how hydrocarbon undergo addition and substitution reaction

In hydrocarbons, saturated hydrocarbons are more stable. So, the unsaturated hydrocarbons undergo addition reaction to achieve this configuration.Example:C2H2+C2-->C2H4.Saturated hydrocarbons may "exchange" one of their element for another because it WONT affect the stability and structure. Only one element will change.Example:CH4+Cl2-->CH3Cl + ClNotice that in the above equation, one chlorine goes and breaks one hydrogen bond and takes its place. The removed hydrogen bonds with the remaining Cl atom to form HCl. There is no net change in structure except for substitutiong of Cl in place of H.