Question

The decreasing order of heat of hydrogenation of the alkenes mentioned in the given compound is:

• A. (c) > (b) > (a)
• B. (c) > (a) > (b)
• C. (a) > (b) > (c)
• D. (b) > (a) > (c)

Answer 1

The correct option is A (c) > (b) > (a)

The addition of hydrogen is called as hydrogenation. The heat released in a hydrogenation reaction is called the heat of hydrogenation.

$\text{Heat of hydrogenation}\propto \frac{1}{\text{Stability of alkene}}$

$\text{Stability of alkene}\propto \text{No. of}\alpha \text{- Hydrogens}$

In given compound, alkene (a) has
10 $\alpha$ - hydrogens, alkene (b) has 9 $\alpha$-hydrogens and alkene (c) has 2 $\alpha$-hydrogens.
Thus, the order of stability of alkene is
$\left(a\right)>\left(b\right)>\left(c\right)$
Hence, the decreasing order of heat of hydrogenation of alkene is
$\left(c\right)>\left(b\right)>\left(a\right)$