91Ó°ÊÓ

The acidity of protons increases with the increase in the electronegativity. The most electronegative atoms are O, F, Cl, Br, etc.

Therefore, COOH, CHO, OH are electron-withdrawing groups,making their adjacent protons more acidic.

${\mathrm{H}}_{\mathrm{a}}\mathrm{and}{\mathrm{H}}_{\mathrm{e}}$must be the two most acidic protons since they are part of carboxylic acids. Loss of a proton forms a resonance-stabilized carboxylate anion with a negative charge delocalized on two O atoms.

${\mathrm{H}}_{\mathrm{a}}$is more acidic than${\mathrm{H}}_{\mathrm{e}}$ because the nearby OH group on the carbon increases acidity by an electron-withdrawing inductive effect.

${\mathrm{H}}_c$is the next most acidic proton because the conjugate base places a negative charge on the electronegative O atom, but it is not resonance stabilized.

The least acidic H’s are${\mathrm{H}}_{\mathrm{c}}\mathrm{and}{\mathrm{H}}_{\mathrm{d}}$ since these are bonded to C atoms. The electronegative O atom further acidifies ${\mathrm{H}}_{\mathrm{c}}$by an electron-withdrawing inductive effect.