hey guys, it's professor dave. let's talk about electrophilic aromatic substitution.
so, we are familiar with the concept of pi bonds interacting with electrophiles in addition reactions.
now, when we look at aromatic systems, there are also pi bonds that are available to interact
with electrophiles, but they will not do addition reactions
they will do something called electrophilic aromatic substitution. so the difference being
above here, we remember addition reactions where the pi bond is going to interact with some electrophile
to yield an addition product, the difference with aromatic structures is that an aromatic system is very stable
so there's a high premium being placed on wanting to maintain aromaticity, to try to maintain that stability
so we can see here that a benzene ring will undergo electrophilic aromatic substitution
that means that this is not an addition reaction product, it is actually a substitution
some electrophile has substituted a hydrogen or a hydrogen was swapped out for some electrophile
so it's not an addition reaction, it's actually a substitution reaction, like an SN2 just on an aromatic structure.
let's look at the generalized reaction mechanism for an electrophilic aromatic substitution.
let's look at benzene because it's a simple system
so let's say we have some molecule where there's an electrophilic atom or portion of the molecule
and then A just represents the rest of it, it doesn't matter what it is. so here's the benzene ring
we're going to see one of these pi bonds interact with this electrophile and the rest of the molecule will leave
and this part is very similar to an addition reaction
and so we're going to see the electrophile now coordinated to the ring, and that left a carbocation
just as we would expect with an addition reaction
so with the benzene ring, what we end up with is the arenium ion intermediate, because this has three
different resonance structures, we could move this bond here and get the cation there
and then we could move this bond here and put the cation there. now the composite resonance structure
shows us that in actuality, we have partial pi electron density delocalized all along the majority of the
molecule, however it does not reach the carbon that now has the electrophile on there, that is sp3 hybridized
and it is not participating in resonance here, so also the cation that can be found on any of these
resonance structures is also delocalized. so delocalized positive charge and pi electron density
about that portion of the molecule, that is the arenium ion intermediate
now the difference here, this is where it becomes different from an addition reaction
because in an addition reaction, some other thing, let's say some halide ion, would go and coordinate
with the cation, that's probably not going to happen
because let's say a bromine atom came and coordinated there, that would be a fine addition product
but then, we've lost aromaticity, we would no longer have a fully conjugated pi electrons
and so that would exist at a higher energy so it's not a thermodynamically favorable pathway.
however, if we do a substitution, the rest of the molecule A- from before, that could go ahead and extract
specifically the proton located on the carbon that now bears the new electrophile
if it goes and extracts that, then the electrons left behind in the carbon hydrogen bond can form a new pi bond
and thus restore aromaticity, we have our benzene ring back, we have the three pi bonds, fully conjugated
and so this is going to be a very stable system as benzene was to begin with, but there's been a
substitution, where a hydrogen once was, there's now an electrophile, that's why we call it a substitution reaction
this is the generalized pathway of an EAS reaction
and then just to talk a little about thermodynamics
let's say we have the reactants over here
we must understand, the first step is certainly the rate-determining step. this is the energetically unfavorable
situation, where we are breaking aromaticity
have aromaticity, we don't have it.
the arenium ion, that's the intermediate here, that is at a much higher energy. so the first step in any EAS reaction
certainly is endothermic, and certainly is the rate-determining step, because once we have generated
the arenium ion here, it is going to be very easy and very energetically favorable to extract that proton
restore aromaticity, bring the whole system to a much lower energy here, these are the products
where they are relative to the reactants is hard to say, but certainly the intermediate is going to be much higher
in energy than either of the two. so let's go ahead and look at some specific EAS reactions.
the first EAS reaction we want to look at is a halogenation. i've drawn a bromination
this could just as easily be a chlorination or something else. we have benzene, once again the circle
in the hexagon signifies benzene, just as well as this does, so you will see that very commonly
so we want to understand that this means benzene, it's meant to resemble the composite structure
of benzene, so it's actually more accurate, so we have benzene, we have a bromination
in the presence of a lewis acid catalyst, so this is iron tribromide, and that is going to give us our
bromo substituted benzene product with HBr byproduct.
so let's try to understand how that might work
the reason we need a catalyst is that it's too energetically unfavorable for benzene to
interact with bromine. in order to break aromaticity which is highly energetically unfavorable
that's not going to work with just bromine, we need the presence of some kind of catalytic complex.
now if we recall, a catalyst is something that lowers the activation energy of the reaction
while not being consumed in the process stoichiometrically. so here we have a molecule
of bromine, this is going to interact with the iron tribromide, so this is acting as a lewis acid
because the iron atom is accepting electron density, it's acting as an electron acceptor, that's what
makes it a lewis acid. and so a lone pair here is going to go ahead and coordinate to this iron atom
so we're going to generate a new covalent bond between the bromine and the iron. this is the catalytic complex.
now the key feature here is that this bromine has a formal positive charge because it's contributing
six electrons to this lewis dot structure, one per covalent bond, and two lone pairs, a halogen atom is
typically participating in one covalent bond. so this is an uncomfortable situation for a bromine atom
it is going to have a formal positive charge, and it also donated an electron to the iron atom, so the iron
has a formal negative charge. so this is the key complex here to promote this EAS reaction.
so benzene, while it could not interact with a molecule of bromine by itself, it will have an easier time
working with this, because if this pi bond interacts with this bromine atom, then these electrons are able
to go and neutralize this bromine atom, so any time you neutralize a formal charge, that is an energetically
favorable situation, and so that's what makes this step have a lower activation barrier than benzene
simply interacting with a bromine molecule. that's why this is able to work. just like any other EAS reaction
we've got this pi bond interacting with this bromine atom, so that's on there, and we have left a cation
so this is our arenium ion intermediate, i didn't draw the other resonance structures but we remember what
those are, and then what happens, so here's the arenium ion intermediate, we've got the remainder of this
complex, this bromine has been neutralized by those electrons, as we saw before, but this iron atom
still bears a formal negative charge. so what that means is that it's very happy to lose a bond, because in losing
a covalent bond, it itself will neutralize. so that means that bromine, along with these electrons in this
covalent bond, inotherwords Br-, or bromide, is going to dissociate from this complex, go ahead and
extract the proton, the electrons in this carbon-hydrogen bond are going to form the pi bond, restore aromaticity
Br- and H+ gives us HBr, and the rest of this, iron is neutralized by losing that electron
and we regenerate our lewis acid catalyst
this is how halogenation would work. if it were chlorination, this would be Cl2 and FeCl3, and the
mechanism would be absolutely identical, just replace every bromine with a chlorine, and so that's how
this is gonna work, if we look closely we can also understand how this looks just like
the generalized mechanism from before, we had benzene, and then we had EA, so this was the
electrophile that was E and the rest of this was A
then later, the rest of this that was simply A- went and grabbed the proton and we restored aromaticity
we do have byproducts, but it does look just like the generalized mechanism we looked at before
and the only other thing we need to understand is what this lewis acid catalyst is, what it's doing
and why it's necessary to be able to promote this reaction.
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