Learn first: Unsaturated hydrocarbons
Alkynes are acyclic unsaturated hydrocarbons whose molecules contain one C≡C triple bond. A triple bond consists of one σ-bond and two π-bonds, so alkynes, like alkenes, undergo addition, but in two stages: first a product with a double bond forms, then a saturated product. In addition, the hydrogen on a triple bond at the end of a chain is weakly acidic, an important property that distinguishes alkynes from alkenes. The first member, ethyne (acetylene), burns in the flame used to weld and cut metals (above 3000 °C) and is a feedstock for ethanal, benzene, chloroethene (PVC) and synthetic rubber.
General formula CₙH₂ₙ₋₂ (n ≥ 2); M = 14n − 2
The triple-bonded carbons are sp-hybridised, bond angle 180°, the C–C≡C–C unit is linear
To form a triple bond, each carbon hybridises one s- and one p-orbital; the two unhybridised p-orbitals form two π-bonds at right angles to each other. The C≡C bond length is 0.120 nm, shorter than a double (0.134 nm) or single (0.154 nm) bond, and its energy is the greatest. But π-bonds are weaker than a σ-bond, so alkynes add reagents readily.
An sp hybrid orbital has 50 % s-character, sp² 33 % and sp³ 25 %. The greater the s-character, the closer the electrons are to the nucleus and the more electronegative the carbon. So the electron density of the ≡C–H bond is shifted towards carbon and the hydrogen can leave as a proton: ethyne is weakly acidic.
To name alkynes, replace the -ane ending with -yne: ethyne C₂H₂, propyne C₃H₄, but-1-yne CH≡C–CH₂–CH₃ and but-2-yne CH₃–C≡C–CH₃ (C₄H₆). The chain is numbered from the end that gives the triple bond the lowest number.
Kinds of isomerism:
Because the triple-bond unit is linear, alkynes have no geometric (cis–trans) isomers.
Physical properties: C₂–C₄ are gases, C₅–C₁₆ liquids and higher members solids. Pure ethyne is a colourless, almost odourless gas; the unpleasant smell of technical ethyne made from carbide comes from impurities (phosphine, hydrogen sulfide). Ethyne forms explosive mixtures with air, so in cylinders it is stored dissolved in propanone.
C₂H₂ + H₂O → CH₃CHO (ethanal); other alkynes give ketones: CH≡C–CH₃ + H₂O → CH₃–CO–CH₃ (propanone)
2.24 L of hydrogen (STP) saturated 2.7 g of an alkyne completely. Find the formula of the alkyne.
If the triple bond is at the end of the chain (R–C≡CH), the terminal hydrogen can be replaced by a metal:
HC≡CH + 2[Ag(NH₃)₂]OH → AgC≡CAg↓ + 4NH₃ + 2H₂O (silver acetylide, off-white precipitate)
HC≡CH + 2[Cu(NH₃)₂]Cl → CuC≡CCu↓ + 2NH₄Cl + 2NH₃ (copper(I) acetylide, red-brown precipitate)
2HC≡CH + 2Na → 2HC≡CNa + H₂ (with excess sodium, NaC≡CNa)
But-2-yne (CH₃–C≡C–CH₃) has no terminal hydrogen, so it does not react — this distinguishes it from but-1-yne. Alkenes do not react either, so the reaction is used to determine a terminal alkyne in a mixture. Dry acetylides explode on impact.
Carbide method: CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂↑; the carbide itself is made in an electric furnace: CaO + 3C → CaC₂ + CO
Pyrolysis of methane: 2CH₄ → C₂H₂ + 3H₂ (1500 °C, very short contact time)
Alcoholic alkali on dihaloalkanes: CH₂Br–CH₂Br + 2KOH → C₂H₂ + 2KBr + 2H₂O
Chain extension by alkylating acetylides: HC≡CNa + CH₃Br → HC≡C–CH₃ + NaBr
Making an alkyne from a dihaloalkane removes two molecules of hydrogen halide, so 2 mol of alkali are needed.
What mass of technical carbide (80 % CaC₂) is needed to make 11.2 L of ethyne (STP)?
In exams alkynes often appear in mixtures: a terminal alkyne gives a precipitate with ammoniacal silver oxide while an alkane does not react; bromine water absorbs alkenes and alkynes but not alkanes.
A mixture of propyne and propane (4.48 L, STP) was passed through ammoniacal silver oxide solution, giving 14.7 g of precipitate. Find the volume fraction of propane.
Uses: the oxy-acetylene flame welds and cuts metals; ethyne is converted into chloroethene and PVC, ethanal and ethanoic acid, acrylonitrile and chloroprene rubber. Alkynes are a link to alkenes, aromatic hydrocarbons and carbonyl compounds.