Amino acids have high boiling point because
A. of the formation of Zwitter ion
B. of dipolar nature.
C. of strong dipole-dipole interaction
D. of strong intermolecular forces
E. of the presence of \(-COOH\) and \(-NH_2\) at \(\alpha \)-position
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A, B, C, D only
In aqueous solution and in the solid, the carboxyl group of an amino acid loses a proton and the amino group accepts it, giving a dipolar ion called the zwitter ion:
\( \ce{H2N-CHR-COOH <=> H3N+-CHR-COO-} \)
Because of this dipolar, salt-like structure, amino acids are crystalline solids with high melting points (they decompose rather than boil), and they behave like salts rather than like simple amines or acids.
(A) of the formation of Zwitter ion — Correct. The zwitter ion is the reason amino acids are salt-like.
(B) of dipolar nature. — Correct. The zwitter ion carries both a positive and a negative charge in one molecule, so it is a dipolar ion.
(C) of strong dipole- dipole interaction — Correct. Neighbouring dipolar ions attract each other strongly (ionic, salt-like attractions).
(D) of strong intermolecular forces — Correct. These strong attractions between molecules require a lot of energy to overcome, so the melting/boiling point is high.
(E) of the presence of \(-COOH\) and \(-NH_2\) at \(\alpha \)-position — Incorrect. The position of the groups is not the reason; the high melting point comes from the zwitter ion that the two groups form, and other amino acids (e.g. β-alanine) with the groups farther apart are also high-melting solids.
NCERT (2026-27 edition):
Classification of Amino Acids – …
