Alkanes definition general formula. International nomenclature of alkanes. Alkanes: structure, properties. Chemical properties of toluene

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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 Isomerism

Starting from butane C 4 H 10, alkanes are characterized by structural isomerism. The number of structural isomers increases with the number of carbon atoms in the alkane molecule. Thus, for pentane C 5 H 12 three isomers are known, for octane C 8 H 18 - 18, for decane C 10 H 22 - 75.

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-:

• Prefixes are used in the names of alkanes neo
• n- means normal (uncorroded) structure of the hydrocarbon chain;
• iso applies only to recycled butyl;
• second- means alkyl of tertiary structure;
• tert- branches at the end of the chain;

used for alkyl with a quaternary carbon atom. second- Prefixes tert- And Prefixes are used in the names of alkanes, n-, iso are written together, and

hyphenated

• 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- If the same alkyl group occurs more than once, then multiplying prefixes are placed in front of it in the name), (before a vowel, three- tetra-

etc. and designate each alkyl separately with a number, for example: It should be noted that for complex residues (groups) multiplying prefixes like, bis-, tris- tetrakis-

• other. If the same alkyl group occurs more than once, then multiplying prefixes are placed in front of it in the name, three- If the side branches of the main chain contain various alkyl substituents, then they are rearranged alphabetically (with multiplying prefixes Prefixes are used in the names of alkanes, n-, iso etc., as well as prefixes

• are not taken into account), for example:
• 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 valency, 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.

1.3 Reduction of haloalkyls. It is possible to reduce haloalkyls with catalytically excited molecular hydrogen, atomic hydrogen, iodine, etc.:

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

Kolbe's preparative method is considered effective in the presence of the corresponding carboxylic acids and the impossibility of using other synthesis methods. 2.2 Fusion of salts of carboxylic acids with alkali. . The reducing agents are the above-mentioned compounds. Most often, iodine is used, which is capable of reducing even ketones: The first four representatives of alkanes from methane to butane (C 1 -C 4) are gases, from pentane to pentadecane (C 5 -C 15 - liquids, from hexadecane (C 16) - solids An increase in their molecular weights leads to an increase in boiling and melting points, with branched-chain alkanes boiling at a lower temperature than normal-structure alkanes. This is explained by the lower van der Waals interaction between the molecules of branched hydrocarbons in the liquid state. The melting point of even-numbered homologues is higher. compared with the temperature, respectively, for odd ones.

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.

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 valencies 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.
1. Type and name of reaction
Substitution reactions A) with halogens (Withchlorine 2 Cl, -in the light 2 Br- when heated ) the reaction obeys Markovnik's rule (Markovnikov's Rules ) - first of all, a halogen replaces hydrogen at the least hydrogenated carbon atom. The reaction takes place in stages - no more than one hydrogen atom is replaced in one stage.	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 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.
One of the hydrogen atoms is replaced by the NO 2 residue (nitro group) and water is released 2. Elimination reactions	A) dehydrogenation
– elimination of hydrogen. Reaction conditions catalyst – platinum and temperature. 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 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	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 react 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).

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.

Chemical properties of alkanes

Alkanes (paraffins) are non-cyclic hydrocarbons in whose molecules all carbon atoms are connected only by single bonds. In other words, there are no multiple - double or triple bonds - in alkane molecules. In fact, alkanes are hydrocarbons containing the maximum possible number of hydrogen atoms, and therefore they are called limiting (saturated).

Due to saturation, alkanes cannot undergo addition reactions.

Since carbon and hydrogen atoms have fairly close electronegativity, this leads to the fact that the C-H bonds in their molecules are extremely low-polar. In this regard, for alkanes, reactions proceeding through the radical substitution mechanism, denoted by the symbol S R, are more typical.

1. Substitution reactions

In reactions of this type, carbon-hydrogen bonds are broken

RH + XY → RX + HY

Halogenation

Alkanes react with halogens (chlorine and bromine) when exposed to ultraviolet light or high heat. In this case, a mixture of halogen derivatives is formed with varying degrees of substitution of hydrogen atoms - mono-, ditri-, etc. halogen-substituted alkanes.

Using methane as an example, it looks like this:

By changing the halogen/methane ratio in the reaction mixture, it is possible to ensure that any specific halogen derivative of methane predominates in the composition of the products.

Reaction mechanism Let us analyze the mechanism of the free radical substitution reaction using the example of the interaction of methane and chlorine. It consists of three stages: initiation (or chain nucleation) is the process of formation of free radicals under the influence of external energy - irradiation with UV light or heating. At this stage, the chlorine molecule undergoes homolytic cleavage of the Cl-Cl bond with the formation of free radicals: Free radicals, as can be seen from the figure above, are atoms or groups of atoms with one or more unpaired electrons (Cl, H, CH 3, CH 2, etc.); 2. Chain development This stage involves the interaction of active free radicals with inactive molecules. In this case, new radicals are formed. In particular, when chlorine radicals act on alkane molecules, an alkyl radical and hydrogen chloride are formed. In turn, the alkyl radical, colliding with chlorine molecules, forms a chlorine derivative and a new chlorine radical: 3) Break (death) of the chain: Occurs as a result of the recombination of two radicals with each other into inactive molecules:

2. Oxidation reactions

Under normal conditions, alkanes are inert towards such strong oxidizing agents as concentrated sulfuric and nitric acids, potassium permanganate and dichromate (KMnO 4, K 2 Cr 2 O 7).

Combustion in oxygen

A) complete combustion with excess oxygen. Leads to the formation of carbon dioxide and water:

CH 4 + 2O 2 = CO 2 + 2H 2 O

B) incomplete combustion due to lack of oxygen:

2CH 4 + 3O 2 = 2CO + 4H 2 O

CH 4 + O 2 = C + 2H 2 O

Catalytic oxidation with oxygen

As a result of heating alkanes with oxygen (~200 o C) in the presence of catalysts, a wide variety of organic products can be obtained from them: aldehydes, ketones, alcohols, carboxylic acids.

For example, methane, depending on the nature of the catalyst, can be oxidized into methyl alcohol, formaldehyde or formic acid:

3. Thermal transformations of alkanes

Cracking

Cracking (from the English to crack - to tear) is a chemical process occurring at high temperatures, as a result of which the carbon skeleton of alkane molecules breaks down to form molecules of alkenes and alkanes with lower molecular weights compared to the original alkanes. For example:

CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3 → CH 3 -CH 2 -CH 2 -CH 3 + CH 3 -CH=CH 2

Cracking can be thermal or catalytic. To carry out catalytic cracking, thanks to the use of catalysts, significantly lower temperatures are used compared to thermal cracking.

Dehydrogenation

The elimination of hydrogen occurs as a result of the cleavage of C-H bonds; carried out in the presence of catalysts at elevated temperatures. When methane is dehydrogenated, acetylene is formed:

2CH 4 → C 2 H 2 + 3H 2

Heating methane to 1200 °C leads to its decomposition into simple substances:

CH 4 → C + 2H 2

When the remaining alkanes are dehydrogenated, alkenes are formed:

C 2 H 6 → C 2 H 4 + H 2

When dehydrogenating n-butane produces butene or butene-2 ​​(a mixture cis- Prefixes trance-isomers):

Dehydrocyclization

Isomerization

Chemical properties of cycloalkanes

The chemical properties of cycloalkanes with more than four carbon atoms in their rings are, in general, almost identical to the properties of alkanes. Oddly enough, cyclopropane and cyclobutane are characterized by addition reactions. This is due to the high tension within the cycle, which leads to the fact that these cycles tend to break. So cyclopropane and cyclobutane easily add bromine, hydrogen or hydrogen chloride:

Chemical properties of alkenes

1. Addition reactions

Since the double bond in alkene molecules consists of one strong sigma and one weak pi bond, they are fairly active compounds that easily undergo addition reactions. Alkenes often undergo such reactions even under mild conditions - in the cold, in aqueous solutions and organic solvents.

Hydrogenation of alkenes

Alkenes are capable of adding hydrogen in the presence of catalysts (platinum, palladium, nickel):

CH 3 -CH = CH 2 + H 2 → CH 3 -CH 2 -CH 3

Hydrogenation of alkenes occurs easily even at normal pressure and slight heating. An interesting fact is that the same catalysts can be used for the dehydrogenation of alkanes to alkenes, only the dehydrogenation process occurs at a higher temperature and lower pressure.

Halogenation

Alkenes easily undergo addition reactions with bromine both in aqueous solution and in organic solvents. As a result of the interaction, initially yellow bromine solutions lose their color, i.e. become discolored.

CH 2 =CH 2 + Br 2 → CH 2 Br-CH 2 Br

Hydrohalogenation

As is easy to see, the addition of a hydrogen halide to a molecule of an unsymmetrical alkene should, theoretically, lead to a mixture of two isomers. For example, when hydrogen bromide is added to propene, the following products should be obtained:

However, in the absence of specific conditions (for example, the presence of peroxides in the reaction mixture), the addition of a hydrogen halide molecule will occur strictly selectively in accordance with Markovnikov’s rule:

The addition of a hydrogen halide to an alkene occurs in such a way that a hydrogen is added to a carbon atom with a greater number of hydrogen atoms (more hydrogenated), and a halogen is added to a carbon atom with a fewer number of hydrogen atoms (less hydrogenated).

Hydration

This reaction leads to the formation of alcohols, and also proceeds in accordance with Markovnikov’s rule:

As you can easily guess, due to the fact that the addition of water to an alkene molecule occurs according to Markovnikov’s rule, the formation of a primary alcohol is possible only in the case of ethylene hydration:

CH 2 =CH 2 + H 2 O → CH 3 -CH 2 -OH

It is through this reaction that the bulk of ethyl alcohol is carried out in large-scale industry.

Polymerization

A specific case of an addition reaction is the polymerization reaction, which, unlike halogenation, hydrohalogenation and hydration, proceeds through the free radical mechanism:

Oxidation reactions

Like all other hydrocarbons, alkenes burn easily in oxygen to form carbon dioxide and water. The equation for the combustion of alkenes in excess oxygen has the form:

C n H 2n+2 + O 2 → nCO 2 + (n+1)H 2 O

Unlike alkanes, alkenes are easily oxidized. When alkenes are exposed to an aqueous solution of KMnO 4, discoloration occurs, which is a qualitative reaction to double and triple CC bonds in molecules of organic substances.

Oxidation of alkenes with potassium permanganate in a neutral or weakly alkaline solution leads to the formation of diols (dihydric alcohols):

C 2 H 4 + 2KMnO 4 + 2H 2 O → CH 2 OH–CH 2 OH + 2MnO 2 + 2KOH (cooling)

In an acidic environment, the double bond is completely broken and the carbon atoms that formed the double bond are converted into carboxyl groups:

5CH 3 CH=CHCH 2 CH 3 + 8KMnO 4 + 12H 2 SO 4 → 5CH 3 COOH + 5C 2 H 5 COOH + 8MnSO 4 + 4K 2 SO 4 + 17H 2 O (heating)

If the double C=C bond is located at the end of the alkene molecule, then carbon dioxide is formed as a product of oxidation of the outermost carbon atom at the double bond. This is due to the fact that the intermediate oxidation product, formic acid, easily oxidizes itself in an excess of oxidizing agent:

5CH 3 CH=CH 2 + 10KMnO 4 + 15H 2 SO 4 → 5CH 3 COOH + 5CO 2 + 10MnSO 4 + 5K 2 SO 4 + 20H 2 O (heating)

The oxidation of alkenes in which the C atom at the double bond contains two hydrocarbon substituents produces a ketone. For example, the oxidation of 2-methylbutene-2 ​​produces acetone and acetic acid.

The oxidation of alkenes, in which the carbon skeleton is broken at the double bond, is used to determine their structure.

Chemical properties of alkadienes

Addition reactions

For example, the addition of halogens:

Bromine water becomes discolored.

Under normal conditions, the addition of halogen atoms occurs at the ends of the 1,3-butadiene molecule, while the π-bonds are broken, bromine atoms are added to the extreme carbon atoms, and the free valences form a new π-bond. Thus, a “movement” of the double bond occurs. If there is an excess of bromine, another molecule can be added at the site of the formed double bond.

Polymerization reactions

Chemical properties of alkynes

Alkynes are unsaturated (unsaturated) hydrocarbons and therefore are capable of undergoing addition reactions. Among the addition reactions for alkynes, electrophilic addition is the most common.

Halogenation

Since the triple bond of alkyne molecules consists of one stronger sigma bond and two weaker pi bonds, they are capable of attaching either one or two halogen molecules. The addition of two halogen molecules by one alkyne molecule proceeds through an electrophilic mechanism sequentially in two stages:

Hydrohalogenation

The addition of hydrogen halide molecules also occurs via an electrophilic mechanism and in two stages. In both stages, the accession proceeds in accordance with Markovnikov’s rule:

Hydration

The addition of water to alkynes occurs in the presence of ruti salts in an acidic medium and is called the Kucherov reaction.

As a result of hydration, the addition of water to acetylene produces acetaldehyde (acetic aldehyde):

For acetylene homologues, the addition of water leads to the formation of ketones:

Hydrogenation of alkynes

Alkynes react with hydrogen in two steps. Metals such as platinum, palladium, and nickel are used as catalysts:

Trimerization of alkynes

When acetylene is passed over activated carbon at high temperature, a mixture of various products is formed from it, the main of which is benzene, a product of acetylene trimerization:

Dimerization of alkynes

Acetylene also undergoes a dimerization reaction. The process takes place in the presence of copper salts as catalysts:

Alkyne oxidation

Alkynes burn in oxygen:

C nH 2n-2 + (3n-1)/2 O 2 → nCO 2 + (n-1)H 2 O

Reaction of alkynes with bases

Alkynes with a triple C≡C at the end of the molecule, unlike other alkynes, are able to enter into reactions in which the hydrogen atom at the triple bond is replaced by a metal. For example, acetylene reacts with sodium amide in liquid ammonia:

HC≡CH + NaNH 2 → NaC≡CNa + 2NH 3 ,

and also with an ammonia solution of silver oxide, forming insoluble salt-like substances called acetylenides:

Thanks to this reaction, it is possible to recognize alkynes with a terminal triple bond, as well as to isolate such an alkyne from a mixture with other alkynes.

It should be noted that all silver and copper acetylenides are explosive substances.

Acetylenides are capable of reacting with halogen derivatives, which is used in the synthesis of more complex organic compounds with a triple bond:

CH 3 -C≡CH + 2NaNH 2 → CH 3 -C≡CNa + NH 3

CH 3 -C≡CNa + CH 3 Br → CH 3 -C≡C-CH 3 + NaBr

Chemical properties of aromatic hydrocarbons

The aromatic nature of the bond influences the chemical properties of benzenes and other aromatic hydrocarbons.

The unified 6pi electron system is much more stable than ordinary pi bonds. Therefore, substitution reactions rather than addition reactions are more typical for aromatic hydrocarbons. Arenes undergo substitution reactions via an electrophilic mechanism.

Substitution reactions

Halogenation

Nitration

The nitration reaction proceeds best under the influence of not pure nitric acid, but its mixture with concentrated sulfuric acid, the so-called nitrating mixture:

Alkylation

A reaction in which one of the hydrogen atoms at the aromatic ring is replaced by a hydrocarbon radical:

Alkenes can also be used instead of halogenated alkanes. Aluminum halides, ferric halides or inorganic acids can be used as catalysts.<

Addition reactions

Hydrogenation

Chlorine addition

Proceeds via a radical mechanism upon intense irradiation with ultraviolet light:

A similar reaction can only occur with chlorine.

Oxidation reactions

Combustion

2C 6 H 6 + 15O 2 = 12CO 2 + 6H 2 O + Q

Incomplete oxidation

The benzene ring is resistant to oxidizing agents such as KMnO 4 and K 2 Cr 2 O 7 . There is no reaction.

Substituents on the benzene ring are divided into two types:

Let us consider the chemical properties of benzene homologues using toluene as an example.

Chemical properties of toluene

Halogenation

The toluene molecule can be considered as consisting of fragments of benzene and methane molecules. Therefore, it is logical to assume that the chemical properties of toluene should to some extent combine the chemical properties of these two substances taken separately. This is often what is observed during its halogenation. We already know that benzene undergoes a substitution reaction with chlorine via an electrophilic mechanism, and to carry out this reaction it is necessary to use catalysts (aluminum or ferric halides). At the same time, methane is also capable of reacting with chlorine, but via a free radical mechanism, which requires irradiation of the initial reaction mixture with UV light. Toluene, depending on the conditions under which it is subjected to chlorination, can give either products of substitution of hydrogen atoms in the benzene ring - for this you need to use the same conditions as for the chlorination of benzene, or products of substitution of hydrogen atoms in the methyl radical, if it how chlorine acts on methane under ultraviolet irradiation:

As you can see, the chlorination of toluene in the presence of aluminum chloride led to two different products - ortho- and para-chlorotoluene. This is due to the fact that the methyl radical is a substituent of the first kind.

If the chlorination of toluene in the presence of AlCl 3 is carried out in excess of chlorine, the formation of trichloro-substituted toluene is possible:

Similarly, when toluene is chlorinated in the light at a higher chlorine/toluene ratio, dichloromethylbenzene or trichloromethylbenzene can be obtained:

Nitration

The replacement of hydrogen atoms with a nitro group during the nitration of toluene with a mixture of concentrated nitric and sulfuric acids leads to substitution products in the aromatic ring rather than the methyl radical:

Alkylation

As already mentioned, the methyl radical is an orienting agent of the first kind, therefore its alkylation according to Friedel-Crafts leads to the substitution products in ortho- and para-positions:

Addition reactions

Toluene can be hydrogenated to methylcyclohexane using metal catalysts (Pt, Pd, Ni):

C 6 H 5 CH 3 + 9O 2 → 7CO 2 + 4H 2 O

Incomplete oxidation

When exposed to an oxidizing agent such as an aqueous solution of potassium permanganate, the side chain undergoes oxidation. The aromatic core cannot oxidize under such conditions. In this case, depending on the pH of the solution, either a carboxylic acid or its salt will be formed.

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 the carbons is nonpolar and weakly polarizable, and is 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, first of all, which 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

Under high temperature conditions, 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 the 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.