See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: To convert p-chloronitrobenzene to p-chlorophenol, we need to replace the nitro group (-NO2) with a hydroxyl group (-OH) while retaining the chloro substituent. Step 1: Reduce the nitro group to an amine using Fe/HCl (a classical nitro reduction). This converts p-chloronitrobenzene into p-chloroaniline. Step 2: Treat with NaOH to neutralize and free the amine (since Fe/HCl gives the ammonium salt, NaOH liberates the free amine: p-chloroaniline). Step 3: Diazotization — treat p-chloroaniline with NaNO2 and H2SO4 (or HCl) at 0–5°C to form the diazonium salt: p-chlorobenzenediazonium hydrogen sulfate. Step 4: Hydrolyze the diazonium salt by warming with water (H2O, heat). The diazonium group (-N2+) is replaced by -OH, yielding p-chlorophenol. Why other options fail: - Option (a): H3PO2 is a reducing agent used to replace -N2+ with -H (deamination), which would give chlorobenzene, not p-chlorophenol. - Option (c): Ethanol as the last reagent would reduce the diazonium salt to give the arene (chlorobenzene), not p-chlorophenol. - Option (d): NaOH/heat on p-chloronitrobenzene could cause nucleophilic aromatic substitution of Cl with OH only if conditions are extreme and the ring is activated, but this is not a reliable or standard synthesis route and would not selectively give p-chlorophenol cleanly from p-chloronitrobenzene. The correct sequence is: reduce NO2 to NH2 (Fe, HCl), liberate free amine (NaOH), diazotize (NaNO2, H2SO4), then hydrolyze diazonium with H2O/heat to get -OH. Therefore, the correct answer is B.