Question

On prolonged nitration of phenol with conc. $HN{O_3}$ and conc. ${H_2}S{O_4}$ mixture, the product obtained is :
A. o-nitrophenol
B. p-nitrophenol
C. picric acid
D. nitrobenzene

Answer 1

Hint : We have studied that on prolonged nitration of phenol with conc. $HN{O_3}$ and conc. ${H_2}S{O_4}$ mixture, is a very important reaction of formation of $2,4,6 -$ trinitrophenol which is also known as picric acid. Sulphuric acid dissociates to form proton and $HS{O^ - }_4$ ion and also helps in generation of nitronium ion $\left( {N{O^ + }_2} \right)$ from nitric acid . Picric acid is very important as an explosive and used by the military.

Complete step by step solution:
When concentrated nitric acid reacts with phenol it produces o-nitrophenol and p-nitrophenol and does not produce picric acid . But the yield of this reaction is very poor. So for enhancing the yield of the reaction phenol is firstly treated with concentrated sulphuric acid which gives phenol$- 2,4 -$ disulphuric acid and then it is further treated with concentrated nitric acid which produces $2,4,6 -$ trinitrophenol as major product. The product is acidic because of the presence of $OH$ group which forms hydrogen ions on dissociation. The reaction of phenol with conc. $HN{O_3}$ and conc. ${H_2}S{O_4}$ is given below;
On prolonged nitration of phenol with conc. Nitric acid and conc. Sulphuric acid mixture, the product obtained is picric acid.

Hence option C is the correct answer to this problem.

Note: The product obtained from the given reaction has various uses. Let's take a look at some uses of picric acid.$2,4,6 -$ trinitrophenol is a yellow crystalline solid which has no odour. Picric acid is a nitrated organic compound. It is useful in military explosives, in medicine such as antiseptic, dyes and burn treatments. It is also used in astringent properties. It is an important component of analytical reagents for testing of blood and urine.