This is a question of JAM 2017. According to the answer key the answer is 4. But I found only the last 3 as aromatic . So which one is the 4th aromatic compound? Please explain enter image description here

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    $\begingroup$ Are you sure about the boron compound? $\endgroup$ – Waylander Sep 3 '18 at 9:39
  • $\begingroup$ No Sir..I was wrong... It's non aromatic.. I got the correct answer . 1,2,4 and 5. @waylander $\endgroup$ – user67074 Sep 3 '18 at 9:40

Actually not all of the last three are aromatic. Look carefully at the third compound; the heteroatom is boron, not nitrogen, and so does not contribute the electron pair that would be needed to make six pi electrons in an aromatic ring. That compound is instead antiaromatic.

So JAM 2017 is apparently rendering both of the first two compounds as aromatic. This, however, is dubious. The first compound in the list is a pyrone, which not everyone would call aromatic. To make an aromatic contributing structure in pyrone we would have to render the carbonyl group as its zwitterionic structure ($\ce{C^+-O^-}$) and then distribute the positive charge around the ring which has an electronegative oxygen atom. We'd have to weigh the favorable effect of aromaticity against the unfavorable effect of putting a positive charge into an electronegative ring.

As for the second compound, we would need to "polarize" two carbonyl groups and concentrate two positive charges into the small ring, and that is not happening even with aromaticity. There is a saving grace, however: if the $\ce{N-H}$ protons are tautomerized onto the oxygen atoms, converting the "keto" structure to its "enol" form, then the tautomerized molecule is 2,4-dihydroxy-1,3-diazine, which would be unambiguously aromatic.

What it boils down to is that aromaticity is often not the simple yes/no question often suggested in introductory chemistry texts. Given this ambiguity, one could potentially defend an answer of 2, 3, or 4 (but not the last three for the two larger numbers) to the JAM 2017 question.

  • $\begingroup$ Actually in keto form of uracil it's maybe second most important mesomeric structure is the one with + on nitrogens and is aromatic. $\endgroup$ – Mithoron Sep 3 '18 at 19:04
  • $\begingroup$ Curious about the source. Wikipedia does not refer to a carbonyl polarized structure (but does indicate that both tautomerization and stability of the given structure without Huckel aromaticity occur). $\endgroup$ – Oscar Lanzi Sep 3 '18 at 19:17
  • $\begingroup$ Meh, it's standard "amidic" stabilisation, but it does lead to aromatic mesomeric structures. also even full 2+ charge on aromatic cycles is possible. Ron made an answer about it, mayybe wold findd it... $\endgroup$ – Mithoron Sep 3 '18 at 19:32
  • $\begingroup$ chemistry.stackexchange.com/questions/28882/… $\endgroup$ – Mithoron Sep 3 '18 at 19:46

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