See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Solution: Diazotization and Coupling Reaction **Identify the reaction type:** This is a diazotization followed by azo coupling reaction of an aromatic amine. **Step 1: Diazotization** An aromatic amine reacts with $NaNO_2$ and $HCl$ to form a diazonium salt: $$Ar-NH_2 + NaNO_2 + 2HCl \rightarrow Ar-N_2^+ Cl^- + 2H_2O$$ **Step 2: Azo coupling** The diazonium ion couples with an activated aromatic compound (phenol or aniline derivative). Coupling occurs at the **ortho/para positions** relative to electron-donating groups. **Step 3: Analyze option A** - Starting material: $p$-nitroaniline ($NO_2$ and $NH_2$ in para positions) - Coupling partner: phenol ($OH$ group) - The diazonium from $p$-nitroaniline couples with phenol - $OH$ is strongly electron-donating (ortho/para director) - Coupling occurs at ortho position relative to $OH$, giving: $4-nitrophenyl$ coupled at ortho to phenolic $OH$ **Why A is correct:** Option A shows $OH$ and $NO$ in para positions on the azo product, which is the expected regiochemistry when a $p$-nitrobenzenediazonium couples with phenol (ortho coupling to the activating $OH$ group). **Why others fail:** - **(B):** $OMe$ would require methylation of the product (not shown in mechanism) - **(C):** Missing substituents inconsistent with reactants - **(D):** Incorrect substitution pattern