Let us take the example of an addition reaction and illustrate the point

• Addition reactions are characteristic of unsaturated compounds.
• Unsaturated compounds have multiple bonds(double or triple bonds)
• Addition takes place across the pi bond .

Note the changes that take place in the following example.

• The entire reagent that is HCl adds on to the double bond (pi bond) of the unsaturated compound.
• The reagent Hydrogen chloride or HCl is divided into two parts, each part adds on to one carbon connected by the double bond.
• Through addition an unsaturated compound becomes a saturated compound.
• A pi bond is broken and two new sigma bonds are formed.
• Thus alkanes may be prepared from alkenes through addition of hydrogen.