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My textbook says "Nowadays picric acid is prepared by treating phenol first with concentrated sulfuric acid which converts it to phenol-2,4-disulfuric acid and then with concentrated nitric acid to get 2,4,6-trinitrophenol"

My question: Why is this a better method than simply treating phenol with concentrated nitric acid to obtain picric acid?

What I think is that maybe because $\ce{NO2}$ is a stronger deactivating group than $\ce{SO3H}$, it is more difficult to add 3 $\ce{NO2}$ to a benzene ring directly.

But we would also have to substitute $\ce{SO3H}$ for $\ce{NO2}$. Wouldn't that be hard?

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    $\begingroup$ Either the book or you have a typo where it says "and then with concentrated picric acid", it should be "and then with concentrated nitric acid" $\endgroup$ – DavePhD Jan 22 '16 at 16:20
  • $\begingroup$ Guess that's because conc nitric acid is too aggressive and would produce a significant amount of other isomeric nitrophenols. $\endgroup$ – Ivan Neretin Jan 22 '16 at 16:32
  • $\begingroup$ @IvanNeretin is this a case of ipso substitution? Is it possible at room temperature? $\endgroup$ – Aditya Dev Jan 22 '16 at 17:48
  • $\begingroup$ Actually it's completely the opposite. Phenol is so strongly activating that you get far too much substitution and tend to end up with a big mess of high molecular weight solids that don't separate easily and aren't picric acid. $\endgroup$ – bon Jan 22 '16 at 19:52
  • $\begingroup$ Phenolic group can be oxidised and polymerisation occurs. $\endgroup$ – Mithoron Jan 22 '16 at 20:25
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The benzene ring in phenol is highly activated toward electrophilic substitution and hence attempts to directly nitrate it result in charring and copious evolution of oxides of nitrogen. The reaction is highly exothermic and difficult to control.

To reduce the reactivity, the phenol is first mono-sulfonated ( some of the product which is substituted may also be used). The products are ortho and para isomers. The para isomer is separated and then nitrated. The nitration is comparatively far smoother (easier to handle). Ipso substitution of -SO3 groups occur.

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Links: Wikipedia, Research paper

Link: (For ipso substitution): PDF, Wikipedia (see subsection: Ipso substitution)

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