I came across this question:

Which is the most stable resonating structure of p-nitrosobenzene among the following?

possible resonance structures of p-nitrosobenzene

I have been taught about the priorities for resonance structure stability as follows:

  1. Complete octet $\rightarrow$ more stable

  2. Uncharged $\rightarrow$ more stable

  3. Lower formal charge $\rightarrow$ more stable

  4. Less separation between opposite charges $\rightarrow$ more stable

  5. Negative charge near more electronegative atoms and positive charge near less electronegative atoms $\rightarrow$ more stable

So according to the above rules, (b) and (c) are the among the most stable. Bu then applying rule #4 above, (b) should be more stable, because here each positive charge is in vicinity of negative charge.

However the answer given is (c). Why is this?

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    $\begingroup$ A -ve charge on oxygen is better than a -ve charge on carbon, so (c). $\endgroup$ – Sawarnik Apr 7 '17 at 6:41
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    $\begingroup$ @user43496 For future reference, please consider the dimension and orientation of the figures included. To render the figure more legible, I rotated and down-scaled your photo. $\endgroup$ – Buttonwood Apr 7 '17 at 13:46
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    $\begingroup$ There is no such thing as a most stable resonance structure. $\endgroup$ – Martin - マーチン Jul 1 '17 at 9:53
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    $\begingroup$ Fyi: None of these is a structure for $p$-nitrosobenzene. Properly the name is $p$-nitroso_nitro_benzene. $\endgroup$ – Oscar Lanzi Jul 1 '17 at 23:18

Because on C; the negative charge should be on oxygen (the more electronegative element) instead of on carbon


The technical terms

There is no such thing as a most stable resonance structure. – Martin

The correct term would be resonance contributor. All these structures contribute, so the charges are spread over the molecule. Also, formal charges are mostly an accounting scheme. For example in the ammonium ion, most of the positive charge is near the hydrogen atoms, but the Lewis structure shows a formal charge on the nitrogen atom.

Here is one explanation of resonance and contributing structures from wikipedia

In chemistry, resonance is a way of describing bonding in certain molecules or ions by the combination of several contributing structures (or forms[1], also variously known as resonance structures or canonical structures) into a resonance hybrid (or hybrid structure) in valence bond theory.

The question

So I edited the question:

Which is the [most stable resonating] major contributing resonance structure of p-nitrosobenzene among the following?

I would agree with @nana: Structures that have a formal charge on nitrogen or oxygen are probably stronger contributors to the resonance hybrid than those with charges on carbon.


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