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I understand that picric acid is acidic because of the $\ce{-NO2}$ electron withdrawing group, but why can't we add more of those on the meta positions to make it more acidic?

Is it because that $\ce{-NO2}$ shows less -R effect when it is on that meta positions?

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    $\begingroup$ We surely can add more nitro groups, albeit with great difficulty. $\endgroup$
    – Ivan Neretin
    Commented Dec 2, 2023 at 17:51
  • $\begingroup$ Would it be more acidic if more NO2 groups are added. $\endgroup$
    – User10001010001000
    Commented Dec 2, 2023 at 17:52
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    $\begingroup$ That is not immediately clear, nor are the experimental data readily available. $\endgroup$
    – Ivan Neretin
    Commented Dec 2, 2023 at 18:09
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    $\begingroup$ I think if more NO2 groups were added the repulsions between them would cause steric hindrance, because of which it would not longer be stabilized by resonance. Can this be a correct explanation? $\endgroup$
    – User10001010001000
    Commented Dec 2, 2023 at 18:19
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    $\begingroup$ Indeed, all NO2 groups will be turned somewhat sideways and the resonance partially broken. Whether this will overcome the increased electron withdrawing effect of five nitro groups, I don't know. $\endgroup$
    – Ivan Neretin
    Commented Dec 2, 2023 at 20:04

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You can certainly make pentanitrophenol but it is way more harder to get the -NO2 group to occupy the last remaining m-places. Historically, it has been achieved previously in the early 1900s but the way to synthesis was not straightforward. They had to make many observations, understand the chemical behavior, analyze, assume things and then come to a conclusion. They have to start with halogen substituted aniline and phenol. They didn't know how the nitric acid will behave towards nitrated m-derivates of aniline and phenol and they understood that there was no way the extra -NO2 can be wedged in between o- and p- position. The only exception noticed was tetranitrophenylmethylnitramine discovered in 1889 which was obtained by direct action of fuming nitric acid on two different trinitrophenyldimethylanilines. They observed that the -NO2 group placed at the m-position is very mobile and very liable to substitutition.

Later they achieved tetranitrophenol by oxidizing oximes. In the same way, they subjected m-nitro and m,m-dinitromethylaniline with conc. nitric acid and proved that it is possible that the pentanitro- derivative is possible along with the tetranitro- derivative. Finally, came to m-nitro and m,m-dinitrophenol where they subjected them with conc. nitric acid and it formed the tetranitro and pentanitrophenol as they predicted (m.p 140 °C and 190 °C resp.).

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Once they achieved this, they went ahead and achieved nitrating m-substituted phenol where the substituted group is something beside -NO2 group.

All of their findings can be found in the below journals:

  1. On pentanitrophenylmethylnitramine and tetranitro and pentanitrophenol by Dr. J.J. Blanksma, 1901
  2. Nitration of meta substituted phenols by Dr. J.J. Blanksma, 1906
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