See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: When a substrate of the form Ph-CH2-C*H2-OTs (where C* is the isotopically labeled carbon bearing the tosylate leaving group) is treated with AcONa (a nucleophile/base) in AcOH, the reaction proceeds via a phenonium ion (neighboring group participation by the aromatic ring), also known as a bridged benzenonium intermediate. Step 1 - Identify the substrate: The starting material is Ph-CH2-C*H2-OTs. The isotopic label (C*) is on the carbon bearing the OTs leaving group (the beta carbon relative to the phenyl group). Step 2 - Neighboring group participation: The phenyl ring acts as an internal nucleophile. As the OTs departs, the phenyl ring bridges across both carbons (C1 and C*2), forming a symmetrical phenonium ion (a spiro-type bridged intermediate). In this bridged ion, both CH2 groups become equivalent carbons connected to the positively charged bridged phenyl ring. Step 3 - Symmetry of the phenonium ion: The phenonium ion is a symmetrical intermediate. The two carbons (C1 and C*2) are now equivalent in terms of their bonding to the bridged ring, meaning attack by acetate can occur at either carbon with equal probability. Step 4 - Attack by acetate nucleophile: AcO- can attack at either carbon of the phenonium ion: - Attack at C1 (the originally non-labeled carbon) gives Ph-C*H2-CH2-OAc: product (a), where the isotopic label ends up on the carbon adjacent to the ring (benzylic position). - Attack at C*2 (the originally labeled carbon) gives Ph-CH2-C*H2-OAc: product (b), where the isotopic label is on the carbon bearing OAc. Step 5 - Both products form: Since the phenonium ion is symmetrical, both modes of attack are equally probable, giving a mixture of both (a) and (b). Why (c) fails: Option (c) shows OAc on the aromatic ring and retention of OTs on the side chain, which would require electrophilic aromatic substitution-type reactivity inconsistent with this SN2/neighboring group participation pathway. Why only (a) or only (b) fails: Either alone would imply the reaction proceeds without the symmetrical bridged intermediate, which contradicts the known mechanism of phenonium ion participation for beta-aryl tosylates. Therefore, the correct answer is D.