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Alkynes

Alkynes contain a C≡C triple bond, made of one σ bond and two π bonds between sp carbons. They undergo addition and reduction reactions and, when the triple bond is terminal, they are acidic (pK_a ≈ 25): their anions are excellent carbon nucleophiles.

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Structural Aspects and Preparation

Examples of alkyne nomenclature: 4-bromo-3,3-dimethylbut-1-yne, (2-butynyl)cyclopentane, (Z)-5-methylhept-4-en-2-yne and 2-methylhex-2-en-4-yne

Nomenclature

Triple bond: two sp hybrids form the σ bond and two pairs of p orbitals the two π bonds; linear molecule (180°) with 1.20 Å C≡C and 1.06 Å C–H bonds

Physical and Bond Properties

Alkynides, good nucleophiles, undergo nucleophilic substitution with halides, epoxide opening and addition to carbonyls

Preparation

Reactivity

Hydrogenation of an alkyne with the Lindlar catalyst (Pd/CaCO₃, Pb(OCOCH₃)₂ and quinoline): cis alkene

Catalytic Hydrogenation

Pieces of sodium metal

Reduction with Metals

Hydration of a terminal alkyne with H₂O/H⁺ and HgSO₄: the more substituted alkenyl carbocation takes up water and the enol formed tautomerizes to the methyl ketone

Electrophilic Addition