The heat of hydrogenation for allene (Problem 7 -61) to...

The heat of hydrogenation for allene (Problem 7 -61) to yield propane is $-295 \mathrm{~kJ} / \mathrm{mol},$ and the heat of hydrogenation for a typical monosubstituted alkene, such as propene, is $-125 \mathrm{~kJ} / \mathrm{mol}$. Is allene more stable or less stable than you might expect for a diene? Explain.

Key Concepts

Heat of Hydrogenation

This is the amount of heat released when an unsaturated molecule (typically containing double bonds) is converted into its saturated analogue through the addition of hydrogen. It serves as a gauge for the inherent stability of the unsaturated system, where a less exothermic hydrogenation indicates a more stabilized (lower-energy) starting material, while a more exothermic reaction suggests that the molecule is higher in energy and thus less stable.

Conjugation and Resonance Stabilization

Conjugation refers to the overlap of p-orbitals across adjacent double bonds, which allows for delocalization of electrons and leads to enhanced stabilization of the molecule. Resonance stabilization is the phenomenon where the electronic energy is lowered by the delocalization of electrons across a conjugated system. In conjugated dienes, this leads to a reduction in the overall heat of hydrogenation compared to isolated double bonds, reflecting their extra stability.

Cumulenes versus Conjugated Dienes

Cumulenes, such as allene, feature adjacent double bonds that are arranged in orthogonal planes, preventing the effective p-orbital overlap necessary for conjugation. This means that unlike in conjugated dienes where electron delocalization provides stability, cumulenes do not gain the same stabilizing effect, and their reactivity and heats of hydrogenation can be quite distinct from what is expected based on the behavior of conjugated systems.

Thermochemical Analysis

Thermochemical analysis uses measured heats of reaction such as hydrogenation energies to infer the relative stabilities of compounds. A more exothermic heat of hydrogenation in a system that might be expected to behave like a stabilized diene indicates that the molecule is actually less stable than anticipated, suggesting the absence of additional stabilizing resonance effects present in conjugated systems.

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