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Ranking Resonance Structures

The following examples have been chosen to help illustrate the ranking of resonance contributors

1.
benzene

These two contributors are equivalent as both structures have the same number of covalent bonds, complete octets and no formal charges.

2.
carboxylate
These two contributors are equivalent as both structures have the same number of covalent bonds, complete octets and the negative charge is located on the same type of atom.
3.
allyl cation
These two contributors are equivalent as both structures have the same number of covalent bonds, one incomplete octets and the positive charge is located on the same type of atom.
4.
but-3-en-2-one
In the left structure, all the C and O have complete octets, we have the highest number of covalent bonds and no formal charges, therefore it is the major contributor. Both two structures on the right have formal charges, a C atom with an incomplete octet and one less covalent bond. The contributor on the right has the + and - further apart which is less favourable than the middle contributor where the + and - are adjacent to each other.

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© Dr. Ian Hunt, Department of Chemistry University of Calgary