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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 $\ce{SO3}$ groups occur.

Picric Acid Preparation


Links: Wikipedia, Research paper

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

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  • $\begingroup$ but how do the nitro groups replace the sulphonic acid ones? I believe desulphonylation requires excess of water hence dilute acidic medium but in the book it's written conc. HNO3. Please help. $\endgroup$ – Ramesh Agarwal Jan 2 at 11:55
  • $\begingroup$ @RameshAgarwal Look up IPSO Substitution. I believe the link is attached in this answer. $\endgroup$ – Vamsi Krishna Jun 27 at 6:28
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The
OH group of phenol is so activating that the nitration mixture rapidly overheats even with dilute nitric acid. The result is oxidation with the evolution of copious amounts of nitrogen oxides.

Instead, you heat the phenol with sulfuric acid to get mostly phenol-2,4-disulfonic acid.The sulfonic acid groups are so electron-withdrawing that nitration can proceed more slowly and at a controllable rate.

The first product is probably 6-nitrophenol-2,4-disulfonic acid.

In subsequent steps, the nitronium ion attacks at the 2- and 4-positions, displacing the sulfonic acid groups and forming 2,4,6-trinitrophenol (picric acid).

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