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

Nitration of Phenol 

When phenol reacts with concentrated nitric acid (HNO₃), it undergoes nitration primarily at the ortho and para positions relative to the hydroxyl group (-OH). The electron-donating effect of the hydroxyl group activates the aromatic ring for electrophilic substitution, making the ring more reactive towards nitration.

Reaction:

The nitration of phenol with concentrated nitric acid produces **2,4,6-Trinitrophenol (picric acid)**, which is a highly reactive compound. The reaction can be represented 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 is an electron-donating group that activates the benzene ring, making it more susceptible to electrophilic attack. During nitration, the nitronium ion (\( \text{NO}_2^+ \)) attacks the ortho and para positions of the phenol ring, resulting in the formation of **2,4,6-Trinitrophenol**, also known as **picric acid**.

Conclusion:

The nitration of phenol with nitric acid results in the formation of **2,4,6-Trinitrophenol (picric acid)**, which is a highly reactive compound due to the electron-donating effect of the hydroxyl group. The major product is formed at the ortho and para positions relative to the hydroxyl group.