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Which of the following has the smallest heat of hydrogenation per mole of H2?

1. 1,3-Butadiene
2. 1-Butene
3. trans-2-Butene
4. cis-2-Butene

Correct answer: 1,3-Butadiene

Solution

Heat of hydrogenation per mole of H2 reflects the relative stability of the unsaturated compound. 1,3-Butadiene is conjugated and resonance-stabilised, so its energy per double bond hydrogenated is the lowest among the choices.

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