The SN1 mechanism is similar to SN2 in that you get a substitution product, but the path to get there is completely different. It’s important that we understand how it’s different from SN2.
Drawing the SN1 Mechanism
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Alright, guys. So now we're pretty much pros at the s and two mechanisms, but it turns out that not all substitution reactions proceed through the S. And two. There's actually another mechanism out there that also produces substitution at the end. And that mechanisms called sn one. All right. So if I were to just generally paraphrase what s and one is, I would say the following sentence, okay. And what you're going to notice is that quite a few of the key words that we use for s and two are going to change. So what it is, is that a neutral nuclear file? So it's a big deal. Neutral is going to react with an inaccessible leaving group. Okay, so what am I talking about here? Well, neutral means that remember, that s and two is negative. So this one means neutral. It's not gonna have that strong nuclear filic property. Okay, Inaccessible has to do with our groups. So we're going to get there to in a second. But you can already start to think That must mean that there's a lot of our groups, maybe. Okay. And that's gonna produce substitution in two steps. So What you can see is that in the sentence, almost every word has changed except for substitution. What that means is that the end. I still get a very similar product, which is a substitution product, but all of the conditions have pretty much changed now, Instead of being negative, it's neutral. Instead of being accessible, it's inaccessible. And instead of happening in one step, it's gonna happen in two. So let's just go ahead and start breaking this mechanism down, okay? The biggest thing that you're going to notice that's different about the S and two versus the S. And one is that my nuclear foul looks totally different. Before my nuclear foul was the initiator in the reaction, my nuclear file was like the arrow. It had a very strong negative charge, and it was looking for any backside to react with. Okay, if I draw that same first step here, that's a huge mistake. Do you know why? Because this is not negatively charged. So it's not very nuclear filic. It's not looking for a backside right now. In fact, it's happy it's neutral. So the nuclear I was just chillin. All right, so to just draw one of the backside would be a huge mistake That nuclear follows chilling. It doesn't need to do anything. It's neutral. So that means that the first step must be something else. And it turns out that the first step is gonna be very unusual. And I know that it's gonna be weird. So I'm just gonna try to get it over with. It's that the X or the halogen believing Group is gonna take off all on its own. All right, So now this is weird, because ever since I've been teaching about mechanisms, I've always said that you make upon first and then after you break a bond. But here, what I'm doing is I'm breaking a bond without making one first. I'm just breaking it by itself. All right. Why is that possible? How does this even make sense? Well, I'm unfortunately, I don't have another reaction in organic chemistry that I can compare this to. We've never done this before, but if you think way back when he took Jen came to, we actually do know reaction that was similar to this. I'll try to make the explanation quick because I know you barely remember Jen come to probably. But remember when we were thinking back on acids and bases and we learned about titrate shins and we talked about how pH had to do with the amount of hydrogen ions you had, the O H. Ions. What we learned is that the K W of water, the dissociation constant of water was one times 10 to the negative. Guys. Remember, it's been such a long time, right? Negative 14. What the hell does that mean? Okay, what it means is that water is normally H 20 right, and that's the way it's happy. But for some random reason, one out of every 100 billion molecules is going to decide to just split apart on its own and make H plus and O H minus okay for a split second. It's going to do that and guess what's gonna happen. It's gonna hate its life, cause now it's two ions and it's way less stable, and then it's gonna come back together. All right, this is a random process that's happening all the time. Okay, you could almost think of it like a divorce, but then they get right back together because they realized that they actually like each other. Okay, so in the same way, alcohol. Hey, lids have a dissociation constant as well. So I'm gonna put here K R X, but that dissociation constant is actually gonna be a lot higher. Why? Because water is very unstable after its associates. Okay, but Al Kyohei, Allied's actually already have a really strong die poll. Not on on top of that. The bond, the carbon halogen bond. It's actually a very, very weak bond in comparison to the water. So what that means is that it's actually gonna be easier for the alcohol. Hey, light to disassociate than water will be. So I don't know the exact number. I'm just gonna make one up. Let's say that instead it's 10 to the negative seventh. Okay, Now, remember that this is on a exponential scale, so that doesn't mean it's twice as good. That means it's like a million times is good. All right. I don't I'm trying to say is that this is gonna wind up giving the ions as well, but at a better rate than I would usually get for or about it equilibrium than I would usually get for water. Okay, So what that means is I'm gonna wind up getting see positive, and I'm gonna wind up getting xnegative, all right? And that's what's happening here. All I'm trying to say is that random processes are going to drive my alcohol. Hey, lied toe ayan eyes. And that's always gonna be the first step. So what that means is that my nuclear house chilling It's neutral. But now, for some reason, my alcohol hill is gonna ionized by itself. All right? And what that's gonna give me is a positive charge and a negative charge. The positive charge is what we're gonna call our carbo cat ion. Okay, now, what are Carbo cat is gonna look like is it's gonna be this carbon right there. Okay, so I'm just gonna draw that carbon. But now it's only a touch to three things. It's attached to an H. It's attached to an ethyl group on one side, and it's attached to a metal group on the other. Okay, now, this carbon wants to have four bonds, but it only has three, so it needs a positive charge. Okay, Now, do you guys remember what the hybridization and geometry waas off a carb academy. Three buns. It should be. I'm just gonna right here. It should be SP two, OK, Because it only has three groups or three bond sites, and it should be tribunal plainer. I'm gonna put TP, but that's just gonna mean plainer tribunal plainer. So notice that before I had my method on a wedge and my hydrogen on a dash, you can't draw it that way anymore. Because tribunal plainer means that it's all on the same plane. Okay, so it's actually wrong to draw it that way. Now we have to draw everything with a stick on the plane. Is that cool? So far, this is what's called our carbo Catalan Intermediate. Okay. And this brings us to the possibility or to the concept of transition state versus intermediate. Remember that the middle step of A S and two was to make, um, a transition state. Well, in this case for a Nessen one, your middle step is an intermediate. What does that mean? You actually can isolate this. This is a molecule that it's not very stable, but it actually can be isolated. And I can make some of it. I could make Carvel Catalans and it could keep them in in a test tube. And I could keep them for a little bit. All right, so now I have my car broke a and intermediate. What's the next step gonna be? Is it just gonna end there? Obviously not. Okay, The next step is gonna be that. Now I have my weak nuclear file. That was just chilling. But now I have a very strong electric file. Ah, carb Acadian is one of the strongest electrifies possible. So when my nuclear file sees that, you know what? It might not be negative, but it has electrons. So then it's going to go ahead and attack the Carvel Catalan. So in the second step, my nuclear file comes in attacks. But now I have a question for you guys. Do I have a front side in a backside that have to worry about? No, because it turns out I don't have a halogen attached anymore. Remember before I always had my halogen attached so I could only go from the back. But now all I have is this plane or structure that I could either attack from the front or from the back equally. It doesn't matter which one. So that means that I have two different ways that I could attack. I could either attack from the back or I could attack from the front. And that could lead. If I have a Cairo center that could lead Thio two different products. Okay, So what that means is that, by the way, is this Cairo? Yeah, it is. It's the same components before. So what that means is that I could get the example where my basically where my nuclear file was here, but everything is exactly the same. Okay, so what that means is that the Cairo ality doesn't change it all. That's if it let's say, attacked from the front. Okay, Because it's just basically it's attacking from the same side that the X attacks from, or the that the ex left from. So basically, the X was here my nuclear fall attacks from the front, so it attaches to the same place, right? But the other product that it could give me is the back product. So what it could also do is it could attack from the back side, which means that the nuclear file would be attached to where the Ethel group used to be. And what that means is that I would get the other Tyrell center. Okay. Is there a way to determine exactly which one I'm gonna get? No. It turns out that because this is tribunal plainer, it's actually a 50 50 shot. I have 50% chance that I'm going to collide into the front. Remember, this is just based on collisions. I also have a 50% chance that I'm gonna collide into the back. So what that means is that I know that I'm gonna have more than one Indian team are possible. If it's if it is a carol center, we're gonna hold that thought. And I'm gonna keep explaining more things about this reaction. But I just want you guys Thio, see how that is right now. Okay. Is there anything else that I should draw? Yeah, I remember that. There's gonna be the X and the X is gonna have a negative site. Okay, So notice that my product came out similar to the s and two. I'm gonna get substitution at the end. The only difference is that I have some differences with the arrows. First of all, the arrows air completely different, and my stereo chemistry seems a little bit off. So we're gonna go ahead and talk about that in a little bit, all right?
Summary: A neutral nucleophile reacts with an inaccessible leaving group to produce substitution in two-step.
Why highly substituted leaving groups favor SN1.
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Now what I want to do is talk about the inaccessible leaving group part, okay? And that has to do with our groups. And it has to do with Carbo Catalans. So basically, there's this trend about carbo cat and stability that says that the more are groups that you have directly attached to a carbo Catalan, the more stable it's gonna be. Okay. And it turns out that since you need Carvel cat ions in order for the nuclear fall to attack it, that means that the more our groups I have on my alcohol hey, lied, the better it's gonna be. So what that means is that a tertiary Al Kyohei lied or tertiary? Yeah, Tertiary. Aqil Hill is gonna be way better at this than a primary. I'm just trying to point that out, okay? Because the tertiary will be the one that's the most stable. And then this would be the side that's the most unstable. Okay, uh, it should be unstable. I'm sorry. Not unstable. Oh, my gosh. All right. Don't worry too much about this other stuff in the middle. I'm going to go through a much more robust explanation of Carvel, Catalans and a little bit, okay? But right now, I just know that the more our groups, the better. The less are groups the worst, Okay?
The slow step of this mechanism is the formation of a carbocation intermediate. These types of intermediates are unstable, so anything that we can do to stabilize them will help them form faster.
-R groups stabilize carbocations through a phenomenon called hyperconjugation. Meaning that the more substituted the carbocation, the more stable it is.
Understanding the properties of SN1.
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So now let's go over our fax. Okay? You can already see. This is gonna be interesting. I got an interesting explanation for you guys. So should the nuclear file be stronger, should it be weak, It has to be weak. The reason is because if it was strong, it would do in S. And two. It would do a backside attack. But since it's week, it's not gonna do anything. It's just gonna chill and wait around for something to happen. Okay. Should my leaving group B unsubs to tooted? Meaning no. Our groups or highly substituted meeting lots of our groups highly. Right? Because we just said that the more our groups you have, the more ours, the more stable your Carvel Cadigan is gonna be on the carbon. Catalan is part of the mechanism, so we want a more Carvel kids. All right. Cool is the transition. Is the reaction coordinate going to be a transition state or an intermediate? It's going to be an intermediate this time because I have distinct two different steps. Okay, So what that means is the thing in the middle I could actually stop there if I wanted Thio and leave it there, but then, later on, I probably don't want to keep it there. So that's why I'm going to go all the way to completion. That means that. Is it concerted? No, it's two step. Okay, Now, I do want to show you guys that we actually have two different steps in terms of, um Well, you know, I'm gonna get to that in a second. Let's just keep going to the rape part, and then I'm gonna explain that. Okay, So now the rates should it be uni molecular or by molecular? Well, it turns out that the name itself tells you the answer to this. Sn one. I'm not gonna write out the whole thing, but it stands for substitution nuclear filic, and then you need molecular. Okay, so that's what s and one means. But how does that actually make sense? So let's go ahead. Circle that. Okay? The only way toe to really know what that means is to think back at the way the mechanism works, Okay? And it turns out that in this mechanism there is a slow step and there is a fast step and the rates of your overall products is gonna be determined by the rate of your slowest step. Kind of like you're only as strong as your weakest link. I know. It's a common phrase. Same thing. You're only as fast as your slowest step. All right, so let's pretend that we owned a theoretical company. We're gonna go into business together, all right? And we're paper and son. Carvel. Catalans. Alright. Our whole entire product is just to put Carvel Catalans inside of boxes and we sell people. Carvel, Catalans. We're gonna make lots of money. Be excited about this business. Cool. Me too. So this is really the cool best thing about this business is that there's only two things that we need. The only thing we need is Carvel cat ions, which come out on this conveyor belt. Okay. And then the other thing that we need is just boxes. Okay, So basically, I'll make the carbo Catalans, you make the boxes, and then at the end, we'll sell these Carvel Catalans for a lot of money. Are you cool with that business strategy, you might want to change the name, but the business sounds pretty good. All right, so I'm gonna make trouble. Captains you're gonna make boxes. Now, let's talk about ways that we could maybe get out more product and make more money. So how about if you make the 100 more boxes than I make carbon cat? So what if I go ahead and I'm making these carbon atoms one by one, and you decide that you're going to just, like, try to increase our sales and just make, like, 100 more boxes? Is that gonna increase the amount of products that we saw at the end? And the answer is no, because this conveyor belt is going at a constant speed and these Carvel cannons air dropping into the boxes. All right, so even if I have ah 100 more boxes here and thats is wow, I'm really bad at drawing cubes. Okay? Even if I added 100 more boxes here, would that change the amount of products that I get? No, because I'm still waiting for the carbon Catalans to drop one by one. Okay, So what that means is that the slow step or the rate determining step here isn't how fast you can make a box. It's how fast the conveyor belt can drop a Carvel cat and into the box. All right. Now, let's ask the other question. Okay, let's say that I were to And by the way, let's also pretend that there's a lot more boxes than there are carved academies. Okay, Because, like I said, the boxes are easy to make. Okay, so what if I were to double the speed at which the conveyor belt drops? Carvel. Catalans? Would that increase the rate of my production or make make more money? Yes, it would, because again, we're assuming that the boxes the easy thing toe have. So we have tons of extra boxes, but the car will cat answer the slow step. So if I were to just speed up the rate by two, let's say now I have twice as many Carvel Catalans. Now I'm gonna make twice as many products because the fact that the boxes of the easy part, the hard part is making the Carvel catacombs, our guys kind of getting that. That's the whole concept of rate determining step. And in this case, the rate determining step Is the Carvel Catalans being generated. Okay, so what that means is that my nuclear file was the box was the weak nuclear follows the box. The leaving group is what makes the Carvel Catalan. Okay, So if I increase the concentration of nuclear fault basically saying let's say that I double the amount of boxes that I have or double the amount of nuclear file the weak nuclear file. Is that gonna increase my reaction? No. How if I double the amount of leaving group, yes, that's going to increase the rate because the leaving group is what turns into a Carvel Catalan. If I have twice as much leaving group, that's going to mean twice as much of a possibility that might leaving group randomly associates and makes a new car Will Catena is that kind of making sense? All right, So what that means is that the rate, um, the rates constant is gonna be just the dissociation, constant times the alcohol allied not times the nuclear fall, because the nuclear fault doesn't matter. Like I said, the nuclear file is just like the boxes I have. I have tons of extra boxes. If I increase the amount of boxes, that just means that I have boxes lying around. It doesn't mean that I actually make more product. Okay, but if I increase the amount of my alcohol Hey, lied. That's like me speeding up the conveyor belt because now I'm gonna make more Carvel Catalans than before. Cool. So that means that if we were to use this information on the top, what I would say is that the first step is my slow step. Okay? And then I would say that the second step or the attack is my fast step. The fast step is my car broke out and getting attacked. That's easy to do. The hard part is making these car will contact. Okay, I hope that my analogy didn't confuse you guys more. I'm just trying to show you guys the difference. Okay, so now what's the stereo chemistry. Is it going to be inversion retention or a Simic? The answer is re Simic. Okay? The reason why the answers, Russi Mick, is because, like I said, this is tribunal plainer the Inter Meters travel planner Soyo of 50% shot of hitting the front. Okay. And I've got a 50% shot of hitting the back. Okay. And in this case, one of these is gonna be our one of them is gonna be s. This would be our this would be s I have a reason, Nick Mixture. So I would just get even in an teamers of both. Okay. And that's also very important to remember about s and one and the nickname for this. It's gonna be a weird nickname, but it's called solve Wallace. Okay. The reason is because our neutral nuclear files are usually just gonna be solvents. The most common neutral nuclear files. Just, you know, are gonna be water, alcohol and a means or ammonia. Okay, thes. They're gonna be our most common re agents that we use as nuclear files. As you can tell, these air not very strong nuclear files. But these air all typically solvents that wind up breaking Savalas. This literally means that the solvent is breaking apart. And that's what winds up happening after reaction. Okay,
You are a manager at Pepe and Son Carbocations Inc, and your boss has asked you to increase production of your product (carbocations in a box- these are awesome).
You have plenty of boxes, but it takes time for the conveyer belt to crank out these custom carbocations.
Will increasing the number of boxes increase the amount of product?
Will increasing the speed of the conveyer belt increase the amount of product?
Properties of SN1 reactions:
Nucleophile = Weak
Leaving Group = Highly Substituted
Reaction coordinate = Intermediate
Reaction = Two-Step
Rate = Unimolecular
Rate = k[RX]
Stereochemistry = Racemic
Nickname = Solvolysis
Predict the product of the following reaction:
The product must be a racemate if the original leaving group is located on a chiral center.
NOTE:Substitution reactions with neutral nucleophiles require an additional deprotonation step.