Question

Write the structures of (A), (B), and (C) in the following reaction:
\[ C_6H_5NO_2 \xrightarrow{Fe/HCl} (A) \xrightarrow[\text{273 K}]{NaNO_2 + HCl} (B) \xrightarrow[\Delta]{H_2O/H^+} (C) \]

Hint:

Temperature control is crucial in Step 2. If the temperature rises above $5^\circ C$, the diazonium salt becomes unstable and hydrolyzes directly to Phenol, evolving Nitrogen gas.

Answer 1

Step-by-Step Analysis: Step 1: Reduction of Nitrobenzene
Reactant: Nitrobenzene ($C_6H_5NO_2$)
Reagent: $Fe/HCl$ (Acidic reduction)
Reaction: Nitro group ($-NO_2$) is reduced to Amino group ($-NH_2$).
Structure (A): Aniline ($C_6H_5NH_2$) Step 2: Diazotization
Reactant: Aniline (A)
Reagent: $NaNO_2 + HCl$ at $0-5^\circ C$ (273 K)
Reaction: The primary aromatic amine reacts with nitrous acid to form a diazonium salt.
Structure (B): Benzene Diazonium Chloride ($C_6H_5N_2^+Cl^-$) Step 3: Hydrolysis
Reactant: Benzene Diazonium Chloride (B)
Reagent: $H_2O/H^+$ with heat ($\Delta$)
Reaction: The diazonium group ($-N_2^+Cl^-$) is an excellent leaving group and is replaced by the hydroxyl group ($-OH$) upon warming with water. $N_2$ gas is evolved.
Structure (C): Phenol ($C_6H_5OH$) Summary of Structures:

• (A): Aniline
• (B): Benzene Diazonium Chloride
• (C): Phenol