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