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?
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1$\begingroup$ 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 2Oct 20, 2016 at 16:57
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$\begingroup$ @ketbra Is o- effect working here? $\endgroup$– ResorcinolOct 20, 2016 at 17:00
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$\begingroup$ It's not a great H-bond. The ideal H-bond is linear. $\endgroup$– ZheOct 20, 2016 at 17:46
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1$\begingroup$ Here is a relevant answered question. $\endgroup$– Nicolau Saker NetoOct 20, 2016 at 21:12
1 Answer
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