Allene, H2 C=C=CH2, has a heat of hydrogenation of -298 kJ / mol(-71.3 kca / / mol). Rank a conj

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Allene, H2 C=C=CH2, has a heat of hydrogenation of -298 kJ /...

Allene, $\mathrm{H}_{2} \mathrm{C}=\mathrm{C}=\mathrm{CH}_{2},$ has a heat of hydrogenation of $-298 \mathrm{kJ} / \mathrm{mol}(-71.3 \mathrm{kca} / / \mathrm{mol})$. Rank a conjugated diene, a nonconjugated diene, and an allene in order of stability.

Key Concepts

Nonconjugated Dienes

Nonconjugated dienes have double bonds that are separated by more than one single bond, which prevents the mutual interaction of their p orbitals. This lack of conjugative stabilization makes them less stable than conjugated dienes, although they may be more stable than compounds with cumulated double bonds.

Heat of Hydrogenation

This is the energy released when an unsaturated compound undergoes hydrogenation. It provides an indirect measure of a compound's stability; a lower (less negative) heat of hydrogenation indicates a more stable unsaturated molecule because it is already lower in energy compared to its saturated form.

Conjugation

Conjugation refers to the overlap of p orbitals across adjacent double bonds, which allows for delocalization of electrons. This electron delocalization stabilizes the compound by lowering its overall energy, making conjugated systems more stable compared to similar nonconjugated systems.

Cumulated Bonds (Allene Structure)

Cumulated double bonds, as found in allenes, involve two consecutive double bonds sharing a common carbon atom. Unlike conjugated systems, the p orbitals in allenes are oriented perpendicular to each other, preventing effective overlap and resulting in less resonance stabilization and lower overall stability.

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