If we count the total number of π electrons, there are 10. But to apply Hückel's rule to this molecule, we should count only the electrons of the outer periphery and there are 8 of these. How can the molecule be aromatic?
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9$\begingroup$ Hückel's rule applies to monocyclic systems only. $\endgroup$– Ivan NeretinApr 16, 2016 at 14:53
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4$\begingroup$ Draw the other tautomer where the six-member ring does not have the double bond in the 'inner'side of the 5-6 ring junction. That will get you a 10-electron system. Or live with Ivan's comment and accept two aromatic systems that are fused together. $\endgroup$– LighthartApr 16, 2016 at 14:54
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1 Answer
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Consider this structure of purine:
The ring on the left contains two nitrogen heteroatoms and is aromatic due to π electron conjugation. Having six π electrons, it obeys Hückel's rule.
For the ring on the right, it contains four π electrons in bonds and the lone pair on the NH group takes part in conjugation. Hence the total number of electrons taking part in conjugation is six, which makes it aromatic by Hückel's rule as well.
Since both the rings of purine are aromatic, the compound is aromatic.
For more information see: Ciesielski, A.; Krygowski, T. M.; Cyrański, M. K.; Balaban, A. T. Defining rules of aromaticity: a unified approach to the hückel, clar and randić concepts. Phys. Chem. Chem. Phys. 2011, 13 (9), 3737–3747. DOI: 10.1039/C0CP01446J.
answered Apr 16, 2016 at 18:06
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8$\begingroup$ It does not obey Hückel's rule. It is not monocyclic, and you must not apply it to each ring independently. Hückel's rule is very strict and can only applied to a handful of systems. $\endgroup$– Martin - マーチン ♦Apr 24, 2016 at 12:30
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1$\begingroup$ @Mart Clarification: Did you mean "must apply" or "must not apply"? Thanks! $\endgroup$ Jan 6, 2018 at 5:49
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1$\begingroup$ @paracetamol I meant not. Hückel's rules apply only to mono cycles $\endgroup$– Martin - マーチン ♦Jan 6, 2018 at 6:01
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$\begingroup$ @Mart Follow-up question: I see, so to employ Hückel's rules the entire system must be a monocycle, but I should not apply the rules to individual cycles in a polycyclic system? (Trying to see if I understood this right) Thanks again! $\endgroup$ Jan 6, 2018 at 7:59
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2$\begingroup$ @paracetamol Hückel's rules are guidelines. They have been used for situations as generalisations, which were not originally covered by the set they were derived from. If used for these occasions they might lead to wrong conclusions. In general: not all aromatic compounds obey Hückel's rules, but all compounds that do obey them are aromatic. $\endgroup$– Martin - マーチン ♦Jan 6, 2018 at 8:02