Alkynes - Preparation of Alkynes

From calcium carbide:

By the action of water on calcium carbide, Cu₂C₂ or BaC_2, acetylene is formed.

CaC₂ + 2H₂O → CH ≡ CH + Ca(OH)₂

\[\ce{CaCO3 ->[][\underset{}{-CO2}] CaO->[2C + Heat][\underset{}{}] CaC2->[H2O][\underset{}{}] CH ≡ CH}\]

Dehalogenation of 1,1,2,2-tetrahaloalkanes:

On treatment with Zn, tetrahalides get dehalogenated to give alkynes.

\[\ce{R - CBr2 - CHBr2 + 2Zn ->[Δ] R - \underset{Alkyne}{C ≡ CH} + 2ZnBr2}\]

\[\ce{R - CBr2 - CBr2 - R + 2Zn ->[Δ] R - \underset{Alkyne}{C ≡ C} - R + 2ZnBr2}\]

By dehydrohalogenation of vic and gem dihalides:

Alkynes are prepared by dehydrohalogenation of vic. and gem. dihalides with alc. KOH + NaNH₂ or KNH₂.

\[\ce{R - \underset{β}{C}H2 - CHX2 ->[(i) NaNH2/Δ][(ii) H^{+}] R - C ≡ CH}\]

\[\begin{array}{cc}
\ce{X}\phantom{.......................}\\
\phantom{}|\phantom{.......................}\\
\phantom{}\ce{R - \underset{β}{C}H2 - C - R ->[NaNH2/Δ] R - C ≡ CH}\phantom{}\\
\phantom{}|\phantom{.......................}\\
\ce{X}\phantom{.......................}
\end{array}\]