Question

What is the order of increasing heat of hydrogenation for the following compounds (lowest first)?

• A. I, II, III, IV
• B. II, IV, I, III
• C. I, III, IV, II
• D. IV, II, III, I

Answer 1

The correct option is B II, IV, I, III

Entalpy of hydrogentation is defined as the change in enthalpy, which occurs when one mole of an unsaturated compound reacts with an excess of hydrogen to become fully saturated at atmospheric pressure and room temperature, the reactants and products being in their natural states under these conditions. This is the enthalpy chang ethat occurs during catalytic hydrogentation. It is used to compare the stability of pi-bonded molecules, and as a probe of alkene stability.
Order of enthalpy of hydrogenation is reverse of stability of alkenes.
II is conjugated and most substituted, thus evolves the least heat, followed by IV. I is more substituted than III, hence more heat is evolved in the hydrogenation of III.
So increasing level of heat of hydrogenation is represented as II, IV, I, III.