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Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

Table of contents
16. Conjugated Systems

Photochemical Electrocyclic Reactions

Photochemical Electrocyclic reactions are pericyclic reactions in which 1 pi bond is destroyed after a light-catalyzed cyclic mechanism.


MO Theory of Photochemical Electrocyclics

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Hey, everyone. In this video, we're going to discuss a type of Paris cyclic reaction called a photochemical electricity click reaction. So photochemical electricity click reactions are simply gonna be intra molecular perry cyclic reactions in which one pie bond is destroyed after a light activated cyclic mechanism. I know that's a mouthful, but you guys should already be really comfortable with all those key terms. It's intra molecular because all electricity click reactions air intra molecular. It destroys one pi bon because all electricity, quick reactions destroy one pi bon, and it's light activated because we're using photochemical energy. So here's an example. We have a molecule that is reacting with itself in the presence of light to form a new ring, and we are changing one pipe on in the process. We start off with three and we end up with two. And this mechanism would be the same exact mechanism for the thermal conversion electricity quick reaction because nothing has changed. All that's gonna happen is that you're gonna form a new Sigma bond and then go through the rest of your concerted mechanism. And basically, in the meantime, you get you make you exchange one sigma bond for one pipe on. Cool. Awesome. So, by the way, just want to throw this out there. Every single conjugated Pauline is capable of doing this. So it's not unique to specific types. Any Pauline could do this, But the stereo chemistry does depend on frontier molecular orbital theory. So we're gonna be doing We're gonna not focus too much on the general mechanism because that's the easy part. We know we're gonna form a ring. We're gonna focus more on the idea of homo on Bloom Oh, frontier orbital's so that we can figure out what the stereo chemistry of the product will be. Okay, now something that is unique to a photochemical electricity click reaction is that light is gonna be involved in exciting ground state electrons and kicking them up one energy level. So it's gonna take those electrons in their ground state, and it's gonna move them toe. Ah, higher energy states. So usually that means we're going to go from a bonding sigh to an anti bonding sigh. And that means that your homo and your loom oh, orbital's are going to change. And since the stereo chemistry oven electricity, click reaction is dependent on understanding the homo orbital. That means that we need to take light into account is going to change the identity of the Homo molecular orbital. Okay, so let's go ahead and look here at just basically a dying, which is a very simple example. And before we even start, why don't we fill in what the molecular orbital's would look like for a dying? Just remember, I'm just gonna go through this very quickly because this is not the point of this video, but very quickly. The first one doesn't change. The last one always changes and my notes keep increasing. So this would be one node and this would be two nodes and this would be three notes. Cool! Awesome. So what we know about a typical dying is that four pi electrons will fill orbital side one and side to making my homocide to usually in my limo side three right. But after I react with light, what's gonna happen is that one of these electrons is going to get kicked upto Ah, higher energy state, meaning that now my molecular orbital diagram will look like this, meaning that my homo and my or blue mo orbital's have changed. Now this is my new home. Oh, Cy three. And this is my new loom. Oh, sigh for quote Now, guys actually loom. Oh is gonna be completely irrelevant for this specific reaction because the electricity click reaction is intra molecular and Onley involves the homo off the molecule. But it's just interesting to see how light has now changed the identity of my homo orbital. So now when I go ahead and consider the stereo chemistry of this molecule, I'm gonna have to draw my orbital differently because light was included. So in the, um, in the next example, in the next video, I'm going to go through an example showing how to draw the stereo chemistry from scratch, often electricity click reaction that's using light.

Predicting Electrocyclic Products

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predict the product of the following electricity click reaction and label. The reaction is either Conroe auditory or destroyed a torrey. So, guys, where should we start? In the same exact place that we started off with last with the thermal electricity Click reaction. Let's see if my pay still works. Oh, hell, yeah, that I got lucky that time so my paste is still working. So I didn't have to draw all my orbital's from scratch. If you haven't drawn these yet, feel free to positive video until you're ready. So we have to fill in our orbital's. And I'm gonna rush through it because you guys should be pros at filling in the orbital's for a dying. At this point, Orbital's don't change these orbital's keep flip flopping and then I fill in my nodes. Cool, Awesome! And we know that we would have our size site one side too. Cy three and psy four. And we know that we would have two electrons in each of side one inside, too. Now, what's gonna happen in the presence of light is that we're gonna kick it up a notch literally, and we're gonna take one of these electrons and excited two Cy three, meaning that my new homo is going to be side three. And what this means is that now, when I draw my three dimensional representation of this molecule, I have to draw side three and not side, too. So let's go ahead and check it out. Let's try to draw a three dimensional representation so it's gonna look something like this like this. And then, like this. Cool, let's trawl are orbital's in one to three and four. Let's also draw our substitue INTs, which should be facing in on the same plane. Cool, in this case because my subsistence air in there in on the same plane. Another way to think about this is that you could draw them into the page out of the page. So that means that this one should be coming out of the page, right, and this one should be going into the page. Cool, Awesome. So now it's fill in our orbital's. And since this is Cy three, that means that this one should be shaded at the bottom. These two should be shaded at the top, and then this one should also be shaded at the bottom So when we go to make our new Sigma Bond, how would we want to rotate this guy? So what we want to do, What we need to do is we're gonna need to rotate dis Rhoda Torrey, right? So that we can get two of the same and overlapping. So that means if this one goes clockwise, this one must go counterclockwise, right? And what that's gonna do is it's gonna form a new thing that looks like this. And I didn't draw this last time, but I'll draw it here really quick. These orbital's will look like this. Now we're now the dark parts are here and here, and the light parts are in the middle. I have a double bond here. I can ignore the other orbital's because they're just sitting there. And then where are my substitute Wint's? Well, let's look at the first one. Okay, The first one is going into the page, and now it is rotating down. So that means that it should be facing down. Cool. Awesome. Now notice that the one on the top was coming out of the page. It's coming out of the page, and what I notice is that it's also rotating down right? So it's coming out of the page and it's rotating down as well. So what that means is that it should face down here. And what that means is that my final product should look like this double bond, dash and dash isn't that cool? And that would be our product. Now the question is, Are there one? Is there one product or are there two products? Should I also draw the product of the other direction? If we would have had gone, one was counterclockwise and clockwise with them both on wedges. The answer is no. There's no an anti more because this is a nice oh compound. So there's Onley one product. You should not draw the other one with wedges because the same exact molecule and your professor could technically give you points off because you drew the same molecule twice. Makes your professor think that you don't know what's going on, and you don't wanna do that. Great. And now we just have to load label this rotation. I'll label it on the equilibrium arrows here. This is what we would consider to be dis road territory because they went in different directions. Isn't that cool? Awesome guys. So now, at this point, you should feel very comfortable with drawing the products off both a thermal and a photochemical electricity. Click reaction. Let's go ahead and move on to the next video.