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Question

What is an instance of a procedure for writing resonance structures for any given compound's name or structural formula?

I would prefer any of these formats to represent the algorithm:

  • Numbered list
  • Flow diagram
  • Pseudocode

Any format that is human-readable and has ordinated the steps is probably okay.

Purpose

For myself, I would like to have such an algorithm because I prefer to systematically solve problems. I may also wish to write a computer program to perform this task in the future, so understanding the logic in more detail would help me create the decision tree.

For others that do not understand how to draw resonance structures, I hope the responses to this question are useful for their learning. I've seen a lot of people struggle with this topic.

Demonstration of Effort

I have been combing through my organic chemistry textbook (Organic Chemistry, Seventh Edition, L.G. Wade, Jr., Whitman College, Pearson, Copyright 2010) for rules to drawing resonance structures.

Here are the general principles I have found:

  • Each resonance structure has the property of being a valid Lewis structure
  • Electrons can be delocalized
  • Nuclei cannot be delocalized (ie don't move them)
  • Prefer placing pairs of electrons to instead of single electrons
  • Never separate pairs of electrons
  • Maximize the number of octets acheived among the atoms that obey the octet rule
  • Maximize the number of bonds in the structure
  • Prefer to place negative charges on electronegative atoms
  • Prefer to produce structures with minimized charge separation
  • Prefer structures with a negative formal charge instead of positive formal charge
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  • $\begingroup$ Never separate pairs of electrons? $\endgroup$ – ringo Mar 9 '16 at 1:31
  • $\begingroup$ A problem here is that your general 'principals' are guidelines, rather than rules. This is clear in your last 3, when you use the term 'prefer'. At best, these rules will give you the 'best' structure, that is the one most descriptive of the molecule as a single picture. However the purpose of drawing resonance structures is to make several, and apply value/weights to them to get an average view of how the molecule will behave. I understand this makes the topic maddeningly frustrating for some. $\endgroup$ – Lighthart Mar 9 '16 at 1:32
  • $\begingroup$ @ringo The text could be wrong, or I misunderstood. It states "Most stable compounds have no unpaired electrons, and all the electrons must remain paired in all the resonance forms". In any case, as long as a proposed algorithm demonstrably works whenever tested, we're in good shape. $\endgroup$ – Galen Mar 9 '16 at 1:47
  • $\begingroup$ @Lighthart I agree, some or all of the points above are guidelines. The implication of your statement about the purpose invokes a thought that there are a subset of structures that are useful for prediction of molecular interaction, as well as a subset that are expected as answers on tests or assignments. For both of those subsets, I don't always find that the combination of resonance structures I draw are in the subset I want to draw resonance structures for. $\endgroup$ – Galen Mar 9 '16 at 2:12
  • $\begingroup$ Sadly, as a former chemistry professor, I find 'experience' is the best help here. For example, don't consider open shell systems unless you are talking about a carbocation explicitly, or aldehydes or ketones. That is something which becomes apparent after awhile studying, but not from the start. From the student's perspective, there are a myriad of these 'exceptions' and chemistry becomes a mess of trivia. Really, what we want you to learn is how to judge the importance of various factors, and then apply the most important ones to the problems. I understand it is frustrating. $\endgroup$ – Lighthart Mar 9 '16 at 17:38

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