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I thought it would be the lower first $\ce{-OH}$ because it's close to the double bond with oxygen, so I thought there would be more electron withdrawal from the oxygen atom due to its electronegativity.But the correct answer is actually the second lower $\ce{-OH}$, which I really don't understand

enter image description here

My attempt to the question:

enter image description here

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  • $\begingroup$ The first lower implies to the one closer to carbon double bond oxygen group $\endgroup$
    – Isha Roy
    Feb 2, 2018 at 14:51
  • $\begingroup$ And the second lower is to the right of first lower $\endgroup$
    – Isha Roy
    Feb 2, 2018 at 14:51
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    $\begingroup$ Try drawing both of the structures. You'll notice something.(add them to the question as well) $\endgroup$ Feb 2, 2018 at 15:19

3 Answers 3

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The correct anion is stabilized by the mighty resonance. enter image description here

That's about it.

Hydrogen bonding is irrelevant, since it affects both variants the same way.

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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.

  1. Removal of either of these $\ce{H}$'s at hydroxyl group A or B does not give a resonance stabilized anion:

$\bf{Scheme \ 1}$ Ascorbic acid: Scheme 1

  1. The proton at D is less acidic, since its conjugate base is less resonance stabilized with two resonance structures:

$\bf{Scheme \ 2}$ Ascorbic acid: Scheme 2

  1. The proton at C is the most acidic proton in ascorbic acid since the conjugate base is most resonance stabilized:

$\bf{Scheme \ 3}$ Ascorbic acid: Scheme 3

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    $\begingroup$ Good answer except for one detail, the molecule in the answer is not oriented the same wah as in the question, so it is not clear that the answer was the lower right hydroxyl group in the original structure. How hard is it to get consistent orientation? Failing that, add text saying that C was the lower right hydroxyl group in the original structure. $\endgroup$ Feb 2, 2018 at 17:51
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    $\begingroup$ @Oscar Lanzi I will reorient the post as you have suggested right I in the morning.Thankyou $\endgroup$ Feb 2, 2018 at 17:54
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enter image description here

As shown, if the hydrogen on the bottom-right position is removed, the negative charge is stabilized by more number of resonance structure (more conjugation) as compared to when the hydrogen on the bottom-left position is removed.

So the hydrogen on the bottom-right position is more acidic.

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