An increase in electron density on an atom and the corresponding decrease in electron density on other atom in the same molecule is called Resonance Effect. It is actually used to explain the structure of a conjugated molecule or an ion (consisting of group of atoms) which cannot be explained by a single Lewis structure.
The actual structure of the molecule is the weighed average of two or more contributing structures and the representation of real structure as weighed average of two or more contributing structures is called Resonance. There are several contributing structures of a molecule and the actual structure corresponds to the structure of lowest possible energy.
For example: In case of benzene the actual structure of benzene corresponds to kekule’s structures not to the Dewar structures i.e.
Choice of correct arrow for the resonance process:
There are three types of arrows use in resonance:
- A double headed arrow on one side is used for the transfer of two electrons. For example:
- A single harpoon arrow is used for the transfer of one electron. For example:
- A double headed arrow on both sides is used to show the interconnectivity of two contributing structures. For example:
Rules for writing resonance structures:
In resonance structures we can move electrons to stabilize the molecule. This electronic movement is called delocalization.
- In all the resonance structures the number of electrons should be same and the skeleton of structure should also be the same. For example: In case of phenol the number of electrons is 94 in all its resonance contributing structures and skeleton of structure is also same in all resonance contributing structures. i.e.
- Number of lone pairs in in all the resonance structures should be same. However, the position of lone pairs may change. For example: In case of phenol there are two lone pairs in all the structures as shown in the above diagram.
Note: Negative charge also show a lone pair of electrons.
- Rules for writing Lewis structures must be followed in all resonance structures and hybridization should also be same.
Rules for estimating resonance structures’ stability:
- Structure having a greater number of covalent bonds will be more stable. The reason for stability is the completion of Octet of more atoms. For example, In fig below, 1 is most stable
- Resonance structure having minimum number of charges is more stable. For example: In fig below, 1 is most stable as it has no charge and all other structures have two charges each.
- Resonance structure having least charge separation is is most stable. For example: In fig below, 3 is least stable as it has greater number of charge separation.
- A resonance structure having negative charge on most electronegative atom is most stable. For example: In fig below, 1 is most stable since nitrogen is more electronegative.
- Resonance structure containing positive charges onto adjacent atoms is least stable. For example: structure 2 in fig below:
- Resonance structure having positive charge on less electronegative atom is more stable. For example: In fig below, 2 is more stable.
- Equivalent resonance forms have same stability and equally contribute to the real structure. For example: Resonance forms of benzene.
It is the combination of all resonance contributing structures the structure with least charge accepted as real structure but the correct structure is the combination of all contributing structures. Resonance hybrid can be determined from the contributing structures by applying following rules:
- Draw all the possible resonance structures of a molecule.
- Draw a dotted line where a new bond (double or triple) can be formed.
- Draw lone pairs only which are available on atoms in all resonance structures. The structure obtained will be resonance hybrid. i.e.
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