In this video, we're going to talk about a really important synthetic pathway called
the Curtius rearrangement.
This reaction does go by another name.
It’s also called the reduction of acyl azide.
That name really describes what's going on here.
An acyl azide is a functional group that has a carbonyl in R group and then N3 on one side.
That’s an azide.
If you can reduce this molecule and get the carbonyl to come off, what we're going to
wind up getting is we’re going to wind up getting a primary amine.
This is going to require a really interesting rearrangement for that to take place.
There's only two reagents in this reaction.
There's heat and there's water.
You might think that it's a really easy mechanism but actually it's really strange because of
the fact that atoms are rearranging and we're going to have a reactive intermediate called
in a nitrene that isn’t really commonly seen in Orgo.
We're going to definitely make sure to handle that in the mechanism part.
What are two steps?
The first step is heat.
Heat is going to drive the rearrangement of this molecule here into isocyanate.
Isocyanate is its own topic within Organic Chemistry.
There are a lot of reactions that we can do with isocyanate.
But for right now, this is just going to be the middle structure that we use the intermediate
structure that we then react with again to get our primary amine.
What's our seconds step?
Our second step is addition of water to that isocyanate.
That's going to result in a decarboxylation reaction and it’s going to liberate two
gases at the end.
It's going to liberate CO2 gas because decarboxylation reactions always liberate CO2 gas.
We're also going to liberate N2 gas from the decomposition of the acyl azide in the first
step.
I have mentioned a few terms here, for example decarboxylation.
That’s a mechanism that I teach in a separate video.
If you want to know more about what a decarboxylation reaction is, you can always just type it into
the Clutch search bar and you'll find that video.
That being said, let's go ahead and just dive into this mechanism and see what it's all
about.
I already showed you guys what an acyl azide looks like.
When you draw it out in bond line structure, this is what you're going to get.
That's the most common way that it's represented.
It's N double bond N double bond N and with the formal charges accordingly.
But this azide has resonance structure.
That resonance structure has to be taken into account.
That resonance would be that the negative charge could make a triple bond.
That would already have three bonds on one side.
That means I only need one more bond on the other or I’d be violating the octet.
If I make this bond, I'm going to break this bond and form a lone pair on that nitrogen.
That means that the resonance structure on the other side looks something like this.
I’ve got a triple bond here, a negative charge here because I added an extra lone
pair, and I still have a positive in the middle because it still has four bonds.
Nothing changed for that middle nitrogen.
What’s interesting here is that usually these resonance structures both exist.
They're both averaging into the hybrid.
But in the presence of heat, one of these resonance structures is going to influence
the character of the molecule more than the other.
The reason is because we can get a decomposition reaction.
Heat is going to make this decompose.
The reason is because this resonance structure of the second one looks a whole lot like N2
gas.
N2 gas is just N triple bond N. It composes 78% of the atmosphere.
If 78% percent of the atmospheres is N2 gas, do you think it's stable or unstable?
It’s highly stable, guys.
I mean, you're breathing it in constantly.
It’s not reacting with you, is it?
Or not too much.
This is a very stable entity.
If you can become N2 gas, you're like in chemical nirvana.
That's like the best thing you can do.
Look at this resonance structure.
It's one bond away from being an N2 gas.
In heat, what you're going to get is a decomposition reaction where these electrons pick up and
leave us a lone pair.
What you wind up getting is now your N which has two lone pairs and nothing else, R, plus
your N¬2 gas.
This molecule that I drew as a product, it's really an intermediate because it has the
right number of valence electrons.
Remember that nitrogen wants to have five.
It has five.
But this is highly reactive because it’s not filling its octet.
This nitrogen only has six octet electrons.
If you were to count lone pair is two, lone pair is two, bond is two, that’s 6.
We need 8.
This is a very reactive intermediate called nitrene.
It turns out that this nitrene is going to want to rearrange.
This is the Curtius rearrangement part.
What you’re going to get is at the R group, the electrons from the R group actually attach
to the N, a lone pair comes down and forms a double bond.
This is just going to be a rearrangement.
What we wind up getting is our isocyanate as a product.
I’m going to have O double bond C double bond N with now an R group coming off of it.
This is a molecule called isocyanate which as I alluded earlier is actually a really
important molecule in Organic Chemistry.
There's a lot of different addition reactions we can do to this.
But for this mechanism, we're only going to do one and that's going to be the addition
of water.
So you could end this here.
If all you did was you added heat to an acyl azide, you’d get isocyanate and you’d
be done.
But we want to get a primary amine.
Essentially we want to liberate this part.
All we want is the NR component.
We don't want the carbon.
We don't want the O. How can we get rid of that?
If we add water.
In my second step, I add water.
Can you guys predict which atom the water will be most attracted to in a nucleophilic
attack?
What do you guys think?
You got it.
We're going to attack the carbon.
Carbonyl carbon, guys.
We’re going to push the electrons down to the N. What this is going to give us is a
molecule that now looks like this.
OH, N, R, and eventually this is going to get an H because it's going to form a negative
charge and it’s going to protonate.
This is called carbamic acid.
What’s special about carbamic acid is that it is very unstable.
It's not a stable molecule, so we can spontaneously decarboxylate.
Again, for more on the mechanism on decarboxylation, search that topic individually.
But we’ll just draw it as it relates to this molecule here.
This is going to be our decarboxylation.
Let me actually draw this H sticking out so it’s going to be easier.
This nitrogen grabs the H. make a bond, break a bond.
We're going to take the electrons from the single bond and donate them to bond between
the O and the C. Now that that carbon has four bonds, the carbonyl carbon, we're going
to use these electrons to make a lone pair on the end.
It’s a weird reaction.
But notice what you get at the end.
What you're going to get is now a nitrogen with an R group.
With how many Hs?
Two.
You’ve got the original H and we've got the new H that we grabbed.
What else do we have?
We also have a carbon with now a double bond O and a double bond O. Do you guys know what
that is?
The product of a decarboxylation is always CO2 gas.
We just created two different gases.
One of them is a greenhouse gas hurting the environment.
We're keeping it in the lab though.
This is really interesting.
We’ve just made CO2 gas.
We’ve made N2 gas here and here.
But most importantly and what your professors will be most interested in is that we made
a primary amine.
That primary amine also lost a carbon.
This is going to be an interesting synthetic reaction that we use when we’re trying to
get an amine and we’re trying to lose a carbon.
I’ll show you guys why you wind up that in a second.
This is a really great reaction to use.
It looks complicated but it’s actually used more often than you’d think in Organic Chemistry.
That being said, that's the whole mechanism.
I hope that made sense.
Let's go ahead and look down with this question.
This question does draw from our aromaticity chapter, electrophilic aromatic substitution.
But that being said, if you have covered those chapters already, go ahead and try to remember,
brush up on those reactions.
See how you could perform this synthesis of making aniline.
Basically, what I’m trying to do here is I’m trying to make aniline.
We've never learned an EAS technique to make aniline before.
I want you to think about how you could use the Curtius rearrangement to make aniline.
Then I'll show you the answer, so go for it.