16. Conjugated Systems
Photochemical Cycloaddition Reactions
Photochemical Cycloaddition reactions are pericyclic reactions in which 2 pi bonds are destroyed after a light-activated cyclic mechanism.
MO Theory of Photochemical Cycloadditions
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So in this video I'm going to introduce you guys to another form of cyclo edition called Photochemical Cyclo Edition. So a photochemical cyclo edition is a Perry cyclic reaction in which two pi bonds are destroyed, just like any cyclo edition. But it on Lee occurs after a light activated cyclic mechanism. So it's not. He activated, its light activated. We're still getting a cycle edition, but it's happening through the energy that's given through light. And as you're going to see, it actually has very different implications on the types of products that you can get. So here's an example. I have to al Canes reacting under light, and what I'm getting is a cyclo butane, right? So how maney double bonds did we start with? We started off with two. How maney double bonds are ending up with zero so we can confirm that this is a cyclo edition because I'm losing to Bonds in the process. I also want to briefly show you the mechanism the mechanism would be cyclic and concerted, and it would make those new single bonds so it would be something like this where this double bond comes in and the tax that carbon to make a new single bond. And then this one does the same thing. Cool. There's no you don't really know where it started and where it ended because it's all happening at the same time, and it's all cyclic. At the end. You get a cyclo butane cool. So that is the basics of photochemical cycle edition. That's you know, that's all. You would need to be able to draw the mechanism. But how do you know what the product is actually favored? Well, for that, we're gonna need Thio once again. Lean on frontier molecular orbital theory, which will give us the tools that are required to really predict if these reactions can happen or not. So in any cycle edition, regardless of whether it's thermal chemical or photochemical, um ah, homo must fill a loom. Oh, okay. And what I usually try to do is make one molecule a molecule b. So homo a fills Lou Moby. Now, according to F M. O. T. The bonding interaction is going to be the strongest when orbital symmetry matches and when orbital energy matches. So once again just to reiterate orbital symmetry means that you're orbital's are lining up nicely to make new single bonds. And energy means that there are isn't a huge home, a limo gap between the Homo and, um oh, that you're choosing to interact with each other, So we're going to be trying to make both of these things happen with a photochemical reaction. Okay, Now, something that is new information here for photochemical versus thermal cycle addition is that we have to remember what does light due to conjugated systems. Remember that light is able to be absorbed by the conjugated system, and the congregation system can turn that radiation energy into kinetic energy and shoot up an electron to, ah, higher energy state. Okay, so light excites those ground state electrons in the conjugated system to a higher energy state. Essentially, what we're gonna have, what's gonna happen is that a bonding sigh is going to transfer electrons to an anti bonding side. That's usually what happens. And that means that your Homo and Lou Mo orbital's are gonna change based on the light. Okay, based on light your home alumal orbital's, we're gonna change versus what would have happened in a thermal cycle addition. Cool. So let's go ahead and look at what would happen. Once again, I'm just going to be using the example that we had above. I haven't all keen on the on the left that I'll call Al Keen A and I have an AL Keen on the right. Then I'm gonna call Al Keen beef And just, you know, I can tell that Al Keen is getting cut off just a little bit. But if you printed your pdf, then you should see it. Just a double bond, just like the one on top. Cool. Awesome, guys. So how would we fill in the electrons for the AL Keen? A. What we know is that sy one would get two electrons inside. Two would get zero electrons, right? Everyone's cool with that. So far. So that means that before light has reacted with anything before radiation energy, um, like, is involved, We just have a homo a and then we also have be okay. Be, um, would do the same thing. It would have basically two electrons inside one and then nothing inside to Okay, So notice that I've already labeled homo and limo. I've labeled that for a I have that The bottom one side one is homo A and that for B Uh, Liu Moby is side to okay, but we haven't taken the light energy into account yet. The radiation energy. So what happens when I involved electromagnetic radiation or light photons? What's gonna happen is that one electron from my home Oh, a From my sigh, one is going to get kicked up to side too. So what it's actually gonna look like afterwards is like this one to Isn't that crazy? So that means that now this is no longer my homo. This is now my home away. Okay, So what that means is that the identity of my orbital's just completely change, which is gonna have massive implications on the symmetry. Now, the symmetry changes because light radiation is involved. Okay, so what that means is that in order to predict if this reaction is gonna be favored or not, I'm gonna have to compare what the molecular orbital looks like for Homo A here versus Lou Moby here. So what that means is that we should draw out what the's orbital's look like. Let's draw them here. So I have to Orbital's here to Orbital's here and then I'm gonna do the same thing over here. This will be be to Orbital's here to Orbital's here. Cool. What do we know about filling Molecular Orbital's? The bottom one should be completely shaded at the bottom and the top ones should have the first one remaining unchanged and the last one changing. So this one should go up, okay. And the orbital's that we're going to be that we really care about with this direction because we're using photochemical energy. It's not this one down here. It's actually gonna be this one because electron got kicked up to that orbital and this one and these, they're gonna be the orbital's that we're gonna bring down to our analysis where we figure out if this is a symmetry allowed or disallowed. So let's go ahead and do that. Now let's go ahead and shade in Homo A which Homo a is now this one right here. And let's also shade in Lieu, Moby Liu Moby Is this one here? Okay, Now, guys, the reason that I have Homo a and Lou Moby one on top of the other has to do with where the Homo orbital started. But what we actually know is that the gap between Homo A and will be is almost zero because they are almost at the same exact height, right? In fact, they could be at identically same heights because they're both the same molecule. So for sure, I'm just gonna say this right now. This is definitely the smallest home, a limo gap possible because I'm that they're there at the same exact energy. But it even turns out that this is the only combination of home a limo that works because notice that the other molecule no longer has a loom. Oh, at all. So molecule a doesn't have a loom. Oh, So if I wanted to go from Homo, be toe Lamo A That just doesn't exist anymore because we've already kicked up our electrons to the highest energy state orbital possible. So that means that I would not even have to really analyze this. But just to reinforce that there's actually very little to no energy difference between these orbital's, which is a great thing. Wonderful. Now I have to look at symmetry. So symmetry is this a symmetry allowed or symmetry disallowed process. Our orbital's match Our orbital's match guys and this is awesome because you may remember that when we were talking about thermal cycle additions to pie and two pi didn't work to pie and two pi would be symmetry disallowed and would lead to no reaction. But when we use lights, since we're kicking electrons up to side too, what that means is that it does work. So that means that this is a symmetry allowed process because world using light isn't that cool. So, guys, that's it for this video. Hopefully, guys now have a understanding of what a, you know, a photochemical cycle edition is, And in the next video, I'm gonna introduce you guys toe a summary that of the things that we learned in thermal chemical and in photochemical cycle additions so that we can put it all together. And we can predict very easily when a reaction will be favored or wanna reaction will not be favored. So let's go ahead and move to the next video
Cycloadditions Summary Chart
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So now that we know about both thermal and photochemical cycle auditions, I just wanted to make a little summary for you guys so that you could easily and quickly tell when a reaction is going to be symmetry allowed or symmetry disallowed. And if you use this, you could always know with certainty. Now, before I get into it, I do have this one disclaimer at the top, which is that this Onley works if you're assuming super a facial interactions. Okay, so there's this concept of an TRA facial and super facial that your professor is gonna mention. Your book is gonna mention examples are gonna mention, but I think it's really silly. And that's because all of the examples that you are actually able to do in your class in a introduction to organic chemistry one and two and three class whatever, Um, sometimes three, you're only going to deal with superficial interactions, because what superficial means is that basically like, let's just look at this, for example, what superficial means is that you on Lee have interactions happening on one side of each m o. So this is one molecular orbital, and then it's on. Lee reacting with this This side of the molecular orbital. This is called Super Facial because basically both of these can just overlap easily and then they can interact. Right and trow. Facial is this other thing that happens with much bigger with much bigger molecules where you would actually have, Let's say, um, one side is interacting here, but it's interacting with the top side of this one. And with the bottom side of this one, that's an tra facial. Yeah, an tra facial and an tra facial could happen. It's basically like almost the idea of CIS versus trans. CIS can happen with all molecules trans. You would need to be a really big molecule before that can happen. So imagine that you're emos. Need to be so long that they actually twist at the end so that the top part of your M O and the bottom part of your M O can link together and link toe other orbital's and share electrons with other orbital's. Basically, if you're dealing with rings that are eight member or smaller, equal ring is equal is eight members or smaller? You can Onley do super facial, and the only way you could do an TRA facial is if it's over 99 or more. So that's why in my videos I'm not even gonna talk about superficial or an tra facial because I'm already assuming that we're gonna be on Lee doing superficial because the examples that your professors could give you could really only be super facial. Okay, that being said, these rules apply to superficial interactions. If you start talking about an tra facial, then you would have to mix all this up because now you're literally twisting your emos. But as long as you're assuming that it's superficial interactions, then this summer is gonna work great on what the summary says. Now let's get to the summary. What it says is that all you have to do is count up all the pie electrons in both Pauline's both the Pauline A and Pauline be if all of the pie electrons equal a multiple four end, which would just be, um, any multiple of end. That's an integer, so that would be for 8, 12 16 etcetera. If your total number of pie electrons equals a multiple for end that can on Lee that a cycle edition can Onley occur under photochemical conditions. Okay, if all of your pie electrons equal add up to a multiple foreign plus two which should be the number six 14 etcetera. Then you could Onley do a cyclo edition in thermal conditions. So a great example of this would be the deals. Alder reaction, which I showed you guys is an example of a thermal cycle edition. Remember? What did it look like? Remember that it was a dying on one side and an AL keen on the other. There were three double bonds total Those three double bonds equal six pie electrons, right? So according to this rule six by electrons can Onley cyclo add in thermal conditions deals alter is a thermal reaction, so we know that it's favorite in the same way. If we were to do a let's say a two pi plus two pi cyclo edition. Remember that and you try to do it. Thermal two plus two equals what number four can a four n number of pie electrons do thermal cycle audition? No, you need photochemical for that. So that's why just with this little chart, you can easily tell what is going to be thermal and what's going to be photochemical? Okay, one last thing, Which is another way to think about this. Like I think the first way is the easiest one. But this is just even another way to think about it. Another way to very quickly determine if a reaction is favorite or not. You can look at the Sai orbital difference. And what this has to do with guys is it has to do with numbers of the side Orbital's that air interacting as homo. And, um Oh, Okay. So, for example, remember that homo a always has to fill loom a B or vice versa. Right? So what you would do is you would actually look at the number of that sigh orbital and take the differences between them. Let's look at an example appear of the one that we just did, which was photochemical. Remember that originally before we, um before we excited electrons using light the original homo that we were gonna use was this one down here and the original loom Oh, that we were gonna use was this one up here? So notice that what are the numbers of the Cy Orbital's that we are reacting with were reacting with Sy one for Homo A and side to for loom will be so If we were to take the difference between those let's just do that very quickly. Sigh to minus. I won. What we get is a number of one Now the number one is odd. So what does that tell us? If the number is odd, that means that the only way it's gonna react is if you use photochemical energy. So that's another way to think about it. That even from the beginning, once I saw that one was one and one was to I know this isn't gonna work unless we use light. Okay, Now, on the other hand, if you go back to our example when we did thermal cycle edition, we actually did two different combinations where homo a reacted with Lou will be and then Homo be reacted with, Um Oh, a And in both of those kids situations, we ended up with even side orbital differences. Let me show you. So for the first one, if you go back and look at it, what we actually did was we subtracted side too, from side to the number we got as a result is zero and zero is even so. Would that work according to a thermal cycle edition? Yes, because even happens with thermal. Then when we did Homo be with loom o a. The difference in the Cy Orbital's was Side three minus I one, which gave us a difference of two, which is also even, which told us once again that that reaction would be favored to go thermal. Okay, so it's just another way to even verify that instead of having to draw your emos and figure out if they're symmetrical or not, you could just do this math very quickly. And if you could just look at your side Orbital's you can already know if this is gonna be symmetry allowed or symmetry disallowed, but again and even faster, faster way than that would be. Just count up your pie electrons. And just by counting up your pie electrons and looking at the activator, you could automatically know if this symmetry allowed or symmetry disallowed. These tricks could save you time on your exam because it's just very quick shortcuts to know if something's gonna be favorite or not. Guys, thanks for watching. I really hope that this summary helped. Let's move on to the next video
- Cycloadditions Summary:
Use FMOT to predict the mechanism and products for the following cycloaddition. If no product is favored, write “symmetry-disallowed” in place of the product.
Use the cycloaddition summary rules to verify that you have come to the correct conclusion.
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Had enough of Cycloadditions? If so don't worry because we will now move on to a new kind of pericyclic reaction:Electrocyclic reactions.
Additional resources for Photochemical Cycloaddition Reactions
PRACTICE PROBLEMS AND ACTIVITIES (1)