In ethylene $H _{2} C = CH _{2}$, the $C - C$ bond distance is $1.34 \,\mathring{A}$, therefore, the $\pi$ electrons of the double bond are easily exposed to the reactant (electrophiles) which readily react. In acetylene molecule $H - C \equiv C - H$, although there are two $\pi$ bonds but these electrons arc not exposed to the attacking reagents due to small bond distance of $C \equiv C$ bond $(1.20 \,\mathring{A})$. A catalyst mercuric ion is therefore needed in reactions with actylene.