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How can we determine which of the above conpounds are aromatic . My teacher taught me to remove a hydrogen and shift the electrons but I did not understand it . Could someone help me so that i can learn and understand this subject better.

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    $\begingroup$ If you're supposed to apply the 4n+2 rule, you can't for the fourth or fifth molecule (2-pyrone, cyclopentadienone). The 4n+2 rule does not apply to systems with pendant pi bonds, and the people who posed this problem should have known that. $\endgroup$ Jan 31, 2022 at 11:48
  • $\begingroup$ @OscarLanzi could you please check if my answers are correct as i am totally confused . ( the first one is aromatic (4n+2 = 6 ) the second and third aren't . the fourth one is aromatic and the fifth isn't ) $\endgroup$
    – Integer
    Jan 31, 2022 at 12:41

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How many of the following are aromatic?

Among these compounds first four are aromatic where fifth compound is not aromatic.

  1. Coming to the first compound we have three π-bonds and a vacant orbital on B. So there are 6 electrons in conjugation and it obeys Hückel's rule. Hence, it is aromatic.
  2. Here we have two π-bonds and a lone pair on N. So there are six electrons in conjugation and hence, it is aromatic.
    Note: Lone pair on right nitrogen is not involved in conjugation.
  3. Similar to 1 and 2, we have a vacant orbital on B, a lone pair on N and two π-bonds. So totally there are six electrons in conjugation and hence, it is aromatic.
  4. Form the following image you can understand why 4th compound is aromatic. Since there are six electrons in delocalisation where four electrons are from two π-bonds and two electrons from oxygen's lone pair.
    resonance structures of 2-pyrone

I hope you can understand why 5th compound is not aromatic as it does not obey Hückel's rule.

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  • $\begingroup$ This is probably what they want in the textbook, but not really the whole story for molecules 4 and 5. See my comment to the question. $\endgroup$ Feb 1, 2022 at 10:43

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