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?

  • 2
    $\begingroup$ 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

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)

  • $\begingroup$ 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 '20 at 11:55
  • $\begingroup$ @RameshAgarwal Look up IPSO Substitution. I believe the link is attached in this answer. $\endgroup$ – Vamsi Krishna Jun 27 '20 at 6:28

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).


Here is the small scale manufacturing process we are giving to our students in their first organic chemistry laboratory, for their synthetizing and destroying picric acid.

Take $1$ mL concentrated nitric acid $\ce{HNO3}$ ($65$%) in a $\pu{20 mL}$ test tube. Add with care, and in several portions, a tiny amount of phenol (maximum $0.02$ g). The reaction is highly exothermic ! Wait two minutes, then cool down the mixture. Add enough water to nearly fill the test tube (about $20$ mL). The picric acid is an oil, that soon gets solid after one minute or two. Throw the liquid away. Half fill the tube with $\pu{10 mL}$ water. Heat to $\pu{100°C}$ : the picric acid gets dissolved. Wait : nice yellow needles are produced in the tube by slow cooling. It is picric acid, which can be filtered in case of necessity. But don't do it presently. Decant the aqueous solution, and keep the solid in the tube

As picric acid is an explosive stuff when dry, don't dry it and don't save it. Instead, add $\pu{1 mL}$ water to the whole sample of picric acid from the preceding experiment. Add $\ce{1 mL NaOH 1 M}$. This produces a yellow solution of sodium picrate. Add one grain $\ce{Na2S}$. Heat for $2$ minutes. The solution darkens and becomes red-orange. The picric acid has been reduced into picramic acid ($\ce{2-amino-4.6-dinitrophenol}$), which is no more an explosive substance. $\ce{Na2S}$ has been oxidized into $\ce{Na2S2O3}$.

  • $\begingroup$ This doesn't answer the question.It just provides a fairly safe small-scale alternative the suggested process in the question. $\endgroup$ – matt_black May 9 at 20:25

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