Question

The carbide of which of the following metals on hydrolysis gives allylene or propyne?

• A. $Be$
• B. $Ca$
• C. $Al$
• D. $Mg$

Answer 1

Answer: (D)

$B{e}_{2}C$ and $A{l}_4{C}_3$ on reaction with water liberate methane gas while $Ca{C}_2$ gives acetylene gas. $Mg$ forms $Mg{C}_2$ either when heated directly with carbon in an electric furnace or when $MgO$ is heated with carbon. When $Mg{C}_2$ is heated, it changes into $M{g}_2{C}_3$. The carbide, $M{g}_2{C}_3$ reacts with $H_{2}O$ to form propyne.
$M{g}_2{C}_3+4H_{3}O\to 2Mg(OH{)}_2+C{H}_{3}C\underset{\text{Propyne}}{\equiv CH}$