See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Catalytic hydrogenation of an alkyne (with excess H2 over Pd/C or similar catalyst) converts the C≡C triple bond to a C–C single bond, adding two hydrogen atoms across the triple bond. The carbon skeleton (connectivity) of the alkyne is preserved; only the degree of unsaturation changes. Therefore, any alkyne that has the same carbon skeleton as 3-ethylhexane will yield 3-ethylhexane upon complete hydrogenation. Step 1: Determine the structure of 3-ethylhexane. 3-Ethylhexane has a hexane main chain (6 carbons) with an ethyl substituent at C3. Total carbons = 8. Skeleton: CH3-CH2-CH(CH2CH3)-CH2-CH2-CH3. Step 2: Any alkyne that gives 3-ethylhexane must have the same 8-carbon branched skeleton, differing only in having a C≡C triple bond somewhere in that skeleton instead of a C–C single bond. Step 3: Analyze each option. (a) The structure shown is a terminal alkyne (HC≡C–) attached to C3 of a branched chain with an ethyl group, consistent with 3-ethylhex-1-yne. Hydrogenation gives 3-ethylhexane. ✓ (b) The structure shown is an alkyne with the triple bond in a different position on the same 3-ethylhexane skeleton (e.g., 3-ethylhex-4-yne or similar). Hydrogenation gives 3-ethylhexane. ✓ (c) The structure shown is another positional isomer of the alkyne on the same skeleton (triple bond near terminus with ethyl branch). Hydrogenation gives 3-ethylhexane. ✓ Step 4: Since all three alkynes (a), (b), and (c) share the same carbon skeleton as 3-ethylhexane and differ only in the position of the triple bond, all three yield 3-ethylhexane upon catalytic hydrogenation. Therefore, the correct answer is D.