acetylenic

Acetylenic is an adjective used in chemistry to describe compounds that contain or resemble the acetylene unit, a carbon–carbon triple bond (C≡C). The term derives from acetylene (ethyne), the simplest member of this class. In practice, acetylenic compounds include alkynes and larger molecules bearing a C≡C fragment or an ethynyl substituent (–C≡CH) or an internal acetylenic linkage (–C≡C–).

Structural features: Acetylenic carbons are sp-hybridized, giving linear geometry about the triple bond. The C≡C bond is typically about 120 picometers long in simple alkynes. Terminal acetylenes contain a hydrogen on an sp carbon, making them relatively acidic (pKa around 25) and able to form acetylide anions that participate in nucleophilic alkynylation. Internal acetylenes lack terminal hydrogens and are less acidic.

Reactivity: The triple bond is reactive toward additions and cycloadditions and serves as a versatile handle in synthesis. Common reactions include hydrogenation, halogenation, and hydrofunctionalization, as well as copper-catalyzed azide–alkyne cycloadditions used in click chemistry. Acetylenic groups enable selective transformations in complex molecules.

Occurrence and applications: Acetylene, the simplest acetylenic hydrocarbon, is produced industrially from calcium carbide and used as a chemical feedstock and in welding. Acetylenic compounds underpin many polymers and materials, provide building blocks for pharmaceuticals and dyes, and are central to various alkyne- and polyynyl chemistry strategies.

Etymology and naming: "Acetylenic" derives from acetylene. The ethynyl group (–C≡CH) is the common acetylenic substituent, while internal acetylenic linkages (–C≡C–) appear in more highly substituted molecules.