How do you explain the amphoteric behaviour of amino acids ?

Answer

Amino acids contain an acidic (carboxyl group) and a basic (amino) group within the same molecule. In aqueous solution, they neutralize each other. The carboxyl group loses a proton while the amino group accepts it. As a result a dipolar or zwitterion is formed.
`H_(2)N-underset(underset(R)(|))(CH)-COOH rarr H_(2) overset(+)(N)-underset(underset(underset("Zwitterion")(R))(|))(CH)-COO^(-)`
In zwitterionic form, (`alpha-` amino acids show amphoteric behaviour as they react with both acids and bases. In the acidic medium, `COO^(-)` ion of the zwitterion accepts a proton to form the cation (I) while in the basic medium, `overset(+)(N)H_(3)` ion loses a proton to form the anion (II)
`H_(2)overset(+)(N)-underset(underset(underset((I))(R))(|))(CH)-COOH underset(H^(+))overset(OH^(-))(hArr)-underset(underset(underset("Zwittertion")(R))(|))(CH)-COO^(-) underset(H^(+))overset(OH^(-))(hArr)H_(2)N-underset(underset(underset((II))(R))(|))(CH)-COOH^(-)`
Thus, `overset(+)(N)H_(3)` group acts as the acid while `COO^(-)` group acts as the base.