Alkynes

Alkynes have a C≡C triple bond and the general formula CₙH₂ₙ₋₂: ethyne (acetylene) C₂H₂, propyne C₃H₄… Each carbon of the triple bond is sp (linear, 180°). The triple bond is one σ + two π bonds, 120 pm long, with a bond enthalpy of 823 kJ mol⁻¹. Alkynes behave much like alkenes; learn alkenes well and alkynes follow easily.

Preparing ethyne:

• From calcium carbide and water: CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂. Calcium carbide itself is made by heating quicklime with coke: CaO + 3C → CaC₂ + CO. This is the industrial method.
• From vicinal dihalides: two molecules of HX are removed, first with alcoholic KOH, then with sodamide (NaNH₂).

Acidity: a hydrogen attached to a triple-bonded carbon is weakly acidic. The sp carbon has 50% s character, so it holds the bonding electrons closely and lets the H⁺ go more easily. So terminal alkynes (with the triple bond at the end of the chain), such as ethyne and but-1-yne, react with sodium or NaNH₂, giving off hydrogen or forming sodium salts. An alkyne with the triple bond inside the chain has no such hydrogen. The acidity order is HC≡CH > CH₃C≡CH ≫ CH₃C≡CCH₃.

Addition reactions, like those of alkenes, but they can happen twice: hydrogen, halogens, hydrogen halides (following Markovnikov's rule), and water, with mercury(II) sulphate and dilute sulphuric acid at 333 K, which gives carbonyl compounds (aldehydes or ketones).

Polymerisation: ethyne can join in a long chain, giving polyacetylene, a plastic that conducts electricity; or three molecules can join in a ring. Passed through a red-hot iron tube at 873 K, ethyne forms benzene.