Question

Refer to the given graph showing state of ionisation of zwitterion.

Select the correct statements regarding zwitterion.
(i) Zwitterions can be formed from compounds that contain both acid groups and basic groups in their molecules.
(ii) A zwitterion can act either as proton donor or proton acceptor.
(iii) A monoamine monocarboxylic a-amino acid is a acid at high $pH$ as both the groups (amino and carboxyl) lose a proton.
(iv) Amino acids in solution at neutral $pH$ exist predominantly as dipolar ions, the amino group is protonated $(-NH_3^+)$ and the carboxyl group is deprotonated $(-COO^-)$.

• A. (iii) and (iv)
• B. (i), (ii), (iii) and (iv)
• C. (i) and (ii)
• D. (i), (ii) and (iii)

Answer 1

Answer: (B)

Zwitterion is an ion that has a positive and negative charge on the same group of atoms. Amino acids in solution at neutral $pH$ exist predominantly as zwitterions. In dipolar form, the amino group is protonated $(-NH_3^+)$ and the carboxyl group is deprotonated $(-COO^-)$.The ionisation state of an amino acid varies with $pH$. In acid solution, the amino group is protonated $(-NH_3^+)$ and the carboxyl group is not dissociated $(-COOH)$. As the $pH$ is raised, the carboxylic acid is the first group to give up a proton. The dipolar form persists until the protonated amino group loses a proton.