Preparation of Alkynes
Alkynes are unsaturated hydrocarbons characteristically containing at least one triple bond between carbon atoms. One practical source of alkynes, particularly ethyne (or acetylene), involves treating calcium carbide with water:
1. From Calcium Carbide
Calcium carbide (CaC2), produced by heating quick lime with coke, reacts with water to yield ethyne:
1. Reaction:
\[ CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2 \]
This reaction provides a significant method for synthesizing ethyne on an industrial scale.
2. From Vicinal Dihalides
Vicinal dihalides, which have halogen atoms attached to adjacent carbon atoms, can also be converted into alkynes through dehydrohalogenation. This involves removing a molecule of hydrogen halide by using alcoholic potassium hydroxide (KOH), leading to the formation of an alkene first, which can then be further treated with sodium amide (NaNH2) to yield the desired alkyne.
- General Steps:
- Dehydrohalogenation:
\[ RCH_2CHBrCH_2Br + KOH \rightarrow RCH=CH_2 + HBr \]
- Formation of Alkyne:
\[ RCH=CH_2 + NaNH_2 \rightarrow RC≡CH + NaBr \]
This method allows for a versatile route to obtain various alkynes based on the starting materials used.