Question

Amino acids show amphoteric behaviour. Why?

Answer 1

In aqueous solution, the carboxyl group of an amino acid can lose a proton and the amino group can accept a proton to give a dipolar ion known as zwitterion.

\[\begin{array}{cc}
\ce{\phantom{........}O\phantom{.......................................}O}\\
\phantom{.......}||\phantom{.......................................}||\\
\ce{R - CH - C - O - H ⇌ R - CH - C - O^-}\\
\phantom{.......}|\phantom{........................................}|\\
\ce{\phantom{.........}\ce{\text{}^\bullet_\bullet}NH2\phantom{................................}^+NH3}
\end{array}\]

Therefore, in its zwitterionic form, the amino acid can act as both an acid and a base.

\[\begin{array}{cc}
\ce{\phantom{........}O\phantom{....................................}O\phantom{.................................}O}\\
\phantom{.......}||\phantom{....................................}||\phantom{.................................}||\\
\ce{R - CH - C - O^- ⇌[H+][OH-] R - CH - C - O^- ⇌ R - CH - C - OH}\\
\phantom{.......}|\phantom{.....................................}|\phantom{...................................}|\\
\ce{\phantom{............}NH2\phantom{.............................}^+NH3\phantom{............................}^+NH3}
\end{array}\]

Thus, amino acids show amphoteric behaviour.