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8.2 Keto-Enol Tautomerism
There is an interesting thing that can happen when a ketone has an alpha proton.
The equilibrium shown above is important because you will see it in many mechanisms.
Let's take a closer look.
If we look at the connections of atoms, we can see that the two compounds differ in the placement of a single protons.
The p bond is in a different location.
When we focus on the atoms, we find that the difference is in the placement of just one protons.
There is a name for the relationship between compounds that differ from each other.
The two compounds shown are not the same compound.
They are not the same compounds.
The two compounds are in equilibrium.
The last two chapters focused on the formation of O bonds as a driving force for reactions.
A ketone and an enol do not have the same bond.
The equilibrium favors the ketone.
The equilibrium can favor the enol.
The enol is an aromatic compound and it is more stable than the ketone.
There are many situations where an enol can be more stable than its tautomer.
Some of these examples can be found in your textbook.
In most cases, the equilibrium will favor a ketone over an enol.
It is very difficult to prevent the equilibrium from being established.
Imagine that you are doing a reaction that produces an enol, and you have to remove all traces of acid or base.
Your hope is that you can prevent the equilibrium from being established so as to avoid the conversion of the enol into a ketone.
You will likely find that your efforts are not successful.
The equilibrium can be established with trace amounts of acid or base on the glassware.
We will look at a mechanism for tautomerism.
tautomers will be different in the position of one protons.
Two separate steps are required.
The order of the steps depends on the conditions.
Under acidic conditions, the first step is protonation.
Deprotonation is the first step.
There is a negatively charged intermediate for which we must draw resonance structures.
The second resonance structure looks like an enol that is missing a protons.
Don't think that the mechanism has more than two steps.
The intermediate is not a step.
The enolate is important for the rest of the chapter.
There will be many examples in the coming sections.
Let's conclude our discussion of tautomerism.
The first step was deprotonation, because the conditions were basic.
Again, there are just two steps.
Don't be fooled by the sound of the intermediate.
It is not a step.
We can't draw the intermediate with only one drawing, that's why we use resonance.
Two drawings are needed to capture its personality.
This intermediate is positively charged if you look at the resonance structures.
The first mechanism has a negatively charged intermediate and the second mechanism has a positively charged intermediate.
The difference between these two mechanisms is small.
There are only two steps to each mechanism.
Both steps are just transfers of protons.
The sequence of events is the only question.
We need to look at the conditions when drawing the mechanism of a tautomerization.
The positively charged intermediate is consistent with acidic conditions.
In basic conditions, deprotonation occurs first, giving a negatively charged intermediate.
It is not possible to purify the compound because it will quickly form a ketone.
Draw a way to convert this enol into a ketone.
The enol will be converted into a ketone through two steps.
We have to decide what order to use.
We look at the conditions to answer the question.
Since we are in acidic conditions, we should first form a positively charged intermediate and then remove the other one.
Students will start the problem with the OH group.
This step will not lead to the formation of the ketone.
Think about why this is happening.
The OH group would lead to an intermediate that is not resonance-stabilized and therefore higher in energy.
HO- and H O+ should never be used in the same mechanism.
Stay consistent with your conditions is the take home message.
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