Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: dvp331 on August 04, 2011, 12:13:05 AM
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I was wondering why this molecule is aromatic... I think it is these double bonds that are throwing me off - are these considered part of the ring system?... I counted only 4 pi electrons and figured it was anti-aromatic... but this is not correct
Can someone please help me understand this
Thanks!
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Check the Hybridisation of each C-atome. sp²!!
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Oh ya... ok that's true
But then how many pi electrons are in this system?
Would it be 6 then, assuming planar and sp2 atoms where possible
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In the ring I sse only 2. Law of Hückel 4n +2
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Let's see if drawing it this way makes the whole thing a little more clear.
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Its 4: the two of the doublebond and the 2 at the negative charged Carbon. So its antiaromatic.
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I count six - two for the double bond, two for the lone pair in a pi orbital on the anion, and one each for the electrons in the exocyclic double bonds.
Ring systems like to be aromatic - if every atom in the ring has an available pi orbital and if there is a resonance structure that you can draw that will make the system aromatic, it is usually a safe bet that the molecule will have substantial aromatic character.
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Looks like it would be isoelectronic with maleimide or maleic anhydride and those are weakly anti-aromatic.
http://pubs.acs.org/doi/abs/10.1021/jo9621985 (http://pubs.acs.org/doi/abs/10.1021/jo9621985)
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The difference lies in the presence of a heteroatom in the ring and in the carbonyls instead of alkenes for exocyclic bonds. Both of these stabilize the localization of the electrons in the anti-aromatic state rather than the delocalization of the aromatic state. The resonance structures placing one exocyclic pi-bond electron within the ring are carbon radical structures, which are more stable than an oxygen radical structure as required for a carbonyl.
Janicki and Patelli demonstrated the effect of a heteroatom stabilizing the localizations of electrons in terms of spin splitting - Janicki, Slawomir Z.; Petillo, Peter A., Journal of Physical Chemistry A (2000), 104(35), 8224-8226.
As for the maleimide paper, the authors point out that the destabilization is very small even for those systems, and that their calculations aren't always consistent. In particular, comparison with the acyclic system shows an unexpected stabilization rather than the slight destabilization. In any case, they point out the the measured destabilization is much less than anticipated by reactivity under Diels-Alder conditions.
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I agree with everything fledarmus says.
To simplify a bit, the molecule has an aromatic resonance structure, where one of the out-of-ring double bonds gives its electron pair to be in the ring system, and the other out-of-ring double bond pulls its electron pair out and places it on the primary carbon. In other words, this resonance structure has a positive charge on one primary carbon and a negative charge on the other primary carbon. This resonance structure is aromatic.
Any time you can draw one resonance structure which is aromatic, and another which is antiaromatic, the one which is aromatic will win. Compare fulvene for an example; it has the same kind of out-of-ring double bond, and has to "choose" whether to keep its lone pair in-ring (aromatic) or put it out of ring (antiaromatic). Naturally the more stable, aromatic resonance structure is a better representation of the true electronic structure.
The interesting thing in your molecule is that the most stable structure is not symmetrical even though the molecule is. Any computational or theoretical chemists among us? What will happen here? Jahn-Teller distortion?
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This is getting a little interesting so I tried some searches to learn more about this molecule. I was starting with looking up the pKa of the site in question (unusually low would likely indicate aromaticity, unusually high would indicate antiaromaticity) but it looks like no one has ever made this molecule*. A further search (based on structure in SciFinder) resulted in: Langler and McBain, Australian Journal of Chemistry 2002 55 (11) 727 - 731 http://www.publish.csiro.au/nid/51/paper/CH02103.htm (http://www.publish.csiro.au/nid/51/paper/CH02103.htm). I can't access the full text, so I don't know what their conclusions are, or if they even actually considered the molecule in question at all (hooray for crazy search results!).
Reaxys turned up C60 of all things as the first result
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Structure databases are really annoying about turning up c60 as a superstructure containing a wide variety of fused polycycles that you might be searching for. They really need to screen for that.
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I am not getting this at all. I can draw the alternating resonance forms of benzene, cyclopentadiene anion, etc., but I cannot draw a similar alternating structure for this anion. If someone were to ask me if maleimide were aromatic, antiaromatic, or nonaromatic, I would say it is nonaromatic. I thought this anion should be non-aromatic as well. Could this be an error?
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Here's the resonance form I was thinking of. Compare to fulvene. Fear my MS Paint structure drawing.
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I'm not sure that invoking fulvene aromaticity is valid in this case because for the sextet to be delocalized around the ring you've got to have a resonance structure where there are negative charges on adjacent carbons. Since fulvene is only weakly aromatic, I doubt that introducing an unfavorable interaction into the game would make it more aromatic.
I think the more likely resonance contributor might be the one you've drawn, but with the adjacent + and - formal charges combined into a pi-bond, with a carbanion on the remaining exo methylene, yielding a structure that is either nonaromatic or antiaromatic.
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Looking at the original structure, I believe the resonance structure should have a negative charge on the carbon one step clockwise from where it is located in Opsomath's structure.
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There are quite a few valid resonance structures for this guy. I believe that if you can draw one that is aromatic, and one that is not, it will favor the one that is...just like cyclobutadiene can be antiaromatic or not aromatic, and distorts to favor the latter case.
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You can't just say it will favor the one that is aromatic if it is introducing other unfavorable interactions. By that logic, (1E,3Z,5E,7Z,9Z)-cyclodeca-1,3,5,7,9-pentaene should be aromatic too.
Your are arguing that this would exhibit fulvene-like aromaticity. My counter is that fulvene is only slightly aromatic* and that the introduction of an unfavorable interaction would make it less likely to be aromatic.
*UV-vis evidence that it is, but he ring current is almost unmeasurable by NMR
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Honestly, I was assuming that the OP was correct, and looking for a reason to call it aromatic. If it ever was generated, probably by flash photolysis of something, it would probably be nonaromatic in terms of NMR. Similarly, a quantum chemical treatment with NICS would probably show no aromaticity.
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I'm going to have to ask the OP what their evidence for aromaticity is. Everything I can work out convinces me that it is either nonaromatic or antiaromatic.
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I'm going to have to ask the OP what their evidence for aromaticity is. Everything I can work out convinces me that it is either nonaromatic or antiaromatic.
I'm with you. I used to make simple mistakes by doing a copy and paste. I can imagine some simple change was introduced without changing the aromaticity label. If this is supposed to be aromatic, I think this is a poor example to explain to rationalize it. Many students are confused by simple things like pyrrole or imidazole, why this example?
Fulvene ring current being virtually non-existant is a really good argument.