Sodium ethanoate on heating with soda lime gives ' $X^{\prime}$. Electrolysis of aqueous solution of sodium ethanoate gives ' $Y^{\prime}$. ' $X^{\prime}$ and ' $Y^{\prime}$ respectively are

But-1-yne on reaction with dil. $\mathrm{H}_2 \mathrm{SO}_4$ in presence of $\mathrm{Hg}^{2+}$ ions at 333 K gives

2-methyl propane can be prepared by Wurtz reaction. The haloalkanes taken along with metallic sodium and dry ether are :

In the given sequence of reactions, identify ' $P^{\prime}, Q^{\prime}, R^{\prime}$ and ' $S$ respectively.