See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the transformation required: The starting material is 4-ethynyltetrahydropyran (a terminal alkyne attached to C4 of a tetrahydropyran ring). The product is a gem-dibromocyclopropane (specifically a 2,2-dibromocyclopropane) fused to the tetrahydropyranyl group, with defined trans stereochemistry of the H atoms on C1 and C2 of the cyclopropane ring. Step 2 - Determine the intermediate needed: To form a gem-dibromocyclopropane from an alkene via carbene addition (Simmons-Smith type or free carbene from CHBr3/NaOH), we first need an alkene (cis or trans). The terminal alkyne must be partially reduced to an alkene. The stereochemistry of the product (trans H's on the cyclopropane, meaning the two substituents—H and the ring—are trans) indicates the alkene precursor must be a trans (E)-alkene, because carbene addition to alkenes is stereospecific (syn addition of :CBr2 to a trans alkene gives a cyclopropane with trans substituents). Step 3 - Choose the reduction method: To reduce a terminal alkyne to a trans (E)-alkene, one uses dissolving metal reduction: Na in liquid NH3 (Birch-type reduction of alkynes). This gives the trans-alkene. To reduce a terminal alkyne to a cis (Z)-alkene, one uses Lindlar's catalyst (H2/Pd-CaCO3, poisoned) or H2/Pd-BaSO4. Since the product requires a trans-alkene intermediate, Na/liq.NH3 is the correct first reagent. Step 4 - Choose the cyclopropanation reagent: The product has two bromines on the same carbon of the cyclopropane (gem-dibromo), which is characteristic of dichlorocarbene or dibromocarbene addition. CHBr3/NaOH(Delta) generates :CBr2 (dibromocarbene), which adds to the alkene to give gem-dibromocyclopropane. CHCl3/NaOH generates :CCl2 (dichlorocarbene), which would give gem-dichlorocyclopropane—not matching the product. Heat (Delta) is needed for CHBr3/NaOH to generate the carbene efficiently. Step 5 - Confirm option A: Option (a): Na/liq.NH3 (first, to give trans-alkene) then CHBr3/NaOH(Delta) (second, to give gem-dibromocyclopropane via :CBr2 addition). This matches the required sequence and the product structure with Br atoms. Step 6 - Eliminate other options: (b) H2/Pd-CaCO3 gives cis-alkene; carbene addition to cis-alkene gives cis H-substituent relationship in cyclopropane, not matching the trans product shown. (c) Na/liq.NH3 gives correct trans-alkene, but CHCl3/NaOH gives :CCl2 → gem-dichlorocyclopropane, not dibromocyclopropane. (d) H2/Pd-CaCO3 gives wrong alkene geometry AND CHCl3/NaOH gives wrong halogen. Therefore, the correct answer is A.