Alkynes are defined as aliphatic hydrocarbons that are unsaturated, have at least one triple bond in the molecule i.e. carbon-carbon.
The general empirical formula for alkynes is written CnH2n-2. They are also called acetylene because the acetylene gas represented by the formula (HC≡CH, belonging to ethyne), is the simplest of them.
The International Union of Pure and Applied Chemistry, by its acronym IUPAC, establishes that to name the alkynes, the ending ane belonging to the homologous alkanes must be replaced to place the ine, specifying with numbers the place of the carbon next to the triple link that belongs to the longest chain found.
Alkynes are obtained by elimination reactions from geminal or vicinal dihaloalkanes. Acetylene is obtained by the simple controlled addition of water with calcium carbide at normal temperature. Since calcium carbide is an inexpensive substance made by heating quicklime with coal, acetylene ends up being an easy and cheap gas to produce.
Electron configuration of alkynes
Orbital hybridization of alkynes is sp. This means that 50% is character p and the rest is character s.
They are two sp-hybrid orbitals and mix with the H atoms of acetylene or with the alkyl groups belonging to alkynes.
The distance that divides H or R is equal to 180 degrees. Also in this way, the pure p orbitals belonging to the carbons can create the triple bond. Therefore, the bond –C≡C– is considered as linear.
General applications of alkynes
Most alkynes are made as acetylene. At the same time, a large part of acetylene is used as a fuel for gas welding thanks to its high temperatures obtained.
In the chemical industry, alkynes are important because the starting materials are made as in the synthesis which is carried out on PVC from artificial rubber. The alkyne group is found in cytostatic drugs. Polymers produced by alkynes, called polyalkynes, are defined as organic semiconductors and can have dopings similar to silicon, although they are long and flexible materials.
Uses of Alkynes
Acetylene or ethyne
Ethyne, also called acetylene, is known as the most useful alkyne. This is a gas that when burned with pure oxygen creates a 2,800 degree Celsius flame used in welding.
Its general uses are:
- Manufacture of acetic acid.
- Welding material thanks to its high temperatures.
- Manufacture of neoprene synthetic rubber and ethanal.
- Production of ethylene chlorine for plastics.
Butyne is known as a highly reactive and flammable alkyne gas at room temperature. It is used in the creation of various synthetic organic compounds. Among its main applications and uses are:
- Production of plastics.
- Production of synthetic organic compounds.
- It is used in industry to make rubber.
This type of alkyne is used in welding. It is a colorless gas, has a characteristic odor, has a higher density than air and extends to the ground. When heated intensely, it decomposes and when pressure is exerted, it produces carbon dioxide and carbon monoxide, creating an explosion or fire hazard.
This alkyne is open chain. It has a semi-liquid state. It is basically used in partial, total and minimum convulsions for autogenous welding. It has a carbon-carbon triple bond.
Pentyne is also an alkyne obtained through the synthesis of ethanol and KOH from the compounds dibromopentane and dihalopentane. The main applications and uses in which pentyne is involved are:
- Synthetic rubber
- metal cutting
Physical properties of alkynes
- It has less density than water. Their boiling points show the usual elevation as well as the increase in the number of carbons and the usual effect of the branching of their chains.
- They cannot dissolve in water but are soluble in common organic solvents which have little polarity, such as ether, ligroin, carbon tetrachloride and benzene.
- The first three terms of alkynes are gases, all others are solids or liquids.
- Using a similar carbon skeleton, the boiling points are nearly the same as in alkenes and alkanes.
- Acetylenes are compounds that have low polarity. For this reason, their physical properties are very similar to those of alkanes and alkenes. Note that acetylenes complete the so-called quartet rule.
- As the molecular weight increases, the density, boiling point, and melting point also increase.