Resonance: Most Important Resonance Structure
Discussion: Many molecules or ions that participate
in an organic reaction have resonance. When writing the mechanism
for the reaction, the best representation of reality would be acheived
by using the resonance hybrid. However, because the resonance hybrid
does not explicitly show electron pairs that are shared by resonance,
its use in mechanisms can be unclear. Thus, we often use a single
resonance contributor instead of the hybrid. When deciding which
resonance structure to use, it makes sense to use the one that makes the
greatest contribution to the resonance hybrid. If we cannot use the
hybrid, then we should use the next closest structure. In addition,
many (but not all) reactions of molecules or ions with resonance proceed
as if this most important resonance structure was the actual reactant.
Thus, we need a set of rules to determine the most important resonance structure.
These rules are based on the idea that if individual resonance structures
did indeed exist, the most thermodynamically stable structures would make
more significant contributions to the resonance hybrid. Factors that
enhance thermodynamic stability are maximization of covalent bonding and
minimization of charge. Resonance increases stability by increasing
the bonding between adjacent atoms and by distributing charge over a greater
number of atoms.
Preference 1: The best structure will have as many full octets as possible. This rule gets priority over the other three rules for determining the most important resonance structure.
The carbon of structure A has an open octet. All the atoms of structure B have full octets. Therefore, structure B is more important than structure A, despite the fact that the positive charge is on the more electronegative oxygen atom instead of the less electronegative carbon atom.
Preference 2: If a resonance structure must have formal charge, the best structure will have these charge(s) on the atoms most willing to accommodate them. Negative charges are best accommodated on more electronegative atoms, whereas positive charges are best accommodated on the least electronegative atoms.
All atoms of resonance structures C and D have a complete
octet, so we turn to other preferences to determine the most important
resonance structure. A negative charge is best accommodated by a more
electronegative atom. Because oxygen is more electronegative than
carbon, structure D is more important than structure C.
(If the ion shown above was a cation, then the resonance structure with
the positive charge on carbon would be more important than the structure
with the positive charge on oxygen.)
Preference 3: The best structure has the maximum number of covalent
bonds. Structure B (above) is more important than structure
A
because B has the carbon-oxygen p bond
absent in A.
Preference 4: The best structure will have the least number of formal charges possible.
Resonance structure F is more important than structure E
because F has no atoms with formal charges, whereas E has two atoms
with formal charges. (Structure F is also favored by Preference
3 as well.)
Exercises: Determine the most important structure for each set
of resonance-contributing structures drawn previously.