See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Isomers include both structural (constitutional) isomers and stereoisomers (optical and geometric). We must count all distinct isomers of chlorobutane (C4H9Cl). Step 1: Draw all structural isomers of chlorobutane. Butane has two carbon skeletons: n-butane and isobutane (2-methylpropane). For n-butane (CH3-CH2-CH2-CH3), Cl can be placed at: - C1: 1-chlorobutane (CH3CH2CH2CH2Cl) - C2: 2-chlorobutane (CH3CHClCH2CH3) These are the only unique positions due to symmetry. For isobutane ((CH3)3CH), Cl can be placed at: - C1 (on a methyl group): 1-chloro-2-methylpropane ((CH3)2CHCH2Cl) - C2 (on the central carbon): 2-chloro-2-methylpropane ((CH3)3CCl) So there are 4 structural isomers total. Step 2: Check for stereoisomers. - 1-chlorobutane: no chiral center, no stereoisomers. - 2-chlorobutane: C2 is a chiral center (attached to H, Cl, CH3, CH2CH3 — all four different). This gives 2 enantiomers (R and S). So this adds 1 additional stereoisomer (the pair counts as 2 isomers, but we already counted one structure, so we add 1 more). - 1-chloro-2-methylpropane: no chiral center. - 2-chloro-2-methylpropane: no chiral center (C2 has three identical CH3 groups). Step 3: Total count. 4 structural isomers + 1 additional enantiomer (for 2-chlorobutane) = 5 total isomers. Why other options fail: - (a) 2: counts only some structural isomers, ignores stereoisomers. - (b) 4: counts only structural isomers, ignores optical isomers of 2-chlorobutane. - (c) 6: overcounts; there is only one chiral center producing one pair of enantiomers. Therefore, the correct answer is D.