See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Concept: The Simmons-Smith reaction uses CH2I2 and Zn(Cu) to convert alkenes into cyclopropanes via carbene/carbenoid addition across double bonds. Each mole of CH2I2 converts one C=C double bond into a cyclopropane ring. Step 1 - Identify the substrate: The starting material is (2E,4E)-hexa-2,4-diene-1,6-diol (or the corresponding diene-diol), i.e., HO-CH2-CH=CH-CH=CH-CH2-OH. It contains two isolated (conjugated) double bonds and two terminal -CH2OH groups. Step 2 - Apply 2 moles of Simmons-Smith reagent: With 2 moles of CH2I2/Zn(Cu), both double bonds undergo cyclopropanation. Each C=C double bond is converted to a cyclopropane ring by addition of a CH2 group across it. Step 3 - Determine the product structure: The substrate has the connectivity: HOCH2-C=C-C=C-CH2OH. After cyclopropanation of each double bond: - The C2=C3 double bond becomes a cyclopropane ring (a CH2 bridges C2 and C3). - The C4=C5 double bond becomes a cyclopropane ring (a CH2 bridges C4 and C5). This gives two cyclopropane rings connected to each other (at C3-C4 bond), each bearing a -CH2OH tail. The product is 1,2-bis(cyclopropyl)ethane-diol equivalent, specifically: HOCH2-CH2-cyclopropyl-cyclopropyl-CH2-CH2OH where the two cyclopropane rings are adjacent (connected via a C-C bond between them), each with a -CH2CH2OH chain. This matches option (b): HO-CH2-CH2-[cyclopropane]-[cyclopropane]-CH2-CH2-OH with two separate cyclopropane rings linked together, each having a 2-hydroxyethyl substituent. Step 4 - Rule out other options: (a) Shows a spiro arrangement - this would require both CH2 insertions at a shared carbon, which does not occur with two separate double bonds. (c) Shows a fused bicyclopropane (bicyclobutane) system - this would require a different connectivity. (d) Shows methoxy groups instead of hydroxyl groups - the reagent does not methylate alcohols. The Simmons-Smith reaction is directed by the nearby hydroxyl groups (hydroxyl-directed cyclopropanation), and with 2 moles of reagent, both double bonds are converted to give two cyclopropane rings in a 1,2-bis(cyclopropane) arrangement with terminal -CH2OH groups, consistent with option (b). Therefore, the correct answer is B.