In this video, I want to talk about specific electrophilic alpha substitutions called alpha-halogenations.
It turns out that alpha-halogenation can proceed through an acid-catalyzed and a base-catalyzed
mechanism.
I'm going to show you both but it's important to know that they actually yield different
products because of what the intermediates look like.
The acid-catalyzed mechanism is always going to yield mono-halogenation.
Let me show you what this mechanism looks like.
For an acid-catalyzed mechanism, this is going to proceed through the enol tautomer.
We're going to be trying to make the enol tautomer.
The way it works is let’s say you’re using just H3O+ and you protonate.
What you're going to wind up getting is something that looks like this.
Then we can make the enol.
The enol would be with the conjugate base.
You take off the alpha-proton with water, my conjugate.
I would do this, this and this, then I get my enol tautomer.
This enol is now a pretty good nucleophile.
I can use it to attack my diatomic halogen in a similar way, not really the same way
but similar to another nucleophile.
You would go ahead, grab one the X's, kick off one of the X’s.
What you wind up getting is a molecule that looks like this, OH with now a new X here.
The mechanism would do this.
You would also bring down the electrons to reform the double bond.
What you would get is something like this.
That would be the intermediate.
Eventually, this gets deprotonated using water and you get back to your keto form.
That's what we call our alpha halogenation.
Notice that we add an X to the alpha opposition but we only had one time.
You might be wondering, “Johnny, why would you only add once?
Why does it not add again?”
It turns out that having the X there to pull electrons away from the alpha-carbon is going
to make it less likely to make an enol next time because the enol is going to require
a positive charge on those carbons.
By pulling away extra electrons, you make it less likely the form an enol, so it only
reacts once.
Cool so far?
Awesome.
This is in contrast to the base-catalyzed mechanism.
In the base-catalyzed mechanism, you may yield polyhalogenation.
The reason is you would basically going to yield as much halogenation as you have alpha
protons.
The reason being is that in a base-catalyzed mechanism, the X’s being there is just going
to make it more reactive the second time.
Let me show you.
In a base-catalyzed mechanism, imagine that I'm using OH negative and that I'm just forming
an enolate right away.
The enolate looks like this, negative charge.
That negative charge can react with X2 to this, to that.
Go here, go there, and give me my halogen.
But now when it sees that second equivalent of base, now we have a dipole pulling away.
It's going to be easier to put a negative charge there because the negative charge is
going to be stabilized by the halogen.
Therefore, we're going to replace every single hydrogen that was there.
We’ll replace all of those alpha hydrogens with X's and we’re going to get polysubstitution.
The only reason that I put may yield, not will yield, is because obviously you need
multiple alpha protons in order for it to get polyhalogenated.
If you only have one, then you’re only going to substitute once.
But you can basically continue to keep reacting until you’re out of alpha protons.
Interesting, right?
In the next video, I want to show you an application of alpha-halogenation called the haloform
reaction.