Picric acid has nitro groups in para and ortho positions, so it has great possibility to go for resonance, thus it is supposed to be a very good acid. But a question arises! The $\ce{O}$ of $\ce{-OH}$ has intramolecular $\ce{H}$-bonding with ortho-nitro groups present on both sides. So will the firstly mentioned theory work well or is the answer something else?

Picric acid

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    $\begingroup$ Good, maybe. Depends on what you want to use it for. Or do you mean if it is a strong acid, like what is the pKa of pricric acid? $\endgroup$ – logical x 2 Oct 20 '16 at 16:57
  • $\begingroup$ @ketbra Is o- effect working here? $\endgroup$ – Resorcinol Oct 20 '16 at 17:00
  • $\begingroup$ It's not a great H-bond. The ideal H-bond is linear. $\endgroup$ – Zhe Oct 20 '16 at 17:46
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    $\begingroup$ Here is a relevant answered question. $\endgroup$ – Nicolau Saker Neto Oct 20 '16 at 21:12

Yes it is very strong, albeit not compared with Inorganic acids like HCl...

As far as organic acids are concerned, it is really one of the strongest ones... as its pKa is 0.3 [ more acidic than acetic, benzoic, formic, oxalic, trichloro-acetic acids...]

Possibility of intramolecular H-bonding, though exists, is offset reasonably as the negative charge of phenolic oxygen is very nicely delocalized over the 3 nitro groups. [In fact, if we compare the pKa's of phenol (9.98) with o-, m-, p- nitro phenols (7.23, 9.88, 7.15 respectively); we can see that H-bonding is not decreasing the acidic nature "upto greater extent"]

It is always possible that due to bulkier -NO2 groups, the repulsions betweens the LP of O atoms in -OH might be leading to the H of -OH group to be oriented far off from the range of effective H-bonding


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