Cope Elimination - Video Tutorials & Practice Problems
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1
concept
General Reaction
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Hey, guys, In the next few videos, we're gonna discuss a really unique type of elimination reaction called the Cope Elimination. So, guys, it turns out that a means are really easily oxidized, even by week oxidizing agents. In fact, if you just leave some mean in your bench at the laboratory and it's exposed to open air, even just the 02 gas that's in room air is enoughto oxidizer means into hydroxy Allah means and even a mean oxides. Okay. And it turns out that tertiary amine oxides are capable of doing something called a self elimination and making a elimination product specifically ah Hoffman product. Now, it might have been a long time since we did elimination, but hopefully this could remember there was a sites of product and the Hoffman product, the Hoffman product being the one that was the less substituted. Okay, so we're gonna be forming less substituted elimination products through a cope elimination. Now, there's a little bit theory you want to get into beforehand, just in case you read it in your textbook. At least I explained it to you one time. So it turns out that first of all, anything that is a tertiary mean oxide can also be referred to as an N oxide. That just makes sense. You're saying that there's an oxygen coming off of an end? Okay, that would be basically let me just circle it for you. That would be this kind of structure right there. So you've got an o directly attached to end, but we notice is that there is a really weird type of bond there. Noticed that I have an arrow pointing to the O, which is something that we don't really talk about. A whole lot in organic chemistry at the college level. Right? But that contains what's called a day tive or a dye polar covalin bond. So what does that exactly mean? Let me just define that In case you forgot. In case you've never heard this before, it's a covalin bonds. That means that there is equal sharing of electrons. It's a strong bond. It's Covalin, just like any other Covalin bond you learned about. But we call it die polar or dated because you've got to charges that air separated on that bond. Okay, you've got a positive and a negative notice that they never go away. so get your positive. You got your negative. And it's known as di polar because one of the species is the one that's giving its electrons away to the other. In this case, we know that the nitrogen has a very nuclear Felix slash basic lone pair. So it's gonna go ahead and donate that lone pair to the oxygen forming that dated bond. Now, it could very well be denoted Justus a straight line because as you guys know, Covalin bonds are always just drawn as a stick. Right? So you could just draw to the stick. That'll be fine. But in some texts it's denoted as an arrow. So would be nitrogen arrow oxygen, Which, for the purposes of this class, I just want you to recognize as a bond this is gonna be ah, Covalin Bond. Just like any other bond. Alright, awesome. So I just cleared that out of the way so we can talk about the reaction and the mechanism. So first of all, before we go into the mechanism, let's just look at the general reaction. Well, we've gotten isn't mean that our first gonna oxidized this is hydrogen peroxide. So hydrogen peroxide not the strongest oxidizing agent. But I just told you guys, even though to gas just room air is enough oxygen oxidizing I mean, so definitely hydrogen peroxide can. Okay, where you're gonna form is that day tive tertiary amine oxide or the N oxide. Now, what's special about these guys is that n oxides can self eliminates what we're gonna wind up. Getting is in heat. He is gonna wind up separating these, and you're gonna wind up getting ah Hoffman Al Keen. Okay, so you can see that. Basically, we had two options. We could have either gone in the red direction here or in the blue direction. Here we went with less substituted version. So that's Hoffman. And you end up getting a hydroxyl amine as a substitue int. I mean, not as a subsidiary as a byproduct. I'm just gonna right here. This is a byproduct. We don't really care about it. Okay, so now in the next video, what I'm gonna do is I'm just gonna break down the cope elimination mechanism, and then we'll do a practice problem.
2
concept
Mechanism
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Alright, guys. So the cope elimination is a concerted mechanism, and you guys might recall concerted means It's all one step. So that means it's gonna have a transition state or an intermediate. What do you think? Transition states. Okay, It's gonna have a transition state that all forms and collapses in one step. Remember that we also denote transition states with a little double dagger. Okay, so let's go ahead and just bring down the structure from the top so we could just see how exactly how the structure would react. So I'm going to draw like this nitrogen. Now, what I am going to do for the sake of the mechanism, guys, is I'm gonna move the methyl groups up. So I'm gonna move this metal here, this metal here and I'll move the lone pair here. Okay, You'll see why in a second. So I'm just rotating it. So when I oxidize, I'm just gonna right here. Oh, in general, that stands for any type of oxidation. What I'm going to get as a product for my first step is I'm gonna get my end. Metal metal. Oh, negative. Charge. Positive charge. Now, you guys might remember this as that die polar or dated bond I didn't draw. It is an arrow because you don't have to. That's just some older text. Draws an arrow. But I'm just gonna draw it as a line because it's the same thing. It's a Covalin bond. Okay. Um, wonderful. Okay, So now how does this work? Well, in this step, what's gonna happen? Is that that on, By the way, I put a positive. There needs to be a negative there. So I'm sorry. I don't know why I put a positive. There should be a negative on the oh. Ah, positive. On the end, like I have above. Now we're going to get is a concerted elimination mechanism. On the least substituted are groups. That means that the least substituted would be this one right here. This hydrogen is the most accessible to the O. The other one to be a little bit more difficult. So we're gonna get some arrows forming here. We're gonna get the o attacks the H. Okay, then we're going to get that the age form doesn't elimination. Reaction makes a double bond and releases the end. Okay, so we're gonna wind up getting is a transition state that looks like the following. It's gonna have now a single bond there. But we're now gonna have a dotted line. Partial bond to end. Those bonds are still there. Now I'm going to get a single bond toe. Oh, why is it a straight line? Because remember that nothing really happened with that bond. The end to the, uh oh, is the Kobe Laban that stays intact. But now I've also got a dotted line down here to H Oops. That's supposed to be an age and another dotted line to this carbon and then finally noticed that we're making a double bond. So there should be a double line here, a dotted line here showing that there's a partial double bond being created. Okay, so now what I'm gonna do, I'm trying to kind of duck out of the way here. Um, is also guys. Yeah, so perfect. Now we're gonna do is we're gonna just draw this in brackets and with our transition state. Okay, so there you have it. That's all gonna kind of form and, uh, and decompose at the same time. And what that's going to do after the transition state is complete is you're going to get your I'm sorry, your Hoffman product. So there's my double bond. Plus, we're going to get our hydroxy Levine, which is, and metal metal with an O. H. And this is what we call hydroxy. I mean and again, guys, this is a byproduct. I don't care too much about it. What I care more about is this product here, the Hoffman elimination product. Okay, so that's your mechanism. Now, what we're gonna do is we're gonna do an example that's drawn a little bit differently. And I love the fact that kind of it's drawn differently. It's to throw you off a little bit. So I want you to think about this mechanism, try to draw it out the best you can. But most importantly, try to get the right product here, and then I will go ahead and give you the answer, So go for it.
3
example
Predict the Product
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All right. So hopefully you were able to recognize that this is first of all, a cope elimination. Why? Because I've got a tertiary amine. I've got an oxidizing agent which is usually hydrogen peroxide. And then I'm adding heat to favor elimination. Now, remember, I didn't really get to mention this in the mechanism above, but why does he favor elimination? You guys remember that, guys, it had to do with entropic effects. I had to do with the fact that when you eliminate, you're creating mawr particles and you're increasing entropy. So, um, he is you guys remember from Gibbs Free Energy hip, he is going to favor, um, high entropy. So that's why we always, um, go towards an elimination reaction. The more you jack up the heat. Okay, so just a little bit of theory there, So we've got our our oxidation. We've got our heat. What does this look like? Well, the first step is gonna be to add the Oh, Okay. Now, just so you guys know, I also didn't get to mention this in the prior video that the oxidation step is a mechanism that is not well described because it turns out that there's a lot of different oxidation mechanisms that could create this mean oxide, meaning that you're not responsible for the first step. So I'm gonna just say here you don't need mechanism. In fact, if you look in your book, your book doesn't show you the mechanism for this because professors air the debate is still raging on in your chemistry department. On what exactly? Mechanism is which one is the major one that creates that oxidation step. Okay, there's a lot of different ways to oxidize those means. So now we've got our n oxide, and we've got a few. We have to figure out what's the next step? Well, it's gonna eliminate with something now. I did forget a metal here, so let's add that method. Perfect. Okay, guys. So what's gonna be the elimination step? Well, we look at the carbon that's attached to the end, so we're gonna look this carbon. I'm going to say out of those out of the carbons extending from that one, which one is the least substituted? What? We're gonna notice that we have really three different choices. We could either make a double bond here on the ring. There I'm gonna make that blue. Or we could make it down the dole bond. The ring there, um, that would be green. Or we could make it down the methyl group here, which will make black. Okay, which of these looks like the least substituted double bond to you? Well, it turns out guys that blue and green are the same thing because this is a symmetrical cycle, plantain. So really, I'm just gonna delete green because the same thing is blue. So now between blue and black, which one's better? The one that's least substituted is gonna be the substitution coming off the chains. That's black. So that means that the mechanism is actually going to take off the hydrogen on this carbon. Okay, so we can go ahead and draw that really quick. Um, it's gonna be a little weird looking, but it's gonna look at this, like, negative to the H, make a double bond kick off the end. Okay, Now you're going to get your transition state, which we don't need to draw. That's you know, that's if it was a full mechanism question. We would draw that, but all I really want is a product here. So now we know that the product is going to be this. It's gonna be a double bond sticking off of the cycle. A painting plus your n um c h 32 And that has an O. H sticking off it. That's your hydroxy I mean, why? Because at first you had, um, just you had Justin. Oh, but now you grab the age, so it's O. H. You might be wondering, Johnny, why is the end neutral? Because, remember, we gave it electrons. We broke clip onto the our group. So now the nitrogen on Lee has three groups around it, so it could be neutral. Okay, So, guys, this is your final product. The one we really care about is the substitution products we made. Ah Hoffman elimination, and we're done with this topic. Okay, Hoffman elimination product. All right, so I hope that made sense, guys, lets me once the next video
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