Question

Which one has the highest heat of hydrogenation?
III
III

• A. I
• B. II
• C. III
• D. None of the above

Answer 1

The correct option is A I

More substituted alkenes are more stable and evolve less heat on hydrogenation.
Also, $\text{Heat of hydrogenation}\propto \text{number of}\pi \text{bond}$
$\text{Heat of hydrogenation}\propto \frac{1}{\text{stability of alkene}}\propto \frac{1}{\text{number of}\alpha \text{hydrogen}}$
I has one $\alpha$ hydrogen
II has nine $\alpha$ hydrogen
III has 11 $\alpha$ hydrogen

Stability order III>II>I
Heat of hydrogenation order I>II>III
Hence, I has highest heat of hydrogenation.