Give the method of preparation of ethyne (acetylene) from vicinal dihalides.

(N/A) Vicinal dihalides are compounds in which two halogen atoms are attached to adjacent carbon atoms.
When vicinal dihalides are treated with alcoholic potassium hydroxide $(KOH)$,they undergo dehydrohalogenation to form vinyl halides,which then react with sodamide $(NaNH_2)$ to form ethyne.
The reaction is as follows:
$CH_2Br-CH_2Br + 2KOH (\text{alc.}) \longrightarrow CH \equiv CH + 2KBr + 2H_2O$
Alternatively,using $NaNH_2$ in liquid ammonia:
$CH_2Br-CH_2Br \xrightarrow{NaNH_2 / liq. NH_3} CH \equiv CH$