Alkynes

Alkynes

Alkynes are unsaturated hydrocarbons with a triple bond.

• General formula: CₙH₂ₙ₋₂.
• The C atoms are sp-hybridized, the bond angle is 180° (linear).
• Triple bond: 1σ + 2π.
• The most important representative: acetylene C₂H₂, H-C≡C-H (3σ + 2π).

Preparation of Acetylene and the Kucherov Reaction

1. CaC₂ + 2H₂O → C₂H₂ (acetylene)
2. Kucherov reaction: H₂O, Hg²⁺
3. CH₃CHO (acetaldehyde)

• CaC₂ + 2H₂O → C₂H₂↑ + Ca(OH)₂.
• Decomposition of methane at 1500 °C: 2CH₄ → C₂H₂ + 3H₂.
• Kucherov: C₂H₂ + H₂O → CH₃CHO (in the presence of Hg²⁺).

Properties

• Addition: H₂, Br₂, HCl, H₂O.
• Alkyne: bromine water and KMnO₄ are decolorized. This is a qualitative reaction for unsaturation.
• Trimerization (Zelinsky): 3C₂H₂ → C₆H₆.
• Ag⁺ precipitate: the triple bond is terminal (with silver ammonia solution).
• C₂H₂ + 2H₂ → C₂H₆ (complete hydrogenation, Ni). C₂H₂ + H₂ → C₂H₄ (partial).

Additional Information

• When acetylene burns, the temperature is ≈ 3000 °C. It is used for cutting and welding metals.
• Isomers of alkynes: alkadienes (CₙH₂ₙ₋₂).
• Vinyl chloride, acetic acid and rubber are obtained from acetylene.