Nowadays picric acid is prepared by treating phenol first with concentrated sulphuric acid and then with concentrated nitric acid, rather than with concentrated nitric acid alone. Which of the following is the reason?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Direct nitration with concentrated nitric acid gives picric acid in poor yield.
Picric acid is 2,4,6-trinitrophenol. Concentrated nitric acid does convert phenol into it, but the yield is poor. So phenol is first sulphonated with concentrated H2SO4 to phenol-2,4-disulphonic acid, and this is then treated with concentrated HNO3 to get 2,4,6-trinitrophenol.
Why the other options are incorrect
Option (1) With concentrated nitric acid, phenol gives mainly a mixture of ortho and para nitrophenols. — Incorrect. The o- and p-nitrophenol mixture is formed with dilute nitric acid at low temperature (298 K), not with the concentrated acid.
Option (2) The –OH group deactivates the benzene ring, so phenol does not react with concentrated nitric acid. — Incorrect. The –OH group activates the ring towards electrophilic substitution, and phenol does react with concentrated nitric acid.
Option (3) With concentrated nitric acid, phenol gives 2,4-dinitrophenol and not 2,4,6-trinitrophenol. — Incorrect. The product with concentrated nitric acid is 2,4,6-trinitrophenol; the problem is only its low yield.
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “With concentrated nitric acid, phenol is converted to 2,4,6-trinitrophenol. The product is commonly known as picric acid. The yield of the reaction product is poor. Nowadays picric acid is prepared by treating phenol first with concentrated sulphuric acid which converts it to phenol-2,4-disulphonic acid, and then with concentrated nitric acid to get 2,4,6-trinitrophenol.”