Claisen Rearrangement - Online Tutor, Practice Problems & Exam Prep

The Claisen rearrangement is a heat-activated 3,3-sigmatropic shift involving an allyl ether, characterized by the transformation of an isolated diene. The mechanism entails breaking a bond between the first two atoms and forming a new bond between the third atoms. The product often contains a carbonyl group, which can tautomerize to an enol form, though the keto form is typically favored. Understanding the conditions and characteristics of this rearrangement is crucial for predicting product outcomes in organic synthesis.

Definition of Claisen Rearrangement

Hey everyone. In this video, we're going to discuss a specific type of sigmatropic shift called a Claisen rearrangement. So guys, what is the Claisen rearrangement? Well, it's a heat-activated, heat-activated, 3,3 sigmatropic shift that involves an allyl ether. So this is interesting. It's a specific type of sigmatropic shift. It's a 3,3, but specifically, the reactant needs to have an allyl ether in the original functional group to then transform into a Claisen rearrangement. If you don't have that allyl ether, you're sunk. It's not going to be a Claisen rearrangement. Okay? I will go ahead and help you identify the allyl ether in a second, but let me just read through a few more points first about it.

So depending on how many pericyclic reactions you've had to learn at this point, you might be kind of confused about how to identify which type is which. Well, something that's easy about the Claisen is that it's one of the few pericyclic reactions that does not begin with any form of conjugation. So, notice that we have a diene in here, right, but this diene is considered to be isolated because they're not next to each other, right? So it's really the only pericyclic reaction that could possibly take place between an isolated diene with an allyl ether. If you think about it with those two things in mind, you can easily determine this is going to be a Claisen rearrangement.

Now, something else to keep in mind is that right now I've drawn this molecule very nicely, so it's very easy to kind of think mechanistically how it would form into this product because the double bonds are close to each other. But just so you know, the way your professor or your homework may give you the problem, it may require some sigma bond rotation for you to get it to look nice and orderly before you do your mechanism. So I'll show you a few examples later about how we're going to have to rotate it first into position before we can then draw the mechanism. Okay. Cool.

So let's go ahead and just identify first of all, what do I mean by allyl ether? So guys, an allyl ether is, like I mean, let's just go all the way back to the beginning. What's an ether? An ether is an R-O-R group, an ether. And specifically an allyl ether would mean that your O is attached to a CH₂ that's then attached to a double bond. Okay, so that's what an allyl group is. This group right here would be allyl. You should be familiar with allyl positions by now because we use them a lot in organic chemistry. But it means you're not directly attached to the double bond, you're one carbon away. So what is the allyl ether in the original reactant? It's this thing. The other thing on the top is actually part of a vinyl ether and that part actually could change. There are Claisen rearrangements that happen with vinyl ethers, there are Claisen rearrangements that happen with phenyl ethers, so don't worry too much about the top part because that part could change based on the specific type of Claisen. But the bottom part, that allyl part, always has to be there. It always needs an allyl ether, so that's why that's like the one defining characteristic of a Claisen rearrangement.

Now once again guys, you guys should already be familiar with how to number sigmatropic shifts in general, but remember that it has to do with the bond that is being broken and the bond that is being made. So in this case, the bond that is obviously broken is the one between the ones because you can see over here it no longer exists. The bond that's being formed is the one between the threes because now that's a new bond over here. So we would then say that this is in the category of a 3,3 sigmatropic shift because a new bond has been created between the threes. Cool. And lastly, what would this mechanism look like? In general terms, it would just be any mechanism you can draw that's pericyclic and concerted that is going to create a bond and break a bond. So what I would do is something like this, something like this, and something like that. Cool? And there you have it, that is your final product.

Now guys, it turns out that it can get a little bit more complicated than this though, and that's because sometimes in your final product, you're going to end up having to decide which tautomer is the most favored because if you'll notice, we actually have a carbonyl, we have a carbonyl as our end product here. Okay. We have a carbonyl and something that we learned about carbonyls a long time ago, or I mean you've definitely come across it at some point in organic chemistry, is that carbonyls are able to tautomerize into enols. So they're able to tautomerize into enols. And this is, so this is called the keto and this is called the enol. Now, most of the time, the keto is favored, almost all the time the keto is favored. So you usually don't need to worry about it. Usually, you can just leave the product the way it is because ketos are favored, but some specific molecules prefer the enol. So what that means is that when you come across a molecule that favors the enol position, you must tautomerize to the enol in the last step. If you don't tautomerize to the enol, you get the question wrong because you picked the wrong tautomer. Okay? So what it says here is that in the final tautomerization, a final tautomerization step is required for molecules in which the enol form is favored. Again, not too many molecules favor the enol form, but if they do, you must tautomerize to the enol. Makes sense? Cool. So what we're going to do in the next practice problem, in the next example, is I'm going to remind you guys of some of the properties of keto-enol tautomerism and we're going to pick the most favored one at equilibrium.

When is Enol Tautomer Favored?

Alright, so here are 2 products of Claisen rearrangements. Now you're going to have to kind of trust me on that because we didn't go through the whole mechanism, but just take my word for it that these are the products of the Claisen rearrangement, which actually is the product we just did, and then this is another Claisen rearrangement, okay? So just take my word for it that these are two products of Claisen rearrangements. Now we need to do, I shouldn't have circled; that's a good thing about having eraser. What we have to do is pick the more stable tautomer of the 2 using what we know about keto-enol tautomerism. Now if you're confused about this or if this isn't too clear to you, I have a whole series of videos on picking the most stable tautomers in tautomerization. So you can just search in the search bar of YouTube for Claisen tautomerization and you're going to get more information on this. This is just a quick review and remember that, so let's just look at the first one, we'll do both of them side by side. So the first one, this is my keto and this is my enol, right? Which one is favored in this case? So guys, just so you know, the rule is that the keto is pretty much always favored. So I'm going to go ahead and circle it. We're done. You would not tautomerize this one. Cool. What about this equilibrium here? I'm going to step outside so you can see both clearly. What I see is that here once again I have a keto and here I have an enol.

Now, out of these 2, which one is more favored? Guys, there are only 2 examples in which the enol is more favored, and I'm going to give them to you here. One is when you can make an aromatic product or an aromatic enol, and two is when you have a beta-dicarbonyl enol. Okay? These are the 2 situations in which enols are actually more favored. If you have it, if either you can make it aromatic or if it's in the form of beta-dicarbonyl, and I do go over both of these situations in my tautomerization videos. So, what do we have here? Do we have any of those 2 situations? Yes, we do, because notice that our first molecule is not aromatic. Aromatic just means that it has alternating single and double bonds, which is going to make it more stable, like our clutch loop, like a benzene ring. So this one does not look like a benzene ring, it's not aromatic, but this one has a benzene ring. It's actually phenyl, this specific enol is called phenyl. So if you have a choice between non-aromatic and aromatic, we pick this one. It turns out that this molecule, if you do not tautomerize to the phenyl, you get the question wrong.

So this is one specific situation or one of 2 situations that you have to make sure that you tautomerize to the enol if it's stable, if it's more stable, but the other one, you could just leave as keto. Does that make sense? This is just to do with the guys; this has to do with the fact that we're using oxygen now and oxygen has the ability to either form an alcohol or either form a carbonyl. So that's why you have to think about this step with a Claisen rearrangement, okay. Awesome. So let's move on to the next video.

Predict the Product

Provide the mechanism and final products for the following reaction. You may skip the tautomerization mechanism if one is required. Cool. So first of all, what are we even dealing with here? Well, all I really see for sure is that I do have an Allyl ether here, so I have an Allyl ether. Another thing I noticed is that my double bond is not conjugated with any other double bond so I'm just going to put here it's not conjugated and I'm reacting with heat. So what this tells me is that this must be a Claisen rearrangement. Remember a Claisen rearrangement can be more technically defined as a 3,3 Claisen rearrangement. That is going to help us determine which bonds to make and which bonds to break.

So just so you know guys, like common sense tells you it's a Claisen because it's on the Claisen topic, but I'm trying to help you think, coach you through how you would see this if it was in the middle of 40 other reactions on your exam. That's how you would distinguish that this must be a Claisen. Okay? Cool. So what else do we need to do? We must rearrange this because, and not just rearrange, we must rotate and then we can rearrange. We must rotate because we need the double bonds to face each other. So in this case what we're going to do is we're just going to rotate this guy up. So let's redraw it here, O, and now let's put the double bond right here. And now we actually can start to visualize these 2 double bonds coming together and making a single bond in between them. This is the way you would rotate it so that you could visualize the Claisen rearrangement.

Now something else to be aware of guys is that remember that I told you that the bottom part of the allyl ether always stays the same, but the top part might change. So this is an example, the top part is different than the other example we did. The other example was a vinyl ether, but specifically, the top part is what we call a phenyl ether. Not phenol, phenyl is different. Phenyl ether because it's directly attached to a double bond, which is fine too because we can use that double bond inside of the aromatic ring to do our mechanism.

Cool. So guys, what's going to happen? Remember this is a 3,3 Claisen rearrangement and we have to break a bond and make a bond. Maybe the most obvious one is the bond that we're going to make, we know that we have to make a new bond here because that's the only area that's missing a gap, which means that this is the 3 and this is the 3. That being said, that means we can count backward and figure out what the one positions were. This is my 2 and this is my 1, this is my 2 and this is my 1. And guys what this means is that I am now ready to break bonds and make bonds using my mechanism. So what I would do is move my electrons here and then move these electrons here and then move these electrons here. And what that's going to do is it's going to give me a new structure that looks like this:

So this appears to now be a carbonyl. This is now a single bond. This is now a single bond. This is now a double bond. That methyl group is still there and these O bonds didn't change. And there you have it, we just broke the bond between the ones and then we made a new bond between the threes and we just did a Claisen rearrangement. Cool? So are you guys happy? Makes sense? Did I miss anything?

Guys, I missed something. We're not done yet because this happens to be the one unique type of tautomer that prefers to be an enol because it wants to be aromatic and turn into a phenyl with an OH. So that means that if you, it says you may skip the tautomerization mechanism if one is required, but I still need to write that it's going to tautomerize. So let's go ahead and do this with equilibrium arrows. Let's write tautomerize. Just so you guys know, in this question it says you can skip it, but almost all professors in all textbooks will not make you draw the tautomerization step for this reaction because it's not the point of this reaction. We have a whole chapter about tautomerization chemistry that you can you can learn about that mechanism if you want, but the point of this, this is a pericyclic reaction, is just to understand that tautomerization will take place. So my final final product would be this with double bond, double bond, double bond, OH and then single bond, single bond, double bond methyl group. Cool and you don't have to draw it just like that, you could also have drawn it sticking out and that also would have been fine. Cool and there you have it guys, we are done with the Claisen rearrangement mechanism. Alright, great. I'm sorry I said Claisen condensation and I meant Claisen rearrangement. Okay. Awesome guys. Let's move on to the next video.

Here’s what students ask on this topic:

What is the Claisen rearrangement in organic chemistry?

The Claisen rearrangement is a heat-activated 3,3-sigmatropic shift involving an allyl ether. It is a specific type of pericyclic reaction where an isolated diene and an allyl ether undergo a concerted mechanism to form a new product. The reaction involves breaking a bond between the first two atoms and forming a new bond between the third atoms. The product often contains a carbonyl group, which can tautomerize to an enol form, although the keto form is typically favored.

What are the key characteristics of the Claisen rearrangement?

The key characteristics of the Claisen rearrangement include: 1) It is a heat-activated 3,3-sigmatropic shift. 2) It involves an allyl ether as the reactant. 3) The reaction does not require conjugation in the starting material. 4) The mechanism is concerted and pericyclic, involving the simultaneous breaking and forming of bonds. 5) The product often contains a carbonyl group, which can tautomerize to an enol form, though the keto form is usually favored.

How do you identify an allyl ether in a Claisen rearrangement?

An allyl ether in a Claisen rearrangement is identified by the presence of an ether group (R-O-R') where the oxygen is attached to a CH₂ group that is then attached to a double bond. This specific structure is crucial for the Claisen rearrangement to occur. The allyl ether must be present in the reactant for the reaction to proceed.

What is the mechanism of the Claisen rearrangement?

The mechanism of the Claisen rearrangement is a concerted, pericyclic process. It involves the simultaneous breaking of a bond between the first two atoms and the formation of a new bond between the third atoms. This 3,3-sigmatropic shift results in the transformation of the allyl ether into a new product, often containing a carbonyl group. The mechanism can be represented by a series of curved arrows showing the movement of electrons during the reaction.

What is the significance of tautomerization in the Claisen rearrangement?

Tautomerization is significant in the Claisen rearrangement because the product often contains a carbonyl group, which can tautomerize to an enol form. While the keto form is typically favored, certain molecules may prefer the enol form. In such cases, the final step of the reaction involves tautomerizing to the enol form to obtain the correct product. Failing to recognize this can lead to incorrect answers in problem-solving.