See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Concept: Gilman reagents (R2CuLi, organocuprates) are well known for performing SN2 substitution reactions on alkyl halides. They do not typically undergo aromatic substitution under normal conditions, and they do not demethylate aryl methyl ethers. Step 1: Identify the substrate. The starting material is 3,5-dimethoxybenzyl bromide, which contains a benzylic C-Br bond (primary alkyl bromide, benzylic position: Ar-CH2-Br). Step 2: Identify the reagent. n-Bu2CuLi is a Gilman reagent (diorganocopper lithium). Gilman reagents are excellent nucleophiles for SN2 reactions on primary and secondary alkyl halides, as well as allylic and benzylic halides. Step 3: Reaction pathway. The cuprate acts as a nucleophile (n-Bu-) and displaces bromide from the benzylic position via SN2 mechanism: Ar-CH2-Br + n-Bu2CuLi → Ar-CH2-n-Bu + LiBr + n-BuCu. The OMe groups are unaffected. The product retains the two OMe groups at 1,3 positions and now has a -CH2-n-Bu group at position 5. Step 4: Match to answer choices. Option (a) shows 3,5-dimethoxybenzyl-n-butane (the CH2Br replaced by CH2Bu), which is exactly the SN2 product at the benzylic carbon. Why other options fail: - Option (b) shows direct attachment of n-Bu to the ring carbon (losing the CH2 linker), which would require a different mechanism not typical of Gilman reagents with this substrate. - Option (c) shows incomplete/wrong substitution with CH2Br still present. - Option (d) shows demethylation of the OMe groups to OH, which Gilman reagents do not cause. Therefore, the correct answer is A.