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For which of the following alkene pairs would hydrogenation release the greater amount of heat (higher heat of hydrogenation)?

1. cyclohexadiene compared with cyclohexene
2. cyclohexene compared with cyclohexadiene
3. CH3-CH=CH-CH3 compared with CH3-CH2-CH=CH2
4. but-2-ene compared with but-1-ene

Correct answer: cyclohexadiene compared with cyclohexene

Solution

Cyclohexadiene has two C=C bonds and so releases far more heat on full hydrogenation than cyclohexene's single C=C. Among the butene pair, the terminal (less substituted, less stable) but-1-ene has the higher heat of hydrogenation, not but-2-ene. So the pair where the first member gives the higher heat is cyclohexadiene vs cyclohexene.

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