Illustrated Glossary of Organic Chemistry

Enthalpy of hydrogenation (heat of hydrogenation): The enthalpy change (ΔH) that occurs during catalytic hydrogenation. Used to compare stability of pi-bonded molecules, and as a probe of alkene stability, alkyne stability, conjugation, and aromaticity.

The enthalpy of hydrogenation of 1,3-butadiene is less than the enthalpy of hydrogenation of 2-butyne. Therefore the pi bonds of 1,3-butadiene are more stable than the pi bonds of 2-butyne.