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 group of hydrocarbons.
In this article we will look at alkynes.
What is this?
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 for acetylenic hydrocarbons is: C H2n-2. The simplest simple alkyne is ethyne (acetylene). It has the following chemical formula - С2Н2. Alkynes also include propyne with the formula C3H4. In addition, butine (C4H6), pentine (C5 H8), hexine (C6H10), heptin (C 7Н12), octine (С8Н14), nonine (С9Н16), Decin (С10Н18), etc. All types of alkynes have similar characteristics. Let's take a closer look at them.
Physical properties of alkynes
In terms of their physical characteristics, acetylenehydrocarbons resemble alkenes.
Under normal conditions, alkynes, whose molecules contain from two to four carbon atoms, have a gaseous state of aggregation. Those in whose molecules there are from five to 16 carbon atoms, under normal liquid conditions. Those whose molecules contain 17 or more atoms of this chemical element are solids.
Alkynes melt and boil at a higher temperature than alkanes and alkenes.
Solubility in water is negligible, but slightly higher than that of alkenes and alkanes.
Solubility in organic solvents is high.
The most widely used alkyne, acetylene, has the following physical properties:
- has no color;
- no smell;
- under normal conditions is in a gaseous aggregate state;
- is less dense than air;
- boiling point - minus 83.6 degrees Celsius;
Chemical properties of alkynes
In these substances, the atoms are connected by a triple bond, which explains their main properties. Alkynes enter into reactions of this type:
- hydrogenation;
- hydrohalogenation;
- halogenation;
- hydration;
- burning.
Let's look at them one by one.
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 a substance attaches additional hydrogen atoms to itself. Here is an example of such a chemical reaction in the case of propyne:
2H2 + C3H4=C3 N8
This reaction occurs in two steps. On the first propyne molecule attaches two hydrogen atoms and on the second - the same number.
Halogenation
This is another reaction that is part of the chemical properties of alkynes. As a result, an acetylenic 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:
С2Н2 + 2СІ2=С2 N2SI4
The same process is possible with other acetylenic hydrocarbons.
Hydrohalogenation
This is also one of the main reactions that go into the chemical properties of alkynes. It lies in the fact that the substance interacts with such compounds as HCI, HI, HBr, etc. This chemical interaction occurs in two stages. Let's look at this type of reaction using ethine as an example:
С2Н2 + NSI=С2Н 3СІ
С2Н2СІ + NSI=С2Н 4SI2
Hydration
This is a chemical reaction that involves interaction with water. It also takes place in two stages. Let's look at it with ethin as an example:
H2O + C2H2=C 2H3OH
The substance that is formed after the first stagereaction is called vinyl alcohol.
Due to the fact that, according to the Eltekov rule, the OH functional group cannot be located next to the double bond, a rearrangement of atoms 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 interaction of alkynes with oxygen at high temperature. Consider the combustion of substances of this group using acetylene as an example:
2C2N2 +2O2=2N2 O + 3C + CO2
With an excess of oxygen, acetylene and other alkynes burn without carbon formation. In this case, only carbon oxide and water are released. Here is the equation for such a reaction using propyne as an example:
4O2 + C3N4=2N2 O + 3CO2
Combustion of other acetylenic hydrocarbons also occurs in a similar way. The result is water and carbon dioxide.
Other reactions
Also, acetylenes are able to react with s alts of metals such as silver, copper, calcium. In this case, the hydrogen is replaced by metal atoms. Consider this type of reaction using the example of acetylene and silver nitrate:
С2Н2 + 2AgNO3=Ag2C2 + 2NH4NO3 + 2H2O
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 charcoal. The equation for this reaction can be expressed as follows:
3S2N2=S6N6
Alkyne polymerization is also possible - the process of combining several molecules of a substance into one polymeric one.
Receive
Alkynes, the reactions with which we discussed above, are obtained in the laboratory by several methods.
The first is dehydrohalogenation. The reaction equation looks like this:
C2H4Br2 + 2KON=С2 N2 + 2N2O + 2KBr
To carry out such a process, it is necessary to heat the reagents, as well as add ethanol as a catalyst.
It is also possible to obtain alkynes from inorganic compounds. Here is an example:
CaC2 + H2O=C2H 2 + 2Ca(OH)2
The next method for obtaining alkynes is dehydrogenation. Here is an example of such a reaction:
2CH4=3H2 + C2H2
This type of reaction can produce not only ethyne, but also other acetylene hydrocarbons.
Use of alkynes
The simplest alkyne, ethyne, is the most widely used in industry. It is widely used in the chemical industry.
- Need acetylene and other alkynes to convert them into other organic compounds such as ketones, aldehydes, solvents andothers
- It is also possible to obtain substances from alkynes that are used in the production of rubbers, polyvinyl chloride, etc.
- Acetone can be obtained from propyne as a result of Kucherov's reaction.
- 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.
- Also, the combustion reaction of ethine is used to weld metals.
- In addition, technical carbon can be obtained using acetylene.
- Also, this substance is used in self-contained fixtures.
- Acetylene and a number of other hydrocarbons of this group are used as rocket fuel due to their high heat of combustion.
This ends the use of alkynes.
Conclusion
As a final part, here is a brief table on the properties of acetylenic hydrocarbons and their production.
| Reaction name | Explanations | Example equation |
| Halogenation | Reaction of addition of halogen atoms (bromine, iodine, chlorine, etc.) by an acetylenic hydrocarbon molecule | C4H6 + 2I2=С4 N6I2 |
| Hydrogenation | The reaction of addition of hydrogen atoms by an alkyne molecule. Occurs in two stages. |
C3H4 +N2=S3N6 C3H6 + H2=C3 N8 |
| Hydrohalogenation | Reaction of addition of hydrohalogens (HI, HCI, HBr) by an acetylenic hydrocarbon molecule. Occurs in two stages. |
C2H2 + HI=C2H3 I C2H3I + HI=C2H 4I2 |
| Hydration | Reaction based on interaction with water. Occurs in two stages. |
C2N2 + H2O=C 2H3OH C2H3OH=CH3-CHO |
| Complete oxidation (combustion) | Interaction of acetylene hydrocarbon with oxygen at elevated temperature. The result is carbon oxide and water. |
2C2H5 + 5O2=2H2 O + 4CO2 2C2N2 + 2O2=N2 O + CO2 + 3C |
| Reactions with metal s alts | Consist in the fact that metal atoms replace hydrogen atoms in the molecules of acetylene hydrocarbons. | С2Н2 + AgNO3=C2Ag2 + 2NH4NO3 + 2H2O |
Alkynes can be obtained in the laboratory in three ways:
- from inorganic compounds;
- by dehydrogenation of organic matter;
- waydehydrohalogenation of organic substances.
So we have considered all the physical and chemical characteristics of alkynes, methods for their production, applications in industry.
