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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.