See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the starting material: The starting material is anthrone (anthracen-9(10H)-one), which has three linearly fused six-membered rings with a ketone at the 9-position of the central ring. Step 2 - Reaction with MeMgI (Grignard reagent): Methylmagnesium iodide adds to the carbonyl carbon (C-9) of anthrone. After aqueous workup with NH4Cl, this gives the tertiary alcohol (A): 9-methyl-9-anthracenol (9-hydroxy-9-methylanthracene, i.e., 10-methyl-9(10H)-anthracenol or 9-methyl-9,10-dihydro-9-anthracenol). Step 3 - Acid-catalyzed dehydration with heat (H+, Delta): The tertiary alcohol (A) undergoes acid-catalyzed dehydration. The hydroxyl group at C-9 leaves as water. The resulting carbocation at C-9 can lose a proton from the methyl group to give an exocyclic alkene, OR more favorably, the system can rearrange/eliminate to give an extended aromatic system. Step 4 - Major product formation: Under acidic conditions with heat, the tertiary alcohol loses water. The elimination can occur such that the double bond forms between C-9 and C-10 of the central ring, restoring aromaticity to the central ring while incorporating the methyl group. However, the more thermodynamically favorable pathway involves the carbocation at C-9 undergoing a 1,2-hydride shift or ring expansion. Actually, the key transformation here is: the 9-methyl-9-anthracenol under H+/Delta undergoes dehydration followed by a [1,5]-H shift or electrophilic cyclization. The methyl carbocation intermediate can undergo intramolecular Friedel-Crafts type ring closure with the adjacent aromatic ring, leading to ring expansion to give a tetracyclic product (naphthacene/tetracene skeleton) with a methyl substituent - this is a known synthesis of substituted naphthacenes from anthrone via Grignard addition and acid-catalyzed ring expansion. Step 5 - Structure of product B: The major product is a linearly fused tetracyclic aromatic compound (tetracene framework) bearing a methyl group at the terminal ring, corresponding to option (d). This is consistent with the acid-catalyzed dehydration and cyclization/aromatization of 9-hydroxy-9-methylanthracene, which gives 5-methyltetracene (or 1-methyltetracene) as the major product via ring expansion. Step 6 - Why other options fail: (a) shows only three rings with exocyclic methylene - this would be a minor product or wrong regiochemistry; (b) shows unsubstituted tetracene - no methyl group, inconsistent with MeMgI addition; (c) shows four rings with methyl but the substitution pattern differs from (d) - (d) is the correct regiochemical outcome of the ring expansion placing the methyl at the terminal position. Therefore, the correct answer is D.