Question

Which from following compounds is obtained when phenol reacts with dilute nitric acid at low temperature?

• A. o-Nitrophenol only
• B. p-Nitrophenol only
• C. 2, 4, 6-trinitrophenol
• D. Mixture of ortho and para nitrophenols

Hint:

Phenol + Dilute $\text{HNO}_3 \rightarrow$ Ortho and Para isomers (Mononitration).
Phenol + Conc. $\text{HNO}_3 + \text{H}_2\text{SO}_4 \rightarrow$ Picric Acid (2,4,6-Trinitrophenol).

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question refers to the electrophilic aromatic substitution (specifically nitration) of phenol under mild conditions (dilute nitric acid and low temperature). We need to identify the resulting product(s).

Step 2: Detailed Explanation:
Phenol contains a hydroxyl group (-OH) attached directly to the benzene ring. The lone pairs of electrons on the oxygen atom strongly engage in resonance with the ring. This $+R$ (resonance) effect significantly increases electron density on the ring, making it highly activated towards electrophilic attack.
Because the electron density increases most prominently at the ortho and para positions, the -OH group is a strongly ortho/para-directing substituent.
When phenol is reacted with dilute nitric acid ($\text{HNO}_3$) at a low temperature (usually around 298 K or 25°C), the mild conditions lead to single nitration. The electrophile ($\text{NO}_2^+$) attacks the activated ortho and para positions, yielding a mixture of ortho-nitrophenol and para-nitrophenol.
(Note: These two isomers can be easily separated by steam distillation because o-nitrophenol is steam volatile due to intramolecular H-bonding, whereas p-nitrophenol forms intermolecular H-bonds and is not steam volatile).
If phenol were reacted with concentrated nitric acid in the presence of concentrated sulfuric acid (a strong nitrating mixture), the severe conditions would force multi-substitution, yielding 2,4,6-trinitrophenol (commonly known as Picric acid).

Step 3: Final Answer:
A mixture of ortho and para nitrophenols is obtained, matching option (d).