Preparation methods
1. From calcium carbide: Ethyne is prepared by calcium carbide with water.
CaC₂ + 2H₂O → Ca (OH)₂ + C₂H₂
2. From vicinal dihalides: Vicinal dihalides on treatment with alcoholic potassium hydroxide undergo, dehydrohalogenation one molecule of hydrogen is eliminated to form alkenyl halide which on treatment with sodalime gives alkyne.
\(\underset{\text{A}\,\text{gem-dihalide}}{\mathop{-\underset{H}{\overset{H}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}}\,-\underset{X}{\overset{X}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}}\,-\,}}\,or\,\underset{\text{A}\,\text{vic-dihalide}}{\mathop{-\underset{X}{\overset{H}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}}\,-\underset{X}{\overset{H}{\mathop{\underset{|}{\overset{|}{\mathop{C}}}\,}}}\,-}}\,\xrightarrow{A/c\,\,\,KOH}KX\,+\,{{H}_{2}}O+\underset{\text{A}\,\text{vinyl}\,\text{halide}}{\mathop{-\overset{H}{\mathop{\overset{|}{\mathop{C}}\,}}\,=\underset{X}{\mathop{\underset{|}{\mathop{C}}\,}}\,-}}\,\xrightarrow{NaN{{H}_{2}}}NaX+N{{H}_{3}}+-C\equiv C-\)
Some Other General Methods of Preparation of Alkynes:
1. By Haloform:Only for acetylene
\(~CH{{X}_{3}}~+\text{ }6Ag\text{ }+\text{ }{{X}_{3}}HC~~\xrightarrow{heat}CH\equiv CH\,+6AgX\)
2. By Kolbe’s electrolytic method:
\(\underset{C}{\mathop{\underset{|}{\mathop{C}}\,}}\,\underset{H{{X}_{[aq]}}}{\mathop{\underset{|}{\mathop{H}}\,}}\,COOK\xrightarrow{\text{Electrolysis}}\overset{\text{Anode}}{\mathop{CH\equiv CH}}\,+2C{{O}_{2}}+\overset{\text{Cathode}}{\mathop{2KOH+{{H}_{2}}}}\,\)