Chemical properties of alkynes. Structure, production, application

Alkanes, alkenes, alkynes are organic chemicals. All of them are constructed of chemical elements such as carbon and hydrogen. Alkanes, alkenes, alkynes are chemical compounds that belong to a group of hydrocarbons.

In this article we will look at alkynes.

What it is?

These substances are also called acetylenic hydrocarbons. The structure of alkynes provides for the presence of carbon and hydrogen atoms in their molecules. The general formula of acetylene hydrocarbons is: C _n H _2n-2 . The simplest simple alkyne is ethyne (acetylene). He has this chemical formula - C ₂ H ₂ . Also, alkynes include propyne with the formula C ₃ H ₄ . In addition, butyne (C ₄ H ₆ ), pentine (C ₅ H ₈ ), hexine (C ₆ H ₁₀ ), heptin (C ₇ H ₁₂ ), octine (C ₈ H ₁₄ ), nonine ( C ₉ H ₁₆ ), decin (C ₁₀ H ₁₈ ), etc. All types of alkynes have similar characteristics. Let's look at them in more detail.

Physical properties of alkynes

According to their physical characteristics, acetylene hydrocarbons resemble alkenes.

Under normal conditions, alkynes, whose molecules contain from two to four carbon atoms, have a gaseous aggregate state. Those in whose molecules there are from five to 16 carbon atoms, under normal fluid conditions. Those whose molecules contain 17 or more atoms of this chemical element are solids.

Alkines melt and boil at a higher temperature than alkanes and alkenes.

The solubility in water is insignificant, but slightly higher than that of alkenes and alkanes.

Solubility in organic solvents is high.

The most widely used alkyne - acetylene - has such physical properties:

• Has no color;
• Has no smell;
• Under normal conditions is in gaseous aggregate state;
• Has a lower density than air;
• The boiling point is minus 83.6 degrees Celsius;

Chemical properties of alkynes

In these substances, atoms are connected by a triple bond, which explains their basic properties. Alkines enter into reactions of this type:

• Hydrogenation;
• Hydrohalogenation;
• Halogenation;
• hydration;
• combustion.

Let's look at them in order.

Hydrogenation

The chemical properties of alkynes allow them to enter into reactions of this type. This is a kind of chemical interaction, in which a molecule of matter attaches additional atoms of hydrogen to itself. Here is an example of such a chemical reaction in the case of propene:

2H ₂ + C ₃ H ₄ = C ₃ H ₈

This reaction occurs in two stages. On the first molecule of propene adds two hydrogen atoms and the second - as much.

Halogenation

This is another reaction that is part of the chemical properties of alkynes. As a result, the molecule of the acetylene hydrocarbon attaches the halogen atoms. The latter include elements such as chlorine, bromine, iodine, etc.

Here is an example of such a reaction in the case of ethyn:

C ₂ H ₂ + ₂ CI ₂ = C ₂ H ₂ CI ₄

The same process is possible with other acetylenic hydrocarbons.

Hydrohalogenation

This is also one of the main reactions, which is part of the chemical properties of alkynes. It consists in the fact that the substance interacts with such compounds as HCl, HI, HBr, etc. This chemical interaction occurs in two stages. Let's consider a reaction of this type with the example of ethyne:

C ₂ H ₂ + HCI = C ₂ H ₃ CI

С ₂ Н ₂ СІ + НСІ = С ₂ Н ₄ СІ ₂

Hydration

This is a chemical reaction, which consists in interaction with water. It also happens in two stages. Let's look at it with the example of ethyn:

H ₂ O + C ₂ H ₂ = C ₂ H ₃ OH

The substance that is formed after the first stage of the reaction is called vinyl alcohol.

In connection with the fact that according to Eltekov's rule the functional group OH can not be located next to the double bond, there is a rearrangement of the atoms, as a result of which acetaldehyde is formed from vinyl alcohol.

The process of hydration of alkynes is also called the reaction of Kucherov.

Combustion

This is the process of interaction of alkynes with oxygen at high temperature. Let us consider the burning of substances of this group with the example of acetylene:

2C ₂ H ₂ + ₂ O ₂ = 2H ₂ O + 3C + CO ₂

With an excess of oxygen, acetylene and other alkynes burn without the formation of carbon. Only carbon monoxide and water are released. Here is the equation of such a reaction with propene as an example:

4O ₂ + C ₃ H ₄ = 2H ₂ O + 3 CO ₂

The combustion of other acetylene hydrocarbons also occurs in a similar manner. As a result, water and carbon dioxide are released.

Other reactions

Also acetylenes are able to react with salts of such metals as silver, copper, calcium. At the same time, hydrogen atoms are replaced by metal atoms. Consider this type of reaction in the example with acetylene and silver nitrate:

C ₂ H ₂ + ₂ AgNO 3 = Ag ₂ C ₂ + ₂ NH ₄ NO ₃ + 2H ₂ O

Another interesting process involving alkynes is Zelinsky's reaction. This is the formation of benzene from acetylene when it is heated to 600 degrees Celsius in the presence of activated carbon. The equation of this reaction can be expressed thus:

3C ₂ H ₂ = C ₆ H ₆

It is also possible the polymerization of alkynes - the process of combining several molecules of matter into one polymer.

Receiving

Alkines, the reactions with which we have discussed above, are obtained in the laboratory by several methods.

The first is dehydrohalogenation. The reaction equation looks like this:

C ₂ H ₄ Br ₂ + ₂ KOH = C ₂ H ₂ + 2 H ₂ O + 2KBr

To carry out such a process, it is necessary to heat the reagents, and also to add ethanol as a catalyst.

It is also possible to obtain alkynes from inorganic compounds. Here's an example:

CaC ₂ + H ₂ O = C ₂ H ₂ + ₂ Ca (OH) ₂

The next method of obtaining alkynes is dehydrogenation. Here is an example of such a reaction:

2CH ₄ = 3H ₂ + C2H ₂

With the help of this type of reaction it is possible to obtain not only ethine, but also other acetylene hydrocarbons.

The use of alkynes

The most common in the industry was the simplest alkyne-ethyne. It is widely used in the chemical industry.

• You need acetylene and other alkynes to get them from other organic compounds, such as ketones, aldehydes, solvents, etc.
• Also from alkynes it is possible to obtain substances that are used in the production of rubbers, polyvinyl chloride, etc.
• From propinum it is possible to receive acetone as a result of Kucherov's report.
• In addition, acetylene is used in the preparation of chemicals such as acetic acid, aromatic hydrocarbons, ethyl alcohol.
• Another acetylene is used as a fuel with a very high heat of combustion.
• Also, the combustion reaction of ethane is used to weld metals.
• In addition, with the use of acetylene, a technical carbon can be obtained.
• Also this substance is used in self-contained luminaires.
• Acetylene and a number of other hydrocarbons of this group are used as rocket fuel due to its high heat of combustion.

This is the end of the use of alkynes.

Conclusion

As a final part, we give a brief table on the properties of acetylenic hydrocarbons and their production.

Chemical properties of alkynes: table

Name of reaction	Explanations	An example of equation
Halogenation	The reaction of the addition of halogen atoms (bromine, iodine, chlorine, etc.) to the acetylene hydrocarbon molecule	C 4 H 6 + 2 I 2 = C 4 H 6 I 2
Hydrogenation	The reaction of the attachment of alkyne atoms to hydrogen atoms. It occurs in two stages.	C 3 H 4 + H 2 = C 3 H 6 C 3 H 6 + H 2 = C 3 H 8
Hydrohalogenation	The reaction of addition of hydrohalogen (HI, HCl, HBr) by an acetylene hydrocarbon molecule. It occurs in two stages.	C 2 H 2 + HI = C 2 H 3 I C 2 H 3 I + HI = C 2 H 4 I 2
Hydration	A reaction based on interaction with water. It occurs in two stages.	C 2 H 2 + H 2 O = C 2 H 3 OH C 2 H 3 OH = CH 3 -CHO
Complete oxidation (combustion)	Interaction of acetylene hydrocarbon with oxygen at elevated temperature. As a result, carbon monoxide and water are formed.	2C2H5 + 5O2 = 2H2O + 4CO2 2C 2 H 2 + 2 O 2 = H 2 O + CO 2 + 3 C
Reactions with metal salts	The conclusion is that the metal atoms replace the hydrogen atoms in the molecules of acetylene hydrocarbons.	C 2 H 2 + AgNO 3 = C 2 Ag 2 + 2 NH 4 NO 3 + 2H 2 O

Alkines can be obtained in laboratory conditions in three ways:

• From inorganic compounds;
• By dehydrogenation of organic substances;
• Method of dehydrohalogenation of organic substances.

So we examined all the physical and chemical characteristics of alkynes, the methods of their production, the field of application in industry.