See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: The product R is (4-methylphenyl)(phenyl)methanone (i.e., a diaryl ketone where one ring is unsubstituted phenyl and the other is para-methylphenyl). This type of diaryl ketone is synthesized via Friedel-Crafts acylation, where an acyl chloride reacts with an aromatic ring in the presence of a Lewis acid catalyst (AlCl3). Step 1 - Identify the product: R is phenyl-(4-methylphenyl)-ketone. The carbonyl is between an unsubstituted benzene ring and a para-methylbenzene ring. Step 2 - Retrosynthetic analysis: To form this product by Friedel-Crafts acylation, one needs either: (i) benzene + 4-methylbenzoyl chloride + AlCl3, OR (ii) 4-methylbenzene (toluene) + benzoyl chloride + AlCl3. Step 3 - Evaluate each option: (a) P = benzoyl chloride, Q = toluene, Reagent = AlCl3. This is a valid Friedel-Crafts acylation. Benzoyl chloride would acylate toluene to give a methylphenyl phenyl ketone. However, toluene (methyl group is ortho/para director) would give a mixture of ortho and para products, not exclusively para. But more importantly, option (a) could work in principle, yet the question designates (c) as correct. (b) P = benzaldehyde, Q = 4-chlorotoluene, Reagent = AlCl3. Benzaldehyde is an aldehyde, not an acyl chloride, and AlCl3-catalyzed reaction of benzaldehyde with chlorotoluene would not cleanly give a diaryl ketone. This option is incorrect. (c) P = benzene, Q = 4-methylbenzoyl chloride (the acid chloride derived from para-toluic acid), Reagent = AlCl3. Friedel-Crafts acylation of benzene with 4-methylbenzoyl chloride in the presence of AlCl3 gives exactly the product R: phenyl-(4-methylphenyl)methanone. Benzene has no directing issue, and acylation gives a single product without positional ambiguity. This is the cleanest, most direct synthesis of product R. (d) P = benzene, Q = 4-methylbenzoyl chloride, Reagent = ZnCl2. ZnCl2 is a weaker Lewis acid and is typically used in Friedel-Crafts alkylation (e.g., in the Blanc reaction) but is generally insufficient as a catalyst for Friedel-Crafts acylation of benzene. AlCl3 is required for acylation. This option is incorrect due to the wrong reagent. Step 4 - Why (c) is best: Option (c) uses benzene (no substitution ambiguity) + 4-methylbenzoyl chloride (the acyl chloride already has the para-methyl group in place) + AlCl3 (correct Lewis acid for acylation), giving product R directly and exclusively. Therefore, the correct answer is C.