Keto–enol tautomerism - Wikipedia
It is necessary, therefore, to achieve complete conversion of aldehyde or ketone reactants to their enolate conjugate bases by treatment with a. 3 EXAMPLES. 4 RATE–EQUILIBRIUM RELATIONSHIPS . E. KE. K. Scheme 2. Acid-, base-, and “uncatalyzed” reaction paths of keto–enol tautomerism. (a) Which isomer, the keto or enol form of cytosine, is the stronger acid? (b) What is the relationship between the conjugate base of the keto form and the.
At this end, we still have this magenta electron, but now it is in a covalent bond with the blue electron, which was now given to the hydrogen proton.
Let me scroll up a little bit. It was given to this hydrogen proton up here. And then this hydronium molecule, it took back an electron, and now it is just neutral water. It took back that magenta electron, so now it has two lone pairs again, so it is just neutral water. Since this oxygen up here in the carbonyl group gave away an electron, it now has a positive charge. But this is actually resonance stabilized. You could maybe see that this would be in resonance, or another resonance form of this would be-- if this guy's positive, so he wants to gain an electron, so maybe he takes an electron from this carbon, the carbon in the carbonyl group right over there.
So if you takes that electron, then the other resonance form would look like this. Let me doing it in the same colors. You have now only a single bond with this oxygen up here. This carbon down here is still bonded to the same carbons, and then this carbon over here, we could call this an alpha carbon. This is an alpha carbon to the carbonyl group. It still has a hydrogen on it right over there. And this oxygen, since it gained this magenta electron, now it has two lone pairs.
It has this pair over there, and then it gained this electron and this electron, so it has another lone pair. And, of course, it has the bond to the hydrogen. Since it gained an electron, it is now neutral.
Tautomers - Chemistry LibreTexts
This carbon lost an electron, so now it is positive. So now this carbon right over here is positive, and these two are two different resonance forms, so they help stabilize each other. And the reality is actually someplace in between.
I could actually draw it in brackets to show that these are two resonance structures.
Now, you can imagine, just as likely-- and actually, I shouldn't just draw this as a one-way arrow, because this guy could take a hydrogen from this hydronium, or a water could take a hydrogen from this guy, so this actually could go in both directions.
So let me make that clear. This could go in both directions. You could say that they're in equilibrium with each other.
You're just as likely to go in that direction as you really, for the most part, are to go on the other direction. But you can now imagine, this has now turned from a carbonyl group, this has now an OH group, this has now turned into an alcohol, although we have this carbocation here, that this does not like being positive.
Keto-enol tautomerization (by Sal) (video) | Khan Academy
And so you could imagine where this electron right here on this hydrogen nucleus might want to go really bad to this carbocation, and it just needs something to nab the proton off for it to go there. And the perfect candidate for that would just be a water molecule. We have this water floating around, so let me draw another water molecule, just like this. It has two lone pairs. It can act as a weak base. It can give one of its electrons to this hydrogen proton. If it does that at the exact same time, bumps into it in the exact same way, this electron can then go to the carbocation.
- 13.1: Tautomers
- Keto-Enol Tautomerism
- Keto-enol tautomerization (by Sal)
And if that happened, you could go in either direction. This reaction is just as likely to happen as the reverse reaction, so we could put this in equilibrium. But if that were to happen, then what started off as our ketone now looks like this. We have a bond to an OH group just like this, and over here-- actually, let me draw the rest of it.
We had our molecule that looked like that, but now, this electron gets giving back to this carbocation. This conversion occurs because the keto form is, in general, more stable than its enol tautomer.
The keto form is therefore favored at equilibrium because it is the lower energy form. Stereochemistry of ketonization[ edit ] See also: Stereochemistry of ketonization of enols and enolates If R1 and R2 note equation at top of page are different substituents, there is a new stereocenter formed at the alpha position when an enol converts to its keto form. Depending on the nature of the three R groups, the resulting products in this situation would be diastereomers or enantiomers.
Phenols[ edit ] In certain aromatic compounds such as phenolthe enol is important due to the aromatic character of the enol but not the keto form. The keto product is kinetically stable and reverts to the enol in presence of a base. The keto form can be obtained in a pure form by stirring the keto form in trifluoroacetic acid and toluene 1: Significance in biochemistry[ edit ] Keto—enol tautomerism is important in several areas of biochemistry.
The high phosphate-transfer potential of phosphoenolpyruvate results from the fact that the phosphorylated compound is "trapped" in the less thermodynamically favorable enol form, whereas after dephosphorylation it can assume the keto form. Rare enol tautomers of the bases guanine and thymine can lead to mutation because of their altered base-pairing properties.
In deoxyribonucleic acids DNA the nucleotide bases are usually in the keto form, which is stabilized by the hydrogen bonding that holds together the two strands of the DNA double-helix.