3.14: Combustion Energy: A Measure of Stability in Alkanes and Cycloalkanes
The low reactivity in alkanes can be attributed to the non-polar nature of C–C and C–H σ bonds. Alkanes, therefore, were initially termed as “paraffins,” derived from the Latin words: parum, meaning “too little,” and affinis, meaning “affinity.”
Alkanes undergo combustion in the presence of excess oxygen and high-temperature conditions to give carbon dioxide and water. A combustion reaction is the energy source in natural gas, liquified petroleum gas (LPG), fuel oil, gasoline, diesel fuel, and aviation fuel. The energy released during combustion, called the heat of combustion (−ΔH°), helps predict the relative stabilities in alkanes and cycloalkanes.
For straight-chain alkanes, the heat of combustion increases gradually with the sequential addition of a CH2 group. However, in higher alkanes, the heat of combustion decreases with increased branching, suggesting that branched isomers have lower potential energies and have greater stabilities compared to straight chain (linear) alkanes.
In cycloalkanes, the relative stability depends on the strain energy, which is the combined outcome of angular, torsional, and steric strains. The strain energy is determined as the difference between the actual and the predicted heats of combustion. A study of strain energy as a function of ring size reveals that the smallest cycloalkane (C3) exhibits maximum strain due to excessive compression of its bond angles. As the ring size increases, the bond angles approach the ideal value of 109° with cyclohexane (C6) being strain-free. Further strains in higher cycloalkanes (C7 to C9) result from their non-ideal bond angles.