Which cycloalkane has the lowest heat of combustion per ${\mathrm{CH}}_{2}$ group

1. A

Cyclopropane

2. B

Cyclobutane

3. C

Cyclopentane

4. D

Cyclohexane

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Solution:

In cyclopropane. the angle strain is maximum hence it is a highly strained molecule and consequently most unstable. The angle strain in cyclobutane is less than that in cyclopropane. hence cyclobutane is more stable than cyclopropane. This stability increases up to 6 membered rings. then decreases from 7 to 11 membered rings and from the 12 membered onwards attains the stability of 6 membered ring. Heat of combustion is a method of measuring chemical stability Hence cyclohexane has the lowest heat of combustion per ${\mathrm{CH}}_{2}$ group.

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