See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Dewar benzene (bicyclo[2.2.0]hexa-2,5-diene) is a valence isomer of benzene. It is a strained bicyclic compound that, upon treatment with a proton acid (H+) or Lewis acid (AlCl3), undergoes acid-catalyzed isomerization (ring opening) to give the thermodynamically most stable C6H6 isomer. Step 1: Identify compound (A). Dewar benzene has the molecular formula C6H6, the same as benzene. It consists of two fused cyclobutene rings (bicyclo[2.2.0]hexa-2,5-diene), which is highly strained. Step 2: Reaction with H+ or AlCl3. Under acidic conditions (H+ or Lewis acid AlCl3), the strained central C-C single bond of Dewar benzene is protonated or activated, leading to ring opening. The carbocation intermediate rearranges to relieve strain and achieve aromaticity. Step 3: Product identification. The major product is benzene (option a), which is the thermodynamically most stable C6H6 isomer due to full aromatic stabilization. The driving force is the gain of approximately 36 kcal/mol of resonance stabilization energy upon aromatization. Step 4: Why other options fail: - (b) Bicyclopropylidene (C6H8) has a different molecular formula and is not an isomer of benzene/Dewar benzene. - (c) The structure shown has a different connectivity and would not be the major product of simple acid-catalyzed isomerization. - (d) The epoxide structure shown is not a plausible major product of acid-catalyzed ring opening of Dewar benzene. Since (A) and (B) are isomers (both C6H6), and the reaction with H+/AlCl3 drives conversion to the most stable isomer, benzene is the major product. Therefore, the correct answer is A.