Question

2-Hydroxybutanedioic acid occurs naturally in apples and other fruits. Rank the labeled protons${\mathbf{H}}_{\mathbf{a}}\mathbf{-}{\mathbf{H}}_{\mathbf{e}}$ in order of increasing acidity and explain in detail the order you chose.

Short answer

Increasing acidity:${\mathrm{H}}_{\mathrm{d}}<{\mathrm{H}}_{\mathrm{c}}<{\mathrm{H}}_{\mathrm{b}}<{\mathrm{H}}_{\mathrm{e}}<{\mathrm{H}}_{\mathrm{a}}$

Step-by-step solution

Acidity of protons

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.

 Step 2: Explanation

${\mathrm{H}}_{\mathrm{a}}\mathrm{and}{\mathrm{H}}_{\mathrm{e}}$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.