Question

Which of the following reagents is used in the conversion of phenol into picric acid?

• A. dil. HNO$_3$
• B. dil. HNO$_2$
• C. conc. HNO$_3$ + conc. H$_2$SO$_4$
• D. conc. H$_2$SO$_4$

Hint:

Although direct nitration with conc. acids gives picric acid, the modern industrial yield is poor due to oxidative degradation of the ring. Industrially, phenol is first sulfonated to phenol-2,4-disulfonic acid, then treated with conc. nitric acid to get a better yield.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Picric acid is 2,4,6-trinitrophenol. To add three nitro groups to the phenol ring, a strong nitrating mixture is required.

Step 2: Formula Application:
The nitrating mixture consists of concentrated nitric acid and concentrated sulphuric acid.

Step 3: Explanation:
Phenol is highly reactive due to the $-OH$ group. When treated with a mixture of conc. $HNO_3$ and conc. $H_2SO_4$, nitration occurs at both ortho positions and the para position simultaneously, yielding picric acid. If dilute $HNO_3$ were used, you would get a mixture of o-nitrophenol and p-nitrophenol instead.

Step 4: Final Answer:
The reagent is conc. HNO$_3$ + conc. H$_2$SO$_4$.