In this article, you are going to learn about Resonance Energy including the calculation of resonance energy for benzene.
What is Resonance Energy in Chemistry?
The difference in energy between the actual molecule and the canonical form of the lowest energy is called the Resonance Energy. The resonance energy of a molecule is expressed in kcal/mole or kJ/mole.
An idea of the resonance energy is given in the figure.
Examples: Resonance energy of some molecules are given in the following table
The resonance energy is directly proportional to the stability of a molecule. So, the stability of a molecule increases with increasing its resonance energy. For example, the resonance energy of benzene is 36 kcal/mole and the resonance energy of pyridine is 28 kcal/mole. It indicates that benzene is more stable than pyridine.
Calculation of Resonance Energy
The resonance energy of a molecule can be estimated by comparing the enthalpy change of hydrogenation (amount of heat is released when one mole of an unsaturated compound is hydrogenated) of the actual molecule and the canonical form of the lowest energy. For example, the estimated resonance energy of benzene from the heat of hydrogenation data is 36 kcal/mole which can be shown in the following way.
Resonance Energy of Benzene = Energy of cyclohexatriene (most stable canonical structure) – Energy of Benzene (Actual Molecule).
The heat of hydrogenation of cyclohexene = 28.6 kcal/mole. Therefore, the heat of hydrogenation of cyclohexatriene is equal to 3 x 28.6 kcal/mole is 85.8 kcal/mole. Thus, Energy of Cyclohexatriene + 3 x Energy of H2 = Energy of cyclohexane + 85.8 kcal/mole
Again, the heat of hydrogenation of benzene (Actual Molecule) is 49.8 kcal/mole. Thus, Energy of benzene +3 x Energy of H2 = Energy of Cyclohexane + 49.8 kcal/mole
By comparing the above two equations, Energy of Cyclohexatriene – Energy of benzene (Actual Molecule) = (85.8 – 49.8) kcal/mole = 36 kcal/mole
Resonance energy of benzene = 36 kcal/mole