See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the starting material: The compound shown is 3-phenylglutaric anhydride. It consists of a benzene ring bearing a CH2 group at one end, connected to a CH center, which bears another CH2 group. These two CH2 groups each carry a carbonyl that together form a cyclic (five-membered) anhydride ring: Ph-CH2-CH(CH2-C(=O)-O-C(=O)) — i.e., the anhydride ring contains -C(=O)-O-C(=O)-CH2-CH(CH2Ph)- closing into a 5-membered anhydride. Step 2 - Friedel-Crafts acylation mechanism: AlCl3 activates one of the anhydride carbonyls to generate an acylium ion (or acyl-AlCl3 complex). Intramolecular Friedel-Crafts acylation occurs when the acylium attacks the pendant benzene ring. Step 3 - Determine which carbonyl reacts and ring size formed: In 3-phenylglutaric anhydride, the two carbonyls are at C1 and C5 of the glutaric chain, with the phenyl on C3. The CH2 adjacent to the phenyl ring (benzylic CH2) connects to one carbonyl. For intramolecular acylation onto the ortho position of the benzene ring: - Attack by the carbonyl closer to the benzene (through the benzylic CH2, i.e., 2 carbons away from the ring) would form a 6-membered transition state and produce a 6-membered carbocyclic ring fused to benzene — giving a tetralone (beta-tetralone type, i.e., 2-tetralone or 1-tetralone skeleton). - Attack by the distal carbonyl would require a longer chain and form a less favorable ring. Step 4 - Product identification: The intramolecular acylation via the benzylic carbon (PhCH2- attacking the ring with the acylium derived from the adjacent carbonyl) forms a 6-membered ring fused to the benzene ring, producing a 1-tetralone framework. The remaining -CH-CO2H (after ring closure the other carboxylate end becomes a free carboxylic acid upon hydrolysis/workup of the mixed anhydride oxygen) is at C3 of the cyclohexanone ring (beta to the ketone carbonyl), giving 3-carboxy-1-tetralone. Step 5 - Why not a 5-membered ring (options c, d): Formation of a 5-membered ring would require direct attack through only one carbon, which is geometrically strained and disfavored compared to the 6-membered transition state. Friedel-Crafts cyclizations strongly prefer 6-membered ring formation (Baldwin's rules and thermodynamic preference). Step 6 - Why option (b) over option (a): In option (a), CO2H is alpha (C2) to the ketone; in option (b), CO2H is at C3 (beta). Given the connectivity of 3-phenylglutaric anhydride, the acylium forms at C1, attacks the ring, the ring closure uses C1-C2(benzylic)-ring bond, and the remaining chain -CH-CO2H ends up at C3 (one carbon away from the benzylic position, which is C2 in the new ring, hence CO2H at C3). This matches option (b). Therefore, the correct answer is B.