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Resonance Energy of Benzene

December 17, 2012

Resonance is also called mesomerism and arises due to localised electrons which make it impossible to represent the chemical structure by single Lewis diagram.

How is resonance developed in benzene?

The empirical formula of benzene is CnHn.

It means it consists of a conjugated planar ring system with delocalized pi electron clouds.

These electtrons which form the double bond keep on ‘hopping’ between subsequent bonds.This also can be expressed in following sentence.

The electrons for C–C bonding are distributed equally between each of the six carbon atoms.

Each carbon atom is attached to one hydrogen atom in addition to two carbon atoms.

The electrons of the pi bonds keep on oscillating forth and back.

In this way an the valencies of the atoms stand satisfied,albeit in a unconventional way.

Thus in order to facilitate this,the structure would be planar and not three dimensional.

This enhanced stability is the fundamental property of aromatic molecules that differentiates them from non-aromatic molecules.

Average length between the C-C and C=C is at 0.139 nm.

This develops resonance in benzene.

The phenomena of resonanace gives extra energy to benzene molecule.
Resonance Energy

Consider the following delocalised and contributing structures.


delocalised contributing structures

contributing structures

contributing structures

The energy required to change the delocalised structure to one of the contributing structures is called resonance energy.

It is calculated by finding the difference between hear of hydrogenation of the resonance and contributing structure.

This being 143.1 kJ (34.2 kcal), is the resonance energy of benzene.

Quantum mechanics also helps to measure the resonance energy. It is noteworthy to mention here that resonance energy and the planar structure contribute to each other.


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