See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Catalytic hydrogenation of alkenes adds H2 across the double bond to give alkanes. Isopentane (2-methylbutane) has the structure CH3CH(CH3)CH2CH3. We need to find all alkene structural isomers (with molecular formula C5H10) that upon hydrogenation yield isopentane (2-methylbutane) specifically. Step 1: Draw the carbon skeleton of isopentane (2-methylbutane): CH3-CH(CH3)-CH2-CH3. It has 5 carbons with a methyl branch at C2. Step 2: To get isopentane by hydrogenation, the alkene must have the same carbon skeleton as isopentane (2-methylbutane) — i.e., a branched 5-carbon chain with the same connectivity — and the double bond can be placed at any position within that skeleton. Step 3: Enumerate all alkenes with the 2-methylbutane skeleton (C5H10) that hydrogenate to 2-methylbutane: 1. 2-Methyl-1-butene: CH2=C(CH3)-CH2-CH3 2. 2-Methyl-2-butene: CH3-C(CH3)=CH-CH3 3. 3-Methyl-1-butene: CH2=CH-CH(CH3)-CH3 Step 4: Check if any other alkenes (e.g., straight-chain pentenes) give isopentane. Straight-chain pentenes (1-pentene, 2-pentene) would give n-pentane, not isopentane. So they are excluded. Step 5: Count: There are exactly 3 alkenes (2-methyl-1-butene, 2-methyl-2-butene, and 3-methyl-1-butene) that on catalytic hydrogenation give isopentane. Why other options fail: - (a) 2: Misses one of the three valid alkenes. - (c) 4 and (d) 5: Overcount; no additional alkenes with the 2-methylbutane skeleton exist. Therefore, the correct answer is B.