TOPIC: 3. Alkanes / Cycloalkanes

SUBTOPIC: 33. Ring strain

5018

The heat of combustion is defined as the enthalpy of the oxidation reaction in which a stoichiometric amount of oxygen is consumed during the conversion of an organic compound to carbon dioxide and water. If the heats of combustion at room temperature of cyclopropane, cyclohexane, and cyclooctane are measured and compared, an interesting result is seen. When the difference in number of carbons in each cycloalkane is considered, we derive a heat of combustion per CH₂ group, relative to cyclohexane and find:

Hydrocarbon Heat of Combustion per CH₂ (Kcal. m^–1)*

Cyclopropane 10.5

Cyclohexane 0

Cyclooctane 2.5

*Relative to cyclohexane

Discuss the source of these values, being as specific as you can about the factors responsible for them.