Explain the addition and substitution reaction with the help of examples. `C_2H_6` undergoes substitution reaction but not addition reaction. Why?

Explanation

Substitution reaction : Those reactions in which an atom or group of atoms of a compound are replaced by other atom or group of atoms are called substitution reactions.
`underset("methane")(CH_(4)) + underset("chlorine")(Cl_(2)) overset(" sunlight ")(rarr) underset("chloromethane")(CH_(3) Cl) + underset("hydrogen chloride")(HCI)`
Addition reaction : Those reactions in which two or more molecules or compounds are combined to form a large molecule or compounds, are called addition reaction. lt takes place in unsaturated hydrocarbon
`CH_(2) = CH_(2) + H_(2) underset("Ni")(rarr) C_(2) H_(6)`
Presence of multiple bonds (double and triple bonds) between carbon atoms in the chain of hydrocarbon is one of the essential condition for addition reaction.
As ethane has no double or triple bond, it does not undergo addition reaction, rather it goes for substitution reaction.
OR
Cleansing Action of Soap : A soap molecule consists of two dissimilar parts :
(i) A short ionic part comprising the carboxylate salt -`"COO"^(-) Na^(+)` which is water soluble.
(ii) A long hydrocarbon chain which is hydrophobic.
When soap is dissolved in water, it forms a colloidal suspension. In this colloidal suspension, the soap molecules duster together to form micelles and remain radially suspended in water with the hydrocarbon end towards the centre and the ionic end directed outward. The dirt particles always adhere to the oily or greasy layer present on the skin or clothes. When a dirty doth is dipped into a soap solution, its nun-polar hydrocarbon end of micelles gets attached to the grease or oil present in dirt and polar end rema.ins in water layer.
The mechanical action of rubbing subsequently, dislodges the oily layer from the dirty surface shaping it into small globules. A stable emulsion of oil in water is formed. The emulsified oil or grease globules bearing the dirt can now be readily washed with water.
(ii) In hard water, soap reacts with calcium and magnesium salts which are present in hard water and form insoluble substances called scum. Thus scum is insoluble and floats on the surface of water.
When soap is shaken with hard water, which contains `Ca^(+2) and Mg^(+2)` ions, very less lather is formed and thus cleaning of clothes equire more soap.