Final answer:
A hydrocarbon is classified as an alkyne by the presence of at least one carbon-to-carbon triple bond, which distinguishes them from alkenes (double bonds) and alkanes (single bonds). Alkynes have the general formula CnH2n-2 and, unlike alkenes, cannot exhibit geometric isomerism.
Step-by-step explanation:
The structural feature that distinguishes a hydrocarbon as an alkyne is the presence of a triple bond between carbon atoms. In contrast, hydrocarbons with double bonds are known as alkenes, and those with only single bonds are called alkanes. The simplest alkyne, ethyne (also known as acetylene), has the molecular formula C2H2 and contains a carbon-to-carbon triple bond. Alkynes are unsaturated hydrocarbons with the general formula CnH2n-2, indicating a lesser number of hydrogen atoms compared to alkenes and alkanes due to the presence of the triple bond.
Unlike alkenes, alkynes do not display geometric isomerism because each carbon atom in a triple bond can only be bonded to one other group; thus, there is only one spatial arrangement possible. Furthermore, aromatic hydrocarbons consist of a special class with closed rings of carbon atoms, like the benzene ring, which is not a feature of alkynes.