During the preparation of picric acid,sulphonation is carried out first and after that nitration.Why?And why does the sulphonic group get replaced by the nitro group?

1 Answer
Ernest Z.
Apr 25, 2018

Here's what I get.

Explanation:

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.

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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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