Illustrated Glossary of Organic Chemistry

Enthalpy of formation (heat of formation; ΔHfo): The hypothetical enthalpy change (ΔH) when a substance is synthesized from the corresponding elements in their standard states. A more negative (or less positive) enthalpy of formation indicates a more stable isomer.

4 C (graphite) + 3 H2 (gas)

1,3-Butadiene: ΔHfo = 29.9 kcal mol-1

4 C (graphite) + 3 H2 (gas)
Methylenecyclopropane: ΔHfo = 47.7 kcal mol-1
ΔHfo 1,3-butadiene < ΔHfo methylenecyclopropane, so 1,3-butadiene is more stable than methylenecyclopropane.