ethynylsubstituted

Ethynyl-substituted refers to a molecule in which one or more hydrogen atoms have been replaced by an ethynyl group, -C≡CH. The ethynyl group is derived from ethyne (acetylene) and attaches to the parent structure through the acetylenic carbon, giving a linear, sp-hybridized substituent. In IUPAC nomenclature, the substituent is usually named ethynyl- (or acetylenyl- in older usage), and its position is indicated with the usual locants in the parent compound's name.

Common examples include ethynylbenzene, also known as phenylacetylene, where a phenyl ring bears an ethynyl substituent; other examples are 4-ethynylpyridine and 1-ethynylcyclohexane. Compounds can contain multiple ethynyl substituents, described as diethynyl-, triethynyl-, etc.

Reactivity and utility: The terminal C–H of an ethynyl group is relatively acidic (pKa around 25), allowing deprotonation to form acetylide anions. These can participate in a range of carbon–carbon bond-forming reactions, notably cross-couplings such as the Sonogashira reaction, to attach various groups via the carbon–carbon bond. Ethynyl groups are also key participants in click chemistry, notably copper-catalyzed azide–alkyne cycloaddition, which enables rapid assembly of complex structures. The ethynyl moiety extends conjugation in many systems, contributing to the properties of organic semiconductors and conjugated polymers, such as polyphenylene ethynylenes.

Applications and considerations: Ethynyl-substituted compounds are widely used in organic synthesis, materials science, and medicinal chemistry as versatile handles for further diversification. Handling requires standard laboratory caution due to the reactive nature of alkynes in some conditions.