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While working out some questions on comparing the basic nature of various compounds, I came across the following two compounds:

A: 4‐nitroaniline; B: piperidin‐2‐one

In case of A I could see several resonating structures, and also the −M effect of the nitro group. So justifying based on the number of resonance structures, I could tell that the lone pairs are more delocalized in A than in B. Hence B is more basic.

However, on verifying my answer I found that A was more basic.

The only reason I could come up with is that in compound A, majority of the resonance includes a negative charge on carbon, but in compound B, the negative charge lands on the more electronegative oxygen, which better succeeds in stabilizing it. But it doesn't seem right for me.

Can anyone please tell me the correct reason for this?

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  • $\begingroup$ I dont know why 2 people downvoted this. Is there any problem with the question? $\endgroup$ – Vamsi Krishna Jun 8 at 17:39
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    $\begingroup$ -1. It's unclear what you're trying to ask. Do you want to find out which compound is more stable? quality vs quantity is like apples vs oranges - you can't compare them. $\endgroup$ – Aniruddha Deb Jun 8 at 18:01
  • $\begingroup$ But then on what aspects do we compare these compounds? $\endgroup$ – Vamsi Krishna Jun 8 at 18:40
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    $\begingroup$ @aniruddhaDeb yes I have edited the question to make it more clear now $\endgroup$ – Vamsi Krishna Jun 8 at 18:43
  • $\begingroup$ You might ask if the hydrochloride salts of these two compounds exist. $\endgroup$ – user55119 Jun 8 at 19:21
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Considering that you're referring to the difference in the number of resonating structures, you're right in that aspect. 4‐Nitroaniline does have more resonating structures than piperidin‐2‐one. But in organic chemistry, you should be knowing that for a phenomenon to happen, there have to be several contributing factors.

While this includes the number of resonating structures as one, you have to take into consideration the other factors as well. And this includes the other factor you have mentioned in your answer — the position of the negative charge.

Since carbon is an atom which has an electronegativity of 2.55 (According to the Pauling scale), a negative charge on it would make it highly unstable, lowering the resonance energy, thus reducing the resonance stabilization in it. Nitrogen and oxygen have electronegativity of 3.04 and 3.44, respectively, thus a negative charge on those atoms would increase the resonance energy, thus increasing the resonance stabilization.

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  • $\begingroup$ @Firefox1921 Thank you for the answer, however isn't the negative charge going to oxygen in BOTH the cases? That is why i do not think it should be the main reason $\endgroup$ – Vamsi Krishna Jun 12 at 7:44
  • $\begingroup$ Yes @VAMSIKRISHNACHILAKAMARRI, the negative charge is going to the oxygen in both cases. But it isn't going on carbon in both the cases, right? $\endgroup$ – Firefox1921 Jun 12 at 11:24

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