In this video, we're going to discuss a type of pericyclic reaction called a sigmatropic shift. So what is a sigmatropic shift? Well, it's an intramolecular pericyclic reaction in which 0 pi bonds are destroyed after a cyclic mechanism. We know this by definition; remember that sigmatropic shifts are the type of pericyclic reactions that occur with no pi bonds being changed between the reactant and the product, and that's exactly what we're going to see in this example. I will get to this example in just a second, but let me give you a few more tips about what a sigmatropic shift is. The pi bonds aren't going to change, but what it does involve every single time is the breaking of 1 sigma bond, so a sigma bond is going to break, and then the making of a sigma bond. What we're basically doing is that the double bonds aren't changing. In fact, they do change position, but they're not changing in terms of quantity. The sigma bonds aren't changing in quantity either, but it's important to note that there's actually a very big change that happens here, which is that one sigma bond is destroyed and another sigma bond is created. This means that these reactions usually take the form of numerous types of rearrangements. You're not going to see a very distinct difference between the product and the reactant where they look very different from each other. It's just that something's going to rearrange or shift, and you're going to say, "I wonder what happened." And afterwards, by looking at counting the number of pi bonds, you can tell this must have been a sigmatropic shift. Because of that, the products are typically constitutional isomers of the reactant. So what that means is again nothing's changing, no atoms are changing, but the way that they are connected to each other is because you're making bonds and you're breaking bonds. They are usually constitutional isomers of each other. Very common examples of these, which you may or may not have to know based on your class and your professor, but some very common examples are the Cope rearrangement and the Claisen rearrangements. We'll go on to learn these in-depth if it's something that your professor and your textbook requires, but I'm just letting you know that even if you don't need to know them, these are examples of sigmatropic shifts. Cool.
So let's look at this reaction and try to figure out the mechanism. Once again, we have this is a heat-activated reaction where I'm basically starting off with 2 pi bonds and I'm ending up with again 2 pi bonds. So we know this is a sigmatropic shift because this is a constitutional isomer of the first one, but no pi bonds changed. Now let's try to figure out what would be a mechanism that would make sense here. How could we create something like this, right? And guys, it's once again it's going to be cyclic. So what you're going to see is that you're breaking a bond and you're making a bond. In this case, which is the bond that we're breaking? We're breaking the one that was right here, okay? So that's actually where we want to start our arrows from, not that you have to because it's concerted. You can start from anywhere, but just to think about it logically, it makes sense that you would take these electrons and make a double bond here, then you would take this pi bond and make a new sigma bond here, and then if you make that bond, you have to break this bond and put the double bond there. So once again, we have this pericyclic concerted reaction, but it's leading to just the formation of a constitutional isomer. In the next video, we're going to talk about how do you name and identify sigmatropic shifts.
