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This post address the acidity of ortho and para hydroxy benzoic acid. However, what effect is there if there are two hydroxy groups ortho to carboxylic group?

I know hydrogen bonding is also at work but won't the two ortho substituents would decrease the acidity by highly destabilising the conjugate base due to steric hindrance and repulsion; as there are three big groups near to each other?

[P.S.:My worksheet claims that ortho di-hydroxy benzoic acid is more acidic(Option-A is correct in below figure). Also, since about the mono-hydroxy substituted benzoic acids are discussed already on the site, so my question is regarding between only di- and mono- hydroxy benzoic acids.)

enter image description here

Thanks!

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The highly electronegative O atom of the carboxyl group (which by itself in an aliphatic compound would be highly stabilised by equivalent resonating structures) forms very efficient H-bonds with the fairly acidic phenolic hydrogen atoms. The ring formation provides enough stability to favour release of a proton. enter image description here

The increased acidity can also be explained by the First Ortho Effect which basically states that steric hindrance forces the carboxyl group to move out of the plane of the ring which essentially stops resonance with the phenyl ring, thus inhibiting the slightly destabilising crossed-conjugation: enter image description here


P.S. I could not find a suitable source for the $\ce{pK_a}$ of $2,6$-dihydroxybenzoic acid. I would be grateful if someone adds them in the comments.

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  • $\begingroup$ But sir I asked for 2,6-dihydroxy benzene mainly because mono hydroxy acids are already been asked. Anyway, I hope you will expand your answer later . Thanks! $\endgroup$ Sep 28, 2022 at 7:32
  • $\begingroup$ What exactly would you want me to add? $\endgroup$ Sep 28, 2022 at 7:33
  • $\begingroup$ That why dihydroxy one is more acidic than mono-hydroxy one. $\endgroup$ Sep 28, 2022 at 7:34
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    $\begingroup$ 2,6 -dihydroxybenzoic has a pKa of 1.64 (computed by MarvinSketch) $\endgroup$ Sep 28, 2022 at 14:43

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