Alkyne Nomenclature
Alkynes are named by replacing the -ane ending of the alkane with -yne. The smallest alkyne is ethyne or acetylene. The longest chain containing the triple bond is chosen as the main chain and is numbered so that the triple bond gets the lowest possible locator.
Structure and Bonding in Alkynes
The triple bond is composed of two perpendicular π bonds, formed by unhybridized p orbitals, and one sigma bond formed by sp hybrids.
Acidity of the Hydrogen in Terminal Alkynes
Terminal alkynes have an acidic hydrogen with a pKa = 25 that can be removed using strong bases, such as sodium amide in liquid ammonia.
The conjugate base (acetylide) is a good nucleophile and can be used in alkylation reactions.
Stability of the Triple Bond
Hyperconjugation also stabilizes alkynes; the internal alkyne is more stable than the terminal one.
Synthesis of Alkynes
Alkynes are obtained through elimination reactions from vicinal or geminal dihaloalkanes.
Hydrogenation of Alkynes
Catalytic hydrogenation converts them into alkanes, although it is possible to stop at the alkene using poisoned catalysts (Lindlar). Sodium in liquid ammonia hydrogenates the alkyne to a trans alkene, a reaction known as a single-electron reduction.
Reactivity of Alkynes
Aqueous sulfuric acid in the presence of mercury sulfate as a catalyst hydrates alkynes via Markovnikov addition, yielding ketones. Hydroboration with hindered boranes, followed by oxidation with hydrogen peroxide, produces enols that tautomerize to aldehydes or ketones. Molecular bromine and HX add to alkynes similarly to alkenes.
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