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 Question:
We must identify the correct nitrating reagent mixture required to convert phenol into picric acid.

Step 2: Detailed Explanation:
Picric acid is the common name for 2,4,6-trinitrophenol.
Because the hydroxyl (-OH) group on phenol is strongly activating and ortho/para directing, the aromatic ring is highly reactive towards electrophilic aromatic substitution.
- If phenol is treated with dilute $HNO_3$ at low temperatures, it undergoes single nitration to yield a mixture of mostly ortho- and para-nitrophenol.
- However, to achieve complete substitution at all available ortho and para positions simultaneously (forming 2,4,6-trinitrophenol), a much harsher nitrating mixture is required.
- Treating phenol with a nitrating mixture of concentrated $HNO_3$ and concentrated $H_2SO_4$ provides the high concentration of nitronium ions ($NO_2^+$) needed to forcefully tri-nitrate the ring.

Step 3: Final Answer:
The conversion requires a mixture of concentrated $HNO_3$ and concentrated $H_2SO_4$, corresponding to option (c).