Unsaturated hydrocarbons with a triple bond are known as alkynes or acetylenes. e.g. ethyne:
Ethyne is the most commonly encountered member of this group. It can be prepared by the following method:
This process of addition of water to calcium carbide is used industrially. Other mechanisms of preparation used industrially include cracking, and the action of ethanolic potassium hydroxide on ethylene dibromide:
One of the major uses of ethyne is in oxy-acetylene welding torches, where the flame temperature exceeds 3000°C.
Because ethyne has a triple bond, and not a double bond, it is more unsaturated than ethene, and it will form addition products with two or four univalent atoms. As discussed before, a triple bond consists of a sigma bond along the line of centres of the two carbon atoms, and two orthogonal pi-bonds.
Ethyne is less reactive towards electrophiles than ethene, which we would not expect when considering the electron densities of both. Ethyne has the higher electron density, and we would therefore expect it to be more reactive towards electrophiles.
The same conclusion is drawn if we look at the relative bond strengths of ethane, ethene and ethyne. We would expect approximately 200kJmol-1 more energy to be released for the complete addition to ethyne than to ethene. The order of reactivity that we expect for the reaction with electrophiles is, however, observed with the catalytic hydrogenation reactions. These are not electrophilic reactions.
One plausible reason for the decreased reactivity of ethyne when compared to ethene is that the bridged carbonium ion formed from ethyne is more strained than the one formed from ethene: