The heat of hydrogenation for allene (Problem 7.52 ) to...

The heat of hydrogenation for allene (Problem 7.52 ) to yield propane is $-295 \mathrm{~kJ} / \mathrm{mol}$, and the heat of hydrogenation for a typical monosubstituted alkene such as propene is $-126 \mathrm{~kJ} / \mathrm{mol}$. Is allene more stable or less stable than you might expect for a diene? Explain.

Key Concepts

Thermodynamic Stability

Thermodynamic stability in organic molecules is assessed by comparing the energy content of the reactants to that of the products. A lower (less exothermic) heat of hydrogenation indicates that a molecule is relatively more stable. In this context, if the sum of hydrogenation heats for individual bonds is more exothermic than what is observed, it suggests the molecule is less stable than expected, releasing extra energy due to its higher intrinsic energy.

Cumulated versus Conjugated Systems

Cumulated systems, such as allenes which have consecutive double bonds arranged perpendicularly, do not benefit from the resonance stabilization typically seen in conjugated dienes. As a consequence, even though allene is a diene by structure, it lacks the added stabilization from delocalized electrons that conjugated systems have. This explains why its heat of hydrogenation is more exothermic than would be expected for a diene, indicating that it is less stable than a conjugated diene.

Heat of Hydrogenation

Heat of hydrogenation is the enthalpy change that occurs when a double bond in an unsaturated molecule is converted into a single bond by the addition of hydrogen. It is used as an indirect measure of a molecule’s stability; lower exothermic values (less heat released) imply a more stable unsaturated system, because the molecule is already at a lower energy state and doesn’t release as much energy upon hydrogenation.

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