See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Reduction of cyclic alkynes can give either cis-alkene or trans-alkene depending on the reagent, and cyclic constraints determine the actual product. Step 1: Identify Reactant P. P is cyclooctyne, an 8-membered ring containing a triple bond (C≡C), with the remaining carbons represented as (CH2)4. Step 2: Identify Products Q and R. Q is the cis (Z) cyclooctene — the 8-membered ring with a cis double bond, shown with both H atoms on the same face. R is a bicyclic compound — specifically, trans-cyclooctene cannot exist as a stable isolated ring in an 8-membered system in a planar sense, so the trans reduction product under Birch (dissolving metal) conditions would normally give trans-alkene. However, in cyclooctyne, the trans-cyclooctene is actually stable enough to exist, but the image of R shows a bicyclic (fused ring) structure, suggesting R is the product of a different pathway. Re-examining: R appears to be bicyclo[6.1.0] type or a carbene-derived product. However, in the context of this problem, the key distinction is: Step 3: Reagent Analysis. - Reagent (II): H2/Pd/CaCO3 (Lindlar's catalyst with quinoline) — this is Lindlar's catalyst, which performs syn addition of H2 to give the cis-alkene (Q, cis-cyclooctene). - Reagent (I): 2Na/liq. NH3 — this is Birch (dissolving metal) reduction, which gives trans addition to alkynes, yielding the trans-alkene. For cyclooctyne, the trans-cyclooctene product is actually the observed product, corresponding to R. - Reagent (III): 2H2/Pd/C — complete hydrogenation to the alkane (cyclooctane), not Q or R. Step 4: Match reagents to products. - Reagent (I) [Na/liq.NH3] → trans-selective reduction → gives trans-cyclooctene = R. This matches option (c). - Reagent (II) [H2/Pd/CaCO3, quinoline] → Lindlar's catalyst → cis-selective reduction → gives cis-cyclooctene = Q. This rules out option (d) (which claims R from reagent II) and option (b) (which claims both R and Q from reagent II). - Option (a) claims Q from reagent (I), but reagent (I) gives trans product (R), not Q. So (a) is incorrect. Step 5: Conclusion. Only option (c) is correct: R (trans-cyclooctene) is obtained on treatment with reagent (I) (Na/liq. NH3, dissolving metal reduction giving trans-alkene). Therefore, the correct answer is C.