CAIE A-Level Chemistry 33.1.5 Acidities of Chlorinated Carboxylic Acids

Amino acids are molecules that contain $-\mathrm{NH}_{2}$ and -COOH functional groups.
Glycine, $\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{COOH}$, is the simplest stable amino acid.

A student proposes a synthesis of hippuric acid by the reaction of benzamide, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CONH}_{2}$, and chloroethanoic acid, $\mathrm{ClCH}_{2} \mathrm{COOH}$.

The reaction does not work well because benzamide is a very weak base.

The $\mathrm{p} K_{\mathrm{a}}$ of chloroethanoic acid is 2.86 whereas the $\mathrm{p} K_{\mathrm{a}}$ of ethanoic acid is 4.76. Explain the difference between these two $\mathrm{p} K_{\mathrm{a}}$ values.