Alkynes: preparation and properties
Alkynes are open-chain unsaturated hydrocarbons with one C≡C triple bond; the general formula is CₙH₂ₙ₋₂ and the first member is acetylene (ethyne) HC≡CH. In systematic names “-ane” becomes “-yne” and the triple-bond position is numbered: propyne, butyne-1, butyne-2; numbering starts at the end nearer the triple bond. In rational nomenclature an alkyne is a derivative of acetylene: methylacetylene (propyne), dimethylacetylene (butyne-2). Alkynes are interclass isomers of alkadienes with the same carbon number. Acetylene is made from calcium carbide and water: CaC₂ + 2H₂O → C₂H₂↑ + Ca(OH)₂ or by heating methane at 1500 °C: 2CH₄ → C₂H₂ + 3H₂. It is a colourless gas, sparingly soluble in water and slightly lighter than air. In the Kucherov reaction water adds in the presence of mercury salts to give acetaldehyde: HC≡CH + H₂O → CH₃–CHO. N. D. Zelinsky passed acetylene over activated carbon to obtain benzene: 3C₂H₂ → C₆H₆. Acetylene adds H₂, Br₂ and HCl in two stages; burned in oxygen it reaches a flame temperature of about 3000 °C and is used for welding and cutting metals.
Look up how acetylene is used in welding and write why welders wear protective goggles. Acetylene is explosive; making or burning it at home is strictly forbidden — only the teacher may demonstrate it.