Question

Write the structures of (A), (B), and (C) in the following reaction:
Reaction Scheme: \[ (C) \xleftarrow{\text{conc. } HNO_3} \text{Phenol} \xrightarrow{\text{dil. } HNO_3} (A) + (B) \]

Hint:

Distinction: o-Nitrophenol is steam volatile due to intramolecular hydrogen bonding (chelation), whereas p-Nitrophenol is less volatile due to intermolecular hydrogen bonding (association). They can be separated by steam distillation.

Answer 1

Analysis of the Reaction:
This question deals with the Nitration of Phenol. The product depends on the concentration of the acid used.

• Reaction with Dilute $HNO_3$:
  When phenol is treated with dilute nitric acid at low temperature (298 K), it undergoes mono-nitration to yield a mixture of ortho- and para- isomers.
  • (A) o-Nitrophenol: The $-NO_2$ group attaches at the ortho position (adjacent to $-OH$).
  • (B) p-Nitrophenol: The $-NO_2$ group attaches at the para position (opposite to $-OH$).
• Reaction with Concentrated $HNO_3$:
  With concentrated nitric acid, the reaction is vigorous, and poly-nitration occurs at all activating positions (ortho and para).
  • (C) 2,4,6-Trinitrophenol: Commonly known as Picric Acid.

Structures:

• (A): o-Nitrophenol ($C_6H_4(OH)(NO_2)$ - ortho isomer)
• (B): p-Nitrophenol ($C_6H_4(OH)(NO_2)$ - para isomer)
• (C): 2,4,6-Trinitrophenol (Picric Acid)