Given :

The enthalpy of hydrogenation of these compounds will be in the order is

Options

A. II gt III gt I
B. II gt I gt III
C. I gt II gt III
D. III gt II gt I

Answer

D

Explanation

Out of the three compounds (I,II,III), compound (I) is aromatic and hence is most stable. Compound (II) is less stable than compound (I) because it is only a cyclic conjugated diene while compound (III) is least stable since it is neither an aromatic compound nor a cyclic conjugated diene. Thus, the stability of alkenes decreases in the order : I gt II gt III.
Since enthalpy of hydrogenation is inversely proportional to its stability, therefore, enthalpy of hydrogenation decreases in the order : III gt II gt I , i.e., option (d) is correct.