Question

The enthalpy of hydrogenation of these 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

The correct option is D III > II > I

Higher is the stability, lower is the enthalpy of hydrogenation.
(I) is most stable due to aromatic character. Hence it has lowest enthalpy of hydrogenation.
(III) is least stable as no resonance is present. Hence, it has highest enthalpy of hydrogenation.
Thus, the decreasing order of the enthalpy of hydrogenation is $III>II>I$.