$\mathrm{HC}\equiv \mathrm{CH}$$\stackrel{\mathrm{Excess} \mathrm{Na}}{\to }\mathrm{A}$ $\stackrel{\mathrm{Excess} {\mathrm{CH}}_3\mathrm{Cl}}{\to }\mathrm{B}$

The final product B of the above conversion 

Acetylene when treated with excess $\mathrm{Na}$, leads to formation of sodium acetylide.

$\mathrm{HC}\equiv \mathrm{CH}$  $\stackrel{\mathrm{Excess} \mathrm{Na}}{\to }\mathrm{Na}-\mathrm{C}\equiv \mathrm{C}-\mathrm{Na}$

This sodium acetylide on further reaction with excess ${\mathrm{CH}}_3\mathrm{Cl}$ gives 2- Butyne.

$\mathrm{Na}-\mathrm{C}\equiv \mathrm{C}-\mathrm{Na}$  $\stackrel{\mathrm{Excess} {\mathrm{CH}}_3\mathrm{Cl}}{\to }{\mathrm{CH}}_3-\mathrm{C}\equiv \mathrm{C}-{\mathrm{CH}}_3$