Alkynen

Alkynes are a class of hydrocarbons characterized by at least one carbon–carbon triple bond. The general formula for acyclic alkynes is CnH2n−2, and they can be classified as terminal (the triple bond is at the end of the carbon chain) or internal (the triple bond is within the chain). The carbon atoms of the triple bond are sp-hybridized, resulting in a linear arrangement with approximately 180-degree bond angles around the triple bond.

Chemical properties and reactivity of alkynes reflect the presence of the triple bond. The C≡C bond is shorter and stronger than a C=C bond, and the pi bonds make the molecule reactive toward additions. Common reactions include hydrogenation to form alkanes, halogenation to give dihalides, and hydrohalogenation to form vinyl halides. Hydration of internal alkynes, and especially terminal alkynes under mercuric sulfate catalysts, yields ketones or aldehydes respectively (terminal alkynes generally give methyl ketones upon hydration). Terminal alkynes are slightly acidic (pKa around 25) and can be deprotonated with strong bases to form acetylide anions, which are useful in nucleophilic alkylation and other synthetic transformations. Hydroboration–oxidation of terminal alkynes provides aldehydes (anti-Markovnikov pathway), while other regioselective methods allow the formation of various carbonyl or alcohol products.

Synthesis and occurrence: Alkynes are prepared by methods such as double dehydrohalogenation of vicinal dihalides or other eliminations, and they can be obtained from natural sources or produced synthetically for use as intermediates in organic synthesis. They also play a significant role in industrial chemistry, with acetylene serving as a key feedstock for welding gases, flame technology, and as a building block for polymers and pharmaceuticals.

Safety and handling: Alkynes range from gases (ethyl or acetylene) to liquids, and many are flammable. Some reactions can be highly exothermic or require careful control of pressure and catalysts.