Illustrated Glossary of Organic Chemistry

Resonance energy: The theoretical difference in molecular energy between a resonance hybrid and the 'most stable' resonance contributor (if this resonance contributor existed as a real molecule). In other words, the stability gain by electron delocalization due to resonance versus the absence of such delocalization. The resonance energy of benzene is 36 kcal mol^-1.

To measure the resonance energy of benzene we start with the enthalpy of hydrogenation for cyclohexene, which is -28.6 kcal mol^-1.

3H2, Pt ΔH = -85.8 kcal mol-1

If benzene is just three alkenes in a ring we expect benzene's enthalpy of hydrogenation to be that of just three alkenes (i.e., three times the enthalpy of hydrogenation for cyclohexene, 3 x - 28.6 kcal mol^-1 = -85.8 kcal mol^-1). The empirical enthalpy of hydrogenation for benzene is -49.8 kcal mol^-1. The difference in these enthalpies of hydrogenation is 36 kcal mol^-1, which is the stabilization of benzene attributed to its resonance and aromaticity.