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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1$\begingroup$ 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. $\endgroup$– Oscar LanziJan 31, 2022 at 11:48
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$\begingroup$ @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 ) $\endgroup$– IntegerJan 31, 2022 at 12:41
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1 Answer
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How many of the following are aromatic?
Among these compounds first four are aromatic where fifth compound is not aromatic.
- 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.
- 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. - 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.
- 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.
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