Questions tagged [aromaticity]

For questions about the concept of aromaticity. Do not use for reactions of aromatic compounds.

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44
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Is buckminsterfullerene aromatic?

According to Wikipedia, The $\ce{C60}$ molecule is extremely stable,[26] withstanding high temperatures and high pressures. The exposed surface of the structure can selectively react with other ...
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Is pyrene aromatic despite failing Hückel's rule?

Pyrene doesn't seem to be aromatic. However, sources claim that it is aromatic. Considerations: Pyrene is cyclic. ✓ Pyrene is flat (planar). ✓ Pyrene has 16 π electrons. Every atom in the ring ...
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Are graphite and hexagonal boron nitride aromatic

Are graphite and hexagonal boron nitride aromatic? Graphite has a planar network of 6-membered rings with each carbon connected to three other carbons. Since the valency of carbon is not satisfied, ...
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What is the justification for Hückel's rule?

I know that experimentally, the rule has been verified but I am having trouble understanding why it works. I see no reason why equivalent cyclic structures can undergo resonance and be just as stable. ...
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What does the circle inscribed in a benzene ring represent?

I have seen benzene being depicted in two different ways: For the right-hand structure, does the inscribed circle represent the movement of double bonds or electrons?
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Is cyclobutadiene antiaromatic?

Cyclobutadiene is very unstable. But, some sources claim that this instability can be attributed to other factors such as ring and angle strain rather than antiaromaticity. According to some, ...
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What is Y-aromaticity? Is the trinitromethanide anion aromatic?

The wikipedia article on trinitromethane claims that: There is some evidence that the anion (which obeys the $4n+2$ Hückel rule) is aromatic. Unfortunately the citation is behind a paywall so I ...
21
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615 views

How does the cyclopropyl group influence conjugation and aromaticity?

While answering the question Why isn't cyclobutane planar?, I recollected that cyclopropane and its derivatives behave like double-bond compounds, e.g. a cyclopropyl group can potentially be ...
20
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What are modern approaches to describe and/or quantify aromaticity?

I know that Schleyer described an approach for evaluating aromaticity using nucleus-independent chemical shifts (NICS, see e.g. Chem. Rev. 2005, 105 (10), 3842–3888) quite some time back. I also ...
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Cyclobutadiene - Jahn–Teller effect or not?

In transition metal chemistry the Jahn–Teller effect arises when the configuration of the metal ion and d orbital splitting set up a doubly degenerate state, which is less stable than a state without ...
19
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1answer
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Is a compound aromatic if it also has anti-aromatic rings?

Is 5,6,11,12-tetradehydrodibenzo[a,e][8]annulene aromatic? If I use Hückel's rule for the whole molecule, I get the answer as anti-aromatic. But the compound has two benzene rings which are ...
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What is three dimensional aromaticity?

I have recently come across a statement which states: The relative stability of the 1,3-dehydro-5,7-adamantanediyl dication is ascribed to its three-dimensional aromaticity. [1] My ...
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Is this sydnone aromatic?

Is the following molecule aromatic? It does seem like it will be aromatic because it is similar to furan. I am confused because of the positive charge on the $\ce{N}$.
17
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Directionality of ring current-induced magnetic fields in purines

What would be the directions of ring current-induced magnetic fields in purine-like heterocycles in a ​strong magnetic field? Would be the antiparallel state be a default state for these substances in ...
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The order of aromaticity of benzene, thiophene, pyrrole, furan, and pyridine

I have been taught that the order of aromaticity is: benzene > thiophene > pyrrole > furan. However, I have been unable to deduce any logical explanation for that order. Also, I wish to compare ...
16
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1answer
527 views

Is [7]сirculene aromatic or anti-aromatic?

This compound is called [7]circulene and is one of the examples of non-planar aromatic systems (1). From the first overview of this compound, it is observed that there are 7 benzene rings so one would ...
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2answers
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Is borazine aromatic?

I came across this structure which has got pi bonds only in its canonical forms (which are also unstable compared to the Ist structure), is it aromatic? Also, is it compulsory for an aromatic ...
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Is the triplet state of the cyclopentadienyl cation really aromatic?

While checking the ASAP articles of the Journal of the American Chemical Society I came across a very interesting contribution by Costa et al. who synthesised a $\ce{BF3}$ stabilised cyclopentadienyl ...
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Is the cyclopropenyl anion antiaromatic?

According to my book, a compound is antiaromatic if it is cyclic, planar, and possesses a fully conjugated system of p-orbitals with $4n$ π-electrons. However, I have also been told that the ...
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Is tropone aromatic?

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 ...
15
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Is cyclopentadiene or cycloheptatriene more acidic?

Which of the following cyclic compounds are more acidic? The most acidic proton is positioned on the carbon that is at the top of the above drawings (the methylene hydrogens) on each of the two ...
14
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Do lone pairs on substituents (e.g. in aniline) count towards Hückel's rule?

Why is aniline aromatic? Doesn't it have 8 π electrons including the lone pair on nitrogen, thereby violating Hückel's rule? The way I see it, there are 6 π electrons from the benzene ring, and an ...
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Can Hückel's rule be applied to inorganic compounds?

Hückel's rule helps us know which molecular orbital, bonding or antibonding contains more electrons, and it is derived from the same. And on the basis of this, we can find out the stability of the ...
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Why is 1H-borepine aromatic?

For a compound to be aromatic, each atom in the ring must be a part of conjugation. Boron here has no lone pair, so how can 1⁠H-borepine be aromatic as I was told by someone?
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1answer
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In Hückel's rule, can n be any integer?

Hückel's rule says that any planar compound with a ring of conjugated p orbitals with $4n+2$ electrons is aromatic. Here, can $n$ be any integer, or does $n$ have to be related to the number of p ...
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Is the phenoxide ion aromatic?

Is the phenoxide ion aromatic? I read that technically it isn't aromatic because it violates some arcane rule, but I can't find any references in the literature. I don't see why it couldn't be ...
12
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2answers
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Why is triptycene aromatic?

I came across this wonderful compound, triptycene. Why is triptycene an aromatic compound, even though it is non-planar? I thought planarity was a requirement for aromaticity.
12
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482 views

What is quasi-aromaticity?

During my study of aromaticity, I came through this topic of quasi-aromaticity. Can you please elaborate what Quasi aromatic compounds are and how are they designated?
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How does a Frost diagram reproduce the solutions to the wave equation?

I came across a Frost diagram for cyclic compounds in my book, and all my book had to offer was that it geometrically reproduces the solutions of the wave equation, and can therefore determine the ...
11
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1answer
417 views

Is phosphorine (C₅H₅P) aromatic?

Phophorine seems aromatic as it has 6 conjugated electrons. But the answer given is that it is not. This seems odd since pyridine has a similar structure and is also aromatic. Thus I ask is ...
11
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1answer
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Is cyanidin aromatic?

The structure of cyanidin is as follows: There are eight double bonds in the entire compound, and hence $8 \times 2 = 16$ π-electrons, which does not satisfy the $4n + 2$ rule. However, the molecule ...
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1answer
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Is cyclononatetraenylium antiaromatic?

I was thinking that it might be non aromatic since the ring is very big and it might bend just like COT (cyclo-octatetraene) does to avoid destabilization due to antiaromaticity but I don't know if ...
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1answer
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Is 1,2-dihydronaphthalene aromatic?

According to Hückel's rule a compound is aromatic if it has conjugation throughout the compound and the number of conjugated electrons is $4n+2$. This compound does not satisfy both: The benzene ring ...
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When do lone pairs on nitrogen contribute to aromaticity?

I've been told that 1,2,4-trimethyl-1⁠H-imidazole is aromatic. As it has four obvious pi electrons (in the double bonds), I assumed that it was anti-aromatic as opposed to aromatic. I've been ...
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1answer
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Is the middle ring of this organic compound aromatic?

Is the middle ring with 5 atoms of (4⁠Z)-4-[(4-methoxyphenyl)methylidene]-5-methyl-2-phenyl-2,4-dihydro-3⁠H-pyrazol-3-one aromatic? The carbonyl group could become a negatively charged oxygen, the ...
8
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1answer
764 views

Applying Huckel's rule to non-planar compounds

Which of the following compounds is aromatic? From the 4n+2 rule, it must be either option 2 or 4. The answer given in my book is option 4. Why? I think It might be due to one being planar and ...
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Why is cyclooctatetraene non planar but the cyclooctatetraenide anion planar?

I was reading some organometallics and I came upon the preparation of uranocene, $\ce{U(C8H8)2}$. It has two cyclooctatetraenide anions, $\ce{C8H8^2-}$, as ligands, with a uranium atom between them. ...
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Is a quinone ring aromatic?

I'm trying to work out which are the aromatic ring systems in the drug aklavinone: I am trying to follow Hückel's rules that the compound must be cyclic, be planar, possess a p-orbital on every atom ...
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How is the aromaticity in graphene different from the aromaticity in benzene?

The web page Aromaticity in Graphene and other 2-D Systems begins: I. Graphene While the σ-bonding in graphene is assumed to be a rigid honeycomb framework built out of two-center two-...
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Determining aromaticity of compounds [closed]

Which of the following planar structures would be aromatic? Could someone help me determining the aromaticity of the compounds? My Attempt: (i) Not an aromatic compound since it only has 4 pi ...
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Is the 9H-fluoren-9-ylium cation aromatic?

In this cation, there are two individual benzene rings which are aromatic. But in the entire π system, there are 12 π-electrons which should make it antiromatic. Which is correct?
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Why do dithiins have 8 π-electrons?

It is mentioned on Wikipedia that dithiins have 8 π-electrons. Why is this so? I was thinking it would suffice for just one of the lone pairs on one of the sulfur atoms to participate in resonance ...
6
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1answer
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Loss of aromaticity vs low entropy of activation

Even if reaction has low entropy of formation does that mean it is favorable to loss aromaticity and form highly strained cyclopropane ring? Why this reaction happens? Maybe someone can explain that ...
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1answer
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Is a pyrylium ring aromatic?

I am trying to figure out which rings in callistephin are aromatic: I know that the benzene rings definitely are aromatic, but I'm not sure about the ring with the positively charged oxygen (...
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Why is only one lone pair in imidazole delocalised?

I was wondering why only one of the nitrogen lone pairs in imidazole is involved in the delocalized π-system. Specifically, I was wondering why it is that lone pair B in particular is delocalized, ...
5
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1answer
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Do lone pairs contribute towards the pi electron count of heterocycles?

I'm not able to understand how the following compounds are aromatic. When should the lone pairs on heteroatoms be taken into consideration when counting the number of π electrons?
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Is cycloheptatriene aromatic or not?

Is cyclohepta-1,3,5-triene aromatic? I think it should not be, because even though the π-system possesses six π-electrons, the conjugation is not complete around the ring.
5
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1answer
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Are the following compounds aromatic?

I know Huckel's rule states that an aromatic species must have $4n+2$ π-electrons. Is the last molecule also aromatic?
5
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1answer
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Is 2a¹H-cyclopenta[cd]indene aromatic?

In this question, 2a1H-cyclopenta[cd]indene is said to be aromatic. My doubt is that the central C atom (connecting the three cyclic rings) is sp³ hybridized. So I think it should be non planar thus ...
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Do the π-electrons from the C=O bond in tropone contribute to Hückel 's rule?

This is a continuation of Is tropone aromatic? I'm looking at the resonance form that does have the carbonyl double-bond, 1a. Can this resonance structure also be considered aromatic? Would the ...