See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the starting material: 1-chloro-4-nitrobenzene (para-chloronitrobenzene). The NO2 group is at C1 and Cl is at C4. Step 2 - Reaction with aq NH3 (A): The chloro group at C4 undergoes nucleophilic aromatic substitution (SNAr) because the nitro group at C1 activates the ring toward nucleophilic attack at C4. The Cl is replaced by NH2, giving 4-nitroaniline (1-amino-4-nitrobenzene) as product A. Step 3 - Reaction with Br2/H2O (B): The amino group (-NH2) is a powerful ortho/para director. Bromination of 4-nitroaniline with Br2/H2O gives tribromination? Actually with 4-nitroaniline, the -NH2 group directs to ortho and para positions relative to itself. The nitro group is already at the para position relative to -NH2. So bromination occurs at the two ortho positions (C3 and C5 relative to NH2, which are C3 and C5 with NH2 at C1 and NO2 at C4). This gives 2,6-dibromo-4-nitroaniline (i.e., NH2 at C1, Br at C2, Br at C6, NO2 at C4) as product B. With excess Br2/H2O, the -NH2 group in aniline undergoes dibromination at both ortho positions predominantly yielding 2,6-dibromo-4-nitroaniline. Step 4 - Diazotization and reduction (C): Treatment with (i) NaNO2 + HCl converts the -NH2 to a diazonium salt (-N2+Cl-). Then (ii) H3PO2 (hypophosphorous acid) replaces the diazonium group with H (reductive dehalogenation of diazonium). This removes the amino/diazonium group entirely, replacing it with H. The product C therefore has: H at C1 (where NH2 was), Br at C2, NO2 at C4, Br at C6. This is 1,3-dibromo-5-nitrobenzene, i.e., NO2 at C1(renumbered), Br at C3 and C5 - which matches option (b): a benzene ring with NO2 at top (C1) and Br at C3 and C5 (meta to each other, both meta to NO2). Why other options fail: (a) shows two Br groups adjacent (ortho to each other) with NO2, which doesn't match the 2,6-dibromo pattern after diazonium removal. (c) shows two Br groups ortho to NO2 and ortho to each other. (d) shows Br at C2 and C4 relative to NO2, inconsistent with the substitution pattern. Option (b) correctly represents 1,3-dibromo-5-nitrobenzene arising from removal of the amino group from 2,6-dibromo-4-nitroaniline. Therefore, the correct answer is B.