11 votes
Accepted

Aromaticity of 1,4-Dioxin

Aromaticity is a fuzzy subject and there are no strict mathematical rules. There is no recognised unilateral theory that does explain it. This becomes a real issue when we are dealing with compounds ...
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11 votes
Accepted

Are 2-pyridone and cycloheptatrienide antiaromatic?

This can be confusing, especially when you are given a molecule that has a pendant carbonyl oxygen like your first molecule. Strictly speaking, the $4n+2$ and $4n$ electron rules apply only when the ...
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  • 38.2k
9 votes
Accepted

Reaction of cyclooctatetraene with sulfuric acid

This is a rather unusual and interesting case of aromaticity, which has been given a special name: homoaromaticity. The Wikipedia page does a quite nice job of explaining what's going on. As you state,...
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9 votes

why is benzene ring ubiquitous in almost all the organic compounds

By "benzene ring", I assume that you mean the aryl functional group: $\hspace{8 cm}$ Molecules with the above motif - called arenes - are common throughout nature and are present in a ...
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7 votes

Aromaticity of 1,4-Dioxin

Assuming the structure is indeed planar*, the presence of two oxygen rather than carbon atoms as one opposing pair breaks the symmetry and therefore breaks the orbital degeneracy on which the 4n+2/4n ...
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  • 38.2k
5 votes

How can Phthalocyanine be aromatic when it is not planar?

Phthalocyanine is flat, for example in the crystal structure (linked on PubChem) or PQR or Wikipedia As noted in the comments, PubChem uses an efficient force field (MMFF94) which does not consider ...
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5 votes

Finding the major contributing tautomer of 1-(propan-2-yl)-1,2,3,5-tetrahydro-6H-pyrazolo[3,4-b]pyridin-6-one

In keto-enol tautomerism, the equilibrium is mostly towards the keto form, as it should be, since $\ce{C=O}$ is a very strong and stable bond. In some cases however we find that the equilibrium ...
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  • 1,418
4 votes
Accepted

Why is selenophene aromatic?

Selenium — like sulfur — is a chalkogen/member of group 16. It is heavier than sulfur, and thus more metalloid. In this regard similar to the trend in the adjacent group 17 (the halogens) where ...
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  • 23.6k
4 votes

Finding the major contributing tautomer of 1-(propan-2-yl)-1,2,3,5-tetrahydro-6H-pyrazolo[3,4-b]pyridin-6-one

Maybe the original structure contained the motif of 2-hydroxypyridine. If this guess is correct then a 2-pyridone is one reasonable tautomer, as shown in the first line of the illustration below. ...
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  • 23.6k
3 votes

Is the lone electron pair of an amide nitrogen part of the π system when building a Hückel matrix?

Setting aside the arguments over umlauts, yes you can add the nitrogen atom to the Hückel matrix; in fact, theoretically you should do so and allow the matrix eigenfunctions and eigenvalues to decide ...
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  • 38.2k
2 votes

Aromaticity in cyclic ketones

Para-benzoquinone is antiaromatic, and tries to get out of antiaromaticity by disrupting the conjugation; i.e. lengthening the "single bonds" so that pi-overlap across these bonds is not ...
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2 votes

why is benzene ring ubiquitous in almost all the organic compounds

To restate the question: what is being referred to as "benzene ring" is a six-membered aromatic carbon ring. The question asks why this structure is ubiquitous, despite the simplest form of ...
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2 votes

Why do electron-withdrawing groups deshield outer ring protons?

I agree that without data one can reason in both ways. However even if you can assume smaller ring current and therefore less deshielding, the electron density is moved toward the electron withdrawing ...
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  • 3,524
1 vote

Effect of substituents on aromatic character

It is also a matter of usage of terms. Electron withdrawing substituents slow the rates of electrophilic substitution and the effect is usually expressed as increasing the energy of the intermediate-...
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  • 643
1 vote

How many π-electrons are there in 2-methylene-2H-pyran?

Yes, the textbook is wrong. With one lone pair from the oxygen capable of conjugation (the other lone pair is a sigma pair in the plane of the ring) plus the three carbon-carbon bonds, we have eight ...
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  • 38.2k
1 vote

Nitrogen with shortest bond length in histidine?

It is correct that the $\ce{N}$ with the double bond has a localized lone pair, because of its $sp^2$ hybridization, due to which the lone pairs are in the same plane as ring, and hence not conjugated....
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1 vote

Why is the polarity of the B-N bonds in borazine the reason for its reduced delocalisation of π electrons?

You can think of it as: polarity causes slight concentration of pi electrons on nitrogen atoms in contrast to boron in the resonance hybrid due to which cyclic delocalization does not occur "...
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1 vote

Why is the polarity of the B-N bonds in borazine the reason for its reduced delocalisation of π electrons?

Let us understand and try to answer your question with a few simple points. First it is important to understand why exactly is borazine aromatic. Boron has an empty p-orbital and Nitrogen has a lone ...
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