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8.3 Reactions Involving Enols
There is a plausible mechanism for the following tautomerization.
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The alpha carbon has a p bond on the enol.
The ability of a ketone to tautomerize is needed in order for the attack to occur.
Not every ketone will be in equilibrium with an enol.
A typical ketone will be in equilibrium with an enol if it has alpha protons.
In the previous section, we saw cases where the equilibrium can favor the enol, but in general, the equilibrium favors the ketone.
You will only have trace amounts of the enol in equilibrium with the ketone.
A small amount of enol is able to react as a nucleophile.
The equilibrium of the keto-enol is re-established by the production of more enol after the attack on the electrophile.
Slowly but surely, most of the ketone molecules convert into enols.
Let's look at how this happens.
A small amount of enol is generated by the tautomerizing of the ketone.
Most of the steps in this mechanism are only for protons.
The pattern of our mechanism is tautomerize, attack, and deprotonate.
"tautomerize" is a new name for a combination of two transfer steps.
When the enol attacks the electrophile is the only step where an attack takes place.
CH COOH can be used as a mild acid to facilitate tautomerization.
carboxylic acids are slower to react in this kind of reaction, so we don't have to worry about that.
The carboxylic acid needs to be converted into an acid halide.
The enol of an acid halide will attack a halogen very quickly.
The Hell-Volhard-Zelinsky reaction is a strategy for halogenating carboxylic acids.
In this section, we have seen two reactions that exploit the nature of enols.
We used some reagents.
The reagents were in acidic conditions.
H O is followed by br and pbr.
In the last step, water is used to convert the acid into a carboxylic acid.
We are starting with a ketone and subjecting it to acidic conditions.
Our product will have a br at one of the alpha positions.
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