Question

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

• A. I < II < III < IV
• B. III < II < II < IV
• C. II < III < IV < I
• D. IV < I < II < III

Answer 1

The correct option is C II < III < IV < I

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