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Resonance forms of tropone

Looking at resonance structure 1a, it doesn't seem that tropone is aromatic. However, the resonance structure 1b with $\ce{C+-O-}$ satisfies Hückel's rule for aromaticity. Is it correct to say that tropone is then aromatic?

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Tropone, or 2,4,6-cycloheptatrien-1-one, is an aromatic, non-benzenoid hydrocarbon. If you look at the resonance structures in the drawing below you'll see that structure B

enter image description here

depicts a molecule with a continuous, planar loop of 7 p-orbitals that contain 6 pi-electrons. Any planar (or near-planar), cyclic system with a continuous loop of p-orbitals is considered aromatic, or stabilized, if it contains 4n+2 pi-electrons and anti-aromatic if it contains 4n pi-electrons. On this basis, molecules like the cyclopropenyl cation (4n+2, n=0), benzene (n=1), cycloheptatriene (tropylium) cation (n=1), cycooctatetraene dianion (n=2) would be considered aromatic systems. Correspondingly, the cyclopropenyl anion (4n, n=1), cyclobutadiene (n=1), cyclopentadienyl cation (n=1), cyclooctatetraene (if it were a planar molecule, n=2) are considered destabilized or anti-aromatic molecules.

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  • $\begingroup$ Does it mean that resonance structure B is far more abundant than A? That is, the real structure of tropone is better represented with structure B than A? $\endgroup$ – RBW Aug 14 '14 at 21:40
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    $\begingroup$ Here is some data. The dipole moment of tropone is 4.17 D while that of cycloheptanone is 3.04 D; the C=O stretch is at 1638 cm-1, weaker than a typical alpha-beta unsaturated ketone (~1665 cm-1); the tropone ring undergoes Diels-Alder reactions and the carbonyl reacts with 2,4-dinitrophenylhydrazine. My guess would be that while structure B is a significant contributor to the overall description of tropone, it is not the major contributor. $\endgroup$ – ron Aug 14 '14 at 22:05
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    $\begingroup$ @ron actually, here I found C13 and H1 data chem.wisc.edu/areas/reich/nmr/c13-data/cdata.htm. Tropone shows C13 shifts that are, unlike in cyclohexenone, pretty uniform through ring atoms except for carbonyl carbon. The same applies to H1-NMR. Moreover, shifts in H1-nmr may be said to be closer to thats of benzene than to cyclohexanone or dienones, though this is debatable to some extent. So, I would say, that according to NMR data tropone has very strong aromatic character. But this aromaticity does not affect its reactivity much, it seems that stabilization is low. $\endgroup$ – permeakra Aug 15 '14 at 2:13
  • $\begingroup$ @Permeakra Are you suggesting that resonance structure B is the major contributor? I agree that the nmr chemical shift data is debatable. However, note that the coupling constants are suggestive of significant bond alternation, "tropone shows larger bond-alternation effects than the aromatic tropylium ion" chem.wisc.edu/areas/reich/nmr/05-hmr-05-3j.htm $\endgroup$ – ron Aug 15 '14 at 2:55
  • $\begingroup$ @ron Afaik, in modern days aromatic ring current is considered as the main sign of aromaticity. Such current creates its own magnetic field, that manifests in specific shifts in NMR data. So, I think that accurate analysis of NMR data should be the method of establishing degree of tropone aromaticity, and I presented my view on the topic. That's not a field of my major interest, though, so I can't provide deep analysis of the subject. $\endgroup$ – permeakra Aug 15 '14 at 3:30
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The best answer to this theme is probably given by Barach in his book

BACHRACH, Steven M. Computational organic chemistry. Annual Reports Section" B"(Organic Chemistry), 2008, 104: 394-426. p 144

there is no single definition of aromaticity but rather a collection of properties associated with aromatic molecules. These properties include the following

Aromatic compounds are stable relative to some nonaromatic reference

Aromatic compounds display magnetic properties related to a ring current. These include unusual chemical shifts, especially protons, large magnetic anisotropies, and large diamagnetic susceptibility exaltations

Aromatic compounds tends to be planar and have equivalent C-C bond lengths,even though formal Lewis structure indicate alternating single and double bonds about the rings.

Even though their Lewis dot structures show C=C bonds, aromatic compounds prefer substitution reaction to addition reactions

According to these definition Tropone can be considered aromatic.

I've found also useful information in SCHERRER, Robert (ed.). Anti-inflammatory Agents Part II: Chemistry And Pharmacology. Elsevier, 2012.

there you can find that there wasn't agreement on the aromaticity of tropone because

... investigation of tropone, tropolone, and their derivatives seemed to confirm the aromatic character, which was attributed to a significant contribution from the dipolar resonance structure. ... Bertelli and Andrews (1969) revaluated the question of aromatic character in tropone and related compounds using data derived from dipole measurements ...Shimanouchi and Sasaada in 1970 .. found that the molecule was almost planar.. The authors pointed our that the average C-C lenght (1.407 Angrstrom) agreed with the standard aromatic value... Deewar and Trinajstic (1970) suggest that tese compounds more closely resemble polyenones

Kalsi in his book KALSI, P. S. Spectroscopy of organic compounds. New Age International, 2007. report that tropone represents a series of dipolar structures which constitute the aromatic tropylium system) and also Agrawal in his book AGRAWAL, O. P. Organic chemistry reactions and reagents. 1996. report that being miscible with water, boils at 113°C, strongly polar ($\mu$ 4.3 D) and lacks of ketonic properties. Since

all these properties may be explained on the basis that tropone is a resonance hybrid of three structures).. the resonance hybryd II of tropone also indicates that the tropone ring contains 6$\pi$-electrons in a cyclic resonating system and thus behave like an aromatic compound. Chemically, tropone behaves partly as an unsaturated compound, viz. it decolourises aqueous potassium permanganate solution, add up chlorine and bromine, and undergoes Diel's-Alder reaction with maleic anhydride. On the other hand it also show benzenoid characteristic as indicated by the formation of 2,4,7-tribromotropone and 2-aminotropone on bromination and amination respectively. enter image description here

So I guess there is not only one answer but depends on the aspect that you are looking at but in order to summarize

pro

  1. is almost planar with an average C-C length that agree with standard aromatic

  2. miscible in water (hint an interaction with $\pi$ electrons)

  3. lack of chetonic propertiers
  4. stable due to the the fact that is a resonance hybrid 5.it reacts to form 2,4,7-tribromotropone and 2-aminotropone by bromination and amination respectively

cons

  1. dipole measurements
  2. decolorise aqueous potassium permanganate solution
  3. add up chlorine and bromine
  4. undergoes Diel's-Alder reaction with maleic anhydride
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