Which of the following compounds can form a zwitterion?

Solution & Explanation

\textbf{Answer:} (D) Glycine bears both $-\mathrm{COOH}$ and $-\mathrm{NH}{}_2$, so it forms a zwitterion. \textbf{Solution:} A zwitterion forms when a molecule carries both an acidic ($-\mathrm{COOH}$) and a basic ($-\mathrm{NH}{}_2$) group that can undergo internal proton transfer to give a dipolar ion with both a positive ($-\mathrm{NH}{}_3{}^{+}$) and a negative ($-\mathrm{COO}-$) charge. Glycine, $\mathrm{H}{}_2\mathrm{N}-\mathrm{CH}{}_2-\mathrm{COOH}$, is an $\alpha$-amino acid: the carboxyl proton transfers to the amino nitrogen to give $\mathrm{H}{}_3\mathrm{N}{}^{+}-\mathrm{CH}{}_2-\mathrm{COO}{}^{-}$. Benzoic acid, acetanilide and aniline each lack the simultaneous acid-plus-base pairing required, so they cannot form an internal zwitterion.