The relative molecular weight of air is taken to be 29 (taking into account the content of nitrogen, oxygen and other gases in the air). It should be noted that the concept of “relative molecular mass of air” is used conditionally, since air is a mixture of gases.

Let's find the molar mass of the gas formed during the chlorination of methane:

M gas = 29 ×D air (gas) = ​​29 × 5.31 = 154 g/mol.

This is carbon tetrachloride - CCl 4. Let's write the reaction equation and arrange the stoichiometric coefficients:

CH 4 + 4Cl 2 = CCl 4 + 4HCl.

Let's calculate the amount of carbon tetrachloride substance:

n(CCl 4) = m(CCl 4) / M(CCl 4);

n(CCl 4) = 1.54 / 154 = 0.01 mol.

According to the reaction equation n(CCl 4) : n(CH 4) = 1: 1, which means

n(CH 4) = n(CCl 4) = 0.01 mol.

Then, the amount of chlorine substance should be equal to n(Cl 2) = 2 × 4 n(CH 4), i.e. n(Cl 2) = 8 × 0.01 = 0.08 mol.

Let us write the reaction equation for the production of chlorine:

MnO 2 + 4HCl = MnCl 2 + Cl 2 + 2H 2 O.

The number of moles of manganese dioxide is 0.08 mol, because n(Cl 2) : n(MnO 2) = 1: 1. Find the mass of manganese dioxide:

m(MnO 2) = n(MnO 2) × M(MnO 2);

M(MnO 2) = Ar(Mn) + 2×Ar(O) = 55 + 2×16 = 87 g/mol;

m(MnO 2) = 0.08 × 87 = 10.4 g.

C n H 2 n -1 Cl 3 .

According to the formula

ω(Cl) = 3×Ar(Cl) / Mr(C n H 2 n -1 Cl 3) × 100%

Let's calculate the molecular weight of trichloroalkane:

Mr(C n H 2 n -1 Cl 3) = 3 × 35.5 / 72.20 × 100% = 147.5.

Let's find the value of n:

12n + 2n - 1 + 35.5×3 = 147.5;

Therefore, the formula of trichloroalkane is C 3 H 5 Cl 3.

Let's compose the structural formulas of the isomers: 1,2,3-trichloropropane (1), 1,1,2-trichloropropane (2), 1,1,3-trichloropropane (3), 1,1,1-trichloropropane (4) and 1 ,2,2-trichloropropane (5).

CH 2 Cl-CHCl-CH 2 Cl (1);

CHCl 2 -CHCl-CH 3 (2);

CHCl 2 -CH 2 -CH 2 Cl (3);

CCl 3 -CH 2 -CH 3 (4);

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 with varying degrees of substitution of hydrogen atoms is formed - 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 a 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:

1. 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 with the formation of alkene molecules 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, butene-1 and butene-2 ​​are formed (the latter in the form cis- And 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 + (3/2) nO 2 → nCO 2 + nH 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 + 2NaNH 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 + NaNH 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.

Alkanes- saturated (saturated) hydrocarbons. A representative of this class is methane ( CH 4). All subsequent saturated hydrocarbons differ by CH 2- a group that is called a homologous group, and compounds are called homologues.

General formula - WITHnH 2 n +2 .

Structure of alkanes.

Each carbon atom is in sp 3- hybridization, forms 4 σ - communications (1 S-S and 3 S-N). The shape of the molecule is in the form of a tetrahedron with an angle of 109.5°.

The bond is formed through the overlap of hybrid orbitals, with the maximum area of ​​overlap lying in space on the straight line connecting the atomic nuclei. This is the most efficient overlap, so the σ bond is considered the strongest.

Isomerism of alkanes.

For alkanes isomerism of the carbon skeleton is characteristic. Limit connections can take on different geometric shapes while maintaining the angle between the connections. For example,

The different positions of the carbon chain are called conformations. Under normal conditions, the conformations of alkanes freely transform into each other through the rotation of C-C bonds, which is why they are often called rotary isomers. There are 2 main conformations - “inhibited” and “eclipsed”:

Isomerism of the carbon skeleton of alkanes.

The number of isomers increases with increasing carbon chain growth. For example, butane has 2 isomers:

For pentane - 3, for heptane - 9, etc.

If a molecule alkane subtract one proton (hydrogen atom), you get a radical:

Physical properties of alkanes.

Under normal conditions - C 1 -C 4- gases , From 5 to From 17- liquids, and hydrocarbons with more than 18 carbon atoms - solids.

As the chain grows, the boiling and melting points increase. Branched alkanes have lower boiling points than normal ones.

Alkanes insoluble in water, but soluble in non-polar organic solvents. Mix easily with each other.

Preparation of alkanes.

Synthetic methods for producing alkanes:

1. From unsaturated hydrocarbons - the “hydrogenation” reaction occurs under the influence of a catalyst (nickel, platinum) and at a temperature:

2. From halogen derivatives - Wurtz reaction: the interaction of monohaloalkanes with sodium metal, resulting in alkanes with double the number of carbon atoms in the chain:

3. From salts of carboxylic acids. When a salt reacts with an alkali, alkanes are obtained that contain 1 less carbon atom compared to the original carboxylic acid:

4. Production of methane. In an electric arc in a hydrogen atmosphere:

C + 2H 2 = CH 4.

In the laboratory, methane is obtained as follows:

Al 4 C 3 + 12H 2 O = 3CH 4 + 4Al(OH) 3.

Chemical properties of alkanes.

Under normal conditions, alkanes are chemically inert compounds; they do not react with concentrated sulfuric and nitric acid, with concentrated alkali, or with potassium permanganate.

Stability is explained by the strength of the bonds and their non-polarity.

Compounds are not prone to bond breaking reactions (addition reactions); they are characterized by substitution.

1. Halogenation of alkanes. Under the influence of a light quantum, radical substitution (chlorination) of the alkane begins. General scheme:

The reaction follows a chain mechanism, in which there are:

A) Initiating the circuit:

B) Chain growth:

B) Open circuit:

In total it can be presented as:

2. Nitration (Konovalov reaction) of alkanes. The reaction occurs at 140 °C:

The reaction proceeds most easily with the tertiary carbon atom than with the primary and secondary ones.

3. Isomerization of alkanes. Under specific conditions, alkanes of normal structure can transform into branched ones:

4. Cracking alkane. Under the action of high temperatures and catalysts, higher alkanes can break their bonds, forming alkenes and lower alkanes:

5. Oxidation of alkanes. Under different conditions and with different catalysts, alkane oxidation can lead to the formation of alcohol, aldehyde (ketone) and acetic acid. Under conditions of complete oxidation, the reaction proceeds to completion - until water and carbon dioxide are formed:

Application of alkanes.

Alkanes have found wide application in industry, in the synthesis of oil, fuel, etc.

Heating the sodium salt of acetic acid (sodium acetate) with an excess of alkali leads to the elimination of the carboxyl group and the formation of methane:

CH3CONa + NaOH CH4 + Na2C03

If you take sodium propionate instead of sodium acetate, then ethane is formed, from sodium butanoate - propane, etc.

RCH2CONa + NaOH -> RCH3 + Na2C03

5. Wurtz synthesis. When haloalkanes interact with the alkali metal sodium, saturated hydrocarbons and an alkali metal halide are formed, for example:

The action of an alkali metal on a mixture of halocarbons (eg bromoethane and bromomethane) will result in the formation of a mixture of alkanes (ethane, propane and butane).

The reaction on which the Wurtz synthesis is based proceeds well only with haloalkanes in the molecules of which a halogen atom is attached to a primary carbon atom.

6. Hydrolysis of carbides. When some carbides containing carbon in the -4 oxidation state (for example, aluminum carbide) are treated with water, methane is formed:

Al4C3 + 12H20 = 3CH4 + 4Al(OH)3 Physical properties

The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a gas without color, taste and smell (the smell of “gas”, which you need to call 04, is determined by the smell of mercaptans - sulfur-containing compounds, specially added to methane used in household and industrial gas appliances, for so that people nearby can detect a leak by smell).

Hydrocarbons of composition from C5H12 to C15H32 are liquids, heavier hydrocarbons are solids.

The boiling and melting points of alkanes gradually increase with increasing carbon chain length. All hydrocarbons are poorly soluble in water; liquid hydrocarbons are common organic solvents.

Chemical properties

1. Substitution reactions. The most characteristic reactions for alkanes are free radical substitution reactions, during which a hydrogen atom is replaced by a halogen atom or some group.

Let us present the equations of the most characteristic reactions.

Halogenation:

СН4 + С12 -> СН3Сl + HCl

In case of excess halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms with chlorine:

СН3Сl + С12 -> HCl + СН2Сl2
dichloromethane methylene chloride

СН2Сl2 + Сl2 -> HCl + CHCl3
trichloromethane chloroform

СНСl3 + Сl2 -> HCl + СCl4
carbon tetrachloride carbon tetrachloride

The resulting substances are widely used as solvents and starting materials in organic syntheses.

2. Dehydrogenation (elimination of hydrogen). When alkanes are passed over a catalyst (Pt, Ni, Al2O3, Cr2O3) at high temperatures (400-600 °C), a hydrogen molecule is eliminated and an alkene is formed:

CH3-CH3 -> CH2=CH2 + H2

3. Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons burn to form carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode. The combustion of saturated hydrocarbons is a free-radical exothermic reaction, which is very important when using alkanes as fuel.

CH4 + 2O2 -> C02 + 2H2O + 880kJ

In general, the combustion reaction of alkanes can be written as follows:

Thermal decomposition reactions underlie the industrial process of hydrocarbon cracking. This process is the most important stage of oil refining.

When methane is heated to a temperature of 1000 ° C, methane pyrolysis begins - decomposition into simple substances. When heated to a temperature of 1500 °C, the formation of acetylene is possible.

4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with a branched carbon skeleton are formed:

5. Flavoring. Alkanes with six or more carbon atoms in the chain cyclize in the presence of a catalyst to form benzene and its derivatives:

What is the reason that alkanes undergo free radical reactions? All carbon atoms in alkane molecules are in a state of sp 3 hybridization. The molecules of these substances are built using covalent nonpolar C-C (carbon-carbon) bonds and weakly polar C-H (carbon-hydrogen) bonds. They do not contain areas with increased or decreased electron density, or easily polarizable bonds, i.e., such bonds in which the electron density can shift under the influence of external influences (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since the bonds in alkane molecules are not broken by a heterolytic mechanism.

The most characteristic reactions of alkanes are free radical substitution reactions. During these reactions, a hydrogen atom is replaced by a halogen atom or some group.

The kinetics and mechanism of free radical chain reactions, i.e. reactions occurring under the influence of free radicals - particles with unpaired electrons - were studied by the remarkable Russian chemist N. N. Semenov. It was for these studies that he was awarded the Nobel Prize in Chemistry.

Typically, the mechanism of free radical substitution reactions is represented by three main stages:

1. Initiation (nucleation of a chain, formation of free radicals under the influence of an energy source - ultraviolet light, heating).

2. Chain development (a chain of sequential interactions of free radicals and inactive molecules, as a result of which new radicals and new molecules are formed).

3. Chain termination (combination of free radicals into inactive molecules (recombination), “death” of radicals, cessation of the development of a chain of reactions).

Scientific research by N.N. Semenov

Semenov Nikolay Nikolaevich

(1896 - 1986)

Soviet physicist and physical chemist, academician. Nobel Prize winner (1956). Scientific research relates to the study of chemical processes, catalysis, chain reactions, the theory of thermal explosion and the combustion of gas mixtures.

Let's consider this mechanism using the example of the methane chlorination reaction:

CH4 + Cl2 -> CH3Cl + HCl

Chain initiation occurs as a result of the fact that under the influence of ultraviolet irradiation or heating, homolytic cleavage of the Cl-Cl bond occurs and the chlorine molecule disintegrates into atoms:

Сl: Сl -> Сl· + Сl·

The resulting free radicals attack methane molecules, tearing off their hydrogen atom:

CH4 + Cl· -> CH3· + HCl

and transforming into CH3· radicals, which, in turn, colliding with chlorine molecules, destroy them with the formation of new radicals:

CH3 + Cl2 -> CH3Cl + Cl etc.

The chain develops.

Along with the formation of radicals, their “death” occurs as a result of the process of recombination - the formation of an inactive molecule from two radicals:

СН3+ Сl -> СН3Сl

Сl· + Сl· -> Сl2

CH3 + CH3 -> CH3-CH3

It is interesting to note that during recombination, only as much energy is released as is necessary to break the newly formed bond. In this regard, recombination is possible only if a third particle (another molecule, the wall of the reaction vessel) participates in the collision of two radicals, which absorbs excess energy. This makes it possible to regulate and even stop free radical chain reactions.

Note the last example of a recombination reaction - the formation of an ethane molecule. This example shows that a reaction involving organic compounds is a rather complex process, as a result of which, along with the main reaction product, by-products are very often formed, which leads to the need to develop complex and expensive methods for the purification and isolation of target substances.

The reaction mixture obtained from the chlorination of methane, along with chloromethane (CH3Cl) and hydrogen chloride, will contain: dichloromethane (CH2Cl2), trichloromethane (CHCl3), carbon tetrachloride (CCl4), ethane and its chlorination products.

Now let's try to consider the halogenation reaction (for example, bromination) of a more complex organic compound - propane.

If in the case of methane chlorination only one monochloro derivative is possible, then in this reaction two monobromo derivatives can be formed:

It can be seen that in the first case, the hydrogen atom is replaced at the primary carbon atom, and in the second case, at the secondary one. Are the rates of these reactions the same? It turns out that the product of substitution of the hydrogen atom, which is located at the secondary carbon, predominates in the final mixture, i.e. 2-bromopropane (CH3-CHBg-CH3). Let's try to explain this.

In order to do this, we will have to use the idea of ​​​​the stability of intermediate particles. Did you notice that when describing the mechanism of the methane chlorination reaction we mentioned the methyl radical - CH3·? This radical is an intermediate particle between methane CH4 and chloromethane CH3Cl. The intermediate particle between propane and 1-bromopropane is a radical with an unpaired electron at the primary carbon, and between propane and 2-bromopropane at the secondary carbon.

A radical with an unpaired electron at the secondary carbon atom (b) is more stable compared to a free radical with an unpaired electron at the primary carbon atom (a). It is formed in greater quantities. For this reason, the main product of the propane bromination reaction is 2-bromopropane, a compound whose formation occurs through a more stable intermediate species.

Here are some examples of free radical reactions:

Nitration reaction (Konovalov reaction)

The reaction is used to obtain nitro compounds - solvents, starting materials for many syntheses.

Catalytic oxidation of alkanes with oxygen

These reactions are the basis of the most important industrial processes for the production of aldehydes, ketones, and alcohols directly from saturated hydrocarbons, for example:

CH4 + [O] -> CH3OH

Application

Saturated hydrocarbons, especially methane, are widely used in industry (Scheme 2). They are a simple and fairly cheap fuel, a raw material for the production of a large number of important compounds.

Compounds obtained from methane, the cheapest hydrocarbon raw material, are used to produce many other substances and materials. Methane is used as a source of hydrogen in the synthesis of ammonia, as well as to produce synthesis gas (a mixture of CO and H2), used for the industrial synthesis of hydrocarbons, alcohols, aldehydes and other organic compounds.

Hydrocarbons of higher boiling oil fractions are used as fuel for diesel and turbojet engines, as the basis of lubricating oils, as raw materials for the production of synthetic fats, etc.

Here are several industrially significant reactions that occur with the participation of methane. Methane is used to produce chloroform, nitromethane, and oxygen-containing derivatives. Alcohols, aldehydes, carboxylic acids can be formed by the direct interaction of alkanes with oxygen, depending on the reaction conditions (catalyst, temperature, pressure):

As you already know, hydrocarbons of the composition from C5H12 to C11H24 are included in the gasoline fraction of oil and are used mainly as fuel for internal combustion engines. It is known that the most valuable components of gasoline are isomeric hydrocarbons, since they have maximum detonation resistance.

When hydrocarbons come into contact with atmospheric oxygen, they slowly form compounds with it - peroxides. This is a slowly occurring free radical reaction, initiated by an oxygen molecule:

Please note that the hydroperoxide group is formed at secondary carbon atoms, which are most abundant in linear, or normal, hydrocarbons.

With a sharp increase in pressure and temperature that occurs at the end of the compression stroke, the decomposition of these peroxide compounds begins with the formation of a large number of free radicals, which “trigger” the free radical combustion chain reaction earlier than necessary. The piston still goes up, and the combustion products of gasoline, which have already formed as a result of premature ignition of the mixture, push it down. This leads to a sharp decrease in engine power and wear.

Thus, the main cause of detonation is the presence of peroxide compounds, the ability to form which is maximum in linear hydrocarbons.

C-heptane has the lowest detonation resistance among the hydrocarbons of the gasoline fraction (C5H14 - C11H24). The most stable (i.e., forms peroxides to the least extent) is the so-called isooctane (2,2,4-trimethylpentane).

A generally accepted characteristic of the knock resistance of gasoline is the octane number. An octane number of 92 (for example, A-92 gasoline) means that this gasoline has the same properties as a mixture consisting of 92% isooctane and 8% heptane.

In conclusion, we can add that the use of high-octane gasoline makes it possible to increase the compression ratio (pressure at the end of the compression stroke), which leads to increased power and efficiency of the internal combustion engine.

Being in nature and receiving

In today's lesson, you became acquainted with the concept of alkanes, and also learned about its chemical composition and methods of preparation. Therefore, let's now dwell in more detail on the topic of the presence of alkanes in nature and find out how and where alkanes have found application.

The main sources for the production of alkanes are natural gas and oil. They make up the bulk of oil refining products. Methane, common in sedimentary rock deposits, is also a gas hydrate of alkanes.

The main component of natural gas is methane, but it also contains a small proportion of ethane, propane and butane. Methane can be found in emissions from coal seams, swamps and associated petroleum gases.

Ankans can also be obtained by coking coal. In nature, there are also so-called solid alkanes - ozokerites, which are presented in the form of deposits of mountain wax. Ozokerite can be found in the waxy coatings of plants or their seeds, as well as in beeswax.

The industrial isolation of alkanes is taken from natural sources, which, fortunately, are still inexhaustible. They are obtained by the catalytic hydrogenation of carbon oxides. Methane can also be produced in the laboratory using the method of heating sodium acetate with solid alkali or hydrolysis of certain carbides. But alkanes can also be obtained by decarboxylation of carboxylic acids and by their electrolysis.

Applications of alkanes

Alkanes at the household level are widely used in many areas of human activity. After all, it is very difficult to imagine our life without natural gas. And it will not be a secret to anyone that the basis of natural gas is methane, from which carbon black is produced, which is used in the production of topographic paints and tires. The refrigerator that everyone has in their home also works thanks to alkane compounds used as refrigerants. Acetylene obtained from methane is used for welding and cutting metals.

Now you already know that alkanes are used as fuel. They are present in gasoline, kerosene, diesel oil and fuel oil. In addition, they are also found in lubricating oils, petroleum jelly and paraffin.

Cyclohexane has found wide use as a solvent and for the synthesis of various polymers. Cyclopropane is used in anesthesia. Squalane, as a high-quality lubricating oil, is a component of many pharmaceutical and cosmetic preparations. Alkanes are the raw materials used to produce organic compounds such as alcohol, aldehydes and acids.

Paraffin is a mixture of higher alkanes, and since it is non-toxic, it is widely used in the food industry. It is used for impregnation of packaging for dairy products, juices, cereals, etc., but also in the manufacture of chewing gum. And heated paraffin is used in medicine for paraffin treatment.

In addition to the above, the heads of matches are impregnated with paraffin for better burning, pencils, and candles are made from it.

By oxidizing paraffin, oxygen-containing products, mainly organic acids, are obtained. When liquid hydrocarbons with a certain number of carbon atoms are mixed, Vaseline is obtained, which is widely used in perfumery and cosmetology, as well as in medicine. It is used to prepare various ointments, creams and gels. They are also used for thermal procedures in medicine.

Practical tasks

1. Write down the general formula of hydrocarbons of the homologous series of alkanes.

2. Write the formulas of possible isomers of hexane and name them according to systematic nomenclature.

3. What is cracking? What types of cracking do you know?

4. Write the formulas of possible products of hexane cracking.

5. Decipher the following chain of transformations. Name the compounds A, B and C.

6. Give the structural formula of the hydrocarbon C5H12, which forms only one monobromine derivative upon bromination.

7. For the complete combustion of 0.1 mol of an alkane of unknown structure, 11.2 liters of oxygen were consumed (at ambient conditions). What is the structural formula of an alkane?

8. What is the structural formula of a gaseous saturated hydrocarbon if 11 g of this gas occupy a volume of 5.6 liters (at standard conditions)?

9. Recall what you know about the use of methane and explain why a domestic gas leak can be detected by smell, although its components are odorless.

10*. What compounds can be obtained by catalytic oxidation of methane under various conditions? Write the equations for the corresponding reactions.

eleven*. Products of complete combustion (in excess oxygen) 10.08 liters (N.S.) of a mixture of ethane and propane were passed through an excess of lime water. In this case, 120 g of sediment was formed. Determine the volumetric composition of the initial mixture.

12*. The ethane density of a mixture of two alkanes is 1.808. Upon bromination of this mixture, only two pairs of isomeric monobromoalkanes were isolated. The total mass of lighter isomers in the reaction products is equal to the total mass of heavier isomers. Determine the volume fraction of the heavier alkane in the initial mixture.

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