We can use reactions we’ve learned in the past (particularly acid-base reactions) to understand substitution. In fact, substitution is just a category of Lewis acid-base reactions!
Remembering general patterns of reactions.
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Hey, guys. So I just want to kick off this topic by describing substitution in the most general terms possible, and also by relating it to reactions that we've already learned previously in or go one. So let's get started. Alright. So previously, when we were talking about acids and bases, we talked about how electrons would always move in a very predictable fashion. They would always move from one thing to the other. Okay, on what we said was that basically, electrons would always travel from regions of high density to regions of low density. Okay, I've been saying that pretty much all semester, but it holds true again for substitution. Okay, So what that means is that nuclear files or things with negative charges are gonna be attacking electro files. All right, so there's actually a lot of different ways that this can happen. Like I said earlier, a lot of organic chemistry can be broken down just into nuclear files and electro files. But the exact way that they react together is going to be what we actually name as a reaction. Okay,
Let’s take a stroll through the reactions we’ve already learned so we can make this connection.
Nucleophiles and Electrophiles can react in Bronsted-Lowry Reactions.
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So let's talk about one of the simplest ways that they could react, which would be to react in an acid base with a Bronston Lowry reaction. Okay, so when a nuclear file an electrified react together to exchange a proton, we call that a bronze, said Lowry reaction. Okay, so this is an example that we used earlier from acids and bases you could see here. I have a negative charge, and I have a neutral substance. If I were to figure out which one is the nuclear file, what would you say that ISS? Well, the nuclear file was always the thing that was good at donating electrons. So I would say that that would be this thing right here. Okay, That species is my nuclear file, which means that invariably, the other thing needs to be my electrified, even if it's difficult for me to see what, like how it's Electra Filic. But it must be because the other thing is a better nuclear file. So let's go ahead and say, this is my electrical here, Okay? Now, in this reaction, I would have to figure out OK, now I know my nuclear found my electric follow Where does the arrow start from? Okay, remember that with mechanisms, we always start from the nuclear file. So I would know that I need to draw on Arrow starting from this negative charge. Now, the question is, where does it go, too? Okay. And to figure out which Adam is going to go to because there's no positive charge directly drawn. Okay, If there's a positive charge, then I would just go ahead and attack that, but the electrical doesn't have a positive. So I'm gonna have to use die polls to figure out what's the most positive Adam on this molecule. So I would say Okay, I've got a few different bonds, have got a carbon sulfur bond. I've also got a hydrogen sulfur bond. Which direction would those disciples go? They would both go towards the sulfur. Okay, So what that means is that eventually my sulfur would have a partial negative, and both of these would have partial positives. Okay, Now notice that I have a positive on a hydrogen. Okay? That means that that's the same thing as me saying that I have in acidic hydrogen. Why is that acidic? Because it's gonna be easily donated because the fact that it already has a partial positive charge, So it's looking for something negative that can attack it. Okay, So to finish off this arrow, since I have an acidic hydrogen, that's gonna be my electric fall right away. I'm gonna go ahead and attack the H, okay? And you guys could have predicted that's what's gonna happen because I just told you were doing a Brown said Laurie reaction. So this is kind of a recap of chapter of the acid base chapter. Okay, so I go ahead and I grabbed that H. Now what's the next question that I asked myself? You guys remember? Well, the next question is always Are we done with the mechanism, or do we need to keep drawing? Okay, So do you think that we're done with the mechanism just the way it's drawn? No, we're actually not. And the reason is because remember that that arrow that I just drew represents the sharing of two new electrons. So this is two electrons that are now going to be attached to that H how maney electrons does the H want tohave in total. It only wants to have two electrons. So now it would have four electrons if I donated this new loan pair. Okay, So what that means is that we're going to follow that predictable rule, which is that if I make a bond, I have to break upon in order to preserve the octet of the hydrogen. Okay, So obviously the only thing that I can break is this bond to the sulfur. So I would go ahead and dump the electrons onto the sulfur. And since this is Brown, said Lowry, I would draw equilibrium, arrows and what I would wind up getting is that now I have my Oh, that now has a new single bond. And that new single bond is to an H because it pulled that h off of the acid. Okay, then I also have to draw my conjugal based. Remember that? And my conjugate base would just be the thing that now it doesn't have a hydrogen anymore. So I draw my ring structure, and then I would draw on s. And then I would ask myself. Okay. How many electrons did the s have before it had eight. It had let me just draw them in. It had a lone pair here and a lone pair there, so this s would still have those blue electrons from before. But now it's gonna have one extra lone pair that came from the breaking bond. So I'm gonna go ahead and add that lone pair here, and then I would use the formal charge rules to figure out what kind of charges these should have. So the oxygen should be neutral because oxygen wants to have six electrons, and right now it does have six. But the sulfur should have a negative charge because the sulfur wants to have six as well. It's actually in the same column as oxygen, but it has seven electrons. Okay, so notice that this is a very predictable Bronx, said Larry Reaction, because what I'm doing is I'm reacting a nuclear file on electric file, and what I'm getting is an exchange of ah, hydrogen and the exchange of a lone pair. That's easy. This is what we've already done in acids and base. This is really just a recap. Is that cool? All right,
In Bronsted-Lowry reactions, a nucleophile attacks an electrophilewith an acidic hydrogen, and removes it.
Nucleophiles and Electrophiles can react in Lewis Acid-Base Reactions.
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So now what I want to show you guys is how does this relate to substitution? Well, remember that we also had the Lewis, um, the Lewis acid in based definition, that didn't have to do with protons. And what that means is that sometimes you're gonna have Electra files and nuclear files that want to react together. But there's no acidic hydrogen is that they can react with. Does that mean that you give up? No. Okay, you still react. And this is an example that I also used when we're talking about acids and bases. We said, like in this compound, which would be the lewis acid, which would be the Louis based Do you remember? Well, remember that Lewis acid. So I'm just gonna right here is actually the definition of a Lewis acid is that it's an electric file. Okay, those two words actually synonymous with each other. Lewis acid means it's a good electron, except, er okay, remember that a Lewis base is synonymous with nuclear file. It means that it's good at giving away electrics. Okay, so in this case, which will be good at giving away electrons the double bond. Okay, I've mentioned this several times during the span of this course. But I keep saying double bonds are really good sources of electric files of electrons not elected valence of electrons because they have these two free electrons in the P in the pie in the pipe on. Okay, so I know that I'm gonna start from there. Okay? On top of that on is born on a good electrical, actually. Yeah, I remember that. Boron and aluminum or two special atoms that I keep pointing out that happened toe have an incomplete octet. Or basically, they don't have eight electrons. They only have six, and they have an empty P orbital. Okay, now, if this is the first time that you're hearing me say that, that's okay, all right? That just means that maybe haven't didn't get to watch the old reviews. Okay, But from now on, this is gonna be very important facts for the rest of or go. You need to remember that aluminum and boron okay, are very good at accepting electrons because they just have this empty P orbital. That's just waiting to have some electrons in it. Okay, so I'm gonna go ahead and draw the rest of my mechanism, my electrons would go straight into the orbital. Okay, so my end products here, Okay, When it's a Lewis acid Lewis base, we actually don't use the equilibrium arrows. We use this to forward arrow, okay? And the reason is because what we're going to get is a new Covalin bond without the exchange of hydrogen. Okay, when you have an exchange of hydrogen to use an equilibrium sign because the hydrogen could go from one place to another, and then it could go back. But with Lewis acid Lewis base, there's no exchange. Okay, So as you can see from my description, I didn't read it. But that's because I wanted to show you guys when a nuclear file on Electra file react with an empty orbital. Okay, that's called a Lewis acid Lewis base. Okay, so this is what we were used to doing in the acid base chapter. When we had an empty orbital, we would just draw this and what I would get now is a square again. Okay, cycle, butane. Now you guys know how to name that, and I would get now be age three. Okay, remember that basically every arrow always has to turn into a Sigma Bond, alright. Or a single bond. So now I have that, and I just have to figure out the formal charges. Are there any formal charges here that I have to worry about? And yes, there are. Because let's look at the double bond for a second. Anytime you break a double bond, what that means is that you are going to be removing electrons for two atoms, not just one. Okay, so this top carbon would have had a hydrogen. This bottom carbon would have also had a hydrogen. Why? Because Remember that Carbon one step four bonds. So obviously according to bond line, they need one hydrogen each. Okay, after the reaction, does that change? Absolutely not. They still have one hydrogen each there and there. Okay. The Onley differences that now one of the carbons is happy. It's octet is filled because, as four bonds, the other one is not happy because it only has three. Okay, so what are the formal charges that I'm gonna have to put here? There's gonna be a positive charge here because that carbon is missing electrons. There's gonna be a negative charge here because born one step three bonds and now has four. Easy. Okay, now, we're gonna leave this right here later on in future chapters, we're going to continue. Okay, I'm just gonna put a question because we don't know what that is yet. Okay, But in the addition chapter, once we get there oops, addition. What we're gonna find is that this is the precursor toe. A very important reaction. Okay, so but we're not there yet, but I just want to show you guys that this is another example of an acid base. But there's the Louis definition. Okay, so once again, you're like, OK, Johnny, I get it. What does this have to do? A substitution. Okay, finally. Let's get to it.
In Lewis reactions, a nucleophile attacks an electrophilewithout an acidic hydrogen, but with an empty orbital, to yield a covalent bond.
Nucleophiles and Electrophiles can react in Substitution Reactions.
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right. So a substitution reaction is simply a reaction that takes place when you have a nuclear file, not purple a nuclear file and electrify like before. Okay. Same thing. The Onley difference is that it does not have an empty orbital. Okay, if it does not have an empty orbital like presents a problem to us. Okay. What that problem is, is that before when I made that bond for this Lewis acid Lewis base, did I have to break upon? No, I didn't. Because what if you have an empty orbital? That means you can accept electrons, and you don't need to break upon after because you're not violating any octet. Okay, that boron was ready to accept some electrons. It didn't need to break anything. But now for substitution reaction if you don't have an empty orbital, what that means is that if you attack on Adam, that Adam is gonna have too many bonds because there were no empty orbital's. So what that means is that in order to make upon, I'm gonna have to break upon, okay? And since you have to break upon what that means, you always have to break upon in substitution What that means is that we're going to get a new type of compound called Believing Group. And Leaving Group is going to simply be the thing that always leaves after the reaction is over. Okay, now, we have dealt with leaving groups before. The name that we used to use for the leaving group was just the college. Get base. Okay. The conjugate base. If you think about it, it's always the thing that leaves. But that's when you have an acid base reaction in these types of reactions. It's not gonna be Bronson acid Brown said Lowry acid base, because we're not gonna be exchanging hydrogen. Okay, But we still are gonna have the general principle of something needs to leave at the end. All right, so I know I've been doing a lot of talking. Let me show you a substitution reaction in action. Okay, that right? Well,
In substitution reactions, a nucleophile attacks an electrophilewithout an acidic hydrogen, or an empty orbital, so a covalent bond MUST be broken to preserve the octet of the electrophile.
The bond that is broken to allow this reaction to take place makes a leaving group.
Predict the product
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So let's go for it. Nuclear file Electra file Which one's the nuclear file? Obviously, my own negative. So let's do that. And you. Negative. So which one's my Electra file? Well, it must obviously be the other thing. But which Adam is the electrical? That's the hard part. Okay, So what do you guys think? We know that the arrow is going to start where it has to start at the O. But then where is it going to go? Is it going to go to a carbon iodine? Ah, Bond, what do you think? In order. Listen, if there was a positive charge present on this molecule, that would be easy. But there's no positive charge, so that means my Onley choice is to used to draw diaper. Okay, So what I have to do here is a have to draw die pole, and the DYP hole is gonna pull in which direction? Well, all of these carbon hydrogen bonds I don't worry about they're all the same. Okay, They all have very They're all equivalent. But I do have a carbon iodine and that die polls gonna pull towards the iodine. So what that means is that I'm gonna have a partial negative here. A partial positive there. Cool. So far. All right. So that means that where is my arrow? Gonna actually attack? It's gonna attack the positive because this is a negatively charged nuclear fall, which that's kind of the definition of a nuclear fall. So now I'm gonna go ahead and attack this carbon. Okay, So that's already the first common mistake the students make. They don't draw the dye polls, and they wind up attacking the iodine instead. That doesn't make sense because the iodine has a negative charge already. Why would a negative want to attack a negative? Right. So now we come to the part with substitution. Okay. Does that carbon have an empty orbital? No, Actually, this carbon already has four bonds where those four well has to to the ring as one to the iodine. And it also has one h that we never drew because it was implied. Okay, so now I've got a new bond being made to that carbon. How maney bonds will that carbon have after that arrow is formed. Five. Does carbon like to be what's called Penta Vaillant or five bonds? Absolutely not. that Carbon hates its life right now. Okay, So what are we going to do to fix the situation? Because it sucks. In order to make this bond, I'm gonna have to break a bond in order to break the bond. That means I have to kick out a leaving group. Okay, so in this case, I've got five bonds I can choose to break any one of them. But the one that's easiest to break is the one that already has a DI pole because the one that already has a DI pool is the one that's already pulling electrons away from it. So the bond that I break is the one to the iodine. Okay, afterwards, what I'm going to get is affords direction, arrow, not equilibrium. Why? Because this is not the exchange of a proton. It's literally just a nuclear file. Reacting with an electrified. Okay, so this is like the Lewis acid Lewis base reaction that we did on top. Okay, so now all we have to do is you have to draw our products. Well, what would they look like? I would have this ring still, so let's go ahead and draw that ring okay. I drew it a little bit different. It's still five numbered ring, though. And what we would notice is that what's attached to that ring? Well, it still has that age, so that h is still there. It doesn't have the eye anymore. So it's not all the I yet, but what it does have, is it. Now it's gonna have a single bond. And that single bond is now going to be attached to my nuclear file, which is Oh, and then in Ethel Group. So there we go. We have a completely new product. In fact, if you look at this functional group, this is an ether. Okay. Did I start off with an ether? No. I start off with an alcohol. Hey, lied. And now I got having ether, so you can see already. We haven't even gotten to the mechanisms, the full mechanisms yet. And you can already see how this would be useful. I could make a completely new, functional group out of this. All right, so I've got that thing connected. Is there anything else that I'm missing? The leaving group, something left to make this direction possible? And that was my I negative. Okay. Why is it in I negative? Because I was gonna have one extra lone pair that it didn't have before. Okay, so congrats. You guys just drew your first substitution reaction because you were able to use I know I coached you a lot through it, but I'm just let you guys know we used all the same principles that we use from acid base, and now we applied it to substitution, and it works, okay? And the reason is because all the substitution reaction is is it's a lewis acid Lewis base electrify on nuclear file. But when you don't have an empty P orbital, so that means you have to break makeup on and then break upon, and the one that you break is simply the I. Okay, by the way, how could you tell this with substitution? Because notice that everything traded places. My iodine used to be on the ring. Now it's in solution. Okay. My negative used to be on the Oh, and now it's on the I. Okay, so what I'm trying to say here is that everything switch places the ring used to have an idea. Another ring hasn't Oh, okay. the oh, you should have a negative. And now that I has the negative, so, like, everything's perfectly switching places. Okay? And that's the definition of a substitution. Substitution means you're trading things. All right, so a few more facts and then we'll be done with this video really quick. Um, in a typical acid based reaction, remember that we would use the stability of the conjugate base to determine if it was favored. Okay, What we would say is we would compare P. K s and would say which one stronger, which was weak or all that stuff. Well, it turns out that because the conjugal Bates gets a new name and these were actually call it a leaving group, we're just gonna use the same principle to figure out the reaction rate. We're going to say that the strength or the stability of the leaving group is going to tell us if this is supposed to be a fast reaction or a slow reaction. If it's going to be favorite to quit, happen quickly or not, favorites happen quickly. All right,