See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Lindlar's catalyst (Pd-BaSO4 with quinoline, or simply Pd-BaSO4) is a poisoned/partially deactivated palladium catalyst used for the partial hydrogenation of alkynes to alkenes. The key feature of this catalyst is that it delivers hydrogen in a syn (cis) fashion to the triple bond, and it stops at the alkene stage rather than fully reducing to the alkane. Step 1: Identify the starting material. 2-Butyne is CH3-C≡C-CH3 (an internal alkyne with the triple bond between C2 and C3). Step 2: Identify the reagent. H2/Pd-BaSO4 is Lindlar's catalyst (a poisoned palladium catalyst). It performs syn addition of H2 across the triple bond and does not over-reduce to the alkane. Step 3: Determine stereochemistry. Because H2 is delivered from the same face of the triple bond (syn addition), both hydrogen atoms add to the same side of the resulting double bond. For an internal alkyne like 2-butyne, syn addition gives the cis alkene product. Step 4: Identify the product. Syn addition of H2 to CH3-C≡C-CH3 gives cis-2-butene (both methyl groups on the same side of the double bond). Why other options fail: (b) trans-2-butene would result from anti addition, which is characteristic of dissolving metal reductions (e.g., Na/NH3, Birch-type conditions), not Lindlar's catalyst. (c) 1-butene would require isomerization of the double bond position, which does not occur under these conditions. (d) 2-hydroxy butane would require addition of water (hydration), not H2. Therefore, the correct answer is A.