As you already know, acetylene is a product of the incomplete decomposition of methane. This process is called pyrolysis (from the Greek. Feast - fire, lysis - decomposition). Theoretically, acetylene can be represented as a product of ethylene dehydrogenation:

In practice, acetylene, in addition to the pyrolysis method, is very often obtained from calcium carbide:

The peculiarity of the structure of the acetylene molecule (Fig. 21) is that there is a triple bond between the carbon atoms, that is, it is an even more unsaturated compound than ethylene, the molecule of which contains a double carbon-carbon bond.

Rice. 21.
Models of the acetylene molecule: 1 - ball-and-stick; 2 - large-scale

Acetylene is the ancestor of the homologous series of alkynes, or acetylenic hydrocarbons.

Acetylene is a colorless, odorless gas, slightly soluble in water.

Consider the chemical properties of acetylene that underlie its use.

Acetylene burns in air with a smoky flame due to the high carbon content in its molecule, therefore, oxygen is used to burn acetylene:

The temperature of the acetylene-oxygen flame reaches 3200 ° C. This flame can be used to cut and weld metals (fig. 22).

Rice. 22.
Oxygen acetylene flame is used for cutting and welding metal

Like all unsaturated compounds, acetylene actively enters into addition reactions. 1) halogens (halogenation), 2) hydrogen (hydrogenation), 3) hydrogen halides (hydrohalogenation), 4) water (hydration).

Consider, for example, the reaction of hydrochlorination - addition of hydrogen chloride:

Why the product of the hydrochlorination of acetylene is called chloroethene is clear to you. Why vinyl chloride? Because the monovalent ethylene radical CH 2 = CH- is called vinyl. Vinyl chloride is a starting compound for obtaining a polymer - polyvinyl chloride, which is widely used (Fig. 23). Currently, vinyl chloride is obtained not by hydrochlorination of acetylene, but by other methods.

Rice. 23.
Application of polyvinyl chloride:
1 - artificial leather; 2 - electrical tape; 3 - wire insulation; 4 - pipes; 5 - linoleum; 6 - oilcloth

Polyvinyl chloride is produced using the polymerization reaction you already know. The polymerization of vinyl chloride to polyvinyl chloride can be described using the following scheme:

or the reaction equations:

The hydration reaction, which takes place in the presence of mercury salts containing the Hg 2+ cation as a catalyst, bears the name of the outstanding Russian organic chemist M.G. Kucherov and was previously widely used to obtain a very important organic compound - acetaldehyde:

The bromine addition reaction - bromination - is used as a qualitative reaction for a multiple (double or triple) bond. When acetylene (or ethylene, or most other unsaturated organic compounds) is passed through bromine water, its discoloration can be observed. In this case, the following chemical transformations occur:

Another qualitative reaction to acetylene and unsaturated organic compounds is the discoloration of the potassium permanganate solution.

Acetylene is the most important product of the chemical industry, which is widely used (Fig. 24).

Rice. 24.
Acetylene application:
1 - cutting and welding of metals; 2-4 - production of organic compounds (solvents 2, polyvinyl chloride 3, glue 4)

New words and concepts

1. Alkyne.
2. Acetylene.
3. Chemical properties of acetylene: combustion, addition of hydrogen halides, water (Kucherov's reaction), halogens.
4. Polyvinyl chloride.
5. Qualitative reactions to multiple bonds: discoloration of bromine water and potassium permanganate solution.

Questions and tasks

When a mixture of propane and acetylene was passed through a bottle with bromine water, the weight of the bottle increased by 1.3 g. With the complete combustion of the same amount of the initial mixture of hydrocarbons, 14 liters of carbon monoxide (IV) were released. Determine the mass fraction of propane in the initial mixture.

Solution: Acetylene is absorbed by bromine water:

НC ≡ СH + 2Вr 2 → НСВr 2 -СНВr 2.

1.3 g is the mass of acetylene. v (C 2 H 2) = 1.3 / 26 = 0.05 mol. During the combustion of this amount of acetylene according to the equation

2С 2 Н 2 + 5О 2 = 4СО 2 + 2Н 2 О

released 2-0.05 = 0.1 mol CO 2. The total amount of CO 2 is 14 / 22.4 = 0.625 mol. When propane is burned according to the equation

С 3 Н 8 + 5О 2 = ЗСО 2 + 4Н 2 О

0.625 - 0.1 = 0.525 mol CO 2 was released, while 0.525 / 3 = 0.175 mol C 3 H 8 with a mass of 0.175 - 44 = 7.7 g entered into the reaction.

The total mass of the mixture of hydrocarbons is 1.3 + 7.7 = 9.0 g, and the mass fraction of propane is:  (C 3 H 8) = 7.7 / 9.0 = 0.856, or 85.6%.

Answer. 85.6% propane.

Experiments on obtaining acetylene and investigations of its properties are demonstrated simultaneously. Preparing acetylene for the lesson in advance and storing it in the gasometer should not be due to the danger of an explosion.

^ Getting acetylene. The most affordable way to obtain acetylene is the interaction of calcium carbide with water:

CaC 2 + 2H 2 O  C 2 H 2 + Ca (OH) 2

The reaction flask should not be of too small a volume, since during the reaction the resulting thick liquid swells and the foam can be driven by gas through the branch pipe. In the case of using an oversized flask, it will take a long time until all the air has been displaced from the device, and if you wait for this moment, then there will be a large loss of acetylene. It is convenient to use a 250 ml flask. In the case of using a flask of a larger capacity, experiments with acetylene can be started with reactions that do not require air displacement, which will make it possible to conduct them quite economically.

When water is added to calcium carbide, the reaction is always very violent; therefore, acetylene is consumed involuntarily, and it may not be enough to show all the experiments planned. You can get a quieter and more uniform stream of acetylene in two ways: add ethyl alcohol to calcium carbide and only then let water in, or use a saturated solution of sodium chloride instead of water.

7-8 pieces of calcium carbide the size of a pea are placed in a flask, a plug with a funnel is inserted tightly, a saturated solution of sodium chloride is poured into the funnel and a few drops of it are put into the flask. Further addition of the solution is carried out so that a uniform gas flow is established at a rate that allows the bubbles to be counted. Students are drawn to the fact that the reaction, in contrast to the production of ethylene and methane, proceeds without heating.

The resulting gas is collected in cylinders by the method of water displacement (after checking for the completeness of air displacement) or directly used for the corresponding experiments. In view of the noticeable solubility of acetylene in water, it is sometimes recommended to collect it over a solution of sodium chloride, however, as experience shows, it is quite possible to use ordinary water.

^ Dissolving acetylene in water ... Acetylene dissolves in water better than methane and ethylene.

1. Pass acetylene through water in a test tube for several minutes. After that, the water is heated to a boil and a lighted splinter is brought to the opening of the test tube. Acetylene released from the water flares up.

2. A cylinder or a test tube with clean (without air) acetylene is tipped with a hole into a glass with tinted cold water. When shaking the cylinder (test tube), the water level in it rises noticeably. This phenomenon will be more evident if the cylinder is fixed in this position in a tripod and left until the next session.

^ Dissolving acetylene in acetone ... Acetylene is readily soluble in acetone. In the form of such a solution, it is usually stored in steel cylinders (with a porous filler).

3-4 ml of acetone are poured into a small cylinder with acetylene. The cylinder is closed with a stopper, shaken several times and thrown into a bath of tinted water. When the plug is opened, water rises in the cylinder.

^ Combustion of acetylene ... Complete combustion of acetylene is expressed by the equation:

2С 2 Н 2 + 5O 2  4СO 2 + 2Н 2 O

The external picture of combustion and the temperature developed in this case strongly depend on the volumetric ratio of gases.

Acetylene collected in a cylinder is ignited by the method of displacing water. The gas burns with a smoky flame. As the flame moves inside the cylinder, the formation of soot increases, since combustion in the cylinder proceeds with an even greater lack of oxygen. Acetylene is ignited and at the outlet tube of the device (Fig. 14) pay attention to the fact that the smaller the tube opening, the less smoky gas burns, and complete combustion occurs in a thin stream of gas.

^ Explosion of acetylene with oxygen ... A mixture of acetylene and oxygen, when ignited, explodes with great force. Therefore, the experiment can be carried out in a steel cylinder or in such a shell, the rupture of which is not dangerous. The best way to show the explosion of acetylene is in soap bubbles.

Acetylene from the device and oxygen from the gasometer are simultaneously passed into soapy water, prepared in advance in an iron cup at the rate of 1 g of soap per 30-40 ml of water and 4-5 ml of glycerin. They take the devices off the table and set fire to the resulting bubbles with a mixture of gases with a long splinter. A strong but safe explosion occurs.

^ Reaction of acetylene with bromine and potassium permanganate solution. Experiments illustrating the unsaturation of acetylene can be carried out in two ways: a) by passing acetylene into bromic water and a solution of potassium permanganate, b) by pouring these solutions into cylinders with acetylene.

When demonstrating experiments by the first method, the decolorization of solutions (especially bromine water) occurs rather slowly. Stirring solutions with a glass rod somewhat speeds up the process.

When demonstrating experiments in the second way, the gas with solutions in the cylinders is shaken; discoloration occurs faster.

Based on the structure of acetylene, students usually correctly formulate the equations for its reactions with bromine:

The reaction of acetylene with potassium permanganate, like the reaction of ethylene, can be considered here only in general terms.

If the reaction of acetylene with bromine and potassium permanganate is demonstrated by passing gas through solutions, then the time during which the solutions will decolor (compare with ethylene) can be used to analyze the reaction. Such an experiment should be carried out under thrust, since a relatively large amount of acetylene (with toxic impurities) is released into the atmosphere. The discussion of the experiment can start something like this: “Based on the presence of a triple bond in the acetylene molecule, we suggest that it should be easily oxidized and enter into addition reactions. On what basis can we determine whether these reactions will proceed in the given experiment? " After the students' answer, the teacher asks the question: "If discoloration of solutions occurs, then how could these reactions be expressed by equations?"

^ Combustion of acetylene in chlorine ... Acetylene, like other hydrocarbons, burns in chlorine, forming hydrogen chloride and coal:

C 2 H 2 + CI 2  2C + 2HCI

1. A curved glass tube containing burning acetylene is slowly introduced into the chlorine cylinder. Acetylene continues to burn in chlorine, forming soot. You can fill the cylinder with chlorine in the classroom by pouring a little crushed potassium permanganate into it and adding concentrated hydrochloric acid.

2. A curved tube is gradually introduced into the chlorine cylinder, through which acetylene enters (previously checked for

Stot). Acetylene is flammable in chlorine. Experience illustrates the great reactivity of acetylene.

3. A little bleach is poured into a glass cylinder and poured over with dilute hydrochloric acid. Several pieces of calcium carbide are thrown there. After a while, flashes of flame are observed in the cylinder, sometimes extinguishing, then reappearing and rolling from top to bottom.

When bleach reacts with acid, chlorine is released:

CaCIOCl + 2HCI  CaCI 2 + H 2 O + CI 2

and in the reaction of calcium carbide with water - acetylene (see p. 51). Acetylene and chlorine enter into an addition reaction, the heat of which ignites acetylene in chlorine (in the cylinder) and in air (at the cylinder bore). Bleach is taken here because it forms chlorine when it reacts with dilute hydrochloric acid, while potassium permanganate releases it when it reacts with concentrated acid. The presence of free water is necessary to obtain a sufficiently large amount of acetylene.

^ Experiments with PVC ... When familiarizing yourself with PVC, you should consider its relationship to heating, to chemicals and solvents. The last two experiments are not described here, since they are set up in the same way as with polyethylene.

A) Pieces of vinyl plastic or PVC film (oilcloth, insulation) are heated carefully in a test tube or porcelain cup.

Ensure that the polymer does not liquefy but decomposes. A wet litmus paper is brought to the emitted gaseous decomposition products and then a stick moistened with ammonia solution. Reddening of the paper is observed in the first case and the formation of a white haze in the second. A conclusion is made about the release of hydrogen chloride during the decomposition of polyvinyl chloride.

B) Pieces of polyvinyl chloride are heated in a test tube closed with a stopper with a branch tube. Gaseous decomposition products are taken into a test tube with water. A little silver nitrate solution and 1-2 drops of nitric acid are added to the resulting aqueous solution. The formation of a precipitate of silver chloride is observed, which indicates the release of hydrogen chloride during the decomposition of the polymer.

Development of a lesson in chemistry

Grade 10

Lesson 8

Lesson topic: Alkyne. Acetylene, its production by methane pyrolysis and carbide method. Chemical properties of acetylene: combustion, discoloration of bromine water, addition of hydrogen chloride and hydration. Acetylene application based on properties. Vinyl chloride polymerization reaction. Polyvinyl chloride and its application.

Lesson objectives:

- Exploregeneral formula, nomenclature, physical properties of representatives of the homologous series of alkynes, their structure, chemical properties of the first representative of the homologous series of alkynes - acetylene, application.

- Develop general educational competencies, logical thinking.

Increase the cognitive interest of students,show the importance of knowledge of organic chemistry.

Lesson type: UPNZ

Information and methodological support: slides, diagrams, collection materials, tables on the topic of the lesson.

Textbook: Chemistry. Organic chemistry. Grade 10 (basic level).Rudzitis G.E., Feldman F.G.,15th ed. - M .: 2012 .-- 192 p.

Characteristics of student activities: frontal, individual, work at the board.

Types of control: Survey.

During the classes

I. Organizational moment of the lesson

II. Knowledge update

Survey on basic concepts:

Hydrocarbons

Saturated and unsaturated compounds

Alkyne: formula of the series, the first representative of the series, basic properties, methods of preparation, application.

III. Homework check

IV. Presentation of new material

Alkyne - unsaturated hydrocarbons, the molecules of which, in addition to single C-C bonds, contain one triple C≡ C-bond.

The general formula of the series isWITH n H 2n-2

Features of the nomenclature alc and new

The belonging of a hydrocarbon to the class of alkenes is reflected by the suffix–In:

WITH 2 N 2 CH≡ CHethine (acetylene)

WITH 3 N 4 CH≡ C-CH 3 propyne

WITH 4 N 6 CH≡ C-CH 2 -CH 3 butin-1

etc.

The rules for naming compounds remain the same as for alkenes, only the suffix is ​​replaced by–In .

Alkenes isomerism

Structural isomerism.

1. Isomerism of the structure of the carbon chain.

   Isomerism of the position of the triple bond.

   Interclass isomerism.

Give examples of isomers of each type, name them!

Features of the structure of molecules (for example, acetylene)

In acetylene, carbon is in the statesp - hybridization(onesand 1p-orbital). Each carbon atom in the ethylene molecule has 2 hybridsp - orbitals and two non-hybrid p-orbitals. The axes of the hybrid orbitals are located in the same plane, and the angle between them is 180 °. Such orbitals of each carbon atom intersect with those of the other carbon atomands-orbitalstwo atoms of a conduit, formingσ -connections C-C and C-H.

Education scheme σ-bonds in a molecule ace tylena

Four non-hybrid p-orbitals of carbon atoms overlap in mutually perpendicular planes, which are located perpendicular to the planeσ -connections. This is how twoπ- communication.

WITH ≡ C = σ + 2 π

Diagram of the formation of π-bonds in a molecule NS tylena

Acetylene molecule structure

Physical properties

Acetylene - gas, lighter than air, slightly soluble in water, odorless. Forms explosive mixtures with air.

In the series of alkynes, the boiling point increases with increasing molecular weight.

Describe the physical properties of acetylene as a table in your notebook!

Receiving

Methods for obtaining acetylene:

Calcium carbide... (Laboratory method)

CaC 2 + 2H 2 O → C 2 N 2 + Ca (OH) 2

Obtaining calcium carbide (in industry):

CaO + 3C CaC 2 + CO

calcium oxide coke calcium carbide

CaCO 3 CaO + CO 2

calcium carbonate calcium oxide

Thermal decomposition of methane.

2CH 4 WITH 2 N 2 + 3H 2

Methods for obtaining homologues of acetylene - hydrocarbons of a number of alkynes:

Dehydrohalogenation - elimination of two molecules of hydrogen halide from dihaloalkanes, which contain two halogen atoms either at the neighboring ones or at one carbon atom:

The reaction proceeds under the action of an alcoholic solution of alkalis on halogen derivatives.

CH 3 - SVr 2 - CH 3 + 2KON

CH 3 -C ≡ CH + 2KVr+ 2H 2 O,

2,2 - dibromopropane propyne

Chemical properties

Addition reactions

1. 6interactions with halogens

A qualitative reaction to the presence of multiple bonds is the discoloration of bromine water!

Addition reactions take place in two stages.

acetaldehyde

1. With metals.

The reaction product - acetylenides - are poorly soluble, unstable, explosive substances!

The formation of a grayish-white precipitate of silver acetylenide or a red-brown precipitate of copper acetylenide is a qualitative reaction to the terminal triple bond!

Oxidation reactions.

1. Combustion.

Acetylene can polymerize to benzene and vinyl acetylene.

Polymerization of vinyl chloride



Durable plastic, artificial leather, oilcloth, dielectrics, etc. are obtained from polyvinyl chloride (PVC).

Use of acetylene

Raw materials in organic synthesis (production of fibers, dyes, varnishes, drugs, PVC, chloroprene rubber, acetic acid, solvents, etc.)

When cutting and welding metals.

V. Consolidation of knowledge.

Solving problems and exercises

p. 54 - exercise 1,3,5,6.

p.55 - exercise 8

p.55 - tasks 1.

V I ... Reflection. Lesson summary

VI I ... Homework

Paragraph 13,

with. 55 - tasks 2, 3

Explain the differences between the molecular structure of ethylene and acetylene.

What hydrocarbon is the closest homologue of ethine?

Make up the reaction equations, with which you can carry out the following transformations, indicate the conditions for the reaction, name the reaction products:

DEFINITION

Unsaturated (unsaturated) hydrocarbons- hydrocarbons containing multiple (double or triple) bonds. Among unsaturated hydrocarbons, alkenes (contain one double bond), alkadienes (contain two double bonds) and alkynes (contain three double bonds) are distinguished.

DEFINITION

Alkyne- unsaturated hydrocarbons, the molecules of which contain one triple bond, in the name of alkynes there is a suffix –in. The general formula for alkynes is C n H 2 n -2.

Table 1. Homological series of alkynes.

To give the alkyne name it is necessary to choose the longest hydrocarbon chain containing a triple bond. The chain numbering starts from the edge closer to which the triple bond is.

Isomerism

For alkynes, starting with pentine, isomerism of the carbon skeleton is characteristic:

CH≡C-CH 2 -CH 2 -CH 3 (pentin-1)

CH≡C-CH (CH 3) -CH 3 (3-methylbutyn1)

starting with butyne, all alkynes are characterized by isomerism of the position of the triple bond:

CH≡C-CH 2 -CH 3 (butyne-1)

CH 3 -C≡C-CH 3 (butyne-2)

For alkynes, interclass isomerism with alkadienes is characteristic, so a substance with the composition C 4 H 6 corresponds to three different substances - butyne-1, butyne-2 and butadiene-1,3.

Alkyne structure

The carbon atoms of the triple bond in the molecules of alkynes are in sp-hybridization: two σ-bonds are located on the same line at an angle of 180C to each other, two π-bonds are formed by p-electrons of neighboring carbon atoms and are located in mutually perpendicular planes. A triple bond is a combination of one σ and two π bonds.

Physical properties of alkynes

Under normal conditions, C 2 -C 4 are gases, C 5 -C 16 are liquids, starting from C 18 are solids. The temperatures of the alkynes are higher than those of the corresponding alkenes.

Receiving

There are industrial and laboratory methods for producing acetylene. So, in the industry, acetylene is obtained by high-temperature cracking of methane:

2CH 4 → CH≡CH + 3H 2

In the laboratory, acetylene is obtained by hydrolysis of calcium carbide:

CaC 2 + 2H 2 O = Ca (OH) 2 + C 2 H 2

To obtain alkynes, the following are most often used:

- reactions of dehydrogenation of alkanes and alkenes

CH 3 -CH 3 → CH≡CH + 2H 2

CH 2 = CH 2 → CH≡CH + H 2

- reactions of elimination of dihalogenated derivatives

(CH 3) 3 -CCl 2 -CH 3 + 2KOH → (CH 3) 3 -C≡CH + 2KCl + 2H 2 O

- reactions of acetylenides with primary haloalkanes

CH≡CNa + CH 3 -CH 2 -CH 2 -Br → CH≡C- (CH 2) 3 -CH 3 + NaBr

Chemical properties of alkynes

Alkynes are characterized by addition reactions proceeding by the nucleophilic mechanism, such as:

- hydrogenation - addition of water in the presence of 18% sulfuric acid, mercury (II) sulfate and heating to 90C (Kucherov's reaction), resulting in the formation of aldehydes

CH≡CH + H 2 O → → CH 3 -CH = O

- halogenation - addition of halogens, proceeding in two stages

CH≡CH + Br 2 → CHBr = CHBr + Br 2 → CHBr 2 -CHBr 2

- hydrohalogenation - the addition of hydrogen halides, also proceeding in two stages, and the addition of the second hydrogen halide molecule proceeds according to Markovnikov's rule (the addition of hydrogen halides to a triple bond is more difficult than to a double one)

CH≡CH + HCl → CH 2 = CHCl + HCl → CH 3 -CHCl 2

For alkynes, which have a terminal triple bond, the presence of weak acidic properties is characteristic. Such alkynes are capable of forming salts when interacting with active metals:

2R-C≡C-H + 2Na → 2R-C≡C-Na + H 2

Silver acetylinide is easily formed and precipitated when acetylene is passed through an ammonia solution of silver oxide:

CH≡CH + Ag 2 O → Ag- C≡C-Ag ↓ + H 2 O

Acetylene is able to trimerize - when acetylene is passed over activated carbon at 60 ° C, benzene is formed:

3C 2 H 2 → C 6 H 6

Alkynes are characterized by oxidation and reduction reactions. Thus, alkynes are easily oxidized by potassium permanganate. As a result of this reaction, carboxylic acids are formed:

R- C≡C-R '+ [O] + H 2 O → R-COOH + R'-COOH

In the presence of metal catalysts, alkynes add hydrogen molecules:

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

Qualitative reactions to the triple bond are the bleaching reactions of a solution of bromine water and potassium permanganate, as well as a reaction with an ammonia solution of silver oxide in the case of the terminal position of the triple bond.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2