46 votes
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What does the circle inscribed in a benzene ring represent?

The circle, if anything, represents the inability of our everyday physical intuition to cope with the quantum phenomena. See, you would often encounter those two pictures with "double bonds this way" ...
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33 votes

Is pyrene aromatic despite failing Hückel's rule?

To clarify @ron's point, the general "cheminformatics" rule for deducing aromatic fused-ring systems is whether there is one path (usually the peripheral one) that satisfies $4n+2$: So for ...
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33 votes
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Is buckminsterfullerene aromatic?

Aromaticity is not binary, but rather there are degrees of aromaticity. The degree of aromaticity in benzene is large, whereas the spiro-aromaticity in [4.4]nonatetraene is relatively small. The ...
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32 votes
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Is pyrene aromatic despite failing Hückel's rule?

Pyrene is aromatic. The Hückel $4n+2$ rule works best with monocyclic ring systems. If you look at the following resonance structure for pyrene with a central double bond, the monocyclic periphery has ...
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  • 82.2k
26 votes
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Is [7]сirculene aromatic or anti-aromatic?

TL;DR: We don't count π electrons here, rather the π conjugated circuits [1]. [7]-сirculene has all conjugated circles of size $4n + 2,$ and none of $2n,$ what makes it an aromatic compound. Strictly ...
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24 votes
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What is Y-aromaticity? Is the trinitromethanide anion aromatic?

Introduction It is fairly obvious that the statement given by Wikipedia cannot be entirely correct. There is some evidence that the anion (which obeys the 4​n+2 Hückel rule) is aromatic. The ...
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23 votes
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Is borazine aromatic?

The modern definition of aromaticity from deep theoreticians is that the π-system needs to support aromatic ring currents. Borazine can support it, so it is technically aromatic. Aromatic systems ...
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22 votes
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Cyclobutadiene - Jahn–Teller effect or not?

Very interesting question, and it kept me up despite daylight saving time cheating me of one hour of sleep last night... A good reference is Albright, Burdett and Whangbo, Orbital Interactions in ...
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21 votes
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Is cyclobutadiene antiaromatic?

If we go back to your earlier question on Frost diagrams (I've reproduced the key figure below), we see why simple molecular orbital theory or the "$4n+2$" rule suggests that benzene is aromatic while ...
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21 votes
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Are graphite and hexagonal boron nitride aromatic

Graphite is definitely aromatic and boron nitride is at least partially aromatic. For instance, in this paper, the authors calculate the percent resonance energy (%RE) of graphite as a comparison to ...
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21 votes
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Is a compound aromatic if it also has anti-aromatic rings?

The molecule actually optimizes to a planar, delocalized $\pi$ system Using B3LYP/6-31G(d) optimizations, we see that the molecule optimizes to become planar. (The initial configuration was ...
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21 votes
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Why is 1H-borepine aromatic?

1H-borepine is isoelectronic to the cycloheptatrienyl cation. In both cases, you have a planar ring of seven atoms, whose hybridisation is close enough to $\mathrm{sp^2}$ to allow for a third p-...
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20 votes
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Do lone pairs on substituents (e.g. in aniline) count towards Hückel's rule?

It is only the electrons from atoms in the ring that count when applying Huckel's rule. Electrons from substituents on the ring are only cross-conjugated with the aromatic π-system. When you think of ...
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19 votes
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Is biphenyl considered to be aromatic?

The two rings in biphenyl are inclined at an angle of 44° to each other because of the large steric repulsion between the ortho hydrogens on each ring. For Hückel's rule to be applied, the molecule ...
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17 votes
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Is cyclopentadiene or cycloheptatriene more acidic?

Essentially it's a case of aromaticity vs number of resonance structures. Well put! Aromaticity is a very strong driving force so aromaticity wins out; Huckel's rule is more important than the ...
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16 votes
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Is the cyclopropenyl anion antiaromatic?

When you have a ring that has $4n$ π electrons it's anti-aromatic if you force it into a fully conjugated cycle. Often such rings find a way to break the unfavorable (or at least, less favored) anti-...
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16 votes

Applying Huckel's rule to non-planar compounds

Number 2 is not aromatic because transannular $\ce{C-H}$ interactions cause the molecule to be non-planar although it satisfies $4n+2$. This problem was solved by the synthesis of number 4, which, ...
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15 votes
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Is tropone aromatic?

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 depicts a molecule with ...
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15 votes

Is buckminsterfullerene aromatic?

There are quite a few conjugated $\pi$-systems out there; some of them are pretty stable, some are less so. The ultimate way to find out is to go and solve the eigenvalues problem for the ...
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15 votes
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Why is triptycene aromatic?

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 ...
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15 votes
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Can Hückel's rule be applied to inorganic compounds?

Huckel's rule can be applied to "inorganic" cyclic molecules but only if they have a delocalised pi system. They also have to have flat (or close to flat) rings. Recently some N5 rings which 6 pi ...
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15 votes
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Is oxepin aromatic?

Oxepin is certainly not aromatic. If it were flat and the oxygen atom was sp2 hybridized, then it would be an 8π-electron anti-aromatic system, but to avoid being anti-aromatic, it exists in a boat-...
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15 votes
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What is quasi-aromaticity?

In general, we can describe a quasi aromatic compound as a compound, which is ionic in nature with a counter ion, and the $\pi$ electrons in such compounds follow Huckel's rule ($4n+2$). In other ...
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14 votes

Is pyrene aromatic despite failing Hückel's rule?

While the previous answers are correct in indicating that pyrene has aromaticity, they are often correct for the wrong reason. The proper way to break down the aromaticity in pyrene is to make the ...
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14 votes
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Is the phenoxide ion aromatic?

I agree absolutely with Max, and while I was still running the calculations, he already provided the answer. The goldbook provides a nice definition for aromatic compounds: In the traditional ...
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14 votes
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Is the triplet state of the cyclopentadienyl cation really aromatic?

TL;DR Yes. For cyclic conjugated systems in the $T_1$ state (i.e. lowest energy triplet excited state), $4n$ electrons means aromatic and $4n+2$ means anti-aromatic. The $T_1$ state of the ...
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14 votes

The order of aromaticity of benzene, thiophene, pyrrole, furan, and pyridine

The main criteria of aromaticity is resonant or delocalization energy. It is difference between energy contained in bonds if they were localized like in Kekule's formula for benzene and actual energy. ...
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  • 141
14 votes
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Why is borole not considered aromatic

A very basic way to do this would be to draw a diagram that shows the p-orbitals going above and below the plane. For Borole, the bonding orbitals would look like as shown below (taken from the ...
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13 votes

Is this sydnone aromatic?

As correctly pointed out in the comments, the compound you present belongs to the class of sydnones. According to Hückel's rule, it should be aromatic, as the molecule is planar and contains $4n+2$ π-...
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