General Methods of Preparation of Alkynes:
- By dehydrohalogenation of vicinal dihaloalkanes:
The removal of hydrogen and halogen is called dehydrohalogenation. When vicinal dihalides are heated with an alcoholic solution of KOH, hydrogen and halogen atoms are removed from adjacent carbons and in two steps alkynes are produced.
- Dehalogenation of Tetrahaloalkanes:
Tetrahaloalkanes on treated with active metals produce alkynes. Usually, Zinc dust is used for dehalogenation.
- Kolbe’s Electrolysis:
When electrolysis of an aqueous solution of sodium or potassium salt of an unsaturated dicarboxylic acid is carried out, alkynes are produced. This method is used to prepare gaseous alkynes with an even number of carbons.
║ ║ _
HC─C─ONa —> HC─C─O + 2 Na+
║ ║ _
HC─C─ONa —> HC─C─O
Di sodium fumarate fumarate ion
Reaction at cathode: 2 Na+ + 2e – -> 2Na
2 Na + 2H2O -> 2NaOH + H2
Reaction at the anode:
The mixture of ethyne and carbon dioxide obtained at the anode is made to pass through KOH(aq) solution, CO2 will be absorbed by the solution while ethyne will pass on and will then be collected over the surface of the water.
- Commercial preparation of Acetylene:
By hydrolysis of Calcium carbide:
Acetylene is commercially prepared by hydrolysis of calcium carbide. For this purpose, calcium carbide is added to water.
Calcium carbide is produced by heating quick lime (CaO) with coke at 3000oC in an electric furnace.