Phenylacetylenes

Phenylacetylenes are aryl-substituted acetylenes in which a phenyl ring is attached to a carbon–carbon triple bond. The simplest member is phenylacetylene (ethynylbenzene, Ph–C≡CH). The class also includes substituted monoaryl variants (for example, o-, m-, and p-ethynylbenzenes) and diaryl acetylenes such as diphenylacetylene (Ph–C≡C–Ph). These compounds are typically characterized by a linear, rigid framework around the C≡C bond and by the terminal alkyne C–H group in the monoaryl examples.

Synthesis and occurrence in chemistry are often achieved by transition-metal-catalyzed cross-coupling. The most common route to simple phenylacetylenes is the Sonogashira coupling of iodoarenes with acetylene or with protected alkynes, followed by deprotection if needed. Diaryl acetylenes can be prepared by analogous couplings between aryl halides and aryl alkynes or by coupling of terminal diaryl alkynes. These methods give access to a wide range of mono- and diaryl phenylacetylenes for use as building blocks in synthesis.

Reactivity is dominated by the terminal alkyne functional group. Phenylacetylenes undergo hydration with Hg2+ catalysts to give acetophenones, and hydroboration-oxidation to yield aldehydes such as benzaldehyde from phenylacetylene. They participate in electrophilic additions to the triple bond, cycloadditions, and in copper-catalyzed azide–alkyne cycloadditions (the “click” reaction) to form triazoles. They also undergo polymerization and serve as precursors to more complex aromatic systems and materials.

Applications include use as versatile intermediates in organic synthesis, the preparation of polyphenylacetylenes and other conjugated polymers, and roles in organic electronics and ligands for metal catalysis. Safety concerns center on their flammability and potential toxicity; standard laboratory precautions apply.