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.