29 votes
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Which is the more stable enol form?

Keto-enol equilibria reflect a delicate thermodynamic balance between the two forms. The bonding differences between the keto and enol structures shown above are: keto: C=O double bond, C-C single ...
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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-...
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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 ...
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10 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 ...
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10 votes
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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 ...
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7 votes
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Dependence of steric hinderance on enol content

How can hydrogen form two bonds? In the case of an enol, hydrogen does not form two full bonds. What the drawing below is trying to show with the enol hydrogen is that one bond (the O-H with the ...
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7 votes

Which is the more stable enol form?

The 2,4-pentanedione enol form shown is the more stable enol, since the lone pair on the enol oxygen can delocalize across the five atoms to the electronegative carbonyl oxygen. Hydrogen bonding is ...
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7 votes
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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 ...
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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: ...
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7 votes
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Is there any possibility of tautomerisation to en-ol form?

If you're asking specifically about the tautomerisation, then I made this comparative computational study using B3LYP theory at the 6-31g* level, and the results are: So as is evident, the bridge ...
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7 votes
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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. (...
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7 votes
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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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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 ...
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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,...
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6 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 ...
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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 ...
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6 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 ...
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6 votes
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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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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 ...
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Can a tertiary enamine tautomerize to an imine?

Tautomerization is a chemical reaction whereby constitutional isomers interconvert. The interconversion process involves the shift of a hydrogen atom accompanied by the shift of a double bond. Here ...
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5 votes
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Dieckmann condensation's final protonation step

why does the enol tautomerizes to the keto form? In 1,3-dicarbonyl systems, like the one pictured above, we would generally expect to have a measurable amount of the enol form present. The keto-enol ...
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5 votes
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Why tautomers are considered to be the same chemical compound?

There are a number of types of rapid chemical equilibria that basically cannot be turned off. Tautomers are probably the best known examples, although fluxional behavior is observed in many inorganic ...
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Why do only ketones exhibit keto-enol tautomerism?

Esters, amides and acids do exhibit tautomerization to the enol form, but the enol content in these compounds is just much less than that found with aldehydes and ketones. Why? The keto-enol process ...
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5 votes
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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, ...
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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 ...
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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 ...
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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. ...
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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 ...
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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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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 ...
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