Hey guys. In this video, we're going to dive deep into a type of pericyclic reaction called a thermal cycloaddition. So, thermal cycloadditions are pericyclic reactions in which 2 pi bonds are destroyed, by the way, we know that it's 2 because it's a cycloaddition, and they always destroy 2 pi bonds, after a heat-activated cyclic mechanism. So, we know that all pericyclic mechanisms are cyclic, but this happens to be a thermally controlled one, not a photochemically controlled pericyclic reaction. It turns out that there's already a very popular example of this type of reaction in your organic chemistry textbook, and that's called a reaction. Now, you may have not studied the reaction yet, or maybe you just got finished studying it, but this reaction is an example of a thermal cycloaddition. So here's just the general kind of rundown of what you would get. You would have 3 pi bonds reacting under heat to form a new cyclic product that only has 1 pi bond. Okay. That's the basics of how it works. Now, very quickly, I do want to show you guys the mechanism. The mechanism would have to be a cyclic, concerted reaction where all of your double bonds are forming new bonds within the system to make a new ring. So, for example, this double bond right here would form a new bond in between the 2, then that's basically going to form a new bond between the diene and the alkene. Then this double bond would come down and form a new double bond here, and then finally, this double bond would come around and attack this one. Now, actually, even though I showed that it's making a bond, typically, this first mechanism arrow is actually drawn to that carbon, just so people understand that it's actually going to form a new link to that carbon, okay? And what you would get as a result is a new cyclic adduct, where now you have these 2 molecules that came together and made a new ring. Okay? Now that's the basics of the mechanism, but to really understand what's driving a cycloaddition, a thermal cycloaddition, you need to go back to frontier molecular orbital theory. We need to understand what frontier orbitals are. We need to understand how HOMO and LUMO are interacting for these 2 molecules. And it turns out that it's a very kind of simple rule, which is that in a cycloaddition, the HOMO from 1 molecule, which I'm calling HOMO a, must fill the LUMO from molecule b. So that means that the HOMO from 1 is going to kick up electrons to fill the LUMO of the other molecule, and that is how they're going to join together and make a new ring. Okay. According to frontier molecular orbital theory, this bonding interaction is the strongest when the symmetry and energy levels between your molecular orbitals match closely. So, what does that mean in terms of symmetry? Symmetry has to do with the lobes. Remember, when you have molecular orbitals, you have lobes facing in different directions, right? So, you want to make sure that the terminal lobes of your HOMO match the terminal lobes of your LUMO, so that there can be good bonding overlap between them. That's what we call symmetry, and I'm going to show you more of an example, but for right now, just know that it has to do with what direction the lobes are facing. You want them facing similar directions, okay? And then, what do we need by energy? The energy also has to match because you want your HOMO and your LUMO to be close in energy, and not far apart. If they're very far apart in energy, one is much higher than the other, it's difficult to make a strong bond. So you're trying to find the HOMO that's closest to the other's LUMO, and that's where you're going to find a really strong interaction. So, just to kind of summarize what I just said, in order for a cycloaddition to take place, you're looking at 2 things. 1, the reaction must be what's called symmetry allowed. Symmetry allowed has to do with the orbitals matching up properly. Okay. Symmetry allowed, not disallowed. Disallowed is also called just, you know, forbidden. This is another term your textbook may use, symmetry forbidden or just forbidden. It needs to be symmetry allowed, okay. And the second one is that you want to try to minimize your gap, which is what I was just talking about in terms of energy levels. The gap gets bigger the further apart those HOMO and LUMO orbitals are in energy. So you want to make sure that you're trying to minimize that gap as much as possible. Cool? Awesome, guys. So now it's time to actually get into the molecular orbital theory of a cycloaddition. I hope you're excited. So here's our diene. Let's start off with our diene. You should know how to draw the molecular orbitals for a diene at this point. We've gone over that in other videos. And what we know about a diene is that it has 4 pi electrons. So that means that psi 1 should get 2 electrons, and I've already gone ahead and labeled our HOMO and our LUMO orbitals for the diene. Once again, a is just to signify that we're dealing with this molecule a. I'm calling it a for this reaction. Okay. Now, in heat, we're going to be exposing that diene to another conjugated molecule. In this case, it's the simplest conjugated molecule which is an alkene, but it doesn't have to be just an alkene. It could be another diene; that would be fine, just something else that's conjugated that has HOMO LUMO orbitals. Okay? So in this case, we should know how to fill in the orbitals for this. How many pi electrons are there? 2. So that means that I should fill in 1₂. And I've also filled in that this is the HOMO for molecule B and on top, we have the LUMO for molecule B. So what did we say about what we're trying to accomplish in a cycloaddition? What we're trying to do is we're trying to take the electrons from HOMO a and use them to fill the orbital of LUMO B. Okay? So that's what the heat energy is going to do. You might say, well, Johnny, why would these electrons raise in energy? Well, that's why you need to use heat.
Table of contents
- 1. A Review of General Chemistry5h 5m
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- 34. Nucleic Acids1h 32m
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16. Conjugated Systems
Thermal Cycloaddition Reactions
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