Alright guys. So now we're pretty much pros at the SN2 mechanism, but it turns out that not all substitution reactions proceed through the SN2. There's actually another mechanism out there that also produces substitution at the end, and that mechanism is called SN1. Alright? So, if I were to just generally paraphrase what SN1 is, I would say the following sentence. Okay? And what you're going to notice is that quite a few of the keywords that we use for SN2 are going to change. So what it is, is that a neutral nucleophile, so that's a big deal, neutral, is going to react with an inaccessible leaving group. Okay? So what am I talking about here? Well, neutral means that, remember, SN2 is negative. So this one means neutral. It's not going to have that strong nucleophilic property. Okay? Inaccessible has to do with R groups. So, we're going to get there in a second, but you can already start to think that must mean that there are a lot of R groups maybe. Okay? And that's going to produce substitution in 2 steps. So, what you can see is that in this sentence, almost every word has changed except for substitution. What that means is, at the end, they still get a very similar product, which is a substitution product, but all of the conditions have pretty much changed. Now instead of being negative, it's neutral. Instead of being accessible, it's inaccessible. And instead of happening in one step, it's going to happen in 2. So let's just go ahead and start breaking this mechanism down. Okay? The biggest thing that you're going to notice that's different from about the SN2 versus the SN1 is that my nucleophile looks totally different. Before, my nucleophile was the initiator in the reaction. My nucleophile was like the arrow. It had a very strong negative charge and it was looking for any backside to react with. Okay? If I draw that same first step here, that's a huge mistake. Do you know why? Because this is not negatively charged, so it's not very nucleophilic. It's not looking for a backside right now. In fact, it's happy. It's neutral. So the nucleophile is just chilling. Right? So to just draw a coin in the backside would be a huge mistake. That nucleophile is chilling. It doesn't need to do anything. It's neutral. So that means that the first step must be something else. And it turns out that the first step is going to be very unusual, and I know that it's going to be weird, so I'm just going to try to get it over with. It's that the X or the halogen, the leaving group, is going to take off all on its own. Alright? So now, this is weird, because ever since I've been teaching you all about mechanisms, I've always said that you make a bond first and then after you break a bond. But here what I'm doing is I'm breaking a bond without making one first. I'm just breaking it by itself. Alright? Why is that possible? How does this even make sense? Well, unfortunately, I don't have another reaction in organic chemistry that I can compare this to. We've never done this before. But if you think way back when you took Gen Chem 2, we actually do know a reaction that was similar to this. I'll try to make the explanation quick because I know you barely remember Gen chem 2 probably. But remember when we were thinking back on acids and bases, and we learned about titrations, and we talked about how pH had to do with the amount of hydrogen ions you had and the OH ions. What we learned is that the K_w of water, the dissociation constant of water, was 1×10-14. What the hell does that mean? What it means is that water is normally H₂O. Right? And that's the way it's happy. But for some random reason, one out of every 10⁹ molecules is going to decide to just split apart on its own and make H⁺ and OH^-. Okay? For a split second, it's going to do that. And guess what's going to happen? It's going to hate its life because now it's 2 ions and it's way less stable, and then it's going to come back together. Alright? This is a random process that's happening all the time. Okay? You can almost think of it like a divorce, but then they get right back together because they realize that they actually like each other. Okay? So in the same way, alkyl halides have a dissociation constant as well. So, I'm going to put here K_RX. But that dissociation constant is actually going to be a lot higher. Why? Because water is very unstable after it dissociates. But alkyl halides actually already have a really strong dipole. Now, on top of that, the bond, the carbon-halogen bond, is actually a very, very weak bond in comparison to the water. So what that means is that it's actually going to be easier for the alkyl halide to dissociate than water will be. So I don't know the exact number. I'm just going to make one up. Let's say that instead, it's 1×10-7. Okay? Now remember that this is on an exponential scale, so that doesn't mean it's twice as good. That means it's like a million times as good. All right? All I'm trying to say is that this is going to wind up giving me ions as well, but at a better rate than I would usually get for, or a better equilibrium than I would usually get for water. Okay? So what that means is I'm going to wind up getting C⁺ and I'm going to wind up getting X^-. And that's what's happening here. All I'm trying to say is that random processes are going to drive my alkyl halide to ionize. And that's always going to be the first step. So what that means is that my nucleophile is chilling. It's neutral. But now, for some reason, my alkyl halide is going to ionize by itself. And what that's going to give me is a positive charge and a negative charge. The positive charge is what we're going to call our carbocation. Okay? Now, what our carbocation is going to look like is it's going to be this carbon right there. Okay, so I'm just going to draw that carbon. But now it's only attached to three things. It's attached to an H. It's attached to an ethyl group on one side. And it's attached to a methyl group on the other. Okay? Now, this carbon wants to have 4 bonds, but it only has 3, so it needs a positive charge. Okay? Now, do you remember what the hybridization and geometry was of a carbocation? Three bonds. It should be sp2. Okay. Because it only has 3 groups or 3 bond sites. And it should be trigonal