Lesson summary: natural gas alkanes. Application of alkanes. II. Expected results

Heating the sodium salt 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, during which a hydrogen atom is replaced by a halogen atom or some group.

Let us present the equations 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 has a very great importance 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 non-polar 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. Laureate Nobel Prize (1956). Scientific research relate to the study of chemical processes, catalysis, chain reactions, the theory of thermal explosion and 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 engines internal combustion. 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 occurring at the end of the compression stroke, the decomposition of these peroxide compounds begins with the formation large number free radicals that “start” the free radical chain reaction of combustion 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 were introduced to the concept of alkanes, and also learned about its chemical composition and methods of obtaining. 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.

Ancans 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 obtained from laboratory conditions, 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 Food Industry. It is used for impregnation of packaging for dairy products, juices, cereals, etc., but also in the manufacture 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.

DEFINITION

Alkanes are called saturated hydrocarbons, the molecules of which consist of carbon and hydrogen atoms connected to each other only by σ bonds.

Under normal conditions (at 25 o C and atmospheric pressure), the first four members of the homologous series of alkanes (C 1 - C 4) are gases. Normal alkanes from pentane to heptadecane (C 5 - C 17) are liquids, starting from C 18 and above are solids. As the relative molecular weight increases, the boiling and melting points of alkanes increase. With the same number of carbon atoms in the molecule, branched alkanes have lower boiling points than normal alkanes. The structure of the alkane molecule using methane as an example is shown in Fig. 1.

Rice. 1. The structure of the methane molecule.

Alkanes are practically insoluble in water, since their molecules are low-polar and do not interact with water molecules. Liquid alkanes mix easily with each other. They dissolve well in non-polar organic solvents such as benzene, carbon tetrachloride, diethyl ether, etc.

Preparation of alkanes

The main sources of various saturated hydrocarbons containing up to 40 carbon atoms are oil and natural gas. Alkanes with a small number of carbon atoms (1 - 10) can be isolated by fractional distillation of natural gas or the gasoline fraction of oil.

There are industrial (I) and laboratory (II) methods for producing alkanes.

C + H 2 → CH 4 (kat = Ni, t 0);

CO + 3H 2 → CH 4 + H 2 O (kat = Ni, t 0 = 200 - 300);

CO 2 + 4H 2 → CH 4 + 2H 2 O (kat, t 0).

— hydrogenation of unsaturated hydrocarbons

CH 3 -CH=CH 2 + H 2 →CH 3 -CH 2 -CH 3 (kat = Ni, t 0);

- reduction of haloalkanes

C 2 H 5 I + HI →C 2 H 6 + I 2 (t 0);

- alkaline melting reactions of salts of monobasic organic acids

C 2 H 5 -COONa + NaOH → C 2 H 6 + Na 2 CO 3 (t 0);

— interaction of haloalkanes with sodium metal (Wurtz reaction)

2C 2 H 5 Br + 2Na → CH 3 -CH 2 -CH 2 -CH 3 + 2NaBr;

— electrolysis of salts of monobasic organic acids

2C 2 H 5 COONa + 2H 2 O → H 2 + 2NaOH + C 4 H 10 + 2CO 2 ;

K(-): 2H 2 O + 2e → H 2 + 2OH - ;

A(+):2C 2 H 5 COO — -2e → 2C 2 H 5 COO + → 2C 2 H 5 + + 2CO 2 .

Chemical properties of alkanes

Alkanes are among the least reactive organic compounds, which is explained by their structure.

Alkanes under normal conditions do not react with concentrated acids, molten and concentrated alkalis, alkali metals, halogens (except fluorine), potassium permanganate and potassium dichromate in an acidic environment.

For alkanes, the most typical reactions are those that proceed by a radical mechanism. Homolytic cleavage is energetically more favorable C-H bonds and C-C than their heterolytic break.

Radical substitution reactions most easily occur at the tertiary carbon atom, then at the secondary carbon atom, and lastly at the primary carbon atom.

All chemical transformations of alkanes proceed with splitting:

1) C-H bonds

— halogenation (S R)

CH 4 + Cl 2 → CH 3 Cl + HCl ( hv);

CH 3 -CH 2 -CH 3 + Br 2 → CH 3 -CHBr-CH 3 + HBr ( hv).

- nitration (S R)

CH 3 -C(CH 3)H-CH 3 + HONO 2 (dilute) → CH 3 -C(NO 2)H-CH 3 + H 2 O (t 0).

— sulfochlorination (S R)

R-H + SO 2 + Cl 2 → RSO 2 Cl + HCl ( hv).

- dehydrogenation

CH 3 -CH 3 → CH 2 =CH 2 + H 2 (kat = Ni, t 0).

- dehydrocyclization

CH 3 (CH 2) 4 CH 3 → C 6 H 6 + 4H 2 (kat = Cr 2 O 3, t 0).

2) C-H and C-C bonds

- isomerization (intramolecular rearrangement)

CH 3 -CH 2 -CH 2 -CH 3 →CH 3 -C(CH 3)H-CH 3 (kat=AlCl 3, t 0).

- oxidation

2CH 3 -CH 2 -CH 2 -CH 3 + 5O 2 → 4CH 3 COOH + 2H 2 O (t 0 , p);

C n H 2n+2 + (1.5n + 0.5) O 2 → nCO 2 + (n+1) H 2 O (t 0).

Applications of alkanes

Alkanes have found application in various industries. Let us consider in more detail, using the example of some representatives of the homologous series, as well as mixtures of alkanes.

Methane forms the raw material basis for the most important chemical industrial processes for the production of carbon and hydrogen, acetylene, oxygen-containing organic compounds - alcohols, aldehydes, acids. Propane is used as automobile fuel. Butane is used to produce butadiene, which is a raw material for the production of synthetic rubber.

A mixture of liquid and solid alkanes up to C 25, called Vaseline, is used in medicine as the basis of ointments. A mixture of solid alkanes C 18 - C 25 (paraffin) is used for impregnation various materials(paper, fabrics, wood) to give them hydrophobic properties, i.e. non-wetting with water. In medicine it is used for physiotherapeutic procedures (paraffin treatment).

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Exercise

Determine the molecular formula of trichloroalkane, mass fraction chlorine in which is 72.20%. Compose structural formulas all possible isomers and give the names of the substances according to the IUPAC substitutive nomenclature.

Answer

Let's write the general formula of trichloroalkean: 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); Municipal budget educational institution"Aktanyshskaya average comprehensive school No. 1" Aktanysh municipal district of the Republic of Tatarstan Chemistry Grade 10 Lesson type: learning new material Lesson format: lesson - travel using a computer (using multimedia teaching aids) Valieva Elvira Fanisovna Lesson topic: Alkanes, preparations, properties and applications Lesson – travel with multimedia accompaniment I. Lesson objectives. 1. Developmental goals. Develop in schoolchildren logical thinking, develop the ability to compose reaction equations involving alkanes. To form intellectual skills: the ability to analyze the properties of alkanes, highlight the main thing, compare, generalize and systematize. Develop will and independence. Develop self-control: self-confidence, the ability to overcome difficulties in learning chemistry. 2. Educational purposes. Ensure that students understand the chemical properties and methods of producing alkanes. Summarize and consolidate, systematize previously acquired knowledge on types of hybridization, on the nomenclature of organic compounds. Develop skills in working with game elements, video clips, and illustrative materials. To create a culture of health in chemistry lessons. Identify underdeveloped topics and correct them educational process and prepare students for the Unified State Exam. 3. Educational goals. Develop a culture of speech among students. Educate ecological culture and students' thinking. II. Lesson type:learning new material. III. Lesson type:lesson using a computer (using multimedia teaching aids). IV. Innovative, informational educational technologies, based on the use of modern advanced technology - computers, interactive whiteboards, projectors. V. Lesson methods: A. Illustrative and gaming B. Teaching - reporting. training – a/ programmed b/ illustrative game 2) teaching – a/ explanatory b/ stimulating 3) teaching – a/ reproductive b/ partially exploratory VI. Facilities:Computer, illustrative material, game elements, laboratory experiments and video demonstration. During the classes: On the projector screen: Travel map of the country "Alkany" Information Halt Warm-up Informational Start C n H2 n +2 Technique security Finish Experiment I station. Warm up. Start. 1. Oral interview 1. Gasoline, household gas, solvents, plastics, dyes, alcohols, medicines, perfumes - all products... 2. Swamp gas. Formed during rotting during dry distillation of coal. It is the main component of natural gases... 3. How many types organic matter? 4. Combs, jewelry, billiard balls, toys, balls, brushes are made from it... 5. Material for making suitcases... 6.Many well-known aromatic substances belong to the class... 7.The world-famous perfumes - French “Soir de Paris” and “Chanel” are made from what substances? 8. Fuel for the body... 9. This substance is a narcotic, not harmless to humans, paralyzes the nervous, cardiovascular system, liver... 10. Who discovered the theory of the structure of organic compounds? 11. Who introduced the concept of “hybridization” 12. What are isomers? 2. Questions and tasks on the projector screen The students answer. After the students answer, the computer immediately gives the correct answer. 1. How many electrons are there in the second level of the carbon atom. 2. Distribute the electrons into the orbitals of carbon in the excited state. 3. Hybridization of atomic orbitals. a) Which electrons overlap? b) Formation of covalent bonds in a methane molecule (medication) c) Formation of G and P bonds in the ethylene molecule (medication) d) Formation of G and P bonds in the acetylene molecule (medication) e) Location of C atoms in space (medication) 4. What class do the following compounds belong to? R-OH, R-C, R-C, R-O-R, R-CI 5. General formulas of which compounds are shown? C n H 2 n +2 , C n H 2 n , C n H 2 n -2 , C n H 2 n +1 COOH, C n H 2 n +1 COH 6.What is a homological series? Screen image H H H H H H H H H H-C - C-H H-C-C-C-H H-C-C-C-C-H H H H H H H H H H 7. Which formula is redundant? C 2 H 6 CH 4 C 6 H 16 C 16 H 34 C 2 H 4 C 12 H 24 C 4 H 10 3. Let us recall the algorithm for naming substances of acyclic structure. The formula of the substance appears on the screen: Meditation with sound: 1. Choose the longest carbon chain 2. Number it on the side to which the radicals, or the senior substituent, or the multiple bond are closest.(numbering occurs on the screen) 3. Indicate the position in the prefix (carbon atom number) and name the radicals, substituents, functional group in alphabetical order. (on screen 2 – methyl -) 4.Name the main hydrocarbon (2-methylbutane on the screen) 5.If there is double bond, then after the root put the suffix -en, for a triple bond -in, if there are no multiple bonds - the suffix -an. II Information station 1. Physical properties of alkanes. On the diagram screen; The teacher says: sulfur-containing compounds - mercaptans - are specially added to methane so that people can detect a leak by smell. Demonstration substances: hexane, paraffin Branched alkanes boil at lower temperatures than straight alkanes. Write in notebook: C 1 - C 4 gases CH 4 - T pl = -182.5 °С 5 – C 15 - liquids C 16 – C n - solids 2. Methods for obtaining alkanes. Alkanes in large quantities obtained from natural gas and oil. From simple substances in an electric discharge: C+2H 2 →CH 4 Hydrolysis of aluminum carbide +3 -4 AI 4 C 3 +6HOH → 4AI(OH) 3 +3CH 4 Heating of monohaloalkanes with sodium metal (Wurtz reaction) C 2 H 5 Br+2Na+Br-C 2 H 5 → C 2 H 5 - C 2 H 5 + 2NaBr If the haloalkanes are different, the result will be a mixture of three products: t ° 3CH 3 Br + 3Na + 3Br-C 2 H 5 →CH 3 -CH 3 + CH 3 -CH 2 -CH 3 +C 2 H 5 -C 2 H 5 5. Decarboxylation. Fusion of sodium acetate with alkali. The alkane produced this way will have one less carbon atom. Demonstration of experience on a computer screen (with sound) 6. Hydrolysis of the Grignard reagent: 7.Alkanes of symmetrical structure can be obtained by electrolysis of salts of carboxylic acids (Kolb reaction) III . Station Prival . (Students relax, listen to music). IV. Information station. 3. Chemical properties alkanes. Since the bonds in alkanes are low-polar, they are characterized by radical reactions and substitution reactions. 1.Substitution reactions. a) With halogens (halogenation). With chlorine in the light, with bromine when heated. In case of excess chlorine, chlorination goes further until the hydrogen atoms are completely replaced. The reaction follows a radical mechanism. 2. Elimination reactions a) Dehydrogenation (elimination of hydrogen) b) Cracking of alkanes: Cracking -0 radical breaking of C-C bonds. Occurs when heated and in the presence of catalysts. Cracking produces a mixture of alkanes with fewer C atoms. The mechanism is free radical. This process is the most important stage of oil refining. c) at a temperature of 1500 0 C methane is pyrolyzed d) at a temperature of 1000 0 C: 3 Oxidation reactions. a) In the presence of excess oxygen, complete combustion of alkanes occurs to CO 2 and H 2 O. The combustion of alkanes releases a large amount of heat, which is the basis for their use as fuel. V.Experimental station - On the screen there is a video fragment with sound recording “Methane combustion” with sound recording: Low alkanes burn with a colorless flame, and with an increase in the number of carbon atoms in the molecule, the flame of alkanes becomes more and more colored and smoky. VI. Station Safety precautions a) Gaseous hydrocarbons with air in certain proportions can explode! b) In conditions of lack of oxygen, incomplete combustion occurs, the product is soot (C) poisonous gas CO c) By mild oxidation of alkanes with atmospheric oxygen on catalysts, alcohols, aldehydes, and acids with fewer carbon atoms in the molecule can be obtained. 4 Isomerization reactions Alkanes of normal structure, when heated in the presence of a catalyst, can transform into branched-chain alkanes. 5. Flavoring. Alkanes with six or more carbon atoms undergo dehydrogenation reactions to form a ring: Finish-fixing station Questions for groups. Homework: Exercise 4,6,7,8 (written), p.81. Chemistry lesson using ICT on the topic "Alkanes" The purpose of the lesson: introduce students to alkanes and identify their important role in industry. Lesson objectives: Educational: consider the homologous series of saturated hydrocarbons, structure, physical and chemical properties, methods of their production during natural gas processing, the possibility of their production from natural sources: natural and associated petroleum gas, oil and coal. Developmental: develop the concept of the spatial structure of alkanes; development of cognitive interests, creative and intellectual abilities, development of independence in acquiring new knowledge using new technologies. Educational: show the unity of the material world using the example of the genetic connection of hydrocarbons of different homologous series obtained from the processing of natural and associated petroleum gases, oil and coal. Equipment: computer, multimedia projector, screen, presentation. During the classes I. Organizing time. (Inform the purpose and topic of the lesson). II. Learned new material. Lesson topic: "Alkanes". Slide number 1 Plan for studying alkanes. Slide number 2 Definition. General formula of the class of hydrocarbons. Homologous series. Types of isomerism. Structure of alkanes. Physical properties. Methods of obtaining. Chemical properties. Application. Alkanes. (Saturated hydrocarbons. Paraffins. Saturated hydrocarbons.) Alkanes are hydrocarbons in molecules in which all carbon atoms are connected by single bonds and have the general formula: C n H 2n+2 Slide No. 3 What are homologues? Homologous series of methane CH 4 methane C 2 H 6 ethane C 3 H 8 propane C 4 H 10 butane C 5 H 12 pentane C 6 H 14 hexane C 7 H 16 heptane C 9 H 20 nonane Homologues are substances that are similar in structure and properties and differ by one or more CH 2 groups. Structural isomerism: Algorithm. 1. Selecting the main circuit: Slide No. 5 2. Numbering of atoms of the main chain: Slide No. 6 3. Formation of the title: Slide No. 7 2 - methylbutane Structure of alkanes. The carbon atom in all organic substances is in an “excited” state and has four unpaired electrons at the outer level. Each electron cloud has a reserve of energy: the s-cloud has a smaller reserve of energy than the p-cloud; in the carbon atom they are in different energy states. Therefore, when a chemical bond is formed, hybridization occurs, i.e., the alignment of electron clouds in terms of energy reserve. This is reflected in the shape and direction of the clouds; a restructuring (spatial) of the electron clouds occurs. As a result of sp3 hybridization, all four valence electron clouds are hybridized: the bond angle between these axes of the hybridized clouds is 109° 28", therefore the molecules have a spatial tetrahedral shape, the shape of the carbon chains is zigzag; the carbon atoms are not on the same straight line, because during rotation atoms bond angles remain the same. All organic substances are built mainly through covalent bonds. Carbon-carbon and carbon-hydrogen bonds are referred to as sigma bonds - a bond formed when atomic orbitals overlap along a line passing through the atomic nuclei. Rotation around sigma bonds is possible, since this bond has axial symmetry. Slide number 13 Physical properties. CH 4:C 4 H 10 - gases Boiling point: -161.6:-0.5 °C Melting temperature: -182.5:-138.3 °C C 5 H 12: C 15 H 32 - liquids Boiling point: 36.1:270.5 °C Melting temperature: -129.8:10 °C Boiling point: 287.5 °C Melting temperature: 20 °C With increasing relative molecular weights saturated hydrocarbons naturally increase their boiling and melting points. Slide number 14 Receipt. In industry 1) cracking of petroleum products: C 16 H 34 - C 8 H 18 + C 8 H 16 2) In the laboratory: a) Hydrolysis of carbides: Al 4 C 3 +12 H 2 O = 3 CH 4 + 4 Al(OH) 3 b) Wurtz reaction: C 2 H 5 Cl + 2Na - C 4 H 10 + 2NaCl c) Decarboxylation of sodium salts of carbon salts: CH 3 COONa + 2NaOH - CH 4 + Na 2 CO 3 Slide No. 15 Chemical properties The following types are typical for alkanes chemical reactions: Substitution of hydrogen atoms; Dehydrogenation; Oxidation. 1) Substitution of hydrogen atoms: A) Halogenation reaction: CH 4 +Cl 2 - CH 3 Cl + HCl B) Nitration reaction (Konovalov): CH 4 + HNO 3 - CH 3 -NO 2 + H 2 O + Q B) Sulfonation reaction: CH 4 + H 2 SO 4 - CH 3 -SO 3 H + H 2 O + Q 2) Isomerization reaction: CH 3 -CH 2 -CH 2 -CH 2 -CH 3 - CH 3 -CH-CH 2 -CH 3 3) Reaction with water vapor: CH 4 + H 2 O = CO + 3H 2 4) Dehydrogenation reaction: 2CH 4 - HC=CH + 3H 2 + Q 5) Oxidation reaction: CH 4 + O 2 - H 2 C=O + H 2 O 6) Methane combustion: CH 4 + 2O 2 CO 2 + 2H 2 O + Q Slide number 20 Application. (Perhaps pre-prepared student speeches.) Widely used as fuel, including for internal combustion engines, as well as in the production of soot (1 - cartridges; 2 - rubber; 3 - printing ink), when obtaining organic substances (4 - solvents; 5 - refrigerants used in refrigeration units; 6 - methanol; 7 - acetylene) Slide No. 21 III. Consolidation. List all possible isomers for heptane and name them. Make the 2 closest homologs for pentane and name them. Determine the saturated hydrocarbon whose vapor density in air is 2. (C 4 H 10). Textbook: No. 12 (p. 33). IV. Homework: Textbook O.S. Gabrielyan (10th grade a basic level of): 3, ex. 4, 7, 8 (page 32). Literature. Gorkovenko M. Yu. Lesson developments in chemistry for educational sets of O. S. Gabrielyan and others, grade 10 (11). M.: "VEKO", 2008 Structure of alkanes Alkanes are hydrocarbons in whose molecules the atoms are connected by single bonds and which correspond to general formula C n H 2n+2. In alkane molecules, all carbon atoms are in the state sp 3 -hybridization. This means that all four hybrid orbitals of the carbon atom are identical in shape, energy and are directed towards the corners of an equilateral triangular pyramid - tetrahedron. The angles between the orbitals are 109° 28′. Almost free rotation is possible around a single carbon-carbon bond, and alkane molecules can take on a wide variety of shapes with angles at the carbon atoms close to tetrahedral (109° 28′), for example, in the n-pentane molecule. It is especially worth recalling the bonds in alkane molecules. All bonds in the molecules of saturated hydrocarbons are single. The overlap occurs along the axis connecting the nuclei of atoms, i.e. it σ bonds. Carbon-carbon bonds are non-polar and poorly polarizable. Length S-S connections in alkanes is 0.154 nm (1.54 10 10 m). C-H bonds are somewhat shorter. The electron density is slightly shifted towards the more electronegative carbon atom, i.e. C-H connection is weakly polar. Homologous series of methane Homologues- substances that are similar in structure and properties and differ in one or more CH groups 2 . Saturated hydrocarbons constitute the homologous series of methane. Isomerism and nomenclature of alkanes Alkanes are characterized by the so-called structural isomerism. Structural isomers differ from each other in the structure of the carbon skeleton. The simplest alkane, which is characterized by structural isomers, is butane. Let us consider in more detail the basic nomenclature for alkanes IUPAC. 1. Main circuit selection. The formation of the name of a hydrocarbon begins with the definition of the main chain - the longest chain of carbon atoms in the molecule, which is, as it were, its basis. 2. Numbering of main chain atoms. The atoms of the main chain are assigned numbers. The numbering of the atoms of the main chain begins from the end to which the substituent is closest (structures A, B). If the substituents are located at an equal distance from the end of the chain, then numbering starts from the end at which there are more of them (structure B). If different substituents are located at equal distances from the ends of the chain, then numbering begins from the end to which the senior one is closest (structure D). The seniority of hydrocarbon substituents is determined by the order in which the letter with which their name begins appears in the alphabet: methyl (-CH 3), then propyl (-CH 2 -CH 2 -CH 3), ethyl (-CH 2 -CH 3 ) etc. Please note that the name of the substituent is formed by replacing the suffix -ane with the suffix -yl in the name of the corresponding alkane. 3. Formation of the name. At the beginning of the name, numbers are indicated - the numbers of the carbon atoms at which the substituents are located. If there are several substituents at a given atom, then the corresponding number in the name is repeated twice separated by a comma (2,2-). After the number, a hyphen indicates the number of substituents (di - two, three - three, tetra - four, penta - five) and the name of the substituent (methyl, ethyl, propyl). Then, without spaces or hyphens, the name of the main chain. The main chain is called a hydrocarbon - a member of the homologous series of methane (methane, ethane, propane, etc.). The names of substances whose structural formulas are given above are as follows: Structure A: 2-methylpropane; Structure B: 3-ethylhexane; Structure B: 2,2,4-trimethylpentane; Structure D: 2-methyl 4-ethylhexane. Absence of saturated hydrocarbons in molecules polar bonds leads to them poorly soluble in water, do not interact with charged particles (ions). The most characteristic reactions for alkanes are those involving free radicals. Physical properties of alkanes The first four representatives of the homologous series of methane are gases. The simplest of them is methane - a colorless, tasteless and odorless gas (the smell of “gas”, when you smell it, 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 so that people , located next to them, could detect the leak by smell). Hydrocarbons of composition from WITH 5 N 12 before WITH 15 N 32 - 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 of alkanes Substitution reactions. The most characteristic reactions for alkanes are free radical substitution, during which a hydrogen atom is replaced by a halogen atom or some group. Let us present the characteristic equations halogenation reactions: In case of excess halogen, chlorination can go further, up to the complete replacement of all hydrogen atoms with chlorine: The resulting substances are widely used as solvents and starting materials in organic syntheses. Dehydrogenation reaction(hydrogen abstraction). When alkanes are passed over a catalyst (Pt, Ni, Al 2 O 3, Cr 2 O 3) at high temperatures (400-600 °C), a hydrogen molecule is eliminated and a alkene: Reactions accompanied by the destruction of the carbon chain. All saturated hydrocarbons are burning with the formation of carbon dioxide and water. Gaseous hydrocarbons mixed with air in certain proportions can explode. 1. Combustion of saturated hydrocarbons is a free radical exothermic reaction, which is very important when using alkanes as fuel: In general, the combustion reaction of alkanes can be written as follows: 2. Thermal splitting of hydrocarbons. The process proceeds according to free radical mechanism. An increase in temperature leads to homolytic cleavage of the carbon-carbon bond and the formation of free radicals. These radicals interact with each other, exchanging a hydrogen atom, to form a molecule alkane and alkene molecule: Thermal decomposition reactions underlie the industrial process - hydrocarbon cracking. This process is the most important stage of oil refining. 3. Pyrolysis. When methane is heated to a temperature of 1000 °C, methane pyrolysis- decomposition into simple substances: When heated to a temperature of 1500 °C, the formation of acetylene: 4. Isomerization. When linear hydrocarbons are heated with an isomerization catalyst (aluminum chloride), substances with branched carbon skeleton: 5. Aromatization. Alkanes with six or more carbon atoms in the chain cyclize in the presence of a catalyst to form benzene and its derivatives: Alkanes enter into reactions that proceed according to the free radical mechanism, since 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 factors (electrostatic fields of ions). Consequently, alkanes will not react with charged particles, since the bonds in alkane molecules are not broken by the heterolytic mechanism. 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