42 votes
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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 ...
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23 votes
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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 ...
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21 votes
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Why is the exo product formed in the Diels–Alder reaction between furan and maleic anhydride?

Great question! It turns out that the rate of formation of the "expected" endo product is actually ~500 times faster than the rate of formation of the exo product. However, the Diels–Alder is a ...
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19 votes
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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 ...
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19 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 ...
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Regioselectivity in electrophilic substitution of pyrrole

The following figure shows the resonance structures we can draw to describe the intermediates produced by electrophilic at the 2- ($\alpha$) or 3- ($\beta$) positions in pyrrole. image source ...
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16 votes
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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 ...
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14 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). ...
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14 votes
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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 ...
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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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13 votes

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 ...
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13 votes

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 ...
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13 votes
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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 ...
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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 --> 5....
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12 votes
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Why is only one lone pair in imidazole delocalised?

The correct answer to this question is more direct, and not listed in your items. The non-bonding electron pair in nitrogen 3 is in an orbital perpendicular to the π-bonding p orbitals of all other ...
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11 votes

When do lone pairs on nitrogen contribute to aromaticity?

Both nitrogens in your drawing are roughly $\mathrm{sp^2}$ hybridized, therefore they each have a p-orbital and 3 $\mathrm{sp^2}$ hybdridized orbitals. All of the p-orbitals, including those on the 3 ...
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11 votes
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Why is pyridine a monodentate ligand, but pyrrole a tridentate ligand?

From what I gathered in the comments you confused a few terms. Both compounds are monodentate, i.e. they can only coordinate one metal at a time. However, pyridine usually donates one electron pair, ...
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11 votes
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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; ...
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11 votes
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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, ...
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10 votes
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Proton NMR signals and rings

Here is the proton nmr spectrum for pyridine. Here is the proton nmr spectrum for furan For comparison, benzene has a single proton nmr signal at 7.27 ppm. In both pyridine and furan there are ...
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10 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 ...
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10 votes

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" ...
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10 votes
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Resonance structures of some aromatic compounds

Resonance structures can be drawn for all aromatic compounds. Tropolone, furan and thiophene are considered to be aromatic compounds because we can draw resonance structures and they are cyclic ...
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10 votes
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Nitration of pyrrole with sulfuric and nitric acids

Would 2,5-dinitropyrrole be a product of nitration of pyrrole with $\ce{H2SO4/HNO3}$? I doubt it and would rather expect a lot of tar to be formed here. Pyrrole tends to undergo acid-catalyzed ...
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9 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 ...
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9 votes
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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'...
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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-...
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9 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 ...
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9 votes

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 ...
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