In ethylene H2C=CH2, the C−C bond distance is 1.34A˚, therefore, the π electrons of the double bond are easily exposed to the reactant (electrophiles) which readily react. In acetylene molecule H−C≡C−H, although there are two π bonds but these electrons arc not exposed to the attacking reagents due to small bond distance of C≡C bond (1.20A˚). A catalyst mercuric ion is therefore needed in reactions with actylene.