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If the heat of hydrogenation of 1-butene is 30 kcal/mol, what is the approximate heat of hydrogenation of 1,3-butadiene?

1. 57
2. 60
3. 30
4. 25

Correct answer: 57

Solution

Two isolated double bonds would release about 2 * 30 = 60 kcal/mol. Because 1,3-butadiene is conjugated and resonance-stabilised by a few kcal/mol, the measured heat of hydrogenation is slightly less than 60, close to 57 kcal/mol.

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