POCl3 Dehydration

So it turns out that there's another set of re agents that could also do a dehydration of alcohol. And these regents aren't gonna involve an acid at all. In fact, the reason that we would use this alternative reaction is because some molecules happen to be sensitive to acid, and in the presence of acid, they could decompose. So instead of using that for these, what we're gonna use this phosphor chloride and what fossil chloride looks like is basically p o c. L three. That's we're gonna talk. All right, now. So it turns out that p o c l three in unison with Pyra Dean, which What is that? That's basically just like a benzene ring with a nitrogen on it. Okay? And you don't have to necessarily memorize that right now. I just want you to recognize what it is. So P L C l three and Pyrah Dean are going to do an elimination reaction on alcohol and make a double bond. Okay, so that's all you need to know right now in terms of the general reaction. Now, any time that we're doing an elimination, that does bring up the whole possibility of Zaitsev Hoffman stuff like that. Okay? And it turns out that for these reactions were always gonna favor decisive product. Okay, Okay. Is it saves? Rule still applies. So what that means is the most substituted product is the one that I favor. Okay, So in terms of the general reaction, as long as you just know that those two regions with an alcohol give you a sites have double bond, that's great.

Now, I just want to show us the mechanism really quick, so you'll know what's going on. Okay, So, basically, in my first step, what I have is a nuclear filic oxygen. Why is that nuclear, Philip? Because it got too long pairs. And then I've got my fostering chloride that looks like this. Now, notice that it kind of looks like a carbon, Neil. Except I have a phosphorus there instead of a carbon. So if I were to draw any dye polls, does that molecule have any? Yeah, there's actually a lot. There's a bunch of die polls pulling away from the phosphorus. Okay, So if I had to give the phosphorus a charge, what do you think it would be? Would it be partially positive? Partially negative. What would make sense? This should have a very strong partial positive. It's gonna be very electro, Philip, because all the electrons were basically getting sucked out of the phosphorus. Okay, so what that means is my oxygen sees this and those lone pairs air nuclear filic, so it attacks the phosphorus. Okay, Now this phosphorus is attached to three c. L's and cl are really good at leaving after they leave and become a conjugal base. They're super stable. So we could just kick out any of the seals that we want. Okay. What? That's gonna lead to his A structure. Looks like this. It's gonna be oh, with an H and then attached to this thing, P O C. L two. Okay. And obviously we'd get the cl leaving group just by itself. Okay? Now, are there any formal charges that I'm missing? Yes. There's a positive charge right here. And in the second step period. Dean or not, the second step. Sure. The second step period in does a deep throat nation. Okay, it turns out that the lone pair on parodying is actually pretty basic. Okay? It's a very, very nuclear feel. Like it's looking for something to give its electrons to, so it can go ahead. And deep protein eight. Okay, So in this next step, what happens is that I basically get Oh, sorry. I forgot this metal group down there. So there we go. What I basically get is oh, p o c. L two with a metal, and I wind up getting period in. That's protein ated and seal Negative. Does that make sense so far. Cool. So now we're back here down to this bottom part where we're going to do an E two beta elimination. It turns out that the P O. Co two just made this oxygen and awesome leaving group. So ohh used to suck as a leaving group. Now it's an awesome leaving group. Okay, so that means that this is my Alfa Carbon. How many different beta carbons do I have? I'm going to do an elimination. Well, actually, this is Beta, and this is beta. Are they both exactly the same? No, they're not. Because one has a metal, one doesn't do they both have a least one hydrogen. Yes, they dio this one has one hydrogen or two, and this one is one. So which one do I pick? Which one is gonna be the one that I eliminate with? And it turns out I'm gonna eliminate with one that's gonna give me the sites of products that would be green because green is going to give you the most substituted double bond. So when this last elimination step, what I do is I use another equivalent of Paradyne. Okay? I'm gonna use another equivalent of parodying. And because it's a basic lone pair, it's gonna allow me to do this elimination step and what I'm gonna wind up getting as my organic product is a double bond like this. Plus, I would get my O p o c l two negative as a leaving group. Okay, I'm all I'm sorry, by the way, for that to draw one arrow the leaving group had I hadn't left yet. This is E too. So if I make that double bond, I need to kick out the leaving group. I'm sorry about that. Obviously I need to kick out the leaving group. I would also get the seal negative from the beginning, and then I'm also going to get to equivalents of protein ated period een I'm sorry. It's just gonna have an h there. And since as an h that nitrogen is actually positive and I'm gonna get two of those because the first one happened in the first step with the Deep Throat nation, and the second one happened in the elimination step, so I would get all of these things. But guess what? What's the part? I actually care about? Just the organic product