Question

Nowadays picric acid is prepared by treating phenol with concentrated Sulphuric acid which converts it into $phenol - 2,4 - disulphonic$ acid, and then with concentrated nitric acid to get $2,4,6 - trinitrophenol$. Can you write the equations of reaction involved?

Answer 1

First of all we should know what a picric acid is. Since the mode of its preparation is given we will look into how the process goes. In the end we will write the equation of it. We will also see the compound it is forming in between.

Complete answer:
Step1. Picric acid is a nitro compound. It is composed of phenol and has three nitro substitutes at $2,4,6$ positions. It is often used in explosives. It is often used in antiseptic drugs and fixatives. It is derived from phenol and $1,3,5 - trinitrobenzene$ . It is also conjugate acid of picrate anion.
Step2. Now when we will react with the Sulphuric acid with the phenol, it will pass two Sulphuric acid at 2 and 5 position. It will convert into phenol - 2,4 - disulphonic acid. We should not use dilute acid with phenol. This reaction is only possible when we use concentrated Sulphuric acid. This happened because the Sulphuric acid behave as an oxidizing agent and protecting group at the same time
Step3: $phenol - 2,4 - disulphonic$ it will be reacted with concentrated Nitric acid which will finally give us $2,4,6 - trinitrophenol$. The $NO_2^ +$ will work as an electrophile and Nitric acid is an oxidizing agent. So it will remove $S{O_3}H$ present and will add $N{O_3}$ at $2,4,6$ positions.

Note:
Picric acid is also called TNP in short. It is extensively used in dyes. When glucose, sodium bicarbonate and picric acids are combined and heated, they form the red color. The red color can be used to measure the glucose level added.