In order to be considered an alkyne (also called acetylenes), a hydrocarbon must possess at least one triple bond between two carbons. A single triple bond results in a hydrocarbon molecule with the general formula of CnH2n-2. In following the general trend where alkenes were more reactive than alkanes, alkynes are more reactive than alkenes, making them the most reactive of the three basic hydrocarbon classes.
The naming convention for alkynes includes the “yne” ending, which notifies one of the presence of a triple bond. It is also the case that official naming conventions are ignored in favor of traditional names for these compounds. For instance, ethyne, the smallest alkyne, is traditionally called acetylene and this is the most commonly used method of referring to the molecule. Every effort is made here to use the appropriate standard name, with reference to the traditional name as necessary.
Ethyne (C2H2) aka Acetylene
This two-carbon molecule is a colorless gas that is exceptionally unstable when it is pure, but is also useful in a number of industrial settings. It is usually mixed with other components to make a solution so that it can be handled without degrading. Because it is so reactive, acetylene is explosive, making it highly undesirable in many applications. Nevertheless, it does find application in some laboratory settings.
The major uses of acetylene include its conversion to ethylene for the production of polyethylene, as a combustion component in acetylene welding, and as a basis for acrylic acids. Acrylic acids are used in the manufacture of everything from paint to plastic to adhesives. In general, the conversion of acetylene to acrylic acid has been supplanted by the conversion of propene, which is safer.
Welding accounts for about 20% of acetylene use where the combustion of acetylene with oxygen produces a flame of roughly 3300 degrees Celsius, making it the hottest burning common fuel available and the third hottest natural chemical burn. Cylinders that contain acetylene also contain porous material (such as diatomaceous earth and, at one point, asbestos) and acetone. Under pressure, these substances ensure that acetylene is absorbed or in solution and that it is only released slowly as pressure is released. This helps ensure safe transport of the explosive gas.
Propyne (C3H4) aka Methylacetylene
Propyne is a three-carbon aklyne that is also a colorless gas. It is often used as a substitute for acetylene in welding because it can safely be condensed to a liquid for transport and storage. Propyne has also been investigated for use as a rocket fuel for space craft intended for low Earth orbit. Its major advantage is that, unlike hydrogen, propyne remains a liquid without the need for cryogenic storage.
The alkynes beyond propyne are generally only useful in niche applications or organic chemistry. The basic structures of the simple alkynes above can be found in a number of more complex alkynes. However, the more complex molecules have properties that are vastly different from the small, explosive alkynes discussed here. In fact, large alkynes, like the contraceptive norethynodrel, are used in pharmaceutical applications. Other medical applications include antiretroviral medications, antifungal medications, and cancer treatment.