Alkynes: triple bond, preparation and reactions

Alkynes contain at least one C≡C and have general formula CₙH₂ₙ₋₂. Ethyne (acetylene) is the first stable member; burnt with oxygen as the oxyacetylene flame, it is used for welding.

IUPAC names replace -ane by -yne, and the locant is the first triply bonded carbon. But-1-yne and but-2-yne are position isomers. C₅H₈ has three alkynes (pent-1-yne, pent-2-yne, 3-methylbut-1-yne) and C₆H₁₀ has seven.

Each carbon of ethyne is sp hybridised: it forms one C–C σ bond and two C–H σ bonds, while its two unhybridised p orbitals, at right angles to each other, overlap sideways with those of the partner carbon to make two π bonds. H–C–C is 180°, so ethyne is linear, and the π cloud is cylindrically symmetrical about the axis.

C≡C (823 kJ mol⁻¹, 120 pm) is stronger and shorter than C=C (681 kJ mol⁻¹, 133 pm) and C–C (348 kJ mol⁻¹, 154 pm).

Industrial ethyne: CaCO₃ → CaO + CO₂; CaO + 3C → CaC₂ + CO; CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂. Alternatively, a vicinal dihalide treated with alcoholic KOH gives an alkenyl halide, which sodamide converts to the alkyne.

The first three alkynes are gases, the next eight liquids, the rest solids. They are colourless, weakly polar, lighter than water and immiscible with it; ethyne has a characteristic odour.

Hydrogen on a triply bonded carbon is acidic. Na or NaNH₂ gives sodium ethynide and H₂; alkenes and alkanes do not react, so this distinguishes them. The sp carbon, with 50% s character, is the most electronegative and holds the C–H electrons tightly. Acidity order: HC≡CH > H₂C=CH₂ > CH₃–CH₃, and HC≡CH > CH₃–C≡CH >> CH₃–C≡C–CH₃.

Alkynes add two molecules of H₂, halogen or HX. HX addition follows Markovnikov's rule through a vinylic cation and ends in a gem dihalide: ethyne + 2HBr gives 1,1-dibromoethane. Bromine in CCl₄ is decolourised here too.

With HgSO₄ and dilute H₂SO₄ at 333 K, one molecule of water adds to give a carbonyl compound: ethyne gives ethanal and propyne gives propanone.

Linear polymerisation gives polyacetylene, which conducts electricity under special conditions and can serve as a light battery electrode. Passing ethyne through a red-hot iron tube at 873 K joins three molecules into benzene.