Compare the $\ce{OH}$ bonds in Vitamin C (ascorbic acid) and decide which one is the most acidic. The most acidic proton in ascorbic acid is the one whose conjugate base is most resonance stabilized.
- Removal of either of these $\ce{H}$'s at hydroxyl group A or B does not give a resonance stabilized anion:
$\bf{Scheme \ 1}$Image may be NSFW.
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- The proton at D is less acidic, since its conjugate base is lessresonance stabilized with two resonance structures:
$\bf{Scheme \ 2}$Image may be NSFW.
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- The proton at C is the most acidic proton in ascorbic acid since the conjugate base is most resonance stabilized:
$\bf{Scheme \ 3}$Image may be NSFW.
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