Question

The alkene that has greatest negative heat of hydrogenation among the following is:

• A.
• B.
• C.
• D.

Answer 1

The correct option is D

$\text{Heat of hydrogenation}\propto \frac{1}{\text{Stability of alkenes}}$
In this case, the stability of alkenes can be compared by hyperconjugation. More the number of alpha hydrogens, more will be the number of hyperconjugating structures and hence more stable will be the alkene. If an alkene is stable its heat of hydrogenation is less. On the other hand, less stable alkene has greater heat of hydrogenation.


Stability order of the given alkenes is: (b) > (a) > (c) > (d)
Thus, the correct order of heat of hydrogenation is:
(d) > (c) > (a) > (b)