3.14 : Combustion Energy: A Measure of Stability in Alkanes and Cycloalkanes

Alkanes exhibit low reactivity due to strong non-polar C–C and C–H σ bonds.

Combustion of alkanes in excess oxygen, under high-temperature conditions, gives carbon dioxide and water.

Combustion reactions form the basis of energy sources for heat and power.

The energy released during combustion — called the heat of combustion — helps predict the relative stabilities of alkanes and cycloalkanes.

For a series of straight-chain alkanes, the sequential addition of a CH₂ group gradually increases the heat of combustion by an average value of 658.5 kJ mol^-1.

Now, consider different isomers of octane undergoing combustion to produce identical moles of products and different experimental heats of combustion.

The straight-chain isomer has the highest negative heat of combustion. The amount of heat released marginally decreases with branching, suggesting that increased branching lowers the potential energy and increases the stability of the isomer.

In cycloalkanes, wherein several CH₂ groups are linked together by C–C bonds, the predicted heat of combustion is “n times the average combustion energy of a CH₂ group.”

For strained cycloalkanes, the actual heat of combustion is slightly higher than the predicted values. The difference between the actual and the predicted value gives the strain energy.

A plot of strain energy as a function of ring size shows that cyclopropane has maximum strain due to excessive compression of its bond angles from 109.5° to 60°.

A decrease in the energy for cyclobutane, followed by cyclopentane, is relative to their reduced overall strain, while cyclohexane is virtually strain-free.

Moderate strain energies in C7 to C9 cycloalkanes mainly result from torsional and steric strains that arise from non-ideal bond angles in their conformations.