Electrophilic Aromatic Substitution

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Electrophilic Aromatic Substitution

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We've already talked about how a benzene ring is very-- let
me draw a better looking benzene ring than that-- that
a benzene ring is very stable, because it's aromatic.
That these electrons in these pi orbitals that form these
double bonds, they're actually just not in this double bond,
they can keep swapping.
This one can go here.
This one can go there.
That one can go there.
Actually, they don't go back and forth.
They actually just completely go around the entire ring.
And when a molecule is aromatic, it stabilizes it.
But we've seen examples of aromatic, or actually, in
particular, we've seen examples of benzene rings that
have other things bumping off of them, whether they're
halides or whether they're OH groups, and what we want to do
in this video is think about how that might happen, how do
things get added on to a benzene ring.
We're going to learn about electrophilic aromatic
substitution.
Let me write that down.
Electrophilic aromatic substitution.
And you might say, well, Sal, you just said you're adding
things to the ring.
But the reality is that there's six hydrogens here.
There's one hydrogen, two hydrogens, three hydrogens,
four hydrogens, five hydrogens and six hydrogens.
They're always there.
If you don't draw them, they are implicitly there.
So what we're actually doing, when you add a chlorine or a
bromine or an OH group, it's actually replacing one of
these hydrogens.
That's why it is substitution.
It's aromatic, because we're dealing with a benzene ring.
We're dealing with an aromatic molecule, and we're going to
see that we need a really strong electrophile in order
to do this.
Let's think about this how this will happen.
Before I do that, let me just copy and paste this, because I
don't want to have to redraw this.
Let me just copy it, just like that.
So let's say we have a really strong electrophile.
And I'll give you particular cases in the next few videos,
so you can better visualize what a really strong
electrophile is.
But just from the word itself, electrophile, you could
imagine it's something that loves electrons.
It wants electrons really, really, really,
really, really badly.
And usually, it has a positive charge.
So it wants electrons badly.
And actually, let me make it very clear.
Instead of saying it wants electrons badly, because when
you're talking about electrophiles or nucleophiles,
you're actually talking about how good something is
reacting, you're not actually talking about the actual
energies involved.
Let me put it a different way: good at getting electrons.
Really, really, really, really, really
good at getting electrons.
So what would happen?
We already said this is already pretty stable.
These guys, these electrons, these pi electrons can
circulate all around.
If it bumps just in the right way to something that's really
good at getting electrons, what might happen-- let's say
we have this electron right here.
The way we've drawn it, it's on this carbon right here.
Obviously, the carbon is just at the intersection.
I never drew the carbon.
But if this electrophile, which is really good at
getting electrons, bumps in just the right way, this
electron can go to that electrophile.
And then it would be left with-- so let me copy and
paste our original molecule.
So then what would we be left with?
So we no longer have this bond right here.
It has now been bonded to the electrophile.
Let me make it clear.
We had this electron right here.
That electron is still on this carbon right over here at this
intersection, but the other end of it, the other electron,
has now been given to the electrophile, the thing that's
good at getting them.
So the other side has been given to this electrophile.
This electrophile now gained an electron.
So it had a positive charge, now it will be neutral.
And once again, I'll show you a particular, or several
particular cases of this in the next few videos.
Let me just make it clear.
So this bond, you could now view it as being this bond.
Now, this carbon right over here, this lost an electron.
So if it lost an electron, it will now
have a positive charge.
Now, this is hard to do to a resonance-stabilized molecule,
to a benzene ring.
So that, once again, and I said, and I'm being a little
bit repetitive, this has to be a very good
electrophile to do it.
But once this is there, this is a actually relatively
stable carbocation.
The reason why it is, it's only a secondary carbocation,
but it's actually a resonance-stabilized
carbocation because this electron right
can be given to that.
If this electron goes there, then it would look like this.
Let me redraw it.
I'll draw the resonance structures quickly.
You have your hydrogen.
You have your electrophile.
That's not an electrophile anymore, but you have that E
that's now been added.
You have that hydrogen.
You have a double bond here.
Let me draw a little bit neater.
You have this hydrogen.
You have this hydrogen, this hydrogen and this hydrogen.
What I said is, this is stabilized.
So an electron here can actually jump over here.
So if this electron jumps over here, the double bond is now
over there If that goes over there like that, the double
bond is now over here.
Now this guy lost his electron and it would
have a positive charge.
And then that is resonance stabilized.
It can either go back to this guy, or this electron over
here can jump over there.
Let me redraw the whole thing over again.
Let me draw all the hydrogens.
This right here, you have the E and the hydrogen.
You have a hydrogen here, hydrogen here, hydrogen here,
hydrogen here.
And normally you don't worry about the hydrogens, but one
of the hydrogens is going to be nabbed later on in this
mechanism, so I want to draw all the hydrogens just so you
know that they are there.
But as I said, this is resonance stabilized.
If this electron right here jumps over there, then this
double bond is now this double bond.
And now this guy over here lost an electron, so it would
have a positive charge.
And again, once you had this double bond up here, this
double bond up there is that double bond.
So we can go back and forth between these.
The electrons are just swishing around the ring.
So it's not going to be maybe as great as the situation that
we had when we had a nice benzene ring that was
completely aromatic.
The electrons can just go around the p-orbitals, around
and around the ring, stabilize the structure, but this is
still a relatively stable carbocation, because the
electrons can move around.
You can kind of view it as a positive charge that gets
dispersed between this carbon, this carbon, and that carbon
over there.
As I said, it's still not a great situation.
The molecule wants to go back to being aromatic, wants to go
to that really stable state.
And the way it can go back to that really stable state is
somehow an electron can be added to this thing.
And the way that an electron can be added to this thing is,
if we have some base flying around, and that base nabs
this proton, this proton right here that's on the same carbon
as where the electrophile is attached.
So if this base nabs a proton, so it just nabs the hydrogen
nucleus, then that electron that the hydrogen had, that
electron-- let me do that in a different color.
That electron that the hydrogen had right over there
could then be returned to this carbon up there.
And maybe that makes it a little confusing
when I cross lines.
It can be returned to that carbon right there.
So what would it look like after that?
After that it would look like this.
Let me draw my-- so if that happened, and we drew it in
yellow, we have our six-carbon ring.
Let me draw all the hydrogens.
What did I do that in?
It likes like a slightly green color I did that in.
So I have all the hydrogens on that ring.
Now, I have to be careful.
This hydrogen right there, just the nucleus of it, got
nabbed by this base.
So that hydrogen has now been nabbed by the base.
This electron right here has now been
given to this hydrogen.
So that electron has now been given to this hydrogen, and
then the other electron in the pair is still with the base.
So now this is the conjugate acid of the base.
It has gained a proton.
And on this carbon, right here, we just have what was
the electrophile.
And I'll do the same colors, just to make it clear.
What was the electrophile right over there, this
bond is this bond.
And then finally, we had-- and I'll color code it here just
to make it clear.
We had this double bond here, which is this double bond
right over here.
We had this double bond.
We had this double bond, which is that double bond there.
And then this electron gets returned to this top carbon
right here.
So that electron-- let me make it very, very clear.
So the bond and that electron are
returned to that top carbon.
So that we have the bond and that electron returned to that
top carbon.
That top carbon is now going to be neutral.
And once again, we are resonance stabilized.
One thing I forgot, just to make the charge stabilized,
maybe this base had a negative charge to begin with.
It didn't have to.
But if this base did have a negative charge to begin with,
it now gave an electron to the hydrogen,
so it is now neutral.
And this should make sense because before we had a plus
charge and a negative charge, and then when everything
reacted, everything is neutral again.
The total net charge is zero.
But this is the electrophilic aromatic substitution.
We substituted one of the hydrogens.
We substituted this hydrogen right here with this
electrophile, or what was previously an electrophile,
but then once it got an electron, it's just kind of a
group that is now on the benzene ring.
And by going through this little convoluted process, we
finally got to another aromatic molecule that now has
this E group on it.
In the next video, I'll show you this with particular
examples of electrophiles and bases.