How are alkynes prepared from the following?
`(i)` Alkenes `(ii)` Gem dihalides
`(iii)` Potassium maleate `(iv)` Calcium carbide

Explanation

`1.` Preparation of alkynes from alkenes :
This process involves two steps :
`(i)` Halogenation of alkenes to form vicinal dihalides
`(ii)` Dehalogenation of vicinal dihalides to form alkynes.
`CH_(2)=CH_(2)+Br_(2)tounderset(1,2-"Dibromethene")(underset(Br)overset(CH_(2))(|)underset(Br)overset(CH_(2))(|))overset("Alocoholic" KOH)(to)underset("Bromoethene")(underset(Br)overset(CH=CH_(2))(|))underset(-NH_(3))(overset(NaNH_(2))underset(-NaBr)(to))underset("Acetylene")(CH-=CH)`
`2.` Preparation of alkene from gem dihalides :
A compound containing two halogen atoms on he same carbon atom is called gem dihalide (Latin word 'Gemini' means twins). On heating with alcoholic `KOH`, gem dihalides give alkynes.

`3.` Preparation of alkenes from electrolysis of salts of Unsaturated Dicarboxylic Acids. (Kolbe's Electrolytic Method)
Electrolysis of sodium or potassium salt of maleic or fumaric acid yields alkynes.
`underset("Potassium maleate")(underset(CHCOOK)overset(CHCOOK)(||)overset("Electrolysis")(to))overset(CHCOO)underset(CHCOO)(||)+2K^(+)`
`overset(CHCOO)underset(CHCOO)(||)tounderset("Acetylene")(underset(CH)overset(CH)(||))+2CO_(2)+2e^(-)`
`4.` Industrial preparation of ethyne :
Ethyne can be manufactured in large scale by action of calcium carbide with water.
`+H_(2)OtoCH-=CH+Ca(OH)_(2)`