The alkyne triple bond is composed of one σ and two π covalent bonds. The π bonds are the structures that preclude any rotation around the σ‐bond axis. As with the alkenes, any rotation on the σ‐bond axis would require disruption of the π‐bond system. Breaking the π‐bond system requires energy and thus would lead to a molecule with a less stable, higher energy state.
Like alkenes, alkynes are unsaturated because they are capable of reacting with hydrogen in the presence of a catalyst to form a corresponding fully saturated alkane. Each π bond signals that two hydrogen atoms have been lost from the molecular formula of the alkane with the same number of carbon atoms. Because alkynes possess two π bonds per molecule, they are said to contain two elements of unsaturation.