Rank the following alkenes in order of decreasing heat of hydrogenation.
A)I II III
B)III I II
C)II I III
D)III II I

Question

Rank the following alkenes in order of decreasing heat of hydrogenation. 
A)I > II > III 
B)III > I > II 
C)II > I > III 
D)III > II > I

Quizplus · Accepted Answer

The heat of hydrogenation depends on the stability of the alkene. The more stable the alkene, the less heat energy it will release upon hydrogenation. In this case, option III has a conjugated system of double bonds, which is more stable than a simple alkene like I or II. Therefore, III will release the least amount of heat energy upon hydrogenation and is the most stable. Thus, option B, III > I > II, represents the correct order of decreasing heat of hydrogenation.

Rank the Following Alkenes in Order of Decreasing Heat of Hydrogenation