ethinyl

Ethinyl is a chemical substituent used in organic chemistry to denote a two-carbon unit derived from ethyne (acetylene) that contains a carbon–carbon triple bond. The group is typically represented as –C≡CH and is bound to the rest of the molecule through one of the acetylenic carbons. In systematic nomenclature this substituent is often called ethynyl, while ethinyl remains a common spelling in older literature and in pharmaceutical naming.

The term ethinyl is widely encountered in pharmacology and historic chemical literature, with ethinylestradiol (ethinyl estradiol) being a well-known example. In modern practice, ethynyl is generally preferred in IUPAC nomenclature, but ethinyl persists in many compound names and in everyday usage.

Chemically, the ethynyl group is linear and sp-hybridized, making it an unsaturated, reactive alkyne moiety. It serves as a versatile handle for chemical transformations, including additions to the triple bond and various cross-coupling or click-chemistry reactions. The presence of the terminal C–H hydrogen also affects the acidity and reactivity of adjacent sites compared with purely saturated hydrocarbons.

Ethinyl is used as a building block in synthetic chemistry and in drugs and biologically active molecules, where the triple bond can influence metabolic stability and molecular conformation. The spelling variant ethinyl appears in historical and pharmaceutical contexts, while ethynyl is the preferred IUPAC spelling.

See also: ethynyl, acetylene, alkyne chemistry.