Talk:Aromatic hydrocarbon

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Aromaticity of pyrrole[edit]

Elsewhere, we call pyrrole an aromatic hydrocarbon, linking to this article for an explanation of the term. But the pyrrole ring has only five atoms, and one of them is nitrogen, so by the restrictive definition used here, pyrrole would not qualify as aromatic. Can somebody with more detailed knowledge provide a less restrictive definition that includes pyrrole, or should we drop the claim that pyrrole is an example?

ACW 21:42, 29 Sep 2004 (UTC)

Pyrrole should not be considered an aromatic hydrocarbon. However, I get too many google hits to change it outright. Granted, most of them were probably copied from here, but it's enough to make me want to ask my professor before I change it.
Darrien 10:17, 2004 Sep 30 (UTC)
I was wrong. Pyrrole is an aromatic compound. Hückel's rule does apply to five membered rings. The article does mention why Pyrrole is aromatic, but not in a direct way: "...the nitrogen atom's lone electron pair becomes delocalized in the aromatic ring and thus not very accessible to acids", which gives Pyrrole six pi electrons and makes it aromatic.
Darrien 03:26, 2004 Oct 1 (UTC)
So, I guess we need to relax the definition we give here in order to agree with standard usage. I'm not qualified to modify the definition — in fact, that's why I was looking at this article in the first place, hoping to find out. If I'm feeling energetic, I'll go out websurfing, looking for likely formalisms; ideally, we want to provide a definition that lets someone distinguish aromatic from non-aromatic compounds 'algorithmically' or 'by rule'.
You sound like you have enough chemical knowlege to change it. A cut and paste from Hückel's rule with a few tweaks should be enough.
I'm worried about the magic number six, as well. Isn't 1,3,5,7-cyclooctotetraene aromatic? It's got all that pi-resonance going on, and it's got eight pi electrons.
ACW 01:43, 11 Oct 2004 (UTC)
Hückel's rule says that a compound must have 4n + 2 pi electrons to be considered aromatic. With eight pi electrons, 1,3,5,7-cyclooctotetraene would not be aromatic according to Hückel's rule.
Darrien 02:47, 2004 Oct 11 (UTC)

Definition, continued (related to pyrrole issue)[edit]

[[1]] has an outstanding account, and explains all the important issues. I'm reading some other sources, and maybe I'll get up my nerve to try an edit. Unless one of you good people beats me to it.

The key concept is dramatic stabilization, which manifests itself as resistance to addition reactions (breaking the putative double bonds), and decreased heat of hydrogenation. OK, maybe those two are really the same thing.

I was exactly wrong about cyclooctotetraene, which is not aromatic.

Pyrrole is considered aromatic these days, although its stabilization is not as dramatic as that of benzene.

ACW 02:56, 11 Oct 2004 (UTC)

See my recent comment at "Aromatic"[edit]

The article aromatic has quite a lot of overlap with this one. Read my comment there. I'm thinking this one needs to shrink and donate a lot of its content to the other article. After all, a lot of the stuff here is about aromaticity in general, and not specifically about hydrocarbons. This article could just say, "An aromatic hydrocarbon is simply a hydrocarbon that is aromatic. The classic example is benzene. Other important aromatic hydrocarbons are ..."

The stuff about PAH's should also go to its own article, which would have the cool recent stuff about interstellar PAH's, and pointers to polycyclic and here for details about the technical nomenclature.

What do you think (if there is any 'you' there)?

ACW 01:35, 7 Nov 2004 (UTC)

Resonance structures[edit]

I think resonance structures should be discussed or at least mentioned in here. Or should this article refer to aromatic compounds where they can be covered? H Padleckas 16:30, 25 Jan 2005 (UTC)

PAH definition conflict[edit]

The Polycyclic aromatic hydrocarbon is clear in defining that PAH have no hetero atoms and no substituents. NIST's database seems to agree, as it contains only pure H and C compounds, though their information doesn't seem to include an explicit definition. Here's another definition that agrees with that. However, aromatic hydrocarbon and Category:Polycyclic aromatic hydrocarbons have a looser definition allowing hetero atoms.

Is one definition more correct than the other? And if the definition of a PAH is the strict one, what is a compound that is aromatic and polycyclic, but has hetero atoms and functional groups called? I'm out of my league with regards to chemical knowledge here, and I don't have any authoritative sources to look it up in at the moment. But it is definitely a conflict among our articles that should be fixed. - Taxman Talk 14:34, July 30, 2005 (UTC)

Also, the "simplest PAH" is listed as benzocyclobutene. If the definition is correct, this is not a PAH because it has one aromatic ring and one cyclobutane ring. Am I missing something or does this compound not fit?-- 04:03, 9 July 2006 (UTC)
Benzocyclobutene is fully aromatic, it just showed the wrong structure image. Cacycle 12:53, 9 July 2006 (UTC)
None of the 'simplest PAHs' are PAHs. Benzocyclopropene has a break in conjugation in its correct structure. Benzocyclobutadiene has 8 pi electrons, making it antiaromatic. Benzocyclobutene is an ambiguous name referring to either benzocyclobutane (one aromatic and one nonaromatic ring) or benzocyclobutadiene. Benzocyclopropane contains once aromatic ring and one nonaromatic ring. Note that Polycyclic aromatic hydrocarbon makes no mention of the previous four compounds. The simplest true PAHs are napthalene and azulene. Itoe (talk) 16:51, 10 March 2009 (UTC)

removed - Epistemological caveat:[edit]

Removed the following:

[[Epistemological caveat: we need a copernician revolution and do not marvel when any new carbon compound is found somewhere in the universe; most of those molecules are irrelevant for the understanding of the origin of life
- It is essential, when investigating the origin of life, to recognize first the necessary ingrediants, and not to witness simply some form of the ubiquitous presence of carbon containing molecules everywhere in the universe. Carbon is but one of the important components, but it is nothing without nitrogen, oxygen, hydrogen, sulfur and phosphorus. Most aromatics, in living organisms, except for the very evolved forms (and hence far from the origin) are heterocyclic in nature, where nitrogen plays an essential role.
- The role of aromatics will appear at two levels: that of major co-enzymes, the small catalysts of the vast majority of essential biological processes, and that of the building blocks of nucleic acids and proteins. In the latter we are now witnessing data suggesting that the usage of aromatics is much often meant to invent new macromolecules, new proteins with unexpected activities. The "orphan" proteins uncovered in all genome projects - those proteins that do not have significant counterparts elsewhere, excepts in very close kins - are particularly rich in aromatic amino acids, and this appears to be a uneversal rule of in silico proteomics.

As it seemed out of place. But, after rereading the section PAHs and the origins of life, perhaps it fits, if so replace it. Vsmith 01:12, 1 August 2005 (UTC)