Alkynes: Molecular and Structural Formulas

The alkynes comprise a series of carbon‐ and hydrogen‐based compounds that contain at least one triple bond. This group of compounds is a homologous series with the general molecular formula of C n H 2 n‐‐2 , where n equals any integer greater than one.

The simplest alkyne, ethyne (also known as acetylene), has two carbon atoms and the molecular formula of C 2H 2. The structural formula for ethyne is



In longer alkyne chains, the additional carbon atoms are attached to each other by single covalent bonds. Each carbon atom is also attached to sufficient hydrogen atoms to produce a total of four single covalent bonds about itself. In alkynes of four or more carbon atoms, the triple bond can be located in different positions along the chain, leading to the formation of structural isomers. For example, the alkyne of molecular formula C 4H 6 has two isomers,



Although alkynes possess restricted rotation due to the triple bond, they do not have stereoisomers like the alkenes because the bonding in a carbon‐carbon triple bond is sp hybridized. In sp hybridization, the maximum separation between the hybridized orbitals is 180°, so the molecule is linear. Thus, the substituents on triple‐bonded carbons are positioned in a straight line, and stereoisomers are impossible.