See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Step 1: Identify the first reaction - allylic bromination. CH3-CH=CH2 reacted with Br2 under hv at low concentration undergoes allylic bromination (NBS-type free radical mechanism with Br2 at low conc/hv). The allylic position is the CH3 group (C1 or C3). The major allylic bromide formed is CH2=CH-CH2Br (allyl bromide, 3-bromoprop-1-ene), since the allylic radical is more stable at the terminal carbon giving the primary allylic bromide as the dominant product (allyl bromide). Step 2: Reaction with Mg in dry ether. CH2=CH-CH2Br + Mg (dry ether) --> CH2=CH-CH2MgBr (allylmagnesium bromide, a Grignard reagent). Step 3: Reaction with acetone (CH3-C(=O)-CH3) followed by NH4Cl workup. The Grignard reagent CH2=CH-CH2MgBr attacks the carbonyl of acetone: CH2=CH-CH2MgBr + (CH3)2C=O --> after NH4Cl hydrolysis --> CH2=CH-CH2-C(OH)(CH3)2 This gives 2-methylbut-4-en-2-ol: H2C=CH-CH2-C(OH)(CH3)(CH3), i.e., option (c) structure. Step 4: Reaction with H+ and heat (acid-catalyzed dehydration, Delta). The tertiary alcohol H2C=CH-CH2-C(OH)(CH3)2 undergoes acid-catalyzed dehydration. Under H+/Delta, elimination of water occurs. The carbocation formed at the tertiary carbon can rearrange or direct elimination. The OH is on a tertiary carbon; protonation and loss of water gives a tertiary carbocation: H2C=CH-CH2-C+(CH3)2. This allylic/tertiary carbocation is delocalized. Elimination (E1) from this carbocation by loss of a proton from the adjacent methyl group gives a conjugated diene. Loss of H+ from CH3 group adjacent to the carbocation gives: H2C=CH-CH=C(CH3)-CH3, which is (E)- or (Z)-penta-1,3-diene with a methyl branch - specifically 2-methylpenta-1,3-diene... actually the product is H2C=CH-CH=C(CH3)2 ... re-examining: carbocation is at C4 (numbered from the vinyl end): C1=C2-C3-C4+(CH3)2. Loss of H from one of the CH3 groups on C4 gives C1=C2-C3=C4(CH3)-CH3, which is H2C=CH-CH=C(CH3)-CH3. This is a conjugated diene and corresponds to option (b). The major product under thermodynamic control (H+, Delta) is the more stable conjugated diene: H2C=CH-CH=C(CH3)-CH3, which is option (b). Why other options fail: - Option (a): Would require a different mechanism producing an exocyclic alkene, not consistent with this pathway. - Option (c): This is the alcohol intermediate before dehydration, not the final product after H+/Delta. - Option (d): Requires a completely different reaction sequence and structural outcome. Therefore, the correct answer is B.