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?
Here's what I get.
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).