How to Choose the More Stable Resonance Structure
If a molecule has more than one Lewis structure, it can be represented by the corresponding resonance forms. They are separated by a double-headed arrow (two arrows are used for equilibrium):
Remember, resonance structures have the same placement of atoms, meaning that they represent the same compound and only the arrangement of electrons is different.
What is important as well, is that not all the resonance structures are equally stable. In fact, the most stable resonance form is the resonance hybrid since it delocalizes the electron density over a greater number of atoms:
However, drawing the resonance hybrid is not very practical, and often, certain properties and reactions of the molecule are better explained by a single resonance form.
And for some of these explanations, we need to determine the more stable resonance form. There are a few principles you need to remember to determine which resonance structure is more stable/important, i.e. it contributes more to the resonance hybrid:
Principle 1. Atoms, in general, “don’t like” charges, so having no charge is better:
Sometimes, it is impossible to avoid charges, so if both resonance structures are charged, then the octet rule needs to be considered.
Principle 2. The resonance structure with a complete octet is more stable:
If the resonance structures have charges and the octet is not a determining factor either, then we need to look at the general trends for stabilizing negative and positive charges.
The negative charge is a high density of electrons, so in order for the charge to be better-stabilized, these electrons need to be on a more electronegative atom.
This observation works best only when the two atoms bearing the formal charge are in the same row of the periodic table since they have comparable atomic sizes.
However, as we go down the group, the atomic sizes increase which helps to handle the negative charge more efficiently because the charge density decreases with large volume/surface.
Therefore, if you are comparing elements from different rows in the periodic table, choose the one where the charge is on the bigger atom as the major resonance structure:
If the negative charge is on the same atom in both resonance structures, then look for other factors that can stabilize it. For example, if the charge is next to an electronegative atom that helps to stabilize it as well (inductive effect):
Check the stability of the conjugate base in acid-base reaction for more details about stabilizing the negative charge.
The stability trends for a positive charge are, as expected, opposite to the ones for the negative charge. The positive charge is a center of electron deficiency, therefore to stabilize it, we need electron-donating groups. Alkyl groups (CH3-, CH3CH2-, etc.) being electron-donating stabilize the positive charge on a carbon atom (carbocation):
1o, 2o, and 3o stand for primary, secondary, and tertiary respectively. This shows the number of carbons (alkyl groups) connected to the politely charged carbon. The more alkyl groups, the more stable the carbocation.
For the same reason, putting the positive charge next to an electron-withdrawing group makes it less stable:
Notice that in none of the examples, we had a structure with more than one formal charge. So, remember that any resonance form with an atom bearing a +2 or −2 charge is very unstable and cannot be a significant contributor to the resonance hybrid.
Check this 60-question, Multiple-Choice Quiz with a 2-hour Video Solution covering Lewis Structures, Resonance structures, Localized and Delocalized Lone Pairs,Bond-line structures, Functional Groups, Formal Charges, Curved Arrows, and Constitutional Isomers.
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1.
Determine which resonance structure makes the greatest contribution to the resonance hybrid. I.e. which resonance form is more stable?
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2.
Use curved arrows to draw all the significant resonance structures for the following molecule and determine the major contributor to the resonance hybrid:
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3.
Rank the following resonance structures from the most to the least stable:
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