Cumulative Substitution/Elimination - Online Tutor, Practice Problems & Exam Prep
Time to test yourself on what we've learned thus far. You are on your own here. We will be predicting mechanisms so keep the flowchart handy. Good luck!
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Intro to Substitution/Elimination Problems
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All right, guys. So now we're literally going to put everything together and do some cumulative problems based on everything we've learned about these four mechanisms and the big daddy flow chart. Okay? So what that means is that for these questions, I'm not going to tell you what the mechanism is. You're expected to use the flow chart to figure that out. And then on top of that, you're expected to actually know what the mechanisms do based on my descriptions of those mechanisms. So if you're weak on that, you might want to brush up on it before you start. And then you have to predict all the products as well. So this is literally a huge amount of practice, but this is going to help you guys solidify this chapter so well. So let's go ahead and get started with problem number 1.
Time for some practice questions. Have a game plan ready and take it step by step. I believe in you all! Let's begin.
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Problem
Problem
Predict the mechanism for the following reactions. Provide the full mechanism and draw the final product
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So for these questions, I expect you guys to have the big daddy flowchart printed out next to you, because I'm not going to be able to move back and forth between pages because I just want to stay on this page. But I will guide you through it as it's next to you. Alright? So what mechanism did you guys get for this question? Hopefully, what you got was SN2. Okay? If you used the flowchart correctly, that should have been the right mechanism. Let me go ahead and walk you guys through it. So what's the first question that we always ask on the flow chart? Okay? I know that I have a nucleophile. I know that I have a leaving group, so I know that I should use that flowchart. And the first question is, is it negatively charged or is it neutral? What do you guys think about NaSH? Is NaSH negative or is it neutral? It should be negative, right? Because remember that sodium, Na, is going to dissociate into Na+ and SH-. So that means that I do have a negatively charged nucleophile. It's SH-, so I'm going to go down the left side of my flow chart. So let's keep going. The second question, is SH- one of my bulky bases? Yes or no? No. Remember there are only 3 bulky bases. You have to memorize those. Don't think about it too hard. Just memorize them. SH- is not one of them. So then let's go to my third question. The third question going down is what type of leaving group do I have? So my leaving group is this bromine here. What type of leaving group is that? That would be secondary. Why? Because the carbon that it's attached to has 2 R's coming off of it. Okay? So that would be secondary. What does that mean? That means we have to ask ourselves one last question because secondary doesn't tell you a mechanism right away. You have to ask yourself one last question. Is it one of my strong bases? Remember that we had a list of 5 strong bases or 4 bases plus heat that we needed to figure out. So do we have any of those conditions? So is SH- on my bases list? No. It's none of the 4 bases. Is 20 degrees Celsius on my heat list? Do you guys know how hot 20 degrees Celsius is? It's probably colder than the room that you're in right now. Okay? Room temperature is 25 degrees Celsius, so unless you like it really cold in your room, it's probably warmer than 20 degrees Celsius. That would not be hot. So we would say that it's not going to be E2. It's not a base. It's going to prefer to be a nucleophile, so this is going to be I'm just going to say here nucleophile is going to equal SN2. Does that logic make sense? So see how that's cool how you didn't actually need to understand that much, even though I know that, like I said, it sounds weird that I don't want you to understand it, but there are so many complexities that a lot of times it's just better if we just let the flowchart take over. Alright. So now that I know it's SN2, now that was just the first step. Now I have to draw the mechanism and I have to predict the products. Thankfully, SN2 is a really easy reaction. All I'm going to do is backside attack. So I take my SH- and I draw my arrow coming from the back of wherever the alkyl halide is. That's actually important. If this is a free response exam and you drew your arrow like this, your professor could actually dock points because that's not a backside attack. That's a frontside attack. You have to draw from the opposite direction. So you do this. Okay. And if we make a bond, we have to break a bond, so we're going to break the bond to the bromine. What you should have gotten for your last product or for your final product is just one product. And what would have been the stereochemistry? Everything else would have been the same. Notice that those were chiral centers as well, but that's not where the reaction took place. Those stay exactly the way they are. But now I would get inversion of configuration on, not with Br, but with SH because SH was my nucleophile. So that was we used the flowchart to determine the mechanism, then we drew the mechanism, and then we drew the product. And if you guys did that, I'm more than impressed. You guys are doing awesome. If you got stuck at one of those steps, that's okay because I still have 5 more practice problems for you. So got plenty of time to get better. Alright. So let's go ahead and do the same thing for problem number 2.
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Problem
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All right. So if you guys went through the big daddy flow chart, this is actually a really easy problem, but I'm going to go through it anyway. I'm not going to spend too much time on it, but I will go through it. The first step was what? Is my nucleophile neutral or is it negative? What do you guys think? Well, I've got CH3OH. Does that have a spectator ion on it that can dissociate? No. Remember that my spectator ions are on that first row of the periodic table. That doesn't have potassium or anything like that, so that would definitely not be a negatively charged nucleophile. This is just neutral. This is alcohol. Okay? And alcohol is a common neutral nucleophile that you use. So this is going to go down the right side. Okay? So now once I go down the right side of my flow chart, I say, okay, is this going to make a good carbocation, meaning that it's going to favor SN1 E1? Or is it going to make a really bad carbocation, meaning that it doesn't favor anything? And so here I'm just going to put neutral. So my second question was what type of alkyl halide or leaving group do I have? What do you guys think? This is actually primary. Okay? Is that going to make a good carbocation? No. This is going to make a bad carbocation. So what does that equal for mechanism? That means no reaction. So the answer here is no reaction. Okay. So that was the correct answer if you just drew NR. That's the right answer for it. So I hope that makes sense to you guys that I have a really bad nucleophile in terms of it's not strong, so it can't do backside attack. And then on top of that, I can't make carbocations either, so I'm stuck. That's why it's no reaction because I'm stuck. There's nothing I can do with it. Alright? So let's move on to the third question.
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Problem
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All right, guys. So for this question, we literally busted out all the weird letters. Okay? There are letters everywhere. So what are we going to do with this? How are we going to interpret it? Use the flowchart. So the first question is, is this negative or neutral? Okay? LDA actually, that's a really complicated question to answer with LDA because it looks like it's neutral. I mean, I just have three letters. But what I didn't teach you is what it actually stands for. LDA is, lithium diisopropylamide. So what it looks like is an n with 2 isopropyl groups and the lithium attached to it. Okay? Are you supposed to memorize that? No, it's okay. Just memorize LDA as a strong base, a strong bulky base. But now that you see it like this, does that help you guys determine if it's negative? It is because remember that Li is going to dissociate. So what I would wind up getting is a negative charge on my n, so this is negatively charged. Does that make sense? So I'm going to go down the left-hand side. Cool? So now my second question is okay. What about my, my nucleophile? Is it bulky? Yes. LDA happens to be one of the 3 bulky bases that I told you guys to memorize. Remember, it's terbutoxide, LDA and litmp. Okay? So the answer is actually yes, it is bulky. So since it is bulky, that's going to be E2. Isn't that easy? So we just got to the mechanism super fast. Now, I did all of that without even looking at my alkyl halide or my leaving group yet. And if you'll notice, I've got some a weird symbol going on there, ts. Like, what is that? Okay? Remember that that's actually a tosylate. Okay? And remember that a tosylate was a type of sulfonate ester. Okay? Now, usually you wouldn't have to draw this out, but just in case you guys were wondering what that would look like if I were to draw it out, what it would look like is like this. It actually is, carbon and then O and then S and then double bond O, double bond O, and then a benzene ring and then a methyl. That's actually what the tosylate stands for. It stands for that entire thing. Is that thing a good leaving group? Actually, yeah. It's a great leaving group because it can resonate. So even though you could usually just draw the mechanism the way it is there with the ts, let's just go ahead and use the molecule since I already drew it out. So now it's time to draw the mechanism. We know that this is E2. So with E2, what are we looking for? We're looking for beta hydrogens. Right? Any elimination is going to look for beta hydrogens. And remember that E2, the first step is that the base has to pull off a proton. Remember that E2 is that 3 arrow mechanism where it all has to happen at the same time. So this is my alpha because it's the carbon that has a leaving group on it. That means that how many beta carbons do I have? Just one. K. I only have 1 beta carbon, one carbon that's directly attached to the alpha. Does that beta carbon have at least 1 hydrogen on it? Now check this out. If it didn't have any hydrogens, I would be stuck. There'd be no elimination possible. But it actually does have 1. Since it has 1 hydrogen, that means I am going to be able to do an elimination, but I'll only get one product because I only have one carbon that can do it. So now let's actually draw this mechanism out. If you guys want to get really technical, I could actually use the LDA molecule that I drew up here. So let's do that. What it would actually look like is the negative charge from the n grabs that h, then where does the next arrow go? You guys remember? It goes between a double bond between the alpha and the beta. Remember? So there. Can I end there? Nope. I need to kick out the leaving group. So I'm going to kick out the o or the tosylate, ts in this case. So now let's go ahead and draw our product. Our product would just be a double bond. Okay? Plus it would have been the negatively charged TS or what we would just draw as OTs negative. Okay? And that would just be in solution. Also, there would be like a lithium positive. But these are the things that professors don't usually care much about because these are not the organic products. The organic product is this. That's what we would try to make in synthesis so that we can actually now react with that. That's the important thing. All right. So I hope that made sense to you guys. We've done a few different mechanisms so far. If you're still trying to keep up, that's okay. We've got a few more reactions to go. So let's go ahead and get started with the 4th reaction.
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Problem
What is the major product for reaction d ?
A
B
C
A and B
D
None of these
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Problem
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It's flowchart time. So what's the first question? And what is the first answer? Is this negatively charged or neutral? Okay? And it has a weird-looking nucleophile. Alright? I don't blame you if you have no clue what that is, but it is negatively charged because there's a k, right? So this would have a negative charge right there. And the k would dissociate, so I'm going to say that it's negative. I'm going to go down the left hand side. Okay? The second question is, is this one of my bulky bases? K? Actually, no. My 3 bulky bases are t-butoxide, LDA, lithium, and potassium. This is not any of those. So I'm going to say no and I'm going to move down to the third. Okay. The third question is what's my leaving group? Okay. So my leaving group, this time it's easy. It's just an alkyl halide. What type of alkyl halide? That would be tertiary. Okay? So there's a tertiary alkyl halide. By the way, this ring is drawn in 3D, but you don't need to draw your answer in 3D. That's just a weird way I'm trying to throw you guys off. Alright? So tertiary, do I know the mechanism already? No. I need to go to my last step, which is, okay, is this one of my good bases? Now, this is the part that a bunch of you guys probably got messed up on. Okay? And in advance, I'm sorry. This is just a tricky problem, but this is the way you learn. A lot of you guys thought that this is actually going to be OR negative, which is an oxide. Okay? That's actually the first strong base that I list because you see O and you see that this is an R. It turns out this is not OR negative. Okay? OR negative would be something like OCH3. See how it's just a single O, it's not 2 oxygens? What this actually is, is something that would be OOR negative. Okay? Because notice that there are 2 oxygens, so this is actually a different type of reagent altogether. Instead of being called an oxide, this is actually called an acetate. It's something complet
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Problem
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All right, guys. So what mechanism did you guys get? It should have been E2. Okay? Why is that? I'll tell you why. We're going to use the flowchart. So the first step was is this negative or neutral? It was negative because I had a sodium. So I had a negative charge right there. Okay? So now my second step was, is this one of my bulky bases? No. It's not. So then my third step was what kind of alkyl halide do I have or a leaving group? Look what I got again. One of those pesky sulfonate esters. All right? And that's going to be secondary. So I have a secondary leaving group. So that means that I'm going to have to make one more decision, which is the fourth one. Is my nucleophile a better base or is it a better nucleophile? So you have to think, is this molecule here on my good bases list? Yes, it is. This was actually the second base on the list. It's an alkynide. Remember that I said that alkynides are strong bases? So what that means is that this is going to be better at doing an elimination than a substitution. So since it's a good base, we're going to consider this E2. Is that okay so far? Cool. So now that this is E2, we have to remember everything about the mechanism. Where do we start with E2? Do you guys remember? We have to find beta protons, just like always. Okay? So where is my alpha? My alpha is here. So I have 2 betas. Right? I've got a beta at the top. I'll call it on the green one. I got a beta at the bottom. I'll call it on the blue one. Are they equivalent? Are they the same thing? No. The top one doesn't have a methyl, the bottom one does. So they're obviously different. Okay? Do they both have at least one hydrogen? Yeah, they do. The beta at the top, notice that the beta at the top actually has 2. It has a hydrogen in the front and a hydrogen in the back. And the blue one just has one, but that's fine. It's a hydrogen in the back. So really, we actually have 3 total hydrogens, but 2 different types of hydrogens. Are you guys following me so far? So here's the big question, how many final products did you guys draw? Did you draw 2? That would be wrong. If you drew 2, that's actually the wrong answer and you got 0 credit on this question and your professor tricked you. Okay? Why is that? Can anyone help me answer that question? I know someone's dying too. It's because of anticoplanar. Remember that I told you guys that every time you hear E2, from before when I told you about E2, you have to remember that anticoplanar is required. That means that your leaving group and your beta hydrogen have to be anti to each other. So how many of these hydrogens, out of the 3 hydrogens I have possible, how many of them are actually anti? This one's on the dash, which means that the little top. So now let's go ahead and draw our mechanism. And at this point, if you got this far, then you're just a rock star and you got the question right. It's just going to be in you grab this H, then you make a double bond between the alpha and the beta, and you kick out your leaving group, and your final answer should look like this with a double bond here. That's where the distinction is going to come, the double bond there and your methyl group there and obviously plus your leaving group. Okay? So anyway, like I told you guys earlier when we were discussing E2, this is one of the hardest types of problems that you guys can get. And that's why you just need to practice it. This is nothing but practice. It's about recognizing this over and over so that by the time you get to your test, it's second nature to look for anticoplanar. Because I know that 90% of you guys forgot to think about that and why you need to practice it. That's why this chapter is hard because you have to practice. Now, one more note I just wanted to make. Notice that in all 6 of these mechanisms, I completely ignored the solvent if there is one. If there was one. Acetone was here. DMF was here. DMSO was here. I completely ignored them. I didn't have them in my decision-making process. That's the way I want you guys to think when you're on the test. It's good to know what the solvents are for conceptual purposes, but for these questions, I don't want you thinking about them. Alright? It's just a distraction. It's not going to actually help you get the mechanism in the end. Alright? So I hope that made sense to you guys. Let me know if this video helped you or if these questions helped you. But also, let me know if there's anything I could do to explain it better. All right? So let's move on.