Question

The heat of hydrogenation for allene (Problem \(7.41\) ) 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.

Short answer

Allene is less stable than expected for a diene due to a higher heat of hydrogenation.

Step-by-step solution

Understand the Reaction

The reaction involves the hydrogenation of allene to form propane. We are comparing this with the hydrogenation of a typical monosubstituted alkene, propene, to propane to analyze stability.

Review Heats of Hydrogenation

The heat of hydrogenation for a typical monosubstituted alkene (propene) is given as \(-126 \, \mathrm{kJ/mol}\), while for allene it is \(-295 \, \mathrm{kJ/mol}\). These values represent the energy released during the hydrogenation process.

Compare Heat Values

Allene releases \( \left| -295 \right| = 295 \, \mathrm{kJ/mol} \) during hydrogenation, while propene releases \( 126 \, \mathrm{kJ/mol} \). More energy released in allene indicates less stability.

Analyze Stability

A higher exothermic heat of hydrogenation means the molecule was higher in energy prior to reaction, implying less stability. Hence, allene is less stable compared to typical alkenes like propene.

Conclusion on Diene Stability

Since allene's heat of hydrogenation is much more exothermic, this indicates that it is less stable than one might expect for a diene, which typically should have intermediate stability between alkenes and polyenes.

Key concepts

Allene Stability

Allene is a fascinating molecule that poses unique challenges when discussing its stability. At first glance, one might expect allene to display considerable stability due to its linear structure and conjugated systems opening many stabilization possibilities. However, this isn't quite the case. The heat of hydrogenation for allene is quite high, at \(-295 \, \mathrm{kJ/mol}\). This high exothermic energy released during its hydrogenation tells us that allene is actually quite "unstable".

Unlike typical dienes, which are generally seen as having stability between alkenes and polyenes, allene defies this expectation. It is less stable than common alkenes such as propene. This lack of stability in allene can be attributed to its unique bonding and arrangement processing two perpendicular double bonds which do not allow for effective overlap of the p orbitals, hence not providing the conjugative stabilization seen in more typical systems.

Always remember that in chemistry, more energy released in the reaction means the starting material is more unstable.

Propene Hydrogenation

The process of propene hydrogenation illustrates a critical concept in understanding alkene chemistry. Propene, a monosubstituted alkene, has a hydrogenation heat of \(-126 \, \mathrm{kJ/mol}\), which is significantly less exothermic compared to compound like allene.

To hydrogenate propene means breaking its double bonds, and forming single bonds as in propane. This reaction is exothermic because energy is released when the new \(\text{C-H}\) single bonds form, which are stronger than the broken \(\text{C=C}\) double bonds. The heat of hydrogenation is a reflection of the potential stability in the original compound: lower energy release means it starts relatively lower in energy, which implies greater stability.

So when energy output is smaller, as seen here with propene compared to allene, the compound is more stable.

Exothermic Reactions

Exothermic reactions are key to understanding many chemical processes. When a reaction is exothermic, it releases energy to the surroundings, typically in the form of heat. This is crucial in hydrogenation reactions, where the energy release provides insight into the stability of the reacting molecules prior to the reaction.

The reaction's exothermic nature signifies the transition of a high-energy, less stable starting material to a lower-energy, more stable product. Therefore, a greater release of energy, as we see with allene compared to propene, means the starting substance is higher in energy and inherently less stable.

The stability of a compound before and after a reaction can be inferred from the heat exchanged. Exothermic reactions are common and important, particularly in organic chemistry where energy considerations help determine reaction pathways and the feasibility of processes.