Question

Why do unsaturated Hydrocarbons undergo addition reaction whereas saturated hydrocarbons do not?

Answer 1

Unsaturated hydrocarbons includes compounds in which carbon atoms form either double or triple bonds between them. They undergo addition reaction to add another atom to the carbon atoms forming double or triple bond so that they can form alkane which is more stable than Alkene or Alkyne.
Saturated hydrocarbons do not undergo addition reaction because the bond between carbon atoms is already a single bond and they are stable.

Answer 2

Addition reaction is a type of reaction in which two or more molecules combine with each other to form a single substance. ... Substitution reaction does not add any atom to the hydrocarbon because it takes place only in saturated hydrocarbons(containing only single bonds).