See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Concept: The heat of combustion of cycloalkanes depends on two factors: (1) the number of CH2 units (more carbons = more energy released per mole), and (2) ring strain (higher ring strain adds extra enthalpy to combustion). Step 1 - Number of carbons: Cyclopropane (C3H6) < Cyclobutane (C4H8) < Cyclopentane (C5H10) More carbons means more bonds to oxidize, so the base heat of combustion increases in the order I < II < III. Step 2 - Ring strain contribution: Ring strain also contributes to the heat of combustion because strained molecules have higher internal energy. Cyclopropane has the highest ring strain (~115 kJ/mol), cyclobutane has significant strain (~110 kJ/mol), and cyclopentane has very low ring strain (~26 kJ/mol). However, this ring strain effect is secondary to the carbon count effect. Step 3 - Overall heats of combustion (experimental values per mole): Cyclopropane: ~2091 kJ/mol (3 carbons, high strain) Cyclobutane: ~2745 kJ/mol (4 carbons, moderate strain) Cyclopentane: ~3291 kJ/mol (5 carbons, low strain) So the order is: Cyclopentane (III) > Cyclobutane (II) > Cyclopropane (I), i.e., III > II > I. Step 4 - Why other options fail: (a) I > II > III: Incorrect; this reverses the actual order. (b) II > I > III: Incorrect; cyclopentane has the most carbons and highest heat of combustion. (d) III > I > II: Incorrect; cyclobutane (II) has more carbons than cyclopropane (I) and releases more heat per mole. Therefore, the correct answer is C.