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For the three isomeric cycloalkanes of formula C6H12 listed below, arrange them in the decreasing order of their molar heat of combustion: (i) cyclohexane (ii) methylcyclopentane (iii) a substituted (dimethyl) cyclobutane

1. (iii) substituted cyclobutane > (ii) methylcyclopentane > (i) cyclohexane
2. (i) cyclohexane > (ii) methylcyclopentane > (iii) substituted cyclobutane
3. (ii) methylcyclopentane > (i) cyclohexane > (iii) substituted cyclobutane
4. (i) cyclohexane > (iii) substituted cyclobutane > (ii) methylcyclopentane

Correct answer: (iii) substituted cyclobutane > (ii) methylcyclopentane > (i) cyclohexane

Solution

For isomers, heat of combustion (HOC) increases with the internal energy of the molecule. A more strained ring stores more energy and therefore liberates more heat when burnt. Ring strain order is: cyclobutane (large angle strain, ~26 kcal/mol) > cyclopentane (~6 kcal/mol) > cyclohexane (essentially strain-free, ~0). Hence the substituted cyclobutane has the greatest HOC and cyclohexane the least. Greater stability corresponds to lower HOC.

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