ethynyl

Ethynyl is a term used in organic chemistry to denote the ethynyl substituent, -C≡CH, a two-carbon unit derived from acetylene by removing one hydrogen. When attached to a larger molecule, the ethynyl group is connected through the acetylenic carbon adjacent to the triple bond. The group is linear due to sp hybridization and provides a versatile handle for further functionalization.

The ethynyl hydrogen is relatively acidic (pKa ≈ 25 for acetylene), enabling formation of acetylide anions by base. This reactivity underlies many transformations: deprotonation to give R−C≡C−, followed by cross-coupling with electrophiles. The Sonogashira coupling is a common method to form C(sp2)–C(sp) bonds between a terminal alkyne (or ethynyl-substituted fragment) and an aryl or vinyl halide, often using palladium catalysts with copper co-catalyst.

Other notable reactions include Glaser coupling (oxidative homocoupling of terminal alkynes to diynes) and Eglinton coupling (a related oxidative method). Terminal alkynes also participate in click chemistry with azides to form 1,2,3-triazoles under copper catalysis.

In materials and bioconjugation, ethynyl groups serve as reactive handles for polymer synthesis and labeling. Ethynylated motifs appear in natural product derivatives, pharmaceuticals, and research reagents; many commercially available building blocks are ethynylated.

In nomenclature, "ethynyl" is the preferred IUPAC substituent name; as a base unit it is part of acetylenic chemistry.