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I was asked which of the following were aromatic: potentially aromatic compounds

My understanding of aromatic nature is, that the molecule must be cyclic, planar, every atom in the ring must be $sp^2$-hybridized and the molecule must have [4n+2] π electrons, where n∈N (Hückel’s rule). The molecular orbitals also have to have the correct orientation and phase for aromaticity.

  • A and B are aromatic since they follow conditions as above (including empty orbital of boron in conjugation)
  • azulene's aromaticity is due to the “fusion” of two aromatic ions, both with 6 π electrons, cycloheptatrienyl (tropylium) cation and cyclopentadienyl anion.

azulene's resonance

Based on the above My answer was a, b, and c.

Given answer: a, b, c, d.

I checked for d. It was aromatic as described in this link. 1. The diagram indicates only π electrons along the blue need to be considered amounting to 18 π electrons in conjugation.

porphyrin

My question is why only consider π electrons in blue and why not others? How do we explain the aromaticity of porphyrin?

References

https://en.wikipedia.org/wiki/Porphyrin

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  • $\begingroup$ Simply that a circuit satisfying those rules is attained. Though one can image system in which fused moiety can pin electrons and reduce the aromaticity of the whole ensemble. $\endgroup$
    – Alchimista
    Commented May 19, 2019 at 10:29
  • $\begingroup$ @Alchimista why cant pi electrons in black be used instead? $\endgroup$
    – Chakravarthy Kalyan
    Commented May 19, 2019 at 12:57
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    $\begingroup$ Simply because that would lead to a higher energy of the system. By the way, as usual and even more with a complex ring, first one assess the aromaticity, then moves to explain. For instance in this case coordination properties and reactivity at the various position point to aromaticity. Then you identify the blue system. Your exercise requires a previous knowledge or a good feeling for aromaticity. Is the same with Hueckel rules. $\endgroup$
    – Alchimista
    Commented May 20, 2019 at 10:15
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    $\begingroup$ Counting electrons in a ring is a rather crude way to determine whether a structure is aromatic. There are far more direct and reliable ways to prove aromaticity, such as unusual NMR chemical shifts due to the presence of aromatic ring currents. Typically, once a compound is shown to be aromatic by these more stringent measures, then a posteriori arguments regarding electron count tend to be made. $\endgroup$
    – Nicolau Saker Neto
    Commented May 21, 2019 at 2:01

1 Answer 1

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The aromatic properties are evident from the heats of combustion, the fact that all bonds have the same length, the ability of porphyrins and metalloporphyrins to undergo electrophilic substitution reactions, the stability of the porphyrin nucleus in electron impact mass spectrometry and the characteristic chemical shifts observed in proton NMR spectra1.

The 18π electron substructure (shown in blue in your figure) was thought to be responsible for the aromatic characteristics. In support of this view it was noted that the outermost bonds in the pyrrolenine rings(the not blue double bonds in your figure) take part in cycloaddition reactions(Fig.1, taken from ref.2) and therefore have a significant double bond character suggesting that their electrons are more localized.

This 18π model is also called "the diaza[18]annulene model" because it was speculated that the aromatic pathway found in porphyrins resembles [18]annulene(fig.2, ref.1). You can also note the similarities in the spectra of [18]annulene(fig.3, ref.3) and porphyrins: both have a major peak in the UV region and a less intense absorption at longer wavelengths.

Other supportive data to the the diaza[18]annulene model are proton NMR studies all mentioned in ref.1

  1. https://www.worldscientific.com/doi/abs/10.1142/S1088424611004063
  2. http://www.scielo.br/pdf/jbchs/v15n6/22664.pdf
  3. Chem. Commun., 2016, 52, 4710

Fig.1

Fig.2

Fig.3

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