alkyne
Summary
An alkyne is a hydrocarbon with a carbon–carbon triple bond (one σ, two π), and its sp carbons give a linear geometry. Most testable: a terminal alkyne's sp C–H is far more acidic (pKa about 25) than alkene or alkane C–H, so a strong base like NaNH₂ makes a nucleophilic acetylide.
Detail
Meaning: An alkyne is a hydrocarbon with a carbon–carbon triple bond: one σ bond and two π bonds. The triple-bonded carbons are sp hybridized, so the geometry around them is linear (180°). A terminal alkyne (R–C≡C–H) has an unusually acidic C–H (pKa about 25, versus about 44 for alkenes) because sp orbitals have 50% s character and stabilize the conjugate base. A strong base such as NaNH₂ forms an acetylide, which acts as a nucleophile.
Equation: CnH2n−2, where n is the number of carbons (unitless), for an acyclic hydrocarbon with one triple bond.
On the MCAT: Expect product prediction. H₂ with Pd/C gives the alkane; H₂ with Lindlar catalyst gives the cis alkene; Na in NH₃(l) gives the trans alkene. Hydration with HgSO₄/H₂SO₄ gives an enol that tautomerizes to a ketone. In IR, the C≡C stretch appears near 2100–2260 cm⁻¹.
Common confusion: Terminal alkynes are acidic only relative to other hydrocarbons. A pKa of about 25 is well above that of water (15.7), so hydroxide does not deprotonate them appreciably. Internal alkynes have no sp C–H, so they lack this acidity.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry (McMurry)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.