Question

Among these compounds, the order of heats of hydrogenation is:

• A. $I>II>III$
• B. $III>II>I$
• C. $II>I>III$
• D. $II>III>I$

Answer 1

The correct option is C $II>I>III$

Heat of Hydrogenation $\alpha \frac{1}{\text{Stability}}\alpha \frac{1}{\text{No. of resonating structures}}\alpha \text{number of pi bonds}$
In between (I) and (II), no. of reasonating structures of (I) is maximum i.e. 2.
$\therefore$ Heat of hydrogenation of (I) is lower than (II).
(III) has only one $\pi$ bond, so least heat of hydrogenation.
$\therefore$ Order of heat of hydrogenation is $\Rightarrow$ $\left(III\right)>\left(II\right)>\left(I\right)$