Question

Arrange the following compounds in increasing order of heat evolved when one mole of each of them is hydrogenated using Pt-catalyst.

• A. $\text{I < II < III < IV}$
• B. $\text{III < II < II < IV}$
• C. $\text{II < III < IV < I}$
• D. $\text{IV < I < II < III}$

Answer 1

The correct option is C $\text{II < III < IV < I}$

More substituted alkenes are more stable and evolve less heat on hydrogenation.
Also, $\text{Heat of hydrogenation}\propto \text{number of}\pi \text{bond}$
$\text{Heat of hydrogenation}\propto \frac{1}{\text{stability of alkene}}\propto \frac{1}{\text{number of}\alpha \text{hydrogen}}$
Structure $\text{I}$ has zero $\alpha$ hydrogen.
Structure $\text{II}$ has six $\alpha$ hydrogen and is in trans form.
Structure $\text{III}$ has six $\alpha$ hydrogen is in cis form.
Structure $\text{IV}$ has three $\alpha$ hydrogen is in cis form.
Between $\text{II}$ and $\text{III}$, structure $\text{III}$ (cis) is more reactive and less stable as compared to II(trans).
Stability order $\text{II > III > IV > I}$
Heat of hydrogenation order $\text{II < III < IV < I}$