Question

Amino acids exist as zwitter ion but $o$ - and $p$-aminobenzoic acids do not. Why?

Answer 1

$o$ - and $p$ - aminobenzoic acids contain both weakly acidic $(-\mathrm{COOH})$ and weakly basic $\left(-\mathrm{NH}_2\right)$ group due to the electron withdrawing nature of benzene ring. Hence, $-\mathrm{COOH}$ group cannot transfer $\mathrm{H}^{+}$ion to $-\mathrm{NH}_2$ group and cannot form zwitter ion.