Question

Conc. HNO\(_3\) 

Hint:

Concentrated nitric acid causes electrophilic nitration of phenol, predominantly at the ortho and para positions, leading to trinitrophenol (picric acid).

Answer 1

Phenol Nitration

When phenol is treated with concentrated nitric acid (HNO₃), nitration predominantly occurs at the ortho and para positions relative to the hydroxyl group (-OH). The hydroxyl group's electron-donating nature activates the aromatic ring, increasing its susceptibility to electrophilic substitution and nitration.

Reaction:

Nitration of phenol using concentrated nitric acid yields 2,4,6-Trinitrophenol (picric acid), a highly reactive substance. The reaction is depicted as: \[ \text{C}_6\text{H}_5\text{OH} + 3\text{HNO}_3 \rightarrow \text{C}_6\text{H}_2(\text{NO}_2)_3\text{OH} + 3\text{H}_2\text{O} \]

Mechanism:

The hydroxyl group on phenol acts as an electron-donating substituent, activating the benzene ring for electrophilic attack. During nitration, the nitronium ion (\( \text{NO}_2^+ \)) targets the ortho and para positions of the phenol ring, leading to the formation of 2,4,6-Trinitrophenol, also identified as picric acid.

Conclusion:

The nitration of phenol with nitric acid results in the synthesis of 2,4,6-Trinitrophenol (picric acid), a compound characterized by its high reactivity due to the activating influence of the hydroxyl group. The primary product is formed at the positions ortho and para to the hydroxyl group.