Resonance: Most Important Resonance Contributor

Discussion:  Many molecules or ions that participate in an organic reaction have resonance. 

  When deciding which resonance contributor to use, it makes sense to use the one that makes the greatest contribution to the resonance hybrid.

  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.

Drawing Resonance Forms

There are several things that should be checked before and after drawing the resonance forms. First know where the nonbonding electrons are, keep track of formal charges on atoms, and do not break sigma bonds. Finally, after drawing the resonance form make sure all the atoms have eight electrons in the outer shell. Checking these will make drawing resonance forms easier.

When drawing a resonance structure there are three rules that need to be followed for the structures to be correct:

1. Only electrons move and the nuclei of the atoms never move.
2. Only electrons that can move are pi electrons, single unpaired electrons, and lone pair electrons. 
3. The total number of electrons in the molecule do not change and neither do the number of paired and unpaired electrons. 

Approaches for moving electrons are move pi electrons toward a positive charge or toward an another pi bond. Move a single nonbonding electron towards a pi bond. Move lone pair electrons toward a pi bond and when electrons can be moved in more than one direction, move them to the more electronegative atom. 

Helpful hints

1. Two resonance structures differ in the position of multiple bonds and non bonding electron. The placement of atoms and single bonds always stays the same. 
2. They must make sense and agree to the rules. Hydrogens must have two electrons and elements in the second row cannot have more than 8 electrons. If so, the resonance structure is not valid. Always look at the placement of arrows to make sure they agree. 
3. Electrons move toward a sp2 hybridized atom. The sp2 hybridized atom is either a double-bonded carbon, or a carbon with a positive charge, or it is an unpaired electron. Electrons do not move toward a sp3 hybridized carbon because there is no room for the electrons. 

After drawing resonance structures check the net charge of all the structures. For example, if a structure has a net charge of +1 then all other structures must also have a net charge of +1. If not, the structure is not correct. Always check the net charge after each structure. These important details can ensure success in drawing any Resonance structure.

Rules for drawing contributing resonance structures.  Some rules must be considered when drawing resonance structures.

Rule 1: All resonance structures must have the same number of valence electrons.  Electrons are not created or destroyed, nor are they lost or gained to other molecules or ions during resonance.





The rule is violated because structure E has 12 valence electrons (four bonding pairs and two lone pairs), whereas structure F has 14 valence electrons (five bonding pairs and two lone pairs).  Therefore these cannot be resonance structures of the same ion.  (Structure F also violates rule 2.)

Rule 2:  The octet rule must be obeyed.  Hydrogen may never have more than two valence electrons.  Lithium through fluorine may never have more than eight valence electrons. (A carbon with five attachments, often called a "pentavalent carbon" has ten valence electrons.  This is forbidden because carbon does not have the space in its orbitals to accommodate ten electrons.  You should take care to avoid this common mistake made by inexperienced organic chemistry students.)  Certain elements commonly encountered in organic chemistry may have ten valance electrons.  These elements are in periods three and higher in the periodic table, and include chlorine, bromine, iodine, phosphorus and silicon.  Of all the atoms commonly encountered in organic chemistry, only sulfur can routinely expand its octet to include twelve valence electrons. These atoms expand their octets so as to improve the importance of the resonance structure.

Resonance structure G is acceptable.  Structure H is not acceptable because the carbon has ten valance electrons.


Structures I and J are both acceptable resonance contributors for bisulfate ion, the conjugate base of sulfuric acid.  The sulfur atom of structure J has 12 valence electrons, an expanded octet.  Sulfur is a third row element, so an expanded octet is allowed.  Structure J is the more important resonance structure because it maximizes the number of covalent bonds and minimizes the number atoms with a nonzero formal charge.

Rule 3: Nuclei do not change positions in space between resonance structures.  Resonance structures differ only in the arrangement of valence electrons.


Structures K and L are both acceptable Lewis structures, but they are not related by resonance because the circled hydrogen atom has changed position in space.

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).

Resonance: Most Important Resonance Structure
examples
When determining the most important resonance structure, we consider full octets above other preferences.  In neither of these resonance structures do all atoms have full octets.  When looking to rank structures, we must look for differences.  In each structure, the positive charge resides on the exactly the same type of atom, each has the same number of covalent bonds, and each has the same number of formal charges.  Thus, there is no preference between these two structures.  Resonance structures which are deemed to be of equal importance (equal energy) are termed degenerate.  Structures A and Bcontribute equally to the resonance hybrid structure.

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Considering full octets first, we see that all atoms of structures C and D have full octets.  The have no formal charge, and the same number of covalent bonds.  These structures are of equal importance.

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Each of these structures has one carbon atom with an open octet (the carbon with the positive formal charge).  The charge always resides on a carbon, and there is an equal number of formal charges and covalent bonds for each structure.  At first glance, we would conclude that all five structures are of equal importance.  (We will learn in section 6.3 of the text that a carbon bearing a positive charge, called a carbocation, is most stable when the carbon bearing the charge has the greatest number of other carbons attached to it.  We will also learn in sections 9.1 and 9.2 of the special stability associated with having three alternating carbon-carbon double bonds in a six-membered ring, called aromaticity.  In this case, aromaticity is the most important factor, so degenerate structures E and F are the most important.)

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The carbon of resonance structure J has an open octet.  All atoms of structure K have complete octets.  Although structure K has formal charges, and the positive charge is on the more electronegative atom (oxygen vs. carbon), the full octet preference dominates.  Structure K is therefore the most important resonance structure for carbon monoxide.

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The carbon bearing the positive charge of structure M has an open octet.  All of the atoms of structures L and N have full octets, so structure M is less important.  Structure M also has more formal charges than either of the other two structures.  Structures L and N have the same number of formal charges and covalent bonds, so the difference is the position of the formal charge.  A negative charge is best accommodated by a more electronegative atom, so structure N is more important than structure L.

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The carbon bearing the positive charge of structure P has an open octet.  All of the atoms of structures O and Q have full octets, so structure P is less important.  Structures O and Q have the same number of covalent bonds.  Structure Q has two formal charges whereas structure O has no formal charges.  Thus structure O is more important than structures P or Q.

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All atoms in all of these resonance structures have a complete octet.  (Recall that while phosphorus can expand its octet to ten or twelve valence electrons, it still needs only eight electrons to fill its octet.)  The negative charges always reside on oxygen and the positive charges always reside on phosphorus, so we cannot rank importance of these structures using Preference 2.  Structure R has four covalent bonds, whereas structures S - V each have five covalent bonds (four single and one double), so structure R is less important than structures S - V.  Structures S - V all have three oxygens with formal charges.  Thus we conclude that structures S - V are degenerate, and are equally important.