Pre-Requisite Reading: Resonance
Resonance is an integral concept that is often used to explain the extra stability that certain molecules possess.
The extra stability is attributed to electron delocalization. It is observed that the molecules that delocalize electrons are more stable, and such molecules also tend to have more than one Lewis structure.
The stability is measured in resonance energy or delocalization energy.
How is resonance energy calculated?
One must use an example like Benzene to understand resonance energy and break it into gaseous atoms, theoretically and experimentally, to observe the difference in the released energy. The difference in energy is attributed to electron delocalization or resonance in the molecule.
Benzene is a six-carbon cyclic molecule with alternating single and double bonds. Theoretically, it is a cyclohexatriene, a cyclic hexagon ring with three alkene bonds.
Cyclohexatriene has three components-
3 Carbon-Carbon double bonds, 3 Carbon-Carbon single bonds, and 6 Carbon-Hydrogen single bonds.
It is assumed that when a cyclohexatriene is heated, and its bonds are broken to liberate gaseous atoms of Carbon and Hydrogen, 5390 kJ/mol of energy is released.
The theoretical value of 5390 kJ/mol is obtained using the data from the previous standardization experiments. The standardization experiments calculated the heat of atomization values of some carbon bond types - C-C single bond, C=C double bond, C-H single bond, etc. by breaking each bond type into gaseous atoms. The energy released indicates the bond strength and these values were then used for the cyclohexatriene molecule.
However, breaking down Benzene into gaseous atoms (heat of atomization) of Carbon and Hydrogen is found to have a value of 5535 kJ/mol. Subtracting the value of cyclohexatriene 5390 kJ/mol with 5535 kJ/mol for Benzene gives 145 kJ/mol. This extra 145 kJ/mol of energy is the resonance energy and is obtained due to electron delocalization or resonance.
Therefore, resonance energy is the prime factor differentiating isolated bonds of cyclohexatriene from Benzene by imparting additional stability.
How is resonance energy calculated in other molecules?
In most cases, the resonance energies are calculated indirectly or are estimations.
If a molecule has multiple resonance structures, in that case, the structure that has the lowest energy is used for calculation. The most stable structure will be the one with the lowest energy. The resonance energy is then the difference in energy between the actual molecule and the most stable hypothetical Lewis’s structure.
One can arrive at the most stable resonance structure by following Resonance structure drawing rules. These rules called the rules of resonance, predict a molecule's most stable, lowest-energy Lewis structures.
Therefore, the resonance energy of the actual molecule is always greater than any of the predicted Lewis structures.
Higher resonance energy always correlates to higher electron delocalization and larger stability.
The resonance energy of some other molecules are-Naphthalene (255 kJ/mol), Pyridine (117 kJ/mol).