Resonance: Most Important Resonance Contributor

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 achieved by using the resonance hybrid structure.  However, because the resonance hybrid does not show 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 contributor 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 contributor was the actual reactant.  Thus we need a set of rules to determine the most important resonance contributor.  These rules are based on the idea that if individual resonance contributors 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 most important contributor has the maximum number of atoms with full octets.

This preference gets priority over the other three rules for determining the most important resonance contributor.

The carbon of structure A has an open octet.  All the atoms of structure B have full octets.  Therefore contributor B is more important than contributor 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 contributor must have formal charge, the most importnat contributor has 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 contributors C and D have a complete octet, so we turn to other preferences to determine the most important resonance contributor.  A negative charge is best accommodated by a more electronegative atom.  Because oxygen is more electronegative than carbon, contributor D is more important than contributor C.  (If the ion shown above was a cation, then the resonance contributor with the positive charge on carbon would be more important than the contributor with the positive charge on oxygen.)

Preference 3: The most significant contributor has the maximum number of covalent bonds.  Contributor B (above) is more important than contributor A because B has the carbon-oxygen p bond absent in A.

Preference 4: The most significant contributor will have the least number of formal charges.

Resonance contributor F is more significant than contributor E because F has no atoms with formal charges, whereas E has two atoms with formal charges.  (Contributor F is also favored by Preference 3 as well.)

Preference 5: The most significant contributor has the least number of unpaired electrons.

For example, contributors G and H each have one unpaired electron, and thus are preferred over contributor I which has three unpaired electrons.  Resonance contributors that include avoidable unpaired electrons are rarely of any consequence and thus should not be considered.  There is one common exception: molecular oxygen.  Due to molecular orbital considerations, molecular oxygen is best described as having two unpaired electrons and an oxygen-oxygen single bond (contributor J) and not as lacking unpaired electrons with an oxygen-oxygen double bond bond (contributor K).

Exercises: Determine the most significant resonance contributor for each set of contributing resonance structures drawn previously.

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