So far we've been learning about radical halogenation mechanisms and how the radical is always going
to be attracted to the spot that has the most R groups.
So the tertiary is going to be more stable than primary.
But what we also know is that according to the radical stability trend, allylic radicals
are the most stable.
So now I want to throw that extra complexity into the mix: what happens if there are allylic
positions available to be halogenated?
And that brings us to allylic halogenation.
Once again, I just want to show you guys, just in case you forgot, that the allylic
and benzylic radicals are more stable than primary, secondary, or tertiary.
What that means is if I have one of these two possibilities, this is going to be my
preferred spot, not those three.
Now, what's interesting about these is that these radicals can resonate.
What that means is that resonance is going to have to play a central role in this mechanism.
We can't just ignore resonance; we're actually going to have to embrace it for this mechanism.
So I just want to show you guys what this could look like.
Basically, I've got a diatomic halogen.
Let's say this is Br2, so it's selective, and we've got a double bond.
Usually we would just say secondary or primary; we're going to go for the secondary spot.
But in this case, I actually have an allylic position.
That allylic position is right here, because that's the one next to a double bond.
That's actually going to be the position that's the most stable, and that's going to be the
one that I want to react with.
Now let's go ahead and draw the whole mechanism out and I'm going to show you guys how things
are a little bit different now.
First of all, my initiation step is really easy.
We're just going to use Br2.
And where do those first arrows go?
We would just make radicals, there and there.
So now what I'm going to get is Br radical plus Br radical.
All right, so that's it for our initiation.
We have our target radical.
Now what we have is the propagation step.
In the propagation step, I'm going to want to pull off the hydrogen that's going to give
me the most stable radical, so obviously it should be this one, right?
So let's go ahead and react that.
I would use three arrows.
I would do one into the middle of nowhere, one into the middle of nowhere, and then one
right there.
What I wind up getting is I wind up getting a radical that looks like this.
And also, we would get HBr.
Now, usually what would happen is that my propagation phase would continue and I would
have to regenerate the Br radical by reacting with Br2.
That's normal; that's what we're expecting.
But wait, hater's a little difference: this radical can now resonate.
What that means is that I can't actually just continue from here.
I need to draw the resonant structure of this radical before I can continue.
What that means is that the resonant structure actually looks like this.
And what that means is that now, my Br that's going to continue in the propagation stage
can attach both there and there.
So it's going to be able to attack both those carbons because the radical is hybridized
between those two.
Remember that resonant structures, it's not an equilibrium; it's a mathematical statistical
average of where that radical is.
So what that means is that I can go ahead and draw my mechanism with the Br here, and
I can draw it from the first one.
Just to keep things easy, I'll just do this there, there, and there.
What I wind up getting is I'm going to wind up getting a Br right here plus Br radical.
But I'm also going to get something else.
I'm going to get a mixture of products because the radical on the end could have also attacked.
What that means is that I'm also going to get a product that looks like this.
And this is one of the complications of allylic halogenation.
If your double bond isn't perfectly symmetrical, you're going to wind up getting a combination
of products, and really, we don't deal with major and minor in this section; we're just
going to say, hey, we're going to get both of them.
So it kind of sucks.
Then finally, what would the termination step look like?
Well, there are a lot of terminations.
We could get a Br and a Br.
We could get just any of these radicals – I mean, any of these – that plus Br.
I'm just going to draw this and this.
And that would give us our products that look like this.
And we would also get the other one, the other radical.
That would look like this, with the radical here, plus Br, and that would give us the
other looking product.
Now, I know that there was another one that was the two R groups.
That is almost never seen in here, but if you wanted to include it, you could just add
R radical R and just say that that's going to give you R-R.
But honestly, for these reactions, usually professors just neglect this because it's
assumed that you're not really going to get much of this at all.
What is important is that now you can detect when you're going to get just one product
or when you're going to get a mixture.
And when you're going to get a mixture is when you're doing an allylic halogenation.
If it was just tertiary or secondary, you'd only get one, but since it's allylic, that
means it can resonate.
And in this case, you would get two different structures.
The way to know how many structures you're going to get is just to draw the resonant
structures yourself.
Now what I want to do is go more into depth on allylic halogenation with the specific
regions that we're going to use.