Generally we compare acidic strengths by comparing relative stability of negative charge formed by removal of acidic hydrogen. However I can't understand the following comparison comparison of acidic strength question

The solution given was

solution to above question

Now I can't understand why the 7-member ring is getting a positive character and the 5-member ring a negative character. I can't get any resources related to it.

Is this related to aromaticity in some way?


1 Answer 1


Yes, it is because of Huckel's rule of aromaticity. This states that for a ring to be aromatic, it must have 4n+2 pi electrons, where n is an integer. For the conjugate bases to be stabilized they would need to be aromatic. For the five-membered ring, the -1 charge is stable as it has 4(1)+2=6 pi electrons. For the seven-membered ring, the +1 charge is stable as it has 4(1)+2=6 pi electrons.


Note: the compound azulene is quite similar and also exhibits dipoles due to aromatic stabilization: https://en.wikipedia.org/wiki/Azulene

  • $\begingroup$ The link you provided states that "Hückel's rule is not valid for many compounds containing more than one ring" in "Polycyclic Ring" section. However i think here different parts of the compound are trying to get partial aromatic character, so it seems correct $\endgroup$ Mar 24 at 18:28
  • 1
    $\begingroup$ @hemantkumar I think for those compounds you have to analyse each ring individually. Usually, some electrons are shared between rings or do not participate in aromaticity so the system cannot be analysed as a whole. $\endgroup$
    – Sticklink
    Mar 24 at 18:39

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