See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Concept: Grignard reagents (RMgBr) react with cyclic sultones (cyclic sulfonic acid esters) via nucleophilic addition. The sultone shown is 1,3-propanesultone (a 5-membered ring: S connected to two oxygens via S=O bonds and to a trimethylene chain -CH2CH2CH2- forming the ring). Step 1: Identify the substrate. The structure drawn is a 5-membered ring with sulfur bearing two oxo groups (SO2), making it 1,3-propanesultone (a cyclic sulfonate ester). The ring has the pattern: S(=O)2 - CH2 - CH2 - CH2 - O (back to S), i.e., the oxygen is part of the ring as S-O-C. Step 2: Grignard reaction with sultones. RMgBr acts as a nucleophile (R-) attacking the electrophilic carbon adjacent to the oxygen (the C-O bond of the sulfonate ester is cleaved). The Grignard attacks the carbon bearing the S-O (ester-type) bond, opening the ring. However, an alternative pathway is ring opening at the C-O bond: R- attacks the carbon bonded to oxygen, breaking the C-O bond and generating a sulfonate anion. Step 3: The key mechanistic point. When RMgBr reacts with 1,3-propanesultone, the nucleophile R- attacks the carbon attached to oxygen (alkyl-oxygen cleavage), opening the ring to give: R-CH2CH2CH2-SO2-OMgBr. Upon workup this gives H-SO2(CH2)3-R... but wait, the correct answer is (c). Step 4: Re-examining option (c). Option (c) shows a 5-membered ring (thiolane/tetrahydrothiophene skeleton) with SO2 and MgBr on the sulfur-bearing carbon. This represents the Grignard reagent attacking sulfur rather than carbon, or more precisely, the RMgBr does NOT open the sultone ring but instead acts as a base or the product retains the ring with MgBr incorporated - i.e., the Grignard reagent deprotonates alpha to SO2 within the ring, generating a carbanion stabilized by SO2, which then coordinates with MgBr. 1,3-Propanesultone has acidic alpha-H next to SO2; RMgBr (acting as a base) abstracts the alpha proton to give the alpha-metalated sultone (the ring-intact carbanion with MgBr at the alpha carbon), releasing RH. This is a deprotonation (metalation) rather than nucleophilic addition. Step 5: Why other options fail. - Option (a): Shows a 4-carbon chain with HSO2 terminal group - incorrect carbon count and connectivity. - Option (b): H-SO2(CH2)3-R would be the ring-opened alkylation product, but this is not what happens preferentially; also the Grignard acts as base here. - Option (d): H-SO2(CH)3-R has incorrect hydrogen count (CH instead of CH2). Step 6: Conclusion. RMgBr acts as a base toward 1,3-propanesultone, abstracting the alpha-hydrogen adjacent to the SO2 group within the ring, producing the alpha-MgBr metalated sultone (ring intact, with MgBr at the carbon alpha to SO2), which is option (c). Therefore, the correct answer is C.