Question

what is resonance energy?

Answer 1

Resonance energy is a measure of stability of a system and is defined as the difference in energy of the actual resonance hybrid and the most stable contributing structure.

E.g
we look at the values of heats of hydrogenation of cyclohexene and benzene.

Heat of hydrogenation of cyclohexene = 28.6 Kcal/mole

Heat of hydrogenation of benzene = 49.6 Kcal/mole

Now, benzene should evolve three times the heat of hydrogenation compared to hypothetical molecule of cyclohexatriene, i.e. 3 x 28.6 = 85.8 Kcal/mol

But, the experimentally determined value is 49.6 Kcal/mol.

This difference ( 85.8 – 49.6 = 36 Kcal/mol) is known as resonance energy of benzene.