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Organic chemistry
Course: Organic chemistry > Unit 9
Lesson 2: Reactions of benzeneElectrophilic aromatic substitution
Electrophilic aromatic substitution. Created by Sal Khan.
Want to join the conversation?
- Why doesn't the electrophile (E+) just take one of the base's (B-) extra electrons? Wouldn't this be easier than stealing an electron from the benzene?(7 votes)
- Check out Sal's other video, "Nucleophilicity vs. Basicity". Just because a base (B-) has extra electrons, that doesn't necessarily make it a good nucleophile (:N) too.
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I also want to add this because all of my chemistry professors emphasize this and they take off points if you do it wrong...
Only Nucleophiles (:N) can give its extra electrons; Electrophiles (E+) do NOT take them! So the arrow should always go from the electrons to the positive charge, not the other way around. (This makes sense if you think about it because the positive charge really means that the atom is lacking an electron and the excess proton is giving the atom the overall positive charge. Since protons are in the nucleus, they cannot actively participate in bonding and reactions. Only the electrons participate.)(9 votes)
- If benzene doesnt have double bonds, then why is it drawn like it does? it confuses things a little?(3 votes)
- It is a compromise between accuracy and utility.
It isn't difficult to draw a benzene ring with a circle or dashed 'half bonds' around the inside of the ring to show that the electrons are actually delocalized, but it is much harder to draw a reaction mechanism that way.
By exaggerating a bit by drawing electrons in one place (and there ARE electrons there, even if they are spread out a bit), we can actually get a feeling for how electrons move during a reaction.(8 votes)
- isnt this also called the (-R) effect?(4 votes)
- There is no such thing as the (-R) effect.
alkyl groups donate electron density but this is no where called or recognized as the "(-R effect)".(1 vote)
- Is it possible for the base to attack the carbocation in a nucleophillic manner as well?(3 votes)
- Nucleophilic reaction have their own specific ways of reacting, and I don't think that a base could react to a carbocation in a nucleophilic way. But you might have to recheck this.(1 vote)
- Can that Base nab a hydrogen atom, other than the hydrogen that got nabbed in this video(The hydogen atom that is attached to the carbon with electrophile)?
And if not, than why?(3 votes) - Is Bromide a nucleophile kind of? Because it loves nucleus or it wants to give away electrons(2 votes)
- Yes Br is a nucleophile, nucleophiles are electron pair donors, and Lewis bases.(3 votes)
- how we prepare electrophile?(3 votes)
- It's a one type of halide or hydroxyl group.(1 vote)
- ... Is one of these three resonance structures more stable than the other two? Just taking a quick glance at it, I can see that the "odd one out" is the middle one, in which the two double bonds are opposite one another (as far away from each other as possible), whereas in the other two diagrams, the double bonds are a bit closer to each other. 7:04(2 votes)
- You are correct. The middle structure has isolated double bonds. The other two structures have conjugated double bonds, and the conjugation makes them more stable than the isolated bonds.(2 votes)
- what happens when a cyclic compound with a double bond reacts with bromine in the presence of sunlight?(2 votes)
- You would get a trans-1,2-dibromocycloalkane. This is because as the Br2 reacts with the double bond a bromonium ion (ring consisting of 2C and 1Br) is formed on one side of the ring, so the left over Br- has to attack from the opposite side.(1 vote)
- How to draw a resonance structure of cyclohexene(2 votes)
- The only other resonance contributors you can draw for cyclohexene will have a + charge on one of the alkene carbons and a lone pair and a negative charge on the other alkene carbon.(1 vote)
Video transcript
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.