Now I want to talk about one of the most interesting but also annoying things that carbocations do, and that's called the carbocation rearrangement. So it turns out that carbocations are going to be able to rearrange to more stable positions if they're adjacent to the carbocation and if it has more R groups than the carbocation has at the moment. That's called a shift. All right? And there are a few different ways that this can happen. So basically what your criteria are is this. Your carbocation, let's say that it's secondary, it's only going to 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 or that it could resonate or something like that.
Carbocation Intermediate Rearrangements - Online Tutor, Practice Problems & Exam Prep
Carbocations will rearrange to an adjacent, more stable position if possible. These have different names based on which atoms are rearranging.
Understanding why carbocations shift.
Video transcript
a. 1,2-Hydride Shift occurs when there is a hydrogen located on an adjacent, more stable carbon.
Hydride Shift
Video transcript
All right. So let's talk about the most common type of shift first, and that's the 1,2-hydride shift.
The 1,2-hydride shift occurs when there is a hydrogen located on an adjacent more stable carbon.
Okay? So here's an example of an alkyl halide. Remember that I told you guys that alkyl halides have the ability to leave. So, my Cl could leave all on its own and make a carbocation that looks like this. Okay? Now that I have it, and by the way, the Cl would just become Cl-. Alright? So there's my carbocation.
Now my question is, is that carbocation the most stable that it can be right now? No, it's primary. Primary isn't the best. 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 we 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 hydrogens 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 1,2-hydride shift.
The way that we draw the arrow for this, which 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. What that means is that you never draw your arrow coming from the positive charge because the positive charge is the thing that's missing electrons. You take the electrons from the bonds of the H and you attack the carbocation with it. What that's going to do is we're just going to write here this is a 1,2-H shift. And what we're going to wind up getting is a new carbocation.
Now let's, I just want to point out some things about how this carbocation is going to move.
First of all, I'm going to circle this carbon right here. You guys see that? The green one? Sorry, the one with the carbocation before, how many hydrogens did that carbon have before the shift? It has a positive charge, so how many hydrogens did it have? Not 3. I know you're thinking 3, so you can fulfill the octet, but it's not 3 because it has the positive charge, right? So it only had 2. I'm going to draw them out. It had one here and it had one here, let's say. It had 2 hydrogens coming off and then had a positive charge, meaning that it's missing 1 hydrogen.
Now that we're moving this hydrogen over, how many hydrogens will that carbon have? Well, it's going to have the 2 original blue ones. Let me draw that in blue. Okay? But now it's also going to have this red one that I just moved over. So is that carbon going to be happy now? Yeah. It's going to have 4 bonds. It's fine.
But now I have this carbon here that used to have 4 bonds and now it only has 3 because the hydrogen moved over. That means that now the carbocation goes there. That's the way that it works. Now I've done a 1,2-shift. I have a tertiary carbocation, and that is a whole lot more stable.
Does that make sense? This is a carbocation rearrangement.
b. 1,2-Alkyl Shift occurs when only small alkyl groups are located on an adjacent, more stable carbon.
Alkyl Shift
Video transcript
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 1,2-alkyl shift. The 1,2-alkyl shift occurs when only small alkyl groups are located on adjacent stable carbons. What that means is that you only do a 1,2-alkyl shift if no hydrogens are available. Why? Because it's a lot easier to move a hydrogen over than it is to move a methyl group or an ethyl group or some alkyl group over. So what you want to do is to do the hydrogen shift first no matter what. But in the case where there are no hydrogens, then you are allowed to do an alkyl shift. Just so you guys know, the two types of shifts that are common with this are the methyl shift and sometimes you will see professors use an ethyl shift. But I've never seen anything higher than that. The reason is that the bigger these alkyl groups get, the more energy it takes to move them over. By the time you get to propyl, the activation energy to make that happen is just overwhelming. It just doesn't happen anymore.
Okay. So here we've got another one. Let's go ahead and make our carbocation first. How do we make our carbocation? Kick out the alkyl halide. So what I'm going to get is a carbocation that looks like this. Are you guys cool with that? Cool. And then plus Bromine ion.Br-
Alright, now I've got my carbocation. 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 would it be if it went to the left? Yeah, that one on the left definitely has a lot more groups than the secondary. So now I have to ask which shift do I use? Well, do I have any hydrogens attached to that carbon? No, I don't. So that means my only choice is to do an alkyl shift. Now all three of these alkyl groups are the same size, so typically I'd want to pick the smallest one. But since they are all the same size, 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 closest to it. But these are all even the same distance away; that's just the way I drew it; it happens to look closer. So now what would the arrow look like? The same exact thing, it would just come from the bond to the carbocation. What this means is that now I'm going to get what we would call a 1,2-Methyl shift.
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 carbocation there. Why? Because the carbon here used to have four bounds, but now it only has three because the methyl group left. So now I just went from a secondary carbocation to a tertiary carbocation. And that's going to be a lot more stable. Does that make any sense? So remember, a hydrogen shift is the easier one, then alkyl shifts come next. Methyl is before ethyl. Ethyl is like your last resort. We rarely see ethyl shifts, but it is possible.
c. Ring Expansion occurs when a carbocation is adjacent to a 3, 4 or 5-membered ring.
Ring Expansion
Video transcript
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 ring expansion occurs when a carbocation is next to or adjacent to a 3-, 4-, or 5-membered 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 can get something called a ring expansion. Now notice that the molecule that I'm using to show this to you is very similar to the molecule that's at the top of your page. At the top of your page, recall that we used a cyclohexane with a carbon and a chlorine. And we said that this molecule, once the carbocation forms, is going to do a hydride shift, a 1,2-hydride 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 carbocation that forms on this carbon. And now because I made my ring size just a little bit smaller, now instead of 6, it's 5. A 5-membered ring counts as a small ring. And 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 5 of a 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 3 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. Now we know that carbons aren't circles or aren't drawn as circles. But just for the sake of drawing the mechanism, I think it's easy to do that. Now notice how many hydrogens each of these has. Red has 2. Blue also has 2. It's always going to have 2 hydrogens, but it's going to have a positive charge after the chlorine leaves. And notice that green is the odd man out, it has just 1. Well, what happens with this mechanism is that, in a ring expansion, the ring is strained. It doesn't like to have those bond angles and that torsional strain. It doesn't like to be a five-member ring. It wants to be bigger. So instead of doing a methyl 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 used them to pull blue in between both of them. So that now, instead of having red and green directly attached, now it's red, blue, green.
Well now I've got a 6-membered ring and where are these carbons? Well, let's say this is still red. Notice that red has 2 hydrogens. So is red going to have a charge now? No. Red is fine because it's got 4 bonds. Now notice that red is attached to blue. Why is it attached to blue? Well, because of this new bond that was created. How many hydrogens did blue have? Still 2. So is it going to 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 4 bonds. But now, we just broke this bond, right? So that bond doesn't exist anymore. And green had how many hydrogens? Just one. So that means that now green with 1 hydrogen is going to have a 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 it to make a bigger ring. Do you still have a carbocation at the end? Yes, but now that carbocation is located on a bigger ring. So now, without drawing all these arrows, I just want to show you that what it's usually going to look like is this: Like a carbocation that looks like this would rearrange to form a carbocation that looks like this. Basically, the carbocation with 1 carbon and a 5-membered ring, see how I have 5+1, it's going to engulf and it's going to become 6. So 6 is just the sum of 5 eating up 1 and making it into the 6th carbon of the ring. All right. So guys, also remember this is only going to happen if you start off with a 5, 4- or 3-membered ring. Over here in the bottom example, the carbocation here would not expand because it doesn't make sense to engulf it and to make a 7-membered ring that's not more energetically stable, so that doesn't really happen a whole lot. All right. 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 may not want to undergo a rearrangment.
Intro
Video transcript
All right guys. So for these rearrangement problems, what I want you to be able to do is answer 2 questions. I want you to first say will this carbocation rearrange or not? Yes or no? And then secondly, if it does rearrange, what's it going to look like? All right. So basically, here are the questions. Go ahead and start on question 1 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?
Which of the following carbocations would be likely to rearrange? Draw each rearranged structure below.
Molecule I
Problem Transcript
Which of the following carbocations would be likely to rearrange? Draw each rearranged structure below.
Molecule II
Problem Transcript
Which of the following carbocations would be likely to rearrange? Draw each rearranged structure below.
Molecule III
Problem Transcript
Which of the following carbocations would be likely to rearrange? Draw each rearranged structure below.
Molecule IV
Problem Transcript
So, how'd you do?
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