Questions tagged [tautomer]

Tautomers are constitutional isomers of (usually organic) compounds that readily interconvert into each other. This tag is also suitable for tautomerization, the isomerization by which tautomers are interconverted. It may synonymously specified for questions about tautomerism.

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Is carbonyl-enol tautomerization only intermolecular or can it be intramolecular?

During workup of a conjugate addition reaction, it is likely that protonation occurs at the oxygen of the enolate formed. See discussion here: Do enolates get protonated at the carbon or oxygen atom? ...
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18 votes
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How large is the energy difference between keto- and enol-form of guanine and uracil?

The nucleobases in DNA and RNA are generally present in the keto-form, and not in the enol-form. As an interesting historical note, James Watson and Francis Crick did initially believe them to adopt ...
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16 votes
2 answers
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Which is the more stable enol form?

If I understand correctly, when we go from a keto to an enol form, we simply abstract an alpha proton and thereby create a double bond to a carbon-bearing the hydroxyl functional group. When ...
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13 votes
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Stability of keto-enol vs. conjugation

I am trying to figure out which tautomer of creatinine: is more stable. We see that the enol tautomer is more highly conjugated: the $\pi$ electrons from the alcohol to the double bond to the $\pi$ ...
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12 votes
4 answers
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Stability of 4-pyridone vs 4-pyridinol

In kryptomerism assignment, there was a question to compare more stable tautomeric form between 4-pyridone (1) and 4-pyridinol (2): Answer: 4-pyridone. My friend gave two plausible reasons: $\ce{C=...
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11 votes
1 answer
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Which of the following has higher enol content?

Which of the following has higher enol content? The enol form of (A) and (B) is: Both the enols have hydrogen bonding. If the oxygen were to accept the pi electrns from carbon forming a carbocation ...
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10 votes
2 answers
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How can an α,β-unsaturated ketone tautomerise?

I was taught that for aldehydes and ketone to exhibit keto-enol tautomerism, it is essential that there must be at least one α-hydrogen. For example, acetophenone and butan-2-one show keto-enol ...
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9 votes
1 answer
359 views

Why is quinone mono oxime more stable than 4-nitrosophenol?

When we do nitrosation of phenols, we get para-nitrosophenol as the major product, which tautomerises to quinone mono oxime. This article states that it largely exists in the quinonoid form. My ...
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8 votes
3 answers
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Is there any possibility of tautomerisation to en-ol form?

In the picture, you can see the bridge whose end points are in para relation (for making the image clear). Now, what my friend said is that there is no possibility of tautomerisation as there is no H ...
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8 votes
1 answer
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Why is the thione tautomeric form of 2-Mercaptopyridine favoured?

During some organic chemistry studies (through an old online course material) I ran into a doubt about the equilibrium in tautomeric forms of some heteroaromatics. First, the context: On the course ...
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Why is the oxime tautomer more stable than its corresponding nitroso isomer?

We know, generally, that the keto tautomer is more stable than the corresponding enol tautomer. I would appreciate an explanation that goes beyond the thermodynamic reasoning, too. In the nitroso-...
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Which tautomer of hexane-2,4-dione is more stable?

Which tautomer will be more stable? Both 1 and 2 show intramolecular hydrogen bonding and form 6 membered cyclic ring, thus increasing the stability. I think I need to compare the effect of the two ...
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7 votes
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Ring chain tautomerization of hydroxyacetone

My chemistry textbook says that hydroxypropane-2-one and 1,2-epoxy-2-hydroxypropane are ring chain tautomers, but it did not mention the steps for conversion of one into the other. I tried to figure ...
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7 votes
1 answer
337 views

On the oxyacids of +III pnictogens and +IV chalcogens

Phosphorus in its $\mathrm{+III}$ oxidation state is known to exist as the phosphonic acid tautomer rather than phosphorous acid one. All salts isolated containing $\ce{H2PO3-}$ are (to the best of my ...
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6 votes
1 answer
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Can a tertiary enamine tautomerize to an imine?

Which of the following compounds does not show tautomerism? The correct answer is (a), and I'm curious if this is because to show tautomerism, there can't be any net charge. For instance, ketones ...
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6 votes
1 answer
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Why tautomers are considered to be the same chemical compound?

Because of the rapid interconversion, tautomers are generally considered to be the same chemical compound. (Wikipedia) But looking at the examples provided on that same Wikipedia article, it is ...
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5 votes
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What is the keto form of a hexahydroxybenzene?

159. The keto isomer of the following compound is: The answer has been given as (c). I cannot seem to understand why is it not possible for the keto form to only have one OH convert into keto form ...
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On the stabler tautomer in amide-imidic acid tautomerism

In the section on acid-catalysed hydrolysis of nitriles, this webpage has the following line of reasoning; ...the more stable amide tautomer predominates the (following) equilibrium. (credit) How ...
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1 answer
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In the bicarbonate ion, why can the hydrogen not bond to the carbon?

I've been studying electrophiles and nucleophiles recently, including how the carbonate ion ($\ce{CO3^2-}$) is stabilised through resonance. While drawing the model for bicarbonate, however, I ...
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5 votes
1 answer
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Dieckmann condensation's final protonation step

This is the Wikipedia image for the reaction mechanism: In the final protonation step, why does the enol tautomerize to the keto form? In the enol form the carbon-carbon double bond is in ...
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Is tautomerism in ketimines possible?

In a reaction between an aliphatic primary amine and a ketone, I could expect a ketimine as a product or intermediate. My question regards tautomerization of this intermediate: has tautomerization ...
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5 votes
1 answer
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What is the most stable structure amongst the keto-enol tautomers of pentane-2,4-dione?

I was assigned the following question and I cannot decide which is the correct answer. Which is the most stable structure? (a) pentane-2,4-dione (b) 4-hydroxypent-4-en-2-one (c) (Z)-4-...
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4 votes
3 answers
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What is the enol form of 4-methylcyclohex-2-en-1-one?

What is the enol form of this compound? I have made an attempt, but it doesn't look quite right. Which hydrogen should be taken off (left or right hand side of the carbonyl)? And I don't think the ...
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4 votes
3 answers
289 views

Finding the major contributing tautomer of 1-(propan-2-yl)-1,2,3,5-tetrahydro-6H-pyrazolo[3,4-b]pyridin-6-one [closed]

I am having a problem generating the correct tautomer for 1-(propan-2-yl)-1,2,3,5-tetrahydro-6H-pyrazolo[3,4-b]pyridin-6-one. I am told by my collaborator that the one I am generating is wrong. I am ...
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4 votes
1 answer
684 views

Dependence of steric hinderance on enol content

The following is extract from my book- To illustrate this, let us consider the enol contents of acetyacetone and $\alpha$ - methylacetylacetone. Although both the enols are stabilized by H bonding ...
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4 votes
1 answer
427 views

What anions does the loss of a proton from HOCN and HNCO produce? Are they same or different?

What anions does the loss of a proton from $\ce{HOCN}$ and $\ce{HNCO}$ produce? Are they same or different? Once, I had asked my teacher if $\ce{HCN}$ and $\ce{ HNC }$ are different or same compunds, ...
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4 votes
1 answer
6k views

Why do only ketones exhibit keto-enol tautomerism?

Why can't esters or acids or their derivatives exhibit tautomerism? I mean if they did, then there would be a conjugation between lone pair electrons of oxygen and pi electrons adding to their ...
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4 votes
0 answers
161 views

Tautomerism between cyclohex-2-en-1-one and cyclohex-3-en-1-one

I think cyclohex-2-en-1-one and cyclohex-3-en-1-one should undergo tautomerism as shown below: I'm calling it tautomerism because of it's similarity with the tautomerism mechanism in basic medium. I ...
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4 votes
2 answers
503 views

Structure of the compound C9H7O2Cl?

A compound having formula $\ce{C9H7O2Cl}$ (O = oxygen) exists predominantly in enol form (A) and also in keto form (B). On oxidation with $\ce{KMnO4}$ it gives m-chlorobenzoic acid as one of the ...
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3 votes
2 answers
507 views

Tautomer of acrolein

Are $\ce{CH2=CH-CH=O}$ and $\ce{CH2=C=CH-O-H}$ tautomers? There is only a shift of a hydrogen atom and a pi bond. So why are these two not tautomers?
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3 votes
2 answers
111 views

2-Pyridone tautomer ratio

The following excerpts from different textbooks about keto–enol tautomerism in pyridones seem to be contradictory. Could someone explain which one is right? MARCH’S ADVANCED ORGANIC CHEMISTRY pg-103 ...
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3 votes
2 answers
9k views

Which has the highest enol content?

Which of the following compound has highest enol content? The answer is an option (c) and I think the reason behind it is that there are two phenyl groups attached to it, hence there will be a good ...
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3 votes
1 answer
1k views

Sulfurous acid tautomerisation

In the structure of $\ce{H2SO3}$ there is a lone pair on sulfur. Why can't sulfur make a bond with one of the hydrogen atoms and a double bond with oxygen to avoid having a lone pair? Such as given ...
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3 votes
1 answer
97 views

Acid hydrolysis of Birch-reduced Anisole

The following is a question from Advanced problems in organic chemistry by MS Chouhan: Product $A$ is rather straightforward: and on acidic hydrolysis, the following reaction takes place: Here the ...
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3 votes
1 answer
69 views

Tautomerism in 3,5‐dimethyl‐4H‐pyrazole

How would tautomerism take place in 3,5‐dimethyl‐4H‐pyrazole? I am aware of nitro–aci nitro and keto–enol, but I haven't seen such a compound before. I recognize that a hydrazone formation was done ...
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3 votes
3 answers
2k views

Why enol content of ethyl acetoacetate is more in toluene than water?

Ethyl Acetoacetate has enol content of 0.4% in water and 19.8% in toluene. What is the reason behind this? Does this anything have to do with the intramolecular hydrogen bonding in the enol form?
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3 votes
1 answer
300 views

Enol or Keto form?

Which will be the major form among the two tautomeric forms? According to me, it should be enol form due to resonance stabilization as well has five membered ring formes througj hydrogen bonding. On ...
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3 votes
1 answer
311 views

What will be the major enol form of methyl acetoacetate?

I presume the best enol form would have a double bond between the two $\ce{C=O}$ so they're conjugated, but I'm not sure how the resonance would play out.
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3 votes
0 answers
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Do ketdione molecules have diol tautomers?

In my chemistry textbook there's a keto-enol tautomerism example: But will it also be in equilibrium with pent-1,3-dien-2,4-diol in a reaction like this or will it just be a functional isomer?
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3 votes
0 answers
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Enol content of acetylacetone and ethyl acetoacetate

Why is the enol content of acetylacetone higher than that of acetoacetic ester? Hydrogen bonding stabilisation is present in enol form of both of them. I cannot figure out which factor makes one ...
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3 votes
1 answer
557 views

Equilibrium Constants For Phenol Tautomers

I encountered the following equilibrium constant values for the keto-enol interconversions of phenol in Organic Chemistry (Solomons, Fryhle, Snyder, 3rd Edition): I wonder what could explain the ...
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3 votes
2 answers
654 views

Compound having the highest enol content will be?

The answer written in book is 3 but no solution is given. After referring to some other question I think that hydrogen bonding can take place in compounds like 3 but am not sure about it. Am I right ...
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2 votes
1 answer
978 views

Rates of Keto-Enol Tautomerism

Are there any databases with rate constants for keto-enol tautomerism in small molecules? I'm looking for rate constants if possible.
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2 votes
2 answers
316 views

How are there three possible tautomers of 2,2,4-trimethylheptane-3,5-dione? [closed]

The answer for the above question is three. But I am able to draw four structures: Where am I getting it wrong?
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2 votes
2 answers
900 views

Can ketones next to a nitrogen hetero atom tautomerise?

I worry that the electronegative N will stabilise keto form so much that equilibrium will be so far towards keto it would be unreasonable to call the enolisiation process a factor
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2 votes
1 answer
401 views

Deuteration of enone - mechanism?

What happens when the above compound is mixed with D2O (deuterated water)? As a result of tautomerism, I'd expect the 4H atoms, one alpha to carbonyl carbon, and three at gamma position; to be ...
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2 votes
2 answers
285 views

Will (3R,4S,5R)-1,3,4,5,6-pentahydroxyhexan-2-one tautomerise in nitric acid?

The following question was sent to me .The question is " Which of the following is A ?" The given answer was a,b,c Background If both ends of an aldose chain are oxidized to carboxylic ...
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2 votes
2 answers
14k views

alpha-hydrogen atom and keto-enol tautomerism

The following is an extract from my book- For an aldehyde or a ketone to exhibit keto-enol tautomerism, it is essential that it must have atleast one $\alpha$-hydrogen atom. Thus, acetophenone, ...
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2 votes
2 answers
259 views

Keto-enol tautomerism in a bicyclo compound

Question Which one is more stable- the keto or enol form. The compound given is this: Solution The enol form would violate Bredt's Rule and would therefore be unstable. Hence the keto form is more ...
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2 votes
0 answers
140 views

Chirality with tautomers

This molecule has 6 peaks in its proton NMR. I think this is due to the following tautomerisation: However, I am unsure as to whether this molecule would be chiral, as it can freely interconvert ...
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