Carbocation Intermediate Rearrangements - Video Tutorials & Practice Problems
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Carbocations will rearrange to an adjacent, more stable position if possible. These have different names based on which atoms are rearranging.
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Understanding why carbocations shift.
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Now I want to talk about one of the most interesting but also annoying things that carbo Catalans do. And that's called the Carb Okada and rearrangement. So it turns out that Carbo Catalans are going to be able to rearrange tome or stable positions if they're adjacent to the carbon Catalan and if it has more are groups than the car Buchanan has at the moment. Okay, that's called a shift, all right, and there's a few different ways that this can happen. So basically what you're criteria is Is this your car? Will Katyn? Let's say that it's secondary. It's on. Lee gonna want to move if it can move one space to the right or the left and become more stable by moving that space, meaning that it could become tertiary all right, or that it could resonate or something like that. All right,
a. 1,2-Hydride Shift occurs when there is a hydrogen located on an adjacent, more stable carbon.
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Hydride Shift
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Alright, So let's talk about the most common type of shift first. And that's the one to hydride shift. The one to hydride shift occurs when there is a hydrogen located on an adjacent, more stable carbon. Okay, so here's an example of an alcohol. Hey, lied. Remember that I told you guys that alcohol highlights have the ability to leave, so my seal could leave all on its own and make a carb Acadian. That looks like this. Okay, now that I have, by the way, the CEO would just become seal negative. All right, so there's my carbo cat ion. Now, my question is, Is that carbon capture and the most stable that it could be right now? No, it's primary. Okay, Primary sucks. Is there a way that if it moved one space over, could it become more stable? And the answer is Yes, because right now it's on a primary position. This is a tertiary position. So if you could just move one carbon over that would make it a whole lot more stable. Okay, Well, how do we do that? Well, are there any hydrogen attached to that more stable position? Yes, there is. There's actually a hydrogen right there. Okay, that means that I'm allowed to do a 12 hydride shift The way that we draw the arrow for this. This is what a lot of students get confused about. You have to draw the arrow from the most negative thing to the most positive thing. Just like any mechanism we've ever drawn. So what that means is that never draw your arrow coming from the positive charge, because the positive charge is the thing that's missing electrons. You don't want to do that. You take the electrons from the bond to the H, and you attack the carb Acadian with it. What that's going to do is we're just gonna right here. This is a 12 h shift. And what we're gonna wind up getting is a new Carvel. Katyn. Okay, Now, let's I just wanna point out some things about how this carbon cat is gonna move. Okay, First of all, I'm going to circle this carbon right here. Okay? You guys see that? The green one. How Maney. Sorry. The one with the Carbon County before how maney hydrogen is Did that carbon have before the shift? It has a positive charge. So how many hydrogen did it have? Not three. I know you're thinking three, so you can fulfill the octet. But it's not three, because as the positive charge. Right. So it only had two. I'm going to draw them out. It had one here, and it had one here. Let's say Okay, I had to. Hydrogen is coming off and then had a positive charge. Meaning that it's missing one hydrogen. All right, now that we're moving this hydrogen over, Okay, How maney hydrogen is Will that carbon have? Well, it's gonna have the two original blue ones. Let me draw that in blue. Okay, but now it's also gonna have this red one that I just moved over. So is that carbon gonna be happy now? Yeah, it's gonna be see, it's gonna have four, Bond. It's fine. Okay, But now I have this carbon here that usedto have four bonds, and now it only has three because the hydrogen moved over. So that means that now the Carbo caravan goes there, and that's the way that it works. So now I've done a one to shift. I have a tertiary Carvel cat. Ion on that is a whole lot more stable. Does that make sense? That is a carbon copy and rearrangement. Okay,
b. 1,2-Alkyl Shift occurs when only small alkyl groups are located on an adjacent, more stable carbon.
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Alkyl Shift
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So now I want to go on to the next type of rearrangement. It's a little bit less common, but you're still going to see it. And that's the one to alcohol shift. Okay, The one to alcohol shift occurs when Onley small alcohol groups okay are located on adjacent stable carbons. So what that means is that Sorry that Z alcohol. Okay. This you Onley do a one to alcohol shift. If there's no hydrogen is available. Why? Because it's a lot easier to move. Ah, hydrogen over than it is to move a methyl group or a ethel group or some alcohol group over. So you want to do is you want to do the hydrogen first, no matter what, but in the case of there being no hydrogen is there, then you are allowed to do an alcohol shift. Alright, Just you guys know the two types of shift that are common with this are the metal shift and sometimes you will see professors use an ethyl shift. But I've never seen anything higher than that. And the reason is just because the bigger these are groups get more energy, it takes to move them over. So by the time you get it to pro Poll. That's just like the activation energy to make that happen. It's just overwhelming. It just doesn't happen anymore. Okay, so here we've got another one. Let's go ahead and make our carbo Catalan first. How do we make our car Will carry on Carry on. Kick out the Al Kyohei lied. So what I'm gonna get is it Carma Cat in that looks like this. Are you guys cool with that? Cool and then plus B are negative. All right, so now I've got my car. Will cat iron? Is that able to shift to a more stable location? Well, let's say it went to the right. Would that make it more stable? No, it would just still be secondary right now. It's secondary. How did it went to the left? Yeah, I mean, that one on the left definitely has a lot more groups than the secondary. So now I have to ask which shift dough I use. Well, do I have any hydrogen attached to that carbon? No, I don't. So that means my only choice is to do an alcohol shift. Now all three of these al que groups are the same size. So typically, I wanna pick the smallest one. But they're all the same size. So it doesn't matter which one I use. I'm just going to use the one closest to it. But, I mean, the one that I drew closer to it. But these are all even the same distance away. That's just the way I drew. It happens to look closer. Okay, so now what the arrow look like? Same exact thing would just come from the bond to the Carvel. Catalan. What this means is that now I'm going to get what we would call a metal shift. Okay? And what I would wind up getting is now that I have an extra carbon coming off of that one down there. And now I have a carbon kata in there. Why? Because the fact that the red carbon here used to have four bonds, but now it only has three because of metal group left snow. I just went from a tertiary. I mean, from a secondary Carvel Catalan to a tertiary Carvel Catalan. And that's gonna be a lot more stable. Is that making sense? So remember, hydrogen shift is the easier, hydride. Easy one, Then alcohol ships come. Next method was before Ethel. Ethel is like your last resort. Okay? We rarely see Ethel ships, but it is possible.
c. Ring Expansion occurs when a carbocation is adjacent to a 3, 4 or 5-membered ring.
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Ring Expansion
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and this brings us to the last type of rearrangement, which is called a ring expansion. So what is a ring expansion and why does it happen? Well, a re expansion occurs when a carbo cat ion is next to or adjacent to a 34 or five member ID ring. Basically, when it's next to a small ring, when you have a positive charge, that's immediately next to a small ring. You could get something called a ring expansion. Now notice that the molecules that I'm using Thio show this to you guys is very similar to the molecule that's at the top of your page at the top of your page. Recall that we use the cyclo hexane with a carbon and chlorine. Okay? And we said that this molecule, once the carbon Catalan forms, is gonna make he's gonna do ah hydride shift. Ah, 128 shift. But now, just by making the ring one size smaller, I'm actually going to make it do something different because notice that after this chlorine leaves, I'm going to get a carb acadian that forms on this carbon. And now, because I made my ring size just a little bit smaller now instead of six. It's 55 counts is a small ring. Remember that small rings like to do what they like to expand. They like to do a ring expansion. So now that this positive charge is next to the fiber ring, I do something completely different, which is that I basically grab that carbon and I pull it into the ring to make the ring bigger. Now, let me show you what the mechanism looks like for this. For the sake of showing you the mechanism, I'm going to draw three different carbons. I'm going to draw that. This is a red carbon. I'll just make it a red dot This is a blue carbon and this is a green carbon. Okay, now we know that carbons aren't circles, but aren't drawn a circles. But just for the sake of drawing the mechanism, I think it's easy to do that now. Notice how maney hydrogen is. Each of these has read. Has to blue on. Well, it has to. Okay, It's always gonna have to hydrogen is, But it's gonna have a positive charge after the chlorine leaves and notice that green is the odd man out. It has one. Okay, Well, what happens with this mechanism is that in a ring expansion, the ring is strained. It doesn't like tohave those bond angles in that torch inal strain. It doesn't like to be a fiber ring. It wants to be bigger. So the ring. And instead of doing a metal shift or a hydride shift, the ring is actually going to donate its electrons to that carbon to make it bigger. Okay, so imagine that the bond between red and green gets broken and those electrons are used to pull the blue one in. Let me show you. Imagine that you took these electrons and you use them toe pull blue in between both of them. Okay, so that now instead of having red and green directly attached now it's red, blue green. Let me show you what this would look like. Well, now I've got a six member bring. And where are these carbons? Well, let's say this is still red. Notice that red has to hydrogen. So is red gonna have a charge now? No, red is fine because it's got four bonds now. Notice that red is a touch too blue. Why is it attached to Blue? Well, because of this new bond that was created, right? I just grabbed the blue. So this is that new single bond here. How many hydrogen is did blue have still to? So is it gonna have a charge? No, it's neutral. But what else happened? Well, the loser in this situation is green. Because notice that Green was happy before he had four bonds. But now we just broke this bond, right? So that bond doesn't exist anymore. And green had how many hydrogen? Just one. So that means that now green with one hydrogen is gonna have what type of charge? Positive charge. And that is a ring expansion. So what happens in a ring expansion, guys, is that you take basically a smaller ring and you expand to make a bigger ring. Do you still have a carbon cat? And at the end, yes. But now that carbon caddying is located on a bigger ring, So now, without drawing all these arrows, I just want to show you or without drawing all these hydrants, I just want to show you that what it's usually gonna look like is like this. Like a Carvel cat in that looks like this would rearrange to form a carbo Karan that looks like this Basically the one carbon the carbon Catalan with a one carbon and a five member ring of C high of five plus one It's gonna engulf and it's gonna become six. So six is just the sum of five eating up one and making it into the sixth Carbon of the ring. Alright. So guys also remember that this is only gonna happen if you start off with a five, four or three or bring over here in the bottom example the car broke out. I am here would not expand because it doesn't make sense. Thio Engulf it in to make a seven member bring That's not more energetically stable. So that doesn't really happen a whole lot. Alright. So I hope that made sense. Guys, let's move on to the next page
I hope we didn't lose you with that last one! Just remember to label your carbons and you will do great.:)
NOW, we will move on to some practice questions. Let's see if we can apply what we just learned to different molecules who may or maynot want to undergo a rearrangment.
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Intro
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Alright, guys. So for these rearrangement problems, what I want you to be able to do is answer to questions. I want you to first say, Will this car broke a and rearrange or not? Yes or no? And then secondly, if it does rearrange, what's it gonna look like? All right, So basically, hear the questions, go ahead and start a question one and figure out. Will it shift and then what it would look like if it does shift?
Which of the following carbocations are likely to rearrange?
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Problem
Problem
Which of the following carbocations would be likely to rearrange? Draw each rearranged structure below.
Molecule I
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Problem
Which of the following carbocations would be likely to rearrange? Draw each rearranged structure below.
Molecule II
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Problem
Problem
Which of the following carbocations would be likely to rearrange? Draw each rearranged structure below.
Molecule III
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Problem
Which of the following carbocations would be likely to rearrange? Draw each rearranged structure below.
Molecule IV
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