See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Concept: Gilman reagent (organocuprate) formation and conjugate (1,4-Michael) addition. Step 1: Reagent identification. CH3MgBr with CuCl forms a cuprate-like species (a Gilman-type reagent in situ: (CH3)2CuLi equivalent using CuCl). Organocuprates are well known to undergo 1,4-conjugate addition to alpha,beta-unsaturated carbonyl compounds rather than 1,2-addition. Step 2: Substrate identification. The starting material is cyclohex-2-en-1-one (2-cyclohexenone), an alpha,beta-unsaturated cyclic ketone with the carbonyl at C1 and the double bond between C2 and C3. Step 3: Reaction pathway. The cuprate reagent delivers the methyl group to the beta-carbon (C3, the carbon farther from the carbonyl in the conjugated system) via 1,4-addition. This generates an enolate intermediate at C1 (the oxygen becomes the enolate). Step 4: Workup. Treatment with H2O/H+ protonates the enolate at C2 (the alpha carbon), giving the saturated ketone product. Step 5: Product. The result is 3-methylcyclohexan-1-one: a cyclohexanone ring with a methyl group at the 3-position (beta-carbon of the original enone). This corresponds to option (b), which shows a cyclohexanone ring with H at C2 and CH3 at C3. Why other options fail: (a) Shows 1-methylcyclohex-2-en-1-ol - this would be a 1,2-addition product to the carbonyl with retention of the double bond; cuprates favor 1,4-addition, not 1,2-addition. (c) Shows a keto-diol or keto-alcohol with both CH3 and OH at C2 - this does not correspond to simple 1,4-addition chemistry. (d) Shows a tertiary alcohol with two methyl groups on the cyclohexane ring - this would require a different reaction mechanism and does not arise from 1,4-addition to 2-cyclohexenone. Therefore, the correct answer is B.