19
votes
Why Is Hydrogen in HSO3- Connected to Oxygen Instead of Sulfur?
It's actually both, at least in solution. Bisulfite ion in solution tautomerizes, existing as an equilibrium of the isomer with hydrogen bonded to sulfur and the isomer with hydrogen bonded to oxygen.
...
- 53.4k
13
votes
How can an α,β-unsaturated ketone tautomerise?
If you have an α,β-double bond next to your carbonyl, a γ hydrogen can also take part in tautomerism. Take crotonaldehyde $\ce{H3C-CH=CH-CHO}$:
$$\ce{O=CH-CH=CH-CH3 <<=> HO-...
- 67.3k
11
votes
What is the keto form of a hexahydroxybenzene?
The question is badly phrased because it uses the term ‘the keto form’, implying that there be only one. In the case of hexahydroxybenzene, you are correct in that there is a monoketo form, at least ...
- 67.3k
11
votes
What is the enol form of 4-methylcyclohex-2-en-1-one?
There are two possible enol forms of this ketone. The first one is:
This is the one you drew, which shows the α hydrogen being abstracted. It is perfectly valid.
The other one is:
Note how the ...
- 71.1k
11
votes
Accepted
Tautomer of acrolein
The molecular geometry argument is pretty common-sensical, and is true to a certain extent. But I don't think it's the cleanest argument. If you look at a typical ketone and its enol, you will notice ...
- 71.1k
9
votes
Accepted
Stability of 4-pyridone vs 4-pyridinol
I don't think this is an either this or that situation. As with most of the equilibria we know it depends very much on the circumstances we look at it. Many examples of keto-enol-tautomerism are ...
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8
votes
Accepted
Which of the following has higher enol content?
The two main factors affecting the stability of the enol forms of (A) and (B) are
Hydrogen bonding
Aromaticity
In the enol form of both compounds, hydrogen bonding is present:
But the resonance ...
- 7,676
8
votes
Accepted
Which tautomer of hexane-2,4-dione is more stable?
TL;DR
The question you are asking is not worth asking. Systems like these are hard to judge and the equilibrium between these structures is extremely fast. Considering any structure as more stable ...
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7
votes
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Why is the thione tautomeric form of 2-Mercaptopyridine favoured?
Assumption (1) doesn't hold too much precedence in this case, just like in the case of 2-pyridone, so it is a minor thing to consider. In the case of the assumption (2), electronegativity is only a ...
7
votes
Accepted
On the oxyacids of +III pnictogens and +IV chalcogens
Disclaimer: This isn't really the answer Jan is looking for.
Holleman, Wiberg "Lehrbuch der Anorganischen Chemie", de Gruyter, Vol. 101, notes that:
Selenium
Selenium forms a weaker, but more ...
- 6,851
7
votes
Sulfurous acid tautomerisation
Most of what I would be saying now is stuff I already highlighted in another answer of mine to a related question.
$\ce{H2SO3}$ exists in two forms, one of which is equivalent to the structure you ...
- 67.3k
7
votes
Accepted
Ring chain tautomerization of hydroxyacetone
At first, even though this is not related to the question, I want to say that you're confused between double-headed arrow between the two resonance forms and double harpoon in reversible reactions. (...
- 498
7
votes
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Rates of Keto-Enol Tautomerism
This is not a database, and I don't know about how small molecules you are looking for, I found that article on Jacs Kinetics and thermodynamics of keto-enol tautomerism of simple carbonyl compounds: ...
- 4,167
7
votes
Tautomer of acrolein
One of the requirements of tautomerism is that the carbon skeleton of the molecule remains unchanged. Only the proton must change it's location with reference to the molecule. Acrolein and propadienol ...
- 6,218
7
votes
On the stabler tautomer in amide-imidic acid tautomerism
The relative stability of the two tautomers (two isomers differing how protons are bound) may be assessed from the equilibrium constant you may formulate for the two. Deriving from above illustration,...
- 28.4k
6
votes
Accepted
What anions does the loss of a proton from HOCN and HNCO produce? Are they same or different?
When they are bonded to hydrogen that is $\ce{HOCN}$ $\ce{HNCO}$. The two compounds will be different:
From Wikipedia
Organic cyanates are called isocyanates when there is a $\ce{C−NCO}$ bond and ...
- 5,504
5
votes
Accepted
Is carbonyl-enol tautomerization only intermolecular or can it be intramolecular?
tl;dr: The intramolecular variant is an impossible rearrangement. Only intermolecular proton transfer can be observed.
This is actually a very nice example to discuss sigmatropic rearrangements, ...
- 67.3k
5
votes
Is there any possibility of tautomerisation to en-ol form?
I can confirm Brian's findings, who beat me to it. On the DF-BP86/def2-SVP level of theory, the energy difference is $\Delta G = 238.8~\mathrm{kJ\, mol^{-1}}$. Which means, that the enol tautomer does ...
- 43.8k
5
votes
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Deuteration of enone - mechanism?
As TAR86 mentioned, you seem to understand where deuteration occurs in crotonaldehyde. The process is usually conducted under alkaline conditions. All of the steps are reversible and need not occur in ...
- 16.4k
5
votes
In the bicarbonate ion, why can the hydrogen not bond to the carbon?
First of all I appreciate your effort to think out of the box.
Now, as you are thinking of a new structure for the bicarbonate ion, you just have to compare the stability factors between the two ...
- 678
5
votes
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Which has the highest enol content?
The extent of enol content can be explained on the basis of acidic hydrogen, intramolecular hydrogen bonding.
The stability of the carbanion formed is dependent on its stability after it is formed. ...
- 4,312
5
votes
Accepted
Why does coumarin predominantly exist in keto form?
The answer is as simple as it will be unhelpful to your original question: There is no tautomer possible.
Coumarin does not have a keto group, and there are no enolisable protons either. Coumarin ...
- 43.8k
5
votes
Tautomerism in 3,5‐dimethyl‐4H‐pyrazole
You are lucky, because not only the pyrazole motif, but the complete structure drawn by you is part of the Tautobase database
with a preference for the second structure drawn.
By the way, the values ...
- 28.4k
5
votes
Accepted
Why is quinone mono oxime more stable than 4-nitrosophenol?
Two factors come into play: (1) the inherent difference in resonance energy is not as great as you might think, and (2) the greater tendency for a dissociable proton to bind more tightly to an oxime ...
- 53.4k
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 ...
- 1,514
5
votes
2-Pyridone tautomer ratio
Why not start with the corresponding entry in Wikipedia as suggested in the comment by @Maurice? There, it discerns the solid state
«The predominant solid state form is 2-pyridone. This has been ...
- 28.4k
5
votes
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Acid hydrolysis of Birch-reduced Anisole
Lost Anisole: Before fulfilling your request for a diagram of the mechanism for the formation of cyclohex-2-en-1-one (9), a few remarks about the problem as presented are in order. Your product A is ...
- 16.4k
5
votes
Why Is Hydrogen in HSO3- Connected to Oxygen Instead of Sulfur?
While in the specific case of bisulfite you were technically correct (at least some of the time, as Oscar Lanzi explains), the reason most sources show the hydrogen only bonded with the oxygen is ...
- 159
4
votes
Enol or Keto form?
Whether a diketo compound is present in solution in its diketo or its enol configuration is not something that can be said a priori — it strongly depends on the solvent and whether it is able to ...
- 67.3k
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