Question

The enthalpy of hydrogenation of those compounds will be in the order as:

A) II > III > I
B) II > I > III
C) I > II > III
D) III > II > I

Answer 1

Hint: Enthalpy of hydrogenation is a measure of the stability of carbon-carbon double bonds. It is defined as the enthalpy change when one mole of an unsaturated compound reacts with an excess of hydrogen to become a fully saturated compound. Order of enthalpy of hydrogenation is reverse to the order of stability of alkenes.

Complete answer:
The phenomenon of addition of hydrogen to the double bonds that is, in unsaturated compounds to convert them into saturated compounds is known as hydrogenation. Enthalpy of hydrogenation is the enthalpy change when one mole of an unsaturated compound reacts with an excess of hydrogen to become a fully saturated compound at atmospheric pressure and room temperature. It is used to compare the stability of pi-bonded molecules. Given all the three structures are pi-bonded molecules and they are as follows:

Mathematically, heat of hydrogenation is inversely proportional to the stability of unsaturated compounds as:
${\text{Heat of hydrogenation }} \propto \dfrac{1}{{{\text{Stability of unsaturated compound}}}}$
It means if the compound which is most stable has the lowest heat of hydrogenation and the compound which is least stable has the highest heat of hydrogenation.
 

From the given compounds, compound (I) is most stable due to aromatic character. Its pi-bonds are in conjugation with each other in the ring and hence, the conjugation makes the compound stable.

The compound (III) is least stable as no resonance is present there. All the pi-bonds are outside the ring hence, no conjugation of pi-bonds.
Hence, stability of compound (II) will be in between that of compound (I) and (III).
Therefore, order of stability of compounds: I > II > III
Hence, order of enthalpy of hydrogenation: III > II > I

Thus, option D is correct.

Note:
During the hydrogenation process, strong $\sigma $-bonds formed at the cost of breaking weak $\pi $-bonds. Thus, energy is released to the surroundings during the process. Hence, the hydrogenation process is exothermic in nature and enthalpy of hydrogenation has a negative value.