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Resonance in Organic Chemistry

November 26, 2012

In chemistry molecules are represented by Lewis dot structures. These structures show valence electrons involved in bonding and lone pair of electrons along with formal charges on atom.

There are certain cases when a single Lewis structure cannot explain the properties of a molecule. In such cases we draw more than one structure to represent the molecule and take the original structure of the molecule as the weighed average of these structures.

These structures are called as resonance structures of a molecule.

Resonance structures are common in organic chemistry where there is possibility of delocalization of electrons.
Features of Resonance Structures:

Resonance organic structure have same position of nuclei with difference in bonding electrons.

The original structure can be considered as a weighed average of the contributing resonance structures

The difference in stability due to resonance is called as resonance energy of the molecule

Resonance is different from tautomerism where one form is continuously changes to another form. Here there is no change in structures. The original structure of molecule is entirely different from the resonance structures.

Resonance structures are hypothetical models to explain the property of molecules

All the resonance structures have same number of electrons and also the net charges on the resonance structures are equal

resonance structures have more energy than that of molecule.
Resonance Effect or Mesomeric Effect:

This is the electron releasing or attracting effect of a group in organic compound. This is due to the resonance structures of a molecule/group. For example nitro group is electron attracting group due to its resonance structures have positive charge on

nitro group

Nitrogennitro group
Example for Resonance in Organic Compounds- Benzene:

A good example for resonance in organic compounds is benzene.

Benzene can be described as a resonance hybrid of 2 structures.

benzene

benzene

The other three structures called as kekule structures are high energetic then actual molecule so they have less significant.

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