Conjugated Hydrohalogenation (1,2 vs 1,4 addition)
16. Conjugated Systems
Conjugated Hydrohalogenation (1,2 vs 1,4 addition) - Video Tutorials & Practice Problems
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Conjugated hydrohalogenation, also known as hydrohalogenation of dienes, or 1,2 vs. 1,4 addition to dienes, is the same reaction as hydrohalogenation, except with a possibility of multiple products due to the presence of a conjugated intermediate.
1
concept
Conjugated Hydrohalogenation - General Mechanism
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So now we're gonna discuss in a little excite reaction called conjugated hydro hallucination. So for conjugated hydrogenation to take place, we're gonna need a double bond and a strong halo Heidrick acid like HCL or HBR. Okay, Now, we're gonna run into a problem, though, because if you guys recall, there's already a reaction that happens between a dull bond and h X. Do you guys remember what that is? It's in addition type reaction where if you guys recall, ah, double bond is nuclear Filic and H X is always highly electron deprived. So you're always going to get our very easily going to get the formation of basically a car broke, a tie in and a Markov Nick off edition. So let's just go through this mechanism really quick so you guys can remember what this mechanism is. Remember that your double bond would hit the H. Now you have two choices. We could either place the car broke down on the primary or on the secondary carbon, and we would definitely choose the secondary because that's the Markov knockoff edition, right? That's the Mark Avakov, Carbo Catalan. And from there, that Carbo Catalan could rearrange if it was unstable. In this case, we're not gonna have a rearrangement possible, but it's something you have to think about any time you make a Carvel cat ion. And then you would get your xnegative attacking toe form an alcohol. Hey, lied. All right. Our Markov, Nick. Aww. Alcohol. Hey, lied. So I'm just gonna put here Mark Alcohol. Hey, Light is our product. Okay? Now, remember that the name of this reaction is simply hydrology Nation. Okay, so if I say the word hydrology Nation, I'm talking about a double born attacking H X. So what's different about hydrology nation and conjugated hydrology? Nation? They sound so similar. And in fact, the re agents look very similar, but there's a huge difference. Let me show you. Well, remember that in the mechanism for hydrogenation, you always get rid of the double bond. You start off with a double bond, you make a couple cutting. And now that double bond is gone forever. Okay, until you do an elimination reaction later, Okay? But notice that for conjugated hydrology nation, we keep one double bond around because you're always going to start off with a dying Okay? So instead of starting off just with one boat double bond. You're always going to start off with two. What does that mean? That means one of the double bonds attacks the H X, and one of them is left over to participate in conjugation. Does that make sense? That's the big difference here that we still have in a little position after the reaction has taken place Now really quick. I just want to interject and say that your textbook might not call this conjugated hydrology nation. There's a few different names for it. It could also be called hydrology. Nation of Dying's. You might also see it called 12 versus 14 addition to dying's thes. They're all the same exact concept. Okay, In fact, if you ever see Hydrology Nation and a Lilic together again, another way to say it so notice that the biggest difference being that one of the double bonds is going to react, but one of them is going to stay left over. We're always gonna be left with one leftover. And that means that we're gonna have a conjugated intermediate. Okay, now, one thing I one kind of misconception that I want to take out of here is that you might be thinking about another reaction that's similar to this called a Lilic halogen nation. And in a little college nation, you always had a radical initiator. Okay, but keep in mind that this is not a radical reaction. This is a Carvel Catalan reaction. So we're not gonna have a radical initiator in this reaction. We're simply going to rely on the fact that you have one double bond and a car Will Catalan present? Okay, let's draw the mechanism really quick. So in this mechanism, what we would expect is that my H X is going to attack one of the double bonds. I'm sorry, one of the old ones will attack the H X, and it will form a carpool cat ion. Now we have the choice of putting the car. Will Catalan on the primary or the secondary position? Obviously, we're gonna choose secondary due to Mark Cobb niqabs rule, but also because it's a Lilic. And we know that a Lilic sites are always more stable than any other sites than their respective, you know, types that aren't a little like Okay, so now we have that Carvell Catalan and we have an h x present, and you might already be. Your wheels might be turning of what's gonna happen next. You're thinking this Xnegative is gonna hit the positive charge just, like always. Just like a normal hydrology nation. You're exactly right. But there's one extra complication, which is that now this positive charge is actually conjugated. So we have to draw a resonance structure in this mechanism. That resonant structure is going to be of one arrow swinging open like a door hinge. So we're gonna get now a double bond here and a positive charge here. Okay, so now we have two different reactive intermediates that we have to react that we could possibly react with. And in general, we're going to react with both. Okay, so we're gonna notice is that the xnegative has the option to attack that carbon, But it also has the option to attack this carbon. So you should be aware of the fact that two different products are possible. Let's go ahead and draw these. So one of the products would be in a Lilic halogen. Okay. On a Lilic alcohol. Hey lied in that position and another one would be and a little collagen in that position. Now you'll notice that this looks very similar. If you're aware of the A Lilic hallucination reaction that uses radicals, this product looks very similar to the product that you would get before and using it. In a little college nation, however, the reaction is completely different. And the mechanism is completely different because one of the reactions uses radicals, and this one uses Carvel Catalan. So it's a very different situation. Now, let me go ahead and give you an extra set of tools to be able to understand these. Um, we actually give these two different products different names based on where the halogen attacks. Okay, in both of these cases, my hydrogen always attacked one of the carbons. We're going to say that the hydrogen attacked this carbon here and this carbon here. If you're wondering why I picked that carbon, look at the original mechanism. The original mechanism has this double bond attacking the H, and the car will carry informing here, meaning that an H must have been attached to that carbon. Okay, the site where the H attacks is called with one carbon. That's your that's your number one carbon. Okay. From there, we can continue to count carbons in order to determine where the halogen adds. Okay. So as you could see this halogen being right next to the first product, this halogen being right next to the hydrogen would be considered too. So this would be what we call a one to product, okay? Or a one to alcohol. Hey, like products. Okay, because of the fact that you're hydrogen and your halogen attacked right next to each other on the same double bond. Whereas we see that it's a little bit different. After you resonate, you get a different distance, right? So this stills my one carbon, because no matter what, the h x to attack the same position. But now notice that 23 this is now attached to the fourth position. So this product is called the one four product. Okay, so it turns out that we don't always have to get uneven mixture of both the 12 on the one for product. You should be aware that they conform. But it turns out that there is a way to selectively favor one over the other. And the way we can do that is by using temperature control. So this is a This is a type of reaction that can use temperature control to prefer one product over another. And in fact, you can get a very high yield of just one of the products if you use the correct temperature. So what are the temperatures we need to know? Well, the temperatures that we need to know our that temperatures above 40. So these were gonna be hot. Temperatures are gonna favor the one four product. Okay. Temperatures below zero degrees Celsius. That's gonna be really cold. Think about it. That's below freezing. Right? Are gonna favor the one to product. Okay, Before I read off any of these other lines, I wanna help you memorize this. How can you remember that 14 correlates to 40 or higher, one to correlates toe lower. I just think of the bigger number. So the hotter the temperature, I'm going to get the 14 product mostly with lower the temperature. I'm going to get more of the 12 So even if you have no clue what's going on, you can at least remember that and use that as a memory trick on your exam. Okay, Now, there's a few other words here that I haven't defined yet. Notice that I'm calling the 14 product the thermal dynamic product. And I'm calling the one to product the kinetic product. Okay, this brings up a type of reaction called thermal dynamic versus kinetic control. And this is actually a really important concept for organic chemistry. This is not the Onley reaction in organic chemistry that we've learned that uses thermodynamic versus kinetic control. So I'm gonna sign an entire set of videos. I'm going to do an entire other set of videos just to explain that process. Okay, so right now, I'm not actually going to define thermodynamic and kinetic because that would be for another video. What I'm really trying to do here is just get you to memorize it and get you to, um basically recognize when we're gonna have different types of products. Okay, So once again, since this is my 12 product, this would be my kinetic product. If can net tick. And since this is my since this is my 14 product, this would be my thermal dynamic dynamic K. Now, notice that in my original reaction, I had no temperature present. Okay, so what happens if your professor doesn't give you a temperature? What do you think you dio do you assume that it's 14? Do you assume that it's 12? No, if there's no temperature. So let's just add a bullet point. You know, this is good learning. If there's no temperature, then assume both products. Okay, so if you're professor does not give you temperature information, it's probably somewhere between zero and 40. And you have to assume that you're just going to get a mixture of products. Okay, No thermal dynamic control, no kinetic control. So, guys, I'm gonna go ahead and let you guys try to solve these. We'll start off with the first one. Eso go ahead and take some time to try to draw the products of the first one and then I'll explain it. Okay, go for it.
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example
1,2 vs. 1,4 products
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3m
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So first, let's just try to recap What type of reaction this is. See how I'm starting off with a dying and a strong halo? Heidrick acid HCL and I have a low temperature. I have a temperature of zero degrees Celsius. So immediately I should be thinking What? Got double bonds? I've got HCL. This should be Hydrology Nation. But wait. Since I have two double bonds, it needs to be conjugated hydrology nation. Okay, awesome. The zero degrees part means that it's not just gonna be a mixture of products, right? I'm actually going to prefer the one to product as my major. Okay, so I'm gonna prefer this is gonna be a 12 edition of H X. Okay, so now here's another good question for you. How do I know which double bond is going to attack the HCL first on my prior mechanisms, I was always starting with one on the right hand side. Do you think it always has to be that one? Nope. In this case, guys, it's not really gonna matter because it's gonna make the same product, no matter what. But in the case of having an asymmetrical dying, you have to worry about lots of different products. Okay. So you could get this HCL. I mean, this Taliban attacking the HCL, but the other one could also attack. Meaning that you have two different cat ions that air possible. We have a cat. I impossible That would look like this in red, right? With a with a line there. But we also have the cat. I impossible. That looks like Oh, my gosh. I'm sorry, guys. I was a mistake. Has to be a carbo cat. Ion. Wow. I just looked like a fool. Okay, so you've got your two different car broke a dance that are possible and notice that in both of these cases, they're the same exact thing. That's the same molecule just flipped. But if this were an asymmetrical dying, then you would have to draw both. Okay, so we said that we're gonna favor a 12 product. So that means that do I resonate this carbon content or just do I just keep it where it iss keep it exactly where it iss. So I'm gonna take my cl, and I'm gonna attach it to both. And look what I'm going to get from my product. I'm gonna get a product that looks like this CEO or like this C l and notice that those were the same exact molecule. Just flipped around so I wouldn't actually draw both. I would just cross one of them out, and I would just keep one of these. And this would be my answer to have just the one to product. Okay, I know it's a little bit cramped for space, but that's what we would dio. Alright. So, again, guys, keep in mind that you it's not really able. You're not able to predict which double bond in this case would attack. You have to draw the products of both. In this case, they happen to be the same. All right, so let's move on to the next one. Try to draw the whole mechanism for that one and predict how many products you would find
3
example
1,2 vs. 1,4 products
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2m
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All right. So, due to limited space, I'm just gonna go ahead and take myself out of the camera so we can use this whole area too, right? All right. So, guys, once again we have HBR and a dying, but notice that my temperature is different. It's 50 degrees, Meaning that I'm going to favor my one for product. Okay, Now, once again, we can't really determine which double bond is going to attack in this case. So I really have to draw both. I have to draw the fact that I would have a Carvel Kata and that looks like this. And I would also have a Carvel Catalan that looks like this fair. It's a little bit low. I'm sorry it's off the screen a little bit, but I think you could tell what I was trying to write. I'll try to make it a little higher next time. Okay, So now, guys, because I'm favoring with 14 products should resonate this cat ion Or should I just keep it where it is? I actually have to resonate it. So we're gonna have to resonate both of these to go to the other position. Because remember We're favoring the 14 products, so I'm gonna resonate this guy over here, Okay? Now, even this one's even easier to tell them the last problem. You can see how that Carbon County is The exact same thing just flipped to the other side. So I'm just gonna go ahead and use one of these for the product. I'll just use the red one, and I'll say that the final product is going to be this br okay? And that's my final product because I used 50 degrees Celsius. So this is a thermal dynamic controlled product. All right. Okay, guys. So that's it for this part of the lesson. Um, Now, I'm actually gonna go into more detail on what it means for it to be kinetically controlled or thermal dynamically controlled. But thankfully, now you have the basics. You can predict the product, no matter what. Now, I'm just gonna explain a little bit more about theory.
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concept
Kinetic vs. Thermodynamic Control
Video duration:
8m
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Now I want to go into a little bit more detail on what it means for a reaction to be kinetically controlled or thermal dynamically controlled. So the reaction, called conjugated hydrogenation is a really good example of a reaction that has thes different types of control. It's not the only one, but it's a great reaction to use as an example to explain this concept. So if you guys recall with this reaction, hot reaction conditions favor the formation of the thermal dynamic product, which we call. They won four product due to the orientation of the hydrogen and the halogen that goes with it. And we learned that cold reaction conditions favored the kinetic product, which we called the one too product for the same reasons. Okay, now what I'm here to do is show you guys on an energy diagram. What these words kinetic and thermal dynamic actually relate Thio. So, first of all, I just wanna say a disclaimer here that notice that I put the word simplified energy diagram here, and that's because this energy is diagram isn't perfectly drawn. Um, it's simplified to make it as easy for me to teach you is possible But this is not the official diagram. In fact, any diagram of a reaction that has a Carvel cat iron should actually have two humps. Notice that here, I only have one hump drawn. And that's because in order for you to understand what this is about, you don't need the full reaction diagram. You just need toe see kind of the intermediate and then the product. And that's gonna help us to really determine why it's called kinetic or thermal dynamic. Okay, so first of all, let's just start at the beginning. Notice that I'm starting off. This is my energy reaction. Coordinate at the bottom. This is my X axis, and my Y axis is on in free energy or Delta G. Okay. Um, so what we notice is that in the reaction coordinate. We're starting off with a dying, okay. And the dying energy level is starting off right here. Okay. So regardless of the type of reaction, conditions were always starting at the same energy. Okay. And notice that there's two different pathways that we can take from the get go. We notice that I have these two activation energies. I have an activation energy. That's called activation energy K or kinetic and an activation energy. That is thermal dynamic. Okay. And what we notice is that the intermediates that air created in these two different reactions actually have different energies. Because I feel notice. Remember, kinetic means one too, right? Remember that thermal dynamic means 14 Well, what we see when we're looking at the different types of intermediates is that the 12 intermediate is a secondary Carvel cat ion. Whereas the 14 intermediate is a primary Carvel cat. I do. You guys remember which one is more stable with a primary or a secondary Carvel cattle and be more stable? Secondary. Okay, so what that means is that the preferred intermediate is actually gonna be the secondary or the one to products. Okay, so the one to product is going toe flow through a an intermediate that is more stable. Okay, so this intermediate is more stable, and this intermediate is less stable. Okay, so that's the first part. But notice that as we move to the products, things kind of change, because we see is that even though the secondary intermediate is more stable, look at the reaction product that it produces, we wind up getting an alcohol. Hey, lied. That looks like this. Whereas for my thermal dynamic product or my 14 I get in alcohol. Highlight, that looks like this. Now the alcohol Hey, lights have equal energy is very similar energy. Halogen is do not benefit from being primary or secondary, so that's not the important part. What we do see, though, is that double bonds have different levels of stability, depending on how Maney are groups air surrounding them. I'm not sure if you guys recall the concept of hyper conjugation, which said that carbo, Catalans and double bonds are stabilized through our groups. That's why a tetra substituted double bond is much more stable than a mono substituted double bond. Okay, this is a concept that we borrow from elimination reactions. Way back in the day, when we were just talking about elimination reactions, we learned that the most stable double bonds are the ones with the most are groups around them. We'll check out these two products. Which of these two products is the more stable one? Well, for my kinetic product, I have a mono substituted product, whereas for my 14 product or my thermal dynamic. I have a di substituted. So notice that these thes thes things are in conflict with each other because my thermal dynamic Intermediate was the least stable. But the thermal dynamic product is the most stable, and it's exactly the opposite. For my kinetic product. My kinetic intermediate was most stable, but my kinetic product was least stable. Okay, so then what does this mean in terms of the temperature control? Where does that come into play? Well, basically, temperature control allows us toe overcome the very high activation energy off the thermal dynamic product. Because guess what? Eventually all the molecules, all the products want to look like this. They want to be the thermodynamic product. That's the best one. However, if they don't have enough energy, they're not gonna be able to overcome the very steep activation energy to become a thermal dynamic 14 product. So if we want to favor the most stable product, we use lots of heat to jack up the energy of the re agents so that we can overcome this very high activation energy and make the most stable product. Whereas if we want to favor the less stable product. Then we make the reaction conditions very cold so that the ambient energy is very low so that it's on Lee gonna be ableto cross, the less the it's only gonna be able to cross the threshold of less activation energy. And it's never gonna be able to form the less stable intermediate because it's so cold that it can't overcome the very steep activation energy of the thermal dynamic product that has to do with the names kinetic and thermal dynamic. Kinetic means that it's the easiest one to make, and it's the one that is formed the the fastest okay, whereas the thermal dynamic product is going to be the one that is overall, the most stable at the end. And the thermal dynamic product is usually gonna require heat to make it possible because of the extra energy that you need to put in for the activation energy. So if we were to summarize this, what we could say is that the kinetic pathway has the more stable intermediate. Let's actually just write that in. It has the more stable intermediate right, the secondary, but it has the less stable product. The mono substituted correct, whereas the thermal dynamic pathway has the less stable intermediate primary. But the most stable product which would be di substituted. Okay, mono substituted and di substituted secondary and primary. Okay, so these things are in conflict with each other and it allows us it gives us the opportunity. Thio use temperature to control the formation of whichever product we want, which is really cool again. This isn't the only reaction we learn in or go one and two that has this idea of thermodynamic versus kinetic control. But it's one of the best examples. So anyway, guys, remember that if anything, you could always just remember that your thermal dynamic product, the hot one is the higher number 14 But now at least hopefully understand mawr why this even exists. Okay, awesome. So let's go ahead and move onto the next topic.
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