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Topic: Missing Resonance Structures  (Read 919 times)

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Offline AcceleratorLvL5

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Missing Resonance Structures
« on: July 04, 2020, 06:43:37 AM »
Hello there,

I'm having trouble determining if I'm having too many or too few resonance structures.

I have some exercises from different sources and some of the solutions seem to lack resonance structures, or is it that I have skipped over a rule and didn't notice?

You can see examples in the images attached. In (1) is the 'official' solution, in (2) is the solution I came up with after working on another worksheet that had similar structures and more 'extensive' solutions, the turquoise marked structure is missing in the 'official' solution.
In (3) is a solution from another worksheet that was one of the 'guides' on how I used to solve similar resonance problems.
The second attachment is even more frustrating as the problem appears twice but has 'two' solutions.

Any help is appreciated:)

Offline Babcock_Hall

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Re: Missing Resonance Structures
« Reply #1 on: July 04, 2020, 09:26:28 AM »
For 3) the middle structure has one atom with less than a full octet of electrons.  What can you say about the other two structures?

Offline hollytara

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Re: Missing Resonance Structures
« Reply #2 on: July 04, 2020, 12:09:04 PM »
Adding the resonance structure where the carbonyl is carbocation and alkoxide anion isn't really wrong. 

Offline AcceleratorLvL5

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Re: Missing Resonance Structures
« Reply #3 on: July 04, 2020, 02:29:14 PM »
For 3) the middle structure has one atom with less than a full octet of electrons.  What can you say about the other two structures?

Well, both, the first and third structures, fulfill the octet rule in regards to nitrogen and oxygen. I know that structures with a full octet are major contributing structures whereas without one aren't. The first structure would be the major while the third the minor (and second the very minor, if it is a valid resonance structure). I'm not quite sure what you are hinting at :-\

Adding the resonance structure where the carbonyl is carbocation and alkoxide anion isn't really wrong. 

That's quite the thing I'm unsure about and what makes me feel uneasy (that I might put extra ones where they don't belong or leave them out).

Offline Babcock_Hall

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Re: Missing Resonance Structures
« Reply #4 on: July 04, 2020, 03:22:06 PM »
I agree with you and hollytara, and I don't think any of the ones you drew are wrong.  The middle structure in 3) is minor contributor relative to the other two.  IMO the ones which violate the Lewis octet rule are clearly wrong; beyond that it gets more difficult.  I don't believe that a bright line between what might be termed minor resonance forms versus insignificant forms can be drawn. 

Offline AcceleratorLvL5

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Re: Missing Resonance Structures
« Reply #5 on: July 04, 2020, 04:02:44 PM »
Ok, I will keep that in mind :)
Thank you Babcock_Hall and hollytara four your input and help.

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