See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: Acid-catalyzed dehydration of allylic/benzylic alcohols and ring expansion via carbocation intermediates, combined with the thermodynamic drive toward aromatic stabilization. Step 1: Identify the starting material. The compound is (cyclopenta-2,4-dien-1-yl)methanol — a cyclopentadiene ring bearing a -CH2OH substituent at the sp3 carbon (C5 position, between the two double bonds). Step 2: Protonation of the hydroxyl group under acidic conditions converts -OH to -OH2+, which is a good leaving group. Loss of water generates a carbocation at the exocyclic -CH2+ position. Step 3: This primary carbocation is actually stabilized because it is allylic/homoaromatic relative to the cyclopentadienyl system. More importantly, the resulting CH2+ adjacent to the cyclopentadienyl system allows for a 1,2-hydride shift or ring expansion. Step 4: Ring expansion mechanism — the exocyclic carbocation (CH2+) is adjacent to the cyclopentadiene ring. A 1,2-carbon shift (ring expansion) occurs: the C-C bond of the ring migrates to the carbocation center, expanding the five-membered ring to a six-membered ring carbocation (cyclohexadienyl cation, i.e., the arenium/cyclohexadienyl cation). Step 5: The cyclohexadienyl cation loses a proton (H+) to restore aromaticity, yielding benzene, which is highly thermodynamically stable (aromatic). Step 6: Why other options fail: - (a) Methylenecyclopentadiene would form if only simple dehydration occurred without ring expansion, but this is not the favored pathway under these conditions. - (c) Cyclohexadiene and (d) cyclohexene would require different starting materials or different reaction conditions and do not arise from this mechanism. - The strong thermodynamic driving force of aromatization (gaining ~36 kcal/mol resonance energy) makes benzene the overwhelmingly favored product. Therefore, the correct answer is B.