Question

can alkynes like C2H2 undergo substitution reaction to give saturated hydrocarbons?

Answer 1

Addition of hydrogen to form saturated hydrocarbons is called 'addition reaction' and not substitution reaction.
Ethyne can add one molecule of H₂ and get converted to ethene that further adds up hydrogen to form ethane.
$$\begin{gathered} \mathrm{CH}\equiv \mathrm{CH}\underset{{150}^{\mathrm{o}}\mathrm{C}}{\overset{\mathrm{Ni}}{\to }}{\mathrm{CH}}_2={\mathrm{CH}}_2 \\ {\mathrm{CH}}_2={\mathrm{CH}}_2\underset{{150}^{\mathrm{o}}\mathrm{C}}{\overset{\mathrm{Ni}}{\to }}{\mathrm{CH}}_3-{\mathrm{CH}}_3 \end{gathered}$$