Which of the following compounds exhibits the highest heat of hydrogenation per mole: ${\text{CH}}_2=\text{CH}-\text{CH}={\text{CH}}_2$, ${\text{CH}}_3-\text{CH}=\text{CH}-{\text{CH}}_3$, ${\text{CH}}_2=\text{C}({\text{CH}}_3{)}_2$, or ${\text{CH}}_2={\text{CH}}_2$?

Solution

1. The heat of hydrogenation depends on the number of double bonds and the stability of the alkene.
2. $$\begin{array}{rl}{\text{CH}}_2& =\text{CH}-\text{CH}\\ & ={\text{CH}}_2\end{array}$$ has two double bonds, while the others have only one.
3. The total energy released for two double bonds is significantly higher than for a single double bond.
4. Hence the answer is (A).