Which of the following amino acids does not exist as a dipolar ion?

Options

A. `p-` Aminobenzoic acid
B. `o-` Amonobenzoic acid
C. `p-` Aminobezensulphonic acid
D. Both (1) and (2) acid

Answer

D

Explanation

p - Aminobenzoic acid (anthranilic acid) or o - aminobezoic acid does not exist appreciably as the dipolar ion, but p - aminobenzenesulphonic acid (sulphanilic acid) does because `K_(b)` for aromatic amines is quite low, about `10^(-10)`. (The `COO^(-)` group, with `K_(b) 10^(-9)`, is actually a slightly stronger base than aromatic `-NH_(2)`). Thus, `- NH_(2)` is not netralized by `- COOH`. But `- SO_(3) H` is strongly acidic and can neutralize an aromatic `- NH_(2)` group.
Sulphanilic acid dissolves in alkalis not in acids because in alkali, sulphanilic acid, `p - H_(3) NC_(6) H_(4) SO_(3)^(-)`, gives up a proton to `OH^(-)` to form a water soluble sulphonate, `p - H_(2) NC_(6) H_(4) SO_(3)^(-)`. But sulphanilic acids are very strong acids, and sulphonate anions are correspondingly weak bases, thus in acid solution `- SO_(3)^(-)` does not accept a proton, and dipolar ion structure of sulphanilic acid remains unchanged.