See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: Heat of combustion depends on two main factors: (a) the total number of carbons/hydrogens (molecular formula, i.e., more carbons = more heat released), and (b) ring strain (more strained molecules have higher internal energy and therefore release more heat upon combustion). Step 2 - Determine molecular formulas: - Substance 1 (methylcyclopentane): C6H12 - Substance 2 (cis-1,2-dimethylcyclobutane): C6H12 - Substance 3 (cyclohexane): C6H12 - Substance 4 (trans-1,2-dimethylcyclobutane): C6H12 All four compounds are isomers with molecular formula C6H12, so the number of carbons and hydrogens is the same for all. Step 3 - Ring strain determines order: Since all are C6H12, the compound with the greatest ring strain will have the highest heat of combustion (it has more stored potential energy). - Cyclobutane ring (4-membered) has significantly more ring strain (~110 kJ/mol total) than cyclopentane (5-membered, ~26 kJ/mol) or cyclohexane (6-membered, ~0 kJ/mol). - Among the dimethylcyclobutanes: cis-1,2-dimethylcyclobutane (substance 2) has greater ring strain than trans-1,2-dimethylcyclobutane (substance 4) due to additional torsional/steric strain from having both methyls on the same face. - Methylcyclopentane (substance 1) has intermediate ring strain (5-membered ring). - Cyclohexane (substance 3) has essentially zero ring strain (chair conformation). Step 4 - Rank by decreasing heat of combustion: cis-1,2-dimethylcyclobutane (2) > trans-1,2-dimethylcyclobutane (4) > methylcyclopentane (1) > cyclohexane (3) This gives order: 2 > 4 > 1 > 3, which matches option (c). Step 5 - Why other options fail: - (a) 1 > 2 > 4 > 3: Incorrectly places methylcyclopentane above both dimethylcyclobutanes; ignores that 4-membered rings have more strain than 5-membered rings. - (b) 3 > 4 > 2 > 1: Completely reverses the ring strain logic; cyclohexane has least strain and should have lowest heat of combustion. - (d) 1 > 3 > 2 > 4: Incorrectly places cyclohexane above dimethylcyclobutanes and methylcyclopentane above all cyclobutane compounds. Therefore, the correct answer is C.