SN1 SN2 E1 E2 Chart (Big Daddy Flowchart) - Online Tutor, Practice Problems & Exam Prep

Understanding nucleophilic substitution and elimination mechanisms is crucial in organic chemistry. The "Big Daddy Flowchart" aids in determining whether to use SN1, SN2, E1, or E2 mechanisms based on nucleophile strength and steric factors. Key questions include assessing nucleophile charge, bulkiness, and leaving group quality. Strong, bulky bases favor E2, while weak nucleophiles with good leaving groups lead to SN1 or E1. Memorizing strong bases and recognizing good leaving groups enhances problem-solving efficiency in organic reactions.

Overview of the flowchart.

So guys, we have a big problem. I've taught you all about SN1, SN2, E1, E2, and I've taught you about all the different conditions that are favored for each one. And in some cases, it's kind of obvious what mechanism we would use. But there are a lot of cases where it's going to be kind of blurry and you're going to be wondering, is it going to be SN2? Is it going to be E2? And I remember having this problem as a private tutor back many years ago when I just used to do private tutoring and trying to put myself and my thinking into my student's head and trying to tell them, guys, it's just this. Just obviously the nucleophile or whatever. And what I realized is that it wasn't getting through. I needed some method or some way to just give my brain to someone else, so that they would be able to see what kind of mechanism we're using. Because many times, sometimes we're going to have to ask ourselves up to 4 different questions to determine what mechanism to use. So that is when I decided to make a solution. And lo and behold, I'm going to introduce you guys to one of the best parts of this whole chapter, which is this awesome Johnny-patented flowchart called the Big Daddy flowchart, and it's just going to change your life. So are you guys excited? Ready to get going? Like I said, this is such a great flowchart. I've even had students that have already taken their MCAT in med school tell me, Johnny, like, the Big Daddy flowchart still saves my life. So I'm like, wow, it must be pretty good. So now I've just hyped it up a crazy amount. Hopefully, you guys like it. Let's go ahead and get started. So as you guys can see, it's very complicated. It's very big, but it's actually pretty easy to use. The way that we are going to use this is like a series of questions that you ask yourself. Okay? So we're just going to ask ourselves, self, is this whatever? And then you say yes or no. And then you keep going down the flow chart until you get to the mechanism that you need. Alright? So what's the most important question? It's actually the same question that we were asking when I was teaching you about the mechanisms at the beginning, which is what kind of nucleophile do I have? Is it strong or is it weak? The way we determine that is by looking at whether it's negatively charged or neutral. So actually, it's the same question that we always start off with. So if it's negatively charged, what that means is that we're going to go down the left part of the pathway. Okay? If it's neutral, then we're going to go down the right part of the pathway. How about if it's positive? That's a trick question. Positive charges are not nucleophiles. Those are electrophiles, right? So it could never be positive. But it could either be negative or it could be neutral. Alright?

How to predict SN2 and E2 mechanisms.

So let's go ahead and go down the negative part first because that's actually the more complicated one. Let's just get it over with. Alright? So the second question that I ask myself, once I've determined that it's negative. Oh, by the way, I'm sorry. There's something that I forgot to tell you guys. There's going to be some negative nucleophiles that don't look negative at the beginning. Okay? And that's because they're attached to a spectator ion. Do you guys remember what spectator ions are? They're just ions that dissociate in solution and don't participate in the reaction. So, spectator ions, there's actually 4 of them. They're in the 1st column of the periodic table. And there are 4 that you should be aware of. It's, we have lithium. Okay? Lithium dissociates into Li⁺. We have sodium. We have potassium. And then finally, we have cesium, which doesn't always show up, but sometimes it does. Okay? These are the 4 cations that are going to dissociate from our nucleophiles and make them negatively charged. So, for example, if I gave you the nucleophile NaOH, guess what's going to happen? A lot of students are going to say that's neutral. But obviously, that's not neutral. Right? You have to dissociate the Na first, and what you're going to get afterward is OH^-. That is negatively charged, so you'd go down the left-hand side. Do you guys see how to use that? So, watch out for these spectators. They're there to make your life just a little bit more complicated.

Now, let's go to the second question. The second question I asked myself is okay, I know that I have a nucleophile that's negatively charged. I know it's strong, but is it a bulky base? Okay? And for this question, we're just going to memorize 3 bulky bases. Alright? These are just 3 bulky bases that I've seen professors use. There really isn't a very long list of them. Okay? So really, if you just memorize this list of 3, you're set. And all it is is tert-butoxide, which is this one, LDA, and LitMP. These are the 3 bulky bases that you could find. And what you might even notice is that your professor might not use all 3. K? A lot of times, professors will have like a pet base that they really like and they'll just stick with it the whole semester. So, for example, some professors love LDA. There's LDA everywhere. Some professors love tert-butoxide. Tert-butoxide everywhere. It just depends on which professor you get. So, I would just say just know them just in case. Also, in case you ever want to look up this stuff online or do some more reading in your book, you want to know what the other ones are, even though your professor might not use it very often. Cool. So those are the 3 that we say they're bulky. And if they're bulky, what did we say about nucleophiles? If I have a very bulky nucleophile, is that going to be a good base or a bad base? That's going to be a really good base. Remember that I said that bulk increases basicity. So that means that automatically right away, we know what the mechanism is. We know that it's going to be E2. Isn't that easy? We're just going to say, oh, this is E2 right away. Now, notice that it has a word Hoffman next to it. Don't worry about that yet. We haven't gotten there yet. This flowchart not only works for this topic, but it works later on as well. So, we're going to get there in a little bit. So, that would be if we said yes, that it is bulky. Okay? Now, but what if it's no? What if, like, for example, O^-? Is that one of the 3 bulky bases? No, it's not. So that means I keep going to now the next question. The next question is question 3. So you can see we've already asked ourselves 2 questions. We're on to the third one. The third one is what type of leaving group do I have? Okay. So remember that leaving group could be a lot of a few different things. Usually, that's going to be an alkyl halide, but that could also be a sulfonate ester. Right? Okay. And then we also said water, but yeah, water too. Sure. So it could also be water. But water doesn't happen quite as much. So I'm just going to put here. Okay? So those are like our 3 main leaving groups. So now the way that we have to think about these leaving groups is we want to separate them into 2 categories. There are the leaving groups that have a good backside. That means they're really accessible. It's easy to do a backside attack. And then we have the nucleophiles that have a bad backside. So if you had to think about the types of nucleophiles that have a really good backside, what would you think? What would you say? Very available. Very down for backside attack. That would be methyl and primary. Right? Because they're, like, have no steric bulk back there. So it turns out that methyl and primary are always going to pretty much give us the same mechanism because they have a good, I'm just going to write it here, good backside. Okay? So we get an SN2 reaction. Everything secondary and the tertiaries, which are right here and here, are the ones with the secondary and the tertiaries, which are right here and here, are the ones with bad backsides. Okay? They aren't as good. In fact, tertiary is impossible. Secondary can happen, but it's kind of bad in some cases. So for secondary and tertiary, we're going to have to ask ourselves another question. This brings us to the 4th question. Let's start off with secondary first. Okay? So now, for secondary, what I want to ask myself is okay, this nucleophile that it's negatively charged, it's not bulky, what I want to know is is it going to be a better nucleophile? Is it going to be better at donating electrons? Or is it going to be a better base? Meaning that it's better at pulling off protons. Okay? For this part, all I want you to do is memorize the good bases. Why? Because it turns out that there are probably 20 different nucleophiles that your professor could use. Lots of different ones. He could basically put anything with a negative charge on it and say that's a nucleophile. Okay? And for you as a student, that could get very confusing trying to memorize every single nucleophile and what it does. Okay? So, instead of memorizing every single nucleophile, let's just memorize the ones that are good bases because that's a much shorter list. And then what that means is that anything that's not on my base list, I'm going to automatically assume is better at being a nucleophile. So what are these bases that are strong bases? The bases are 1, oxides. That means any molecule that I have OR^-. Okay? So that's the first one. The second one is called an alkynide. An alkynide is just a triple bond with a negative charge at one side. Okay? That negative charge has to be directly on the C. That's also a very, very strong base. It's not very stable. Okay? Then we have 2 bases that are very similar, which is NH₂^- and H^-. These are both going to be small, very strong bases because they're not very stable in solution at all. And then finally, we have one more thing that's not really a base, but it favors basic reactions, and that's heat. It turns out that heat is going to favor elimination for a variety of reasons. So these five things are the things that I want you guys to memorize as favoring an E2 mechanism on a secondary alkyl halide. Okay? If you have one of those 5 things or even more than one of those 5 things, then for sure, it's going to be E2. Now, what's this word next to it? Zaitsev? Again, don't worry about that. We're not going to get to that until the next topic or until a few topics from now. But for right now, you should just know that it's E2.

Alright? So now what if I gave you a nucleophile that you really don't know what it is? For example, if I gave you something that looks like this, N₂^-. Alright? So what if I gave you a nucleophile that looks like that? K. Oh, I'm sorry. This is supposed to be a negative 2. So then, just so you know, this is actually called N₃^-. If you added up all the formal charges, it would be negative at the end. And I gave you N₃^- on a secondary alkyl halide. Okay? So my question to you is what would the nucleophile be? I mean, what would the mechanism be? And I would just ask myself, okay, is N₃^- on my base list? Is it an oxide? No. Is it an alkynide? No. No. No. No. There's no heat. So that means it must be in my nucleophile category, that it's not a good base, and that's going to be SN2. And that's going to apply for a lot of different nucleophiles. So also, like, for example, SH^-. Okay? SH^-, not on this list. Right? So that means it must be a better nucleophile, and it's going to do SN2. Do you guys get the point? Okay. So basically, I'm just going to go with whatever those bases are, that's E2. If it's not on that list, it's going to prefer SN2. Alright? Are you guys cool with that? Awesome. So now let's go to tertiary.

So for tertiary, we get a similar problem where we need to figure out if it's a nucleophile or a base. So now for the base list, it's actually going to be the same as the other list, so the same 5 compounds, except that now I'm going to add O^- to the mix. Okay? Okay? So, O^- was actually not on my list before because my list before has strong bases. It only had oxides. O^- is not an oxide because it doesn't have an R group attached to the O. That's a hydroxyl. It's hydroxide. It's not an oxide. So, but now I'm going to treat the hydroxide as one of the strong bases, so it's actually going to be those 5 things I told you plus O^- are going to favor E2.

How to predict SN1 and E1 mechanisms.

Then what I want to do finally is I'm not going to do this last pathway. I'm going to save that one for the end. We're done with the negative one for now. Now. Okay? Now I want to go to the neutral pathway and then finish up with this last little stick. Okay? So let's go to the neutral pathway now. I know that was a mouthful, but now we have to do the neutral pathway. What if we have something like instead of O negative, how if we just have water? Okay. Water is neutral, right? So now, my second question is actually this pathway is a lot easier. All I'm going to ask myself is okay, what type of leaving group do I have? Because if this is a neutral, then that's going to prefer what kind of mechanisms. That's going to prefer that it's going to be mechanisms that aren't bimolecular, that don't have the nucleophile attacking at the beginning. So this is going to favor SN1 and E1 mechanisms. Right? Because it's going to be waiting around for a carbocation to be generated. Remember that first step? So then I just have to ask myself two things. I just have to ask one thing actually for the second question. I'm going to say, what type of leaving group do I have? Do I have a leaving group that can make a good carbocation in the first step? Or do I have a leaving group that wouldn't make a great carbocation in the first step? Okay? So it turns out that if your leaving group doesn't make a good carbocation in the first step, that would be what type of alkyl halide? Well, remember that primaries and methyls are really bad at making carbocations. So I'm going to say here bad carbocation. Okay? The two mechanisms that are good at making carbocations because there's a lot of R groups, so it's going to stabilize it, remember that I said R groups stabilize carbocations, would be secondary and tertiary. So these would be good carbocations. Okay? And the mechanisms are really just going to determine on which side you land. If you're

Here’s what students ask on this topic:

What is the difference between SN1 and SN2 mechanisms?

SN1 (unimolecular nucleophilic substitution) involves a two-step mechanism where the leaving group departs first, forming a carbocation intermediate, followed by nucleophilic attack. It is favored by tertiary carbons and weak nucleophiles. SN2 (bimolecular nucleophilic substitution) is a one-step mechanism where the nucleophile attacks the substrate simultaneously as the leaving group departs, leading to an inversion of configuration. It is favored by primary carbons and strong nucleophiles. The key difference lies in the reaction steps and the type of carbons and nucleophiles involved.

How do you determine if a nucleophile is strong or weak?

A nucleophile's strength is determined by its charge and the presence of electron-donating groups. Strong nucleophiles are typically negatively charged (e.g., OH^-, CN^-) and have high electron density, making them more reactive. Weak nucleophiles are usually neutral molecules (e.g., H₂O, NH₃) with lower electron density. Additionally, the solvent and steric hindrance can affect nucleophilicity; polar aprotic solvents enhance nucleophilicity, while bulky groups can hinder it.

What factors favor E2 over E1 mechanisms?

E2 (bimolecular elimination) is favored by strong, bulky bases (e.g., t-BuO^-, LDA) and occurs in a single step where the base removes a proton as the leaving group departs. It is favored by high temperatures and occurs with primary, secondary, and tertiary substrates. E1 (unimolecular elimination) involves a two-step mechanism with a carbocation intermediate and is favored by weak bases and good leaving groups. E1 is more common with tertiary substrates and lower temperatures.

How does the Big Daddy Flowchart help in determining reaction mechanisms?

The Big Daddy Flowchart simplifies the process of determining whether a reaction will follow SN1, SN2, E1, or E2 mechanisms. It guides you through a series of questions about the nucleophile's charge, bulkiness, and the quality of the leaving group. By following the flowchart, you can systematically narrow down the possible mechanisms based on the given conditions, making it easier to predict the outcome of the reaction. This tool is especially useful for students preparing for exams and needing a structured approach to problem-solving in organic chemistry.

What are some common bulky bases used in E2 reactions?

Common bulky bases used in E2 reactions include tert-butoxide (t-BuO^-), lithium diisopropylamide (LDA), and lithium tetramethylpiperidide (LiTMP). These bases are large and sterically hindered, making them effective at abstracting protons but poor nucleophiles. Their bulkiness increases their basicity, favoring the E2 elimination mechanism over substitution reactions.