91Ó°ÊÓ

The stability of alkene is increased by making it more substituted by the alkyl group due to the hyperconjugation effect; e.g., a disubstituted alkene is more stable than a monosubstituted alkene.

The stability of alkene can be connected to the heat of hydrogenation; a more stable alkene produces less heat (having less energy) while the less stable (having more energy) alkene produces more heat.

The more the heat of hydrogenation $\left(\mathrm{Δ}{\text{H}}_{\text{hydrog}}\right)$, the less is the stability of alkene.

Allene having two double bonds, the excepted heat of hydrogenation data-custom-editor="chemistry" $\left(\mathrm{Δ}{\text{H}}_{\text{hydrog}}\right)$is given as:

$$\begin{gathered} \left(\mathrm{Δ}{\text{H}}_{\text{hydrog}}\right)\text{=}\text{- 126 + ( - 126)} \\ \text{=}\text{- 252}\text{kJ/mol} \end{gathered}$$

Expected role="math" localid="1663240392459" $\mathrm{Δ}{\text{H}}_{\text{hydro}}$for allene is $-252\mathrm{kJ}/\mathrm{mol}$when the heat of hydrogenation for each bond is the same as that of the isolated double bond.

The measure$\mathrm{Δ}{\text{H}}_{\text{hydro}}$for allene is $-298\mathrm{kJ}/\mathrm{mol}$, more than that of the expected value $(252\mathrm{kJ}/\mathrm{mol})$.

Thus, the allene has higher energy than a nonconjugated diene, which is higher than a conjugated diene.

The stability order for the given alkene is as follows:

role="math" localid="1663240895312" $$\begin{gathered} \mathrm{Allene}<\mathrm{Non}-\mathrm{conjugated}\mathrm{diene}<\mathrm{conjugated}\mathrm{diene} \\ \mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\mathit{}\stackrel{\mathit{→}}{\mathit{}\mathit{}\mathit{}stability\mathit{}\mathit{}\mathit{}} \end{gathered}$$