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43 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 ...
Jan's user avatar
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20 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 ...
orthocresol's user avatar
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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 ...
Mathew Mahindaratne's user avatar
17 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 ...
electronpusher's user avatar
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). ...
NotEvans.'s user avatar
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15 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 ...
user55119's user avatar
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14 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 ...
Martin - マーチン's user avatar
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 ...
orthocresol's user avatar
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14 votes
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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 $\...
user55119's user avatar
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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 ...
user55119's user avatar
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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 ...
orthocresol's user avatar
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12 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, ...
Eli Jones's user avatar
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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; ...
orthocresol's user avatar
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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" ...
orthocresol's user avatar
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10 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 ...
user55119's user avatar
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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-...
bon's user avatar
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9 votes
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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 ...
orthocresol's user avatar
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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 ...
Waylander's user avatar
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8 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 ...
ringo's user avatar
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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 ...
ron's user avatar
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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, ...
orthocresol's user avatar
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8 votes
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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 ...
Oscar Lanzi's user avatar
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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 ...
NotEvans.'s user avatar
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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 ...
andselisk's user avatar
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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 ...
Zhe's user avatar
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8 votes
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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 ...
Buttonwood's user avatar
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7 votes

Side chain hydroxylation of pyrrole

I'd like to propose a dark, non-radical pathway :) Pyrroles, supposed that they are not fully substituted, undergo electrophilic aromatic substitutions with halogens. For the given starting compound,...
Klaus-Dieter Warzecha's user avatar
7 votes
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Comparing basicity of imidazole and 2-imidazoline

I was unable to find experimental $\mathrm{p}K_\mathrm{a}$ values of imidazoline. However, the tools one can use to calculate and thereby predict $\mathrm{p}K_\mathrm{a}$ values are pretty good ...
Jan's user avatar
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7 votes
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How to tell whether lone pairs in heterocycles contribute to aromaticity?

For the most common situations use this rule: a conjugated ring atom needs either a ligand or a localized electron pair to replace a missing ligand. Any remaining electron pairs are then part of the ...
Oscar Lanzi's user avatar
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7 votes
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Why does nucleophilic aromatic substitution occur at C-2 and C-4 of pyridine?

Nucleophilic aromatic substitution on pyridines (or related heterocycles such as quinolines or pyrimidines) regioselectively occurs at the 2- and 4- positions. These can be thought of as being "ortho" ...
orthocresol's user avatar
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