See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1: Determine the degree of unsaturation (DoU) for C9H18. DoU = (2×9 + 2 - 18)/2 = (18 + 2 - 18)/2 = 2/2 = 1. So there is exactly one degree of unsaturation, which is satisfied by the cyclohexane ring itself. This means no additional rings or double bonds are present. Step 2: The cyclohexane ring accounts for 6 carbons. The remaining carbons = 9 - 6 = 3 carbons must be attached as alkyl substituents on the ring, with no additional unsaturation. Step 3: We need to distribute 3 carbon atoms as alkyl groups on the cyclohexane ring (with no double bonds or additional rings). The possible alkyl combinations are: Case A: One substituent containing all 3 carbons (n-propyl or isopropyl group on cyclohexane). - n-propylcyclohexane: 1 isomer - isopropylcyclohexane: 1 isomer Total: 2 isomers Case B: Two substituents totaling 3 carbons (one C1 + one C2, i.e., methyl + ethyl). The ring positions for two substituents (1,1), (1,2), (1,3), (1,4): - 1,1-methyl,ethyl (1-methyl-1-ethylcyclohexane): 1 isomer - 1,2-methyl,ethyl: 1 isomer - 1,3-methyl,ethyl: 1 isomer - 1,4-methyl,ethyl: 1 isomer Total: 4 isomers Case C: Three substituents each with 1 carbon (three methyl groups on cyclohexane). The distinct positional isomers of trimethylcyclohexane: - 1,1,2-trimethylcyclohexane: 1 isomer - 1,1,3-trimethylcyclohexane: 1 isomer - 1,1,4-trimethylcyclohexane: 1 isomer - 1,2,3-trimethylcyclohexane: 1 isomer - 1,2,4-trimethylcyclohexane: 1 isomer - 1,3,5-trimethylcyclohexane: 1 isomer Total: 6 isomers Step 4: Sum all structural isomers (constitutional isomers only, ignoring stereoisomers as these are structural isomers): 2 + 4 + 6 = 12 Therefore, the correct answer is 12.