Alkanes, alkenes, alkynes are organic chemicals. All of them are built from such chemical elements as carbon and hydrogen. Alkanes, alkenes, alkynes are chemical compounds that belong to the hydrocarbon group.
In this article we will consider alkynes.
What it is?
These substances are also called acetylene hydrocarbons. The structure of alkynes provides for the presence of carbon and hydrogen atoms in their molecules. The general formula for acetylene hydrocarbons is: C n H 2n-2 . The simplest simple alkine is ethine (acetylene). He has such a chemical formula - C 2 H 2 . Propyn with the formula C 3 H 4 also belongs to alkynes. In addition, acetylene hydrocarbons include butine (C 4 H 6 ), pentin (C 5 H 8 ), hexine (C 6 H 10 ), heptin (C 7 H 12 ), octine (C 8 H 14 ), nonin ( C 9 H 16 ), decin (C 10 H 18 ), etc. All types of alkynes have similar characteristics. Let's take a closer look at them.
Physical properties of alkynes
In their physical characteristics, acetylene hydrocarbons resemble alkenes.
Under normal conditions, alkynes, in the molecules of which contain from two to four carbon atoms, have a gaseous state of aggregation. Those with five to 16 carbon atoms in their molecules under normal liquid conditions. Those with 17 or more atoms of this chemical element in their molecules are solids.
Alkynes melt and boil at a higher temperature than alkanes and alkenes.
The solubility in water is negligible, but slightly higher than that of alkenes and alkanes.
Solubility in organic solvents is high.
The most widely used alkine - acetylene - has the following physical properties:
- has no color;
- odorless;
- under normal conditions it is in a gaseous state of aggregation;
- has a lower density than air;
- boiling point - 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 type of chemical interaction in which a molecule of matter attaches additional hydrogen atoms. Here is an example of such a chemical reaction in the case of propine:
2H 2 + C 3 H 4 = C 3 H 8
This reaction occurs in two stages. On the first, the propine molecule attaches two hydrogen atoms, and on the second, the same.
Halogenation
This is another reaction that goes into the chemical properties of alkynes. As a result, an acetylene hydrocarbon molecule attaches halogen atoms. The latter include elements such as chlorine, bromine, iodine, etc.
Here is an example of such a reaction in the case of ethine:
2 2 + 2Π 2 = 2 2 Π 4
The same process is possible with other acetylene hydrocarbons.
Hydrohalogenation
It is also one of the main reactions that enter into the chemical properties of alkynes. It lies in the fact that the substance interacts with such compounds as HCl, HI, HBr, etc. This chemical interaction occurs in two stages. Let's look at this type of reaction using the ethine example:
2 2 + Π = 2 3 Π
2 2 Π + Π = 2 4 Π 2
Hydration
This is a chemical reaction that involves interacting with water. It also occurs in two stages. Let's look at it with an example of ethin:
H 2 O + C 2 H 2 = C 2 H 3 OH
The substance that forms after the first stage of the reaction is called vinyl alcohol.
Due to the fact that according to Eltekovβs rule the OH functional group cannot be located next to the double bond, atom rearrangement occurs, as a result of which acetaldehyde is formed from vinyl alcohol.
The process of hydration of alkynes is also called the Kucherov reaction.
Combustion
This is the process of the interaction of alkynes with oxygen at high temperature. Consider the combustion of substances of this group on the example with acetylene:
2 2 2 + 2 2 = 2 2 + 3 + 2
With an excess of oxygen, acetylene and other alkynes burn without the formation of carbon. In this case, only carbon monoxide and water are released. Here is the equation for such a reaction using propine as an example:
4 2 + 3 4 = 2 2 + 3 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 metals such as silver, copper, calcium. In this case, the hydrogen is replaced by metal atoms. Consider this type of reaction using acetylene and silver nitrate as an example:
2 2 + 2AgNO3 = Ag 2 C 2 + 2NH 4 NO 3 + 2 2
Another interesting process involving alkynes is the Zelinsky 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 as follows:
3C 2 H 2 = C 6 H 6
Polymerization of alkynes is also possible - the process of combining several molecules of a substance into one polymer.
Getting
Alkines, the reactions with which we examined 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 + 2KOH = C 2 H 2 + 2H 2 O + 2KBr
To carry out such a process, it is necessary to heat the reagents, and also add ethanol as a catalyst.
It is also possible to obtain alkynes from inorganic compounds. Here is an example:
CaC 2 + H 2 O = C 2 H 2 + 2Ca (OH) 2
The next method for producing alkynes is dehydrogenation. Here is an example of such a reaction:
2CH 4 = 3H 2 + C 2 H 2
Using a reaction of this type, not only ethine, but also other acetylene hydrocarbons can be obtained.
The use of alkynes
The most widespread in industry was the simplest alkine - ethin. It is widely used in the chemical industry.
- Acetylene and other alkynes are needed to obtain other organic compounds from them , such as ketones, aldehydes, solvents, etc.
- Also, from alkynes, it is possible to obtain substances that are used in the manufacture of rubbers, polyvinyl chloride, etc.
- Acetone can be obtained from propine as a result of Kucherov's raction.
- In addition, acetylene is used in the production of chemicals such as acetic acid, aromatic hydrocarbons, ethyl alcohol.
- Acetylene is also used as a fuel with a very high heat of combustion.
- The ethine combustion reaction is also used to weld metals.
- In addition, carbon black can be obtained using acetylene .
- Also, this substance is used in stand-alone lamps.
- Acetylene and a number of other hydrocarbons of this group are used as rocket fuel due to its high heat of combustion.
This ends the application of alkynes.
Conclusion
As a final part, we give a brief table on the properties of acetylene hydrocarbons and their preparation.
Chemical properties of alkynes: table| Reaction name | Explanations | Equation example |
| Halogenation | The reaction of the addition of halogen atoms (bromine, iodine, chlorine, etc.) by an acetylene hydrocarbon molecule | C 4 H 6 + 2I 2 = C 4 H 6 I 2 |
| Hydrogenation | The reaction of the addition of hydrogen atoms by an alkyn molecule. 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 a molecule of acetylene hydrocarbon hydrohalogens (Π, Π, HBr). Occurs in two stages. | C 2 H 2 + HI = C 2 H 3 Π C 2 H 3 I + HI = C 2 H 4 I 2 |
| Hydration | The reaction, which is based on the interaction with water. 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) | The interaction of acetylene hydrocarbon with oxygen at elevated temperatures. The result is carbon monoxide and water. | 2C 2 H 5 + 5O 2 = 2H 2 O + 4CO 2 2 2 2 + 2 2 = 2 + CO 2 + 3 |
| Reactions with metal salts | They consist in the fact that metal atoms replace hydrogen atoms in acetylene hydrocarbon molecules. | 2 2 + AgNO3 = C 2 Ag 2 + 2NH 4 NO 3 + 2 2 |
You can get alkynes in the laboratory under three methods:
- from inorganic compounds;
- by dehydrogenation of organic substances;
- the method of dehydrohalogenation of organic substances.
So we examined all the physical and chemical characteristics of alkynes, methods for their preparation, and applications in industry.