Illustrated Glossary of Organic Chemistry

Resonance contributor preference rules: A series of rules used to judge the relative significance (i.e., contribution to the resonance hybrid) of resonance contributors. The greater number of rules violated (or the greater number of times a particular rule), the less significant the given resonance contributor is.

Rule 1: The most significant resonance contributor has the greatest number of full octets (or if applicable, expanded octets).




Open octet on carbon
Less significant resonance contributor
  
All atoms have full octets
More significant resonance contributor

Rule 2: The most significant resonance contributor has the least number of atoms with formal charges.



Two formal charges
Less significant resonance contributor
  
No formal charges
More significant resonance contributor

Rule 3: If formal charges cannot be avoided, the most significant resonance contributor has the negative formal charges on the most electronegative atoms, and the positive formal charges on the least electronegative atoms.



Negative formal charge on carbon (EN = 2.5)
Less significant resonance contributor
  
Negative formal charge on oxygen (EN = 3.5)
More significant resonance contributor

Rule 4: The most significant resonance contributor has the greatest number of covalent bonds.

Three covalent bonds
Less significant resonance contributor
  
Four covalent bonds
More significant resonance contributor

Rule 5: If a pi bond is present, the most significant resonance contributor has this pi bond between atoms of the same row of the periodic table (usually carbon pi bonded to boron, carbon, nitrogen, oxygen, or fluorine).



Carbon-chlorine double bond
Less significant resonance contributor
  
Carbon-fluorine double bond
More significant resonance contributor

Rule 6: Aromatic resonance contributors are more significant than resonance contributors that are not aromatic.



Nonaromatic
Less significant resonance contributor
  
Aromatic
More significant resonance contributor