Friedel Crafts Acylation Friedel Crafts Acylation
Friedel Crafts Acylation
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- 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 resident stabilized.
- You say, how is it resident 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 residence 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 residently
- 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.
- And we're done!
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