See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Friedel-Crafts acylation using an anhydride opens the cyclic anhydride ring to give a keto-acid product. When a cyclic anhydride reacts with an arene under AlCl3 conditions, one carbonyl acts as the electrophile attaching to the ring, and the other carbonyl becomes a carboxylic acid after workup. Step 1 - Identify the target compound: The target molecule is 4-(4-methylphenyl... wait, examining the structure more carefully: the top ring has methyl groups at para positions (p-xylene-derived ring), and the side chain is -C(=O)-CH2-CH2-CO2H. This is a 4-oxo-4-(p-tolyl or p-xylyl)butanoic acid, the classic product of Friedel-Crafts acylation with succinic anhydride. Step 2 - Identify the arene needed: The target ring is p-dimethylbenzene (p-xylene), which has two methyl groups in para positions. Step 3 - Identify the acylating agent needed: The side chain -C(=O)-CH2-CH2-CO2H comes from succinic anhydride (a 5-membered cyclic anhydride). Under AlCl3, succinic anhydride acts as the acylating agent, opening to give the keto-acid chain -C(=O)-CH2-CH2-CO2H. Step 4 - Match with options: - Option (a): Uses p-xylene but with a branched diacid chloride (has a methyl branch on the chain), which would NOT give the straight -CH2-CH2- chain in the target. Eliminated. - Option (b): Uses p-xylene (correct arene) + succinic anhydride (correct acylating agent, shown as the cyclic anhydride ring). This correctly gives 4-(3,4-dimethylphenyl or 4-methylphenyl)-4-oxobutanoic acid upon Friedel-Crafts acylation. This matches the target. - Option (c): Uses m-xylene (methyl groups at 1,3 positions), which is the wrong regioisomer of xylene. Eliminated. - Option (d): Uses a xylene isomer + a diketone (cyclobutane-1,3-dione, no ring oxygen), which is not an anhydride and would not undergo Friedel-Crafts acylation in the same manner to give a keto-acid. Eliminated. Step 5 - Conclusion: Option (b) — p-xylene + succinic anhydride with AlCl3 — gives the correct Friedel-Crafts acylation product matching the target compound. Therefore, the correct answer is B.