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Organic Chemistry
Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.
9. Alkenes and Alkynes
Dehydrohalogenation
The elimination reaction is exactly what it sounds like. Use a base to take away (de-) one hydrogen and one halogen. Voila! We’ve got a double bond.
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concept
The dehydrohalogenation mechanism.
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Hey, guys. So now we're going to talk about a named reaction called D Hydrology Nation. I know the name sounds tricky, but actually it turns out that you already know all the parts of this mechanism already. So it's actually pretty easy. Let's go ahead and check it out. So as you can see, the name is pretty long. But all this really is is it's an e two mechanism because if you think about the name, it's saying De hydro, we're taking away one hydrogen and we're taking away one halogen. Well, that's exactly what happens with the typical e two mechanism. Remember that we always break those to signal bonds and make a pie bond at the end, and that's exactly what we're gonna do. So let's go ahead and check it out. So, basically, you would have a calculus. Hey, lied in this case, Do we prefer that if alcohol light to be like primary or tertiary, what do you think is better? Well, we just said this is an E two reaction, so that means that we're gonna prefer the mawr substituted alcohol. Hey, lied. That's gonna favor elimination more. Okay, so that means that Hopefully we have, like a secondary or tertiary alcohol. Hey, lied. And they're reacting in that with some kind of base. Now notice here. I'm just using the word base in general, remember, the type of base could lead to a different type of product. Okay, Because we had Zaitsev and we had Hoffman on the type of base that you use could prefer one product over another. But let's just go ahead and just draw the General E to elimination product right now, I would take my base. And where would those arrows go to? Do you remember? Remember that you'd always take off a beta hydrogen. This is actually called beta hydrogen elimination. So I take my minus grab a beta hydrogen. Now, notice that the geometry of that beta hydrogen is in a special position and it's in the antico plainer position. Remember that That's important, because if you were to make a Newman projection out of this guy, you would want to make sure that your group's heir facing opposite directions or in the anti position so that they can be in the most favorable orientation to eliminate. Okay, so I would take that. But remember that elimination always has three arrows. So take the electrons from here and make a double bond. And finally, I would kick out my ex and what I'm gonna get it. The end is just a new double bond where basically these two methyl groups here are now located here, and these two methyl groups here are now located there. Plus, I would get obviously my base with the new hydrogen on it, so that would be a conjugal acid. And I would also get the leaving group X minus. Okay, so that was really easy. But now you just I just understand that that's the name associated with this type of reaction. When everywhere, using a strong base to eliminate a alcohol. Hey, lied through an e two mechanism that's called D Hydrology Nation and have to think of all those things in terms of anti complainer. In terms of sites of Hoffman, All of that is fair game. Okay, so now I have a practice problem for you guys. I want you guys to take your time, try to draw the products based on exactly what re agents you see, and then I'll give you the answer. So anyway, go for it.
Even more simply put, this is simply the name given to an E2 mechanism with a base an alkyl halide.
Supply the mechanism and major/minor products for the following dehydrohalogenation reaction:
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example
Dehydrohalogenation mechanism and products
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so right. When we look at this problem, we noticed that we have a nuclear file, which would be in this case, terp. You talk side and we have a leaving group, which in this case, is an alcohol fluoride. So what that means is that this is a perfect situation to use a flow chart. Okay, The flow chart that I use for substitution elimination reactions because right away we don't know exactly what this is. We're gonna need to identify it first. So let's go through the flow chart. The first question is, is my nuclear file negatively charged or neutral? And in this case, because I have a Okay, there. I have a potassium that's gonna leave a spectator ion, so it is negatively charged. So that means we're gonna go down the left side of my flow chart. I'm gonna go to to to do I have a bulky base. Yes, I dio in this case, this is Turkey talk side and turkey. Talk side is one of my bulky bases. So what that means is that I'm going to say yes here, and that's going to indicate that I have a certain type of e to. I haven't e to and it's gonna be Hoffman. Why is that? Well, my flow chart tells you that. So in case you just want to use the flow chart, you could. But on top of that, you know that it's Hoffman because of the fact that we have a bulky base. And bulky bases prefer that kinetic product or the one that's easiest to form. Okay, the one that's fastest to form. So what that means is that if I have more than one option possible, I'm gonna go with less substituted option. All right, so now we have to go ahead and identify beta carbons, and we have to see how many different ones there are. So this is a beta carbon here. I'm gonna call that beta, and this is a beta carbon here. So now my next question is, do both of these beta carbons have at least one beta hydrogen on them? Yes, they dio, both of them do. So then my last question is, Do they have hydrogen is in the antico plainer position. And now it turns out that I don't need to ask that question in this case. Do you remember Why? Because I have not been given stereo chemistry of the alcohol flooring. Alcohol florid. So it says No Kyra Ality given. Okay. And since there's no Kyra Ality given, I don't really have to worry about if the hydrogen is in the anti position or not, because I don't even know what position the Florian is it. All right, So it turns out that that last question I could ignore I can say that I'm gonna get both of these products. Now it's saying that provide the major minor products and the mechanism for this reaction. So I'm gonna go ahead and draw the mechanism for what I think is gonna be the major product, and then I'll draw the other one is well, so the major product is going to go along in the less substituted direction, so it's probably gonna be this age right here. Okay, so let's go ahead and draw our arrows. It's gonna go basically TV talks. It looks like this. Okay. And I'm going to do the following I'm gonna do grab the beta hydrogen, make a double bond kick out the f, and what I'm gonna get for that blue product is this. Okay, cool. But now we also have another product. That's possible. If it would have attacked the red position, then I would have gotten a double bond that looked like this. Okay, now I just have to figure out which one's gonna be Major. Which one's gonna be minor? So I look at how substituted each double bond is. This one is Di Substituted. This one is Try substituted. How do you know that? Um, actually wow. Okay, I messed up. This one is not di Substituted. This one is keeping you on your toes. This double bond, only one chain coming off of it. So it's actually only mono substituted. Sorry about that. And then the red one is try substitute because it's got three different branches coming off of it. So one is way less astute than the other. This is gonna be my Hoffman product, and this is gonna be my sights of product. And when I'm using this base, which one do I prefer? I actually prefer the Hoffman. So this is gonna be my major. Okay? And then obviously that means that this is my minor. Does that make sense, guys? So really, we haven't changed anything from the E two mechanism. It's just that now we have a name for it When you do an E two with just a knock. Yohei lied and a base. It's called D Hydrology Nation. Alright, cool. So I hope that made sense. Let's go ahead and move on to the next topic.
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