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I came across this wonderful compound, triptycene.

Obligatory image

Why is triptycene an aromatic compound, even though it is non-planar? I thought planarity was a requirement for aromaticity.

pentavalentcarbon
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Gaurang Tandon
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    Related:https://chemistry.stackexchange.com/questions/37641/is-buckminsterfullerene-aromatic?rq=1 – Tyberius Oct 18 '17 at 16:43
  • @Tyberius I don't really see how this is analogous to fullerenes. – bon Oct 18 '17 at 16:44
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    Each benzene ring is aromatic so the whole molecule is described as aromatic. I doubt the aromaticity extends over the bridging carbons though. – bon Oct 18 '17 at 16:45
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    @bon it gets at misconceptions that the OP is having, like that aromaticity is a binary variable and that planarity is an absolute requirement for aromaticity. – Tyberius Oct 18 '17 at 16:54
  • @bon I believed planarity is required as (1) that's what I was taught (2) that's what I found on Wikipedia (3) third party source (i believed barrelenes to be unrelated here) with these three source confirming each other I had a valid question in mind, hence my post. – Gaurang Tandon Oct 18 '17 at 17:00
  • @GaurangTandon I would look at Ivan Neretin's answer in the linked post. As to your question, I think bon gets at the right idea that the aromaticity doesn't extend over the whole molecule. For example, something like tricyclobutabenzne is also considered aromatic even though the aromaticity doesn't apply to the cyclobuta groups. – Tyberius Oct 18 '17 at 17:10
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    For future reference: Aromaticity is not at all an easy concept and there was/is/will be a lot of discussion especially in ambiguous cases. Hückel's rules (plural!) only strictly apply to monocyclic-hydrocarbons, better not overuse it. – Martin - マーチン Oct 19 '17 at 09:57
  • In addition to the reading of all comments and answers (incl. those of the related question) I suggest you to take it easy. Again definition of aromaticity is still subject of research. Simple rules developed for well defined systems can present exceptions, etc. In this case the meaning is simple. Each peripheral ring display a chemistry similar to that of benzene. That for sure. One could look at the behavior of the Hs at the bridge to get an idea. I think (but can't prove) that any chemistry typical of aromatics will take place at the peripheral rings. The molecule is stabilize, so in a sens – Alchimista Oct 19 '17 at 12:37
  • It is aromatic, independent of the behaviour of the bridge. C6H5-C6H11 is an aromatic molecule, at the end. – Alchimista Oct 19 '17 at 12:39
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    Think also of triphenylamine. Is a different example of complexity and variety of chemistry. You will classified as aromatic at first glance (correctly). But there are no inner ring, "just a common substituent" for each ring. However, triphenylamine is also not pyramidal as the N lone pair delocalises indeed! It has a almost planar geometry but whit a tilt of the rings (propeller shape). However you will say it is aromatic independent of this.... – Alchimista Oct 21 '17 at 11:13

2 Answers2

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The confusion arises because of a simplistic definition of aromaticity.

What matters for aromaticity is the planarity of the relevant part of the molecule. In triptycene there are three planar benzene groups joined together. Each is individually aromatic, but the whole molecule is not. Similarly, triphenyl-methane has three individually planar benzene groups, each of which is aromatic, but the molecule cannot be planar because of steric interactions between the groups. There are many other such examples of complex molecules containing an aromatic component attached to other things.

matt_black
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First of all, there's one thing you must know, aromaticity is a property of a ring system and not always of the molecule.

Small molecules like benzene and pyridine have only one ring, so it's "ok" to call them as aromatic, since they have only one ring system. But this approach can't be taken if you want to analyze, say, biphenyl.

enter image description here

Here, if you (incorrectly) try to analyze the whole molecule for aromaticity, you'd get 12 $\pi$ electrons, which is clearly a non-Huckel number, and hence you might infer it as non-aromatic. But however that approach is incorrect. However if I ask you Is biphenyl aromatic? what would you say? The answer is that it's meaningless to ask if biphenyl is aromatic or not. You may however ask Does biphenyl have aromatic ring systems in it?, which is a perfectly valid question.

Pritt says Reinstate Monica
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