Q7-62E - Organic Chemistry (Mcmurry)

Question

The heat of hydrogenation for allene (Problem 7-61) to yield propane is -295 kJ/mol, and the heat of hydrogenation for a typical monosubstituted alkene, such as propene, is -125 kJ/mol. Is allene more stable or less stable than you might expect for a diene? Explain.

Step-by-Step Solution

Answer

Allene is less stable than that diene.

Step 1: Heat of hydrogenation

The heat of hydrogenation depends on the double bonds there in alkene and helps measure the alkene stability. The lower the value of heat of alkene hydrogenation, the more stable is the alkene.

Step 2: The stability of allene

The heat of hydrogenation for diene is,

 $\begin{array}{l}=2\times \Delta H_{hydrog}^0\\ =-252kJ/mol\end{array}$$\begin{array}{l}=2\times \Delta H_{hydrog}^0\\ \\ =-252kJ/mol\end{array}$

 

The heat of hydrogenation for allene is -295 kJ/mol, which has a 43 kJ/mol higher energy than that of a diene. Thus, it is less stable because of higher energy.