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Organic chemistry
Course: Organic chemistry > Unit 9
Lesson 2: Reactions of benzeneFriedel-Crafts acylation
Friedel-Crafts acylation reaction. Created by Sal Khan.
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- at about, you show the chlorine in AlCl4 giving its' electron to the hydrogen, then the hydrogen giving it's electron back to the benzene ring so it can form a double bond and become aromatic again. You don't mention what happens to that chlorine atom and hydrogen after? do they then form HCl? thank you! 9:00(12 votes)
- I just watched the next video, Friedel Crafts Acylation Addendum, that answers my question!(7 votes)
- I might have misheard my teacher, but is there something about the first stage of this reaction, which mustn't occur in a polar solvent (such as water) because it will dissolve the AlCl3 salt?(3 votes)
- Not exactly. AlCl3 is a powerful Lewis acid. It reacts vigorously and irreversibly with Lewis bases. With water it forms the hexaaquoaluminum(III) ion, Al(H2O)_6^3+. This means that it can no longer behave as a Lewis acid and is therefore unable to catalyze the Friedel-Crafts reaction.(5 votes)
- I know this is irrelevant to the chemistry, but as an historical note I believe that it was James Crafts who was the president of MIT, and not Charles Friedel as Sal stated. James Crafts was the one who never received a PhD but went on to become a professor at Cornell as well as MIT, and then president at MIT (1897-1900), in recognition of his ground breaking work in organic chemistry. :)(4 votes)
- At 6 minutes, why is the resonance structure a triple bond and not a single bond with positive charge on oxygen, i.e. electrons form the double bond transferred to the carbon.(2 votes)
- All three structures are permissible but high-energy structures.
CH₃-C⁺=O ⟷ CH₃-C≡O⁺ ⟷ CH₃-C²⁺-O⁻
The middle structure is unfavourable because it puts a formal positive charge on the electronegative O atom.
Your structure is even more unfavourable because it creates a separation of charge.
This takes a lot of energy, so the third contributor is the least important of the three.(5 votes)
- I thought that AlCl3 is an ionic compound and that aluminium loses 3 electrons to reach an octet of electrons.
But Sal shows AlCl3 to be covalent.
Please point out where is my mistake(1 vote)- The view of Al losing three electrons to form an octet is a useful one, but it is an oversimplification. Chemical bonds range over a whole spectrum from pure covalent to polar covalent to ionic. There is a formula that allows us to calculate the polarity (i.e., the % ionic character) in a bond. It depends on the differences in electronegativities (ΔEN) between the two atoms. For Cl, EN = 2.5; for Al, EN = 1.5. Therefore ΔEN = 1.0. The formula then tells us that the Al-Cl bond is polar covalent with a 22% ionic character. So the bonds in AlCl₃ are really polar covalent.
We usually consider bonds with more than 50% ionic character to be ionic. For this to happen, we must have ΔEN > 1.6. The Na-Cl bond has ΔEN = 2.1. This corresponds to 67% ionic character, and we usually consider NaCl to be an ionic compound. See, for example,
http://www.avogadro.co.uk/structure/bondnature.htm(4 votes)
- I'm also wondering if this is a step-wise or concerted reaction as well.(2 votes)
- Does the AlCl4- always react as a base and take the hydrogen away? Because in our book it says that there is a base taking away the hydrogen, it never says that the AlCl4- can do this too.(1 vote)
- at about, What happened to the HCl that combined at the end? 10:15(1 vote)
- Correct. HCl is aproduct of the reaction:
Ar-H + RCOCl → ArCOR + HCl
During the reaction, hydrogen chloride is formed and some
provision has to be made to prevent it from being released into the laboratory. Usually, you employ a gas trap. A vacuum adapter is placed on top of the reaction apparatus and a hose to a water aspirator is connected to it (NOTE: The system is left open to the atmosphere). Air is drawn through the apparatus and sweeps the hydrogen chloride gas out with it. The hydrogen chloride dissolves in the water at the aspirator and goes down the drain.
To see the details of a typical Friedel-Crafts alkylation, visit http://www.chem.wisc.edu/courses/344/Clauss/Fall2004/Friedel-Crafts%20Alkylation.pdf(3 votes)
- what does "Aromaticity of benzene ring" means?(1 vote)
- The cyclic arrangement of the π electrons in a benzene ring makes the compound much more stable than ordinary alkenes. We call this "aromatic stabilization".
"Aromaticity of the benzene ring" refers to this extra stability.(3 votes)
- i just want to know, do phenols give friedel crafts reactions?
like we know aniline and derivatives do not give these reactions, while anisole does. i'm confused about phenol.(1 vote)- No, phenols do not give Friedel-Crafts reactions.
The lone pairs on the O atoms are much stronger Lewis bases than the π electrons in the ring.
So the FeCl₃ reacts preferentially with the O atoms of the phenols.(2 votes)
Video transcript
We now know how to name
aldehydes and ketones, and what I want to do in this video
is show a mechanism for actually creating one. In particular, we're going
to create a ketone. So let's say we've got ourselves
some benzene, and in the first step of this reaction,
the benzene is just going to sit and watch. We've got some benzene
and we've got some of acetyl chloride. So it looks almost like an
aldehyde or a ketone, but instead of having a carbon chain
or a hydrogen, we're going to have a chlorine
atom right over there. So this is acetyl chloride, sometimes called acyl chloride. This is acetyl chloride, and
we're going to have an aluminum chloride catalyst. And
a catalyst means that it participates in the reaction,
but it enters the reaction and it exits the reaction as
the same molecule. So it just catalyzes it. It doesn't disappear. It just changes halfway,
but then goes back to what it was before. So we have some aluminum
chloride and it's bonded to one, two, three chlorines. Now, the first step of this
reaction is to turn this acetyl chloride into a good
electrophile, turn it into something that's really good at
nabbing electrons, so good that it can break the
aromaticity of the benzene ring and essentially add itself
to the benzene ring. This is actually the same
mechanism we saw with electrophilic aromatic
substitution. I always have trouble
remembering the name, but I always imagine it's
electrophilic substitution. Either way, but it's a very
similar mechanism. And actually, what we're going
to show in this video is called Friedel-Crafts acylation,
because this right here is called an acyl group and
we're essentially going to acylate the benzene ring. We're going to add this
group right here to the benzene ring. So enough of what's
going to happen. Let's actually see it happen. So the first thing to realize,
this aluminum chloride, the aluminum in it is electron
deficient. And at first, if you just look
at the Periodic Table, you have these chlorines over here,
pretty electronegative. Aluminum is in the same row, but
chlorine's way more to the right, so it's more
electronegative, so the chlorines are going to hog the
electrons in this molecule. The chlorines are going to hog
electrons, so the aluminum is going to have a partial
positive charge. Chlorines will have slightly
partial negative charge. On top of that, you see aluminum
is a Group 3 element, one, two, three, so it has
three valence electrons. You see that right here, one,
two, three, nowhere close to the magic number of eight. Even when it covalently bonds
with these chlorines, it can only pretend like it
has six electrons. It can kind of pretend like it
has these chlorine electrons over here, but that only
gets it to six. So it would really like to have
more electrons to get closer to that magic
eight number. So what you can imagine is a
situation where this chlorine on the acetyl chloride, it's
already hogging this green electron from this carbon. It was already doing that. It's more electronegative, so
this thing over here will actually be given
to the aluminum. And so it will then have a
bond with the chloride. So if that happened, what does
our reaction look like? So if that happened, what does
everything look like? Our aluminum, our aluminum
chloride, or what was formerly aluminum chloride, now just
gained an electron, and with it, it is now bonded to
another chlorine. So it is now bonded to another
chlorine, and since it gains an electron-- let me
make it very clear. This is an L. My penmanship is
deteriorating. That's an L. And since this aluminum gained
an electron, it now has a negative charge. And normally, a negatively
charged thing isn't that stable, but these guys are
electronegative, so they might hog a lot of that
negative charge. And on top of it, aluminum
can now pretend like it has eight electrons. It has one, two, three, four,
five, six, seven, eight. When you covalently bond to
someone, you can kind of pretend like you have their
electrons as well. So now you have this anion
that was formed from the aluminum chloride, and now
the acetyl chloride will look like this. Scroll down a little bit. Let me make it clear. We're in the next step
of the reaction. What was formerly the acetyl
chloride has now lost the chloride, so it's now really
just an acyl group. So you have the carbonyl bonded
to a CH3, a methyl, just like that. This guy lost his electrons,
so now he has a positive charge. And this is actually
not that stable. you're going to see it's
actually highly reactive. It's a very good electrophile. It wants to steal other
people's electrons. But it can exist for a short
amount of time, especially because it is resonant
stabilized. You say, how is it resonant
stabilized? Well, this oxygen over here has
two electron pairs that I didn't draw before. And let me draw the second
one in a different color. It has two electron
pairs like that. So you can imagine
the situation. This carbon is already-- he has
a positive charge, and his oxygen is more electronegative. It's already hogging
his electrons. Maybe he wants to give
back a little bit. Say, hey, this is positive. All of the electrons are
hanging out here. They'd be attracted to the
positive, and you could imagine one of these
electrons being given back to the carbon. And if that happened then we
have another resonance form that looks like this. So this was our original
molecule. That's our original, or
what it looked like. We still have this double bond
right over there or that pair of electrons. Now, this pair of electrons
now forms another bond. This electron here is now
on the oxygen end. This electron over here is now
on the carbon end, and now they have a triple bond. And what happened? This positive carbon gained an
electron, so it's now neutral. And the neutral oxygen
lost an electron, so it is now positive. And you could imagine, this
is not a very stable-- you wouldn't see this just floating
around by itself, but it does stabilize this
entire configuration. It stabilizes this molecule. So you can show that these
are alternate resonantly stabilized structures
right there. But as I said, these aren't
super stable. This guy really, really,
really wants to react. And now this is where benzene
comes into the mix. And actually, let me draw a
little dividing line here, just so we know that
this was a separate stage of our reaction. So that was the first stage. Then we go over here and now
benzene comes into the mix. The benzene was floating
around. So we have our benzene floating around, just like that. And then I'm going to draw
one of the hydrogens on one of the carbons. All of these carbons have
hydrogens on them. I just won't draw them all. It just make things
complicated. But this guy we said is a really
good electrophile, and you have to be a really good
electrophile to attract electrons from a benzene ring,
to break it's aromaticity. But if it bumps into this guy in
just the right way, at just the right angle, you could
imagine that this electron on this carbon right here gets
swiped by the acyl group. So then what do we have? So now I will go back
in this direction. So you have what was
a benzene ring. We can draw the double
bonds here and here. And we, of course, have
this hydrogen. But now this bond, which was
a double bond there, is now bonded to the acyl group. So it has that blue electron
that the acyl group nabbed. And let me draw the
acyl group. And I'll flip it over so that
we have the methyl on the right-hand side. So it's carbonyl bonded
to a CH3. It was positive. It now gained an electron. It is now neutral. This carbon over here lost
an electron, so it is now positive, so it is now
positively charged. Now, we mentioned the aluminum
chloride is a catalyst, so it won't just sit around
there as the anion. It has to go back to being
aluminum chloride, so let's bring the aluminum chloride
back into the scene. So we have our aluminum
chloride. Let me copy and paste it. So we have our aluminum chloride
here, and so you can imagine that the benzene ring
wants to go back to being aromatic, so this electron right
here on the hydrogen might really want to go back
to this carbon right over here, this carbocation. At the same time, if this anion
now passes the hydrogen at just the right angle at the
right time while this guy's attracted to this carbon, this
chlorine can give this green electron to the hydrogen
nucleus, which is really just a proton. And then the hydrogen's electron
can be taken up by what was this carbocation. And then what do we have? Then we have a situation where
our benzene ring is reformed. We have the aromaticity again. We have that double bond, that
double bond, and now we have this double bond again, although
now it's using the electron from the hydrogen. And now we've substituted this
hydrogen with essentially this acyl group right over here. So we have a carbon double
bonded to an oxygen bonded to a methyl group. And now the aluminum, or this
anion, lost its electron, so it goes back to just being
straight-up aluminum chloride. It goes back to just being
straight-up neutral, electron-deficient aluminum
chloride. And we're done. We've just acylized this benzene
ring, and that's why this mechanism is called
Friedel-Crafts acylation. And Friedel is actually a former
president of MIT, and I did some reading on this. Apparently, he did not have a
PhD, but because he discovered Friedel-Crafts acylation and
this Friedel-Crafts alkylation as well, they said, hey, you
know, this guy's a smart dude. Let's make him the
president of MIT. But I just wanted to show you
that this is a reaction for creating a ketone. So this ketone that we've
created is acetophenone, which we've seen before, which we've
learned is a common name for this molecule that we learned
in the first ketone video. And I'll write it in purple. Acetophenone. And we're done!