Hofmann Rearrangement - Video Tutorials & Practice Problems
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General Reaction
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Hey, guys, in this video, we're gonna break down the mechanism for a reaction called the Hoffman Rearrangement. So, guys, the Hoffman rearrangement is also known as the Hoffman degradation. So if you hear that term, just consider it synonymous with Hoffman rearrangement. And it's a method to turn am it's into primary amines. Now, guys, I do wanna let you guys know that this is similar to another mechanism that you may or may not know at this point called the courteous rearrangements. I'm just gonna put here. It's similar to courteous rearrangement. Okay, so just letting you know if you are. If you have seen that my video in that reaction or if you've learned about it already, this mechanism is going to remind you a whole lot of that one. Okay, they kind of kind of similar. Now, if you don't know that reaction yet, it's fine, because I'm gonna teach you the mechanism anyway, so you don't need to know the courteous to understand this reaction, okay, but similar to courteous, there's gonna be two really similar things here, which is that this is a reaction that goes through an isocyanate. Um, intermediate. Okay, now I use the term intermediate here. Very precautious, Lee, because I don't like to say intermediate, actually. Sounds like it's got a charger, that it's like a highly energized species. I society, it's pretty stable. I'm just saying It's like an intermediary structure where we make the isocyanate first, and then we add something to it. Okay, So just, you know, make sure that we're clear on that. And then also similar to courteous. It's gonna liberate C 02 gas as a byproduct. Okay, but other than that, it's a mechanism all on its own. Okay, So, guys, here's the general reaction. We've got an amad, and you acted with two different steps. One is you have a base. Okay, so you have some kind of base that's gonna deep protein ate the nitrogen, turn it into a nuclear file, and we've got an Electra filic br to. Okay, The B R two is gonna be with the nitrogen, then attacks. Okay, Um, after your ableto add one equivalent of the bro Mean, what we're gonna see is a rearrangement take place and the D card box elation that's gonna produce our gas or co two gas and the part we really care about the mean Now notice that one thing that happens here just talking in general terms is that the our group that was originally on one side of the carbon eel eventually gets attached directly to the end. So notice that before I had them separated by a carbon, and now I have them directly attached to each other. That's because I'm able to get rid of the carbon in the middle through my dick, our box elation that you're gonna see later. Okay, so in the next video, I'm gonna go through the whole mechanism of popular arrangement. So you guys know exactly what to expect?
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Mechanism
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Alright, guys. So we're going to start off with our Amad and H H and our our group on one side. Okay, well, that's part of the amad. And in our first step, we're gonna react it with our base. So usually we use sodium hydroxide. Oh, each negative. So my first step, what I'm gonna do is I'm gonna deep protein eat the amad to make it nuclear filic. So I'm gonna take out an h, make a double bond, kick my electrons up to the O. This is now going to give me a nuclear Felix structure. Looks like this. Okay, now, in this structure interacts with my B R two. Remember, that guy's BR two is a pretty good electrify. I'll remember that we have a lot of instances of negative charges attacking BR, too. So we're going to do that. We're gonna kick our electrons back down, use this nuclear feel like double bond to attack one of the BRS and kick out one. What this is going to give us is a new structure that looks like this. It's a nitrogen with now. One hydrogen, actually. Let me flip that around a little bit. Let me put the bro mean here. That's the new bro. Mean. And the hydrogen there. Okay, so this is what we would call an n Romo a mind. Okay, Because we have ah, bro. Mean, coming straight off of the nitrogen. Okay, well, that's the thing about the end. Bromide is that it can react with my O h negative again because it has a new H Still, So we're going to do that whole step again. We're gonna take our another equivalent over O H minus. And you can predict what's gonna happen. We're gonna form the same type of structure where you get a nuclear filic molecule. So let's go ahead and bring the structure down. We've got own negative. We've got a double bond tow n to bro. Mean, and that's pretty much it. And then, you know, we're always getting water here, so I mean, you would always you have water here and you'd have water here that was created because you're ohh. Grabbed in each awesome guys. Well, this is the part that gets a little bit weird. This is the rearrangement step. This is the hard part. This is why this is a tricky reaction. So I'm gonna put here. This is the rearrangement. Okay? Because instead of doing what we did before, we're going to grab a bro. Mean we're going to get a strange rearrangement. Were eventually this Our group on this side is going to attach to the end and make our is assigning. How does that happen? Well, because these electrons are gonna make a double bond. Now, notice that this carbons in a tricky situation right here, this carbon, let me just rate that really big. Okay? Why? Because if you make a double bond now, you've got your four carbons. So you would assume that you have to make if you make up on, you, have to break a bond, and you'd have to break upon in order to preserve the offset of the carbon. Now, if it were me predicting this mechanism and I was just drawing from my past experience, I would think that the next bond that breaks is this one. Don't draw it. But that's what I would think I would think makeup on break upon, make a negative charge in the end. Great. Awesome. But that's not what happens. This is where the rearrangement part comes in. Instead of breaking that double bond, it's actually gonna be more energetically favorable to break this single bond and attach that carbon so that you don't notice that by doing that I still preserve my octet. But now I'm going to get that the nitrogen is attached to our group. Okay, So once I do that, what's gonna happen? I get my isocyanate. Okay, so Oh, by the way, we still got a problem. If you make of new bonds at the end of the end, is gonna have a formal charge. If you don't kick something out, we're gonna kick out the PR. Okay, Snow, we get our isil, sign it. Which is oxygen Double bond. Carbon. Okay, that's from the electrons coming down marking a double bond now, double bond, nitrogen. And then the nitrogen is attached to in our group. Cool, Right. And this is our is assigning. Now, guys, isocyanate is an important molecules on its own. And there's a ton of different things that isocyanate contract with a contract with the means and contract with alcohol. It can react with water to make all kinds of structures, but in this case, we're only interact with one thing, which is the base that we used for a deep throat nation. We're going to react with base. So when we react with based, where do you think a nuclear feel like attack would add to my eye society? What do you think is the most Electra, Filic, Adam? Or the most positively charged atom on the is a Sydney. Good job. You guys got this one? It's the carbon, right? So we've got that positively charged carbon because of our strong die polls pulling away from it we're gonna make up on and break upon. Okay, What that's going to give us is now a dull bon o c with an O. H on one side, right? And then on this side, we're gonna have a nitrogen within our group. Now, I'm going to skip a step if you guys don't mind. Because notice that I would have gotten a negative charge on the end, right? I would have gotten a negative, but I'm just gonna protein ate it because eventually that gets propagated. So I'm gonna say this Plus h right would give me a proton there. Awesome guys. So now we've basically got what we want. Okay? Because, look, now what I have is a nitrogen that's directly attached to in our group. And remember that what I'm trying to make is a primary mean, So I'm just thinking, Is there any way I could get rid of this car? Books like acid? As long as I could get rid of the car books like acid, I could keep the amine, and I would get my product. Okay, what reaction could use to get rid of it? De car box elation. Okay, so I can de car Boxley, and I could get rid of this thing. Okay, So what this is gonna look like is we're gonna take our O H minus. We're gonna grab an H, make a double bond break up onto the end. Okay, this is gonna make our CO two. So what you see is that we're gonna get now is our products we're gonna get and h h our This is our primary mean. Okay. Plus, we're gonna get carbon double bond. Oh, Taleban. Oh, this is your C 02 gas. Okay. Plus, you're gonna get water, which we made plenty of. Okay. Now the one I really care about is the primary means. Okay, always I care about the organic product. That's the one your professor cares about. That's the one that's important to draw as a product. Co two Gas is a byproduct. Lot of reactions make water nothing special. Okay, so guys, I hope that made sense. Also, if you are familiar with the courteous tree arrangement, hopefully you were able to pick up on some of those similarities because the more similarities and patterns you guys can recognize the more of an organic can beast, you're gonna be OK. So that's it for this video. Let's move on to the next.
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