Question

Given:

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

To start with, all these compounds, be it I, II or III yield the same hydrocarbon on hydrogenation (3 eq. $H_2$ ).

Clearly I – which is also known as mesitylene – has an aromatic arrangement with 6 $\pi$ electrons and thus would be the most stable.
II has a reasonable deal of conjugation and this makes it less stable than I but definitely more stable than III.
III is by far the least stable – in other words, it will be at the top in an energy diagram.
Hydrogenation is usually exothermic and thus the enthalpy order is III > II > I.