule interacts with oxygen, resulting in the release of active radicals. Next, another O 2 molecule interacts with the alkyl particle, resulting in ∙ROO. An alkane molecule comes into contact with the peroxide radical of the fatty acid, after which hydroperoxide is released. An example is the auto-oxidation of ethane:

C 2 H 6 + O 2 → ∙C 2 H 5 + HOO∙,

∙C 2 H 5 + O 2 → ∙OOC 2 H 5,

∙OOC 2 H 5 + C 2 H 6 → HOOC 2 H 5 + ∙C 2 H 5.

Alkanes are characterized by combustion reactions, which are among the main chemical properties when determining them in the composition of fuel. They are oxidative in nature with the release of heat: 2C 2 H 6 + 7O 2 → 4CO 2 + 6H 2 O.

If a small amount of oxygen is observed in the process, then the final product may be coal or carbon divalent oxide, which is determined by the O 2 concentration.

When alkanes are oxidized under the influence of catalytic substances and heated to 200 °C, molecules of alcohol, aldehyde or carboxylic acid are obtained.

Ethane example:

C 2 H 6 + O 2 → C 2 H 5 OH (ethanol),

C 2 H 6 + O 2 → CH 3 CHO + H 2 O (ethanal and water),

2C 2 H 6 + 3O 2 → 2CH 3 COOH + 2H 2 O (ethanoic acid and water).

Alkanes can be oxidized when exposed to three-membered cyclic peroxides. These include dimethyldioxirane. The result of the oxidation of paraffins is an alcohol molecule.

Representatives of paraffins do not react to KMnO 4 or potassium permanganate, as well as to

Isomerization

For alkanes, the type of reaction is characterized by substitution with an electrophilic mechanism. This includes isomerization of the carbon chain. This process is catalyzed by aluminum chloride, which interacts with saturated paraffin. An example is the isomerization of a butane molecule, which becomes 2-methylpropane: C 4 H 10 → C 3 H 7 CH 3.

Flavoring process

Saturated substances that contain six or more carbon atoms in the main carbon chain are capable of dehydrocyclization. This reaction is not typical for short molecules. The result is always a six-membered ring in the form of cyclohexane and its derivatives.

In the presence of reaction accelerators, further dehydrogenation and transformation into a more stable benzene ring takes place. Acyclic hydrocarbons are converted into aromatic compounds or arenes. An example is the dehydrocyclization of hexane:

H 3 C−CH 2 − CH 2 − CH 2 − CH 2 −CH 3 → C 6 H 12 (cyclohexane),

C 6 H 12 → C 6 H 6 + 3H 2 (benzene).

Saturated hydrocarbons are compounds that are molecules consisting of carbon atoms in a state of sp 3 hybridization. They are connected to each other exclusively by covalent sigma bonds. The name "saturated" or "saturated" hydrocarbons comes from the fact that these compounds do not have the ability to attach any atoms. They are extreme, completely saturated. The exception is cycloalkanes.

What are alkanes?

Alkanes are saturated hydrocarbons, and their carbon chain is open and consists of carbon atoms connected to each other using single bonds. It does not contain other (that is, double, like alkenes, or triple, like alkyls) bonds. Alkanes are also called paraffins. They received this name because well-known paraffins are a mixture of predominantly these saturated hydrocarbons C 18 -C 35 with particular inertness.

General information about alkanes and their radicals

Their formula: C n P 2 n +2, here n is greater than or equal to 1. The molar mass is calculated using the formula: M = 14n + 2. Characteristic feature: the endings in their names are “-an”. The residues of their molecules, which are formed as a result of the replacement of hydrogen atoms with other atoms, are called aliphatic radicals, or alkyls. They are designated by the letter R. The general formula of monovalent aliphatic radicals: C n P 2 n +1, here n is greater than or equal to 1. The molar mass of aliphatic radicals is calculated by the formula: M = 14n + 1. A characteristic feature of aliphatic radicals: endings in the names “- silt." Alkane molecules have their own structural features:

• The C-C bond is characterized by a length of 0.154 nm;
• The C-H bond is characterized by a length of 0.109 nm;
• the bond angle (the angle between carbon-carbon bonds) is 109 degrees and 28 minutes.

Alkanes begin the homologous series: methane, ethane, propane, butane, and so on.

Physical properties of alkanes

Alkanes are substances that are colorless and insoluble in water. The temperature at which alkanes begin to melt and the temperature at which they boil increase in accordance with the increase in molecular weight and hydrocarbon chain length. From less branched to more branched alkanes, the boiling and melting points decrease. Gaseous alkanes can burn with a pale blue or colorless flame and produce quite a lot of heat. CH 4 -C 4 H 10 are gases that also have no odor. C 5 H 12 -C 15 H 32 are liquids that have a specific odor. C 15 H 32 and so on are solids that are also odorless.

Chemical properties of alkanes

These compounds are chemically inactive, which can be explained by the strength of difficult-to-break sigma bonds - C-C and C-H. It is also worth considering that C-C bonds are non-polar, and C-H bonds are low-polar. These are low-polarized types of bonds belonging to the sigma type and, accordingly, they are most likely to be broken by a homolytic mechanism, as a result of which radicals will be formed. Thus, the chemical properties of alkanes are mainly limited to radical substitution reactions.

Nitration reactions

Alkanes react only with nitric acid with a concentration of 10% or with tetravalent nitrogen oxide in a gaseous environment at a temperature of 140°C. The nitration reaction of alkanes is called the Konovalov reaction. As a result, nitro compounds and water are formed: CH 4 + nitric acid (diluted) = CH 3 - NO 2 (nitromethane) + water.

Combustion reactions

Saturated hydrocarbons are very often used as fuel, which is justified by their ability to burn: C n P 2n+2 + ((3n+1)/2) O 2 = (n+1) H 2 O + n CO 2.

Oxidation reactions

The chemical properties of alkanes also include their ability to oxidize. Depending on what conditions accompany the reaction and how they are changed, different end products can be obtained from the same substance. Mild oxidation of methane with oxygen in the presence of a catalyst accelerating the reaction and a temperature of about 200 ° C can result in the following substances:

1) 2CH 4 (oxidation with oxygen) = 2CH 3 OH (alcohol - methanol).

2) CH 4 (oxidation with oxygen) = CH 2 O (aldehyde - methanal or formaldehyde) + H 2 O.

3) 2CH 4 (oxidation with oxygen) = 2HCOOH (carboxylic acid - methane or formic) + 2H 2 O.

Also, the oxidation of alkanes can be carried out in a gaseous or liquid medium with air. Such reactions lead to the formation of higher fatty alcohols and corresponding acids.

Relation to heat

At temperatures not exceeding +150-250°C, always in the presence of a catalyst, a structural rearrangement of organic substances occurs, which consists of a change in the order of connection of atoms. This process is called isomerization, and the substances resulting from the reaction are called isomers. Thus, from normal butane, its isomer is obtained - isobutane. At temperatures of 300-600°C and the presence of a catalyst, C-H bonds are broken with the formation of hydrogen molecules (dehydrogenation reactions), hydrogen molecules with the closure of the carbon chain into a cycle (cyclization or aromatization reactions of alkanes):

1) 2CH 4 = C 2 H 4 (ethene) + 2H 2.

2) 2CH 4 = C 2 H 2 (ethyne) + 3H 2.

3) C 7 H 16 (normal heptane) = C 6 H 5 - CH 3 (toluene) + 4 H 2.

Halogenation reactions

Such reactions involve the introduction of halogens (their atoms) into the molecule of an organic substance, resulting in the formation of a C-halogen bond. When alkanes react with halogens, halogen derivatives are formed. This reaction has specific features. It proceeds according to a radical mechanism, and in order to initiate it, it is necessary to expose the mixture of halogens and alkanes to ultraviolet radiation or simply heat it. The properties of alkanes allow the halogenation reaction to proceed until complete replacement with halogen atoms is achieved. That is, the chlorination of methane will not end in one stage and the production of methyl chloride. The reaction will go further, all possible substitution products will be formed, starting with chloromethane and ending with carbon tetrachloride. Exposure of other alkanes to chlorine under these conditions will result in the formation of various products resulting from the substitution of hydrogen at different carbon atoms. The temperature at which the reaction occurs will determine the ratio of the final products and the rate of their formation. The longer the hydrocarbon chain of the alkane, the easier the reaction will be. During halogenation, the least hydrogenated (tertiary) carbon atom will be replaced first. The primary one will react after all the others. The halogenation reaction will occur in stages. In the first stage, only one hydrogen atom is replaced. Alkanes do not interact with halogen solutions (chlorine and bromine water).

Sulfochlorination reactions

The chemical properties of alkanes are also complemented by the sulfochlorination reaction (called the Reed reaction). When exposed to ultraviolet radiation, alkanes are able to react with a mixture of chlorine and sulfur dioxide. As a result, hydrogen chloride is formed, as well as an alkyl radical, which adds sulfur dioxide. The result is a complex compound that becomes stable due to the capture of a chlorine atom and the destruction of its next molecule: R-H + SO 2 + Cl 2 + ultraviolet radiation = R-SO 2 Cl + HCl. The sulfonyl chlorides formed as a result of the reaction are widely used in the production of surfactants.

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Chemical properties of alkanes.

All bonds in alkanes are low-polar, which is why they are characterized by radical reactions. The absence of pi bonds makes addition reactions impossible. Alkanes are characterized by substitution, elimination, and combustion reactions.

Type and name of reaction	Example
1. Substitution reactions
A) with halogens(With chlorineCl 2 -in the light, Br 2 - when heated) the reaction obeys Markovnik's rule (Markovnikov's Rules ) - primarily a halogenreplaces hydrogen inleast hydrogenated new carbon atom. Reaction takes place in stages - in one stage replaced no more than one hydrogen atom. Iodine reacts most difficultly, and moreover, the reaction does not go to completion, since, for example, when methane reacts with iodine, hydrogen iodide is formed, which reacts with methyl iodide to form methane and iodine (reversible reaction):	CH 4 + Cl 2 → CH 3 Cl + HCl (chloromethane) CH 3 Cl + Cl 2 → CH 2 Cl 2 + HCl (dichloromethane) CH 2 Cl 2 + Cl 2 → CHCl 3 + HCl (trichloromethane) CHCl 3 + Cl 2 → CCl 4 + HCl (carbon tetrachloride).
B) Nitration (Konovalov reaction) Alkanes react with 10% nitric acid or nitric oxide N 2 O 4 in the gas phase at a temperature of 140° and low pressure with the formation of nitro derivatives. The reaction also obeys Markovnikov's rule. ABOUT dyne of the hydrogen atoms is replaced by an NO residue 2 (nitro group) and water is released
2. Elimination reactions
A) dehydrogenation– elimination of hydrogen. Reaction conditions: catalyst – platinum and temperature.	CH 3 - CH 3 → CH 2 = CH 2 + H 2
B) cracking the process of thermal decomposition of hydrocarbons, which is based on the reactions of splitting the carbon chain of large molecules to form compounds with a shorter chain. At a temperature of 450–700 o C alkanes decompose due to bond cleavage S–S (stronger connections S–H are preserved at this temperature) and alkanes and alkenes with a smaller number of carbon atoms are formed	C 6 H 14 C 2 H 6 + C 4 H 8
B) complete thermal decomposition	CH 4 C + 2H 2
3. Oxidation reactions
A) combustion reaction When ignited (t = 600 o C), alkanes react with oxygen, and they are oxidized to carbon dioxide and water.	C n H 2n+2 + O 2 ––>CO 2 + H 2 O + Q CH 4 + 2O 2 ––>CO 2 + 2H 2 O + Q
B) Catalytic oxidation-at a relatively low temperature and with the use of catalysts, it is accompanied by the rupture of only part of the C–C bonds approximately in the middle of the molecule and C–H and is used to obtain valuable products: carboxylic acids, ketones, aldehydes, alcohols.	For example, with incomplete oxidation of butane (cleavage of the C 2 –C 3 bond), acetic acid is obtained
4. Isomerization reactions are not typical for all alkanes. Attention is drawn to the possibility of converting some isomers into others and the presence of catalysts.	C 4 H 10 C 4 H 10
5.. Alkanes with a main chain of 6 or more carbon atoms also reactdehydrocyclization 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).

1. 
   Alkanes are obtained in large quantities from natural gas and oil.
2. 
   From simple substances in an electric discharge:
3. 
   Hydrolysis of aluminum carbide
4. 
   Heating of monohaloalkanes with sodium metal (Wurtz reaction)
   If the haloalkanes are different, the result will be a mixture of three products:
5. 
   Decarboxylation. Fusion of sodium acetate with alkali. The alkane produced this way will have one less carbon atom.
6. 
   Hydrolysis of Grignard reagent:
7. 
   Alkanes with a symmetrical structure can be obtained by electrolysis of salts of carboxylic acids. (Kolb reaction)

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: homologous 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:

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 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:

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).