See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

To synthesize 4-isopropylbenzonitrile, we need a benzene ring with an isopropyl group and a nitrile (CN) group in the para position relative to each other. Concept: The best strategy involves introducing the isopropyl group first (as a directing group), then introducing the nitrile via a diazonium intermediate. Step-by-step reasoning for option C: Step 1: Benzene + (CH3)2CHCl, AlCl3 → isopropylbenzene (cumene) via Friedel-Crafts alkylation. The isopropyl group is an ortho/para director. Step 2: HNO3, H2SO4 → nitration of isopropylbenzene. The isopropyl group directs the nitro group predominantly to the para position, giving 4-isopropylnitrobenzene. Step 3: Fe, HCl → reduction of the nitro group to an amino group, giving 4-isopropylaniline. Step 4: NaNO2/HCl (cold) → diazotization of the amine to form the diazonium salt, 4-isopropylbenzenediazonium chloride. Step 5: KCN → Sandmeyer-type reaction (or simply reaction with KCN/CuCN) replacing the diazonium group with CN, giving 4-isopropylbenzonitrile. Why other options fail: - Option (a): Direct bromination followed by KCN substitution on aryl bromide is not straightforward under simple conditions; nucleophilic aromatic substitution with KCN on aryl bromides requires harsh conditions and does not selectively give nitrile. Also, bromination would give a mixture of ortho/para products. - Option (b): Steps 4 and 5 use NaOH followed by NaNO2/HCl/H2O, which would convert the diazonium to a phenol (reaction with water), not a nitrile. - Option (d): Starting with nitration first, then Friedel-Crafts alkylation is problematic because the nitro group is a strong deactivator and meta-director, and Friedel-Crafts reactions do not work well on deactivated rings. Additionally, step 5 uses CuCN (Sandmeyer), which could work, but the overall sequence is flawed due to the ordering issue. Option (c) is the best and most reliable sequence: Friedel-Crafts alkylation → para-nitration (directed by isopropyl) → reduction to amine → diazotization → replacement with CN via KCN. Therefore, the correct answer is C.