43
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Why is pyrimidine less basic than pyridine?
It is not the number of lone pairs that in any way explains basicity. Take a random sugar and it will have ten times the number of lone pairs (albeit on oxygen, not on nitrogen) without being ...
- 67.3k
24
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Dipole moments of pyrrole and furan
Both pyrrole and furan have a lone pair of electrons in a p-orbital, this lone pair is extensively delocalized into the conjugated pi framework to create an aromatic 6 pi electron system.
Where ...
- 84.2k
20
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What is the mechanism of the reaction between hexane-2,5-dione and nitrous acid?
As you may know from diazotisation reactions, $\ce{HNO2}$ is a generator of the nitrosonium cation, $\ce{NO+}$. Electrophiles like these can react with the enol tautomer of hexanedione:
Following ...
- 71.1k
20
votes
Does folic acid contain a benzyl or a phenyl group?
Does folic acid contain a benzyl or a phenyl group?
This is the question asked in the title. At the first glance to the structure, one would say folic acid consists of phenyl function but not benzyl ...
- 39.5k
16
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Which carbons in pyrrole have the highest electron density?
Interesting question, more subtle than I realized!
Theoretical approach: I would turn to resonance and induction as the rationale for electronic properties. I think all of your resonance forms may ...
- 2,194
15
votes
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Multiplet shape in proton NMR of morpholines
It looks as if the NMR of morpholine is an AA′XX′ spectrum (the chemical shift difference is 0.80 ppm, or 320 Hz on your spectrometer, two orders of magnitude larger than the coupling constant).
...
- 17.1k
15
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What is the mechanism of the reaction between hexane-2,5-dione and nitrous acid?
Orthocresol did a great job describing how to get to the isoxazole methyl ketone. However, nitrous acid can be an oxidant. The methyl ketone in the presence of mineral acid, which you need to get the ...
- 16.4k
14
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Reaction of substituted furan with methyl vinyl ketone with subsequent rearomatisation
Your Diels–Alder adduct is pretty much correct. Without any further evidence we cannot definitely say whether the exo or the endo adduct is formed (you have drawn the exo adduct, which generally ...
- 71.1k
14
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What is the mechanism of azulene synthesis?
The @orthocresol link to the Zincke reaction is a good start to the mechanism. You are correct about the first step. Dimethylamine adds to the electrophilic pyridinium salt 1 as illustrated in 1 $\...
- 16.4k
13
votes
How to check for geometrical isomerism in cyclic compounds?
I hope the statutes of limitation haven't run out. Structure 1 is a (Z)-alkene. Using Cahn-Ingold-Prelog (CIP) protocol, the rings are deconstructed as shown in diagram 2. The larger red arrow marks ...
- 16.4k
13
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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-...
- 10.8k
13
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Which carbons in pyrrole have the highest electron density?
That is really not a fair question and even with a computational approach the differences are more or less marginal. The approach by electron pusher is certainly the way to go, and here is the ...
- 43.8k
13
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Are nucleophilic aromatic substitutions stepwise or concerted?
TL;DR Most SNAr reactions proceed via concerted mechanisms without a discrete anionic intermediate. This is typically the case when there is a good leaving group (e.g. chloride or bromide) and if the ...
- 71.1k
12
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What's special about the purine scaffold?
It is important to remember that the purine scaffold is one of the most fundamental organic structures in all lifeforms. From an evolutionary standpoint, this means that there must have been a simple, ...
- 1,905
11
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Regioselectivity in Electrophilic Aromatic Substitution of Pyrrole and Indole
The traditional explanation for the reactivity of indole at C–3 is that attack at C–3 does not disrupt the aromaticity of the benzene ring in the cationic intermediate (see any organic chemistry book; ...
- 71.1k
10
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What's the mechanism of the reaction of (E)-methyl 3-(furan-2-yl)acrylate in acidic methanol?
Unfortunately I cannot agree with the currently accepted answer. This 1,5-hydride shift that was invoked seems to have a very strained transition state (the cyclic, planar system means that the "1" ...
- 71.1k
10
votes
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Reaction of furan with chloroform in base
The key to this question lies in realising that the Reimer–Tiemann reaction is not an electrophilic aromatic substitution but it is in fact
addition of a carbanion to a strongly electron-deficient ...
- 15.3k
10
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Pyridine synthesis by tin(II) chloride reduction of 5-nitronorbornene
This reaction was conducted by Ho and Liao1. Nitro compound 1 is reduced to the nitroso compound 2 which undergoes a [3+3] electrocyclization to form 3 (4). Tautomerization of 4 to 5 is followed by a ...
- 16.4k
9
votes
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Why can't oxygen in furan be sp-hybridized?
Is this only because the overall structure will not be consistent, because sp-hybridization will make the COC line straight, and thus the OCC angles would have to be lesser than the carbons' sp2-...
- 15.3k
9
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Why does selenophene not undergo aromatic substitution?
The Se atom has such diffuse valence orbitals compared with carbon that there is not good pi overlap, so we don't really have a conjugated cycle. By definition when there isn't a conjugated cycle, it'...
- 53.1k
9
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Is cyanidin aromatic?
No doubt it is aromatic.
The $4n+2$ rule is intended for a single cycle of π-electrons, which may possibly include multiple rings but can't have any interior atoms or pendant π- bonds (which would ...
- 53.1k
9
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What is the reaction mechanism between acetophenone and ethyl phenylpropiolate to form a 2-pyrone?
The general idea in the answer above is correct, but some of the details need to be looked at more closely.
The first step is as drawn in the original post. Deprotonation of the ketone, followed by ...
- 71.1k
9
votes
Acidity of metronidazole
There are no acidic groups in metronidazole. The $\mathrm{p}K_\mathrm{a}$ you are quoting relates to the protonated form of the molecule (usually hydrochloride or benzoate). The most easily removed ...
- 21.5k
8
votes
Is this sydnone aromatic?
One of Paul Schleyer's students, Matthew Duvall, has studied the aromaticity (or lack thereof) of mesoionic compounds, such as sydnones and münchnones, using various techniques.[1] In his thesis, ...
- 71.1k
8
votes
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Regioselectivity in electrophilic aromatic substitution of benzofuran and indole
In electrophilic substitution of aromatic systems sigma complexes are typically formed as reaction intermediates. Let's look at the sigma complexes formed in the reaction of benzofuran and indole ...
- 84.2k
8
votes
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Is the term "heteroatom" reserved for exclusive elements?
The IUPAC Goldbook provides no 'official' definition of the term heteroatom,and as such you're somewhat free to use (and abuse) the term as you see fit.
Since no IUPAC definition exists, it may be ...
- 17.1k
8
votes
Why do dithiins have 8 π-electrons?
TL;DR Ab initio HF molecular orbital studies of the potentially antiaromatic $8\pi$-electron systems of distorted and planar 6-membered rings revealed that conformational rigidity (calculated or ...
- 37.4k
8
votes
Why are carbons of Inositol chiral centers?
Consider 2,3-butanediol.
This compound has 2 stereocenters. The meso version of this compound is achiral. However, there are still two stereo centers. As drawn, the left one is (R) and the right one ...
- 17.3k
8
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Comprehensive Functional-Group-Priority Table for IUPAC Nomenclature
Blue book would be indeed the reference to go to. Chemistry.se's page resources for learning chemistry though includes a contribution about nomenclature in general, including the freely accessible ...
- 28.4k
7
votes
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1H-NMR spectrum for furfural
They are not coupling with the aldehyde proton. Because the three protons around the ring are locked in such close proximity to one another, they will all couple each other. Let us consider these ...
- 23.9k
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