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.