Alkyne

Alkyne One of a group of organic compounds containing a carbon-to-carbon triple-bond linkage (-C≡C-). They are termed acetylenes or alkynes. While exhibit­ing many of the characteristics of alkenes as regards unsaturation, the acetylenes have many unique properties. Since the bonding in alkyne molecules is linear. R-C≡C-R, cis-trans isomerism is not possible. In the simplest alkyne, acetylene (HC≡CH), or in monosubstituted acetylenes, the hydrogen attached to triply bonded carbon is acidic to such a degree that it is replaceable with metals such as sodium.

General methods for the preparation of alkynes depend on dehydrohalogenation of a,B-dihaloparaffins, conversion of aldehydes or ketones to dihaloparaffins with subse­quent dehydrohalogenation, and alkylation of metallic acetylides with alkyl halides in liquid ammonia. Grignard reagents of 1-alkynes behave similarly.

The reactions of triply bonded carbon compounds are in general similar to those of compounds containing ethylenic bonds. Addition reactions proceed in two stages to form first a vinyl compound or substituted ethylene, and second the substituted paraffin. In the presence of catalytic quantities of alkoxides, acetylene adds to alcohols and phenols to give vinyl ethers. Reactions of this type, in which addition takes place by replacement of hydrogen with a vinyl group, are termed vinylation. Another form of addition reaction known as ethynylation involves the addition of acetylene to un­saturated compounds. A general reaction of alkynes involves the addition of carbon monoxide and water, or other compounds having an active hydrogen, in the presence of nickel carbonyl.

Polymerization of alkynes may yield acyclic, aromatic, or alicyclic derivatives. In the presence of cuprous chloride, acetylene dimerizes to vinyl acetylene, which adds hy­drogen chloride to give chloroprene (2-chloro-1,3-butadiene). Polymerization of chloro­prene in the presence of free radical initiators gives the commercially important synthetic rubber, neoprene. Thermal polymerization of acetylene yields benzene as the major product and also a wide variety of polynuclear aromatic compounds. Of great theoret­ical and practical importance is the polymerization of acetylene to the cyclic tetramer, cyclooctatetraene.