With the development of the study of organic chemistry, a separate class was distinguished among a large group of hydrocarbons - “alkynes”. These compounds are usually called unsaturated hydrocarbons, which contain in their structure one or more triple (other names are triple carbon-carbon or acetylene) bonds, which distinguishes them from alkenes (compounds with double bonds).
In various sources, you can also find a rational common name for alkynes - acetylenic hydrocarbons, and the same name for their "residues" - acetylene radicals. Alkynes are presented in the table below with their structural formula and various names.
| Structural formulas | Nomenclature | |
| International IUPAC | Rational | |
| HC ≡ CH | ethyn, acetylene | acetylene |
| H3C ‒ C ≡ CH | propine | methylacetylene |
| H3C ‒ CH2 ‒ C≡CH | butin-1 | ethylacetylene |
| H3C ‒ C≡C ‒CH3 | butin-2 | dimethylacetylene |
| H3C ‒ CH2 ‒ CH2 ‒ C ≡ CH | pentin-1 | propylacetylene |
| H3C ‒ CH2 ‒ C ≡ C - CH3 | Pentin-2 | methylethylacetylene |
International and rational nomenclature
Alkynes in chemistry, according to the IUPAC nomenclature (transliteration from English. International Union of Pure and Applied Chemistry), are given names by changing the suffix "-an" to the suffix "-in" in the name of the related alkane, for example ethane → ethyne (example 1).
But you can also use rational names, for example: ethyne → acetylene, propyne → methylacetylene (example 2), that is, attach the name of the radical located near the triple bond to the name of the smaller representative of the homologous series.
It should be remembered that when determining the name of complex substances, where there are both double and triple bonds, the numbering should be such that they get the smallest numbers. If there is a choice between the beginning of the numbering, then they start with double bonds, for example: pentene-1, -in-4 (example 3).
A special case of this rule is double and triple bonds equidistant from the end of the chain, as, for example, in the molecule of hexadiene-1, 3, -in-5 (example 4). Here it should be remembered that the numbering of the chain will begin with a double bond.
For long chain alkynes (more than С5-С6) it is recommended to useIUPAC international nomenclature.
The structure of a molecule with a triple bond
The most common example of the structure of an acetylenic hydrocarbon molecule is presented for ethine, the structure of which can be viewed in the table of alkynes. For ease of understanding, a detailed drawing of the interaction of carbon atoms in an acetylene molecule will be given below.
The general formula of an alkyne is C2H2. Therefore, in the process of creating a triple bond 2 carbon atoms are involved. Since carbon is tetravalent - the excited state of the atom - in organic compounds, there are 4 unpaired electrons in the outer orbital - 2s and 2p3 (Fig. 1a). In the process of creating a bond, a hybrid cloud is formed from electron clouds of the s- and one p-orbital, which is called the sp-hybrid cloud (Fig. 1b). Hybrid clouds in both carbon atoms are strongly oriented along one axis, which causes their linear arrangement (at an angle of 180°) relative to each other with smaller parts outward (Fig. 2). Electrons in most parts of the cloud, when connected, form an electron pair and create a σ-bond (sigma bond, Fig. 1c).
The unpaired electron, located in the smaller part, attaches the same electron to the hydrogen atom (Fig. 2). The remaining 2 unpaired electrons on the outer p-orbital of one atom interact with 2 other similar electrons of the second atom. In this case, each pair of two p-orbitals overlaps according to the π-bond principle (pi-bond, Fig. 1d) and becomes oriented relative tothe other at a 90° angle. After all interactions, the general cloud takes a cylindrical shape (Fig. 3).
Physical properties of alkynes
Acetylene hydrocarbons are very similar in nature to alkanes and alkenes. In nature, they practically do not occur, except for ethine, so they are obtained artificially. Lower alkynes (up to C17) are colorless gases and liquids. These are low-polar substances, as a result of which they are poorly soluble in water and other polar solvents. However, they dissolve well in simple organic substances such as ether, naphtha or benzene, and this ability improves with increasing pressure when compressing the gas. The highest representatives of this class (C17 and above) are crystalline substances.
Read more about the properties of acetylene
Since acetylene is the most used and widely used, the physical properties of alkyne are well understood. It is a colorless gas with absolute chemical purity and odorless. Technical ethine has a pungent odor due to the presence of ammonia NH3, hydrogen sulfide H2S and hydrogen fluoride HF. This liquefied or gaseous gas is highly explosive and ignites easily even from static discharge from fingers. Also, due to its physical properties, alkyne, mixed with oxygen, gives a combustion temperature of 3150 ° C, which makes it possible to use acetylene as a good combustible gas in welding and cutting metals. Acetylene is poisonous, so extreme care must be taken when working with this gas.
Acetylene dissolves best in acetone, especially in a liquefied state, therefore, when stored in a liquid state, special cylinders are used filled with a porous mass with evenly distributed acetone under pressure up to 25 MPa.
And when used in a gaseous state, gas is released through special pipelines, guided by regulatory and technical documentation and GOST 5457-75 “Acetylene dissolved and gaseous technical. Specifications”, which describes the formula of alkyne and all procedures for the verification and storage of the above gaseous and liquid hydrocarbon.
Production of acetylene
One of the methods is the partial thermal oxidation of methane CH4 with oxygen at a temperature of 1500 °C. This process is also called thermal oxidative cracking. A practically similar process occurs during the oxidation of methane in an electric arc at a temperature above 1500 ° C with rapid cooling of the evolved gases, since due to the physical properties of alkyne, acetylene in a mixture with unreacted methane can provoke an explosion. Also, this product can be obtained by reacting calcium carbide CaC2 and water at 2000 °C.
Application
Among the homologues, as described above, only acetylene has received large-scale and permanent use, and it has historically developed that the rational name is used in production.
Due to its physical and chemicalproperties and a relatively cheap method of obtaining this hydrocarbon is used in the production of various organic solvents, synthetic rubbers and polymers.
